Wear mechanisms of laser-hardened martensitic high-nitrogen-steels under sliding wear

High-nitrogen tool steels (Fe, 15% Cr, 1% Mo, 0.3% C, 0.3% N) are applied, e.g. in bearings and gears in aeronautics and space technology. Their advantage compared to conventional, nitrogen-free tool steels is a superior corrosion resistance, which can be attributed to Cr, Mo, and N dissolved within the solid solution. In order to gain a sufficient toughness for application, these steels are tempered above 600°C bringing about precipitated carbides and nitrides, which bind Cr and N and, therefore, deteriorate the chemical properties. Within a DFG (German Research Council)-funded research project the authors show, that by means of laser hardening it is possible to dissolve a part of these precipitates — mainly nitrides resulting in improved properties under fatigue, wear and corrosion. This is brought about by a newly generated martensite with compressive residual stresses (fatigue, sliding wear), dissolution of Cr and N (corrosion) and a higher mechanical stability of the surfaces (sliding wear). This contribution focuses on the acting wear mechanisms under dry sliding wear. The investigations are carried out with pin-on-disk tests, with the disk as the actual specimen and a pin made of conventionally hardened 52100 bearing steel (100Cr6). It can be shown, that the wear properties of the high-nitrogen-steel are better than those of comparable conventional tool steels and that a laser treatment leads to a further improvement. Due to the fact that there is a tempered zone between overlapping laser-hardened areas, there is a change of acting mechanisms and, thus a distinct difference in wear rates. For the conventional corrosion resistant martensitic tool steel the difference between the tempered and the hardened zone is not as marked. Neither the wear mechanisms nor the wear rates differ distinctly. These effects and their influence on the wear behaviour is correlated with the microstructure of both steels before and after laser-hardening.