X80 linepipes are going to be widely used for gas transmission pipelines in China. The welding procedure for girth weld is very critical to the safety of the X80 pipeline. In this paper, different welding procedures are employed to obtain three different mismatched girth welds, with which tests were carried out to analyze effect of mismatch on the properties of girth welds. Plastic deformation and critical defect size (δc) is calculated and discussed for the different mismatched girth welds utilizing both FAD and FEM methods. Overmatched or evenmatched girth weld is recommended for high grade linepipes because overmatched weld has the advantages of improving limit load and enhencing fracture resistance of pipe girth welds. But the increase of weld strength may reduce the resistance of the weld to HIC and SCC, as well as make the field weld operation more difficult. The level of mismatch shouldn’t be too high because too much weld seam strength will not only be a way of waste, but also decrease weld resistance to HIC cracks.

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