41

Engineering Alloys

(Ferrous and Non-Ferrous) UNIT 2 ENGINEERING ALLOYS (FERROUS

AND NON-FERROUS)

Structure

2.1 Introduction

Objectives

2.2 Production of Iron and Steel

2.3 Casting of Ingots

2.4 Continuous Casting

2.5 Steels

2.6 Heat Treatment of Steel

2.7 Hardenability of Steel

2.8 Tempering

2.9 Special Treatments

2.10 Surface Hardening

2.11 Heat Treating Equipment

2.12 Alloy Steels

2.13 Cast Iron

2.14 Non-ferrous Materials

2.15 Aluminium

2.16 Copper and its Production

2.17 Copper Alloys

2.18 Magnesium and its Alloys

2.19 Titanium Alloys

2.20 Bearing Materials

2.21 Alloys for Cutting Tools

2.22 Summary

2.23 Key Words

2.24 Answers to SAQs

2.1 INTRODUCTION

Out of solid materials used in engineering practice metals, plastic and ceramics are very

common. Then metals may be used in their elemental forms like aluminium, copper and

titanium. When a metallic element has additives much smaller in quantity than base

element, the resulting material is called an alloy of base element. Out of all metallic

elements it is iron whose alloys are used in largest quantity. All such alloys in which iron

forms the base are grouped as ferrous material. The other alloys are grouped as

non-ferrous materials.

Ferrous materials, both metal and alloys have iron as their base and due to wide range of

their properties are most useful for use in engineering machines and structures. Owing to

the advents in steel technology and casting technique ferrous metals are cast, shaped and

42

Engineering Materials machined in various shapes and sizes. Several standard shapes of sections are variable

commercially which make the job of designer and constructor much easy. They are used

for making trusses, bridges, ships and boilers. For such construction standard section and

sheets of plats of steel are available. The other machine parts like shafts, gears, bearings,

pulleys and bodies of machines can be made in steel through forming, cutting or casting

processes or combination thereof. Metal cutting tools, dies, punches, jigs and fixtures are

also made in ferrous metal. One of the largest consumer of steel is automobile industry.

Despite the modern trend of making light cars nearly 60% of weight of car is still due to

steel and an average passenger car contains about 500 kgf of steel in India. Perhaps in

countries like USA where cars of bigger size are in use this weight could be as high as 800

kgf/car.

The first human effort in the direction of making tools was based upon meteoritic iron

obtained from meteorite that had struck the earth. This happened more than 3000 BC. In

India the well known Ashoka Column in Delhi was constructed more than 4000 years ago.

The blast furnace was invented in 1340 AD and then it became possible to produce large

quantities of iron and steel. The future trend is to replace steel by plastics in many

machines and equipment. This target has been achieved in a number of home appliances.

The demand for steel is level since 1965. Cost fluctuations in most metals have been

controlled. The same is true for steel whose cost is increasing at constant rate since early

eighties. The comparative price of various metals with piece of gold at 1000 is given in

Table 2.1.

Table 2.1 : Approximate Comparative Prices of Various Metals

with Gold Piece of 100 as Base (per Weight)

Steel 0.0476

Aluminium 0.2078

Copper 0.3140

Magnesium 0.3528

Zinc 0.1840

Gold 1000

Lead 0.075

Nickel 1.5151

Tin 1.0823

Titanium 1.1363

Silver 15.1515

Objectives

After studying this unit, you should be able to

• know how iron and steel are produced,

• know what are different classifications and applications of steel,

• understand how different types of steel are formed as alloy,

• identify different constituents of steel and their effects on properties,

• know how steel can be treated,

• understand an alloy steel and effects of alloying on properties,

• distinguish between steel and cast iron and properties and uses of cast iron,

• know about alloys of copper, aluminium and their properties and uses,

• identify bearing materials, and

43

Engineering Alloys

(Ferrous and Non-Ferrous) • identify creep resistant materials.

2.2 PRODUCTION OF IRON AND STEEL

Reviewing the principles of the iron and steel making processes, beginning with new

materials is taken up first. This knowledge is essential to an understanding of the quality

and characteristics of the steels produced by different processes.

2.2.1 Raw Materials

The three basic materials used in iron and steel making are iron or, limestone and coke.

Iron does not occur in a free state in nature, yet it is one of the most abundant elements,

making up about 5 percent of the earth’s crust in the form of various ores. The principal

iron ores are taconite (a black flintlike rock), hematite (an iron oxide mineral) and

limonite (an iron oxide containing water). After it is mined, the iron ore is crushed into

fine particles, the impurities are removed by various means (such as magnetic separation),

and it is formed into pallets, balls, or briquettes using binders and water. Typically, pellets

are about 65 percent pure iron and 25 mm in diameter. The concentrated iron ore is

referred to as beneficiated. Some iron-rich ores are used directly without palletising.

Coke is obtained from special grades of bituminous coal, which are heated in vertical coke

ovens to temperatures of 1150oC and cooled with water in quenching towers. Coke has

several functions in steel making. One is to generate the high level of heat required for

chemical reactions to take place in iron making. Second, it produces carbon monoxide (a

reducing gas) which is then used to reduce iron oxide to iron. The chemical by-products of

coke are used in making plastics and chemical compounds. Coke oven gases are used as

fuel for plant operations, and power generations.

The function of limestone (calcium carbonates) is to remove impurities from the molten

iron. The limestone reacts chemically with impurities, action as a flux which causes the

impurities to melt at a low temperature. The limestone combines with the impurities and

forms a slag, which is light and floats over the molten metal. Slag is subsequently

removed. Dolmite (an ore of calcium magnesium carbonate) is also used as a flux. The

slag is later used for making cement, fertilizers, glass, building materials, rock wool

insulation, and road ballast.

2.2.2 Iron Making

The three raw materials are charged into blast furnace by carrying them to the top of and

dumping into the furnace. The principle of this furnace was developed in Central Europe,

and the first furnace began operating in 1621. The first steel plant in India begins its

operation in the early part of twentieth century. The blast furnace is basically a large steel

cylinder lined with refractory (heat-resistant) bricks and has the height of about a ten-

storey building.

The charge mixture is melted in a reaction at 1650oC with air preheated to about 1100oC

and blasted into the furnace (hence the term blast furnace) through nozzles (or tuyeres).

Although a number of reaction may take place, basically the reaction of iron oxide with

carbon produces carbon monoxide, which in turn reacts with the iron oxide, reducing it to

iron. Preheating the incoming air is necessary because the burning coke along does not

produce sufficiently high temperature for the reactions to occur.

The molten metal accumulates at the bottom of the blast furnace, while the impurities float

to the top of the metal. At intervals of four or five hours, the molten metal is tapped, into

ladle cars. Each ladle car can hold as much as 160 tons of molten iron. The molten metal

at this stage has a typical composition of 4 percent carbon, 1.5 percent silicon, 1 percent

manganese, 0.04 percent sulphur, and 0.4 percent phosphorous, with the rest being pure

iron. The molten metal is referred to as pig iron. Use of the world pig comes from the

44

Engineering Materials early practice of pouring molten iron into small sand molds, arranged like a litter of small

pigs around a main channel. The solidified metal is called pig and is used in making iron

and steels. The blast furnace is shown in Figure 2.1.

Figure 2.1 : Blast Furnace

2.2.3 Steel Making

Steel was first produced in China and Japan in about 600-800 AD. The process is

essentially one of refining the pig iron obtained from the blast furnace. The refining of pig

iron consists of reduction of the percentage of manganese, silicon, carbon and other

elements, and control of its composition by the addition of various elements. The molten

metal from the blast furnace is transported into one of three types of furnace. The steel

making furnaces are open hearth, electric, or basic oxygen. The name open hearth derives

from the shallow heart shape that is open directly to the flames that melt the metal.

Developed in the 1860s, the open-hearth furnace is being replaced by electric furnace and

by the basic-oxygen process. These newer methods are more efficient and produce better

quality steels.

The electric furnace was first introduced in 1906. The source of heat is a continuous

electric arc formed between the electrodes and the charged metal (Figure 2.2).

Temperature as high as 1925oC are generated in this type of furnace. There are usually

three graphites electrodes in direct arc electric furnace, and they can be as large as

750 mm in diameter and 1.5 to 2.5 m in length. Their height in the furnace can be adjusted

depending on the amount of metal present and water of the electrodes.

Figure 2.2 : Direct Arc Electric Furnace

Hopper

380C

4800C

12050C

16500C

Molten Slag Molten Iron

Reduction Zone

Combustion Zone

Fusion Zone

Heat Absorption Zone

Gas

Iron Ground Slag

Tuyere

Hot Blast

Coke Ore

Lime Stone

Coke + Ore + Lime Stone

Power Leads

Carbon Electrodes

Door

Slag

Metal Rammed Hearth

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Engineering Alloys

(Ferrous and Non-Ferrous) Steel scrap and a small amount of carbon and limestone are dropped into the electric

furnace through the open roof. Electric furnaces can also use 100 percent scrap as its

charge. The roof is then closed and the electrodes are lowered. Power is turned on, and

within a period of about two hours the metal melts. The current is shut off, the electrodes

are raised, the furnace is titled, and the molten metal is poured into a ladle, which is a

receptacle used for transferring and pouring molten metal. Electric-furnace capacities

range from 60 to 90 tons of steel per day. The quality of steel produced is better than that

of open-hearth or basic-oxygen process.

Figure 2.3 : Indirect Arc Electric Furnace

The induction type electric furnace (Figure 2.4) is used for smaller quantities. The metal

is placed in crucible, made of refractory material and surrounded with a copper coil

through which alternating current is passed. The induced current in the charge melts the

metal. These furnaces are also used for re-melting metal for casting.

Figure 2.4 : Induction Type Electric Furnace

The basic-oxygen furnace (BOF) is the newest and fastest steel making process.

Typically, 200 tons of molten pig iron and 90 tons of scrap are charged into a refractory

lined barrel shaped vessel called converter [Figure 2.3(a)]. Pure oxygen is then blown into

the furnace for about 20 minutes under a pressure of about 1250 kPa, through a water-

cooled lance, as shown in Figure 2.5(b). Fluxing agents, such as burnt lime are added

through a chute.

Electrodes

Trunion

Roller Metal

Crucible

Copper Induction

Coil

Refractory Cement

Molten Metal

(c) Tapping the (d) Pouring the

Lance

(b) Blowing with

(a) (i) Charging Scrap

into Furnace

(ii) Charging

Molten

Iron

(iii) Addition of

Burnt Lime

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Engineering Materials

Figure 2.5 : Basic Oxygen Process of Steel Making Illustrated through Various Operations

The vigorous agitation by the oxygen refines the molten metal through an oxidation

process in which iron oxide is first produced. The oxide reacts with the carbon in the

molten metal, producing carbon monoxide and carbon oxide. The iron oxide is reduced to

iron. The lance is retracted and the furnace is tapped by tilting it. The opening in the vessel

is so provided that the slag still floats on the top of the molten metal as seen in Figure

2.5(c). The slag is then removed by tilting the furnace in the opposite direction.

The BOF process is capable of refining 250 tons of steel in 35 to 50 minutes. Most BOF

steels, which are of better quality then open-hearth furnace steels and have low impurity

levels, are processes into plates, sheets, and various structural shapes, such as I-beams and

channels.

Steel may also be melted in induction furnaces from which the air has been removed,

similar to the one shown in Figure 2.4. The vacuum melting produces high quality steels

because the process removes gaseous impurities from the molten metal.

2.3 CASTING OF INGOTS

After the molten steel has been poured from the steel making furnace it has to be converted

in solid shapes called ingot. The ingot is further processed by rolling it into shapes, casting

it into semi-finished forms, or forging. For eliminating the need for ingot the shaping

process is being rapidly replaced by continuous casting, thus improving efficiency. The

molten metal is poured (teemed) from the ladle into ingot moulds in which the metal

solidifies. Moulds are usually made of cupola iron or blast-furnace iron, with 3.5 percent

carbon, and are tapered in order to facilitate the removal of the solid metal. The bottoms of

the moulds may be closed or open; if open, the mould are placed on a flat surface. The

taper may be such that the big end is down.

The cooled ingots are removed (stripped) from the moulds and lowered into soaking pits,

where they are reheated to a uniform temperature of about 1200oC for subsequent

processing by rolling. Ingots may be square, rectangular, or round in cross-section, and

their weights ranges from a few hundred kgf of 40 tons.

Certain reactions take place during the solidification of an ingot, which in turn have

important influences on the quality of the steel. For example, significant amounts of

oxygen and other gases can dissolve in the molten metal during steel making. However,

much of these gases is rejected during solidification of the metal because the solubility

limit of gases in the metal decrease sharply as its temperature decreases. The rejected

oxygen combines with carbon, forming carbon monoxide, which causes porosity in the

solidified ingot.

Three types of steel are produced depending on the amount of gas evolved during

solidification. These types are : killed, semi-killed, and rimmed.

Killed Steel

Killed steel is usually deoxidized steel; from which oxygen has been removed and

porosity eliminated. In the de-oxidation process the dissolved oxygen in the molten

metal is made to react with elements such as aluminium, silicon, manganese, and

vanadium that are added to the melt. These elements have an affinity for oxygen and

form metallic oxides. If aluminium is used, the product is called aluminium-killed

steel. The term killed comes from the fact that the steel lies quietly after being

poured into the mould. The oxides in the slag. A fully killed steel is thus free of any

porosity and blowholes caused by gases. The chemical and mechanical properties of

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Engineering Alloys

(Ferrous and Non-Ferrous) killed steel are relatively uniform throughout the ingot. However, because of metal

shrinkage during solidification, an ingot of this type develops a pipe at the top. It is

also called shrinkage cavity and has the appearance of a funnel-like shape. This

pipe can comprise a substantial portion of the ingot and has to be removed.

Semi-killed Steel

Semi-killed steel is partially deoxidized steel. It contains some porosity, generally in

the upper section of the ingot, but has little or no pipe; thus scrap is reduced.

Although piping in semi-killed steels is less, it is compensated for by the presence of

porosity in that region. Semi-killed steels are economical for deoxidation process is

quite costly.

Rimmed Steel

Rimmed steel, generally having a low carbon content (less than 0.15 percent), have

the evolved gases only partially killed or controlled by the addition of elements such

as aluminium. The gases form blowholes along the outer iron of the ingot – hence

the term rimmed. Blowholes are generally not objectionable unless they break

through the outer skin of the ingot. Rimmed steels have little or not piping, and have

a ductile skin with good surface finish. The blowholes may break through the skin if

they are not controlled properly. Impurities and inclusion tend to segregate toward

the centre of the ingot. Thus, products made from this steel may be defective and

should be inspected.

Refining

The properties and manufacturing characteristics of ferrous alloys are adversely

affected by the amount of impurities, inclusions, and other elements present. The

removal of impurities is known as refining, much of which is done in melting

furnaces or ladles, with the addition of various elements. The cleaner steels having

improved and more uniform properties and consistency of composition are

increasingly being demanded. Refining is particularly important in producing high-

grade steels and alloys for high-performance and critical applications, such as in

aircraft. Moreover, warranty periods on several machine parts such as shafts,

camshafts, crankshafts for diesel trucks, and other similar parts can be increased

significantly using higher-quality steels.

The trend in steel making is for secondary refining in ladles and vacuum chambers.

New methods of ladle refining (injection refining) generally consist in melting and

processing in a vacuum. Several methods of heating and re-melting have been

introduced for their efficiency and cleanliness. These are normally used in controlled

atmosphere. Some methods are : electron-beam melting, vacuum-arc re-melting,

argon-oxygen decarburisation, and vacuum are double-electrode

re-melting.

2.4 CONTINUOUS CASTING

The traditional method of casting ingots is a batch process. Each ingot is stripped from its

mould after solidification and processed individually. Additionally the defects like piping

and micro-structural and chemical variations are present throughout the ingot. These

problems are alleviated by continuous casting process, which produce better quality

steels.

Continuous or strand casting was first developed for casting non-ferrous metal strip. The

process is now used for steel production, with major efficiency and productivity

improvements and significant cost reduction. A system for continuous casting is shown in

Figure 2.6. The molten metal in the ladle is cleaned and nitrogen gas through it is blown

for five to ten minutes to equlise the temperature. The metal is then poured into a

refractory-lines intermediate pouring vessel called tundish where impurities are skimmed

off. The tundish can hold as much as three tons of metal. The molten metal vessels through

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Engineering Materials water-cooled copper moulds and begins to solidify as it travels downward along a part

supported by rollers.

Before starting the casting process, a solid starter, or dummy, bar is inserted into the

bottom of the mould. The molten metal is then poured and freezes onto the dummy bar

(Figure 2.6). The bar is withdrawn at the same rate the metal is poured. The cooling rate is

such that the metal develops a solidified skin to support itself during its travel downward

at speed maintained at 25 mm/s. The shell thickness at the exit end of the mould is about

12 to 18 mm. Additional cooling is provided by water sprays along the travel path of the

solidifying metal. The moulds are generally coated with graphite or similar solid lubricants

to reduce friction and adhesion at the mould walls. Vibration of moulds may further reduce

friction and adhesion tendency.

The continuously cast metal may be cut into desired lengths by shearing or touch cutting,

or it may be fed directly into a rolling mill for further reduction in thickness.

Whether the steel is obtained in form of ingots, stationary moulds or in form of slab from

continuous casting process, it is converted into blooms, billets and slabs. The subsequent

hot rolled products are described as follows :

Blooms

Beams and angle sections, rails, bars of different sections.

Billets

Wire nails and wire mesh, pipes and tubes.

Slabs

Plates, strips and further cold rolled for reducing thickness.

Figure 2.6 : Continuous Casting Process of Steel Schematically

SAQ 1

(a) Distinguish between an elemental metal and alloy.

Starting Dummy

Oxygen Lance

(For Cutting)

Pinch Rolls

Catch Basin

Air Gap

Argon Platform

Electric Furnace

Tundish

Oil

Cooling Water

X – Ray Transmitter

Molten Metal

Solidified Metal

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Engineering Alloys

(Ferrous and Non-Ferrous) (b) What are the raw materials used in blast furnace in iron making process?

(c) What are electric heating processes for making steel?

(d) Describe BOF and its advantages.

(e) Distinguish between killed and semi-killed steels.

(f) List steel products obtained from blooms, billets and slabs.

2.5 STEELS

Steel perhaps is the oldest material of construction which has weathered and history and

not only maintained its foremost place in industrial application but also enhanced it

greatly. The enhancement of its position in industrial world is mainly due to its capacity to

be produced in several alloyed varieties and response to various heat treatments. The

cutting properties of sharp edges of steel was recognised long back when man began to use

swords and knives made in steel. The very same properties of this material have been

exploited to create cutting tools which wear very little and such machine parts as gears,

shafts, bearing, etc. The steel is now being used in every conceivable engineering structure.

Machine bodies, railways and rail road rolling stocks, ships, bridges and boilers are a few

examples. Several forms of steel are now available commercially and each is produced to

serve some specific purpose. The requirements of steel very largely and more often than

one involve consideration of their tensile strength, impact strength and hardness. Table 2.2

describes some applications and requirements of some steels.

Table 2.2 : Typical Carbon Steels and their Applications

Requirements % Composition Application

C Mn Si P S

Axles, shafts,

small gears

Availability in form of bar stock. Good

strength in bending and torsion. Heat

treatable for improvement of surface

resistance.

0.4 0.8 0.1 0.05 0.05

Helical

springs

Availability in rod form, this is obtained

through rolling. Good ductility for

coiling. Good response to heat treatment

for spring properties.

