The effect of microstructure evolution on fatigue property of SA508Gr.4N steel for nuclear reactor pressure vessels

The effect of tempering temperature on the microstructure evolution of SA508Gr.4N steel as well as the subsequent effect on the low cycle fatigue property were studied. The tempering temperature was carried out from 595 °C to 675 °C. The experimental results showed that more M/A islands were decomposed at 630 °C, and carbides were dissolved when the tempering temperature increased to 640 °C. The intercritical tempering temperature of the steel reached at a higher temperature (675 °C). After the intercritical range cooling, the granular bainite transformed into martensite, quasi-polygonal ferrite and a small number of undissolved carbides. Subsequently, the tempered specimens at 595 °C, 630 °C and 675 °C were subjected to low cycle fatigue tests under the strain amplitudes of ± 0.45% and ± 0.6% at 300 °C, respectively. The fatigue life of the steel increased first and then decreased with the increase of tempering temperature. The fatigue stripes separation exhibited a contrary trend to the fatigue life with the increase of tempering temperature. Moreover, the fatigue crack initiation points were transferred from around the M/A islands to the grain boundaries triple point with the increase of tempering temperature from 595 °C to 630 °C. Subsequently, the fatigue crack initiation points were transferred from the grain boundaries triple point to the ferrite phase and the boundary between martensite and ferrite when the tempering temperature increased to 675 °C. The obtained results can provide meaningful reference for improving the fatigue life of nuclear reactor pressure vessels.