Corrosion behavior of laser hardened 50CrMo4 (AISI 4150) steel: A depth-wise analysis

Surface laser treatments are often used on metals and metal alloys to improve the material's hardness and corrosion resistance. Investigating the reasons for this property enhancement is not trivial because the resulting microstructure changes gradually from the surface to the bulk. Here we report a simple serial-sectioning technique to assess the hardness and corrosion behavior of a laser hardened chromoly steel at depth intervals of 30 mu m. The method consists of indenting the sample surface to a controlled depth and then mechanically polishing the sample until the indents are no longer visible. By performing corrosion tests and recording the surface hardness at each depth interval, we assess the corrosion behavior and hardness of the material throughout the laser-induced microstructure gradient. Our measurements show that the improved corrosion resistance stems from the chromium-enriched oxide layer at the top surface of the steel alloy. Surprisingly, the phase content and grain size, whose distribution varies significantly throughout the sample thickness, has little to no effect on the corrosion behavior of the steel.