Effect of Carbides on Reheat Cracking Sensitivity : Study of Reheat Cracking of Cr-Mo Steels, Report 3

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This study is devoted to the influences of chromium and molybdenum on the reheat cracking sensitivity of Cr-Mo steels from view points of the behavior of their carbide phases. X-ray analyses, electron microscopic observations and hardness measurements were carried out on synthetic 12 Cr-Mo steels in order to clarify the carbide reactions and the change of hardness due to carbide precipitation during tempering at 500 to 700゜C for 1 to 100 hours. The results are summarized as follows: 1) Molybdenum carbide M_2C precipitates in Cr-Mo steels with relatively low chromium content and high cracking sensitivity. Precipitation of M_2C causes a delay of softening in the temperature range of 500 to 550゜C, which corresponds to that of crack initiation. Therefore, M_2C prevents the softening of grain interior and promotes cracking along the grain boundary (prior-auslenite grain boundary). 2) Chromium carbide M_7C_3 precipitates in Cr-Mo steels with higher chromium content and lower cracking sensitivity. Precipitation of M_7C_3 also causes a delay of softening in the temperature range of 450 to 500゜C, which is much lower than that of crack initiation. 3) It has been found that certain carbides will precipitate along the grain boundary and cause the brittleness of steel. But it is less probable that M_2C preferentially precipitates at grain boundary and mechanically embrittles the steel.
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1984-04-01