Abstract: Analyzed here in the essay are the teeh nological characteristics of the final welding process associated with a 1.5 spiral space between the internal and external welding points.Based on chemical composition,carbon equivalent and hardness of the steel coil,the welding process parameters are designed.And the mechanical properties,macro morphology and micro structure,and hardness of the weld seam of the pipe as welded are compared.The test result indicates that there is little change with type of the weld seam metallurgical structure as resulting from the influence by internal weld pre-heating and welding heat cycle.However,it is observed that there are some changes in shapes,sizes and quantity of different kinds of structures as well as sizes of the original austenite grain.The Charpy impact of the weld seam with a 1.5 spiral space between the internal and the external welding points is about 30 J higher than that with a 0.5 spiral space.And in particular,the influence on hardness is rather obvious,having met the specified requirement that weld seam hardness of the X70M HSAW pipe should be less than 250 HV 10.