Effects of thermal treatment on the mechanical and electromagnetic properties of 9Cr-1Mo steel

Abstract

This paper presents the effects of thermal treatment conditions on the mechanical properties of 9Cr-1Mo steel. Each sample was normalized at 1,040 °C for 1 h prior to tempering at different temperatures for 2 hours. Three levels of tempering conditions, 600, 650, and 700 °C, were tested in this study. After treatment, the metallurgical structures of the samples were observed and characterized by optical microscope (OM), scanning electron microscope (SEM), energy dispersive X-ray spectrometer (EDS) and X-ray diffraction (XRD). Hardness and static tensile of the samples at room and elevated temperatures were also measured and analyzed. Non-destructive testing (NDT) associated with the eddy current method was employed to detect the microstructural changes of samples. The results revealed that the lath martensitic structure, lath boundaries, coarse precipitates, and prior austenite grain boundaries were found at grain boundaries and inside the matrix region. The average size of the particle and area fraction of carbide precipitates decreased with tempering temperature increases from 600 − 650 °C. The true yield stress and true ultimate tensile stress decreased while true strain increased when the tempering process was applied after the normalization. In addition, the use of the eddy current method at 100 kHz with proper conductivity could more accurately detect the microstructural changes of 9Cr-1Mo steel specimens than using the phase angle approach. The correlation coefficient of conductivity and true ultimate tensile strength was found to be 0.9459, indicating a high correlation between them.