02 heat treatment

-Heat Treatment-

© Rohan Desai- Automobile Department - New Polytechnic, Kolhapur.

Q. Define heat treatment. “Heat treatment is defined as an operation involving heating and

cooling of metals or alloys in its solid state with the purpose of changing the

properties of the material.” The physical and mechanical properties of the

materials depend upon the size, shape and form of the micro-constituents

present. The micro-constituents generally present in steel are ferrite,

troostite, sorbite, austenite and cementite.

Steel possesses many properties like strength, cheapness and

workability in addition to toughness, stiffness creep resistance, fatigue

resistance, impact strength, etc. Proper heat treatment of steel plays an

important part in engineering. Heat treatment of all components, whether

cast forged or rolled, is necessary before actual use.

Q. Which factors are to be considered in Heat Treatment Processes?

1. Chemical composition of the material.

2. Mode of manufacture of the material, i.e. cast, ingot, rolled or forged,

etc.

3. Whether any previous heat treatment operation has been carried out

on the material and what is its structure.

4. Heat treatment operations to be performed and properties and

structure required.

Q. Which are the advantages or objects of heat treatment process?

Or

Why heat treatment is done?

The following are the main objects of the heat treatment of steel.

1. To soften the steel that has been hardened by the previous heat

treatment or mechanical working.

2. To harden the steel and increase its strength.

3. To adjust its other mechanical and physical properties like ductility,

malleability, permeability corrosion resistance, etc.

4. To stabilize the dimensions of the steel instruments so that they do not

expand or contract with time.

5. To refine the grain size of the steel and to reduce internal stresses.

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Q. Draw cooling curve of pure iron.

Fig 2.1: Cooling curve of pure iron

Q. Draw Fe-C Phase transformation diagram. (Iron-Iron carbide

equilibrium diagram)

The various phases existing in the diagram are as below:

(i) α (Ferrite): Ferrite is a solid solution of carbon in low temperature B.C.C. α

iron. It is almost pure iron and the name ferrite comes from the Latin word

ferrum which means iron. It is a relatively soft and ductile phase

(ii) γ (Austenite): Austenite is a solid solution of carbon in F.C.C. γ - iron. It

can dissolve upto 2.0% carbon at 1147°C. The phase is stable only above

727°C. It is a soft, ductile, malleable and non-magnetic (paramagnetic) phase

(iii) δ (δ - ferrite): It is a solid solution of carbon in high temperature B.C.C. δ-

iron. It is similar to α-ferrite except its occurrence at high temperature.

(iv) Fe3C (Cementite): It is an intermetallic compound of iron and carbon with

a fixed carbon content of 6.67% by weight. It is extremely hard and brittle

phase. It is also called Iron Carbide.

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Fig 2.2: Iron-Iron carbide equilibrium diagram

The above diagram contains three different transformations which are

described below:

(i) Peritectic transformation

The peritectic region is the upper left hand corner of Fig. 2.2. In Fe-C

system, this transformation occurs at point P and is as below:

δ of 0.1% C combines with liquid of 0.55% C at 1492°C and forms γ of

0.18%C.

(ii) Eutectic transformation

In Fe-C system, this reaction occurs at 1147°C and 4.3% C and is as

below:

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Liquid of 4.3% carbon transforms at constant temperature of 1147 °C

and gives a eutectic mixture of austenite (of 2% carbon) and cementite. This

eutectic mixture of austenite and cementite is called ledeburite.

(iii) Eutectoid transformation

The eutectoid region is in the lower left hand side of Fig 2.2. Eutectoid

transformation in Fig. 2.2 occurs at point E and is as below:

Q. Explain with sketch TTT Curve.

Time Temperature Transformation diagrams or Isothermal diagrams

are also called S curve or C curve due to their shape. For each steel

composition, different IT diagram is obtained. Fig 2.3 shows TTT diagram of

eutectoid steel (i.e. steel containing 0.8% C).

Austenite is stable above eutectoid temperature 727 °C. When steel is cooled

to temperature below this eutectoid temperature, austenite is transformed

into its transformation product. TTT diagram relates transformation of

austenite to time and temperature conditions. Thus, TTT diagram indicates

transformation product according to temperature and also time required for

complete transformation.

