CHAPTER 2

FERROUS MATERIAL STRUCTURE AND BINARY

SYSTEM

• Most widely used structural metals due to their wide

range of mechanical, physical and chemical properties.

• Contains iron as their base metal.

• Ferrous alloys are produced as

o Sheet steels for automobiles and containers

o Plates for boilers, ships and bridges

o Structural members – I-beam, bar products, crankshafts, railroad

rails.

o Gears, tools, dies and molds

o Fasteners – bolts, rivets and nuts

Ferrous Metals and Alloys

Iron-carbon alloy consist 0.005% C called pure iron. Characteristics : soft, ductile, low strength Used in : magnetic device and enameling steels. Invention of blast furnace made production of iron in large quantities. 3 basic materials used in iron production :

◦ Iron ore◦ Limestone◦ Coke

Production of Iron

a) Iron ore Is one of the most abundant element in world, making up about

5% of the earth’s crust ( in the form of various ores) 4 compositions of iron ores :

Magnetite Hematite ( an iron-oxide mineral) Limonite (an iron-oxide containing water) Siderite (Carbonate iron)

Magnetide Category : Oxide mineral Colour : black, gray with brownish tint in reflected sun Hardness : 5 – 6

Hematite Category : oxide mineral Colour : metallic gray, dull to bright red Hardness : 5.5 – 6.5

Limonite Category : amorphous, mineraloid Colour : varoius shades of brown and yellow Hardness : 4 – 5.5

Siderite ( carbonate iron) Category : carbonte mineral Colour : yellow, gray, brown, black and sometimes

nearly colourless Hardness : 3.75 – 4.25

Iron ore processing

◦ Crushing into fine particles

◦ Removal of impurities by various means ( such as magnetic

separation)

◦ Pellets, balls or briquettes formation by using water and

binders. ( some iron-rich ores used directly without pelletizing)

Iron Ore

Obtained from a soft coal rich in volatile hydrocarbons and tarry

matter that are heated in vertical oven and then cooled with water

Functions of coke :

◦ Generating the high level of heat required for the chemical

reactions in iron making to take place.

◦ Producing carbon monoxide ( a reducing gas to removes

oxygen) which is used to reduce iron oxide to iron.

b) Coke

Function of limestone:

◦ Remove impurities from the molten iron

◦ Reacts chemically with impurities, acting as flux (flow as

fluid) that causes the impurities to melt at a low

temperature.

◦ Combine with impurities to form slag, which is light, floats

over the molten metal and is removed

c) Limestone

Blast Furnace pg 151

Iron ore, limestone and coke added into the blast furnace. (charging process)

The charge mixture is melted in a reaction at 1650˚C, with the air preheated

(to produce sufficient high temperature) to about 1100 ˚C and blasted into

furnace through nozzles (tuyeres)

Oxygen reacts with carbon to produce carbon monoxide.

Produced carbon monoxide reacts with iron oxide and reduces it to iron.

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

impurities floats to the top of the metal.

The molten metal in tapped into ladle cars.

The molten metal at this stage is called pig iron or hot metal.

Composition of pig iron : 4%C, 1.5%Si, 1%Mn, 0.04%S, 0.4%P and the rest

is pure iron.

Blast Furnace processing

an alloy of iron

Carbon content between 0.002% and 2.1% by

weight

Used in : buildings, infrastructure, tools, ships,

automobiles, machines and weapons

Steel can be produced by Basic Oxygen Furnace

Electric Arc Furnace

Vacuum Furnace

Production of Steel

Is the fastest steel making process Processes in BOF :

a) Molten pig iron and scrap charged into vessel

b) Pure oxygen in blown into furnace through lance(long tube)

c) Fluxing agents (lime) are added through a chute

d) Oxygen refines the molten metal by an oxidation process in which iron oxide is

produced

e) The oxide reacts with the carbon in the molten metal, producing carbon

monoxide and carbon dioxide.

f) The furnace is tapped by tilting

g) The slag is removed by tilting the furnace in opposite direction

Basic Oxygen Furnace

Source of heat : continuous electric arc formed between the

electrodes and the charged metal. (temperature generated :

1925˚C)

Steel scrap, a small amount of carbon and limestone dropped

into the electric furnace through the open roof

Roof closed and electrodes are lowered

Metal melts after around 2 hours the power has been turned on

Current turned off, and the electrodes raised.

Furnace is tilted and the molten metal is poured into a ladle

(used for transferring and pouring molten metal)

Electric Arc Furnace

Carbon composition in iron, steel and cast iron :

Pure iron : up to 0.008% C

Steels : 0.008 - 2.14% C

Cast Iron : 2.14 - 6.67% C ( most cast iron contain less than 4.5% C)

Pure iron experiences 2 changes in crystal structure before it melts

at a temperature of 1537˚C.

