Microstructural observation of tempered martensite in a medium-carbon low-alloy steel by atomic force microscopy

Microstructures of JIS-SCM440 steel tempered at 723 K were observed by atomic force microscopy (AFM). Two kinds of specimen surface were prepared by electrically polishing and by chemically etching. The quality of AFM images was much better on the electropolished surface because the high vertical resolution of AFM could be fully utilized. Since the electropolishing rate depends on the crystal phase and the surface orientation, prior austenite grains, packets, blocks and small precipitates were clearly distinguished on the electropolished surface, and the average block width was determined to be 0.6 μm. The density of the heterogeneously precipitated carbides along prior austenite grain boundaries was not uniform, suggesting that the nucleation of the carbides depends on the grain boundary characteristics. The apparent size of the carbides observed by AFM was larger than the actual size observed by TEM due to the effect of the tip curvature.