The growing demand of the transport of gas and oil under severe conditions, such as in deepwater and onshore in cold climates, requires heavy wall thickness line pipes with both high strength and excellent toughness at low temperature in order to reduce the cost of gas transportation and constructions. In particular, when developing and efficiently manufacturing high strength line pipes with heavy wall thickness, achieving excellent heat affected zone (HAZ) toughness of seam weld is one of the key subjects. In general, steel plate for heavy wall thickness line pipes relatively contains large amount of alloy elements to secure required mechanical properties, whereas addition of alloy elements cause deterioration of HAZ toughness of seam weld. This paper deals with the method of improving HAZ toughness in UOE pipe seam weld. Relationship between microstructure and toughness of simulated HAZ was investigated, and volume fraction of martensite-austenite constituent was reduced by optimization of chemical composition of steel plates. Moreover, low heat input double submerged arc welding (DSAW) process with using welding wires with smaller diameter was newly developed to enhance HAZ toughness. With this new DSAW process, decrease of HAZ width and reduction of austenite grain size were also achieved. Based on the above knowledge, the manufacturing condition was optimized and heavy wall thickness X70 UOE pipe with excellent toughness was successfully developed.

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