Evolution of surface to interior microstructure of SCM435 steel after ultra-high-temperature and ultra-high-pressure cavitation processing

The microstructure and hardness on and just beneath the surface of Cr-Mo steel (SCM435) processed by multifunction cavitation (MFC), a technique that combines conventional water jet cavitation (WJC) processing technology and ultrasonic cavitation, were investigated using scanning electron microscopy and a micro Vickers hardness meter. The dependence of the microstructure and hardness on the MFC processing time was also investigated, the results were compared to those for WJC. The corrosion resistance of a specimen surface processed by MFC was improved due to selective oxidation within the bulk interior of the specimen. The specimen surface treated by WJC or MFC was improved from tensile residual stress to compressive residual stress. Compressive residual stress applied by WJC processing and MFC processing affected the surface hardness and microstructure. The surface was hardened from the topmost surface to the depth of ca. 1 mm by MFC processing. Regarding the microstructure of WJC processed specimen, protrusions and the disappearance of cementite in pearlite grains was observed at the specimen surface, and voids and cracks were present at depths of 0.5-1 mm from the specimen surface. No voids or cracks were observed at depths of 2-3 mm from the specimen surface. Microstructural observation indicated that cementite becomes spherical within pearlite grains at the surface of the MFC processed specimen, and the spheroidization of cementite was observed at depths of 0.5-1 mm beneath the surface. No voids or cracks were evident in the specimen interior.