1.0 0.6 0.3 0.05 0.05

Automative

bodies and

panels

Availability in form of thin sheets which

can be pressed into accurate shapes. Good

ductility and low yield strength.

0.0

8

0.3 − 0.05 0.05

Ship hulls Availability in thick plate forms. Good

strength to withstand stresses during

forming and service, particularly good

ductility at low temperatures. Good

weldability since most ships have welded

structure.

0.1

8

0.8 0.1 0.05 0.05

The carbon content of steel plays an important role in deciding its properties. If no carbon

is present in iron, it crystallises in form of ferrite which is bcc soft material and very

ductile. Pure iron without impurities is perhaps used only in laboratories and may be as

costly as gold. Pure iron is though but not very strong. With addition of carbon, increasing

amounts of cementite (Fe3C) crystallise in the structure. Cementite being hard reduces

ductility considerably. Table 2.3 would illustrate how ductility (measured as % elongation)

of steel is reduced with increasing amount of carbon. When carbon is as much as 6.67% in

iron, the entire structure crystallises as cementite and is not at all usable commercially

because it neither has ductility nor is machinable.

50

Engineering Materials Table 2.3 : Ductility of Plain Carbon Steel

Carbon Content

(%)

% Elongation

Pure iron 42

0.2 37

0.4 31

0.6 22

0.8 17

1.2 3

The carbon content in steel serves to classify steel according to its application. Table 2.4

describes the application of different iron-carbon alloys. Figure 2.7 illustrates the effect of

carbon percentage on tensile strength of steels mainly. The tensile strength of pure iron

(0% C) is around 250 N/mm2 which increases to about 850 N/mm

2 at a carbon percentage

of 0.8. It has already been pointed out that a very low carbon content the entire metal is

made up of ferrite grains which are soft and ductile. As the percentage of carbon increases

more and more material is made up of pearlite, a substance that appears to have colour of

mother-of-pearl, hence the name. At 0.4% C, pearlite appears to be almost half the area

viewed through microscope. At this level of carbon the tensile strength is about 540

N/mm2. At 0.8% C almost total area consists of pearlite when a strength of 850 N/mm

2 is

reached.

Table 2.4 : Carbon Percentage in Plain Carbon Steels and Application

Range of Carbon

(%)

Application

0.1 – 0.8 General engineering

purposes

0.0 – 1.2 Wear resistance steel

1.3 – 2.2 Not used normally

2.4 – 4.2 Cast iron, casting

2.5.1 Plain Carbon Steel and Applications

Some applications of carbon steels have been described already in Table 2.2. Here some of

the applications will again be described after emphasising the manner in which plain

carbon steels are classified. Plain carbon steels are those which contain carbon as principal

alloying element. These steels may also contain small amounts of such impurities as

manganese, sulphur, phosphorous, silicon and nickel. The sulphur and phosphorous are

mainly undesirable impurities and attempts are make to keep them at as low level as

possible. Their levels beyond 0.05% are not permissible.

According to carbon percentage (or microscope structure) the steel is divided into three

groups.

Eutectoid Steels

Such steels contain ideally 0.83% of carbon and have entirely lamellar peralite

structure. In practice fully pearlite structure appears in all steels containing about

0.8% carbon. Moreover many alloying elements influence the carbon contents of

eutectoid steels. For instance, Mn to the extent of 1% reduces carbon in the

eutectoid to 0.7%.

Hypo-Eutectoid Steels

These steels contain carbon between 0.08% to just below 0.83%. They contain

grains of ferrite together with grains of pearlite. The strength increases with

increasing carbon content due to increasing proportion of strong pearlite formed but

ductility decreases proportionally.

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Engineering Alloys

(Ferrous and Non-Ferrous) Hyper-Eutectoid Steels

These steels contain carbon significantly in excess of 0.8%. The structure contains

pearlite and cementite. Cementite forms along the grain boundaries of pearlite as an

inter-granular network and increase brittleness.

With respect to range of carbon content the plain carbon steels are divided into following

groups. The carbon percentages may overlap in many cases.

Dead Mild Steels

The carbon range for this steel is between 0.07% to 0.15%. These steels are highly

ductile and hence can withstand large amount of plastic deformation through cold

working. Solid drawn tubes are made out of deal mild steels.

Mild Steels

The carbon percentage for mild steels very between 0.15 to 0.25. This steel does not

harden appreciably when quenched. It has very good weldability and is commonest

of the steels used for structural purposes.

Medium Carbon Steels

These steels contain carbon between 0.25 and 0.55%. These steels respond to

suitable types of heat treatments.

High Carbon Steels

The carbon content in these steels vary between 0.55% to 0.9%. Very high

hardnesses can be achieved through heat treatment. They develop extreme resistance

to wear and hence are good for tooling applications.

In yet another classification steels containing upto 0.3% C are known as low carbon steels

thus including both dead and mild steel. Those containing carbon between 0.3 and 0.6%

are medium carbon steels and those containing carbon in excess of 0.6% carbon are high

carbon steel thus including carbon tool steels.

Table 2.5 describes various applications of plain carbon steels, whereas Table 2.6

describes plain steels for several applications.

Table 2.5 : Some Applications of Plain Carbon Steels

Steel % Carbon Applications

Dead mild steel 0.07 – 0.15 Rivets, nails, tin plates, stamping,

chains, seam welded pipes, automative

body sheets, other materials subject to

drawing and pressing.

Mild steel 0.10 – 0.20

0.20 – 0.30

Structures, rolled steel sections, drop

forgings, screws, case hardening

purposes.

Machine structures, shafting and

forging, gear.

Medium carbon steel 0.30 – 0.40

0.40 – 0.50

0.50 – 0.60

Shafting, axles, crane hooks,

connecting rods, general purpose

forgings.

Axles, gear, shafts, rotors, tyres, skip

wheels, crank shafts.

Rails, loco tyres, wire ropes.

High carbon steel 0.60 – 0.70 Drop hammers, dies, saws, screw

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Engineering Materials

0.70 – 0.80

0.80 – 0.90

drivers.

Hammers, anvils, wrenches, leaf

springs, cable wires, band saws, large

dies for cold presses.

Shear blades, punches, rock drills, cold

chisels.

Tool steels 0.90 – 1.10

1.10 – 1.40

Drills, knives, taps, picks, screwing

dies, axes.

Razors, files, broaches, boring and

finishing tools, such machine parts

where wear resistance is required, ball

bearings.

Table 2.6 : Plain Carbon Steels for Different Applications

Sl. No. Application Properties Steel

1 Nails, rivets,

stampings

High ductility, low

strength

Low C (AISI 1010)

2 Beams, rolled

sections, reinforcing

bars, pipes boiler

plates, bolts

High ductility, low

strength, toughness

Low C (1020)

3 Shafts and gears Heat treatable for

good strength and

ductility

Med C (1030)

4 Crank shaft, bolts,

connecting rod,

machine component

Heat treatable for

good strength and

ductility

Med C (1040)

5 Lock washers, valve

springs

Toughness High C (1060)

6 Wrenches, dies,

anvils

High toughness and

hardness

High C (1070)

7 Chisels, hammers,

shear

Retaining sharp edges High C (1080)

8 Cutters, tools, taps,

hacksaw blades,

springs

Hardness, toughness,

heat treatable

High C (1090) tool

steel

SAQ 2

(a) What are distinguish features of eutectoid, hypoeutectoid and hypereutectoid

steels?

(b) How are plain carbon steels classified as low, medium and high carbon

steels?

(c) Describe uses of low, medium and high carbon steels.

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Engineering Alloys

(Ferrous and Non-Ferrous) 2.5.2 Iron-Carbon System Phase Diagram

Iron and carbon make a series of alloys which include a number of steels and cast iron.

Steels are the alloys which contain upto 1.2% of carbon while cast irons contain carbon

within range of 2.3% to 4.2%. Alloys with carbon greater than 4.6% have poor properties

and hence not used.

The carbon atom is smaller than the iron atom (the diameters being 1.54 o

A and 2.56 o

A

respectively) and dissolves interstitially in iron. Pure molten iron solidifies at 1539oC in

bcc structure. At 1400oC, on further cooling this structure changes into fcc and again at

910oC back to bcc. These three structures in reverse order (i.e. form room temperature)

are named as α, γ and δ.

The α-iron is ferromagnetic but loses its ferromagnetism when heated above 770oC (curic

temperature). The non-magnetic α-iron was earlier known as β-iron until X-ray studies

showed that structures of α − and δ − irons are same. The γ-iron is the structure of closest

packing while α − and β − irons are not. There will be abrupt changes of dimensions at

transitions α to γ and γ − β. Figure 2.7 depicts the cooling curve of pure iron.

Figure 2.7 : Cooling Curve of Pure Iron with Steady Rate of Heat Loss

Alloys in iron-carbon systems also undergo complex structural changes which play an

important role in deciding their characteristics. Changes that occur in iron-carbon system

are illustrated in iron-carbon phase diagram of Figure 2.8. Strictly speaking the diagram

refers to the iron-iron carbide system but still the phase relationship can be expressed in

terms of carbon percentage, hence the name.

Iron-carbon diagram is divided in various phase fields characterised by the existence of

one or mixture of two phases. The liquids above which only molten metal in liquid state

can exist is identified as ABCD. Iron and dissolved carbon exist in the liquid state. The

solidus below which iron-carbon exist in solid state is identified as AEPGCH. Between

these two lies there exist mixtures of solid and liquid.

α and γ phases can dissolve carbon and the solubility changes with temperatures. The solid

solution of carbon in α-iron is called ferrite while the solid solution of C in γ-iron is

known as austenite. Ferrite and austenite are also designated as α and γ respectively. the

solubility of C in austenite is upto 0.2% while ferrite can dissolve C only upto 0.025%.

The C solubility in austenite changes along GK in austenite and along LN in ferrite.

Beyond point C on solidus, the compound Fe3C which is cementite, separates. Cementite

is hard and brittle whereas ferrite on the extreme left of phase diagram is soft and ductile.

Austenite also transforms into cementite along the ling GK and into ferrite along IK.

0

500

1000

1500

Liquid

1539

1400

δ (bcc)

γ (fcc)

910 β (Non-magnetic α)

770

α (bcc)

Time

54

Engineering Materials The line IK and GK are respectively designated as A3 and Acm. Austenite is unstable below

the lines A3 and Acm if carbon content is less than 0.8%. Austenite begins to transform into

ferrite on cooling and gets enriched in carbon along line A3 until point K is reached.

Similarly for carbon content between 0.8 and 2.0% iron carbide will precipitate and

carbon in austenite will vary until point K (carbon 0.8%) is reached. At point K austenite

will transform into pearlite. Pearlilte is an intimate mixture of ferrite and cementite (Fe3C)

having a characteristics lamellar structures composed of alternate platelets of ferrite and

cementite. The transformation reaction at K wherein a single solid phase splits into two

phases is termed eutectoid. The equation is written as

(Austenite) (Ferrite) (Cementite)

Solid phase ( ) Solid phase ( ) Solid phase ( )A B C→ +

Apparently at point K three phases exist (P = 3) and three are two components (C = 2),

hence using Gibb’s phase rule the degree of freedom, F can be calculated.

Gibb’s rule at constant pressure,

P + F = C + 1

F = 2 – 3 + 1 = 0

Thus the eutectoid point like eutectoid point is non-variant.

Slightly above 110oC at point C on solidus, the eutectoid transformation occurs. The liquid

state at C contains 4.3% C and this liquid begins to transform into two solid phase. One

phase is called Ledeburite which is eutectic mixture of austenite and cementite while the

other phase is cementite. On further cooling eutectic austenite transforms gradually into

cementite, changing composition along GK until it changes into pearlite and cementite at

eutectoid point K.

Peritectic

Peritectic transformation occurs in steel having carbon upto 0.55%. This

transformation occurs at point P and is characterised by transformation of liquid

and solid phase into a single solid phase. For example, the transformation occurring

at P is represented by the following equation,

Liquid (0.5% C) + δ iron (0.08% C) → γ iron (0.18% C)

Explanation of Reading Iron-Carbon Phase Diagram

Assume that in liquid state mixture of iron and carbon contains 0.4% carbon and is

represented by point 1 in Figure 2.8. The line xx shows the path of cooling.

As cooling begins the freezing starts in intersection point 2 of xx and liquidus ABC

at 1510oC. At a temperature of 1470

oC at point 3 the liquid is solidified completely

into solid austenite. This phase gradually cools until point 4 on line IK is reached.

The precipitation of proeutectoid ferrite begins and the carbon content of austenite

varies along line A3. The composition of ferrite varies along the line IL. In the region

enclosed between IL and IK he percentage of austenite and ferrite can be determined

by using lever rule. The remaining austenite transforms into peralite (88% ferrite

and 12% cementite) at eutectoid temperature of 723oC. The % weight of (ferrite +

cementite) can be determined by lever rule along line LKM. Further cooling will

effect no change in microstructure since carbon percentage in ferrite practically

remains constant of 0.008. The microstructure is built by pearlite matrix embedded

with ferrite crystals.

Taking example of alloys with 3% carbon and considering its cooling from a point

above liquids along yy. The composition of course is in the region of cast iron. The

austenite separates from liquid and its content will increase along the solidus PG

and the amount of liquid reduces with composition varying along liquidus BC.

When the temperature of 1130oC is reached, the mixture will consists of austenite

containing 2% carbon and liquid of eutectic composition.

55

Engineering Alloys

(Ferrous and Non-Ferrous) The liquid will solidify at constant temperature into ledeburite – a mixture of

austenite and cementite. On further cooling eutectic austenite decomposes to

cementite along the line GK. At temperature of 723oC (eutectoid). The remaining

austenite will transform to pearlite. Below the temperature of 723oC all ledeburite

will be transformed into a mixture of pearlite and cementite.

Transformation Reactions

Several transformation were described in above paragraphs while explaining how to

read iron-carbon diagram. These transformation are summed up afresh here.

Eutectoid Reaction

Eutectoid reaction takes place when austenite containing 0.77% C

decomposes into ferrite and cementite at 723oC (point K in the phase diagram

of Figure 2.8).

o

0.77% C

3723 C

Solid austenite ( ) Ferrite ( ) Cementite (Fe C)γ → α +

Solid phase splits into two solids phases. Steel containing carbon between

0.008 and 0.8% is called hypoeutectoid and those containing between 0.8

and 2.0% carbon are hypereutectoid.

Eutectic Reaction

Eutectic reaction occurs when liquid solution containing 4.3% C

transformations into austenite (γ) and cementite (Fe3C) at 1130oC. Point C in

Figure 2.8 is eutectic point.

o

4.3% C

31130 C

Liquid Austenite ( ) Cementite (Fe C)L → γ +

Liquid phase transforms into two solid phases.

Figure 2.8 : Iron-Carbon Phase Diagram

Peritectic Reaction

Peritectic reaction occurs when a liquid and a solid phase freeze to form a

solid phase. In iron-carbon system peritectic reaction takes place when alloy

containing 0.55% C and containing liquid and solid δ iron transforms at

1495oC into solid austenite (γ).

o(1495 C)(0.99% C)(0.55% C) (0.17% C)

Liquid -Iron Solid austenite+ δ →

56

Engineering Materials 2.5.3 Time Temperature Transformations

The phase diagram of iron-carbon system evokes much interest for engineers to use the

information for useful purposes of increasing strength and eliminating locked in stresses by

retaining or avoiding a particular phase. However, important aspect of such a diagram to

keep in mind is that it represents equilibrium cooling. Such equilibrium cooling is neither

obtained in practice nor it is conducive to develop desired strength and structure in a

particular steel. Increasing cooling rates reduces the transformation temperature as was

highlighted and may result information of metastable phase. As an example very high

cooling rate of iron-carbon system in steel range causes development of a metastable phase

called martensite. This phase does not appear in Figure 2.8. The steel and cast iron

normally carry some alloying elements, though in varying amounts. These alloying

elements have great deal of influence on precipitation of all the phases and this is also not

represented by phase diagram of Figure 2.8.

The metastable phase marteniste has been introduced here and other phases like pearlite

ferrite, and cementite were mentioned earlier. More about their structure and properties

will be discussed now but before that transformation curves are described.

Experimental determination of isothermal transformation curves goes as under. A number

of samples of the size of a 50 paise coin are austenised just above the temperature of

723oC (eutectoid temperature). The samples are rapidly cooled in a salt bath maintained at

a temperature slightly below 723oC. After allowing various time intervals the specimens

are taken out one by one from the salt both and quenched into water at room temperature.

The resulting microstructure is then examined at room temperature. The experiment is

repeated with isothermal transformation of eutectoid steel (both regions of hypo- and

hypereutectoid steel) at progressively reducing temperatures. If temperature of

transformation is plotted as function of time of transformation the resulting curve as

shown schematically in Figure 2.9 is called isothermal transformation (IT) curve, or

temperature-time-transformation curve (TTT). This is also known as Bain curve after the

metallurgist who first introduced the idea of S curve because of its shape.

Figure 2.9 : Time Temperature Transformation Diagram for Plain Carbon Steel

The line marked Ps shows the beginning of transformation of austenite into pearlite and the

line Pf represents the completion of such transformation. Figures 2.10 and 2.11 show the

TTT diagrams respectively for 0.8% C steel and 0.3% steel. The difference between the

two can be noted. Even the fastest cooling rate will not be able to mess the nose of the S

curve and hence 100% martensite will not be retained at room temperature in structure of

steel. The important feature of TTT curve is its bending backwards at nose. Below the nose

the austenite transforms into Bainite. Whether it is pearlite or bainite, at a temperature

called Ms the transformation into a transition phase Martenstite takes place.

Both binite and martensite may be retained in steel by controlling the rate of cooling. If

cooling is sufficiently fast so that nose is avoided then austenite transforms into bainite. If

the part is now held at this temperature for sufficiently long time the bainite is stabilised.

Unlike, pearlite, in bainite the cementite is in particle form, distributed uniformly in the

Eutectoid Temp

Coarse Pearlite

Fine Pearlite

Bainite

Pf Ps

PS

Pf

Time

Martensite

Tem

pera

ture

(0 C

)

57

Engineering Alloys

(Ferrous and Non-Ferrous) matrix of ferrite. Bainite is harder, stronger and tougher than pearlite. Bainitic steel is

more ductile than pearlitic steel for some level of hardness.

Figure 2.10 : TTT Diagrams for 0.8% C Steel with 0.76 Mn

If heated steel is cooled sufficiently fast the nose of Ms temperature, martensite is formed.

Its transformation is complete at temperature Mf. Martensite has C dissolved in Fe

whereby its bcc structure changes into body centered tetragonal (bct) structure and is

marked by high hardness because of :

(a) distortion of iron lattice,

(b) very fine size of martensite plates, and

(c) high density of dislocations associated with twining.

Figure 2.11 : TTT Diagrams for 0.3% C Hypoeutectoid Steel

Figure 2.12 schematically shows the cooling pattern to produce different phases in steel.

Heat treatments given to steel will be governed by the cooling rates producing final phases

with pearlite, bainite or martensite. Many refinements are possible with control of cooling

rate and soaking time.

4sec 0-5min 1min 4min 1 hr 15 hr

MF

Ms Martensite

γ

γ Ps

PF

Bs BF Bainite

Fine Pearlite

Coarse Pearlite

727

550

230

110

Tem

pera

ture

4sec 0.5min 4min 1 hr 15 hr

MF

Ms Martensite

Ps

PF

Bs BF

Ferrite+Pearlite

727

340

210

Tem

pera

ture

(0 C

)

Ferrite + Pearlite + Bainite +

Time

230

110

727

1 2 3 4

Ms

MF

I II III

1sec 2sec 4min 2hr

Time

Tem

pera

ture

(0 C

)

58

Engineering Materials

Figure 2.12 : TTT Curves for Steel and Different Cooling Rates

At this stage, before taking up different methods of heat treatments in any detail, it will be

worthwhile to first describe formation and characteristics of different phases.

