高温焼もどし型ボルト用鋼の耐遅れ破壊性に関する研究
1994 年 65 巻 1 号 p. 31-40
詳細
1994 年 65 巻 1 号 p. 31-40
We have developed the CrMoV steel with low P and S content for connecting rod bolts of 1400MPa strength level. To clarify the mechanism of improvement in delayed fracture resistance of this precipitation hardening steel, a bending type delayed fracture test was carried out.
The tensile strength of the specimens which are quenched from 1148∼1223K and tempered at 823∼923K varies from 1350MPa to 1600MPa. It is confirmed that the delayed fracture resistance improves with increase in tempering temperature. And fracture mode of the delayed fracture specimens predominantly exhibits transgranular.
From the change in carbide morphology and mechanical properties with increase in tempering temperature, the superior delayed fracture resistance of this CrMoV steel is explained as follows.
(1) The spheroidizing of the cementite reduces the delayed fracture susceptibility, especially at prior austenite grain boundaries.
(2) The precipitation of M2C and MC type carbide within grains and the decrease in coherency between these carbides and the matrix increase the amount of trapped hydrogen.
