Improvement of stress-relief cracking resistance in coarse-grained heat-affected zone of T23 steel by refining sub-structure through second thermal cycle

Abstract

Simulated coarse-grained heat-affected zone (CGHAZ), second reheated CGHAZ (UA-CGHAZ), super-critical reheated CGHAZ (SCR-CGHAZ) and inter-critical CGHAZ (ICR-CGHAZ) of T23 steel were produced via thermal simulation of welding. Their corresponding stress-relief cracking (SRC) susceptibility were assessed using isothermal slow strain rate tensile test. The fracture features and microstructures were characterized to reveal the cracking mechanisms. The simulated CGHAZ of T23 steel was highly susceptible to SRC at temperatures of 550–750°C and the fracture mode exhibited micro-void coalescence. M23C6 carbides precipitated at grain boundary may promote the micro-void nucleation and weaken the grain boundary, which resulted in grain boundary cracking preferentially. The strain was concentrated on the weakened grain boundary under tensile load, and the micro-voids gradually grew and coalesced into micro-crack, which ultimately propagated along grain boundary until complete fracture occurred. The SRC susceptibility of SCR-CGHAZ was reduced partially and the SRC susceptibility of ICR-CGHAZ was eliminated. After a second thermal cycle with a peak temperature slightly higher than Ac3 (SCR-CGHAZ), the grain size of CGHAZ was significantly refined and the crack resistance was elevated. When the peak temperature of the second thermal cycle was limited between Ac1 and Ac3, partial grains near grain boundaries were austenitized and transformed to fine ferrite grains along the prior austenite grain boundaries. The fine ferrite grains inhibited M23C6 precipitation and improve the resistance of cracking propagation. Thus, the ductility was improved and the SRC susceptibility was reduced.