Influence of Different Bainitic Pretreatment Microstructures on the Nitriding Response of EN42CrMo4 and EN31CrMoV9

Abstract Surface treatments are used to improve the surface strength of mechanical loaded components. The strength increase due to nitriding is caused by forming nitrides, but achieving desired core strength requires a quenching and tempering (QT) preheat treatment. Austempering, which is used as an alternative to traditional QT treatment in bearing applications, offers improved toughness with comparable hardness, potential energy savings, and reduced dimensional changes. The investigations are concerned with different bainitic preheat treatment microstructures on EN42CrMo4 and EN-31CrMoV9 steels. Three austempering variants were explored for each, producing lower to upper bainite isothermally. Afterwards the specimens were nitrided to evaluate the impact of different pre-heat treatment microstructures on the characteristics of the nitrided layer. Compared to the tempered martensite formed during QT pre-heat treatment, the lower bainite provides similar hardness values and can be formed with shorter treatment durations and lower temperatures. The upper bainite positively affects hardening depth and compound layer thickness, but core hardness is lower, and treatment times can be economically disadvantageous for isothermal treatment in the upper bainite region.