Simultaneously enhancing strength and fracture toughness via tailoring the microstructure in X80 girth weld metal

Brittle fracture of girth weld metal is the main failure form of high strength steel oil and gas pipeline facilities, which is related to the deterioration of mechanical properties of weld metal at low temperature, especially the fracture toughness. In order to improve the strength and fracture toughness, tempering and intercritical quenching-tempering treatment are used in ×80 girth weld metal. The results demonstrate that the heat treatment processes not only decomposed M-A constituent, but also increased grain size, reduced dislocation density and increased precipitated phase content. In the intercritical quenching-tempering treatment, small size retained austenite was produced, which was rich in carbon and manganese elements. The tempering and intercritical quenching-tempering treatment process significantly improved the tensile properties and fracture toughness, so that the Rt0.5 increased from 598 MPa to 629 MPa and 648 MPa, and the value of crack tip opening displacement (CTOD) increased from 0.05 mm to 0.12 mm and 0.23 mm, respectively. (Nb,Ti)C precipitated during heat treatment could effectively improve the yield strength, and the increase of precipitation strengthening is much higher than the strength loss caused by grain size increase and dislocation density reduction. Transformation induced plasticity effect of stable retained austenite formed by intercritical quenching-tempering treatment could effectively inhibit crack propagation and improve fracture toughness. The combined effect of retained austenite and nano-precipitated (Nb,Ti)C significantly improves the strength and fracture toughness of the weld metal. Generally, the research results have important scientific value and engineering significance for explaining the strength and toughness mechanism of girth weld and practical pipeline engineering application.