2.5.4 Pearlite

The tensile strength of pure iron (0.00% C) is around 250 N/mm2 which increases to about

850 N/mm2 at a carbon percentage of 0.8. At very low carbon percentage the entire metal

is made up of ferrite grains which are soft and ductile. With increasing carbon percentage

more and more material is made up of pearlite, a substance that appears to have colour of

mother-of-pearl, hence the name. At 0.4% carbon pearlite occupies almost half the area

viewed through microscope. At this level of carbon the tensile strength is about 540

N/mm2. At 0.8% carbon almost total area consists of pearlite with a strength of 850

N/mm2.

Examination of pearlite at higher magnification (X 1500) reveals that it has a laminated

structure. It is a composite consisting of alternate layers of ferrite and cementite.

Cementite provides the strength while ferrite retains ductility of steel. At carbon content

above 0.8% the quantity of Fe3C begins to increase and steel begins to lose its ductility.

Figure 2.13 shows relationship between pearlite and carbon content. Outside the region of

Figure 2.13, within the range of carbon between 0.9 and 1.2% pearlite structure still exists

but grains are surrounded by cementite network. These cementite networks not being parts

of grains are referred to as free cementite and are the cause of brittleness in steel. In

general the strength of steel does not increase due to the presence of free cementite, later it

may reduce below the level of 0.8% carbon steel.

Since pearlite is an important constituent of steel is control becomes important if

properties of steel are to be controlled. It will be worthwhile to understand how it is

formed. With reference to Figure 2.7 three forms of iron were identified. These forms are

referred to as allotropic forms of iron and at each transformation a thermal arrest is

experienced.

Figure 2.13 : Relationship between Pearlite % and Carbon Content

During the cooling of pure iron the transformation from γ-iron (fcc) to α-iron (bcc) occurs

at a unique temperature of 910oC. However, carbon is added to the iron to form steel this

transformation takes place over a range of temperatures. The limits of this range are

known as the critical points and the higher temperature at which transformation starts

during cooling is designated A3. The lower point is designated as A1. It was stated in

0.0 0.2 0.4 0.6 0.8 1.0 1.2

50

100

Ferrite + Pearlite Fe3 C + Pearlite

Carbon Content (%)

Are

a o

f M

icro

gra

ph O

ccupie

d

By P

earlite

59

Engineering Alloys

(Ferrous and Non-Ferrous) Section 2.7 that the lower temperature A1 remains constant at 723oC irrespective of the

carbon content while the higher temperature A3 gradually reduces as carbon percentage

increases (Figure 2.8). At a carbon percentage of 0.8, A3 and A1 are equal to 723oC. If

carbon percentage further increases, A3 also increased. The equilibrium diagram will be

obtained if A3 and A1 temperatures are plotted against carbon percentage as seen in

Figure 2.14. The detailed phase diagram has already been presented in Figure 2.8. This

diagram shows how γ-iron which is austenite is transformed into pearlite.

Figure 2.14 : Critical Points for Carbon Steel

It may be noted that carbon solubility is different in austenite than in ferrite. The fcc

(γ-iron) can hold as much as 1.7% carbon in solution at 1130oC. Within the range of

carbon under discussion the structure is entirely austenite at this temperature and all the

carbon is in solution. The carbon is dissolved interstitially or in other words, a carbon

atom does not replace an iron atom to form part of the cube (Figure 2.15(a)). When the

lattice is arranged to have a bcc structure the carbon atoms is ejected from its site

(Figure 2.15(b)) because space is insufficient. This is the reaction that takes place at A3.

Figure 2.15 : Interstitial Solution of Carbon

The sequence of events between temperatures A2 and A1 is illustrated in Figure 2.16 during

which austenite transforms into pearlite. The bcc ferrite grains nucleate at the boundaries

of austenite grains. As the temperature reduces the ferrite covers more and more are in

austenite grain which reduces in size. The separation of ferrite from austenite will result in

rejection of carbon which will be absorbed by remaining austenite. Thus the remaining

austenite gets richer in carbon until the amount of carbon reaches 0.8% causing

precipitation of layers of cementite in the austenite region. Thus, the austenite transforms

into pearlite consisting of layers of cementite and ferrite. Thus transformation of austenite

Ferrite + Pearlite Pearlite + Cementite

Austenite + Cementite

Cem

A

A1

A1

A1

A1

A1

A1

723

910

A3

A3

A3

A3

0.8

Carbon Content (%)

Tem

pera

ture

( 0

C )

Cementite

Carbon atom in Space between Iron Atom

Insufficient Space Between Iron Atom to Accommodate Carbon Atom

60

Engineering Materials is similar to eutectic reaction but since it occurs in the solid state it is termed eutectoid

transformation. This eutectoid reaction (or breaking of solid phase austenite into ferrite

and cementite as already described in Section 2.9) takes place at temperature of 723oC

which is termed A1. For this reason pearlite is also often referred as eutectoid.

Figure 2.16 : Transformation of Austenite in 0.4% C Steel, as Cooling

occurs from Molten State above A3 Temperature

2.5.5 Martensite

Carbon atoms cannot move fast through the lattice between temperatures of A3 and A1.

The formation of pearlite and ferrite depends upon allowing sufficient time for the

movement of carbon atoms. This means that such transformation is possible only under

equilibrium condition. It may be realised that the edge of fcc cell is 23% greater than that

of bcc cell. If a carbon atom is trapped between two iron atoms, they are kept apart and

are not able to take up the position in bcc cell. Carbon atoms in martensite occupy the

position on edge of unit cell between two iron atoms and thus elongate the edge and cause

distortion. Figure 2.17 compares austenite, ferrite and martensite unit cells at (a), (b) and

(c) respectively. At (d) the size of edge of martensite unit cell on which the carbon atom

size is compared with other edge on which there is no carbon atom. As a result a distorted

lattice is obtained. It may be emphasised that such distortions resulting into regions of high

strain energy, would impede the movement of dislocations whereby the material would lose

its ductility or increase its hardness. This trapping of carbon atom between atoms of Fe

occurs when the steel is not allowed to cool in equilibrium condition or in other words it is

suddenly cooled from A3 to room temperature or to about 300oC. This process of sudden

cooling from A3 is called quenching which may be achieved by plunging a heated steel (A3

temperatures) into water or some other quenching medium. During this process the carbon

does not have sufficient time to diffuse or carbon does not occur below a temperature

300oC hence a quench treatment in which temperatures is suddenly dropped from A3 to

300oC is sufficient to create permanent hardening in steel. If the resulting structure is

examined under microscope it appears to be needle like, often referred to as acicular, and

is called martensite.

Figure 2.17 : Comparison of Unit Cells of Austenite, Ferrite and Martensite

Also Dimension are Compared at (d)

Different carbon contents will naturally result in different hardness. At low carbon levels

there is very little change in hardness. The noticeable change in hardness is achieved when

carbon content is at least 0.4% and reaches a maximum at eutectoid composition of 0.8%.

The hardness that can be achieved through martensite transformation at 0.4 and 0.8% C

are respectively 255 VHN and 530 VHN. Also the formation of martensite depends upon

the cooling rate. As the carbon content increases the critical cooling rate becomes slower.

Fe Atom C Atom

Fe

Fe

C

a

c

C Atom Fe Atom

61

Engineering Alloys

(Ferrous and Non-Ferrous) The critical cooling rates for plain carbon steels vary between 400oC/sec and 500oC/sec. It

means that it is easier to harden steel containing 0.8% C than the one containing 0.4% C

(Also refer to Figure 2.12). Quenching in water results in fastest cooling rate and

corresponds to critical cooling rate for 0.35% carbon. This is not good for hardening.

Figure 2.18 shows the critical cooling curves for some plain carbon steels.

Figure 2.18 : Critical Cooling Rate for Different Carbon Content

Example 2.1

A 0.4% C hypoeutectoid plain carbon steel is slowly cooled from 1540oC to

(i) slightly above 723oC and (ii) slightly below 723

oC.

Calculate the weight percent

(a) austenite present in the steel,

(b) ferrite present in the steel in case (i),

(c) proeutectoid ferrite prevent in the steel, and

(d) eutectoid ferrite and eutectoid cementite % present in the steel in

case (ii).

Solution

Refer to Figure 2.8. Point 1 above the liquidus represents the state of liquid steel.

The cooling occurs along the line xx and an equilibrium cooling is assumed.

Freezing begins at point 2 which is intersection of liquidus and line xx. Temperature

at 2 is 1510oC. The steel solidifies completely at point 3 where temperature is

1471oC. The whole alloy is now composed of austenite (γ-phase) as indicated by

first of Figure 2.19. No change occurs until point 4 on line A3 is reached. At this

point the precipitation of ferrite begins out of solid austenite. Further cooling

increases the amount of ferrite and austenite decreases. The amount of austenite

varies along IK. The composition of ferrite varies along the line IL.

Calculation of % content will be made by lever rule.

The amount of austenite slightly above 723oC is calculated from the line LK itself.

i.e. taking LK as tie line.

(a) Weight % of austenite 5 0.4 0.025

0.8 0.025

L

LK

−= =

−

0.375

0.484 or 48.4%0.775

= =

. . . (i)

Weight % of ferrite 5 0.8 0.4

0.8 0.025

K

LK

−= =

−

0.4

0.516 or 51.6%0.775

= = .

. . (ii)

Time

0.4% 0.6%

C = 0.8%

Tem

pera

ture

(0C

)

62

Engineering Materials (b) Weight % of proeutectoid ferrite slightly below 723oC is same as that

slightly above, i.e. 48.4%. . . . (iii)

For calculating eutectoid ferrite, the weight of carbide will have to be

subtracted form total mass of ferrite and cementite. Just below

isothermal line LKM ferrite and pearlite are present and lever arm will

extend upto ordinate representing 6.67% C.

Weight % of total (ferrite + cementite) just below 723oC

6.67 0.4 6.37

6.67 0.025 6.645

−= =

−

0.96 or 96%=

Weight % of Fe3C just below 723oC

0.4 0.025 0.375

6.67 0.025 6.645

−= =

−

0.0564 or 5.64%=

Weight % of eutectoid cementite = total ferrite – proeutectoid ferrite

96 51.6 44.4%= − =

Weight % of eutectoid cementite (by difference)

100 48.4 5.64 44.4 1.56%= − − − = . . .

(iv)

Example 2.2

A hypoeutetoid steel which was cooled slowly from γ-state to room temperature was

found to contain 10% eutectoid ferrite. Assume no change in structure occurred on

cooling from just below the eutectoid temperature to room temperature. Calculate

the carbon content of steel.

Solution

Refer to phase diagram of Figure 2.8 and let the vertical line xx cross the isotherm

at 5 such that 5 is at a distance x′ from temperatures axis. Then by lever rule

% total ferrite 6.67 6.67

6.67 0.025 6.645

x x′ ′− −= =

−

% proeutectoid ferrite 0.80 0.80

0.80 0.025 0.775

x x′ ′− −= =

−

% eutectoid ferrite = % total ferrite – % proeutectoid ferrite

or 10 6.67 0.80

100 6.645 0.775

x x′ ′− −= =

0.51,498 5.169 0.775 5.316 6.645x x′ ′= = − +

0.51,645 5.87 x′=

∴ 0.51645

0.088%3.87

x′ = =

The steel has 0.088% C.

Example 2.3

Heat treatments as mentioned below are given to thin steel strips for which TTT

diagram is as shown in Figure 2.19. What will be the resulting structure of steel in

each case.

63

Engineering Alloys

(Ferrous and Non-Ferrous) Treatments

(a) Water quench to room temperature.

(b) Hot quench in molten salt to 690oC hold for 2 hours and water quench.

(c) Hot quench to 610oC and hold 3 minutes, water quench.

(d) Hot quench to 580oC, hold for 25 minutes, water quench.

(e) Hot quench to 450oC, hold for 1 hour, water quench.

(f) Hot quench to 300oC, hole for 30 minutes, water quench.

(g) Hot quench to 300oC, hold for 5 hours, water quench.

Figure 2.19 : Cooling Scheme of Example 2.3

Solution

From Figure 2.19 the final structures of steel can be determined

(a) Martensite

(b) Coarse pearlite

(c) Fine pearlite

(d) 50% fine pearlite and 50% martensite

(e) Bainite

(f) 50% fine bainite and 50% martensite

(g) Fine bainite.

SAQ 3

(a) Describe cooling curve for pure iron. Will this curve change in presence of

impurity.

(b) Explain eutectoid and peritectic transformation by the help of Fe-C phase

diagram.

(c) Describe the following phases in iron-carbon phase diagram. Pearlite, ferrite,

cementite, austentite and ledeburite.

(d) What is an S-curve? What are its other names?

(e) What are allotropic forms of iron? Correlates these forms with temperatures

of iron cooling form molten state.

(f) What is martensite and how is it formed? Explain using unit cell structure.

1 10 102 10

3 10

4

200

300

400

500

600

700

Time (sec)

(a)

(d)

(f)

(g)

Ms

300 0 C

4500C

580

680 0 C

690 0 C

(e) Bainite

Pearlite

(c)

TTT Curves

Tem

pera

ture

( 0

C )

64

Engineering Materials 2.6 HEAT TREATMENT OF STEEL

A range of properties may be produced in steels because the structure of various phases of

microstructure depend upon the rate of cooling. Some aspects in this connection have

already been explained. In this section specific treatments will be outlined. All heat

treatment processes consist of three main steps.

(a) The heating of metal to predetermined heat treating temperature.

(b) The soaking of the metal at that temperature until the structure becomes

uniform throughout the section.

(c) The cooling of the metal at some pre-determined rate such as well cause the

formation of desirable structure.

The heat treatments are normally applied to hypo-eutectoid carbon steels. These are :

annealing, normalising and hardening. The temperature to which heating is done in all

three cases is about 50oC above A3 temperature as indicated in Figure 2.20.

Figure 2.20 : Heat Treatment Range for Carbon Steels

The heat treatment of other steels will be discussed in specific section where these steels

are described.

2.6.1 Annealing

It is a heat treatment basically to soften the steels. The heating and cooling both at

controlled rate are performed in a furnace. Hypoeutectoid steels are heated above the

upper critical temperature (A3 line) while hypereutectoid steels are heated only above the

lower critical temperature (A1 line). The cooling is so done that γ-to-α transformation goes

to completion to each temperature. The resulting structure consists of large grains of

ferrite with coarse pearlite in which thick plates of ferrite and carbide are present. This is

softest possible structure and is ideal point for starting mechanical working of steel. Low

yield strength and tensile strength are associated with this treatment.

Different purpose which are achieved through annealing are listed below :

(a) the relief of all internal stresses within the metal.

(b) the production of uniform grain structure throughout the metal.

(c) the softening of the metal.

The annealing processes are classified either as full annealing or process annealing. Full

annealing is essentially the process described above. However, cooling may be done in

furnace, in ashes of sand or in specially built cooling pits lines with refractory and covered

with refractory lid. If heated to too high a temperature or soaked for too long a time the

austenitic phase undergoes grain growth resulting into coarse peralite grains. Such a

structure is termed “overheated” and exhibits low mechanical strength.

0.2 0 0.4 0.6 0.8 1.0 1.2 1.4 1.6

600

700

800

900

1000

A1

Stress Relief Spheroidizing

738 0 C

A3

Normalizing

Hardening

Annealing

Full Annealing

Acm

Composition, % C

Tem

pera

ture

(0

C)

65

Engineering Alloys

(Ferrous and Non-Ferrous) One main problem to overcome during annealing process is the decarburisation and

oxidation on the surface. Packing the steel into special boxes are used a neutral

atmosphere in the furnace may overcome this problem. For example, low carbon steel

parts could be packed into boxes filled with sand, line, ground mica or cast iron swarf

while higher carbon components are usually packed into charcoal and other carbonaceous

materials.

Full annealing is not usually desirables as it results into considerable loss of mechanical

strength. Further it is too slow and costly process.

Process annealing, also known as commercial annealing or referred as stress relieving is

performed by heating steel to a predetermined temperature which is below the A1

temperature. The metal is air-cooled or quenched in a suitable pickling bath. Mild steels

(or hypoeutectoid steel containing less than 0.3% C) after having undergone the

mechanical treatment are softened by this process by heating to a temperature between

550oC and 650

oC. The distorted grains of ferrites in steel are fully recrystallised by the

process. The pearlite grains are not affected by process annealing so that the structure

consists of stress free ferrite matrix with distorted pearlite.

2.6.2 Normalising

Normalising is used as a finishing treatment for carbon steels giving higher strength than

annealing. There is no serious loss of ductility also. In this process the heating and soaking

is same as in full annealing but part is allowed to cool in air so that cooling are much

faster. The finer grains are produced because there is lesser time available for them to

grow. The finer grain structure increases the yield and ultimate strengths, hardness and

impact strength. The ductility is, however, slightly reduced.

The effect of small grain size is more noticeable in impact strength than in tensile strength

at low carbon content, say 0.2%. The tensile strength improves only by 5% while impact

strength may improve by as much as 20% by normalising than by annealing. At higher

carbon content, say 0.4% C it is only the impact strength but also the tensile strength that

improves marked by about 15%.

Normalising often applied to castings and forgings is stress relieving process. It increases

strength of medium carbon steel to some extent. When applied to low carbon steel it

improves machinability.

Alloy steels in which the austenite is very stable can be normalised to produce hard

martensitic structure. Cooling in air produces high rate of cooling which can decompose

the austenitic structures in such steels and martensite is produced. This increases the

hardness to a great extent. Table 2.7 describes the hardness obtainable in various

normalised steels. Table 2.8 describes variation of annealing and normalising temperature

and resulting hardness for different carbon contents of plain carbon steels.

Table 2.7 : Hardness of Annealed and Normalised Carbon Steels

Hardness BHN

Structural Steel

Conditions

Commercial

Iron Low C Medium C High C

Total Steel

Annealed 80 – 100 125 160 185 220

Normalised 90 – 100 140 190 230 270

Table 2.8 : Variation of Annealing and Normalising Temperature and Resulting

Hardness for Different Carbon Contents

Sl. No. C % Annealing

Temperature oC

Normalising

Temperature oC

Hardness

after

Annealing

(BHN)

Hardness

after

Normalising

(BHN)

1 0.18 – 0.22 860 – 900 900 – 925 110 – 149 120 – 160

66

Engineering Materials 2 0.23 – 0.28 850 – 890 890 – 910 130 – 180 140 – 190

3 0.29 – 0.38 840 – 880 880 – 900 140 – 206 150 – 220

4 0.39 – 0.55 820 – 870 840 – 870 150 – 217 180 – 230

5 0.56 – 0.80 790 – 840 810 – 840 160 – 230 210 – 270

6 0.81 – 0.99 790 – 830 810 – 840 170 – 230 260 – 300

2.6.3 Hardening

If a steel part is heated 30-50oC above A3 temperature complete austenising is permitted

by soaking at that temperature and then cooled suddenly (quenched), the breakdown of

austenite is suppressed. The new phase that forms is martensite in which all the dissolved

carbon is held in form of body centered tetragonal structure shown in

Figure 2.21. Martensite is only metastable phase and may be tempering. It is extremely

hard and brittle and has a characteristic acicular appearance when examined under

microscope under high magnification. In the steels upto eutectoid composition, the

martensite formed by this drastic quenching operation contains all the carbon that was

contained in austenite. In higher eutectoid steels, however, some carbon is converted into

carbide particles also.