At rt, the stable form called ferrite(α iron) has BCC crystal structure.

Ferrite undergoes polymorphic transformation to FCC austenite (γ

iron) at around 912 ˚C.

At 1394 ˚C, austenite reverts back to a BCC phase known as δ

ferrite which finally melts at 1537 ˚C

Plain Carbon Steel

Phase diagram for iron-iron carbide system

IRON-IRON CARBIDE PHASE DIAGRAM

IRON-IRON CARBIDE PHASE DIAGRAM

Fe-C liquid solution

•C is an interstitial impurity in Fe.

•It forms a solid solution with α, γ, δ phases of iron

•Maximum solubility in BCC α-ferrite is limited

(max.0.022 wt% at 727 °C) - BCC has relatively

small interstitial positions

•Maximum solubility in FCC austenite is 2.14 wt% at

1147 °C - FCC has larger interstitial positions

γ-austenite - solid solution of C in FCC Fe

• The maximum solubility of C is 2.14 wt % at 1148 °C

• Transforms to BCC δ-ferrite at 1394 °C

• Is not stable below the eutectoid temperature (727° C) unless cooled rapidly

Phases In Iron-carbon System

α-ferrite - solid solution of C in BCC Fe• Stable form of iron at room temperature.• The maximum solubility of C is 0.022 wt% at 727˚C.• Transforms to FCC γ-austenite at 912 °C•Soft, magnetic at temp below 768 ˚C

δ-ferrite solid solution of C in BCC Fe• The same structure as α-ferrite• Stable only at high T, above 1394 °C• Melts at 1538 °C

727˚C

Fe3C (iron carbide or cementite)• This intermetallic compound is metastable• it remains as a compound indefinitely at room T, butdecomposes (very slowly, within several years) into α-Fe and C (graphite)if heated at 650-700°C• hard, brittle

Pearlite•When alloy of eutectoid composition (0.76 wt % C) iscooled slowly it forms perlite,• a lamellar or layeredstructure of two phases: α-ferrite and cementite (Fe3C)•Prop intermediate between soft,ductile ferrite and the hard, brittle cementite

HypoeutectoidAt about 875°C microstructure consist entirely of grain of the γ phase

At about 775°C, small α particles will form along the original γ grain boundaries

At this point, α particles grown larger.

As temperature lowered below eutectoid , point d, all γ phase transform to pearlite, but there is no change in α phase. (pearlite is not a phase!!)

Types of carbon steels Carbon content (%) Application

Low-carbon Steel

(Mild Steel)

Less than 0.30% Bolts , nuts, sheets, plate, tubes and for

machine components that do not require

high strength.

Medium-carbon Steel 0.30 – 0.60% Machinery. Automotive, agricultural

equipment parts, railroad equipment and

parts for metalworking machinery.

High-carbon Steel More than 0.60% Cutting tools, cable, springs, cutlery

The Differences And The Uses Of Carbon Steels

Remember : The higher the carbon content of the steel, the higher is its hardness, strength and wear resistance

A small increase in carbon has significant impact on

properties of the steel. As Carbon increases the steel:

◦ becomes more expensive to produce and less ductile, more

brittle

◦ becomes harder and less machinable and harder to weld

◦ has higher tensile strength and a lower melting point

Alloy Steels

Alloy Steels

A ferrous alloy that contains alloying elements ( other than C and

residual amounts of Mn, Si, S and P).

These alloying elements are added to improve mechanical and

corrosion resistance properties in steel.

Characteristics : high strength, hardness, creep and fatigue

resistance

Alloy steels are widely used in

o Construction and transportation industry for their high strength.

ELEMENT INFLUENCE

Manganese (Mg) Form stable carbide and increasing hardenability.

Silicon (Si) Fluidity and heat resistant.

Copper (Cu) Reduce rusting

Aluminum (Al) Reducing grain size which adds toughness, increase machinability

Boron (B) Increasing hardenability

Chromium (Cr) Stabilize α, Corrosion resistant, heat resistant and increases hardenability

Cobalt (Co) Permanent magnet

Molybdenum (Mo) Increase strength and hardenability.

Nickel (Ni) Stabilize , Grain refiner, corrosion,heat resistant, increase toughness,

strength and impact resistance

Tungsten (W) Stabilize , form very hard carbide, increase toughness and strength and

impact resistance

Vanadium (V) Increase hardenability, increase toughness and strength and impact

resistance

Alloying element and their influence in steel properties

Steel alloys Main class Contents (%) Applications

Structural steel Carbon and low alloy

0.55C, 0.70Mn Gears, cylinders and machine-tool parts requiring resistant to wear.

