Influence of laser surface hardening on the corrosion resistance of martensitic stainless steel

Martensitic high nitrogen stainless steels offer a combination of wear-, corrosion- and fatigue properties. But for some applications a higher surface hardness is required. A laser hardening with rapid heating (without smelting) and cooling (quenching) rates can improve the surface hardness with compressive residual stresses in the near surface layer. Yet, some cases of pitting corrosion in chloride media are reported. In this study, the influence of process parameters, composition of the atmosphere and the overlapping ratio, has been investigated. With complementary surface analytical methods, and electrochemical techniques the relation between surface structure and composition and corrosion behavior in chloride media has been studied. It has been shown that, during the laser treatment the surface must be shielded with argon in order to avoid the formation of a porous layer of iron oxides, which is dramatically detrimental to the corrosion resistance. After the laser treatment a mixture of martensite and retained austenite is obtained, depending on the surface temperature and overlapping ratio. With a surface temperature of 1200 degrees C and a minimal overlapping ratio(10%), a thin surface layer of retained austenite, wherein nitrides are dissolved, improves the corrosion resistance. The hardness increases with the amount of distorted martensite and reaches a maximum at 1000 degrees C. (c) 2007 Elsevier Ltd. All rights reserved.