Microstructure effect on nanohardness distribution for medium-carbon martensitic steel

Nanoindentation measurement was performed to evaluate the local hardness distribution for quenched and tempered martensitic steel with ferrite-cementite constituent. The ratio of standard deviation to the average nanohardness was 15.4% when nanoin-dentation tests were carried out at the peak load of 1000 μN, while that of the Vickers hardness was only 1.5% at 9.8 N. Electron backscatter diffraction analysis and SEM ob-servation all showed that the large scattering of nanohardness did not depend on the crystallographic orientation of each grain but from the inhomogeneous microstructure in the sub-micron scale such as cementite distribution. Compared with the results on tung-sten single crystals with different surface orientations as well as another martensite proc-essed by modified-ausforming with the same chemical composition but more homogene-ous cementite distribution, the nanohardness showed smaller scattering, supporting the conclusion.