Hot Ductility Loss and Recovery in the CGHAZ of T23 Steel during Post-weld Heat Treatment at 750°C

Abstract

The coarse grained heat affected zone (CGHAZ) of T23 steel was assessed by strain-to-fracture (STF) tests during post-weld heat treatment (PWHT) at 750°C to investigate the microstructure changes and corresponding hot ductility. Scanning electron microscope (SEM), transmission electron microscope (TEM), and small angle x-ray scattering (SAXS) combining with JmatPro was used to study the characteristics of carbides, dislocation density, and lath size. Voids and micro-cracks were also analyzed by SEM. The results showed that sample with 1 min PWHT lost its hot ductility and intergranular cracking occurred. The hot ductility recovered with increasing PWHT time. However, the mechanisms were quite distinct during different time periods. Within PWHT time range from 0.5 h to 1.5 h, the recovery of hot ductility resulted from decreasing dislocation density. The hot ductility exhibited an opposite tendency with hardness. By contrast, the hot ductility and the hardness increased together after 2 h PWHT. It was resulted from the formation of lots of V-rich MC carbides in grain and coherent/semi-coherent smaller M₂₃C₆ carbides at the prior austenite grain boundaries (PAGBs). The strength of the grain interior and PAGB increased together. Plastic deformation was accommodated only by the cracking or shearing of low strength blocks. The fracture surface exhibited quasi-cleavage cracking together with dimples, which represented of good hot ductility. It could be concluded that the drastic changes of microstructure altered the manner of plastic deformation accommodation during stress relief, which led to the hot ductility loss and recovery during PWHT at 750°C.