A liquid mercury target system for MW-class spallation neutron source is installed in Japan Proton Accelerator Research Complex (J-PARC). High power proton beams bombard the mercury at 25Hz, the mercury target vessel suffers the repeated stress due to the pressure waves induced by thermal expansion of the mercury. Cavitation will be generated in the propagation of pressure waves, and pitting damage will be formed on the inner wall of the target vessel by the cavitation bubble collapse. In previous research, it was confirmed that the surface hardening treatment is effective to suppress the pitting damage at some level, although crack and/or separation occurred due to poor interface strength. A novel surface modification technique was developed based on the previous research : i.e. double stratified surface modification by plasma nitriding and carburizing. The bending fatigue tests on modified stainless steels with pitting damage were performed. As results, damage suppression and fatigue limit improvement due to the modification were confirmed.
