By the increase in Mo content, the addition of microalloying elements V and Nb and by reducing the contents of Mn, P and S based on the composition of steel 42CrMo, we have developed a 1300 MPa-grade high strength steel (ADF1) for bolts. The sustained load bending test, sustained load tensile test and stress corrosion cracking test have been carried out to evaluate the delayed fracture resistance of steel ADF1 and commercial steel 42CrMo. The results showed that steel ADF1 has superior delayed fracture resistance to that of 42CrMo steel. It's concluded that the superior delayed fracture resistance of ADF1 is mainly due to the increase of tempering temperature, fine homogeneously distributed MC carbide and fine prior austenite grain size.
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