Volume 80, Issue 2

Low-pressure carburizing (LPC) is a recipe-controlled heat treatment process for surface layer hardening. The combination of the process parameters determines the chemical gradients established within the case, the local microstructure and the depth distribution of the resulting process-induced residual stresses. Here, the objective was to quantify the influence of the process parameters on the resulting material state of the case hardening steel AISI 5120 (EN 20MnCr5). Therefore, the carburizing and quenching processes were examined using a self-built process chamber specially designed for in situ synchrotron X-ray diffraction experiments. A method for synchrotron X-ray diffraction in reflection geometry was developed to investigate the LPC process. It could be demonstrated that such experiments enable the analysis of microstructural changes, phase transformations and stress evolution at the immediate surface layer with high temporal resolutions. From a material science perspective, the investigations showed, among others, a carbon saturation of the austenite phase within 10 seconds during the boost segments of the LPC process and the formation of compressive residual stresses below -180 MPa after subsequent quenching.

Case hardening is the dominant heat treatment process for modern gear applications due to decades of continuous development. It is used to produce a hard and wear-resistant surface layer while maintaining a ductile core structure. In conventionally case-hardened components, the surface layer consists mainly of martensite and a remaining proportion of retained austenite as well as some possible precipitates. It is known from previous studies that surface layer structures with higher amounts of retained austenite can exhibit load carrying capacity potentials compared to conventionally case-hardened gears. However, these tend to form precipitations due to the higher carbon content during the carburization process. The experimental investigations carried out on gears confirmed that surface layer conditions with finely distributed precipitations in particular have a positive influence on the tooth root and tooth flank load carrying capacity of case-hardened gears. Endurance tests on the tooth root load carrying capacity of conditions with high amounts of retained austenite led to significantly increased strength values compared to the conventionally case-hardened condition. With regard to massive network-like carbide precipitations, a negative influence has tended to emerge, but this can be compensated for by appropriate blasting treatment of the gears.

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.