Microstructural analyses of grain boundary carbides of tempered martensite in medium-carbon steel by atomic force microscopy

The microstructures of JIS-SCM440 steel (containing 0.4%C, 1%Cr and 0.2%Mo; mass%) that had been tempered at 723 K were observed by atomic force microscopy (AFM). In an AFM image of an electropolished surface, carbides and blocks were clearly distinguished, since the rate of electropolishing depends on the crystal phase and crystal orientation on the surface. However, the prior austenite (γ) grain boundaries could not be clearly recognized. Therefore, an AFM image of the surface that had been etched with picric acid was taken at the same location on the surface. Since picric acid selectively etches prior γ grain boundaries, the boundaries were clearly visible in the image. The two AFM images were then superimposed to reveal prior γ grain boundaries, carbides, and blocks. Finally, the number and sizes of the carbides on the prior γ grain boundaries were examined.