Evolution of volume fraction of plasticity-induced martensitic phase during fatigue process of SUS304 stainless steel at room temperature in air

The present study has investigated plasticity-induced martensitic phase transformation in fatigue of unnotched SUS304 plates. Martensitic phase transformation occurred in uunotched SUS304 plate specimens fatigued at room temperature in air. Volume fraction Vα' of α' martensite in the uunotched potion of fatigued specimens was measured by ferrite scope. The relations between the maximum value of Vα', Vα'max, and the number of load cycles N were represented by reverse sigmoidal curves for all the applied stress range Δσ levels tested in this study. For the most portion of fatigue life, the value of Vα'max remained almost constant. This value was increased with increase in the value of Δσ. The spatial distribution of Vα' in the specimens varied with N: i.e., continued cycling of stress made α' transformation localized near the central portion of specimens where the Vα' value reaches as high as 35-40%. This value is more than doubled compared to be highest Vα' value found in the tensile tests of SUS304 at room temperature. Invisible cracks of 200 μm in length were found in the high Vα' value region. These results imply that the measurement of Vα' in fatigued SUS304 components may detect crack initiation sites and may predict residual fatigue life. (author)