1800 MPa 級ばね鋼のギガサイクル疲労特性

抄録

Fatigue tests up to (10)⁸ cycles were carried out for two spring steels (heats A and D 1) and one valve spring steel (heat F) with tensile strength, σ_B, of 1720, 1725 and 1764 MPa, respectively. The size and composition of inclusions in heats D 1 and F were controlled. The surface type fracture occurred at the shorter life below (10)⁶ cycles, while the fish eye type fracture occurred at the longer life. The fatigue limit, σ_W, at (10)⁸ cycles was 640 MPa for heats A and D 1, and 700 MPa for heat F. Al₂O₃ inclusion for heat A and both of TiN inclusion for heat A and matrix crack, i.e., internal facets, were observed at the fracture sites, while only matrix cracks were observed for heat D 1. ODA, i.e., optically dark area, which is considered to be related to hydrogen embrittlement were formed around Al₂O₃ and TiN inclusions. Fatigue tests were also conducted after specimens were heated up to 300°C in the high vacuum of 2×(10)⁶Pa. The heat treatment eliminated matrix cracks for heat D 1 and the fatigue limit at (10)⁸ cycles recovered to the estimated value of 920 MPa from the equation σ_W=0.53σ_B for the surface type fracture. These results suggest that the inclusion control and hydrogen embrittlement influence the giga-cycle fatigue properties for the high strength steel. In addition, it is expected that the creation of martensite structure with the high resistance against hydrogen enbrittlement in the inclusion controlled steel could achieve the higher fatigue limit estimated for the surface type fracture.