Metalurgy 2

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Formation of CO is an endothermic reaction and the temperature in this region falls to 1100OC.

CO is the main reducing agent in the upper portion of blast furnace.

Overall reactionFe2O3 + 3CO 2Fe + 3CO2

The liquid iron runs downward to the bottom of the furnace and is withdrawn through tap hole.CO reduces most of the iron oxide into iron metal.

Simultaneously, with the process of reduction, part of metallic iron reacts with CO to form

ferric oxide

The backward reaction can be prevented by a continuous supply of hot blast of air.Flux (lime) forms slag with impurities (silica).

Thus, blast furnace has two functions :

To reduce the ore to metallic iron and

To remove the impurities in the form of slag.

Slag formationLime stone on heating decomposes to CaO and CO2.CaCO3 CaO + CO2CaO now reacts the impurities of ore called GANGUE to form slag.

Slag is the mixture of CaSiO3 and Ca(AlO2)2.

The slag floats over the top of molten iron. Slag is a useful byproduct.It is used in road making, cement manufacturing a light weight building materials.

Flux + Gangue Slag

CaO + SiO2 CaSiO3

CaO + Al2O3 Ca(AlO2)2

Reactions in the blast furnaceDuring extraction, different temperatures are attained at different heights

of the furnace known as different zones of the furnace.

Different chemical reactions take place in these zones.The descending charge first melt with the ascending CO, to partly reduce the ore.

Then at higher temperature it is reduced by coke.

Zone of reduction : (300-800 C i.e. dul l red heat)Carbon monoxide reduces ferric oxide to metallic iron.

Fe2O3 + 3C0 i 2Fe + 3CO2

This reduction of Fe2O3 continues with increasing speed as the charge descends to the hotterpart of the furnace. The reduction is done as given below :

Fe2O3> Fe3O4> FeO> Fe

1) TOP OF THE FURNACE, temperature range 300C to 500C

Fe2O3 is reduced to Fe3O4 (ferroso ferric oxide).

3Fe2O3 + CO> 2Fe3O4 + CO2 T

This Fe3O4 is stable upto 650C in the presence of CO, CO2 and


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2) At the temperature between 650-700C, Fe3O4 is reduced to FeO (ferrous oxide).Fe3O4 + CO -> 3FeO + CO2 T

3) At temperature between 700-800C, FeO is reduced to metallic iron

FeO + co --> Fe + CO2 T

4) Simultaneously the limestone present in the charge is also decomposed to produce lime.

CaCO3 --> CaO + CO2 T5) The metal produced is spongy. Simultaneously a part of metallic iron reacts with CO to

form Fe2O3 or Fe3O4.2Fe + 3CO -> Fe2O3 + SC

3Fe + 4CO> Fe3O4 + 4C

Zone of heat absorption : (800-1200 C i.e. bri ght red heat)In the middle part of the furnace.

Following reactions take place

Any oxide of iron - escaped from reduction is reduced by red hot carbon.Fe2O3 + 3C> 2Fe + 3CO

Carbon dioxide is reduced to carbon monoxide by heat absorptionCO2 + C> 2C039 kcal

The hot spongy iron melts the ascending CO and decomposes a part of it to 500C D , produce

finely divided carbon.2CO --> CO2 T + C

This finely divided carbon thus remains enclosed in the spongy iron.

Lime obtained in the zone of reduction forms slag with silica.

CaO + SiO2 -----> CaSiO3flux gangue slag

At about 1200C, non-metallic and metallic oxides are reduced to respective elements, they arereduced by coke.

SO2+2C> S+2COT

SiO2+2C> Si+2COTP205 + so --> 2P + 5CO T

MnO2 + 2C-> Mn + 2C0

Thus, in this zone, iron is mixed with C, P, S, Si and Mn and becomes impure.

Due to presence of impurities, the melting point of iron is lowered and it starts melting at about

1200C.

