Objective Girth welds are critical joints in long-distance liquid ammonia pipelines. However, complex microstructures and residual stress distributions render these welds highly susceptible to Stress Corrosion Cracking (SCC) in impure liquid ammonia environments, posing a severe threat to pipeline integrity. Consequently, it is imperative to determine the SCC susceptibility of X52 seamless steel pipe girth weld microstructures under liquid ammonia service conditions.

Methods The base metal, heat-affected zone, and weld center of X52 seamless steel pipe girth welds were selected as the primary research subjects. Slow Strain Rate Tensile (SSRT) tests were conducted in air and two distinct impure liquid ammonia environments. Through metallographic observation, fracture surface scanning electron microscopy, electron backscatter diffraction analysis, and surface residual stress testing, the stress corrosion susceptibility indices were systematically evaluated based on tensile strength and elongation after fracture. The results were correlated with microstructural characteristics, and the mechanisms underlying variations of SCC susceptibility in liquid ammonia environments were elucidated.

Results SSRT test results demonstrated that susceptibility indices based on tensile strength and elongation after fracture followed a consistent trend, with elongation after fracture exhibiting higher susceptibility to SCC in liquid ammonia environments. The SCC susceptibility of the girth weld regions decreased in the following order: weld center ＞ heat-affected zone ＞ base metal. Microstructural analysis indicated that the weld center exhibited grain refinement, non-uniform size distribution, the highest kernel average misorientation, and a peak surface residual tensile stress of 54 MPa. These characteristics increased the electrochemical activity of the weld center. Under the coupled influence of impure liquid ammonia and tensile stress, pitting corrosion initiated preferentially, inducing brittle cleavage fracture and distinct brittle fracture surface features. Conversely, the base metal exhibited superior SCC resistance, characterized by a uniform microstructure and low residual stress of only 5 MPa.

Conclusion The SCC susceptibility of X52 seamless steel pipe girth welds in liquid ammonia environments is primarily governed by microstructural inhomogeneity and subsequent residual stress concentration. To ensure the operational safety of liquid ammonia pipelines, welding procedures must be strictly controlled during fabrication and construction. Post-weld heat treatment should be performed when necessary to reduce the microstructural gradient and residual stress—particularly within the weld center and heat-affected zone—thereby effectively mitigating SCC risk.