Microstructure, microhardness and fracture driving force are investigated experimentally and numerically for the heat affected zone of girth welds of X80 pipes in the paper. A soft region exists in the heat affected zone of the girth welds, based on the experiment results of microstructure and microhardness distribution. By simplifying the girth welds with mechanical heterogeneity into base metal, weld metal and heat affected zone, the fracture driving force of cracked girth welds have been studied numerically with variation of weld matching property, weld width and soft zone in heat affected zone. The results show that the fracture driving force decreases with increase of weld strength, and the softened region in the undermatched weldment has stronger effects on the fracture driving force of the girth weld than that in the overmatched weldment.
With the strength increasing of the linepipe used for oil and gas transmition, there are some positive effects for energy saving, but the requirement of the linepipes reliability is increasing as well. Not only good strength and toughness of the pipeline steels are required, such as anti-fatigue performance, fracture performance, corrosion resistance, and so on, but also good weldability is required. Due to the complex geological structure of the building area and imperfect weld detection and repair, weld and HAZ of the girth weld become to be defect sensitive area where caused pipeline failure extremely easily. The average accident rate was 0.0004 times/km.a, and research shows that pipe welding defects was one of the main reasons which caused failure [3]. Pipeline steel welding joint is an area where the microstructure and the mechanical properties are highly non-uniform, and its toughness and fracture behavior is extremely complex. Weld area often becomes the weak area or failure occurred place of whole structure.
