Abstract
Welded joints of poor welding surface quality are sensitive to stress concentrations, affecting both the tensile strength of workpieces and the fluidity of liquids and gases in pressure and liquid containers. Orthogonal experiments involving the laser welding of 1-mm-thick duplex stainless steel sheets were conducted using different electric current, pulse width and frequency values in order to analyse the effect of welding properties on the surface characteristics of the welded joints. Rapid judgement regarding the welded joint properties was made based on the observed welding surface quality. The results show that an even phase proportion and grain refinement are not necessarily guaranteed to provide good welding surface quality. A satisfactory welding surface quality characterised by a smaller spot pitch or spot pitch difference, smaller weld width, reduced surface roughness and valley depth of surface waviness implies better welded joint mechanical characteristics and a more even microstructure. The specimen with the most suitable welding parameters and the greatest heat input can reach the lowest volume fraction of ferrite phase of 42.5% and the highest tensile strength of 848 MPa, and its surface quality is the best.
Highlights
2205 duplex stainless steels (DSS), which contain a generally equal volume fraction of ferrite and austenite, are widely used in undersea pipelines carrying petroleum, chemicals or other highly corrosive substances due to their higher strength than austenitic stainless steels, better resistance to chloride stress corrosion and lower embrittlement tendency after welding.[1,2,3,4]Laser welding is a highly efficient material connection technology that has only a small impact on workpiece deformation and the heat-affected zone (HAZ)
Proper welding parameter values ensure sufficient heat absorption and the full melting of the material, which is accompanied by an increased depth of fusion and higher tensile strength
It is obvious that the weld defects caused by improper welding parameters, such as lack of penetration and surface burn, are the main reasons for the decrease of tensile strength of the weldments
Summary
2205 duplex stainless steels (DSS), which contain a generally equal volume fraction of ferrite and austenite, are widely used in undersea pipelines carrying petroleum, chemicals or other highly corrosive substances due to their higher strength than austenitic stainless steels, better resistance to chloride stress corrosion and lower embrittlement tendency after welding.[1,2,3,4]Laser welding is a highly efficient material connection technology that has only a small impact on workpiece deformation and the heat-affected zone (HAZ). Advances in Mechanical Engineering microstructure and alloying elements that take place during laser welding, with the microstructure of laserwelded DSS very much dependent on heat input, cooling rate, composition and other welding parameters.[5] In addition, weld spot overlap type and the cooling rate are considered to determine the solidification zones of the welded joint; by increasing the overlapping distance, the solidification pattern governed by heat extraction to the previous weld spot can form continuously without disruption, leading to a central array of axial grains.[6] Some researchers have concentrated on processing improvement, with the identification of appropriate welding parameters,[7] the use of shielding gas as welding protection[8,9] and other welding processing conditions[10,11] employed to obtain better welds and to maximise DSS material performance Such studies compare welding performance based on the parameters including welding microstructure, ferrite–austenite ratio and corrosion resistance. These latter studies provide a good reference with which to study DSS welding surface quality
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