In literature's the Austenite solidified phase of Iron has a varying hardness from 500 HV to 1000 HV, depending on the quantity of Martensite in the phase. But similarly processed Nickel Austenite phase exhibits a maximum hardness of 350 HV and the Iron Austenite phase similarly formed has hardness of 460 HV. What are the factors responsible for this ?
Dear Roshan
during fast cooling, (solidification of austenite without diffusion) a martensite is formed which has a hexagonal packed crystal grid in which carbon atoms are captured.
Martensite is tough and brittle.
To be usable should be relaxed (tempered).
super alloys based on nickel has austenite at room temperature - and can be hardened with intermetallic phase (precipitation hardening)
see Metals handbook ASM METALS HANDBOOK VOL 9
Hope so this will help you
You can't simply compare the phases of two much different metals and try to compare the hardness values. Even in steels you can get a variability of more than 100 percent.
Moreover in the two phases you describe you have different alloying elements, different cooling rates, different stacking fault energies... Similarly the martensitic microstructures in steel are much harder than the martensitic microstructures in titanium.
I am not fully covering all aspects; just a few of them. Hope that helps.
Tentative interpretation
1- Carbon is more soluble in Ni than in Fe
2- Iron martensite is bcc iron, carbon supersaturated, whereas Ni low and high temperature phase is fcc
3- therefore Ni austenitic phase has more slip planes (fcc vs bcc) and less slip planes blockage by precipitated carbon
4- the highest you can slip planes, the lowest hardness is (plastic deformation is easier)
Sir, I like to disagree with point 3, as both the material is Austenite and thus FCC. In our study, martensite phase presence was confined to the end sections of Iron Austenite, but the hardness measured are from regions containing only Iron Austenite.
As you have said, Inconel is Austenite at lower temperatures also.
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