The Mechanism Underlying the Influence of Temperature on the Fracture Toughness of Dissimilar Steel Welded Joints in Nuclear Power Plants

In this study, the J-integral method was used to evaluate the fracture toughness (JQ) of the isolation layer at the top of SA508-III-309L/308L-316L dissimilar metal welded joints (DMWJs) of a pressure vessel. Tests were carried out at varying temperatures, from room temperature to 320 °C, to study the mechanism underlying temperature effects on unstable crack propagation. The results show that failure occurs in the middle position of the weld isolation layer of the welded joint at all test temperatures. The JQ of the inner diameter of the joint decreases with increased temperature, with a maximum decrease of 31.8%. The analysis shows that the lath ferrite structure in the isolation layer provides a favorable path for crack propagation. The increase in temperature enlarges the difference in thermal expansion between SA508-III steel and the isolation layer, making it easier for second-phase particles in the isolation layer to induce crack initiation and propagation, thus reducing the JQ of the steel. In addition, at high temperatures, the dislocation density of the isolation layer, the deformation resistance of the material, and the difference in the yield ratio of the joint weld all decrease, which is not conducive to the redistribution of the stress field at the tip of the fatigue crack, leading to further reduction in the JQ.