Low-temperature carburizing/nitriding of austenitic stainless steels - Influence of alloy composition on microstructure and properties

Sammanfattning: Austenitic stainless steels are among the most used materials in applications where corrosion resistance is important, as the food, pharmaceutical, chemical, oil and gas industries. However, low hardness and poor tribological properties is often an obstacle for their applicability. Conventional surface hardening techniques, such as high-temperature carburizing (T > 850°C) and nitriding (T > 550°C) are not suitable for 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 have been 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 and improved wear resistance, while maintaining corrosion resistance.The aim of this thesis is to increase the understanding of the microstructure-properties relationship of the surface modifications obtained by treating austenitic stainless steels with low-temperature carburizing and nitriding processes. In particular, the research focus is on the influence of alloy composition and surface finish on the microstructural evolution, phase constituents, thermal stability, strain, hardening, wear and corrosion resistance of the expanded austenite layers. The investigations were carried out by means of different analytical techniques, such as XPS, AES, XRD, SEM, GDOES and EBSD among others. It has been found that alloy composition and surface finish have a paramount influence on the microstructural characteristics and properties of the expanded austenite layers. High molybdenum content and plastic deformation enhances interstitial diffusion and supersaturation, while nickel decreases both. On the other hand, nickel prevents the formation of detrimental nitride/carbide-containing compound layers, enhances the thermal stability of the alloys and directs the expanded austenite decomposition towards a discontinuous route. During thermochemical treatments, precipitates tend to form preferentially at the surface, in correspondence of slip-planes or ferritic/martensitic regions. A hard, expanded austenite layer prevents severe adhesive and abrasive wear during dry sliding tests, improving wear resistance at low load. N-stabilised expanded austenite potentially possesses stronger hardening effects, but is more brittle. The strong interaction between chromium and carbon in expanded austenite alters the iron/chromium ratio within the passive film, making the surface more noble and conductive. When expanded austenite is thermally decomposed, hardening and corrosion resistance are at least partially compromised.