Microstructure and corrosion resistance property of laser transformation hardening pre-hardened AISI P20 plastic die steel

[Display omitted] •The effects of laser transformation hardening (LTH) on AISI P20 steel was explored.•Uniform and compact microstructure without cracks could be obtained by LTH.•LTH induced higher microhardness and compressive residual stress in hardened layer.•The balance of rich and poor Cr regions could be improved effectively by LTH. The current surface properties of the pre-hardened AISI P20 plastic die steel prevent from further increasing the service lifetime of plastic molds. Surface modification experiments under different parameters were carried out on the pre-hardened AISI P20 steel through the laser transformation hardening process. The microstructure evolution, mechanical properties, corrosion resistance properties and the corrosion mechanism were discussed and analyzed systematically. The results indicated that the surface roughness was reduced and there existed insignificant damage to the raw materials. Based on a comprehensive analysis of the microstructure and potentiodynamic anodic polarization curves, more high-carbon martensite and less carbide particles were the positive factors for this surface modification process. The corrosion potential of all the laser transformation hardened samples shifted positively and their hardness exhibited a gradually decreasing distribution from the top surface to substrate with a highest value at the surface of the laser transformation hardened layer. Especially, the sample with parameters of 1.8 kW 3 mm/s had the highest microhardness and the best corrosion resistance property (in 3.5 wt% NaCl solution).