Consideration of Phase Transformation due to High Stress Amplitude of Ultrafine Grain SUS304 Steel

Abstract

Rotating bending fatigue tests were performed on thin round bar of SUS304 steel with five levels of ultra-fine grains with an average grain size of 0.9 to 7.1 μm. Since the fatigue limit was higher than the 0.2% yield strength, it was inferred that the austenite phase underwent stress-induced transformation to the martensite phase during the fatigue process. Therefore, X-ray diffraction measurements were performed on the gauge points of specimens, and the changes in the austenite half-value width and the martensite integrated intensity, and changes in the magnetic flux density during the fatigue process were investigated. In addition, the EBSD analysis of the cross section of the run-out specimen was conducted to examine the aspect of transformation. As a result of the EBSD analysis, it was confirmed that the crystals in the surface layer of the specimen with relatively large grain size were refined, and the structure was found to be martensite phase. Since the martensite integrated intensity increased rapidly from 104 cycles, it was presumed that the stress-induced transformation with refinement started at this timing.