Figure 2.21 : The Unit Cell of Distorted Body-centered Tetragonal Lattice of Martensite

The hardness of martensite is dependent upon the percentage of carbon present in

structure. Figure 2.22 illustrates how this hardness varies with carbon content. It can be

seen that hardness of plain carbon steels increases rapidly until the eutectoid composition

is reached. After this composition the hardness increases very normally. The fact is that

the hardest martensite is formed at eutectoid composition while hardness remains at this

level in hypereutectoid steels. The slight increase in hardness of hypereutectoid steel is due

to formation of carbide particles which are hard and brittle.

Figure 2.22 : Variation of Hardness of Martensite with Carbon Content of Steel

a

a

c

C - Atom

Fe Atom

800

600

400

200

0

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

% Carbon

Hardness as Annealed

Hardness as Quenched

% Hardness Increase After Quenching

Brinell

Hard

ness

67

Engineering Alloys

(Ferrous and Non-Ferrous) 2.6.4 Cooling Rate and Quenching Media

Rate of cooling plays an important role in determining the final structure after quenching

in case of eutectoid steel. Figure 2.23 schematically illustrates four different structures

that are obtained cooling rates when austenised steel is cooled in four different media.

Figure 2.23 : Mircostructure Resulting from Different Cooling Rates

Applied to Austenitised Samples of Eutectoid Steel

Only the drastic water quench produces a fully martensite structure. When quenched in oil

the austensite transforms into very fine pearlite. Fine pearlite also results if the austenised

eutectoid steel is air-cooled. However, if allowed to cool in furnace coarse pearlite is

formed. These effects have already been considered under the headings of annealing and

normalising. However, it may be mentioned that very fine pearlite structure that is

obtained form oil quenching is named primary troostite. Table 2.9 describes the effects of

various cooling media on mechanical properties of eutectoid steels.

Table 2.9 : Mechanical Properties of Eutectoid Steels

after Cooling in Different Quenching Media

Cooling

Media

Structure UTS

(N/mm2)

Y. S.

(N/mm2)

Hardness

(Rc)

Elongation %

(50 mm g. L)

Water Martensite 1700 − 65 Low

Oil Troostite 1100 550 35 5

Air Fine pearlite 850 270 25 8

Furnace Coarse pearlite 520 140 15 12

Water is the cheapest quench media for plain carbon steels. However, while using water

care must be exercised that water is properly agitated during this treatment otherwise air

bubbles may be trapped on the surface and insulate the spots from which heat flow will be

delayed. Such spots will develop softness. Salt water or brine is more severe quenching

media as it removes heat faster. However, in this case the steel must be thoroughly cleaned

after quenching otherwise the surface begins to rust. For very low carbon steels hydroxide

solutions are often used instead of brine. If slower cooling rates are desired, the steel may

be quenched in oil with high flash points. Various grades of quenching oil are available.

High carbon steels are invariably quenched in oil since water quenching will develop

cracks in such steels.

2.7 HARDENABILITY OF STEEL

When a piece of metal is quenched its loss of heat is determined by several factors but ore

face can be easily understood that the loss will not be uniform from total volume. That

part of the metal which is in direct contact with quenching media will lost heat faster than

Heat

Ferrite and

Water Quench

Oil Quench

Cementite

Air Cool

Furnace Cool

0.83%

Austenite and Cementite

Ferr

ite

Austenite

Eutectoid Martensite (Black)

Cementite (White)

Very Fine

Pearlite

Fine Pearlite

Coarse Pearlite

68

Engineering Materials the inner side. This will bring thickness or diameter of the part into focus to play an

important role in hardening. Figure 2.24 shows the cooling curves for surface, for material

just below the surface and the core. The cooling rate inside the material is governed by

thermal conductivity of the steel. It is also the function of the thermal gradient existing

within the piece to be hardened. There is always a possibility that at the inner core the rate

of cooling is less than the critical rate resulting into unhardened material there.

Figure 2.24 : Depth of Hardening in Steel

The depth of hardened layer is the measure of hardenability. A good hardenability will

mean even thicker sections are uniformly hardened. On the other hand a poor hardenability

will produce a soft core inside the piece of steel. The hardenability is dependent upon

following factors :

(a) The steel composition plays an important role. The steels with high carbon

content harden to a greater depth than steels with lower carbon for the former

have lower critical cooling rate. Alloying element such as chromium improve

the hardenability.

(b) The quenching medium.

(c) The section dimensions.

The standards describes the steel hardenability by stating the ruling section which is

maximum thickness which can be used and still achieve the stated properties throughout

the section. Size of thickness also plays an important role in terms of distortion that may

occur in the material. Variations in the rate of cooling within the component can lead to

different amount of contraction at different points across the section. This differential

contraction will result into the distortion of the component.

2.7.1 Measurement of Hardenability

Two testing methods have been developed to measure the hardenability of steels. The first,

known as cylinder series test, gives a single value of hardenability. The value is stated in

terms of percentage of martensite at the centre when quenched in a certain manner. The

second, known as Jominy and quench test results into a curve.

2.7.2 Cylinder Series Test

A series of round bars of different diameters are austenised and quenched in oil or water.

The bars are long enough so that the cooling of section at the middle of the length is not

affected by the ends. After hardening each bar is cut into half and hardness measured at

various points along a diameter. The graph between hardness and distance from the centre

is then prepared (Figure 2.25). From this graph the diameter at which 50% of the structure

is martensite is determined. When this graph the diameter is plotted against bar diameter, it

becomes possible to determine the bar diameter in which 50% martensite would form at

the centre. This is called critical diameter for that quenching medium. Since the rate of

cooling is less for an oil quench than for water quench, the critical diameter, of any steel

Tem

pera

ture

Cooling Curves

Tem

pera

ture

Tem

pera

ture

Hardened Region

69

Engineering Alloys

(Ferrous and Non-Ferrous) will be less for oil quenching than for two water quenching. Severity of quench is an index

that quantitatively defines the quenching condition. This index denoted by H is defined as

following ratio.

Heat transfer coefficient between steel and fluid

Thermal conductivity of steelH =

Figure 2.25 : Variation of Hardness with Depth in Water-quenched Cylindrical Bars of

(a) Plain Carbon Steel, (b) 1% Cr-V Alloy Steel

Naturally, when H → ∞ it represents the severest condition of quench, meaning that

surface of steel immediately reaches the temperature of the quenching medium. The critical

diameter for such an ideal and unrealisable condition is called ideal critical diameter. For

infinite H-value the critical diameter and ideal critical diameter will be same. For other H

values the critical diameter will be smaller. Figure 2.26 shows the relationship between

critical and ideal critical diameters for various H-values. Table 2.10 describes relative

values of H that can be obtained in various quench media under different condition with

value of one for still water as base.

Figure 2.26 : Relation between Critical Diameter, Ideal Critical Diameter and Severity of Quench

Table 2.10 : Relative Quench Severities

Severity of Quench Agitation of

Quenching

Medium

Movement of

Pieces Air Oil Water Brine

None None 0.02 0.3 1.0 2.2

None Moderate − 0.4 – 0.6 1.5 – 3.0 −

None Violent − 0.6 – 0.8 3.0 – 6.0 7.5

Violentor spray − − 1.0 – 1.7 6.0 – 12.0 −

2.7.3 Jominy Test

More convenient laboratory test for hardenability is Jominy test. A standard specimen of

steel (Figure 2.27) is austenised in normal manner. The lower end of the specimen is then

quenched by a standard jet of water, resulting into a varying rate of cooling.

50 25 0 25 50 200

300

400

500

600

700

25 mm

D.P

.N.

75 mm

125 mm

50 25 0 25 50 200

300

400

500

600

700 25 mm

75 mm

D.P

.N.

125 mm

0 25 50 70 100 125 150 175 200

0

25

50

75

100

125

150 3 2 1

0.3

0.1

0.02

Quench Severity ∞

Ideal Critical Diameter (mm)

Critical D

iam

ete

r (m

m)

Thin End Cools Slowly

70

Engineering Materials

Figure 2.27 : The Jominy Test of Hardenability

The rate of cooling at the jet end is about 300oC/sec while that at the other end is about

3oC/sec. This varying cooling rate produces a wised range of hardness along the length of

Jominy specimen. A flat portion is ground along the length and hardness measured at

various points. The plot of hardness along the length gives Jominy index of hardenability.

The best use of the Jominy curve (Figure 2.28) is made by drawing a horizontal line

corresponding to hardness of the semi-martensite zone. The hardness of semi-martensite

zone is described in Table 2.11. The point where this line needs the Jominy curve

determines the distance from the quenched end which can be inserted in Figure 2.29 to

determine the diameter for a particular steel which will be fully hardened in water or oil.

Figure 2.28 : Hardenability Curves Plotted from End Quench Test Data

(a) For Shallow; and (b) For Deep Hardening Steel

0 3 6 9 12 15 18

10

20

30

40

50

60

336 105 55 42 28 16.5 13.5 10

1.5 4.5 7.5 10.5 13.5 16.5 19.5

Distance From Quenched End (mm)

Hard

ness,

RC

a

b

0 6 12 18 24 30 36

20

40

60

80

100

Dia

mete

r of

Work

pie

ce (

mm

)

42

120

140

Water

Oil

71

Engineering Alloys

(Ferrous and Non-Ferrous)

Figure 2.29 : Determining the Diameter of Fully Hardened Articles

according to the Distance from the Quenched End

Table 2.11 : Relationship between the Hardness of the

Semi-Martensite Zone and the Carbon Content

Hardness of the Semi-martensite (Rc) Carbon-content %

Carbon Steel Alloy Steel

0.08 – 0.17 − 25

0.13 – 0.22 25 30

0.23 – 0.27 30 35

0.28 – 0.32 35 40

0.33 – 0.42 40 45

0.43 – 0.52 44 50

0.53 – 0.62 50 55

2.8 TEMPERING

The hardening treatment given to steel increases the hardness but introduces internal

stresses because of different cooling rates. The internal stresses are also created because of

transformation from austenite to martensite. Tempering treatment aims at reducing these

stresses. The treatment consists in heating the hardened component to between 200oC and

600oC and holding it at that temperature for a predetermined period of time and then

cooling slowly to room temperature. Since martensite itself is metastable phase, structural

changes induced by tempering proceed fairly rapidly. All structures resulting from

tempering are termed martensite. The changes occurring during various temperature

ranges are described below :

100o – 220

oC

Very little change occurs in the micro-structure. However, this heating helps remove

considerable amount of internal stresses. The stress relieving treatment is given

when maximum hardness is desirable and brittleness is not a problem. The strain is

relieved because of removal of carbon atoms from their trapped positions.

240o – 400oC

In this range martensite decomposes rapidly into emulsified form of pearlite known

as secondary troostite. This material is very fine in nature and hence provides good

shock resistance. The fine edge tools are tempered in this range but more precisely

within 270oC-300

oC.

400o – 550

oC

The precipitate troostite begins to coalesce forming a coarser from of globular

pearlite known as sorbite. It may be recalled both troostite and sorbite are now

preferably called tempered martensite. This treatment is desirable in such

components as beams, springs and axles.

600o – 700oC

72

Engineering Materials Heating hardened steel in this range causes spheroidisation, the structure being

known as spheroidite. This structure is formed because of further coalescence of the

carbide within the alloy. Spheroidised steels show fairly good machinability since

the hard carbide particles are embedded in the soft ferrite matrix and consequently

do not have to be cut by the cutting tool. If the spheroidised steel is heated to just

above its lower critical temperature the pearlite present will alter to austenite and

cooling to room temperature will yield a structure of lamellar pearlite plus pro-

eutectoid ferrite or cementite depending upon carbon content.

Judging the temperature of tempering by colour appearance is a tradition which is helpful

on shop floors. However, for accuracy the exact temperature measurement are to be

preferred. Table 2.12 describes the colour appearance and temperature in connection with

several tools.

Table 2.12 : Tempering Temperature and Colours of Tools

Tool Temperature oC Colour

Planning tools 230 Paste straw

Milling currents 240 Dark straw

Taps and dies 250 Brown

Punches, drill bits 260 Purplish-brown

Press tools 270 Purple

Cold chisels 280 Dark Purple

Wood saws, springs 300 Blue

Changes in Mechanical Properties with Tempering

Tempering improves the ductility and toughness of quenched steel while decreases

hardness. Figure 2.30 illustrates how these properties are influenced by tempering.

The tempering temperature is so chosen that it results in the desired combination of

the properties. Some steel show drop in impact values in the certain tempering

temperatures range. Third drop is an indication of brittleness and such range should

be avoided. For mild steel this brittleness (termed blue brittleness) occurs at a

tempering temperature of about 300oC.

Figure 2.30 : Tempering Diagram of Property Chart for Water Quenched 26 mm Diameter

Bars of Eu-12 Steel (Scale for Stress Only)

2.9 SPECIAL TREATMENTS

In cases of large sections where the water quenching will most likely produces cracks

special treatments are used for hardening. The cracks on the surface are produced because

0 100 200 300 400 500 600 700

250

400

550

700

850

1000

1150

1300

Tempering Temperature (0 C)

Izod,

N –

m,

Brinell

Hard

ness

Str

ess,

N/m

m2,

Ductilit

y,

%

Elong BHN

Y.P. 0.1 % P.S.

U.T.S

Reduction Of Area

Izod

73

Engineering Alloys

(Ferrous and Non-Ferrous) the skin cools faster and changes into martensite while the inner core cools slowly and

transforms later accompanied by dilation. This dilation causes outer skin to crack. To

avoid this type of cracking special treatments have been developed. Before they are

described it will be worth-while to revise Section 2.8 wherein the isothermal

transformation was described.

2.9.1 Isothermal Transformation

Austenite is not usually converted into martensite instantaneously but the process

continues for sometime. Different steels take different time for full transformation and the

time depends upon the temperatures from where cooling is begun. How to obtain

isothermal transformation diagram was discussed in detail in Section 2.8. The selected

specimen is austenised and then quenched in liquid bath held at temperature to be

investigated. The specimen is held for a different length of time in the bath and then

quenched in the water. The resulting structure can be studied under microscope or any

other associated property like hardness may be studied. It is seen that definite times are

required for the initiation and completion of the transformation and these times vary with

the temperature. The progress of transformation, say for 10%, 50% or 90% may also be

found.

Figure 2.31 which is Figure 2.30, reproduced with more details, illustrates complete

transformation diagram of eutectoid steel. As the transformation temperature is lowered

from A1 temperature to about 550oC, the nucleation and completion time decreases and

pearlte and lamellae become finer. From about 550oC to 250

oC, the nucleation and

completion time increase. The transformation product here is termed bainie which is

composed of two equilibrium phases that are ferrite and cementite. The time of minimum

nucleation is identified as the nose or knee. Below about 250oC the transformation

product is martensite. This forms almost instantaneously, but the amount formed depends

upon the temperature. The upper and lower limits of martensite transformation

temperatures are termed MS and MF temperatures. These diagrams are also known as

Time-Temperature-Transformation or TTT diagrams and were discussed earlier. They

are also referred to as S-curve because of their shape.

Figure 2.31 : Isothermal Transformation Diagram for a Eutectoid Steel.

Structures Present after 105 Seconds are given on the Right-Hand Side

1 101

102

103

104

105

0

100

200

300

400

500

600

700

Austenite

Austenite

Pearlite Course Pearlite

Fine Pearlite

Upper Bainite

Martensite +

Lower Bainite

Bainite

Martensite + Austenite Martensite

100 %

50%

0

Mf

Insta

nta

neous

Tra

nsfo

rmation t

o

Mart

ensite

MS

Time (sec)

Tra

nsfo

rmation T

em

pera

ture

( 0

C)

Start of Transformation

Finish of Transformation

50 % Transformation

+

74

Engineering Materials 2.9.2 Austempering

The component to be hardened is first austenised and then quenched into a lead or salt bath

held at just above the martensite transformation temperature. The component is held in the

bath until the bainite transformation is completed. It is then removed from the bath and

cooled in air to room temperature. The bainite so produced is somewhat softer than

martensite of same carbon content and distortion is minimum. Also the austempered steel

has improved shock resistance and low notch sensitivity. The process of austempering is

depicted in Figure 2.32. Austempering is often limited to section thickness of 20 mm.

Austempering is applicable to a few plain carbon steel and requires facility of molten salt

bath. This may be regarded a disadvantages over quenching and tempering.

Figure 2.32 : Austempering Shown on the TTT Curve

2.9.3 Martempering

The piece to be hardened is fully austenised and then quenched into a lead of salt bath held

at a temperature just above the at which martensite would begin to form. It is kept at this

temperature until its temperature becomes uniform throughout (i.e. outside and inside

temperatures do not remain different) and is then water quenched to form complete

martensite structure and bainite formation is prevented. This process successfully

separates the cooling contraction from the austenite-martensite expansions and thus

prevents quench cracking in large articles. The process of martempering is shown in

Figure 2.33.

Figure 2.33 : Martempering Shown on the TTT Curve

The steel can be tempered to low temperatures to further refine the structure.

Table 2.13 describes a few properties obtained from quenchtemper, austemper and

martemper treatments.

Table 2.13 : Some Mechanical Properties of 0.95 C, 0.40 Mn Steel at 20oC

after Different Treatment

Heat Treatment Hardness

(Rc)

Impact

(J)

Elongation %

(on 25 min gl)

Water quench, Temper 53.0 8.22 0

Water quench, Temper 52.5 9.60 0

Martemper, Temper 53.0 19.20 0

Martemper, Temper 52.8 16.44 0

Austemper 52.0 30.83 11

Time (Log Scale)

Tem

pera

ture

M

MS

Surf

ace

Centre

Time (Log Scale)

Tem

pera

ture

Surf

ace Centre

75

Engineering Alloys

(Ferrous and Non-Ferrous) Austemper 52.5 27.40 8

2.10 SURFACE HARDENING

In many situations surface hardening instead of through hardening only is sufficient to

serve the purpose. Gears are examples. Surface hardening is achieved through case

carburising, nitriding or induction heating. Steels containing 0.1 to 0.25% C are best

suited for case carburising. Good combination of tough core (lesser hardness) and high

surface hardness is achieved by case carburising of nickel steel. Case hardness of 60 RC

with a core hardness of 33 to 38 RC gives best results in case of gears. The case hardness

is due to residual compressive stress introduced on the surface by penetration of C

and N2.

Surface hardening is classified into two types :

(a) Without addition of any element from outside but only transforming outer

layer to martensite. This could be achieved by heating the surface by gas

flame or causing magnetic induction so that complete austenite

transformation occurs on surface. On quenching martensite and retained

austenite form on surface while on the inner side peralite-ferrite is the main

phase.

(b) The second method is called case hardening in which C and/or N2 are

introduced in the surface layer. In carburising the part is surrounded by

material or atmosphere rich in C and on heating this C is released and

absorbed in steel. Recently case carburising is more effectively performed by

heating steel part in the atmosphere or natural gas, coke oven gas, butane or

propane or the valatised form of liquid hydrocarbons like terpenes and

benzene. Volatilised form of alcohol and glycols or ketones are also used. In

these cases the thickness of hardened layer is proportional to root of the time

of treatment in hour.

Liquid carburising consists in dipping the part in fused mixture of chlorides, carbonates

and cyanides. Baths maintained at 840oC to 900

oC produce a case depth of 0.075 to

0.75 mm. 0.5 to 3.0 mm case depth is attainable if bath is maintained at 900 to 950oC.

Plain carbon steel and low alloy steel can be carburised in liquid bath.