Curve 1 is transformation begin curve while curve 2 is transformation end

curve. The region to the left of curve 1 corresponds to austenite (A‟). The

region to the right of curve 2 represents complete transformation of austenite

(F+C). The interval between these two curves indicates partial decomposition

of austenite into ferrite and Cementite (A‟+F+C).

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Fig 2.3: TTT diagram of eutectoid steel

At temperatures just below eutectoid temperature, austenite

decomposes into pearlite; at lower temperatures (600 °C) sorbite is formed

and at 500 – 550 °C troostites is formed. If temperature is lowered from

500 °C to 220 °C acicular troostite or bainite is formed. In eutectoid steels,

the martensite transformation begins at MS (240 °C) and ends at MF (50

°C). The change in the hardness of the structures is shown in Rockwell units

(RC) at the right hand side of the diagram.

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Q. List common heat treatment processes.

Common heat treatment processes can be classified as follows:

Q. Why annealing is done? Explain their types briefly.

The annealing operation is carried out mainly to obtain the following

properties.

1. To soften the steels.

2. To improve machinability.

3. To relieve internal stress induced by some previous treatment (rolling,

forging, extrusion, uneven cooling).

4. To remove coarseness of grains.

5. To produce a completely stable structure.

Annealing treatment is applied to castings, forgings, cold worked sheets and

wires. The operation consists of (i) heating the steel to- a certain

predetermined temperature (ii) soaking at a constant temperature for a

sufficient time to allow the necessary changes to occur and (iii) cooling at a

predetermined very slow rate.

1. Annealing

(i) Full annealing

(ii) Process annealing

(iii) Isothermal annealing

(iv)Spheroidize annealing

(v) Homogenizing

2. Normalizing

3. Hardening

4. Tempering

5. Surface hardening

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1. Full Annealing:

Purpose:

(i) To reduce internal stresses produced due to cold working, welding etc.

(ii) To reduce hardness and increase ductility.

(iii) To refine the grain size.

(iv) To increase machinability.

(v) To make the steel suitable for further heat treatment.

Process:

Hypoeutectoid steel (steel containing less than 0.8 % C) is heated to

30-50 °C above the upper critical temperature and hypereutectoid steel (steel

containing more than 0.8 % C) is heated to 50°C above the lower critical

temperature. The steel is soaked at the annealing temperature (soaking time

depend upon the thickness of steel parts). Then these steel parts are slowly

cooled at the rate of 20 to 40°C per hour. The cooling is carried out in the

furnace.

2. Process Annealing:

It is also known as sub- critical annealing or recrystalization.

Purpose:

(i) To soften the component to restore the ductility.

(ii) To remove the internal stresses produced in the casting by welding or

by previous heat treatment.

Process:

Steel is heated to a temperature from 600 to 650 °C, holding at that

temperature, and then cooling in air or in furnace. By this process, high

degree of softening takes place due to removal of stress from pearlite. No

phase change takes place and the ferrite & pearlite simply rearrange

themselves to induce softening in materials.

3. Isothermal Annealing:

This process is suitable for small rolled and forged components and not for

large components. It is faster than full annealing and saves much time.

Purpose:

(i) To obtain stable structure

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(ii) To save the time required for heat treatment

Process:

The process is similar to ordinary annealing but it is first cooled rapidly in air

or by blast in furnace to temperature 600-700 °C. The steel is held

isothermally at this temperature for certain duration then it is rapidly cooled in

air.

4. Spheroidize Annealing:

The process of producing a structure of globular pearlite is known as

Spheroidizing or spheroidizes annealing.

Purpose:

(i) To improve machinability of the steel

(ii) To reduce hardness

(iii) To prevent chances of cracking during cold working.

Process:

This operation is generally applied to the hypereutectoid steels. Steel is

heated just above the lower critical temperature (740 to 770 0C), held for the

required time and cooled very slowly upto 600 0C in furnace. Further cooling

is conducted in still air. The cooling rate varies from 20 to 25 0C per hour. It

should be noted that heating much above Acm will produce lamellar pearlite

instead of granular cementite.

5. Homogenizing:

It is also known as diffusion annealing.

Purpose:

(i) To remove non uniformity of castings this is caused by coring. Coring

means variation in the composition from centre to surface of a grain.

(ii) To improve the structure of steel.

Process:

The steel is heated as rapidly as possible up to 1150 °C and is held at this

temperature for sufficient time so that diffusion takes place. It is then cooled

in 6 to 8 hours to a temperature of 800 to 850 °C and then further cooled in

air. After homogenizing, the full annealing is done to refine the grain

structure.

