Giga-Cycle Fatigue Properties for 1800MPa-Class Spring Steels.

Abstract

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, σB, 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, σW, 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 σW=0.53σB 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.