Different degrees of tensile deformation were applied on austenitic stainless steel (AISI304), resulting in various amount of martensite in austenitic matrix. Our purpose was to determine the effect of austenite hardness on the total hardness . Standard microhardness measurement was conducted on samples . Identical hardness was achieved with different loading, but the austenite hardness was close to the total hardness and the mixture law estimated the total hardness in higher value than the real hardness. Our guess is that the thin martensite lathes in austenite matrix which couldn't be observed with the low resolution of optical microscopy, and/or the presence of martensite in subsurface of austenite, bring their contribution as non-relevant factors on the hardness measurement.
Hello Esmaeel,
If the relative volume fraction of strain induced martensite is less than 30%, then the macroscopic strength the material equals to that of the austenite. This strange load partitioning between the phases is discussed in detail in the PhD thesis of Juho Talonen. Link:
http://lib.tkk.fi/Diss/2007/isbn9789512287802/isbn9789512287802.pdf
Please have a look at Fig. 42 in the page 75.
Thanks dear mr manninen.
In my case, variation of martensite is in the range of 3 to 80%. And the stated matter is valid in the whole range.
Hi Dear Esmaeel. I agree with your guess. The strain-induced martensite nucleates generally at the intersection of shear bands thus can be so small and fine dispersed that you will need to use the TEM in order to observation. You can find several papers in this field.
Thanks mr sadeghpour. Did you have the same experience?and have you seen any paper indicating the strange low difference between austenite and total hardness?
Yes, I have worked on strain-induced martensitic transformation in metastable austenitic stainless steels. In fact, this low difference is not strange because you are measuring the hardness of both austenite and martensite phases (total hardness). As I mentioned before, this behavior arises from the mechanism of this transformation, therefore I doubt the reliability of your approach. I suggest the papers published by R.D.K Misra,Talonen, Tomimura, Spencer and ...
All The Best.
thanks. can you please give me the full title or DOI of that paper. i couldn't find it with that authors names.
best regards.
1) http://dx.doi.org/10.1179/174328408X293603
2) http://dx.doi.org/10.1016/j.actamat.2007.07.015 and ...
See also "Characterisation of strain-induced martensite in a metastable austenitic stainless steel by nanoindentation" that is close to your work but have used the Nanoindentation instead of conventional micro-hardness test.
Adding some thoughts to Mr. Sadeghpours last Answer ... when nanoindetaion is used to measure the austenite hardness one should be aware of the grain orientation. If we have a look at a solution annealed structure, different grain orientations may show different hardness without the grain itself beeing deformed.
Maybe EBSD could be a easier to use technique in order to get a first impression of your microstructure than TEM. You probably would not find the small martensite needels within your austenitic grains, but the degree of defomation of the grains yould be easily estimated from.
dear Michael,
measurement applied at least on seven grains, so i think different orientation doesn't affect the result.
the problem is the low difference between deformed austenite and total hardness, and i think even by considering the highest deformation for austenite, it couldn't be that close to the macrohardness.
regards.
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