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Q. Explain the normalizing process.

Purpose:

1. To eliminate coarse-grained structure.

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

3. To improve the mechanical properties of the steel.

4. To increase the strength of medium carbon steels to a certain extent

(in comparison with annealed steels)

5. To improve the machinability of low carbon steels

Normalizing is frequently applied as a final heat treatment for items which are

to operate at relatively high stresses.

Process:

1. Heating the metal to temperatures within the normalizing range usually

40°C to 50°C above Ac3 (for Hypoeutectoid steels) and Acm (for

hypereutectoid steels)

2. Holding at this temperature for a short time (about 15 minutes).

3. Cooling in air.

Normalized steels have a higher yield points, tensile strength and

impact strength than if they were annealed, but ductility and machinability

obtained by normalizing will be somewhat lower.

Q. Give Difference between annealing and normalizing

Annealing Normalizing

Less hardness, toughness.

For plain carbon steel the

microstructure shows pearlite.

Pearlite is coarse and usually

gets resolved by the optical

microscope.

Grain size distribution is more

uniform.

Internal stresses are least.

Slightly more hardness,

toughness.

Microstructure shows more

pearlite.

Pearlite is fine and appears

unresolved with optical

microscope.

Grain size distribution is slightly

less uniform.

Internal stresses are slightly

more

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Q. What are hardening and quenching processes? Hardening:

Objectives:

(i) To improve mechanical properties, like elasticity, strength, ductility,

toughness, etc.

(ii) To enable the metal to cut other metals,

(iii) To develop desired hardness.

Process:

The process consists of heating the metal to a temperature above

critical point. The metal is held at this temperature for a considerable time

and then it is rapidly cooled. The cooling media used varies between water,

oil or molten salt.

Hardening is applied to tools and machine parts to perform the

operations more efficiently.

Quenching:

The rapid cooling of a metal in a bath of liquid during heat

treatment is known as quenching, e.g. Steel is heated above its critical

temperature and plunged into water to cool it, an extremely hard, needle

shaped structure known as „martensite‟ is formed. The rapidity with which

heat is absorbed by the quenching bath has different effects on the hardness

of the metal. Cold clean water is used as quenching media, while addition of

salt increases the hardness considerably. Oil gives the best balance between

hardness, toughness and distortion. Special soluble oils are used as quenching

media.

The parts which are subjected to hardening have good tensile

strength, but poor ductility, toughness and impact strength.

Q. What is tempering and why it is done after hardening?

Objectives:

(i) To reduce internal stresses developed during previous heating,

(ii) To reduce the hardness developed during hardening,

(iii) To give the metal a right structural condition (To stabilize the

structure).

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After hardening, when a metal is removed from the quenching media,

it is very hard and brittle and there are several other inequalities in the

structure of the metal. Tempering is done to restore ductility and reduce

hardness. The process involves re-heating of the metal below critical point,

then holding it for a considerable time and then slowly cooling it. Tempering

should be done immediately after quenching in order to relieve hardening

strains. The temperature at which tempering is done varies with the carbon

content of the metal and mechanical properties desired in the finished article.

Lathe tools, chisels in which only the cutting ends need hardening

may be hardened and tempered in one operation only. The whole tool

is heated to the hardening temperature and the cutting end is quenched.

When the cold end is rubbed bright and the heat from unquenched portion

causes tempering, when the colour is satisfactory, the whole tool is quenched.

Three types of tempering processes are classified as:

(i) Low temperature tempering: This type of tempering is done in the range

of 200 - 250° C. At this range, hardness changes to a very small extent.

Tensile strength is increased. Internal stresses are reduced comparatively.

(ii) Medium temperature tempering: Tempering done in this case at a range

of 350° to 450° C. In this case, the properties of the structure are improved,

mostly employed for coil and laminated springs. Highest elastic limit and

toughness are achieved.

(iii) High temperature tempering: This tempering is performed in the range of

550° C to 600° C. Eliminates the internal stresses completely. Comparatively

high strength and toughness are achieved.

