Abstract: Objective Girth welds, as critical connection points in long-distance liquid ammonia pipelines, exhibit a high risk of stress corrosion cracking (SCC) in environments containing impurities due to their complex microstructure and residual stress distribution, seriously threatening the service safety of the pipeline. This study aims to reveal the influence mechanism of the microstructure of the girth weld in X52 seamless steel pipe on its SCC susceptibility in environments containing impurities. Methods The base material, heat-affected zone, and weld center of the girth weld of X52 seamless steel pipe were selected as the research objects. Slow strain rate tensile (SSRT) tests were carried out in air and two environments containing impurities (condition 1: 0.20 wt.% H₂O, 0.0005 wt.% O₂, 0.002 wt.% CO₂, 0.0005 wt.% N₂; condition 2: 0.10 wt.% H₂O, 0.20 wt.% O₂, 0.15 wt.% CO₂, 0.40 wt.% N₂). By comprehensively utilizing metallographic observation, electron backscatter diffraction (EBSD) analysis, surface residual stress testing, and scanning electron microscopy (SEM) morphology characterization of the fracture surface, the stress corrosion susceptibility index based on tensile strength () and elongation after fracture () was systematically evaluated, and correlated with microstructural characteristics to elucidate the intrinsic mechanism of the difference in stress corrosion susceptibility of liquid ammonia. Results SSRT results showed that the trends revealed by the two susceptibility indices were consistent, but was more sensitive to SCC of liquid ammonia. The SCC susceptibility ranking of different regions of the girth weld was: weld center > heat-affected zone > base metal. Microstructural analysis showed that the weld center had refined grains, uneven grain size distribution, the largest average nucleus orientation difference, and the highest surface residual tensile stress (54 MPa). These factors collectively lead to enhanced electrochemical activity. Under the synergistic effect of impurity-containing liquid ammonia and tensile stress, pitting corrosion preferentially initiates and induces brittle cleavage fracture, resulting in a fracture surface exhibiting significant brittle characteristics. In contrast, the base metal exhibits a uniform microstructure and low residual stress (5 MPa), demonstrating good resistance to SCC. Conclusion The SCC susceptibility of the girth weld of X52 seamless steel pipe in a liquid ammonia environment is mainly controlled by the microstructure inhomogeneity and the resulting residual stress concentration. The weld center, due to its refined microstructure, high dislocation density, and high residual tensile stress, becomes the most SCC-sensitive area. To ensure the safe operation of liquid ammonia pipelines, strict process control should be implemented during welding manufacturing and construction, and post-weld heat treatment should be performed when necessary to reduce the microstructure gradient and residual stress in the girth weld, especially in the weld center and heat-affected zone, thereby effectively suppressing the risk of SCC.