Zone of Fusion: (1200 C - 1500 C i.e. whi te heat)In this zone, fusion of iron and slag take place and only chemical reaction taking place is the

formation of CO2.

The coke present in the charge gets oxidised by hot blast of air introduced through twyers andforms CO2 alongwith the liberation of heat.

C + O2 -> CO2 + 97 kcal

Melting point of impure iron is reduced to 1250-1300C (m.p. of pure iron is l530C).


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Melting of impure iron starts,Temperature of the blast furnace is not sufficient to melt the pure iron.

Impure iron melts in the zone of fusion and trickles down

Collected in the hearth.

The slag formed in the zone of absorption also melts at a temperature of

1200-1250CCollected in the hearth of the furnace.

This molten slag floats over the molten metal as it is lighter than molten iron.The slag, when it forms the upper layer automatically protects the iron from oxidising action of

the hot air blast.

The molten iron known as pig iron - withdrawn - from the lower outletThe slag is taken out from the upper outlet.

While gases formed during chemical reactions known as flue gases go out through the outlet

near the top of the furnace.

PRODUCTS OF Blast Furnace

The products of the blast furnace are :pig iron,

slag,

flue gases.

Pig ironThe molten iron from the blast furnace is collected in moulds and solidification, forms pig iron

or cast iron.

It is the most impure form of iron containing highest percentage of (25%) which makes ithard and brittle and hence unsuitable for commercial use.

Compositi on of Pig ir on

The amount of impurities vary in pig iron. Hence, its composition is not fixed.

It contains :Iron90-95%

Carbon2.5-5%

Silicon>1-3%ManganeseO.21% 4

SulphurO.1O.3%

Phosphorus0.5-1%

Properties of Pig iron/ Cast ironBrittle

Low tensile strength.

Melting point is 1 150C to l250C.Cannot be forged, rolled and welded.

Only suitable for casting which are not subjected to sudden strain and shock.


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Uses of Pig iron/ Cast ironCasting metal objects (stoves, lamp posts, drainage covers, pipes, railings, fire gates etc. )

Cheapness is more important than strength.

SLAG

Usually contains silicates of calcium and aluminiumA fusible slag contains approximately

SiO2 - 55%,CaO - 30%

Al2O3 - 15%.

UsesFiller (ballast) for rail roads, for metalling roads when moxed with pitch

Manufacturing of cement,

Powder - fertilizer and for soil conditioning.

FLUE GASESThe gases leaving the furnace through the flue are known as flue gases.The gas mixture contains approximately

CO30% '

CO210%

N259%Hydrogen and hydrocarbon traces.

This gaseous mixture is burnt for the following reasons :

(i) Burning carried out in a special device, produces heat required to heat the air which is

introduced through the twyers into the furnace.The poisonous carbon monoxide is removed as carbon dioxide.

Uses(i) Since this gaseous mixture contains large proportion of CO, it possesses a considerable heat

value (calorific value 750 kcal/ m3), hence used as a fuel.

(ii) This mixture is used for preheating of air for the blast furnace

COMMERCIAL FORMS

PIG IRON OR CAST IRON

PIG IRON

Most impure form of iron.

Average composition of pig iron is as follows :Fe=9295% 11=O.5-1%

C=2.54.5% Mn=0.2l%

Si=O.73% S=O.1O.3%

Purified in cupola furnace with coke and lime.Molten Metal poured into moulds of desired shape and the

iron obtained is known as cast iron.


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STEELOrdinary steel is an alloy of iron and carbon. Hence, it is known as steelor plain carbon steel.

Manufactur ing of Steel

Depending on the quality of steel to be manufactured, steel is prepared from cast iron by

reducing its carbon content from about 5% to 0.2-1The following methods are used for manufacturing of steel.

OPEN HEARTH PROCESS

Also called Siemen-Martin's process is based on the regenerative principle of heat economyproposed by William Siemens (1857).

the waste heat of the products of combustion of a gaseous fuel is utilized.

