Abnormal metallographic structure found in gas-carburized and directly quenched carburizing steel (JIS SCM22) was investigated. Abnormal metallographic structure was consisted of extremely large martensite bundles formed within few abnormally coarse austenite grains grown during carburizing heating. In this study, the influence of prior heat treatment (isothermal annealing) temperature and subsequent heat treatment (carburizing) temperature on abnormal grain growth of austenite was investigated and following results were obtained:
(1) Abnormal grain growth of austenite has close relation to isothermal annealing temperature and subsequent heat treatment temperature. That is to say, abnormal grain growth of austenite initiates in extremely short time at 930°C or so after isothermally annealing at 550°C or so. No abnormal grain grows at higher temperature above 930°C, and initiation time of abnormal grain growth at 930°C after isothermally annealing at other than 550°C is longer than that after I.A. at 550°C.
(2) Prior isothermal annealing temperture makes influence on the distribution of AIN, and initial grain size of austenite during subsequent heating. That is to say, after isothermally annealing at 550°C or so, AIN particles are few and large, and initial grain size of austenite is small.
(3) The influence of isothermal annealing temperature and subsequent heat temperature on abnormal grain growth of austenite can be explained by the theory proposed by Hillert, or by Ogino et al..