In-Situ Synchrotron X-ray Diffraction Investigation of Microstructural Evolutions During Low-Pressure Carburizing

Abstract

An experimental heat treatment chamber and control system were developed to perform in-situ X-ray diffraction experiments during low-pressure carburizing (LPC) processes. Results from the experimental chamber and industrial furnace were compared, and it was proven that the built system is reliable for LPC experiments. In-situ X-ray diffraction investigations during LPC treatment were conducted at the German Electron Synchrotron Facility in Hamburg Germany. During the boost steps, carbon accumulation and carbide formation was observed at the surface. These accumulation and carbide formation decelerated the further carbon diffusion from atmosphere to the sample. In the early minutes of the diffusion steps, it is observed that cementite content continue to increase although there is no presence of gas. This effect is attributed to the high carbon accumulation at the surface during boost steps which acts as a carbon supply. During quenching, martensite at higher temperature had a lower c/a ratio than later formed ones. This difference is credited to the early transformation of austenite regions having lower carbon content. Also, it was noticed that the final carbon content dissolved in martensite reduced compared to carbon in austenite before quenching. This reduction was attributed to the auto-tempering effect.

Investigation of the Effects of Low‐Pressure Carburizing Process Parameters on Microstructural Evolution by Means of In Situ Synchrotron X‐Ray Diffraction

“…One reason for this rate decrease for both samples can be the accumulation of carbon at the surface following the formation of a very fine continuous cementite (Fe 3 C) layer in the first few microns of the surface. Carbide formation after the accumulation of surface was already observed in previous investigations, [ 13 ] and is confirmed for further process parameters. If the concentration of carbon at the surface during boost step exceeds the solubility limit of austenite, M 3 C carbides, M being mostly Fe and Cr, can be generated on the surface.…”

“…In addition, in a previous study by the authors, values determined by the given formula were compared with electron microprobe analyses results and the values were found to be very close. [ 13 ]…”

“…This conclusion correlates with a previous study. [ 13 ] 2) Higher temperature led to higher carbon absorption during the boost step. The effect of high temperature was more dominant at the early stages of the process and its effect decreased in each following step.…”

“…[ 5,10–12 ] In a previous study, this could be confirmed by first evaluation of synchrotron diffraction data and it could be demonstrated that for a specific acetylene flow rate and sample geometry, the saturation limit of austenite in boost steps was reached in less than 10 s and cementite immediately formed at the surface of the material. [ 13 ] Due to the fast carbon accumulation and formation of cementite, the degree of carbon transfer from the atmosphere to the sample surface reduced, so the carbon‐enrichment process was limited by the material. In the following diffusion step, the formed carbides dissolved within several minutes and contributed to the carbon content of the case.…”