High-cycle fatigue properties up to 108 cycles were investigated for a modified-ausformed spring steel SUP 12 with the tensile strength δB=1772 MPa. The surface-type fracture occurred at the higher stress level. On the hand, some specimens had the fish-eye-type fracture origin at the inclusion, and others endured 108 cycling at the lower stress level. The fatigue limit at 108 cycles δw=900 MPa for the fish-eye fracture was higher than δ=886 MPa estimated from δw=0.5δB, which is the empirical equation for the surface-type fracture of ordinary quenched and tempered low alloy steels. The optically dark area was not found around the inclusion. Excluding the fish-eye-type fracture, the higher fatigue limit δw=1090 MPa for the surface-type fracture was expected. It corresponds to δW=0.62δB. It is concluded from these results that the modified-ausforming improves the fatigue properties for both surface and fish-eye type fractures of the high strength steel.
