Effect of long-term aging on the microstructure and mechanical properties of T23 steel weld metal without post-weld heat treatment

T23 steel welds without post-weld heat treatment (PWHT) were aged for 3000 h at temperatures in the range 773–923 K. Microstructural evolution in the aged weld metal (WM) was observed and its influence on the hardness and Charpy impact toughness were investigated. The as-welded WM exhibits a martensitic/bainitic microstructure with moderate hardness less than 350 HB and impact toughness value above 110 J/cm2. Aged WM exhibits deterioration of mechanical properties dependent on the temperature of aging. When the aging temperature is below 823 K, the WM shows little change in hardness but its impact toughness drops significantly. When the aging temperature is above 823 K, the hardness of the WM starts to decrease quickly while its impact toughness increases gradually. As the aging temperature increases to 923 K, its impact toughness further increases and recovers to a value comparable to that of the as-welded condition. Embrittlement occurring during aging at 773–823 K results from tempered martensite embrittlement (TME), which is caused by the decomposition of retained austenite. When the aging temperature exceeds 823 K, M23C6 carbides precipitate rapidly inside grains and at grain boundaries and MX carbides begin to precipitate inside grains. These cause the release of carbon from the matrix, reduction of lattice distortion, and accelerated recovery of laths causing the hardness to decrease rapidly and the impact toughness to increase. W-rich carbides are observed in a minority of grain boundaries in the WM aged at temperatures higher than 823 K, which may cause intergranular fracture and retard the improvement of impact toughness.