Fundamental analyses with regard to the effect of chemical composition, heat treatment conditions, microstructure and fracture mode on SSC were conducted in order to develop quenched and tempered 70 kgf/mm2 yield strength steel with superior SSC resistance for induction heat treatment. Results obtained were as follows.
1) In the case of short time tempering, the optimum tempering temperature to give resistivity to SSC is a high temperature range below Ac1.
2) At the optimum tempering conditions, spheroidized carbides are uniformly dispersed both in grains and at grain boundary.
3) The steels tempered in the above-mentioned range exhibit the transgranular cracking, where as the steels tempered at lower temperature cracked by intergranular cracking.
4) Grain boundary segregation of phosphorus which promotes grain boundary embrittlement is suppressed by the addition of molybdenum. It is reduced more by tempering at high temperature for short time than at low temperature for long time.
From the above results, it was found that in the case of short time tempering the addition of small amount of molybdenum in steel and tempering at high temperature below Ac1 led to the high resistivity to SSC.
