I am working with H13 tool steel. In the microstructure I found some hard and softer parts. The hard parts as far as I can identified has high percentage of retained austenite, whereas the softer parts mainly consists of tempered martensite. Can anyone give any reference or logic as why the microhardness is high at the retained austenite parts? Thank you.
If your identification of phases is correct, then following are the possibilities of retained austenite (RA) having higher hardness than the tempered martensite. These are only speculations:
a) The RA phase has a very high dislocation density due to residual stresses (developed by volume change of the surrounding martensitic phase), b) the RA transforms to martensite upon loading during hardness measurement (TRIP mechanism), c) the RA phase is saturated with alloying elements and highly solid solution strengthened, d) fine carbides such as M7C3, Mo2C, M23C6 etc preferentially precipitate along the austenite boundaries, and e) the lath size of RA is finer than that of martensite.
You have to carry out a detailed microstructural analysis before and after hardness testing using SEM, TEM, and XRD, if possible. You have an interesting result to publish!
Dear Krishnan Raja . Thank you for your answer. I actually got some fine carbide particles precipitated on the RA. But I am not sure how can I correlate the carbide precipitation with hardness? Are you suggesting precipitation hardening of the RA phases?
Thanks again for your comment. However, I will go for publication with some good results in future.
if Fine Carbides are present in RA, certainly it can cause increase in Hardness.
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