Martensite coarsening in low-temperature plasma carburizing

This work shows microstructural evidences of morphological transformation from as-hardened martensite to C-expanded martensite as a result of the low-temperature plasma carburizing treatment carried out in AISI 420 martensitic stainless steel. Treatments were carried out at different treatment temperatures (350, 400, 450, and 500 °C, for a 12 h fixed time) and times (12, 24, 36, and 48 h, at 400 °C fixed temperature). Confocal laser scanning microscopy and SEM results show martensite coarsening in the treated surfaces regarding the respective substrate bulks, which is related to the surface C-alloying, thus being dependent on the treatment time and temperature. Hardness measurement and XRD results strongly suggest that the martensite coarsening is related to the transformation from lath- to plate-type martensite due to the enhancement of the martensite C content, in accordance with the Zener ordering theory. For samples carburized at relatively low temperatures (namely 350, and 400 °C), as C diffuses in paraequilibrium, strain-induced transformation mechanism would govern the changes in the initial martensite morphology as a result of its C content increase. On the other hand, as the chromium carbide precipitation occurs for samples treated at relatively high temperatures (higher than 450 °C for 12 h), and long times (longer than 36 h at 400 °C), C atoms would leave the supersaturated (expanded) martensite phase solid solution, leading the plate martensite to transform back to lath martensite.