Delayed Failure in Steels

Abstract

Since various sorts of high strength steels became to be used for various services, the problem of delayed failure in the steels has become more and more important. According to many studies so far made, man has impression that the cause of such failure can be attributed to hydrogen. In this paper, taking notice of the effect of hydrogen, a part of many investigations on the delayed failure in steels is briefly reviewed.From the detailed studies dealing with the delayed failure in hydrogen-charged high strength steels, it is seen that susceptibility to delayed failure in the steels increases remarkably by the charging of hydrogen. The increase is as larger as the amount of the charged hydrogen. Moreover, some steels with very high strength become susceptible to the delayed failure by absorption of an extremely small amount of hydrogen which has no effect on the usual mechanical properties, e.g., the reduction in area. Temperature dependency of incubation time for delayed failure cracking makes us know that the activation energy for the crack initiation is 9120cal/mol which is equal to the activation energy for diffusion of hydrogen in steel.It is generally known that the delayed failure in steels is very sensitive to using atmosphere. The failure is hastened by the atmosphere containing moisture, chloride, sulfide, etc., but is suppressed by oxygen. Although the effects of atmospheres are very complicated, they can be qualitatively understood considering that hydrogen generated by some electro-chemical reactions between steel and atmosphere may permeate to the steel and may cause to fail.The delayed failure in steels may thus be concluded to have close correlation with hydrogen. However, we can also find some experimental results which suggest us the importance of anodic resolution-reaction. Therefore, it must be said that the detail of the mechanism of the delayed failure remains uncertain.