Microstructural Characterization of Expanded Austenite in 304L and 904L Austenitic Stainless Steels

Licentiatavhandling, 2015

Austenitic stainless steels are among the most used materials in modern industry, mainly because of their superior corrosion resistance. However, low hardness and poor tribological properties often restrict their applicability. Conventional surface hardening techniques such as high-temperature carburizing (T > 850°C) and nitriding (T > 550°C) are not applicable to these alloys. Rapid precipitation of chromium-rich carbides/nitrides at the grain boundaries would in such cases induce chromium depletion in the alloy and compromise the corrosion resistance. Since the middle of the ´80s, low-temperature thermochemical treatments (T < 500°C) were developed for surface hardening of austenitic stainless steels, including gas carburizing and plasma nitriding. These processes can induce formation of a precipitate-free interstitially supersaturated metastable expanded austenite, also known as S-phase, having superior hardness (800-1500 HV), and improved wear resistance while maintaining corrosion resistance. In this study, industrial low-temperature carburizing and nitriding were performed on two austenitic stainless steels, 304L and 904L. The focus of this thesis is the material response to these low temperature treatments and thermal stability of the metastable structure formed. The aim is to investigate the influence of the alloy composition and surface finishing as well as thermal annealing on the microstructure, phase constituents, strain evolution and hardening effect of the modified layer by means of combined analytical techniques such as XPS, XRD, SEM, GDOES and EBSD. It has been found that surfaces with a high amount of plastic deformation (original surface finishing) show higher supersaturation of interstitials, generally accompanied by a faster diffusion of interstitials and thicker expanded austenite case layer. The formation of expanded austenite is accompanied by carbides/nitride for as-carburized 304L. However, the highly alloyed 904L exhibits mainly S-phase with larger degree of lattice expansion. Presence of ferrite/martensite make the formation of S-phase less favorable and might promote nitride/carbide precipitation in 304L. It has also been found that the expanded austenite decomposes upon vacuum annealing (600°C for 150 h) following a eutectoid route for 304L and a discontinuous route for 904L. Low temperature treatment induced significant enhancement of surface hardness, more effectively on 904L. The hardening mechanisms have also been discussed.