Two typesthe acid hearth and basic hearth process

depending on the silica or lime or magnesia lining.Choice of refractory linings = the nature of impurities present.

Hearth is lined with :Silica bricks (acidic lining) to remove C, Si and Mn.Lime or magnesia bricks (basic lining) to remove P and S.

MetallurgyThe charge consists of pig/cast iron (cold or molten), scrap iron/ steel and haematite (ore).

The process consists of heating the charge on the hearth of furnace by the heat produced by

burning fuel in air (both preheated) or by producer gas.

During the first phase of cycle, producer gas / air is passed through previously heated

regenerator (R1) while the products of combustion flow through the regenerator.The charge is fed through a charging door and heated to 1600C to 165OC. Mainly by meansof producer gas. Fuel is fired through nozzles.

The hot gases formed in R1 pass over the hearth to its opposite end and metal charge supportedon the hearth is openly exposed to the flames and is converted into molten metal.

Besides being directly exposed to the flames, metal charge is also heated by the radiations from

the walls.After passing over the hearth, the products of combustion pass through one checker chamber

(R2) and heat it.

After about 25 to 30 minutes the second phase of cycle starts and the idle burner fires the fuel,over the hearth flame passes from the opposite direction and initially active burner becomes

idle.

Checker chambers (i.e. regenerators R1, R2) store and release large quantities of heat which

would have escaped to the atmosphere and thus

wasted.


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In this furnace, tap hole in the lowest part of the hearth which always closed with a refractory

plug until the metal is ready to be poured. Before tapping the molten metal into the ladle, a

sample of the same may be tested as regards its chemical composition. The process takes 8-10

hours.

Following chemical reactions occur in the furnace.Oxidation of impurities of Mn, P and Si by haematite :

2Fe2O3 + 3Mn> 2Fe + 3MnO

5Fe2O3 + 6P> 1OFe + 3P2O5

2Fe2O3 + 3Si> 4Fe + 3SiO2

The charge consists of pig/ cast iron (cold or molten), scrap iron/ steel and haematite (ore).

The process consists of heating the charge on the hearth of furnace by the heat produced by

burning fuel in air [both preheated] or by producer gas.

Procedure :

a) During the first phase of cycle, producer gas/air is passed through previously heatedregenerator (R1) while the products of combustion flow through the regenerator.

b) The charge is fed through a charging door and heated to 1600C to 1650C mainly bymeans of producer gas. Fuel is fired through nozzles.

c) The hot gases formed in R1 pass over the hearth to its opposite end and metal chargesupported on the hearth is openly exposed to the flames and is converted into molten metal.

Besides being directly exposed to the metal charge is also heated by the radiations from the

walls.

d) After passing over the hearth, the products of combustion pass through one checkerchamber (R2) and heat it.

e) After about 25 to 30 minutes the second phase of cycle starts and the idleburner fires thefuel, over the hearth flame passes from the opposite direction and initially active burner

becomes idle.

f)

Checker chambers (i.e. regenerators R1, R2) store and release large quantities of heatwhich would have escaped to the atmosphere and thus wasted.

g) In this furnace, tap hole in the lowest part of the hearth which always closed. With arefractory plug until the metal is ready to be poured.

h) Before tapping the molten metal into the ladle, a sample of the same may be tested asregards its chemical composition. The process takes 8-10 hours.


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Following chemical reactions occur in the fu rnace.

