Influence of Hardness Varied by Surface-Treatment on Delayed Fracture Strength of High Tension Steel

Abstract

In the previous paper, the results of delayed fracture tests on Cr-Mo high strength steel (SCM 435) subjected to plastic deformation by surface-treatments were presented and the influence of surface roughness and compressive residual stress due to surface-treatments were discussed.
In this study, cantilever bending delayed fracture tests and static bending tests were performed to examine the effect of hardness on the delayed fracture strength of SCM 435 surface-treated specimens. The methods used for surface-softening were decarburization and induction heating, and those for surface-hardening were nitriding, carburizing and induction hardning. The results for these specimens were compared with those without surface-treatment.
The main results obtained are as follows;
(1) The delayed fracture life of the decarburized specimens was largely prolonged under high applied stress.
(2) In the case of the induction softened specimens after the heat-treatment, the delayed fracture limit increased remarkably without reducing static strength level so much.
(3) On the other hand, the susceptibility to hydrogen embrittlement for all the surface-hardened specimens mentioned above increased and it was affected by both hardness and its distribution in the vicinity of the surface. The effect of compressive residual stress caused by surface-treatments was not recognized.