Nitriding of steel surface is the absorption of N2 in the surface. Nascent N2 for this

purpose is obtained from ammonia. Molten cyanide (sodium cyanide) bath maintained at

560oC is quite effective in nitriding particularly if thin case is desired. Plain C steel are not

good for nitriding because iron nitride so formed is very brittle. Steels alloyed with Al and

Cr and Ni, Cu, Si and Mn are better nitrided than plain C steel.

Carbonitiriding, nitrocarburising or gas cyaninding is a process similar to gas carburising

in which ammonia is also added to carburising atmosphere. This process produces better

hardened case than carburising.

2.11 HEAT TREATING EQUIPMENT

The main equipment for heat treatment is furnace. There are two major types of furnaces –

batch and continuous. The furnace selection requires consideration of several points

particularly for the reason that they consume a good amount of energy. The efficient use of

energy in furnaces and hence their design and use of proper insulation need consideration.

The man power requirement for operation, the initial and cost and convenience of

maintenance and repair are other important considerations in selecting furnace for heat

treatment. Since temperature control and temperature cycle control are very important in

heat treatment, electronic and computer controlled furnace are taking precedence over

older type.

76

Engineering Materials Heating in furnace is done by burning oil or gas or by electrical resistance or inductance

heating. Gas or oil furnace have distinct disadvantage over electrical furnaces because the

former often introduces products of combustion in the heating space thus affecting the part

to be treated. The electrical heating, on the other hand, has slower start-up and is not easy

to control.

2.11.1 Batch Furnaces

An insulated, chamber for placing the job, the heating system and a door or several doors

for placing the job in place are the requirement of these furnaces. The parts to be heat

treated are loaded and unloaded in individual batches. A furnace which is easy to use,

simple to construct in several sizes and having versatility to accommodate several size is a

box furnace which could be horizontal rectangular. Many times a flat platform on wheels

is used to carry the parts in the furnace.

A pit furnace is made in form of a vertical pit below the ground level. The parts to be

treated are lowered in the pit. Long parts like rods, bars, tuning, shafts, etc. can be

suspended in the space of the pit furnace. These parts are susceptible to distortion if placed

in horizontal position in the box furnace.

A bell furnace does not have bottom and is lowered on the stack of parts to be treated.

The furnace chamber could be round or rectangular.

In elevator furnace the parts to be treated are placed on the rolling platform which is

rolled in the proper position and lifted to the heating chamber of the furnace. By placing a

quenching tank directly below the surface the savings on space and quenching time are

made.

2.11.2 Continuous Furnace

Parts to be treated are placed on some sort of conveyor which move into the furnaces

according to programmed heating cooling cycle. The time for loading-unloading is greatly

saved and handling of job is reduced. For high production rate and better control of heat

treating cycle these furnaces are most suitable.

2.11.3 Salt Bath Furnaces

Salts baths used for heating ensure good control uniform temperature and high heating

rates are compared to air or gas. The molten salts or metals have higher conductivity than

air or gas. The salt may be heated from outside if it is non-conducting or by passing a low

voltage alternating current between electrodes placed in the salt. Direct current is not used

because it is likely to cause electrolysis of salt. Among metals, lead is commonly used.

Wide range of temperature may be obtained from such baths.

2.11.4 Induction Heating

Alternating current through induction coil surrounding the part to be treated induces eddy

current through the part which is thus heated. The advantage of such heating is that no gas

or liquid source for heating is used and coil can be shaped to surround the part

geometrically. Figure 2.34 shows example of induction coils, which normally are made in

copper or copper based alloy which are water cooled. The coil is also designed for

quenching the part after heating. For surface hardening requiring local heating induction

heating is suitable.

Slideway to be Surface Hardened

Travel

Shaped Coils

Induction Coils

Parts to be

Cooling Water

77

Engineering Alloys

(Ferrous and Non-Ferrous)

Figure 2.34 : Coils for Induction Heating

Furnace Atmospheres

If the heating is not through salt bath, then the job to be treated is subject to varying

atmosphere which could be atmospheric air or any one of several gases. Surface

oxidation, tarnishing and decarburisation are the problems which the metals face.

Oxygen can cause oxidation, rusting and scaling. Carbon dioxide may cause

decarburisation depending upon its concentration in furnace atmosphere. Thin blue

film, is formed on the surface in the presence of water vapour. Bluing of surface is

done for improving surface appearance. Nitrogen provides a neutral atmosphere.

Vacuum furnaces often used for small and accurately finished parts provide

complete safety from effects of atmosphere.

SAQ 4

(a) What are different heat treatment given to steel?

(b) Differentiate between annealing and process annealing.

(c) What is quenching? Why should quenched steel be tempered?

(d) Do you think the term martempering is misnomer? Suggest a better term.

(e) Differentiate between austempering and martempering.

(f) Describe different methods of surface hardening. Give examples of surface

hardened parts.

(g) Describe how heating is done for heat treatment.

2.12 ALLOY STEELS

Carbon steels in their commercial forms always contain certain amounts of other elements.

Many of these elements enter the steel from the ores and it is difficult to remove them

during the process of steel making. All commercial steels contain varying amounts of Mn,

Si, S and P and frequently varying amounts of such elements in Cr, Ni, Mo and V. If

alloying elements other than carbon are present only in small amounts (e.g. Mn upto 0.8%,

Si upto 0.3%, etc.) then the steel is usually called low alloy steel or plain carbon steel.

Sulphur and phosphorus when more than 0.05% of either is present, tend to make steel

brittle, so that during steel making these elements are reduced to at least this value. Si has

little effect on strength and ductility if less than 0.2% is present. As the content is rasied to

0.4% the strength is raised without effecting ductility, but above 0.4% Si, the ductility is

impaired. Si is added as deoxidiser and that part which does not make silicon dioxide

remains in steel as impurity.

Mn is another alloying element which is present in most steels. If it exists in solid solutin

in the ferrite it has a strengthening effect. It may also exist in forms of Mn3C which forms

part of the pearlite of MnS. Upto 1% of Mn has strengthening effects on steel and its

78

Engineering Materials presence in excess of 1.5% induces brittleness in steel. Excess Mn is added to melt during

steel making to bring its level to desired value. It also acts as a deoxidiser.

Intentional addition of many other elements modifies the structure of steel and hence

improves its properties. Steels to which such intentional additions have been made

(including those steel which contain Mn in excess of 1% or Si in excess of 0.3%) are

known as alloy steels. One particular effect of alloying is that it enables martensite to be

produced with low rates of cooling and permits larger sections to be hardened than is

possible with plain carbon steel.

The important elements that are used to alloy with steel in varying quantities are Ni, Cr,

Mo, W, Mn and Si. The bcc metals like Cr, W and Mo when alloyed with steel tend to

form carbides which reduce the proportion of Fe3C in the structure. On the other hand the

fcc elements like Ni, Al, Cu and Zr do not form carbides. Mn which has three allotropic

complex structures also forms carbide.

Several advantages in terms of improved mechanical properties and corrosion resistance

are obtained by adding one or several alloying elements.

The various advantages of alloy steel are :

(a) Higher hardness, strength and toughness on surface and over bigger

cross-section.

(b) Better hardenability and retention of hardness at higher temperature (good for

creep and cutting tools).

(c) Higher resistance against corrosion and oxidation.

The alloying elements affect the properties of plain C steel in four ways :

(a) By strengthening ferrite while forming a solid solution. The strengthening

effects of various alloying elements are in this order : Cr, W, V, Mo, Ni, Mn

and Si.

(b) By forming carbides which are harder and stronger. Carbides of Cr and V are

hardest and strongest against wear particularly during tempering. High alloy

tool steel use this effect.

(c) Ni and Mn lower the austenite formation temperature while other alloying

elements raise this temperature. Most elements shift eutectoid composition to

lower C percentage.

(d) Most elements shift the isothermal transformation curve (TTT) to lower

temperature, thus lowering the critical cooling rate. Mn, Ni, Cr and Mo are

prominently effective in this respect.

2.12.1 Effect of Individual Alloying Elements

Sulfur

Sulfur is not a desirable element in steel because in interferes with hot rolling and

forging resulting in hot-shortness or hot embrittlement. Sulfur however, is helpful

in developing free cutting nature. Thus sulfur upto 0.33% is added in free cutting

steel. Otherwise, sulfur is restricted to 0.05% in open hearth or BOF steel and to

0.025% in electric furnace steel.

Phosphorous

It produces cold shortness which reduces impact strength at low temperature. So its

percentage is generally restricted to level of sulfur. It is helpful in free cutting steels

and is added upto 0.12%. It also improves resistance to corrosion.

Silicon

79

Engineering Alloys

(Ferrous and Non-Ferrous) Silicon is present in all steels but is added upto 5% in steels used as laminates in

transformers, motors and generators. For providing toughness it is an important

constituent in steel used for spring, chisels and punches. It has a good effect in steel

that it combines with free O2 and form SiO2 and increases strength and soundness of

steel casting (upto 0.5%).

Manganese

1.2 to 1.4% of Mn produces extremely tough, wear resistant and non-magnetic steel

called Hadfield steel. It is important ingredient of free cutting steel upto 1.6%. Mn

combines with S, forming MnS. For this purpose Mn must be 3 to

8 times the S. Mn is effective in increasing hardness and hardenability.

Nickel

It is good in increasing hardness, strength and toughness while maintaining ductility.

0.5% of Ni is good for parts subjected to impact loads at room and very low

temperatures. Higher amounts of Ni help improve the corrosion resistance in

presence of Cr as in stainless steel. Nickel in steel results in good mechanical

properties after annealing and normalising and hence large forgings, castings and

structural parts are made in Ni-steel.

Chromium

Chromium is common alloying element in tool steels, stainless steel, corrosion

resistant steel (4% Cr). It forms carbide and generally improves hardness, wear and

oxidation resistant at elevated temperature. It improves hardeanbility of thicker

sections.

Molybdenum

Molybdenum is commonly present in high speed tool steel, carburising steel and

heat resisting steel. It forms carbide having high wear resistance and retaining

strength at high temperatures. Mo generally increases hardeability and helps

improve the effects of other alloying elements like Mn, Ni and Cr.

Tungsten

It is important ingredient of tool steel and heat resisting steel and generally has same

effects as Mo but 2 to 3% W has same effect as 1% Mo.

Vanadium

Like Mo, V has inhibiting influence on grain growth at high temperature. V carbide

possesses highest hardness and water resistance. It improves fatigue resistance. It is

important constituent of tool steel and may be added to carburising steel.

Hardeability is markedly increased due to V.

Titanium

Addition of Ti in stainless steel does not permit precipitation of Cr carbide since Ti

is stronger carbide former and fixes are carbon.

Cobalt

It imparts magnetic property to high C steel. In the presence of Cr, Co does not

permit scale formation at high temperature by increasing corrosion resistance.

Copper

Atmospheric corrosion resistance of steel is increased by addition of 0.1 to 0.6%

copper.

Aluminium

80

Engineering Materials Aluminium in percentage of 1 to 3 in nitriding steels is added to improve the

hardness by way of forming Al nitride. 0.01 to 0.06% Al added during solidification

produces fine grained steel castings.

Boron

Very small percentage (like 0.001 to 0.005) of B is effective in increasing hardness,

particularly in surface hardening boriding treatment.

Lead

Less than 0.35% Pb improves machinability.

The effects of alloying element in respect of various desired effects are summarized below

:

(a) Hardenability – Si, Mn, Ni, Cr, Mo, W, B

(b) Toughness – Si, Ni

(c) High temperature strength – Cr, Mo, W

(d) Corrosion resistance – Cr, Mo, W

(e) Wear resistance – Cr, Mo, W, V

(f) Low temperature impact strength – Ni

(g) Atmospheric corrosion resistance – Cu

(h) Machinability – S, P, Pb

(i) Fatigue strength – V

(j) Surface hardening – Al

2.12.2 Some Important Alloy Steels

Structural Steels

Low alloy steels are used for structural purposes. Such steels are required to

possess high yield stress, good ductility and high fatigue resistance. The high yield

stresses result in direct weight saving in part of the structure.

A typical low alloy structural steel will have following composition :

C – 0.12%, Mn – 0.75%m Si – 0.25%, Cu – 0.3%

This alloy steel has a yield strength of 350 N/mm2 and about 15% elongation after

hot rolling. The presence of Cu improves corrosion resistance while Mn and Si

improve weldability by preventing weld embrittlement.

Small amounts of Ni, Cr and V added to these steels may improve the yield strength

to 625 N/mm2 if used in quenched and tempered conditions. Additions of Ni, Cr and

V do not effect the weldability.

Stainless Steels

Stainless steel are particularly known for their resistance to corrosion. This

resistance is obtained because of formation of protective oxide layer which spreads

all over the surface. This layer does not allow the surrounding atmosphere to further

react with the steel which retains its luster and appearance. The oxide layer on the

stainless steel surface is formed by the oxide of Cr when it is present in large

proportions. This oxide film is impervious to both metal ions and atmospheric

oxygen. Improved corrosion resistance is obtained with increasing percentage of Cr,

provided that Cr is in solid solution and not combined as carbide. The corrosion

resistance is further enhanced by addition of certain amounts of nickel. According to

structures obtainable at room temperatures the stainless steels are subdivided into

three groups.

81

Engineering Alloys

(Ferrous and Non-Ferrous) Ferrite stainless steels contain only chromium as alloying element in addition to

small percentage of carbon. The carbon varies between 0.05% to 0.15%, while Cr

varies between 13% to 30%. This alloy contains only α-phase at all temperatures.

Some Cr precipitates in form of carbides along with ferritic grains at room

temperature. This alloy is very ductile and used where outstanding formability in

complicated shapes is required. Many deep drawn objects are produced from ferritic

stainless steel. This material possesses excellent resistance to corrosion.

When alloy steel contain at least 24% Cr and Ni together but not less than 8% of

either element, the γ-phase is retained on cooling at normal rates. At very low

cooling rates the α-phase may separate fully. Austenitc phase is obtained when

quenched from upper critical temperature. The commonest of these steels contain

18% Cr, 8% Ni and 0.1% C. It is called 18 : 8 steel. Austenitic steel is used for

construction of chemical plants, decorative purposes and household utensils.

Neither of above two groups is heat-treatable. If steel contains Cr and Ni in such

proportions that it has a γ-phase at high temperature and an α-phase on cooling at

normal rates, it can be quenched to give a martensitic structure. Such

heat-treatable steels are known as martensitic steels even when not in heat treated

conditions. For developing martensitic these steels are oil-quenched from above

upper-critical temperature. Three types of martensite steels are available

commercially. These are :

(a) 0.07% − 0.1%, C 13% Cr,

(b) 0.2% − 0.4% C, 13% Cr, and

(c) 0.1% C, 18% Cr, 2% Ni.

These steels are used for turbine blades, surgical instruments, springs, ball bearings,

pump shafts, aircraft fittings, etc.

While martensitic steel can be heat treated to obtain high strengths, the strength of

ferritic and austenitic steel can be improved only by mechanical working. Various

precipitation hardening stainless steels have also been developed.

At high temperatures somewhere between 500 and 700oC, the stainless steels lose

their resistance to corrosion. This happens mainly because the chromium has a

tendency to separate from solid solution and precipitate in form of carbides at grain

boundaries. This makes welding of the stainless steels difficult and causes what is

known as weld decay. If welded part is reheated to a temperature of about 900-

1000oC the carbides are re-dissolved and can be converted into stable solid solution

on quenching.

High Carbon Tool Steels

Tools are implements that are used to shape, deform or cut other materials. They

are largely made in steel, though other alloys have also been developed. The

common tool steels contain C, W, Cr, Mo, V, Mn, Si in the range of 0.6 to 1.0%.

They have hardness and wear resistance. For shock resistance C is restricted to

0.5%. W and Mo between 2 to 18% provide high temperature strength. V between

0.1 to 2% enhances hardenability while Si adds to toughness.

Though the tool and die steels are not produced in as large amount as other steels

are, yet they are industrially very important. A variety of steel differing widely in

composition and treatment is used for varying purposes. They are used in such

operations as cutting, shearing, forming and rolling. These operations require

adequate hardness, strength, toughness, wear resistance and heat resistance. For

many purposes near-eutecoid and hyper-eutectoid steels have been used for metal

cutting but these plain carbon steels have tendency to loose hardness through

tempering when rise in temperature occurs during cutting. To overcome this

problem high steep tool steel have been developed. The 18.4.1 type of high steel

82

Engineering Materials contains 18% W, 4% Cr and 1% V. These steels retain sufficient hardness due to

carbide formation which is a complex compound Fe4W2C. A tough matrix is

provided by Cr. These steel may retain hardness upto a temperature of 500oC.

When 5-12% of cobalt is also added, in addition, the hardness through a secondary

hardening process is increased at temperature around 600oC.

High Duty Tool and Die Steels

High duty punching tools or dies are made out of many different steels of fairly high

alloying contents. High-carbon high-chromium die steel contains 2% C,

0.3% Mn and 12% Cr. Though extremely hard and unmachinable in its hardened

form, small and large carbide particles get dispersed in ferritic matrix on annealing

from 800oC. In annealed state this steel can be machined with difficulty. This is an

important requirement because tools and dies have to be machined before they are

hardened for final use.

Magnetic Alloy Steels

These steels are divided into two groups. Those which retain their magnetism and

those which do not. The steels that retain their magnetism are termed hard

magnetic steels. The other group is magnetically soft.

1% plain carbon steel in its fully hardened condition was the earliest permanent

magnetic material. Later developments occurred when W, Cr and cobalt were added

as alloying elements. The most useful of permanent magnetic steels contain high

proportions of Ni, Co, and Al, with small amount of W. Alnico is a good example

which contains 10% Al, 18% Ni, 12% Co, 6% Cu, Rest Fe.

Magnetically soft materials are required to demagnetise quickly. In earlier days soft

iron was used as a soft magnetic material but later iron-silicon alloys containing

upto 4.5% Si were developed. However, modern high duty soft magnetic materials

are iron-nickel alloys such as Permalloy which contains

78% Ni. Another soft magnetic material is mumetal containing 75% Ni. These

alloys are used for transformer cores and as shield material for submarine cables.

Alloy steels find a wide range of application and a few of them are described in

Table 2.14.

Table 2.14 : Application of Alloy Steels

Sl. No. Application Desired Properties Composition (%)

1 Rail steel Strength, ductility, impact and

fatigue strength

C – 0.4 to 0.6 Mn and

Cr – upto 1

2 Spring steel (tension

compression, torsion)

Good elongation high elastic limit

(20 to 30%, 1200-1400 MPa).

Good surface finish for fatigue

strength

(a) C – 0.6, Mn – 0.9,

Si – 2.0

(b) C – 0.5, Mn – 0.8,

Cr – 1.0, V – 0.15

(c) C – 0.5, Ni – 0.9,

Cr – 0.5, Ni – 0.6,

Mo – 0.2

3 Structural steel (bridges,

building, cars, gears,

clutches, shafts)

High strength, toughness, high

temperature strength, corrosion

resistance

Wide range of alloy steels

containing several alloying

elements

4 Weldable steel for

welded structures

Weldability, high resistance to

atmospheric corrosion, resistance

to brittle fracture

C – 0.15 to 0.3 with some

Cu and V

5 Concrete reinforcing

steel

Bend 90o – 180o, tor-steel with

ribs for greater surface area.

