In-situ Observation of Abnormal Grain Growth of Austenite in Case Hardening Steel

In order to clarify the mechanism of abnormal grain growth of austenite in case hardening steel, in-situ observation using high temperature EBSD was performed with JIS SCM420 containing Nb (0.2C-0.2Si-0.8Mn-1.1Cr-0.2Mo-0.024Nb steel in mass%). After heating at 1 143 K, two growing grains several times larger than the surrounding grains were observed. These growing grains grew abnormally at 1 193 K and became adjacent to each other. The abnormal grains were observed as if they were one larger abnormal grain since the boundary between the two abnormal grains is a twin boundary. The growth rate of abnormal grain growth of austenite is faster in the early stage of growth and negligibly small in the latter stage of growth. The grain size in abnormal grain growth is decided by rapid grain growth during a short period in the early stage. The formation of growing grains in abnormal grain growth of austenite is not affected by orientation and strain distribution. When distantly located grains with a twin relationship inherently are adjacent to each other during the grain growth process, the new interface becomes twin, resulting in austenite grain connection, which doubles the area surrounded by the high angle grain boundary excluding twin boundaries. In addition to encroachment of surrounding grains by larger grains, austenite grain connection also affects abnormal grain growth of austenite.