Influence of the Inherited Structure Induced by Al and Si Alloying on Microstructure Evolution and Mechanical Properties of 100Cr6 Steels

The fatigue lifetime of high-strength 100Cr6 steels can be improved by an increased content of retained austenite that can be induced by Al and Si alloying. The related deformation-induced retention of the austenite-martensite transformation during cyclic loading increases their local strain hardening capacity. However, for those 100Cr6 steels containing retained austenite, sufficient dimensional stability must be ensured. In this study, two standard 100Cr6 steels alloyed either with 1.5 wt% Al or 1.5 wt% Si (to diminish carbide formation and accordingly promote austenite retention) are laboratory melted and processed to adjust a microstructure of bainite, retained austenite, and carbides. The segregation simulation in the as-cast condition and the corresponding microstructures in the forging and heat-treating conditions are investigated. The inheritance of chemical heterogeneity leads to structural heterogeneity on both the nano- (nm) and micro (mu m) scales. This heterogeneity is much more pronounced in the Al-alloyed steel, which can be attributed to inheritance from the as-cast state. While the results of the quasistatic tensile tests are comparable for both alloys, the cyclic load increase tests indicate a higher fatigue strength of the Si-alloyed steel, which can be explained with the more homogenous microstructure and the finer distribution of the retained austenite.