Reduction of Delayed Fracture Susceptibility of Tempered Martensitic Steel through Increased Si Content and Surface Softening

Improvement of the surface layer as well as the microstructure has been needed to develop high-strength steels, since delayed fracture cracks initiate in the surface layer. In the present study, two approaches were taken to reduce the delayed fracture susceptibility of tempered martensitic steel with tensile strength of 1 450 MPa. One was by increasing the Si content, which was intended to improve the microstructure. The other was by a surface-softening treatment, which was for improving the surface layer. Delayed fracture susceptibility was evaluated by conducting constant strain rate tensile tests (tensile tests) and constant load tests in a NH4SCN aqueous solution. It was found that increasing the Si content from 0.2 mass% to 1.88 mass% prevented intergranular fracture and reduced delayed fracture susceptibility. One reason for this improvement is that the Fe3C particle size on prior-grain boundaries and in the matrix decreases with increasing Si content, which implies that Si stabilizes dislocation structures. When the surface strength of surface-softened steel specimens was lowered to 1 150 MPa, delayed fracture. susceptibility was reduced further. This is attributed to not only a reduction of the Vickers hardness of the surface layer but also a reduced hydrogen concentration at the surface layer. The rearrangement and annihilation of dislocations and also the spheroidizing and coarsening of Fe3C particles at the surface layer subjected to a high tempering temperature lead to a reduction of the hydrogen concentration at the surface layer.