Corrosion resistant steel

Stainless steel 0.04C, 0.45Mn, 14.00Cr

Kitchen tools (forks and spoons)

Heat resistant steel

Heat resisting steel

0.15C, 20.00Cr, 25.00Ni

Conveyers chair and skids, heat treatment box, recuperator valve, and other furnace part.

Tool and die steel

Alloy tool steel 0.35C, 1.00Si, 5.00Cr, 1.50Mo, 0.40V, 1.35W

Extrusion die, mandrels and noses for aluminum and copper alloy. Hot forming, piercing, gripping and heading tools.

Magnetic steel Hard magnetic material

1.88Ni Electric motor

Characteristics of Alloy Steel

Cast Iron

27

Cast Irons

•Composed of iron, carbon (2.1% ~ 4.5%), and silicon (1% ~

3.5%) – ferrous alloy.

•Cast irons classification according to the solidification

morphology from the eutectic temperature are :

o Gray cast iron or gray iron

o White cast iron

o Black Malleable cast iron

o White Malleable cast iron

o Nodular Cast Iron ( Ductile Cast Iron)

Gray Cast Iron

•Composition of 2.5% to 4% carbon and 1% to 3%) silicon.

•Graphite exists largely in the form of flakes.

•Properties of gray iron :

o Low (negligible) ductility

o Weak in tension

o Strong in compression

o Good vibration damping

•Products from gray iron include automotive engine blocks and heads,

motor housings, and machine tool bases.

Nodular Cast Iron (Ductile Iron)

• This is an iron with the composition of gray iron in which the molten metal

is chemically( added with magnesium) treated before pouring to cause the

formation of graphite spheroids rather than flakes.

• shock resistant , stronger

and more ductile iron.

• Applications include machinery components requiring high strength and

good wear resistance.

White Cast Iron

•Due to large amounts of iron carbide presence, the structure of

white iron is very hard, wear resistance and brittle.

•It is obtained either by cooling gray iron rapidly or by adjusting the

composition by keeping the carbon and silicon content low.

•Products from white iron include railway brake shoes.

Malleable Iron

•Obtained by annealing white iron in an atmosphere of carbon monoxide and

carbon dioxide, between 800oC~900oC, for several hours.

•2 types of malleable iron :

• Pearlite malleable(white malleable) – upon fast cooling of white iron

• Ferrite malleable (black malleable) – upon slow cooling of white iron

•The structure has good ductility, strength and shock resistance.

•Typical products include pipe fittings and flanges, railroad equipment parts.

The Structure, Properties And Uses Of The Various Cast Iron.

Cast Irons Structure Properties ApplicationGray Cast Iron Ferrite and Pearlite

with free graphite flakes

High strength and hardness.

Pipe, engine blocks, machine tools

White Cast Iron Pearlite and Cementite Low Machinability and Brittle

Wear resistant component such as rolls for steel making.

Black Malleable

Ferrite and fine Carbon partical

Higher in machinability, lower melting point and higher fluidity.

Hardware, pipe fitting

White Malleable

Ferritic structure near the surface and Pearlitic structure near the center.

Higher in machinability, lower melting point and higher fluidity. (Higher hardness).

Railroad equipment, couplings

Nodular Cast Iron

Ferrite and Pearlite with Spheroidal Graphite.

Higher strength, reduce fatigue failure

Pipe, crankshaft, gears, rolls for rolling mills

Influences Of Process Variable To The Cast Iron Structure

Process Variable Condition Influences

Cooling rate

Slow Produced ferrite and large flakes of graphite together with fine flakes of graphite formed by the decomposition of the cementite after solidification.

Moderate The structure will consist of flake graphite in a matrix which is entirely pearlitic.

Fast (Chilling) The structure will consist of pearlite and cementite.

Carbon contentLow to high High quantity of graphite will be produced in cast iron by increasing

the carbon content.

Cross-section size

Thin to thick Thin cross section will make the solidification rate faster then the thicker one. Longer solidification time will cause the differences of mechanical properties and shrinkage effect.

Element content

Silicon Affect the properties in the different of percent in content. Ferritic gray cast iron: with 3% silicon Ferritic/pearlitic cast iron: with 2% silicon Pearlitic cast iron: with 1.5% silicon

Sulphur Stabilizing the cementite and preventing the formation of flakes graphite. Thus, sulphur harden the cast iron.

Manganese Remove the sulphur. Thus, softens the cast iron.

Phosphorus High phosphorus content produced a great fluidity. Cause hardness and embrittlement.

THE END