Relationship between nanohardness and microstructures in high-purity Fe-C as-quenched and quench-tempered martensite

The relationship between the nanohardness and the microstructures in the Fe-C martensite was studied to understand the contributions of the matrix and the grain boundary to the macroscopic strength. As-quenched martensite was examined for five kinds of Fe-C alloys with various carbon contents in the range of 0.1-0.8 mass%, while quench-tempered martensite was investigated for an Fe-0.4% C alloy. The ratio of the nanohardness to the macrohardness H_n/H_v was much smaller for the Fe-C martensite than those for the single crystals, indicating that there is a significant grain-boundary effect for the martensite. The ratio H_n/H_v of the as-quenched martensite decreased with an increase in the carbon content since the size of the block structure decreased with increasing carbon content. For the quench-tempered specimens, a significant reduction of the grain-boundary effect occured at the tempering temperature of 723 K. It is mainly due to the depression of the locking parameter caused by the disappearance of the film-like carbides on the boundaries.