Q. What is case hardening? List some of them.

A large number of industrial components like cams, change-over switch

shafts, drive worms brake drums, gears, etc. require a hard wear resistant

surface (also called case) and a soft core, so that it is tough and shock

resistant too. No plain carbon steel and even alloy steels possess both the

requirements, i.e. hard surface and tough core to resist shock. It is noticed

that steel containing 0.1% carbon is tough whereas the steel containing 0.8

%C is very hard and brittle. Both these properties are obtained by the case

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hardening process. The heat treatment process of producing a hard wear-

resistant carbon rich case (surface layers) on a tough and soft core of steel

part is known as case hardening. Low carbon steel is used for the case

hardening processes except in induction, hardening, where medium carbon

steel or high carbon steel is used. The processes generally employed for case

hardening are as follows.

1. Carburising

2. Cyaniding

3. Nitriding

4. Carbonitriding

5. Flame hardening

6. Induction hardening.

Q. Explain the process of case carburizing

Process: Curburising is a method of depositing carbon on the surface

layer of low carbon steel in order to produce a hard case. Carburising is also

known as cementation. Roughly, the machined parts of the low carbon steel

are packed with carburising mixture in a steel box as shown in Fig. The

carburising mixture contains 70% charcoal, 10% barium carbonate, 10%

calcium carbonate and 10% sodium carbonate. A layer of the carburising

mixture of nearly 25 mm thickness is placed at the bottom. Then the

components are so placed that no component touches one another or even

the sides of the box. The box is covered and the lid tightly sealed with fireclay

to avoid the entry or escape of gases.

Fig: Packing components for solid carburising

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The portion which is not to be case hardened is protected by electroplating on

the surface which does not absorb carbon. The boxes are placed in a furnace

and heated to a temperature of 900 to 980°C for 6 to 8 hours. Temperature

and time of heating depends upon the depth of the case required. After

heating, the box is allowed to cool along with components inside the furnace.

Carbon percentage increases on the surface as the austenite has a tendency

to absorb carbon at high temperatures.

Depth of the case obtained in this case varies from 1 mm to 1.5 mm with the

carbon content on the outer surface at 1.1 to 1.2%.

Q. Explain Cyaniding process for case hardening.

The process of providing a hard wear resistant case with a tough core

to the low carbon steels by liquid cyanide bath is called cyaniding.

Process: The cyanide mixture (20 to 50 % Sodium cyanide and 40% Sodium

carbonate) is heated to a temperature of 870 to 930°C, and the work pieces

contained in a wire basket are immersed in the molten bath of cyanide. The

soaking period varies from component to component depending on the depth

of the case, but generally, it varies from-10 minutes to 3 hours.

Nitrogen produced in atomic form also dissolves on the surface and increase

in hardness takes place due to the formation of nitrides. In nitriding, a portion

of the surface to the parts to be kept soft is coated with such materials which

are not affected by the bath. Careful handling of cyanides is needed as these

salts are very poisonous.

Q. Explain the Nitriding process.

The heat treatment process which produces a hard-wear resistant layer

of nitrides on a tough core of low carbon steel is known as nitriding.

Process: The process is suitable for the steels containing 1%

aluminium, 1.5% chromium and 0.2 per cent molybdenum. The percentage of

carbon in these steels varies from 0.2 to 0.5.

The process consists of heating machined and heat treated components to a

temperature of 500°C for 40 to 90 hours in a gas tight box through which

ammonia gas is circulated. The essential requirement of the operation is close

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adherence to a temperature of 500°C. The component is allowed to cool in

the furnace after switching of the supply of ammonia.

When ammonia vapours come in contact with the steel, they get dissociated

NH3 = 3H +N and nascent nitrogen so produced diffuses into the surface of

the workpiece forming hard nitrides.

Q. Give difference between Carburizing and Nitriding.

Carburizing Nitriding

1) Carburizing is a method of heat treatment by which carbon content at

the surface of a ferrous material is increased.

1) Nitriding is a case hardening process by which nitrogen content at

the surface of steel is increased.

2) High temperature (930°C).

Quenching is done.

2) Temperature employed ≤=600°C.

Quenching is not required.

3) Hardening and tempering is needed.

3) No need of hardening and tempering.

4) This process is very simple and inexpensive.

4) This process is complex and expensive.

5) Grain refinement is not necessary. 5) Before nitriding, grain refinement is necessary.

6) Inferior surface finish as compare

to nitriding.

6) Surface finish is very good.

Q. Write short note on Carbonitriding.

The process of producing a hard case by the addition of carbon and nitrogen

on the surface of the steel.

