Influence of Thermomechanical Treatment on Delayed Fracture Property of Mo-Bearing Medium-Carbon Steel

A 0.4C-2Cr-1Mo-2Ni steel (in mass%) was subjected to warm tempforming at 773 K or ausforming at 873 K using multi-pass caliber rolling with a rolling reduction of 78% after austenitizing at 1123 K for 1 h. The warm tempformed (TF) and ausformed (AF) bars were annealed and tempered at 843 K for 1 h, respectively, after which their delayed fracture properties were investigated and compared at an ultrahigh tensile strength level of 1.6 GPa. The TF sample had an ultrafine elongated grain (UFEG) structure with a strong //RD fiber texture evolved through the deformation of the tempered martensite, and the transverse grain size of the UFEG structure was finer than that of the isotropic block structure in the AF sample. Slow strain rate testing of pre-hydrogen-charged notched bar specimens and immersion testing demonstrated that the TF sample had a higher resistance to delayed fracture than the AF sample, based on the balance between the hydrogen embrittlement susceptibility and the hydrogen uptake from an atmospheric corrosive environment. The difference in the delayed fracture properties of the TF and AF samples is discussed in association with the microstructural evolution, fracture behavior, and tensile anisotropy.