Effect of simulated stress-relieving heat treatment on microstructure and tensile properties of CLAM steel

Post weld heat treatment (PWHT) is indispensable in manufacturing process of the test blanket modules for international thermonuclear experimental reactor (ITER). Multiple PWHTs are required to reduce the residual stress that comes from the concentrated welded seams of the test blanket modules. As the number of treating cycles increases, considerable strength degradation of the base metal occurs. In this work, simulated stress-relieving heat treatment (SSRHT) was performed to investigate the effect of multiple PWHTs on the microstructure and tensile properties of China low activation martensitic (CLAM) steel. The results showed that its hardness decreased by 31 HV and yield strength decreased by 146 MPa for a 10-cycle treating at room temperature. The softening rate decreased and the ductility increased with increasing number of cycles at all temperatures. This phenomenon could be explained by the alterations in the martensitic lath width, volume fraction of the precipitates and the particle size. Considering the service safety of the blanket components fabricated with CLAM steel, the safety factors should be higher than 1.33, 1.29 and 1.28 for SSRHT within 10 cycles at room temperature, 450 °C and 550 °C, respectively.