Abstract: The micro-mechanism of reheat crack formation in coarse grain heat affected zone (CGHAZ) of welded joint of T23 water wall tube without heat treatment after welding was analyzed by high temperature aging method.It reveals the internal reason that T23 water wall joint without heat treatment is easy to crack and leak in short-term operation after unit startup.The hardness, microstructure and precipitates of welded joints of water wall after unaged and high temperature aging treatment were systematically analyzed by means of material characterization.The results show that after aging at 530 ℃ for 100 h, the hardness of CGHAZ appears secondary hardening caused by intragranular dispersion strengthening. With the increase of aging (running) time, the hardness of CGHAZ gradually decreases, but after aging for 1000 h, the hardness of CGHAZ is still 319 HV, which is higher than the standard requirement.After aging at 600 ℃, the hardness of CGHAZ decreases with the increase of aging time. The hardness of CGHAZ decreased due to the recovery of microstructure,recrystallization, broadening of martensite lath, reduction of dislocation density, and precipitation of C and alloy elements from the matrix, which is higher than the hardness increased due to dispersion and precipitation of MX carbide in the grain. M23C6 carbide gradually precipitates and grows at grain boundaries and subgrain boundaries.