2020 Volume 106 Issue 12 Pages 924-933
In order to clarify the mechanism of abnormal grain growth of austenite in case hardening steel, in-situ observation by high temperature EBSD was performed using JIS SCM420 containing Nb (0.2C-0.2Si-0.8Mn-1.1Cr-0.2Mo-0.024Nb steel in mass%). After heating at 1143 K, two growing grains several times larger than the surrounding grains were observed. These growing grains grew abnormally by holding at 1193 K and were adjacent to each other. Since the boundary between the two abnormal grains is a twin boundary, the abnormal grains were observed as if they were one larger abnormal grain. The growth rate of abnormal grain is as high as the initial stage of growth and negligibly small at the latter stage of growth. That is, the grain size of abnormal grain growth of austenite is mainly decided by rapid grain growth for a short time after the start of grain growth. The generation of growing grain of abnormal grain growth of austenite is not affected by orientation and strain distribution. When distant grains which have a twin relationship are adjacent to each other in the grain growth process, grain connection occurs. It doubles the area surrounded by high angle grain boundaries without twin boundary. In addition to the encroachment of surrounding grains by larger grains, austenite grain connection through twin boundary also affects abnormal grain growth of austenite.
