Improved process understanding of case hardening with low pressure carburizing based on in-situ X-ray diffraction experiments

• Applicants: Dr.-Ing. Jérémy Epp; Dr.-Ing. Jens Gibmeier
• Subject Area: Metallurgical, Thermal and Thermomechanical Treatment of Materials
• Term: since 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 399551201

In the research project, a fundamental understanding of the process of case hardening with low pressure carburization to be obtained on the basis of highly time-resolved in situ X-ray diffraction data. In the continuation of the project, the understanding gained in the first funding period will be extended to include in-depth investigations of carbide formation and now supplemented to include the usual post-treatment by tempering and the precipitation processes that take place in the process. While the focus in the first funding period was on the case-hardening steel 20MnCr5, in the continuation of the project the transfer to other typical case-hardening steels, the steels 14NiCrMo13-4 and 18CrNiMo7-6 with increasing Ni and Cr contents, is planned. Due to the significantly different chemical composition, it is expected that, depending on the composition, a different behavior will occur in the carbide formation/dissolution during carburizing. It is expected that the process kinetics and carbon uptake will change significantly due to the formation and dissolution of carbides during the boost and diffusion phases. In the continuation of the project, both process chamber setups (transmission and reflection geometry) will be used to obtain fundamental time- and spatially-resolved information on the development of the microstructure and stresses for the other materials. Through the extension of the analyses, the mechanisms occurring around the influence of the initial alloy composition are fundamentally analyzed and thus an extended comprehension of the process is gained.Furthermore, during the continuation of the project, not only the carburizing and quenching process steps are to be investigated, but the investigations are to be extended to the consideration of the subsequent tempering step in order to come closer to the industrial case. In combined in situ WAXS (wide angle X-ray scattering) and SAXS experiments (small angle X-ray scattering), the formation of carbide precursors and cementite, together with further microstructure and stress changes, will be investigated using the existing process chamber. In this combined in situ experiment during tempering, time-resolved diffraction data can be recorded simultaneously with small-angle scattering signals, which allows not only structural analysis but also simultaneous analysis of the size and distribution of the carbide precursors in the nanometer range. Furthermore, the process will be transferred to samples with locally complex geometries in the continuation of the project. Among other things, blind holes with different aspect ratios are to be investigated in situ by using adapted measurement strategies. With the help of comprehensive analyses of the final state, accompanying high-resolution ex situ investigations will also assess how the residual stresses and the microstructure are influenced locally.

• DFG Programme: Research Grants
• Co-Investigator: Dr.-Ing. Matthias Steinbacher