Improved cavitation resistance of structural materials in pulsed liquid metal targets by surface hardening

Cavitation damage of structural materials due to pressure waves is expected to be one of the major life-time limiting factors in high power liquid metal spallation targets under pulsed operation. Two methods are developed for the European Spallation Source (ESS) to mitigate this damage: Introduction of gas bubbles to suppress the pressure pulse, and surface hardening of structural materials to reduce their vulnerability. Surface hardening of four 8-13% Cr martensitic steels was performed by thermal treatment with pulsed or scanned electron- and laser beams as well as by nitriding in a r.f. plasma. Parametric dependences of these processes were investigated and partially optimised. Thermal treatment with electrons reached HV-hardness values up to about 600, laser treatment up to 710, and nitriding above 1200. After mechanical and metallographical characterisation, specimens of 12% Cr steel were tested in liquid mercury under pulsed proton irradiation, and under mechanical pulse-loading. Surface damage was analysed by optical, confocal laser, and scanning-electron microscopy, showing in all tests much better resistance of the hardened material compared to standard condition. Application of these techniques to a spallation target is outlined.