Low temperature plasma nitriding of 316 stainless steel - Nature of S phase and its thermal stability

Conventional plasma nitriding of stainless steels (at approximately 550°C) confers significant hardening of the treated case but at the expense of the corrosion resistance. Attempts have been made in the past decade to achieve combined improvements in wear and corrosion resistance of austenitic stainless steels (ASS) via low temperature thermochemical processes. However, varying degrees of success have been achieved in applications, which may be largely attributed to the lack of scientific understanding of the nature of the S phase and its thermal stability. The present work concerns two aspects of low temperature plasma nitriding of austenitic stainless steel: the study of the nature of the S phase and the thermal stability of the low temperature plasma nitrided layers on austenitic stainless steel. To achieve these tasks, systematic low temperature plasma nitriding of austenitic stainless steel has been carried out. The structure of the S phase was systematically investigated. The thermal stability of the low temperature plasma alloyed layers on stainless steel was studied employing both long term isothermal annealing and TEM in situ heating observations.