Effect of molybdenum on delayed fracture behavior of high strength steel

Four heats of V and Nb microalloyed 40Cr steel containing 0 to 1.54% Mo were used to study the effect of Mo on delayed fracture resistance. The results of both notched tensile sustained load test and stress corrosion cracking test show that the delayed fracture resistance increases with increasing Mo content. Maximum delayed fracture resistance is obtained at a Mo concentration of about 1.15%. The result of EDS (energy dispersive spectroscopy) analysis shows that Mo tends to segregate in the grain boundary in a scale of no more than a few nanometers. EELS (electron energy loss spectra) measurement indicates that the segregation of Mo in grain boundary tends to increase the strength of grain boundary. The pronounced effect of Mo in raising the tempering resistance and the ability to strengthen prior austenite grain boundary are the main reasons for the beneficial effect of Mo on delayed fracture resistance. Hydrogen trapping effect caused by fine Mo2C precipitation could also improve the delayed fracture resistance. It is also confirmed that the tested steels could have a much higher delayed fracture resistance when the secondary hardening carbides of V and Mo are in the condition of slight over-raging.