Effects of post-weld heat treatment on microstructures and tensile properties of K-TIG weld metals of Ni–12Mo–7Cr–2Nb alloy

Abstract

The effect of post-weld heat treatment (PWHT) on the microstructures and tensile properties of Ni–12Mo–7Cr–2Nb weld metal has been investigated systematically. Three kinds of PWHT (as-welded, PWHT at 871 °C and PWHT at 1177 °C) were carried out, and then the accelerated aging treatment at 750 °C was carried out to simulate the long-term service state. The results show that PWHT can mitigate the elemental segregation and increase the grain size in the as-welded metal, especially at 1177 °C. After the accelerated aging treatment, many nanoscale MC carbides form within the matrix of all three samples, and dense MC carbides form on the grain boundaries (GBs) of the 1177 °C PWHT samples. The tensile tests of three aged samples were carried out at 700 °C with the as-welded one as a control. The yield strength of the as-welded and 871 °C PWHT samples after aging increases by ∼14 % due to precipitation strengthening. The yield strength of the 1177 °C PWHT samples does not change significantly after aging, which is attributed to grain coarsening and reduced dislocation density. The high elongation of the 1177 °C PWHT samples is mainly attributed to the dense MC carbides at the GBs and dislocation annihilation, which can relieve strain concentration along GBs and thus restrain intergranular cracking.

Keywords

• Superalloy
• Carbides
• Molten salt reactor
• Post-weld heat treatment