SEMINAR ON

HEAT TREATMENT PROCESS(TTT,CCT & CCR)

NAME- KISHOR T.

GHEGAD

ROLL NO-2663

CLASS-S.E.(MECH.)

WEL-COME

INTRODUCTION

The heat treatment is a very broad term and

includes any heating and cooling operations or

any sequence of heat of two or more such

operation applied to any material in order to

modify its internal structure or to alter its physical,

mechanical or chemical properties. Heat treatment

is carried out by heating the material to some

specific temperature, holding at that temperature

for a define period and cooling to room

temperature or below room temperature with a

definite rate.

HEAT TREATMENT OF STEEL:-

The heat treatment is a very

broad term and includes

any heating and cooling

operation or any sequence

of two or more such

operation.

OBJECTIVE OF HEAT TREATMENT OF STEEL:-

1) To increase hardness, wear and abrasion resistance.

2) To remove internal stresses.

3) To increase machinability.

4) To refine grain structure.

5) To change the composition of steel.

TTT DIAGRAMS

TTT diagram stands for “time-temperature-

transformation” diagram. It is also called isothermal

transformation diagram

Definition: TTT diagrams give the kinetics of

isothermal transformations.

TIME TEMPERATURE TRASFORMATION (TTT) DIAGRAM:-

The iron-iron carbide equilibrium diagram is having

little value in the study of the steels cooled under non

equilibrium conditions. It has been found in the study of

steel that, Austenite is unstable below the lower critical

temperature A1.Below A1 temperature austenite may

transform to pearlite, bainite or martensite, It is

necessary to know that, How these transformation

occurred. To study the transformation of austenite,

eutectoid composition of 0.8% carbon steel bar is cut

into large number of specimens

Fig. TTT diagram for eutectoid steel

CONTINUOUS COOLING TRANSFORMATION (CCT) DIAGRAM:-

A cooling curve is determine experimentally

by placing a thermocouple at a definite

location in steel sample and then measuring

the variation of temperature with time. Since

the coordinates of the TTT and CCT diagram

are same those for a cooling curve. It is

possible to superimpose variation cooling

curve on them.

Fig. Continuous cooling transformation (CCT)

CRITICAL COOLING RATE (CCR)

When cooling curve tangent to the nose of TTT

curve, it is called as critical cooling rate(CCR).

CCR can be defined by two ways ‘The slowest

cooling rate at which unstable austenite can be

transformed into martensite.’

or

‘The faster cooling rate or within minimum time at

which unstable austenite can be transformed into

perlite.’

FACTOR AFFECTING ON COOLING RATE:-

(1) Carbon content.

(2) Austenite temperature.

(1) Carbon content:-

With higher carbon content or alloying element,

decrease critical cooling rate and shift TTT curve towards

right. It rated the transformation of austenite to perlite or

bainite.

(2) Austenite temperature:-

Higher the austenitic temperature, it will coarsen

the structure and make it softer. Higher austenitic

temperature gives lower critical cooling time and increase

CCR.

REFERENCE:-

1) Material technology textbook by S.B.

BARVE

2) http://www.slideshare.net

3)http://www.chegg.com

4)http://en.wikipedia.org/wiki/Isothermal_tran

sformation_diagram

Thank you