Fatigue and fracture behaviour of a laser surface heat treated martensitic high-nitrogen tool steel

High nitrogen tool steels offer a superior corrosion resistance compared to conventional nitrogen-free tool steels at similar mechanical and tribological properties. For this reason they are nowadays applied for ball bearings in the aircraft industry. A laser-surface treatment can lead to a further improvement of this combination of properties. The aim of this research project is to improve the tribological properties and at the same time retain or enhance the corrosion resistance as well as the fatigue behaviour. The laser heat-treatment leads to a mixture of martensite and retained austenite. Thus, the martensitic regions show an increased hardness of about 700 HV10, while the austenitic regions are soft around 400 HV10. In order to determine the influence of a laser-surface treatment 4-point-bend fatigue tests in laboratory air as well as in artificial sea water (3%NaCl, 25°C) have been performed. The fatigue and fracture behaviour are mostly influenced by the refinement of the microstructure and the generation of compressive residual stresses. In addition the retained austenite and its stress induced transformation into martensite affects the fatigue behaviour. The residual stresses generated during heat treatment have a significant influence on crack initiation, while those generated during the transformation of the retained austenite have only a minor influence on crack propagation.