Unit-2(EM)

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

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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

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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 earths 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

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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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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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(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.

0.08

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.

0.18

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.

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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/mm2 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/mm2 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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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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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 , and . The -iron is ferromagnetic but loses its ferromagnetism when heated above 770oC (curic temperature). The non-magnetic -iron was earlier known as -iron until X-ray studies showed that structures of and irons are same. The -iron is the structure of closest packing while and irons are not. There will be abrupt changes of dimensions at transitions to and . 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.

and phases can dissolve carbon and the solubility changes with temperatures. The solid solution of carbon in -iron is called ferrite while the solid solution of C in -iron is known as austenite. Ferrite and austenite are also designated as and 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

(bcc)

(fcc)

910 (Non-magnetic )

770

(bcc)

Time

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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 Gibbs phase rule the degree of freedom, F can be calculated.

Gibbs 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) + iron (0.08% C) 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 1470oC 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


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 CSolid austenite ( ) Ferrite ( ) Cementite (Fe C) +

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 () and cementite (Fe3C) at 1130oC. Point C in Figure 2.8 is eutectic point.

o

4.3% C

31130 CLiquid Austenite ( ) Cementite (Fe C)L +

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 iron transforms at 1495oC into solid austenite ().

o(1495 C)(0.99% C)(0.55% C) (0.17% C)Liquid -Iron Solid austenite+

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


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

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


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 -iron (fcc) to -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 (%)

Area

of

M

icrog

raph

O

ccupi

ed

By Pe

arlit

e

59


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 -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 (-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


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 723oC. 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 (-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 0.0250.8 0.025

LLK

= =

0.375 0.484 or 48.4%0.775

= =

. . . (i)

Weight % of ferrite 5 0.8 0.40.8 0.025

KLK

= =

0.4 0.516 or 51.6%0.775

= = .

. . (ii)

Time

0.4% 0.6%

C = 0.8%

Tem

pera

ture

(0 C

)

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 6.376.67 0.025 6.645

= =

0.96 or 96%=

Weight % of Fe3C just below 723oC

0.4 0.025 0.3756.67 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 -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 6.676.67 0.025 6.645

x x = =

% proeutectoid ferrite 0.80 0.800.80 0.025 0.775

x x = =

% eutectoid ferrite = % total ferrite % proeutectoid ferrite

or 10 6.67 0.80

100 6.645 0.775x x

= =

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

0.51,645 5.87 x=

0.51645 0.088%

3.87x = =

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


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 10 102 103 104

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 -to- 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


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 650oC. 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

Brin

ell H

ardn

ess

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

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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

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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 fluidThermal conductivity of steel

H =

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

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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

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30

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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)

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Oil

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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 220oC 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-300oC.

400o 550oC 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

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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 250oC, 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

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Pearlite Course Pearlite

Fine Pearlite

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+

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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 Ha

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