In-situ Analysis of Carbide Formation and Stress Generation during Low Pressure Carburizing by Synchrotron X-ray Diffraction

Low pressure carburizing (LPC) is a thermochemical process for steels, which modifies the chemical composition of the near-surface region through carbon enrichment. While LPC is already used in industry, there are still aspects that offer opportunities for optimization. The present study aims to quantify the effect of LPC process parameters on the resulting material state of different steel grades. For this purpose, carburizing and quenching were examined using a self-built process chamber specifically designed for in-situ synchrotron X-ray diffraction experiments. These investigations were conducted at the Petra III -Beamline P07 German Electron Synchrotron (DESY) facility in Hamburg, Germany. During the carbon enrichment phase, carbon saturation and subsequent carbide formation were observed experimentally, which slowed the acetylene decomposition at the surface. Carbides formed already after a few seconds at the surface during the enrichment steps and is again dissolved during subsequent diffusion steps. The kinetics of carbide formation and dissolution were found to depend significantly on the steel grade and the size of the carbides. Additionally, the evolution of phase-specific stresses during quenching was systematically analyzed in both surface and subsurface regions. The effect of martensite start temperature being directly related to the carbon gradient could be observed on the maximum compressive stresses. Additionally, the impact of the local amount of formed martensite and the generated stresses was identified within the carburized layer.

This contribution is to be seen in connection with another contribution on this topic with the same participating institutions. In the complementary contribution, however, the focus is more on the methodical approaches used (applied set-ups and data evaluation strategies). In addition, the following tempering step is also analyzed.