The Influence of Test Temperature on the Crack Instability Propagation Behavior of Dissimilar Steel Welded Joints in Nuclear Power Plants

For the failure issue of the weak part of the safety end of the nuclear power pressure vessel connection, the J-integral method was used to test the fracture toughness of the weak part at the bottom of the dissimilar metal welded joints (DMWJs) of SA508-III and 316L in the temperature range of 25 °C to 320 °C, and the mechanism of temperature-induced crack instability and propagation was studied. The research results indicate that at all test temperatures, the position of the weld near the 316L steel is the failure site of the welded joint. The fracture toughness of the joint decreases with increasing temperature, with a maximum decrease of 42.0%. Analysis shows that as the temperature increases, the dislocation density decreases, the tensile strength decreases, and the yield strength ratio decreases, making it easier for secondary cracks to initiate near the crack tip, thereby accelerating the unstable propagation of cracks. At the same time, as the temperature increases, the number of twin crystals that can promote crack turning and prolong the crack propagation path decreases, the energy absorbed before fracture decreases, and the fracture toughness value decreases accordingly, further accelerating the unstable propagation of cracks.