Microstructure Evolution and Alloying Features of a Developed High Strength and High Toughness Weld Metal Used for Pipeline Steels

Lei, X.; Wu, K.; Wang, H., and Yu, L., 2015. Microstructure evolution and alloying features of a developed high strength and high toughness weld metal used for pipeline steels.The microstructure evolution and continuous cooling transformation curves of the simulated heat-affected zone of a developed high strength and high toughness weld metal was investigated by means of thermal simulation on a Gleeble 3,500. This weld metal was picked up from the welding joint of a high strength and high toughness pipeline steel which was welded with commercial welding materials. Results showed that fine acicular ferrite was the main microstructure during a wide range of cooling rates. Actual welding experiment was also carried out with the heat input of 21 kJ / cm. The yield strength of the weld joint in weld metal is 875 MPa. The average impact toughness are about 104 and 183 J at –30° C in weld metal and the fusion line, respectively. The observed results indicated that the developed weld metal had superior mechanical properties compared to the required properties in API Spec 5L standard of high strength and high toughness pipeline steels X120, which was attributed to low carbon design and suitable alloying.