Effect of heat treatment temperature on CGHAZ microstructure of T23 steel in long-term service

Butt joints of T23 water-cooled wall pipes with a service time more than 6 × 10⁴ h were taken as research object, and heat treatment tests with six different parameters were carried out on welded joints, namely no heat treatment (as welded), 520 ℃ × 1 h, 580 ℃ × 1 h, 650 ℃ × 1 h, 720 ℃ × 1 h, and 760 ℃ × 1 h. In order to study microstructure changes of CGHAZ after different heat treatments, analyze distribution law of alloy carbides in grain boundaries and explore internal relationship between formation of CGHAZ reheat crack and precipitates, microhardness analysis, metallographic structure analysis, scanning electron microscope morphology analysis, and energy spectrum analysis of coarse-grained region were emphasized. The results showed that as-welded T23 steel CGHAZ was a mixture of coarse lath martensite and bainite, the maximum grain size was about 100 μm, and most of grain size was 30～50 μm. During heat treatment at 580 ℃, due to grain refinement and dispersion strengthening effect of precipitates in the grain, CGHAZ hardness showed a secondary hardening phenomenon. According to theory of reheat crack formation, the difference between intragranular strength and grain boundary strength was large at this time, stress relaxation was mainly concentrated in the grain boundary, and CGHAZ tended to generate reheat cracks. During heat treatment at 720 ℃, dense voids and alloy carbides appeared at grain boundary of CGHAZ, and there was an associated relationship between voids and alloy carbides. However, whether the holes could develop into reheat cracks under normal service conditions (560～600 ℃) in a short period of time remained to be discussed.