Real-time monitoring of the steel AISI 5120 during low-pressure carburizing and tempering using synchrotron X-ray diffraction

Low-pressure carburizing (LPC) is a case hardening process that has become increasingly important in recent years and enables the combination of a ductile core and a hard surface. The LPC process offers many advantages over conventional case hardening methods, such as high environmental compatibility, efficiency, uniform carburization and the absence of surface oxidation.

Although the LPC process is already used in industry, its process parameters are usually determined empirically, and the process therefore offers opportunities for optimization. The present research study aims to investigate the complete heat treatment process, using the case hardening steel AISI 5120 (EN 20MnCr5) as model material. For this purpose, the individual processing steps carburizing, quenching and tempering were examined using in situ synchrotron diffraction analyses and subsequent ex situ laboratory X-ray analyses. A self-built process chamber enabled complementary analyses in transmission and reflection geometry, which was used for complementary analyses of the materials volume in the near-surface region affected by LPC processing and of the very surface, respectively. The investigations focused on the evolution of chemical gradients, phase transformation kinetics and the temporal evolution of phase-specific stresses.

The induced thermal and transformation stresses of the as-quenched sample states significantly depend on the chosen parameter variation. The variation of tempering temperate leads to the formation of fundamentally different microstructures, including the formation of different types of carbides.

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 analysis of the processes during the diffusion and boost phases (e.g. carbide formation and dissolution).