Austenite ( nuclear ) =========== Eutectiod temp ( Ac1 )

● Grow in to primary austenite grains.

● Primary austenite grains are present in the structure original grain size / actual grain size.

● Original grain size depends on or effect by two reason
      1. Temperature of heating
      2. Holding time at this temperature.

● Primary austenite grains are two types
      1. Inherently coarse grain size
      2. Inherently fine grain size.

● Inherently coarse grain steel grow sharply with increasing the temperature but inherently fine grain steel as former at very slow from Ac1 temperature to 1080℃ and the growth is restricted with the rising the temperature.

austenite grain size index
formula of grain size

n = averages no. of grain per square inch
N = grain size index
ASTM = American society of testing of material

Important of grain size :

hall patch equation
hall patch equation

● Finer the grain size with higher yield stress
● Impact transition temperature.
● An increase in impact transition temperature makes the steel more propose to brittle fracture.
● Higher the impact transition temperature, higher the finer grain size.
● Fine grain size stress has a better creep strength at above equi-cohessive temperature.
● Silicon steel is the fine grain steel than aluminum.
● Fine grain steel have a higher fatigue strength as compare to coarse grain size.
● Coarse grain steel has a better hardenability then the finer grain size.
● Fine grain contain more grain boundary than coarse grain boundary.
● Rate of Diffusion is very high in coarse grain steel.
● Coarse grain steel have a better mechaniability then fine grains


There product are obtained by the decomposition of austenite I.e.
Martensite  ( diffusion less product )
Bainite  ( Diffusion based product )
Pearlite ( Diffusion based product )

T T T  DIAGRAM (Isothermal Cooling)

Time Temperature Transformation

In Cubation Period :  The time during which transformation don't proceed is known as in Cubation Period. This incubation period also gives qualitive idea about the relative stability of super cooled austenite.

If any steel transformation below the critical temperature and makes no transformation us known as super cooling of austenite. During this super cooling the austenite become more stable smaller incubation period corresponds to lesser stability of austenite.

Limitations :

1. If only ten related the amount of transform austenite with different transformation time for a constant temperature. Which shows " when a sample is heated at T4 temp and resulted with a minimum incubation period. This temperature known as Critical transformation temperature.

TTT diagram
TTT Diagram

2. It can be understand that the higher the transformation temperature the more is the incubation period and more time will be required for completion of this transformation.

3. In Cubation Period and transformation time decreased with lowering of transformation temperature. However after a particular temperature the decrease is reversed and both incubation period and transformation time increase again with further lowering the transformation temperature.

Important of TTT DIAGRAM :

1. It indicate the phase existing in Steel at various temperature and various time.

2. It gives a useful information in heat treatment of steel.

3. One can choose proper heating and cooling cycle to obtained different transformation product as well as about required property in this component.

4. Where as iron-carbon equilibrium dia given a limited usduin the study of steel cooled under non equilibrium condition. But this dia gives a principle source of information in the actual process if austenite decomposition under non equilibrium condition.


TTT curve is graphically summary of Isothermal transformation of steel and different data are obtained as

●  A large no of small specimen at equilibrium temperature and are cooled below a critical temperature in liquid both maintaned at that temperature.

● Specimen are then withdrawn at regular interval of time and quenched in ice bring to prevent further transformation.
TTT curve
TTT Curve

●  Microstructure are examine to determine the beginning of the transformation and the end of the transformation at given temperature.

●  A series of test at several temperature are conducted and then allowed to construct a diagram whose shape is procurved as S-shape or S-curve, S-curve and more popular known as TTT curve.
TTT dia for hypo-eutectoid steel
TTT dia for hypo eutectoid steel

TTT dia for hyper-eutectoid steel
TTT dia for hyper eutectoid steel

●  Transformation of austenite Steel just below the critical temperature is pearlite which is obtained at very slow rate on further as transformation time decreased and rate increase rapidly to about 540℃.

●  Here transformation of austenite to pearlite is a nucleation and grain growth process which is dependent on the Diffusion and therefore fine pearlite is obtained. Nucleation rate increase and growth rate decrease with decreasing in temperature.

●  High temperature nucleation is high fall of temperature grain growth is high.

●  Transformation temperature also influence both reaction rate and micro structure pearlite. The pearlite structure become progressivly unit transformation temperature reaches about 300-600℃, below this nose region are austenite transformation to bainite.

References :

1. Heat treatment principle and techniques  by : T.V. Rajan,  C.P. Sharks,  Ashok Sharma.
2. Physical Metallurgy Principle and Practice by : Raghavan V.,
Lectures of IITs & BPUT (ODISHA).

Author :
Subir Kumar Sahu.

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