Elongation = 16%,

UTS = 500-650 MPa,

Y. S. = 35 MPa.

Elongation = 13%,

C – 0.3 to 0.4,

Mn – 0.5 to 0.8

C – 0.45 to 0.6,

Mn – 0.7 to 1.1

83

Engineering Alloys

(Ferrous and Non-Ferrous) UTS = 600 MPa,

Y. S. = 350 MPa

6 High speed steel for

cutting tools

Resist temperature upto

550-600oC. Cutting tools

requiring high hardness at

working temperature 18 : 4 : 1

steel and

6 : 5 : 4 : 2 steel

C – 0.8, W – 18,

Cr – 4, V – 1,

C – 0.8, W – 6,

Mo – 5, Cr – 4,

V – 2

7 Creep resisting steel Application in pipelines upto

400-550oC. Other parts upto

550oC

Mo – 0.4 to 6

V – 0.25 to 1.0

Cr – upto 6.0, C – low

8 Hadfield Mn steel,

excavating and crushing

m/c. rail road crossing,

oil well, cement, mining

industries. Used as

casting and hot rolled

Resistance to abrasion and shock,

high toughness strength and

ductility

C – 1 to 1.4,

Si – 0.3 to 1.0,

Mn – 10 to 14

9 High strength low alloy

steel (HSLA) for

automotive parts

High strength/weight ratio.

Balanced properties such as

toughness, fatigue strength,

weldability and formability

C – 0.07 to 1.3, Ti, V, Al,

Co less than 0.5

10 Ball bearing steel Rolling element, inner and outer

races. High hardness

61-65 Rc high fatigue strength

C – 0.9 to 1.1,

Cr – 0.6 to 1.6,

Mn – 0.2 to 0.4

2.12.3 Heat Treatment of Stainless Steels

Stainless steels, like other steels, respond to heat treatments over wide range. They are

subjected to one or more of following heat treatments.

Stress Relief

Stress relieving treatment removes undesirable residual tensile stresses that are

induced in the material as consequence of mechanical working.

Stress relieving of stainless steel is carried out by heating it to 370oC and then

cooling at a low rate. This treatment also reduces susceptibility to corrosion.

However, resistance against corrosion cracking is improved by stress relieving

at 770oC.

Annealing

Since ferritic stainless steels are prone to form patches of transformation products

particularly after welding it is subjected to annealing treatment. The treatment is

carried out by heating ferritic steel to 770oC and then cooling it in furnace or air.

This treatment will stress relieve and homogenise the structure.

Solution Annealing

This treatment is given to austenitic stainless steel. When this steel is heated to

1000oC to 1120oC the austenite acts as a powerful solvent for chromium carbide

and makes a homogenous structure. This structure is retained by quenching heated

steel in water, oil or air depending upon the thickness of the section. Air is best

quenching media for thin sections. The process is also referred to as quench

annealing.

Hardening and Tempering

This is a treatment very much similar to one for plain carbon steel but only meant

for martensitic stainless steel. The steel is heated to a temperature of 950 to 1050oC

and quenched in air or oil resulting in formation of hard marteniste. The quenched

steel is tempered between 100 and 700oC depending upon the required hardness.

Stabilising Treatment

84

Engineering Materials This is special treatment for stabilised grade of auetenitic steel. The steel is heated

to 870 to 900oC and held for 2 to 4 hours. It is then quenched in air, oil or water

which causes precipitation of titanium or columbium carbide. These carbides do not

permit precipitation of chromium carbide during service life.

Post Weld Treatment

The welding induces undesirable residual stresses and reduces corrosion resistance.

Stress relieving, annealing or solution annealing are the treatments recommended for

weldments before putting them to use. Weld decay is a common welding defect in

stainless steel which is caused by precipitation of carbide of chromium in the weld

region and HAZ. The result of this precipitation is great susceptibility to

intercycstalline cracking when a corrosive media comes in contacts.

2.12.4 Heat Treatment of Tool Steels

Heat treatment in case of tool steels in an important step before actually using the tool.

Most properties are achieved after heat treatment. There are certain special but necessary

precautions to be observed in heat treating tool steels. Such precautions may be clear

understanding of austenising temperature which is affected very much by addition of

alloying elements.

Normalising and Annealing

Normalising becomes necessary in case of forged tools. Forging induces undesirable

residual stresses, coarse grains and non-uniform structure. These defects are

removed by normalising treatment. The normalised tools are machined to exact

dimensions. Such machining increases the hardness because of strain hardening.

The intermediate annealing from just below A1 temperature is applied upon the tool

to remove the effects of strain hardening.

The tools steels have lower thermal conductivity and if the tool is heated at a faster

rate distortion and cracking may occur. To avoid such distortion and cracking the

tool before any treatment is preheated slowly to temperature between 600 to 800oC.

The tool is held at the preheat temperature sufficiently long to allow whole body to

come to uniform temperature and then heat to required temperature for any

treatment.

Austenising

Following preheat the tool is austenised by heating to correct austenising

temperature to obtain fine grained austenite in which carbides are dissolved.

Quenching

The austenised tool is quenched in brine, water, oil, air or molten salt bath

depending upon the hardenability and thickness of cross-section. The correct rate of

cooling will decide the quenching media and cooling faster than the correct rate will

result in under stresses.

The distortion of tools in more conveniently avoided by martempering or

austempering.

Tempering and Stablisation

Tempering of tool steel is again an important step in heat treatment. The quenching

of such steels causes the existence of untempered martensite, retained austenite and

carbide in the structure. This happens because of finer changes in composition do

not permit to establish correct austenising temperature. Tempering besides removing

these structural deficiencies also help induce secondary hardness. Figure 2.35 shows

the presence of secondary hardness manifested by increased hardness at certain

tempering temperature in case of high alloy steels. Both medium and high alloy steel

decrease in hardness respectively upto about 460oC and 500

oC and thereafter they

85

Engineering Alloys

(Ferrous and Non-Ferrous) increase in hardness. High alloy steels show greatest increase in hardness with

increasing tempering temperature.

Figure 2.35 : Tempering Characteristics of Tool Steels

After tempering initially the treatment is repeated two or three time in

multi-tempering practice. Such treatment stabilises the microstructure by reducing

the amount of retained austenite. Yet another method of stabilization is to cool the

tempered steel to subzero temperature by placing it in solid carbon dioxide

(– 75oC) or dipping it in liquid nitrogen (– 196

oC).

Normalised quenched, tempered and stabilised tool steel possesses maximum

hardness, wear resistance and dimensional stability.

2.13 CAST IRON

Cast iron is an important alloy of Fe and C are largely used in industry for its convenience

to casting in intricate and good mechanical properties. Steels could also be cast but

process is often costlier. Figure 2.8 clearly shows the region of cast iron in equilibrium

diagram. The diagram shows that there is a eutectoid at 4.3% (point C) which has

solidification temperature of 1135oC. Alloys within carbon range of 2.3% to 4.2% are

sometimes referred to as hypo-eutectic irons, since their carbon content is below the

eutectic composition. Although, the melting points of cast irons are much higher than

several non-ferrous alloys, yet they are within the reach of simple melting furnaces, several

of which are commercially available. This is one reason as to why cast irons are so

popular as structural material.

To understand various phases that are present in a cast iron one may consider cooling of a

typical Fe – C mixture from melt, say one having y % C (Figure 2.8). The first material to

solidify is austenite which is a solution of carbon in fcc iron. The line PG will give the

amount of C in solution in austenite during solidification. This amount is always less than

the average percentage in the melt, which means carbon is rejected out of austenite while

the liquid phase is enriched in carbon. The last liquid to solidify has the eutectic

composition, i.e. 4.3%. The eutectic contains the austenite and carbon.

If the liquid is cooled slowly maintaining near equilibrium conditions, the carbon solidified

as flakes of graphite in a matrix of soft ferrite, in pearlite or in a mixture of ferrite and

pearlite. If liquid is cooled rapidly then ferrite is suppressed and pearlite and cementite

precipitate.

Casting is a process in which molten is poured in a mould and on solidifying the casting of

the shape of mould is obtained. Cast iron, as already stated, in a good material for casting.

General properties of cast iron are :

(a) Cheap material.

(b) Lower melting point (1200oC) as compared to steel (1380-1500

oC).

300 400 500 600

1

2

3

4

Tempering Temperature ( 0 C)

Hard

ness

86

Engineering Materials (c) Good casting properties, e.g. high fluidity, low shrinkage, sound casting, ease

of production in large number.

(d) Good in compression but CI with ductility are also available.

(e) CI is machinable is most cases.

(f) Abrasion resistance is remarkably high.

(g) Very important property of CI is its damping characteristics which isolates

vibration and makes it good material for foundation and housing.

(h) Alloy CI may be good against corrosion.

2.13.1 Contents of Cast Iron

Cast Iron (CI) is prepared form melting pig iron in electric furnace or in cupola furnace.

Electric furnace gives better quality.

CI contains different elements in addition to Fe. The carbon content of CI is more than

2%. Si varies between 0.5 to 3.0%. It is very important because it controls the form of C

in CI. S content in CI varies between 0.06 to 0.12% and is largely present as FeS which

tends to melt at comparatively low temperature causing hot shortness. Mn inhibits

formation of FeS.

Though P increases fluidity of CI – a property helpful for pouring – it has to be restricted

to 0.1 to 0.3% because it reduces toughness. P is present in the form of FeP.

Mn in CI varies from 0.1 to 1.0% through such a small Mn does not affect properties of

CI it certainly helps improve upon hot shortness by taking care of S.

Several other alloying elements like Ni, Cr, Mo, Mg, Cu and V may be added to CI to

obtain several desirable properties.

2.13.2 Classification of Cast Iron

CI containing C in form of cementite is called white cast iron. Microstructure of such CI

consists of pearlite, cementite and ledeburite. If C content is less than 4.3% it is

hypoeutectic CI and if C is greater than 4.3% it is hypereutectic CI. White cast iron has

high hardness and wear resistance and is very difficult to machine. It can be ground,

though. Hardness of white CI varies between 300-500 BHN and UTS between

140-180 MPa. White CI is normally sand cast to produce such parts as pump liners, mill

liners, grinding balls, etc.

Cast iron containing carbon in form of graphite flakes dispersed in matrix of ferrite or

pearlite is classified as gray cast iron. The name is derived from the fact that a fracture

surface appears gray. Gray cast iron differs in percentage of Si from white cast iron while

C percentage is almost same. The liquid alloy of suitable composition is cooled slowly in

sand mould be decompose Fe3C into Fe and C out of which C is precipitated as graphite

flakes. Addition of Si, Al or Ni accelerates graphitesation. The graphite flakes vary in

length from 0.01 to 1.0 mm. The flakes provide an easy passage to cracks thus not

allowing softer microstructure to deform plastically. Larger flakes reduce strength and

ductility. The best properties of gray cast iron are obtained with flakes distributed and

oriented randomly. Inoculant agents such as metallic Al, Ca, Ti, Zr and SiC and CaSi

when added in small amount, cause formation of smaller graphite flakes and random

distribution and orientation.

Gray cast iron is basically brittle with hardness varying between 149 to 320 BHN and

UTS of 150 to 400 MPa. Different properties are obtained by varying cooling rate and

quantity of inoculant agents. It has excellent fluidity, high damping capacity and

machinability. If gray CI is repeatedly heated in service to about 400oC suffers from

permanent expansion called growth. Associated with dimensional changes are less of

ductility and strength as a result of growth. When locally heated to about 550oC several

times this material develops what are called fire cracks resulting into failure.

87

Engineering Alloys

(Ferrous and Non-Ferrous) High strength gray cast iron is obtained by addition of strong inoculating agent like CaSi

to liquid metal before casting process. UTS in the range of 250 to 400 MPa is obtained.

This cast iron is called Meehanite iron and can be toughened by oil quenching treatment

to a UTS of 520 MPa.

If graphite in cast iron is present in form of nodules or spheroids in the matrix of pearlite

or ferrite the material is called nodular cast iron. This cast iron has marked ductility

giving product the advantage of steel, and process the advantage of cast iron. It is

basically a gray cast iron in which C varies between 3.2 to 4.1%, Si between 1.0 to 2.8%

while S and P are restricted to 0.03 to 0.1% respectively. Ni and Mg are added as alloying

elements. Crank shafts, metal working rolls, punch and sheet metal dies and gears are

made out of nodular CI. The defects like growth and fire cracks are not found in this

class of iron. This makes it suitable for furnace doors, sand casting and steam plants. It

also possesses good corrosion resistance making it useful in chemical plants, petroleum

industry and marine applications.

White CI containing 2.0 to 3.0% C, 0.9 to 1.65% Si, < 0.18% S and P, some Mn and

< 0.01% Bi and B can be heat treated for 50 hours to several days to produce temper

carbon in the matrix of ferrite or pearlite imparting malleability to CI. This class is known

as malleable cast iron and can have as high as 100 MPa of UTS and 14% elongation. Due

to such properties as strength, ductility, machinability and wear resistance and

convenience of casting in various shapes, malleable CI is largely used for automotive parts

such as crank and cam shafts, steering brackets, shaft brackets, brake carriers and also in

electrical industry as switch gear parts, fittings for high and low voltage transmissions and

distribution system for railway electrification.

Addition of alloying elements such as Ni and Cr provide shock and impact resistance along

with corrosion and heat resistance of cast iron. These are called alloyed CI 3 to

5% Ni and 1 to 3% Cr produce Ni-hard CI with hardness upto 650 BHN and modified

Ni-hard CI with impact and fatigue resistance is produced by adding 4.8% Ni and

4.15% Cr. Ni-resist CI with 14 to 36% Ni and 1 to 5% Cr is alloy CI having good

corrosion and heat resistance.

Most castings in CI must be stress relieved at 400-500oC because CI has a property to

relieve locked in stresses after sometime. CI can be annealed by heating to 800-900oC to

improve machinability. Cast iron can be quenched in oil to improve hardness. Such

quenching treatment is often followed by heating to 300oC and cooling slowly

(Table 2.15).

2.13.3 Heat Treatment of Cast Iron

Castings in iron are often heat treated for improving mechanical properties and

microstructure. The treatments given to cast iron are described briefly here.

Stress Relieving

Internal stress in cast material is very common because every casting undergoes

cooling which is non-uniform. After certain time period has passed, these internal

stresses tend to be relieved almost spontaneously. Such self-relieving of stresses

may cause changes in dimensions which may not be permitted for working of parts

of machine. Therefore, it is imperative that cast material (parts) must be stress

relieved before bringing such parts in service. Cast iron is stress relieved by heating

it to a temperature in the range of 400-500oC and kept at this temperature for a few

hours. The cooling is done slowly or as per the rate for a particular structure. Stress

relieving of cast iron is referred to as seasoning of casting.

Annealing

Annealing of cast iron is heating it to a temperature between 800 and 900oC and

cooling slowly. This process decomposes iron carbide into ferrite and graphite and

machinability is improved. This may be necessary for such parts that require

machining.

Quenching and Tempering

88

Engineering Materials Cast iron pearlite structure may be heated to lower critical temperature and then

quenched to effect very rapid cooling. This treatment causes the precipitation of

hard martensite phase and casting is thus capable of providing high wear resistance.

The casting after quenching treatment may be further heated to 300oC and cooled

slowly to restore original toughness.

Surface Hardening

Many applications of cast iron necessitate high surface hardness. This may be

achieved by surface hardening treatments such as nitriding and induction hardening.

Table 2.15 : Typical Mechanical Properties and Applications of Cast Iron

Cast Iron Composition

wt. %

Condition Structure UTS

MPa

YS

MPa

Elongation

(%)

Typical Application

Ferrite 3.4 C,

2.2 Si,

0.7 Mn

Annealed Ferrite matrix 180 − − Cylinder blocks and

head clutch plates

Pearlite 3.2 C,

2.0 Si,

0.7 Mn

As-cast Pearlite matrix 250 − − Truck and tractor

cylinder blocks, gear

box

Gray Cast

Iron

Pearlite 3.3 C,

2.2 Si,

0.7 Mn

As-cast Pearlite matrix 290 − − Diesel engine castings

Ferrite 2.2 C,

1.2 Si,

0.75 Mn

Annealed Temper

carbon and

ferrite

345 224 10 General engineering

service machinability

Pearlite 2.4 C,

1.4 Si,

0.75 Mn

Annealed Temper

carbon and

pearlite

440 310 8 General service with

dimensional tolerance

Melleable

Cast Iron

Martensitic 2.4 C,

1.4 Si,

0.75 Mn

Quenched

and

Tempered

Tempered

martenstie

620 438 2 High strength parts,

connecting rods, yokes

for universal joints

Ferrite 3.5 C,

2.2 Si

Annealed Ferritic 415 275 10 Pressure casting as

valve and pump bodies

Pearlite 3.5 C, 2.2 Si

As-cast Ferritic pearlite

550 380 6 Crank shaft, gears and rollers

Ductile

Cast Iron

Martensitic 3.5 C,

2.2 Si,

Quenched

and

Tempered

Martensitic 830 620 2 Pinions, gears, rollers

and slides

SAQ 5

(a) What is an alloy? Give the range of composition of alloying elements.

(b) State effects of following alloying elements in steel. Tungsten, nickel,

chromium, vanadium and cobalt.

(c) Which elements will improve the following properties of alloy steels?

Hardenability, toughness, machinability, corrosion and wear resistance,

fatigue strength.

(d) What is stainless steel? Mention those properties which distinguish stainless

steel from plain carbon steel.

(e) Describe heat treatments for tool steel.

(f) Classify cast iron. What are Ni-hard and Ni-resist cast irons?

2.14 NON-FERROUS MATERIALS

Modern technology has been highly dependent upon non-ferrous and alloys for in certain

cases they present the advantages of high strength and low weight and for certain other

cases they surpass the mechanical strength of ferrous metals. In certain cases the

non-ferrous meals like copper and aluminium alloys have no alternative in wide range of

steel. Electrical conduction and aircraft bodies are examples. A jet turbine engine is a good

example of application of these materials. A typical engine of this type contains 38%

89

Engineering Alloys

(Ferrous and Non-Ferrous) titanium, 12% chromium, 37% nickel, 6% cobalt, 5% aluminium, 1% niobium and 0.02%

tantalum. Though steel is the largest consumed metal, good amounts of

non-ferrous metals are coming into demand for mechanical, electrical, elevated

temperature and corrosion resistance. Typically aluminium alloys are used for cooking

utensils, aircraft bodies and as building materials, copper is used as electrical conductor in

electrical machines and power transmissions, copper alloys are also used at tubing

wherever good thermal conductivity is desired. And there are several other examples.

In Table 2.16 the prices of several metals was compared with gold price as base at 1000.

Comparing these prices one must be careful that the densities of various metals and their

alloy will vary widely. The application of material generally can be seen in the machine

and structure as its volume and not as weight. Table 2.8 compares the prices on weight

basis. Table 2.16 compares the prices on the basis of both weight and volume.

Table 2.16 : Comparison of Prices of Various Non-ferrous Alloys

Price Metal

Per Volume Per Weight

Mo alloys 3.3 – 4.170 6.24 – 7.8

Ti alloys 0.33 – 0.660 1.37 – 3.71

Cu alloys 0.083 – 0.166 0.30 – 0.36

Zn alloys 0.025 – 0.115 0.67 – 0.158

Stainless steel 0.055 – 0.150 0.082 – 0.37

Mg alloys 0.032 – 0.640 0.31 – 0.74

Al alloys 0.032 – 0.048 2.65 – 3.97

Low alloy steel 0.023 0.097

Gray cast iron 0.020 0.050

Carbon steels 0.0167 0.041

Gold – 1000 (per weight and per volume)

2.15 ALUMINIUM

Aluminium was first produced in 1825. Presently it is produced in quantity second only to

steel. It is the most abundant metallic element on the crust of the earth easily comprising

about 8% of the crust. Bauxite, an hydrous oxide of aluminium and several other oxides,

is the principal ore of aluminium. Aluminium is extracted from its ore mainly through

electrolytic process. The ore is first washed off to remove clay and dirt, the ore is crushed

into powder and treated with hot sodium hydroxide (caustic soda) to remove impurities.

