Observations of prior austenite grain boundaries and carbides in the same area of tempered martensite in medium-carbon steel by atomic force microscopy

Microstructures of JIS-SCM440 steel tempered at 723 K were observed on an electropolished and a picric-acid etched surfaces by atomic force microscopy (AFM). At first, AFM image was taken on an electropolished surface. Carbides and blocks were clearly distinguished on the image, because the electropolishing rate depends on the crystal phase and its surface orientation. However, prior austenite grain boundaries could not be completely recognized. Secondly, AFM image was taken in the same area on a picric-acid etched surface. Prior austenite grain boundaries were observed, because the boundaries are preferentially etched. Thirdly, the both of AFM images were overlapped. This procedure gave us AFM image where prior austenite grain boundaries as well as carbides and blocks were clearly recognized. Finally, number and sizes of the carbides along prior austenite grain boundaries were examined.