Surface Modification of Cr-Mo Steel by a New Water Jet Cavitation Technology

Low-alloy steels with improved corrosion resistance are obtained by alloying carbon steel with elements in amounts of several mass% or less or 1 mass% or less in various combinations. They cost slightly more than carbon steel, but have significantly improved resistance to specific types of corrosion. At present, the improvements obtained in fatigue characteristics and corrosion resistance by conventional surface modification techniques are not always satisfactory, because the operating environment of this material becomes increasingly harsh with time. Surface modification technologies such as water jet cavitation (WJC) have been applied to improve the stress corrosion cracking and fatigue strength of alloy steels. The tensile residual stress applied to the weld zone and the surface grinding face is improved to a compressive residual stress by the WJC technique, and the fatigue strength and stress corrosion cracking resistance are excellent. However, it is difficult to improve corrosion resistance. The authors have developed multifunction cavitation (MFC), a novel technique combining ultrasonication (UC) and WJC. It can utilize the same high-temperature and high-pressure microjet featured in both WJC and UC. In this study, the microstructure, hardness, and corrosion resistance on and just beneath the surface of Cr-Mo steel processed by MFC 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, and the results were compared to those for WJC. The corrosion resistance of the specimen surface processed by MFC was improved owing to selective oxidation within the interior of the specimen. The residual stress at specimen surfaces treated by WJC or MFC was improved, changing from tensile to compressive. The compressive residual stress induced by WJC and MFC affected the surface hardness and microstructure. MFC processing caused the surface to harden, down to a depth of ca. 1 mm. As for the microstructure of the WJC-processed specimen, surface protrusions were observed and the cementite in the pearlite grains on the surface disappeared. In addition, voids and cracks were found at depths of 0.5–1 mm from the surface. No voids or cracks were observed at depths of 2–3 mm from the surface. Microstructural observation indicated that the cementite within the pearlite grains at the surface of the MFC-processed specimen became spherical and that this 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.