Improved microstructure heterogeneity and low-temperature fracture toughness of C-Mn weld metal through post weld heat treatment

The Crack Tip Opening Displacement (CTOD) test was used to investigate the effect of post-weld heat treatment (PWHT) on the low-temperature fracture toughness of C-Mn weld metal. The microstructure observations, instrumented nano-indentation test and microstructure-based finite element simulation were conducted to analyze the toughening mechanisms. The study found that the CTOD value increased significantly from 0.241 mm to 1.754 mm after PWHT. The hardness and strength between acicular ferrite (AP) and proeutectoid ferrite (PF) both get closer, which improves the extent of microstructure heterogeneity. The simulation indicated that the distributions of the strain and stress on AF and PP are homogenized with the improvement of microstructure coordination after PWHT, making the level of plastic strain localization within PF decreased and the stress concentration on the interphase of AF and PF lessened. As a result, the resistance of crack initiation and propagation increases, and the low-temperature fracture toughness is improved. Moreover, the dislocation densities in AF and PF both decrease, the dislocation morphology evolves from dislocation tangles into movable dislocation lines, and amounts of fine carbides precipitate and spheroidize, which also contribute to the toughening of microstructures.