Alumina (the oxide of aluminium) extracted from this solution is dissolved in molten

sodium fluroride and aluminium fluoride bath at 940-980oC. This mixture is then

subjected to direct current electrolysis by passing direct current between carbon anode and

cathode. The metallic aluminium forms in liquid state and sinks to bottom of the cell. This

liquid aluminium is tapped off from time to time. The aluminium so obtained is 99.5 to

99.9% pure with iron and silicon as the major impurities.

Aluminium, then is taken to large refractory lined furnaces for refining before casting. The

chlorine gas is used as purging agent to remove the dissolved hydrogen gas, and the liquid

metal surface is skimmed off to remove oxidized metal. The molten metal is then cast into

ingots for remelting or rolling.

2.15.1 Wrought Aluminium Alloys

Sheets and extrusion ingots are cast through semi-continuous direct chill method. The

sheet ingots are scalped wherein about 12 mm of ingot surface is removed. The scappled

ingots are preheated to homogenise the structure by heating to a high temperature and

90

Engineering Materials soaking there for 10-24 hours. The preheating is done at a temperature below the lowest

melting point of the constituents. The ingots are then hot rolled to about 75 mm thickness

in 4 high reversed rolling stand. Thereafter the rolled sheet is further reheated to the same

temperature and further hot rolled to 18 mm to 25 mm thickness. Further thickness

reduction may be achieved through cold rolling. The products obtained this way are termed

wrought alloys and normally are inform of sheet, plate, rod, wire and extruded sections.

The wrought alloys are identified by a four digit code out of which the first digit signifies

the aluminium purity (if pure aluminium) or the major alloying element. The second digit

indicates the modification of alloy. The third and fourth digits indicate the minimum

amount of aluminium in the alloy. The first digit indicates following :

(a) Aluminium is pure no alloying element.

(b) Alloying element copper but magnesium is also added.

(c) Alloying element manganese.

(d) Alloying element silicon.

(e) Alloying element magnesium.

(f) Main alloying elements are magnesium and silicon.

(g) Main alloying elements are zinc, magnesium and copper.

2.15.2 Aluminium Cast Alloys

Aluminium alloys the cast by any one of the following processes.

Sand Casting is the simplest and most versatile process small castings, complex castings

with intricate cores, large castings and structural castings are produced by sand casting

with equal case.

In permanent mould casting a metallic mould is used which may be gravity6 filled or

rotated for centrifugal action. The castings from permanent mould are fine grained as

compared to sand cast products. In die casting maximum rate of production is achieved.

The molten metal is forced into die which is split but sufficiently strong to withstand

pressure. One important characteristic of die casting is close tolerance in parts. Fine

grained structure and automation of process are other advantages.

Aluminium casting alloys need such element for alloying which will not only impart

mechanical strength but will also increase fluidity and feeding ability. Therefore their

chemical composition must differ from wrought alloys. Silicon is the most preferred

alloying element in aluminium cast alloys for its improves fluidity and feeding ability as

well as its mechanical strength. Normal silicon content varies between 5 to 12%.

Magnesium in the range of 0.3 to 1% provides strength mainly through precipitation. Mg,

Zn, Sn, Ti are also added sometimes.

2.15.3 Properties of Aluminium Alloys

Among the various properties of aluminium alloys following are notable :

(a) low density (2.7 gm/cc)

(b) high electrical and thermal conductivity, only next to Cu

(c) good resistance to atmospheric, water and seawater corrosion

(d) good machinability, formability and castability

(e) maintains good light reflectivity

(f) non-toxic, non-magnetic and non-sparking.

Aluminium is a soft but weak material whose strength is increased by strain hardening and

several heat treatments. Aluminium is used as a matrix in several fibre reinforced

composites. Al2O3 an oxide of Al is very hand and strong and can be dispersed in the

matrix of Al by powder metallurgy to produce SAP (sintered aluminium product). Other

91

Engineering Alloys

(Ferrous and Non-Ferrous) reinforcing elements used in softer aluminium matrix are boron whiskers, stainless steel

fibres and whiskers of Al3Ni.

Alminium alloys are divisible in three groups :

(a) cast Al alloys

(b) wrought Al alloys

(c) aluminium composite reinforced with fibres or particles.

Cast Al Alloys

Low melting temperature, insolubility to gases except H2 and good surface finish

are characteristics of these alloys. Important drawback of cast aluminium alloys is

their shrinkage after solidification and hence careful mould design is called for.

Mechanical properties are inferior to wrought alloys except in creep. Alloys can be

sand cast gravity die cast, and cold chamber pressure die cast. Si, Cu, Mg and Sn

increase fluidity when casting thin sections. Mechanical properties of cast Al alloys

is improved by adding Cu which induces age hardening to impart hardness and

stability upto 250oC. Alloys used for die casting are : 380 (Al, Si, 3.5 Cu) and 413

(Al, 11.5 Si). Alloys preferred for permanent mould casting are : 332 (Al, 9 Si, 3

Cu, 1 Mg) and 319 (Al, 6 SI, 4 Cu). Y-alloy containing 4% Cu and 2% Ni retain

strength at high temperatures. It is used for piston and cylinders of IC engines.

Wrought Al Alloys

Wrought aluminium alloys are obtained by addition of Mn and Mg. The Al-Mn and

Al-Mg alloys cannot be heat treated. Al-Mn alloy combines high ductility with

excellent corrosion resistance. Beverage cans, cooking utensils and roofing sheets

are made in Al-Mn alloy.

Al alloy that responds to heat treatment by age hardening are Al-Cu, Al-Cu-Mg and

Al-Mg-Si. Some Al alloys, their composition and applications are described in

Table 2.17. Duralumin is one such alloy which contain 4% Cu and small amounts

of Mg, Mn and Si. After heat treatment this alloy develops a UTS of 450-550 MPa

and finds use in aircraft structures.

Apart from cast and wrought alloys the greater tonnage (about 85%) of Al is used

in commercially pure form in which impurities are less than 1%. Al extrusions,

tube, rods, wire, electrical conductors, chemical process equipment, foils and many

architectural fittings are made in commercially pure Al. The properties of

aluminium are described in Table 2.18.

Table 2.17 : Some Aluminium Alloys – Properties and Applications

Alloy

Designation Composition (%)

UTS/Elongation (%)

N/mm2 Characteristics and Applications

EC – O 99.5 Al (mini) 75/50 Ductile, high electrical conductivity

3003 – O

3003 – H16

98.8 Al, 1.2 Mn

98.8 Al, 1.2 Mn

130/140

190/140

Good formability and corrosion

resistance weldable, storage and

utensils

2024 – T4 93 Al, 4.5 Cu, 1.5 Mg,

0.5 Si, 0.5 Mn

500/19 High strength, aircraft parts,

bridges, rivers

5056 – H18

5056 – O

94.6 Al, 5.2 Mg, 0.3 Mn

94.6 Al, 5.2 Mg, 0.3 Mn

450/10

300/35

Good corrosion resistance to sea

water good finish when buffed or

anodized, marine parts, cooking

utensils, bus bodies

6061 – T6

6061 – O

98 Al, 1 Mg, 0.6 Si, 0.4 Cu

98 Al, 1 Mg, 0.6 Si, 0.4 Cu

320/17 Good corrosion resistance and

formability, general structure,

anodized articles, marine and

transport parts

7075 – T6

7075 - O

90 Al, 5.5 Zn, 2.5 Mg,

1.7 Cu, 0.3 Cr

90 Al, 5.5 Zn, 2.5 Mg,

600/11

240/16

High strength and corrosion

resistance, aircraft parts, bridges

92

Engineering Materials 1.7 Cu, 0.3 Cr

Table 2.18 : Typical Properties of Aluminium

Sl. No. Property Value

1 Purity % 99.5 Al, 0.25 Si, 0.25 Fe

2 Melting point oC 660

3 Sp. Gravity 2.70

4 Tensile strength, N/mm2

O – Temper

H – 18 Temper

72

135

5 Elongation %

O – Temper

H – 18 Temper

60

17

6 Hardness BHN

O – Temper

H – 18 Temper

19

35

7 Electrical conductivity* % IACS

O – Temper

H – 18 Temper

62

61

8 Thermal conductivity

J/m2/oC/s at 25oC

234

9 Corrosion resistance Very good in rural marine and

industrial, atmosphere

* Compare with copper, 62% of copper electrical conductivity.

In Table 2.17 aluminium alloys have been assigned certain temper like O-Temper and H-

18 Temper. The temper designation indicates the condition and heat treatment of any given

alloy. Generally the temper designation must follow the alloy designation and separated by

a dash. For example, the alloys in Table 2.17 must be described as 3003-O, 2004-T4. The

temper designations are described below. There are four basic tempers :

(a) F – As fabricated

(b) O – Annealed

(c) H – Strain hardened

(d) T – Heat treated

H is always followed by two or more digits. The first digit indicates basic operations while

the following digit stands for the final degree of strain hardening.

H1 – only strain hardened

H2 – strain hardened and partial annealed

H3 – strain hardened followed by stabilization

The second digit stands for amount for cold work. The digit 8 represents fully cold worked

or full hard. The digit of 4 means half hard and 2 means quarter hard. Thus, H18 means

full hard by strain hardening only.

T designation is followed by numbers 2 to 9. Their meanings are :

T2 – Annealed (only for castings)

T3 – Solution heat treated and then cold worked

T4 – Solution heat treated and naturally aged to stable condition

T5 – Artificial ageing after any one of the following : Elevated temperature, rapid

cool fabrication such as casting or extrusion

T6 – Solution heat treated and fabricated

T7 – Solution heat treated and stabillised

T8 – Solution heat treated, cold worked and then artificially aged

T9 – Solution heat treated, artificially aged and then cold worked.

93

Engineering Alloys

(Ferrous and Non-Ferrous) 2.15.4 Age-hardening of Aluminium Alloys

In certain alloys precipitation from a single phase may occur. The precipitate phase may

be in form of find sub-microscopic particles distributed both around the grain boundaries

and throughout the grains. In certain alloys of Al-Cu, Mg-Si and Be-Cu such phases

precipitate after suitable heat treatment. These precipitated phases have strengthening

effects of the alloys. This hardening of alloys is termed age hardening or precipitation

hardening.

Here the process of age-hardening will be described with particular reference to aluminium

alloys containing 4% Cu. Figure 2.36 depicts the equilibrium diagram of

Al-Cu system. It is seen that the solubility of Cu in α-phase solid solution decreases

steadily and quite considerably with decrease in temperature. At temperature

corresponding to point 3, copper forms copper aluminide (CuAl2) which is deposited as

coarse particles in and around the grains of α-solid solution. CuAl2 is extremely hard and

brittle. If the alloy is now reheated to about 550oC, between the points 2 and 3, CuAl2 is

reabsorbed in α-solid solution resulting into single-phase alloy. If alloy from this state is

quenched to room temperature, there is insufficient time for CuAl2 to form and Cu atoms

are now held in a super-saturated solid solution within the aluminium.

Figure 2.36 : The Aluminium-rich Portion of the Copper Aluminium Equilibrium Diagram Showing

the Mechanism of Precipitation Hardening for a 4% Copper Alloy, Over Aging Causes a

Coalescence of the CuAl2 Particles and Consequent Loss of Strength in the Alloy

When this alloy is allowed to stay at room temperature for five to seven days, the strength

improves significantly because of slow precipitation of find submicroscopic particles.

These particles are almost uniformly distributed around the grains. The time of this

precipitation may be reduced to a few hours by heating the quenched alloy to 120oC. This

is known as artificial age-hardening. Closed control of both time and temperature is

essential in precipitation hardening for this purpose. Salt baths at constant temperatures

are used 4% Cu aluminium alloy is most suitable for this type of treatment. However, this

alloy loses its corrosion resistance in hardened state and must be protected by cladding.

Age-hardening alloys containing Si and Mg behave in a similar manner. However, the

submicroscopic particles that provide strengthening are made of magnesium silicide

(Mg2Si). Thus the age-hardening effect of CuAl2 is reinforced by Mg2Si.

2.16 COPPER AND ITS PRODUCTION

Copper is marked by a host of good engineering properties. The foremost is its good

electrical conductivity and bulk of copper is used as electrical conductor. It also has a high

thermal conductivity and coupled with its resistance to corrosion it is largely used as heat

exchanger tubes particularly under circumstances when corrosive atmosphere exists. Its

medium tensile strength and ease of fabrication are added advantages in its industrial

application.

0 2 4 6 8 10 % Copper

Cu Al2

10

20

30

40

50

600

66

3

2 a + Liquid

Re-Heat Slow Cool

Quench Overage

Age

Tem

pera

ture

(0

C)

1 Slow Cool

As Cost

94

Engineering Materials Copper is extracted from its sulfide ore. Such ores also contain sulfides of iron. Low grade

ore is converted into sulfide concentrate which is smelted in reverberatory furnace to

produce a mixture of sulfides of iron and copper, called mate. The slag is separated from

matte. The copper sulfides is then chemically converted into impure or blister copper of

98% purity, by blowing air through the matte. The iron sulfides is oxidized and converted

in slag. The blister copper is then transferred to refining furnace where most of impurities

are converted into slag and removed. This fire refined cooper is called tough pitch copper

and is further refined electrolytically to produce 99.95% pure copper called electrolytic

tough pitch (ETP) copper.

ETP copper is used for production of wire, rod plate and strip. These products serve

several industrial purposes. But ETP copper contains 0.04% oxygen which forms

interdendritic Cu2O when copper is cast. If copper is heated to a temperature of 400oC in

the atmosphere of hydrogen, then hydrogen reacts with densritic Cu2O and produces

steam. These H2O molecules being large in size do not diffuse readily and cluster around

grain boundaries thus causing internal holes. This phenomenon is called hydrogen

embrittlement. The methods of avoiding hydrogen embrittlement are adding phosphorous

in the alloy copper and thus allowing P2O5 to form. The other method is to cast ETP

copper under a controlled reducing atmosphere to produce copper which is oxygen free

high conductivity (OFHC) copper.

2.17 COPPER ALLOYS

Several alloys of copper are used in industry for varying purposes. Copper forms alloys

with zinc (the brasses), tin (the bronzes), with tin and phosphorous (the phospher bronzes),

aluminium (the aluminium bronzes) and with nickel (the cupronickels).

2.17.1 The Brasses

70/30 brass also known as cartridge brass contains 70% Cu and 30% Zn. It is used for

cartridge cases, condenser tubes, sheet fabrication and for general purposes. Its ultimate

tensile strength varies between 350 and 600 N/mm2. It is soft and ductile in and annealed

form can withstand severe cold working.

60/40 brass of Muntz metal contains 60% Cu and 40% Zn. Its UTS varies between 400

and 850 N/mm2. It is suitable for hot working operations as well as for casting. Many cast

valves and marine fittings are made out of this brass. Addition of 2% Pb improves its

machinability.

Small additions of Fe, Al, Sn, Mn and Ni to 60/40 brass improves its strength

considerably. Marine propellers and shafts, pump rods, autoclaves, switch gears and high

strength fittings are made out of these brasses.

Brazing alloys are essentially the brasses of 50/50 composition with small additions of Sn,

Mn and Al. These brasses are hard and brittle.

2.17.2 The Bronzes

The coinage bronze used for making coins in earlier days contains 95% Cu, 4% Sn and

1 % Zn. The Zn acts as a deoxidiser. This alloy is soft and ductile.

Admiralty gun metal contains 88% Cu, 10% Sn and 2% Zn. This bronze is normally cast

to produce steam and water fittings and bearings. The addition of Pb improves the

pressure tightness of the alloy.

Phosphor bronzes are commonly used in manufacture of bearings, hard drawn wires and

bronze springs. In addition to tin they contain small percentage of phosphorous as alloying

element. 0.2% P forms Cu3P which is a hard compound. It acts as deoxidiser and improves

fluidity.

Copper aluminium alloys posses high strength with good resistance to fatigue, corrosion

and abrasion and are golden in colour. Aluminium can dissolve in copper to the extent of

9% and greater content than this induces brittleness. Wrought alloys which are good for

95

Engineering Alloys

(Ferrous and Non-Ferrous) hot and cold working applications contain 5 to 7% Al. Casting alloys contain 10% Al.

Small percentage of Fe, Ni and Mn are added to casting alloys to make them more easily

heat treatable. Aluminium bronze is well known for its colour and often called Imitation

gold. Al bronze compares well with the strength of steel.

Bronzes in general are known for the following characteristics :

(a) costlier than brass,

(b) better corrosion resistance,

(c) stronger than brass, and

(d) bearing material.

2.17.3 Copper-Nickel Alloys

Complete solubility occurs between copper and nickel. All alloys have similar

microstructure and can be cold or hot worked.

Cupro-nickel also known as German silver is extremely malleable and ductile. It is good

against corrosion due to salt water. Condenser tubes are main parts made out of this alloy.

It is also used for coinage. 70/30, 80/20 and 75/25 alloys are very common.

Monel metal is essentially 70% Ni and 30% Cu with small amounts of iron and other

elements. Alloy is well known for its high strength and corrosion resistance. This alloy is

largely used for chemical and food processing plants. It also finds great use as turbine

blades, valves corrosion resistance bolts, screws and nails. It is known for its

characteristics silver luster.

Tables 2.19 and 2.20 respectively describe Brasses and Bronzes with their applications.

Table 2.19 : Composition, Properties and Applications of Brasses

Gliding metal (95 Cu, 5 Zn) High ductility and corrosion resistance, coins,

medals, gold platings

Red brass (85 Cu, 15 Zn) Good corrosion resistance, workability, heat

exchanger tube, radiator cores

Cartridge brass (70 Cu, 30 Zn) Good strength and ductility, rivets, springs,

automotive radiator cores

Yellow brass (65 Cu, 35 Zn) Screws, rivets reflectors, plumbing accessories,

automotive radiator cores

Muntz metal (60 Cu, 40 Zn) Soundness and good machinability, condenser

tubes, architectural work

Leaded red brass (85 Cu, 5 Zn, 5 Sn, 5

Pb)

Fair strength, soundness and good machining in

cast state, pressure valves, pipe fittings, pump

fittings

Leaded commercial bronze

(89 Cu, 9.25 Zn, 1.75 Pb)

Screws, screw machine parts, electrical connectors,

builder’s applications

Admiralty brass (71 Cu, 28 Zn, 1 Sn) Condenser, evaporator and heat exchanger tubes,

marine applications

High leaded brass (64 Cu, 33 Zn, 2 Pb) Flat products, gears, wheels

Table 2.20 : Composition and Applications of a Few Bronzes

Phosphor bronze (94.8 Cu, 5 Sn, 0.2 P) Bolts, electric contracts, spring, bearing

Phosphor bronze (89.8 Cu, 10 Sn, 0.2 P) Such applications where high strength and

resistance to salt water is desired, bushing and gears

Gun metal (88 Cu, 10 Sn, 2 Zn) Sand cast, sued under heavy pressure such as gears

and bearings

Aluminium bronze

(88 Cu, 10.5 Al, 3.5 Fe )

High UTS

Beryllium bronze (98 Cu, 1.7 Be, 0.3

Co)

Very high mechanical strength, springs, used

against fatigue, wear and corrosion

(UTS – 1200 MPa)

96

Engineering Materials 2.17.4 Copper-Beryllium Alloys

Copper-beryllium alloys contain between 0.6 to 2% Be and 0.2 to 2.5% Co. These alloys

can be precipitation hardened and cold worked to develop a tensile strength as high as

1460 MPa. This is the highest strength among the copper alloys. Cu-Be alloys are used as

tools requiring high hardness and non-sparking characteristics for the chemical industry.

