Austensite to Martensite Transformation

[d01mt14usman]

Dear Fellows

Can anyone answers these question.

Why Austensite to Martensite Transformation is incomplete?

Bainite is the most toughest microstructure whereas martensite is the hardest structure, but if we develope a soft microstructure in martensite it toughness increases, explain this phenomena?

Please also give some website links and microstructure to justify your answer.

Thanks

 

[Astronuc]

This seems like a homework problem.

One should attempt an explanation for each question.

What is the process for Austensite to Martensite transformation? What physically happens?

With regard to the second question - as hardness or strength increases, toughness usually decreases. What is the significance of toughness, and what is relationship with hardness/softness?

 

[tospok]

Reoly

Austensite to Martensite transformation is time independent, because its a diffusionless transformation. It only depends on temperature. If the temperature of quenching medium is not low enough, i.e. if its not lower than martensite finish temperature, complete transition will not occur.

Toughness is not directly related to strength (somewhat), for instance ceramics are very strong but brittle. Same is with martensite. If you don't have enough dislocation motion in a metal, or twinning, than it ends up being brittle. Martensite is strong but needs toughness to be useable (martensite can explode from internal stresses!) upon annealing we form some pearlite, thus having a composite like structure, where pearlite gives ductility and toughness, martensite gives strength.

In Austensite to Martensite transformation the carbon atoms in FCC lattice of iron (there are many other martensites in other materials but steel is most important) move to a different position in the lattice and distort it generating high stresses. Diffusion does not take place as the distances are like 1-2 atomic radius

Toughness is very significant as it is the measure of how much energy the material can absorb before failure. Strength denotes how much stress can the material bear before breaking or yielding. A material can be strong and bear tons of load, but it will break at a impact if its brittle.