Microstructure Evolution of a Simulated Coarse-Grained Heat-Affected Zone of T23 Steel During Aging

In this work, simulated CGHAZ of T23 steel was produced via a thermomechanical simulator, and then the CGHAZ specimens were aged at 650 °C for 0 to 240 hours to simulate the microstructure evolution of as-welded CGHAZ during service. Microstructure change and carbide precipitation were observed by OM, SEM, EBSD and TEM + EDS. Carbide precipitation kinetics in T23 steel at 650 °C was calculated for comparison with the experiment results. The hardness change of CGHAZ during aging was detected, and the effect of microstructure evolution on hardness was analyzed. The results showed that the CGHAZ of T23 steel exhibited a mixed microstructure of martensite and bainite with high hardness in as-welded condition. After aging at 650 °C, the microstructure recovered, recrystallization occurred, the dislocation density decreased, and the lath width increased. Consequently, the hardness dropped, the drop depending on the aging time. In the early stage of aging (before 24 hours), the precipitations inside the grain were mainly M₃C, M₇C₃ and a small number of M₂₃C₆ carbides, while the precipitation at the grain boundaries was M₂₃C₆. The precipitation of M₂₃C₆ caused the hardness to drop rapidly. When aged for 24 to 48 hours, MX precipitated inside grains extensively. The precipitation hardening produced by MX could slow down the decline of hardness. As the aging proceeded, carbide precipitated and transformed as follows: M₃C → M₃C + M₇C₃ + M₂₃C₆ → M₃C + M₇C₃ + M₂₃C₆ + MX → M₂₃C₆ + MX + M₆C. W-rich carbides precipitated in some grain boundaries of CGHAZ during aging, which may be related to the W segregation at those grain boundaries.