These alloys are very useful for making springs, gears, valves and diaphragms for their

excellent corrosion resistance, fatigue properties and strength. These alloys, however, are

costlier.

2.18 MAGNESIUM AND ITS ALLOYS

Magnesium is a light metal with density of 1.74 g/cm3. Magnesium is much costlier than

aluminium (density 2.74 g/cm3) with which it compares for lightness. Magnesium in its

molten state burns readily, hence it is difficult to cast the alloys of magnesium. Magnesium

alloys have low corrosion resistance and show poor fatigue and creep behaviour. Their h.

c. p. structure does not permit to deform readily at room temperature since only three slip

systems exist in h. c. p. at room temperature. The best advantage that magnesium alloys

offer is that of low density and many aircraft parts are made in these alloys.

Al when added to Mg in the range of 3 to 10% with small amounts of Zn and Mn increases

strength, hardness and castability. Addition of Mn (1.2%) with small amount of C does not

increase strength but improves corrosion resistance. Mg-Al-Zn alloys have good

mechanical strength and corrosion resistance. These alloys are good casting material and

generally used at high temperature like 250oC. Extrusions and forgings for general

purpose are made in these alloys are used in aircraft, automotive, radio and instrument

industries.

Some magnesium alloys are described in Table 2.21 along with their applications.

Table 2.21 : Magnesium Alloys

Composition (%) Condition UTS

N/mm2

YS

N/mm2

Elongation

%

Application

Mg, 3 Al, 1 Zn, 0.2 Mn Annealed 228 − 11 Air borne cargo

equipment

Mg, 2 Th, 0.8 Mn T8 228 198 6 Missile and

aircraft sheets

upto 427oC

Wrought

alloys

Mg, 6 Zn, 0.5 Zr T5 310 235 5 Highly stressed

aerospace uses,

extrusions,

forgings

MG, 6 Zn, 3 Al, 0.15

Mn

As-cast 179 76 4 Sand casting

requiring good

room temperature

strength

Cast

alloys

Mg, 3 Re, 3 Zn, 0.7 Zr T6

T5

235

138

110

97

3

2

Pressure tight and

permanent mould

castings used at

150-260oC

2.19 TITANIUM ALLOYS

Pure titanium is a strong ductile and light weight metal. It is very strong, highly resistant

to corrosion of all types but has the drawback that it readily reacts with common gases at

around 300oC. It reacts readily with C, O2, N2 and theses elements cause embrittlement of

Ti. It melts at 1725oC, has a UTS of 600-800 MPa and precent elongation of 25%.

Ti 6 Al 4 V alloy develops a UTS of 1300 MPa and had good creep, fatigue and oxdiaton

resistance. Aero engines gas turbine blades and other parts of engine and components of

97

Engineering Alloys

(Ferrous and Non-Ferrous) air frame are made of this alloy. Ti 5 Al 2.5 Sn is also a strong alloy (900 MPa UTS)

which is used in aircraft engine components at 470 to 500oC.

2.20 BEARING MATERIALS

In general it can be said that a good bearing material should posses following

characteristics :

(a) it should be strong enough to sustain bearing load.

(b) it should not heat rapidly.

(c) it should show a small coefficient of friction.

(d) it should wear less, having long service life.

(e) it should work in foundary.

Generally it is expected that the journal and bearing would be made of dissimilar materials

although there are examples where same materials for journals and bearings have been

used. When the two parts are made in the same material the friction and hence the wear are

high.

Cast iron has been used as bearing material with steel shafts in several solutions.

However, the various non-ferrous bearing alloys are now being used largely as bearing

material because they satisfy the conditions outlined above more satisfactory.

Bronzes, babbitts and copper-lead alloys are the important bearing materials that are

widely used in service. Certain copper zinc alloys, that is brasses, have been used as

bearing materials, but only to limited extent. Since brass in general is chapter, it has

replaced bronze in several light duty bearings.

2.20.1 Bearing Bronzes

Bearing bronzes are the copper-tin alloys with small additions of other constitutions.

Under conditions of heavy load and severe service conditions, bronzes are especially of

great advantages. They possess a high resistance to impact loading and, therefore, are

particularly used in locomotive and rolling mills bearings. However, they get heated up

fast as compared to other bearing materials, such as babbitts. Bronze lined bearings are

easily removed and finished bushings are generally available in stocks. A few of the

bronzes that are widely used are described in Table 2.22.

Table 2.22 : Bearing Bronzes

Mechanical Properties Bronze and

SAE Number Composition (%) UTS

N/mm2

YS

N/mm2

Elongation

%

Application

Leaded gun

metal, 63

Cu 86-89, Sn 9-11,

Pb 1-2.5, P 0.25

max. impurities 0.5 max.

200 80 10

Bushing

Phosphor

bronze, 64

Cu 78.5-81.5, Sn 9-11,

Pb 0.05-0.25, Zn 0.75

max. impurities 0.25 max.

167 80 8

Heavy loads

Bronze backing

for lined

bearings 66

Cu 83-86, Sn 4.5-6.0,

Pb 8-10, Zn 2.0,

impurities 0.25 max.

167 80 8

Bronze backed

bearings

Semi-plastic

bronze, 67

Cu, 76.5-99.5, Sn 5-7,

Pb 14.5-17.5, Zn 4.0 max.,

Sb 0.4 max., Fe 0.4 max.,

impurities 10.0 max.

133 − 10

Soft and good

antifriction

properties

2.20.2 Babbitts

The alloys of tin, copper, lead and antimony are called babbitts. The tin provides the

hardness and compressive strength of babbitts, copper makes them tough, antimony

prevents shrinkage while lead contributes to ductility. Bearing liners are extensively made

in babbitts for their better antifriction properties than bronzes.

98

Engineering Materials When Babbitt is backed up a solid metal of high compressive strength it gives good service

under high speeds, heavy pressure, impact loads and vibrations. The backing material

could be bronze or steel. A thin layer of high-tin Babbitt thoroughly fused to a tinned

bronze or a steel shell has exceptional load carrying capacity and impact strength. In case

of cast iron bearings the Babbitt in anchored in place by dovetail slots or drilled holes,

because Babbitt does not fuse with cast iron. Babbitt bearing linings of dependable

strength and life are made by pouring molten material into bearing, allowing to solidify

and fuse thoroughly and then machining to finished sizes. While the melting point of

Babbitt varies between 180 to 245oC, depending upon composition, the pouring should be

done when metal is in fully fluid state. For example, SAE 10 babbitt has a melting point of

223oC, it should not be poured below 440oC.

Some Babbitt materials are described in Table 2.23.

Table 2.23 : Babbitts (White Bearing Metals)

SAE No. Composition (%) Applications

10 Sn 90; Cu 4-5; Pb 0.35 max.; Fe

0.08 max.; As 0.1 max.;

Bi 0.08 max.

Thin liner on bronze backing

11 Sn 86; Cu 5-56; Sb 6-7.5;

Pb 0.35 max.; Fe 0.08 max.; As

0.1 max.; Bi 0.08 max.

Hard Babbitt good for heavy

pressures

12 Sn 59.5; Cu 2.25-3.75;

Sb 9.5-11.5; Pb 26.0 max.;

Fe 0.08, Bi 0.08 max.

Cheap Babbitt good for large

bearings under moderate loads

13 Sn 4.5-5.5; Cu 0.5 max.;

Sb 9.25-10.75 max.;

Pb 86.0 max.; As 0.2 max.

Cheap Babbitt for large bearing

under light load

2.20.3 Copper-Lead Alloys

Copper-lead alloys, containing a large percentage of lead have found a considerable use as

bearing material lately. Straight, copper-lead alloys of this type have only half the strength

of regular bearing bronzes. They are particularly advantageous over Babbitt at high

temperature as they can retain their tensile strength at such temperature. Most babbitts

have low melting point and lose particularly all tensile strength at about 200oC. Typical

copper-lead alloys contain about 75% copper and 25% lead and melt at 980oC. The room

temperature tensile strength of copper-lead alloy is about 73 MPa and reduces to about 33

MPa at about 200oC.

2.20.4 Other Bearing Materials

An extensively hard wood of great density, known as lignum vitae, has been used for

bearing applications. With water as lubricant and cooling medium its antifriction

properties and wear are comparable with those of bearing metals. Lignum vitae has been

used with satisfactory results particularly in cases of step brings of vertical water turbine;

paper mill machinery, marine service and even roll neck bearings of rolling mills.

More recently, in such cases where use of water as lubricant is necessary, especially if

sand and grit are present soft vulcanised rubber bearings have been used. A soft, tough,

resilient rubber acts as a yielding support, permitting grit to pass through the bearing

without scoring the shaft or the rubber. Longitudinal grooves in the rubber lining allow

free passage of the cooling water with any foreign matter present. With feathered edges

these grooves are also very effective in forming passages in the front of which the

supporting pressure is built up in the fluid film. These bearings have coefficient of friction

which compares well with roller bearings and pressure of 4.0 to 5.5 MPa may be carried if

journal is very smooth and load is applied after it has attained a peripheral speed of 150

m/min. The cooling water temperature in case of rubber bearings must always be below

boiling point. In some cases rubber bearings have been found to give as much as ten times

the service as bearings of lignum vitae or metals.

99

Engineering Alloys

(Ferrous and Non-Ferrous) Rubber bearings have been successfully used in centrifugal and deep well pumps, and

washers and several other applications where water must be used as lubricant. The

resilience and cushioning properties of rubber may be exploited in reducing vibrations of

high speed shafts.

Synthetic and neutral composite materials, plastic and reinforced plastic are also being

used as bearing materials now-a-days. However, their characteristics are not well

established as yet. Powder metallurgy bushing permits oil to penetrate into the materials

because of its porosity and is good for its antifriction properties.

Bearings are frequently ball-indented in order to provide small basins for the storage of

lubricant while the journal is at rest. This supplies some lubricant during starting. The

bearing walls may some time be indented and filled with graphite to provide lubricating

effect at the start.

2.21 ALLOYS FOR CUTTING TOOLS

Apart from tool steels described in Unit 11, may alloys which contain wholly

non-ferrous elements have been developed. Such alloys behave better than tool steel in

many respects and are widely used in industry. These alloys are mainly divided into two

groups : stellites and cemented carbides.

Stellite

Stellite is an alloy of Co (40-60%), Cr (25-35%), W (4-25%) and C (1-3%). It is a

cast alloy containing C, Cr, and W in Cobalt matrix. Its main characteristic is low

coefficient of friction and it possesses high hardness, red hardness, high wear and

corrosion resistance. Desired size and shape is achieved by casting and no heat

treatment is required. They are mainly used for cutting tools and can cut steel at

twice the cutting speed of HSS stellite can be used to cut all types of materials like

steels, as high speed steels because they are cast but perform better than HSS with

higher life.

Satellites are used for cutting hard die faces, can surfaces wear plates and crushes.

The hardness varies between 40 to 60 RC and they retain their hardness upto high

temperature because they do not undergo phase changes.

Cemented Carbide

These are small pieces with cutting edges and mechanically jointed or brazed to tool

shank. Cemented Carbide tool tips are produced by process of powder metallurgy

by sintering the powder carbides of W, Ta, Ti in Co powder. The contents are 40-

95% WC, 3-30% Co, 0-30% TaC and TiC and hardness of tips is in excess of 65

RC compared to 60 RC of stellite. High hardness, high compressive strength at high

temperatures are the main characteristics.

Cermets

Cermets are the variation of cemented carbides when the carbides of W and Ti are

solidified in the softer matrix of Co and Ni to obtain high hardness, resistance to

oxidation and thermal shock and resistance to high temperature abrasion.

Ceramic Tools

Aluminium oxide (Al2O3) is pressed and sintered in a powder metallurgy technique

in various shapes of cutting edges which are fastened to mechanical shanks. The

hardness of this ceramic tool is above 65 RC and has chemical inertness and high

resistance to wear. Ceramic tools are made in small pieces of various geometrical

shapes and can be disposed off when not usable.

100

Engineering Materials 2.22 SUMMARY

Engineering alloys can conveniently be subdivided into two types : ferrous and

non-ferrous. Ferrous alloys have iron as their principal base metal, whereas non-ferrous

alloys have a principal metal other than iron. The steels, which are ferrous alloys, are by

far the most important metal alloys mainly because of their relatively low cost and wide

range of mechanical properties. The mechanical properties of carbon steels can be varied

considerably by cold working and annealing. When the carbon content of steels is

increased to about 0.3%, they can be heat-treated by quenching and tempering to produce

high strength with reasonable ductility. Alloying elements such as nickel, chromium, and

molybdenum are added to plain-carbon steels to produce low-alloy steels. Low-alloy steels

have good combination of high strength and toughness and are used extensively in the

automotive industry for uses such as gears, shafts, and axles.

Aluminium alloys are the most important of the non-ferrous alloys mainly because of their

lightness, workability, corrosion resistance, and relatively low cost. Unalloyed copper is

used extensively because of its high electrical conductivity, corrosion resistance,

workability, and relatively low cost. Copper is alloyed with zinc to from a series of brass

alloys which have higher strength than unalloyed copper. Bronzes are other series of alloys

when Cu is alloyed with tin or aluminium.

Stainless steels are important ferrous alloys because of their corrosion resistance in

oxidising environments. To make a stainless steel “stainless”, it must contain at least

12% Cr.

Cast irons are other industrially important family of ferrous alloys. They are low in cost

and have special properties such as good castability, wear resistance, and durability. Grey

cast iron has high machinability and vibration damping capacity due to the graphite flakes

in its structure. White and iron, yet another variety having carbon in cementite form and is

harder.

Other non-ferrous alloys briefly discussed in this unit are magnesium, titanium, and nickel

alloys. Magnesium alloys are exceptionally light and have aerospace applications and are

used in radio and instrument industry. Titanium alloys are expensive but have a

combination of strength and lightness not available from any other metal alloy system and

so are used extensively for aircraft structural parts. Nickel alloys have high corrosion and

oxidation resistance and are therefore commonly used in the oil and chemical process

industries. Nickel when alloyed with chromium and cobalt forms the basis for the nickel-

base superalloys which are necessary for gas turbines in jet aircraft and some electric-

power generating equipment.

In this unit, we have discussed to a limited extent the structure, properties, and

applications of some of the important engineering alloys. However, it must be pointed out

that may important alloys have been left out due to the limited scope of this unit.

2.23 KEY WORDS

Austenite (γγγγ Phase in Fe-Fe3C : An intersitial solid solution of carbon in FCC iron;

Phase Diagram) the maximum solid solubility of carbon in

austenite is 2.0%.

Austenitising : Heating a steel into the austenite temperature range

so that its structure becomes austenite. The

austenitising temperature will vary depending on

the composition of the steel.

αααα Ferrite (αααα Phase in the : An intersitial solid solution of carbon in BCC

Fe-Fe3C Phase diagram) iron; maximum solid solubility of carbon in BCC

iron is 0.02%.

101

Engineering Alloys

(Ferrous and Non-Ferrous) Pearlite : A mixtue of a ferrite and cementite (Fe3O) phases

in parallel plates (lamellar stsructure) produced by

the eutectoid decomposition of austenite.

Eutectoid Ferrite : A ferrite which forms during the eutectoid

decomposition of austenite.

Eutectoid Cementite (Fe3C) : Cementite which forms during the eutectoid

decomposition of austenite; the cementite in

pearlite.

Eutetoid (Plain-carbon Steel) : A steel with 0.8% C.

Hypoeutectoid : A steel with less than 0.8% C.

(Plain-carbon steel)

Hypereutectoid : A steel with 0.8 to 2.0% C.

(Plain-carbon Steel)

Proeutectoid Ferrite : A ferrite which forms by the decomposition of

austenite at temperature above the eutectoid

temperature.

Proeutectoid Cementite (Fe3C) : Cementite which forms by decomposition of

austenite at temperature above the eutectoid

temperature.

Maenstie : A supersaturated interstitial solid solution of

carbon in body-centered tetragonal iron.

Bainite : A mixture of a ferrite and very small particles of

Fe3C particles produced by the decomposition of

austenite; a non-lamellar eutectoid decomposition

product of austenite.

Spheroidite : A mixture of particles of cementite (Fe3C) in an a

ferrite matrix.

Isothermal Transformation : A time-temperature-transformation diagram which

(IT) Diagram indicates the time for a phase to decompose into

other phases isothermally at different

temperatures.

Continuous-cooling : A time-temperature-transformation diagram which

Transformation (CCT) Diagram indicates the time for a phase to decompose into

other phases continuously at different rates of

cooling.

Martempering (Marquenching) : A quenching process whereby a steel in the

austenitic condition is hot-quenched in a liquid

(salt) bath at above the Ms temperature, held for a

time interval short enough to prevent the austenite

from transforming, and then allowed to cool slowly

to room temperature. After this treatment the steel

will be in the martensitic condition, but the

interrupted quench allows stresses in the steel to be

relieved.

Austempering : A quenching process whereby a steel in the

austenitic condition is quenched in a hot liquid

(salt) bath at a temperature just above the Ms of

the steel, held in the bath until the austenite of the

102

Engineering Materials steel is fully transformed, and then cooled to room

temperature. With this process a plain-carbon

eutectoid steel can be produced in the fully bainitic

condition.

Ms : The temperature at which the austenite in a steel

starts to transform to martensite.

Mf : The temperature at which the austenite in a steel

finishes transforming to martensite.

Tempering (of a Steel) : The process of reheating a quenched steel to

increase its toughness and ductility. In this process

martensite is transformed into tempered martensite.

Plain-carbon Steel : An iron-carbon alloy with 0.02 to 2% C. All

commercial plain-carbon steels contain about 0.3 to

0.9% manganese along with sulfur, phosphorus,

and silicon impurities.

Hardenability : The ease of forming martensity in a steel upon

quenching from the austentic condition. A highly

hardenable steel is one which will form martensite

throughout in thick sections. Hardenability should

not be confused with hardness. Hardness is the

resistance of a material to penetration. The

hardenability of a steel is mainly a function of its

composition and grain size.

Jominy Hardenability Test : A test in which a 1 inch (2.54 cm) – diameter bar

by 4 inch (10.2 cm) line is austenitised and then

water-quenched at one end. Hardness is measured

along the side of the bar up to about 2.5 inch

(6.35 cm) from the quenched end. A plot called the

Jominy hardeability curve is made by plotting the

hardness of the bar against the distance from the

quenched end.

White Cast Irons : Iron-carbon-silicon alloys with 1.8-3.6% C and

0.5-1.9% Si. White cast irons contain large

amounts of iron carbide which make them hard and

brittle.

Gray Cast Irons : Iron-carbon-silicon alloys with 2.5-4.0% C and

1.0-3.0% Si. Grey cast irons contain large amounts

of carbon in the form of graphite flakes. They are

easy to machine and have good wear resistance.

Ductile Cast Irons : Iron-carbon-silicon alloys with 3.0-4.0% C and

1.8-2.8% Si. Ductile cast irons contain large

amounts of carbon in the form of graphite nodules

(spheres) instead of flakes as (about 0.05%) before

the liquid cast iron is poured enables the nodules to

form. Ductile irons are in general more ductile than

gray cast irons.

Malleable Cast Irons : Iron-carbon-silicon alloys with 2.0-2.6% C and

1.1-1.6% Si. Melleable cast irons are first cast as

white cast irons and then are heat-treated at about

940oC (1720

oF) and held about 3 to 20- h. The iron

103

Engineering Alloys

(Ferrous and Non-Ferrous) carbide in the white iron is decomposed into

irregularly shaped nodules or graphite.

2.24 ANSWERS TO SAQs

Please refer preceding text for answers of all the SAQs.