Metallurgical Engineering

 

RETAINED AUSTENITE

 

·     As we know, for ferrous alloys the austenite to martensite transformation is never practically completed and a certain amount of austenite always remains even after the completion of the process. It is the untransformed austenite. It is usually more found in Hardened and High carbon steels.

·     The amount of retained austenite depends largely on the M_s and M_f temperature which are the martensitic start and martensitic finish temperature respectively.

Both M_s and M_f temperatures depend upon the carbon content

 and both are inversely proportional to the amount of carbon.

As the carbon content increases the M_s and M_f temperature 

lowers down and the amount of retained austenite increases. It

 is shown in the figure below.

                           Figure: Effect of carbon on retained austenite.

·     Except for cobalt and Aluminium, all other alloying elements lower the M_s and M_f temperatures and so the alloy steels are found to have a greater amount of retained austenite. The formula for the determination of Ms temperature is: 

Ms= 539 - 423(%C) - 30.4(%Mn) -12.1(%Cr) - 17.7(%Ni) - 7.5(%Mo)...

Where, C=carbon,

Mn=Manganese,

Cr=Chromium ,

Ni=Nickel,

Mo=Molybdenum

·     The following two figures depicts the above discussion.

Figure: Effect of alloying elements on M_s temperature.

Figure: Effect of carbon content on M_s and M_f temperature.

·     High carbon and hardened steel find their use in cutting tools and wear-resistant components due to their high alloy and carbon content which leads to lowering of M_s and M_f temperatures and an increase in the austenite amount. In addition to this, the stresses developed during quenching also reduces the M_s and M_f temperatures.

·     The retained austenite differs from original austenite as it tends to have a higher density of imperfections like stacking faults, dislocations, etc.

·     The soft and ductile retained austenite when present in a greater amount affects the hardness and strength of steel and so the very purpose of performing the heat treatment process is lost.

 

While some advantages of the retained austenite are:

1) It reduces the danger to distortion and cracks.

2)Make straightening of components possible after hardening.

3)It balances the transformational volume changes during heating as well as cooling cycles.

 

Majorly the presence of retained austenite is disadvantageous, it is because:

1)It decreases the hardness of steel.

2)Austenite is non-magnetic so it affects the magnetic properties of the steels.

3)Austenite may convert to lower bainite during service which causes volume changes and can be hazardous and may cause failure of Component.

 

Prepared By,

Urvish Gadkari,

Metallurgical and Materials Engineering,

Faculty of Technology and Engineering, MSU, Baroda.