Process: Hydrocarbons, carbon monoxide and ammonia gases are used for

Carbonitriding. Carbonitriding is carried out at a temperature of 800 to 875°C

for 6 to 10 hours and the case depth obtained is 0.5 mm. Carbonitriding is

applied to the low carbon steels (steels used for carburising). Nitrogen in the

surface layer of the steel increases its hardenability and permits hardening in

oil quenching. Thus, chances of distortion and cracking are eliminated. The

portion of components which is not to be carbonitrided is protected by copper

plating.

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Q. Explain any one surface hardening.

Surface hardening involves the following two methods.

1. Flame hardening:

The process of heating the metal with a flame of an oxyacetylene torch and is

then almost immediately quenched is called as flame hardening.

Fig: Principle of flame hardening

Process: The surface to be case hardened is heated by means of an

oxyacetylene torch for sufficient time and Quenching is achieved by sprays of

water which are integrally connected with the heating device. The heating is

generally accomplished for sufficient time so as to raise the temperature of

the surface of the specimen above the critical temperature. As the

temperature desired is achieved immediately, spraying of water is started. In

mass production work, progressive surface hardening is carried out where it is

arranged to have the flame in progress along with quenching.

Advantages:

Selective surface can be hardened even on very large components.

There is less distortion than in ordinary methods.

Disadvantages:

Temperature can not be precisely controlled.

Hardening is restricted to parts which are affected by wear.

2. Induction hardening:

The process of the surface hardening by inductive heating is

known as induction hardening.

Process: A high frequency current is passed through the inductor

blocks which surround the surface to be hardened without actually touching

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it. The inductor block current induces current in the surface of the metal

which the block surrounds. The induced eddy current and hysterisis losses in

surface material effect the heat required. When the surface, to be hardened,

is heated upto a proper length of time, the circuit is opened and water is

sprayed immediately on the surface for quenching. It is extensively used for

hardening of crank shaft, cam shafts, axles and gears.

Advantages:

(i) Time required for this process is less

(ii) Deformation is reduced.

(iii) Hardening can be controlled by controlling the current

(iv) Depth of hardening can be controlled.

Disadvantages:

(i) High equipment cost

(ii) High maintenance cost

(iii) Method is suitable only for large scale production.

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Q. Give difference between Flame and Induction hardening.

Flame Hardening Induction Hardening

Material is heated with

oxyacetylene flame at a required temperature, and then it is followed by water spraying.

Holding time is required. Oxidation and decarburization

are minimum. Irregular shape parts can be

flame hardened.

Flame hardening requires more care in control of temperature.

Material is heated by using high

frequency induced current and then it is followed by water spraying.

Due to very fast heating, no holding time is required.

No scaling and decarburization.

Irregular shape parts are not

suitable for induction hardening.

Easy control of temperature by control of frequency of supply voltage.

Q. Give applications of heat treatment processes.

(a) Heat treatment of steel castings

(b) Heat treatment of forgings

(c) Heat treatment of gears

(a) Heat treatment of Steel Castings:

Cause: After solidification, steel castings have a coarse grained structure.

Coarse grained structures have poor mechanical properties, poor

machinability, high hardness and high internal stresses.

Application: Steel castings are subjected to full annealing. Steel castings are

charged in the furnace at a temperature of 300 to 400 °C and heated at slow

rate to the annealing temperature. Steel castings are held at this temperature

for sufficient time and then cooled to low temperature.

Cooling media: Air for low carbon steels while medium and high carbon

steels are cooled in furnace.

(b) Heat treatment of Forgings:

Cause: After forging operation, component loses its mechanical properties

(which were improved by adding different alloying elements).

Application: Forging components are placed in the furnace and heated at a

slow rate to the annealing temperature. Then they are normalized. After

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normalization, the forged components are subjected to hardening and

tempering to get desired properties.

Cooling media: Air- for low carbon steel forgings

In furnace- for high carbon steel forgings

(c) Heat treatment of Gears:

Cause: Gear teeth are subjected to severe stresses when in use. Thus they

must possess high strength to withstand large torques combined with very

high wear resistance to protect them from wearing away in service.

Application: Plain carbon steel gears (containing 0.4 to 0.5% C) are

hardened and quenched in water from 820 to 850 °C, followed by tempering

from 500 to 550 °C to obtain a Brinell hardness of 220 to 260.

02 heat treatment

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