Delayed Fracture Resistance of High Temperature Tempered NiCrMoV Steel for Bolts (2)

Abstract

For fasteners the author had designed the steel NiCrMoV(0.4C-0.1Si-0.3Mn-1Ni-1Cr-1Mo-0.3V) tempered at 873K, which showed the delayed fracture life at a higher stress level in a static bending test substantially longer than SCM440(0.4C-0.3Si-0.7Mn-1Cr-0.2Mo) tempered at 703K to the equivalent tensile strength 1500MPa. The cathodic charge introduced a larger amount of diffusible hydrogen into NiCrMoV. The finely dispersed M₂C or MC precipitates in NiCrMoV trapped tightly some portion of the hydrogen, which didn't diffuse at all at RT. The obtained results suggest that the hydrogen trapping effect of the carbides retards the initiation of hydrogen-induced cracking and so extends the delayed fracture life of NiCrMoV tempered at higher temperatures.