Structural stability and hardness of carburized surfaces of 316 stainless steel after welding and after neutron irradiation

Surface hardening treatments offer promise of mitigating the threat of liquid cavitation pitting erosion at the interior surfaces of the austenitic 316 stainless steel vessel that will hold the liquid mercury target of the Spallation Neutron Source. One treatment is a commercial carburization process in which carbon is impregnated at low temperature at concentrations up to 6 wt% in supersaturated solid solution to depths of about 33 μm. The surface hardness of 316L steel is raised from 150 to 200HV_0.05 (micro-Vickers hardness number at a 50 g load) to 1000-1200HV_0.05. It is shown that during subsequent electron beam welding the supersaturated carburized layer in the heat affected zone decomposes to a tiered microstructure of carbide phases in austenite. The hardness of this complex decomposition microstructure is in the range 530-1200HV_0.05, depending on the exposure temperature, the local carbon level, and the size of the carbide particles. To test whether the carburized solid solution layer would break down under atomic displacements from proton and neutron irradiation in service, specimens of annealed and 20% cold-rolled 316LN steel were neutron irradiated to 1 dpa at 60-100 °C. No softening of the layer was detected. Rather, the hardness of the layers was increased by 2-12%, compared to increases of 81% and 43% for the annealed and 20% cold rolled substrate materials, respectively. Optical microscopy examinations of the surfaces of the as-carburized-and-irradiated specimens revealed no sign of decomposition attributable to irradiation.