AUSTENITE PHASE TRANSFORMATION

IRON - CEMENTITE PHASE


iron carbon phase diagram
iron carbon diagram

1. Desirable of mechanical property by a alternative the size and shape and distribution of various constituent of or micro constituent. This is achieve in the practice of heat treatment.

2. All transfrom product are obtained from austenite regions by austenite. [ Heating to any steel at a temperature to austenite region on to get a austenite structure.]

3. Austenite can be transformed by cooling system of pearlite, bainite, martensite these are obtained by the rate of cooling.

Austenite Phase - Austenisation :

This process is produce on heating to any grade of steel and to get a final product or structure in a following manner.

1.  Hypoeutectoid steel is the combination of ferrite and pearlite.
2.  Eutectiod steel is the combination of ferrite and Cementite or carbide Fe3C.
3.  Hypereutectoid steel is the is the combination of Cementite and pearlite.
For the heat treatment we know the
@ transformation of austenite
@ transformation of pearlite
@ transformation of martensite
@ transformation of bainite.



A.  On heating to eutectiod temperature here BCC iron alpha iron.

B.  Austenite is FCC structure gama iron.

C.  On heating to a eutectiod temperature BCC iron alpha-Fe lattice changes to FCC iron (gama-Fe) lattice. The maximum solubility of carbon in FCC iron is about 2%.  Therefore at this temp region around the Cementite area will be enriched with carbon because of Diffusion.

austenite microstructure
austenite formaion
D.  The microstructure of pearlite which consists of alternative layer of ferrite and Cementite.

E.  Size of austenite nucles grows further because of dissolving of ferrite.

F. It continues further grow and it was marks as a spot or dissolved.

G.  Finally it make in to austenite.

Mechanism of formation of austenite :

The formation of austenite on heating always occurs at temperature higher than specific temperature is iron carbon equilibrium diagram. Generally heating of steel to austenitic temp is fast and for most steps for all heat treatment process.

The grains size of austenite at different heat treatment temperature largely control the final mechanical property after the heat treatment. Therefore the study of mechanisim of austenite is greater important.
● Heat the number of steel sample to different temperature above eutectiod temperature ( Ac1 temp. Or 723℃ ).

FORMATION OF AUSTENITE PHASE

● Heat is done by dipping those sample in a constant temperature both furnace.

● Then the sample are taken out by one after a definite interval of time followed by immediately quenching, which will result the transformation of martensite,  form this transformation of austenite.

austenite formation accoding to time
austenite formation accoding to time
Transformation of pearlite to austenite as a function of time. The amount of martensite formed will depends on amount of transformed austenite, which enter also depends on temperature on which steel ends holding on this temperature.

It is concluded form of this fig lower is the transformation temperature, more is the time required to complete transformation (pearlite to austenite) change in properly like hardness, magnetic and internal stresses usally accross due to transformation of austenite and formation of martensite by quick cooling.
begin of austenite transformation
begin of austenite transformation 


 Disadvantages:

●This fig. doesn't so the effect of super heating
start of austenite transformation
starts of austentine transformation

  • on the Mechanism of transformation in fact under partial condition of heating. ●Transformation occurs at temperature above the eutectiod temp not at eutectiod temp.

● Effect of temperature on the time required for start and composition of transformation of pearlite to austenite.


CONCLUSION OF AUSTENITE PHASE TRANSFORMATION

● Transformation is completed in a short period at high transformation temperature.

● For higher heating rate transformation will start at higher temperature where as for slower heating rate, transformation will started slowly and at lower temperature.

● For any given partial rate of heating the formation of austenite will occurs over a range of temperature and not at a constant temperature.

● For the formation of austenite at constant temperature the heating rate should be slow and two curve line will converse to a single point located at eutectiod temperature line.

The process of austenite formation on heating proceed by nucleation and growth reaction. Therefor the reason which can be either change the rate of nucleation or the rate of grain growth or both will change the Mechanism of austenite.

● It is also mentioned austenite nucles are form at the interface of ferrite and Cementite forms as soon as eutectiod temperature(Ac1) is reacted

Phenomenon : The no of possible will increase with increasing the interfacial area both ferrite and Cementite. This increase of interfacial area is generally obtained by two ways

1. By increasing the Cementite constant.
2. By increasing the inter lammelar space.

Why high carbon steel more no of nuclei than the low carbon steel?

Ans-- Increase in Cementite content can be met by increasing carbon contain and very closer a ferrite-cementite lammela. Therefore higher will be rate of nucleation and carbon atom have to defuse for a smaller distance in order to enreach non carbon reason. Therefore rate of growth of primary austenite grain will be higher. Pearlite structure with less inter lamelar a spacing is transformation faster to austenite.






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.
Metallurgist.

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