Characterization of low cycle fatigue damage in 9Cr–1Mo ferritic steel using X-ray diffraction technique

Abstract

X-ray diffraction (XRD) technique has been used to characterize the low cycle fatigue (LCF) damage in 9Cr–1Mo ferritic steel. In this study, full-width at half-maximum (FWHM) of the XRD peak has been measured for assessing the fatigue damage. Fully reversed total-axial-strain controlled fatigue tests have been performed at ambient temperature (300 K) at strain amplitudes of ±0.25%, ±0.50% and ±0.75%. FWHM measurement of {3 1 0} plane has been carried out on specimens interrupted at different fatigue life fractions, which represent the various stages of deformation and fracture such as cyclic hardening, cyclic softening, saturation and surface crack initiation and propagation. The cyclic hardening, which occurred in the early stages of fatigue deformation, exhibited broadening of diffraction profile and a rapid increase in the FWHM at all strain amplitudes. Also, large oscillations were observed in the d(2 θ) vs. sin 2Ψ curves. With further cycling, FWHM remained almost constant in the softening and saturation stages. Finally, at the onset of rapid stress drop in the cyclic stress response and cusp formation in the compression portions of stress–strain hysteresis loops, which indicate surface crack initiation and propagation, a rapid decrease in FWHM was observed. This drop in FWHM is attributed to relieving of microstresses owing to surface crack initiation and propagation. Relieving of micro stresses has also been confirmed by a significant reduction in oscillations in the d(2 θ) vs. sin 2Ψ curves.