Oxidation of impurities of Mn, P and Si by haematite 1

2Fe2O3 + 3Mn> 2Fe + 3MnO

5Fe2O3 + 61> _> 101% + sP2o52Fe2O3 + 3Si -~> 41% + 3SiO2

(b) Formation of slag for the removal of Mn, P and Si :

MnO + SiO2 -> MnSiO3

(slag)

P205 + 3CaO -> Ca3(PO4)2

(slag)

SiO2 + CaO -> CaSiO3

(slag)(c) Finally C and S form gaseous oxides which leave the furnace as flue gases :

2Fe2O3 + ac -a 4Fe + 3co2T

2Fe2O3 + as -> 4Fe + sso2 T

Basic oxygen steel (BOS) process :

I It is the most modern method for manufacturing of steel. The another name of this process is L-D

(two separate plants in Austria in 1953 at Linz and Donawitz).

LD converter is charged with molten pig iron and steel scrap (15%). Then the converter is turnedto vertical position for the oxygen blow, but before that lime (to increase slag fluidity) are added in

the converter.

High purity oxygen at a pressure of 100-150 psi (pounds per inch square) is introduced at

supersonic speed on to the surface of the iron bath through water-cooled lance, which is suspended

in the vessel and kept at few feet above the bath.

Immediately very rapid chemical reactions take place in the area just below the lance where

oxygen comes in contact with metal and impurities like Si, Mn, C, Fe and P take place giving out

considerable heat.

The blowing of oxygen produces intense heat at about 2500C to 3000C. Thus the metal in this

zone gets refined.

The surrounding metal at the top takes the place of the refined metal andin turn gets refined.


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Thus, a vertical rotating motion sets in the vessel and in about 20 to 30 minutes from the

commencement of the blow all impurities including S and P are oxidised and removed. The entire

cycle of operation takes about one hour.

Advantages over open hearth [ 0-H ) process :

(1) It requires much less investment than open-hearth process.

(2) L-D process is about ten times faster than O-H process.

(3) It does not use external sources of heat or fuel.

(4) L-D process is operationally more efficient than O-H process.

(5) Oxygen is used rather than air in L-D process. This causes its elimination

of harmful effect of nitrogen.

Properties of Steel :

The properties of steel depend upon the foll owing three factors :

(1) Percentage of carbon in steel.

(2) Presence of other elements.

(3) Heat treatment.

1. Percentage of carbon in steel :

I The presence and percentage of carbon in steel is the guiding factor in imparting desired

properties to steel. In general, lower the carbon content, the softer and more ductile is the steel.

Carbon also impart hardness and increases tensile strength but it reduces malleability of steel.

If the percentage of carbon in steel is low, it is not useful for engineering purposes as it is very soft,

ductile and weak.

If the percentage of carbon in steel is high, there is decrease in plasticity and the properties similar

to those of cast iron are developed in the steel i.e. develop hardness and brittleness and reduces

malleability and ductility.

According to the percentage of carbon and presence of other alloying elements, steel is broadly

classified into two classes :

[i) Plain carbon steels, (ii) Alloy steels.

Plain carbon steel : Plain carbon steel is an alloy of iron and carbon only.

It has its own limitations of use.

On the basis of percentage of carbon, it is further classified as

mild carbon

steel, medium carbon steel and

high carbon steel.


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Alloy steels : Alloy steels are those steels, prepared by addition of small amount of one or more

alloying elements (such as nickel, chromium, cobalt, vanadium, tungsten, molybdenum,

manganese etc.) into plain carbon steels. Alloy steels are further classified on the basis of [i]

chemical composition, (ii) according to structure and (m) according to purpose of use.

Presence of other elements in steel :

Since steel is mainly the alloy of iron and carbon, the properties of steel mainly depend on carbon

content.

Along with carbon other elements such as silicon, sulphur, phosphorous and manganese are also

present in the steel as impurities.

Presence of these constituents may alter the properties of steel. The effect of these constituents on

the properties of steel are as follows :

Carbon : It increases the tensile strength, hardness, but it lowers ductility and malleability with the

increase in carbon content.

Silicon : Silicon in very small amount is used as principal deoxidizer (to remove dissolved gases).

Hence, metal becomes sound and free from blow-holes. It improves tensile strength and deep

hardening properties.

