Gigacycle fatigue properties of 1800 MPa class spring steels

Abstract

ABSTRACTFatigue tests up to 108 cycles were carried out for two spring steels (Heats A and D1) and one valve spring steel (Heat F) with tensile strength, σB, of 1720, 1725 and 1764 MPa, respectively. The size and composition of inclusions in Heats Dl and F were controlled. The surface‐type fracture occurred at shorter lives below 106 cycles, while the fish‐eye‐type fracture occurred at longer lives. The fatigue limit, σW, at 108 cycles was 640 MPa for Heats A and D1 and 700 MPa for Heat F. Al2O3 inclusions for Heat A and both TiN inclusions and matrix cracks, i.e. internal facets, for Heat F were observed at the fish‐eye‐type fracture sites, while only matrix cracks were observed for Heat Dl. ODA, i.e. optically dark area, which is considered to be related to hydrogen effects, were formed around Al2O3 and TiN inclusions. Fatigue tests were also conducted after specimens were heated up to 573 K in high vacuum of 2 × 10–6 Pa. The heat treatment eliminated matrix cracks for Heat D1 and the fatigue limit at 108 cycles recovered to the estimated value of 920 MPa from the equation σw= 0.53 σB for the surface fracture. These results suggest that inclusions control and hydrogen influence the gigacycle fatigue properties for these high strength steels. In addition, it is expected that the creation of a martensite structure with a high resistance to hydrogen effects in the inclusion‐controlled steel could achieve the higher fatigue limit estimated for the surface‐type fracture.