Stress-relief cracking mechanism in simulated coarse-grained heat-affected zone of T23 steel

Abstract

Simulated coarse-grained heat-affected zone (CGHAZ) of T23 steel was produced via thermal simulation of welding, and its stress-relief cracking (SRC) susceptibility was evaluated using isothermal slow strain rate tensile test. The evolution of inter-granular and intra-granular precipitates in CGHAZ during tempering, and the depletion of alloy elements adjacent to grain boundaries were characterized to clarify the cracking mechanism. The simulated CGHAZ of T23 steel was slightly susceptible to SRC at temperatures of 675–750 °C and the fracture mode exhibited micro-void coalescence. Massive coarse inter-granular M23C6 carbides precipitated during tempering, promoting the nucleation of micro-voids on grain boundaries as well as the depletion of Cr, W, and Mo near grain boundaries, thus weakening the grain boundaries. Many relatively fine intra-granular M23C6 and M7C3 carbides precipitated, enhancing the strength of the grain interiors. Therefore, 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 intra-granular MX carbide was not a major factor for the SRC generation in T23 steel because very few MX carbides precipitated during short-time tempering.