The maximum of silicon present in the steel is about 0.25 %.

Manganese : It is used as deoxidiser. It combines with sulphur to form manganese sulphide.

Manganese makes the metal ductile. It is present in the metal about 0.8 % but when it is more it

increases hardness and brittleness.

Classification of Plain Carbon Steel

Plain carbon steel is classified according to the percentage of carbon present in it.

Under this category there are three types of carbon steels, namely

(a) mild or low carbon steel0.05 to 0.3% carbon

[b) medium carbon steel- 0.3 to 0.6% carbon

(c) high carbon steel0.6 to 1.5% carbon.

The various applications of steel are governed by the percentage of carbon present in it, because as

the percentage of carbon in steel increases, the hardness of steel also increases but there is a

corresponding decrease in the ductility.


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Properties and Applications of Different Plain Carbon Steel

Mild or Low carbon steel (0.05-0.3% carbon] :

Properties : It is quite tough, ductile, weldable, can be magnetised

permanently, can resist shock and impact, difficult to heat treatment and undergo corrosion

quickly.

Applications : Mild steel is used for thin soft wires, wires for ropes, bolts, rivets, nails, chains,

tubes, plates for boilers and ships, fan-blades and for powerful magnets. .

Medium carbon steel (0.3-0.6% carbon] :

Properties : It is tougher and harder than mild steel, shock-resistant, can be welded with care, canbe hardened by heat treatment, has high tensile strength than mild steel.

Applications : Medium carbon steel is mainly used in railway engineering for rail roads, wheels,

axles and fish plates, also used in making machine and gun parts, springs, rotors and various shafts

like armature and crack shafts.

High carbon steel (0.6-1.5% carbon) :

Properties : It is very hard and tough, has high tensile strength, cannot be welded, can be hardened

by heat treatment and resistant to wear, hence it can produce a sharp cutting edge.

Applications : It is mainly used for making tools like files, chisel, hammers, punches, wooden

working tools and various cutting instruments like cutters, knives, saw, blades, razors etc.

(d) Sulphur : It should not be present above 0.06 % in the metal; otherwise steel becomes very

brittle or red short i.e. the condition of iron and steel in which it cannot be worked by hammering

or rolling at or above a dull red heat. When it is present along with manganese, it improves the

machinability of steel.

(e) Phosphorus : This should not be present above 0.06 % in steel, otherwise the steel becomes

cold short, i.e. the condition of iron and steel in which it cannot be worked by hammering or rolling

at or below a dull red heat. In low carbon steels, phosphorus improves yield strength and tensile

strength.

Heat treatment of steel :

When metals and alloys are heated and cooled, their physical properties are changed. These

changes take place because of the change in their structure due to heating and cooling. In case of

steel, these changes are marked.


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The changes like physical properties and structure depend upon the way in which heating and

cooling of steel is carried out.

Thus, in general, heat treatment of steel may be defined as the process ofheating steel to a certain

high temperature and then cooling it at a controlled rate, in order to develop certain desirable

physical properties in it without changing its chemical composition.

All the heat treatment purposes may be considered to consist of three main parts :

1. The heating of steel to a particular temperature.

2. The soaking of the steel at that temperature until the structure becomes uniform throughout the

section. The cooling of the metal at some specific rate to cause the formation of desirable structures

within the metal for the desired properties.

Purpose of heat treatment : The improvements resulting from heat treatment are mainly due to the

development of certain micro~constituents which have the desired qualities.

Following are the main purposes of heat treatment of steel :

l. To change the structure of steel.

2. To alter magnetic properties of ste-el.

3. To increase surface hardness.

4. To increase resistance to heat and corrosion.

5. To vary strength and hardness.6. To make steel easily workable.

7. To remove the trapped gases.

8. To improve machinability and mechanical properties.

1.2.5 Heat Treatment of Steels

The important processes involved in heat treatment of steel are

1. Hardening or Quenching,2. Tempering,3. Annealing,4. Nonnalising.5. Hardening or Quenching :


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The operation of hardening is applied to tools and some important machine parts made of alloy

steel.

