Abstract: This paper aims to study the microscopic mechanism of reheating cracks formed in the Coarse-grained heat-affected zone (CGHAZ) of the welded joint of T23 water-wall pipe without heat treatment in the process of service by means of high temperature aging method, and to reveal the internal reasons why the joint of T23 water-wall pipe without heat treatment in engineering is prone to cracking and leakage during short running after machine start-up. In this paper, the CGHAZ hardness, microstructure and precipitate equality of water-cooled wall welded joints after unaged and high-temperature aging were systematically studied by means of microhardness testing, metallographic structure (OM) observation, scanning electron microscope (SEM) morphology observation, energy dispersive spectroscopy (EDS) analysis and transmission electron microscope (TEM) phase analysis. The results show that after aging at 530 ℃ for 100h, the hardness of CGHAZ appears secondary hardening caused by intragranular dispersion strengthening. After that, the hardness of CGHAZ decreases gradually with the increase of aging (running) time, but after 1000h, the hardness of CGHAZ is still 319HV, which is higher than the standard requirements. When aging at 600 ℃, CGHAZ hardness decreases with the increase of aging time. These results indicate that the hardness reduction of CGHAZ caused by the microstructure recovery, recrystallization, martensitic slat widening, dislocation density reduction, and the precipitation of C and alloy elements from the matrix is higher than the hardness increase caused by the dispersion precipitation of MX carbides in the crystal.M₂₃C₆ carbon (nitrogen) compounds gradually precipitate and grow at grain boundaries and subgrain boundaries.The research results of this paper provide theoretical support for revealing the internal reasons of cracking and leakage of unheat treated T23 water-wall joint in short run after machine starting.