2 heat treatment - diploma engineering college-new...

2 HEAT TREATMENT 2015-16

Rohan Desai, Auto. Engg. Dept.NPK.

Chapter Name of the Topic Marks

02

2 HEAT TREATMENT

Specific Objectives:

Study various methods of Heat treatment processes as

applied to automobile components. Understand iron-

carbon phase equilibrium diagram.

Contents:

2.1 Introduction:

• Concept of phase and phase transformations

• Iron-Iron carbide phase (Fe-Fe3C) equilibrium diagram.

2.2 Common heat treatment processes and their

applications: Annealing, Normalizing, Hardening,

Tempering. Surface hardening processes:- Case

carburizing, Nitriding, Cyaniding, Induction and Flame

hardening.

12



2.1 INTRODUCTION

Phase:

Atoms are similar to a bunch of balls, and these balls can be arranged in

many different ways. A simple example is that the billiard balls are arranged

in a hexagonal close-packed fashion to start with, whereas the keyboards you

are using are arranged in another way. Although both of these examples are

two dimensional, atoms can certainly also be arranged in many different ways

three dimensionally.

The reason that atoms have a regular or homogeneous way to stack

themselves is because such arrangement results in a stable or low energy

configuration. The homogeneous arranged portion of atoms is called a phase.

A phase may be defined as a homogeneous portion of a system that has

uniform physical and chemical characteristics. Every pure material is

considered to be a phase; so also is every solid, liquid, and gaseous solution.

The graph below is the phase diagram for pure H2O. Parameters

plotted are external pressure (vertical axis, scaled logarithmically) versus

temperature. In a sense this diagram is a map wherein regions for the three

familiar phases—solid (ice), liquid (water), and vapor (steam) are described.

The three curves represent phase boundaries that define the regions. A

photograph located in each region shows an example of its phase—ice cubes,

liquid water being poured into a glass, and steam that is spewing forth from a

kettle.



One other interesting example is the different forms of carbon: graphite is

one; diamond is another. Diamond can only be formed at extremely high

temperature and pressure, but it stays almost `unbreakable' once it is

formed. Therefore, it is possible to transform graphite into diamond as long

as enough high temperature and pressure are exerted. You may also imagine

that not too large of a diamond can be formed, because the high pressure is

required throughout the whole diamond. If the graphite body is big, it is

difficult to transfer the applied pressure to the inside of that graphite body.

Phase Transformation

In the history, metals stand for a very special class of materials,

because metals are stable enough to remain solid at room temperature, but it

is also unstable enough so that we can burn coals to melt them and to shape

them into different tools. On the contrary, it is almost impossible to melt rocks

or sands, and these are categorized as ceramics. Therefore, metals have

long been used to understand the nature of materials. The science and

technology of metals are defined as metallurgy.

Part of the advantages for metals in scientific research is due to some of

their crystal structures. For example, pure iron has body center cubic

(bcc) structure at room temperature. Bcc structure has eight atoms sitting on

the corners of a cube and one atom in the center of this cube. Other metals

that have this structure include Chromium (Cr), Tungsten (W), etc.

The other important family of structure is face center cubic (fcc). Fcc

structure also has eight atoms on the corners of the cube, but it has six other

atoms sitting at the centers of the cube faces. Fcc metals are famous for their

ductility. Many ductile metals, such as Gold (Au), Silver (Ag), Copper

(Cu), Aluminum (Al), Nickel (Ni), etc., all have this structure.

It should be noted that although pure iron has a bcc structure at room

temperature, iron does not melt but changes from the bcc structure into fcc

structure at temperatures above 910 degree centigrade. Such crystal

structure change between phases is phase transformation.



Heat treatment: It 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.

Factors in Heat Treatment Processes:

Before carrying out any heat treatment operation the following factors

need consideration:

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.

Objectives of Heat Treatment:

When steel is subjected to the heat treatment operations, it undergoes

many structural changes due to which the properties of the material change.

In studying the effects of heat treatment therefore, the following points need

consideration.

1. Structural condition of the object before heat treatment.

2. Structural changes occurring during the heat treatment.

3. Structural conditions permanently retained.

In addition to the above, the chemical composition of the material

plays a vital role in controlling the properties of the materials.



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.

6. To reduce the internal stresses. To eliminate gases.

7. To produce a hard surface on a ductile interior.

8. To improve electrical and magnetic properties.

COOLING CURVE OF PURE IRON

Fig 2.1: Cooling curve of pure iron



• 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.



• TTT DIAGRAM / ISOTHERMAL TRANSFORMATION

DIAGRAM

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).



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.



• 2.2 COMMON HEAT TREATMENT PROCESSES

Common heat treatment processes can be classified as follows:

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



ANNEALING

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. 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

(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.

NORMALIZING

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

6. To improve the structure of welds

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.

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

HARDENING AND QUENCHING 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.

TEMPERING

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).

Why tempering is done after hardening?

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.

2.2.1 CASE HARDENING OF STEELS

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

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.

1. Carburising

Process: Curburising is a method of enriching in carbon 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 50 to 70% charcoal, 5 to 15% barium carbonate,

2 to 15% calcium carbonate and 3 to 13% 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

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%.

2. Cyaniding

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.

3. Nitriding

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

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.



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.

4. 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.

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

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.



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.

2.4 SELECTION AND APPLICATION OF HEAT TREATMENT PROCESS

TYPES OF HEAT TREATMENT PURPOSE COMPONENT

Annealing Softening and removing residual stress for post processes

Forged blanks for gearing

and misc. parts

Normalizing Control microstructure and hardness for machining

Railroad wheels, axles

and some bar products.

Hardening high strength and wear

resistance

Heavy gears, heavy duty

crankshafts

Tempering Optimize hardness for strength

and toughness Fasteners and Rods

Carburizing Fatigue strength and wear

resistance improvement.

Gears and shafts

Nitriding To impart wear and corrosion

resistance.

Cam shafts, oil pump

gears, valves

Induction hardening

Heat up by inductive power

and quench to get hard case

locally.

Cam shafts, Drive shafts,

steering knuckles

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