Abstract
High Strength Low Alloy (HSLA) steels are the materials of choice in pipeline construction with the API X70 grade as the steel for the majority of pipeline networks constructed during the late 20th and early this century. This paper reports on the influence of Post-Weld Heat Treatment (PWHT) on the reduction of residual stresses, resulting changes in the microstructure, and mechanical properties of a multi-pass, X70 HSLA steel, weld joints made by a combined Modified Short Arc Welding (MSAW) and Flux Cored Arc Welding (FCAW) processes. Neutron diffraction results highlighted high magnitude of tensile residual stresses, in excess of yield strength of both parent and weld metal, in the as-welded specimen (650 MPa), which were decreased substantially as a result of applying PWHT (144 MPa). Detailed microstructural studies are reported to confirm the phase transformation during PWHT and its interrelationship with mechanical properties. Transmission Electron Microscopy (TEM) analysis showed polygonization and formation of sub-grains in the PWHT specimen which justifies the reduction of residual stress in the heat-treated weld joints. Furthermore, microstructural changes due to PWHT justify the improvement in ductility (increase in the elongations) with a slight reduction in yield and tensile strength for the PWHT weld joint.
Highlights
Amongst a range of welding techniques employed in manufacturing complex structures, the combination of MSAW and Flux Cored Arc Welding (FCAW) processes are claimed to render a more economical advantage through reduction in welding time and less demand on welder skill [1]
This paper reports on the influence of Post-Weld Heat Treatment (PWHT) on the reduction of residual stresses, resulting changes in the microstructure, and mechanical properties of a multi-pass, X70 High Strength Low Alloy (HSLA) steel, weld joints made by a combined Modified Short Arc
It is worth mentioning that the Charpy, tensile and hardness samples were taken from both samples after neutron diffraction residual stress measurements
Summary
Amongst a range of welding techniques employed in manufacturing complex structures, the combination of MSAW and FCAW processes are claimed to render a more economical advantage through reduction in welding time and less demand on welder skill [1]. The MSAW brings about low-spatter droplet transfer, high process stability, and the ability to deposit weldments with greater control of the heat input [12] Despite such technological advantages, high level of tensile residual stresses, greater than SMAW, were found to be generated during the multi-pass welding with combined MSAW and FCAW for HSLA steel welds [6,7,13]. The current study, as part of a major study on the welding of X70 line pipe steel, was carried out to firstly explore the resulted residual stresses generated for this MSAW-FCAW combined welding route and characterize in detail the effect of the applied PWHT procedure on the changes in the residual strains/stresses and the microstructural and mechanical properties of X70 HSLA multi-pass welded joints
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