Professor, Shibaura Institute of Technology (SIT), Japan

Tatsuhiko Aizawa is a professor of Shibaura Institute of Technology (SIT), Japan. He received his PhD from The University of Tokyo in 1980. He became a research associate, Institute of Aerospace and Aeronautics in 1980, a lecturer in 1985, an associate professor in 1986 and a professor in 1996 till 2004 in the University of Tokyo. After the research professor in the University of Toronto from 2005 to 2008, he works as a professor in SIT from 2009 now. His research interests include the micro-manufacturing, the innovations in manufacturing and materials processing, and, the materials science and engineering. He has published over 350 papers in Japanese and International Journals. He has received lots of awards from academic societies including the Distinguished Achievement Award, Japan Institute of Metals, 2011, and, the Gold Medal, Japan Society of Technology of Plasticity, 2017.

Speech Title: Homogeneous and heterogeneous micro-structuring of austenitic stainless steels by the low temperature plasma nitriding

Aims: Homogeneous and heterogeneous nitriding behavior is described by SEM-EDX, XRD and EBSD together with mechanical characterization.

Methods: High density RF/DC plasma nitriding process was applied to characterize the nitrided AISI316 and AISI304 microstructure and to describe the essential mechanism of inner nitriding at the lower temperature than 680 K.

Results: In case of the nitrided AISI316 at 673 K, the nitrided layer thickness became 60 m with the surface hardness of 1700 HV and the surface nitrogen content of 7 mass %. This nitriding affected layer consists of two layers; homogeneously and heterogeneously nitrided zones. This inner nitriding process to form these two zones is commonly governed by the synergetic interrelation among the nitrogen super-saturation, the -lattice expansion, the phase transformation, the plastic straining, the microstructure refinement and the nitrogen diffusion. When this interrelation is sustained during the nitriding process, the original austenitic microstructure is homogeneously nitrided to have fine grains with the average size of 0.1 m and the high crystallographic mis-orientation angles and to have two phase (γ + α’) structures with the plateau of nitrogen content by 5 mass%.

Conclusions: The lower temperature plasma nitriding than 680 K provides a tool to homogeneously refine the original austenitic stainless steel microstructure down to 0.1 m and strengthen by formation of fine two-phase structure. This homogeneous nitriding follows the heterogeneous process to locally modify the original stainless steel microstructure.