Local deformation and transformation behavior of retained austenite in 18CrNiMo7-6 after high-carbon carburizing treatment

The objective of this investigation is to investigate the retained austenite deformation and transformation behavior during nanoindentation of 18CrNiMo7-6 steel after a high-carbon case-hardening process. We conducted a high-carbon carburizing treatment that led to approximately 1.2 mass% carbon at the surface. Further heat treatment consisted of austenitization, quenching in oil, and low-temperature annealing. To transform large amounts of retained austenite into martensite and to stabilize the existent retained austenite, some specimens were cryogenically treated in liquid nitrogen. The resulting microstructures were analyzed as a function of carbon content in the gradient by means of scanning electron microscopy with energy dispersive X-ray spectroscopy and electron backscatter diffraction analysis. Nanoindentation was used to study locally the mechanical behavior and stability of retained austenite at the carburized surface. To better analyze the deformation behavior during indentation, atomic force microscopy was applied to measure the topography of individual indentation imprints and the deformation pattern around the indents. The results show that nanoindentation is suitable for mechanically induced phase transformation of retained austenite into martensite, which leads to pronounced pop-ins in the load-displacement curves at specific critical loads. The occurrence of pop-ins, and, thus the phase transformation is highly dependent on the local microstructure of the carburized specimens. The main influencing factors are the local amount of retained austenite in the deformed region and, predominantly, the crystallographic orientation of retained austenite. (001)-Oriented retained austenite is highly metastable, whereas (111) orientations show mostly stable austenitic deformation behavior with only isolated mechanically induced retained austenite transformation during indentation. Cryogenic treatment transformed larger amounts of retained austenite into martensite. However, even after cryogenic treatment, (111)-oriented retained austenite could undergo mechanically induced transformation into martensite. Furthermore, we used the progression of hardness as a function of indentation depth and topography images to analyze the extent of retained austenite transformation and the complex compound deformation/transformation behavior of preexisting martensite and retained austenite.