Microstructure modifications and associated hardness and corrosion improvements in the AISI 420 martensitic stainless steel treated by high current pulsed electron beam (HCPEB)

The surface of the AISI 420 martensitic stainless steel was subjected to High Current Pulsed Electron Beam (HCPEB) treatment. The microstructure in the melted layer consisted of a three phase mixture: (i) fine δ-Fe grains formed via epitaxial growth from the substrate, (ii) larger γ-grains nucleated from the top surface of the melt and (iii) some needles-like variants issued from the solid state martenitic transformation. Despite this complex multi-phase microstructure, the corrosion performance, tested in a sulfuric acid solution, was significantly enhanced by the HCPEB treatment. The increase in corrosion potential and delayed pitting are essentially attributed to an increase in Cr content, rising from 13.3 wt. % in the bulk to about 14 wt. % at the surface, together with a very limited amount of surface craters. This low density of craters did not give rise to significant deep hardening in the sub-surface but the top surface melted layer hardness was increased by more than 50% because of the triggering of the martensitic transformation.