Development of an Aluminium-Reduced Niobium-Microalloyed Case Hardening Steel for Heavy Gear Manufacturing

Abstract

Abstract Fatigue strength is a key mechanical property for heavy transmission components, which are subjected to dynamic loading conditions. Oxidic steel cleanliness is known to greatly influence the fatigue strength of steel. The underlying research project work aimed to improve the oxidic cleanliness of the case hardening steel grade 18CrNiMo7-6 (DIN EN ISO 10084), whilst maintaining a fine austenite grain size during high temperature carburising. The hypothesis thereby is to lower the aluminium content of steel in order to reduce the probability of formation of aluminium oxide inclusions. In the absence of aluminium nitride precipitates, the prevention of undue austenite grain coarsening has been ensured in this work by alloying the steel with the appropriate amount of niobium. The optimum content of niobium was selected based on thermodynamic calculations using Thermo-Calc®. The results indicate the basic feasibility of obtaining fine grain stability on an industrial scale for application to high temperature carburising at 995°C for 14.4 hours.