The purposes of hardening are

(1) To increase its resistance to wear or abrasion and ability to cut other metals.

(2) To improve strength, elasticity, ductility and toughness.

In this process, steel is heated to high temperature (800-900C) and then suddenly cooled by

dipping or quenching in some suitable medium.

The quenching medium generally used is either cold water or mineral or animal or vegetable oil,

4-6 % caustic soda, 6-20 % sodium chloride solution or a stream of compressed air or in mercury.

This process is known as Hardening or Quenching and steel is then called hardenedor quenched

steel.

By this process, steel becomes extremely hard and brittle like glass. The hardness developeddepends upon :

(1) The rate of cooling and (2) The medium used for cooling.

(2) Tempering :

If a piece of steel is taken out of quenching medium, then it becomes hard, brittle and has

unequally distributed stresses. Therefore, the primary objects of tempering are :

1. To stabilise the structure of metal by relieving stresses and strains.

2. To reduce some of the hardness produced during hardening.

3. To improve ductility of metal.

4. To reduce brittleness and improve tensile strength.

By quenching, the hardness of the steel is increased but it becomes very brittle and liable to crack

while in use. To minimise this defect, steel is tempered.

Tempering is the process of reheating the hardened or quenched steel to adefinite temperature

(generally between 200600C} and then cooling it at a suitable rate.

By this process of heat treatment the properties like hardness, brittleness and internal stresses

developed in a quenched steel are reduced. On the other hand, it imparts toughness and elasticity to

steel.


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[3] Annealing (means softening) :

It is one of the most important widely used operation in the heat treatment.

The purposes of annealing are as follows :

1. To soften the steel.

2. To improve machinability.

3. To increase or restore ductility and toughness.

4. To refine grain size and to have uniform structure.

Articles made by casting, forging etc., and hard steel develop internal stresses and strains, making

them weak, less tough, less ductile and more brittle.

These defects are removed by heating them to about 760-925C (the temperature depending upon

the carbon content of steel) by holding it at that temperature for some time and then cooling at a

controlled and slow rate. This is known as annealing.

In annealing, the article after heating is cooled in the furnace along with furnace.

Due to annealing, steel becomes more soft, pliable (i.e. it can be easily worked upon with a

machine), malleable and ductile and acquires many new properties which are magnetic and

electrical in nature.

Moreover, its grain structure becomes uniform.

Since the annealing temperatures depend on percentage of carbon, the annealing temperature for

various kinds of steel are different. They are given in the following Table 1.3.

4) Normalising : It is a process of heat treatment just similar to annealing process. The process

consists of heating steel to a definite temperature i.e. about 50C above the critical temperature. (In

case of steel, the critical temperature is about 725C).

Then it is allowed to cool freely in air. The cooling rate is generally about 5C per second.

By this method, steel becomes homogeneous and more soft.

After normalising treatment, the ultimate structure in the steel consists of fine grains.

Cold working is done at a temperature below critical temperature. By cold working, the steel

develops some hardness and looses ductility due to distortion. .


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By normalising, the hardened articles with distorted grains regain their fine grained structure.

Normalising is used particularly for the following purposes :

1. To remove coarse grained structure.

2. To remove internal stresses that may have been caused by working.

3. To give ductility and toughness.

4. To improve the mechanical properties of the steel.

Normalising is similar to annealing. In normalising, the steel is heated to slightly higher

temperature than annealing.

Since steel is cooled in air, in normalising, the rate of cooling is faster as compared to annealing.

Normalising has some advantages over annealing. In normalising, the mechanical properties of

steel somewhat improved.

The time required for normalising is less than annealing. In annealing, the consumption of fuel or

electric power is greater than normalising process.

Metalurgy 2

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