1

COLLEGE OF ENGINEERING ROORKEE

Title :-Heat Treatment

Submitted by:- Submitted to :- Avinash Kumar Mr. Vjay KumarRoll-49 Asst. Prof. ME ‘M’ (MECHANICAL DEPT.) (2010-14)

2

HEAT TREATMENT

3

CONTENTS

• Introduction• Classification Of various heat

treatment.• Microstructure of steel and iron• Heat Treatment Processes• Chemical Heat treatment process

4

What is Heat Treatment?

• It is defined as control heating and cooling of metals for altering their properties and to make them suitable for specific application.

• The rate of cooling and the manner of cooling is the controlling factor of heat treatment processes.

• By this process we can increase Toughness, Hardness as well as Tensile strength of any material.

5

Purpose of Heat treatment

1. Produce hard surface and tough interior.

2. Increase resistance to wear and corrosion.

3. Modify magnetic and electrical properties.

4. Improve mechanical properties such as tensile strength, ductility, shock resistance etc.

5. Improve machinability.

6. Refine grains.

7. Change the chemical composition.

6

Classification of Heat treatment processes

1. Tempering:- (a) Austempering

(b) Martempering

2. Annealing:- (a) Process annealing

(b) Full annealing

3. Normalizing

4. Hardening or Quench hardening

5. Chemical heat treatment process:-

(a) Carburizing (b) Cyaniding (c) Nitriding

7

Microstructures of steel and iron

Due to the presence of carbon in different form in iron and steel various microstructure founds in them –

1. Ferrite :- It is soft and ductile. Low carbon steel and wrought iron consist of ferrite. It is in the form of flakes.

2. Cementite:- It is in the form of iron carbide (Fe3C) and is extremely hard, carbon is in the form of knot, presence of carbon in iron increases the hardness but decreases the ductility.

8

3. Pearlite:- It is a mixture of ferrite and cementite, and is found in low and medium carbon steels.

4. Austenite:- The solid solution of iron carbide(Fe3C)

in ᵞ-iron is known as austenite.

5. Bainite:- It is a mixture of ferrite and cementite having finer crystals then paralite.

6. Martensite:- it is the hardest constituent of steel and has needle like structure.

martensitepearliteferrite

9IRON CARBON PHASE DIAGRAM

10

Reaction at Eutectoid point:-The Eutectoid temperature is 723: C and Eutectoid composition 0.83% carbon.

Reaction at Eutectic Point:- The eutectic temperature is 1175: C and composition is 4.3% of carbon.

11TTT CURVE

12

Phase transformation in Iron Carbon diagram

AusteniteRapid quench

PearliteBainite

Martensite

Tempered Material

Slow

cool

ing

mod

erat

e

Reheat

13

Effect of rate of Cooling

• The rate of cooling ,slow or fast decides the transformation processes. These processes may be summarized as follows:-

1. Annealing, when rate of cooling is very slow as 10: C/s or less.

2. Normalizing, when rate of air cooling is slow, as slow as 50: C/s or less.

3. Hardening, when rate of cooling is faster then the critical rate of cooling.

14

HardeningProcess:-1. Steel is heated to a temperature above its critical

range.

2. It is held at this temperature for a considerable time and then allowed to cool by quenching in water or brine solution.

3. On heating above the critical temperature, the basic structure changes into austenite which contain considerable part of cementite.

4. On rapid cooling this austenite changes into martensite that imparts hardness.

15

• OBJECTIVE:-1. To improve the strength of steel.

2. To develop hardness in the metal to resist wear, abrasion and to enable it to cut other metals.

FACTORS AFFECTING THE HARDNESS:-(i) Carbon content

(ii) Quenching rate.

(iii) Quenching medium

(iv) Work size

16

Effect of quenching Rate:-• The hardness in steel depends on the quenching

rate, a very rapid quenching is necessary to harden low and medium plain carbon steels.

• Quenching in water bath is commonly employed• For high carbon steel and alloy steels, oil is

generally used as quenching medium.• The structure obtained with different rates of

cooling differ in appearance and properties like tensile strength, hardness, yield point etc.

• Faster the cooling rate greater is the hardness of steel.

17

Effect Of Quenching mediaQuenchingMedium

Type of Microco-nstituent

Elongation (%)

HardnessNumber(HRC)

UltimateTensile Strength(mpa)

Water martensite 1-3 60-70 1700

Oil Pearlite (very fine)

4-6 30-40 1100

Air Pearlite(fine)

7-9 20-30 800

Cooling within Furnace

Pearlite(coarse)

10-12 10-12 500

18

Tempering

• Process :-• Tempering is the essential process performed after

hardening.• This process involves reheating of the hardened

steel to a certain temperature below lower critical temperature followed by a slow cooling rate.

• Reheating permits partial transformation of martensite, and relieving of the internal stresses.

• With the increasing tempering temperature, breakdown of martensite occurs at faster rate.

19

• Purpose Of Tempering:- (i) It reduces brittleness of hardened steel.

(ii) It increase ductility.

(iii) It relives internal stresses.

(iv) It improves toughness of steel.

There are some special tempering process:-

1. Austempering:-

20

2. Mar tempering:-

- It minimizes crack

-Remove distortion

and internal stresses

-Toughness increases.

21

AnnealingPROCESS:-1. Heating of steel above the critical temperatures,

and holding it there about 1hr.

2. It is then allow to cool slowly in the furnace at the rate of 30 to 50: C/hr.

3. On cooling steel changes to ferrite and pearlite in case of hypoeutectoid steels, and into pearlite and cementite for hypereutectoid steels.

22

• Purpose :-1. It softens the metal.

2. It improves ductility of steel.

3. It enhances machinablity

4. It refines the grain structure.

5. Internal stresses, if any, are also relived.

Types of Annealing process:-

(i) Process annealing (ii) Full annealing

(iii) Spheroidise annealing (iv) Diffusion annealing

(v) Recrystallization annealing

23

Normalizing

PROCESS:-•It is a process of heating steel 40: to 50: C above the upper critical temperature.•Then holding it for certain duration and then allow it to cool in the surrounding air.

24

PURPOSE:-1. To refine the grain structure completely.

2. To improve the machinablity.

3. To eliminate any leftover internal stresses.

4. To increase the strength of medium carbon steel.

5. To maintain granular homogeneity.

6. To enhance the toughness

25

26

Chemical heat treatment process

1. Carburizing or case hardening:-• This is a method of producing steel having

tough inner core and hard outer surface.• Components such as gears, pins ,piston

and camshafts are hardened by this process.

2. Cyaniding:- • It is the special case hardening process in

which the mild steel absorbs carbon and nitrogen to obtain hard surface.

27

• Component is immersed in a bath of sodium cyanide(NaCN) at about 400: C.

• It increases the Fatigue limit of steel.

3. Nitriding:-• The process involves heating of steel to about 650: C

and holding it in the atmosphere of ammonia(NH3) • The nitrogen from ammonia penetrates into the

surface of steel and forms very hard nitrides on the surface.

Objective:-

1. It improves corrosion resistance.

28

2. It provides very high hardness.

3. Components need not to be machined after the process.

4. Any other heat treatment is not required to be done.

5. Scale formation does not occur.

29

REFRENCES

1. Material science and engineering (Dr. K.M GUPTA)

2. Production technology (RK Jain)

3. Callister’s Material science engineering

30

Thank You