高強度ボルト用鋼の遅れ破壊特性に及ぼすS量の影響

概要

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A study has been made on the effect of sulphur contents on the tempering behavior and delayed failure susceptibility of high strength steels for bolts at a strength level of 130kgf/mm2.By means of bending type test, it is confirmed that delayed failure resistance improves with decreasing sulphur content.Fracture modes of delayed failure specimens of 0.015% and 0.024% sulphur steels exhibit intergranular fracture, and sulphur segregation to fracture surface is detected by Auger electron spectroscopy, while 0.003% sulphur steel shows transgranular fracture. On the other hand, Charpy impact fracture modes are all transgranular ones regardless sulphur contents. Therefore, it is concluded that the intergranular fracture in delayed failure is induced by both sulphur segregation to prior austenite grain boundaries and hydrogen degradation.Further study indicates that increased resistance to delayed failure of low sulphur steel is mainly caused by prolonging the incubation period to slow crack initiation.
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