Effect of inhornogeneity of carbide precipitation on nanohardness distribution for martensitic steels

Nanoindentation technique was applied to evaluate nanohardness distribution in a submicron scale for two kinds of martensitic steels: Fe-0.4C binary steel and Fe-0.05C-0.22Ti steel with a stoichiometric composition of TiC. AFM images showed that Fe-C steel includes relatively coarse cementite particles with about 100 similar to 200 nm in diameter and a couple of hundreds nanometer in average spacing, while high-resolution TEM observation showed that the Fe-C-Ti steel has fine TiC precipitates with 5 nm in diameter and 15 nm for average spacing. Nanoindentation results revealed that the standard deviation was much higher for the Fe-C than that for the Fe-C-Ti. Since the typical indent size was a couple of hundreds nanometer, which was about two orders larger than the size of the TiC and comparable to the cementite size, the small distribution of nanohardness of the Fe-C-Ti was attributed to the homogeneous microstructure in sub-micron scale, while the inhomogeneity of cementite particles in the Fe-C steel leaded to large nanohardness.