ISIJ International
Online ISSN : 1347-5460
Print ISSN : 0915-1559
ISSN-L : 0915-1559
Surface Treatment and Corrosion
Grain Size Effect on the Nitrogen Super-Saturation Process into AISI316 at 623 K
Tatsuhiko AizawaTomoaki YoshinoTomomi ShiratoriSho-Ichiro Yoshihara
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2019 Volume 59 Issue 10 Pages 1886-1892

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Abstract

Coarse and fine grained AISI316 substrates were prepared to describe the grain size effect on the inner nitriding behavior at 623 K by using the high density plasma nitriding without precipitation of nitrides. In case of coarse grained AISI316, the nitriding process advanced homogeneously in one part of nitrided layer with high nitrogen content, and, heterogeneously in its other part. In the former, γα’ two-phase, fine microstructure was uniformly formed by the phase transformation and plastic straining with the nitrogen supersaturation. In the latter, the nitrogen super-saturation localized to selectively modify the coarse grains to form the transformed α’-phase zones with the plastically strained γ-phase ones, even below the nitriding front end of 30 µm. In case of fine-grained AISI316, the nitriding took place homogeneously to form fine, two-phase microstructure down to the nitriding front end of 40 µm. This difference in the inner nitriding behavior came from the synergetic relationship between the nitrogen diffusion and super-saturation processes.

EBSD analysis of fine grained AISI316 substrate after plasma nitriding at 623 K for 14.4 ks. (a) Phase mapping, (b) KAM distribution, and (c) Inverse pole figure in the normal direction. Fullsize Image
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© 2019 by The Iron and Steel Institute of Japan
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