| The balanced ratio of ferrite(α)to austenite(y)in Super Duplex Stainless Steel(SDSS)2507 provides excellent mechanical properties and corrosion resistance,making it an ideal material for marine structural applications.It has been widely used in shipbuilding,gas pipelines,and other applications.The most popular method of welding SDSS 2507 plates at the moment is gas tungsten arc welding.The thick plate of a high efficiency,high quality welding process presented a challenge due to its lower welding efficiency,large heat input,ease of changing the two-phase microstructure of the joints,precipitation of a large number of harmful intermetallic compounds,such as σ phase,χ phase,secondary austenite(γ2),Cr2N,etc.,serious damage to the mechanical properties and corrosion resistance of the joints.To solve the problem of high efficiency and high quality SDSS 2507 plate welding at the same time,this paper conducts welding experiments using key-hole plasma arc welding(K-PAW)and rotating arc narrow gap argon gas tungsten arc welding(NG-GTAW)to study the microstructure and properties of joints under different welding thermal cycle conditions,which has important theoretical significance and engineering practical value for SDSS 2507 high efficiency welding production.K-PAW is developed on the basis of the common tungsten arc welding technology.The arc has a higher stiffness and energy density after forced contraction,which enables single-sided welding and dual-side forming of medium-thick plates,and its unique perforation process leads to dramatic changes in the thermal circulation and microstructure of the liquid metal.SDSS 2507 with a thickness of 10 mm was joined using K-PAW and the microstructure and properties of the joints were investigated.The weld zone(WM)in the joints was found to be composed of equiaxed austenite and columnar ferrite,with a WM austenite content of about 67%and an average grain size of about 11.54 μm;the HAZ formed massive ferrite and different forms of austenite,such as grain boundary austenite,intracrystalline austenite and widmanstatten austenite,with a ferrite content of 68%,under a large heat input.The higher austenite content in the weld reduces the hardness of the WM;the large amount of massive ferrite and Cr2N in the HAZ increasing the microhardness of the joints.The room temperature impact toughness value of 175 J/cm2 for the joints is higher than that of laser and electron beam welding,but the crack expansion path provided by oxide inclusions deteriorates the impact toughness of the joints,resulting in a lower impact toughness for WM than the base material.The smaller grain size makes WM’s pitting resistance comparable to BM;HAZ deteriorates its pitting resistance under the influence of γ2,Cr2N and coarse ferrite grains.The large amount of oxide inclusions in the joints,which are shed during grinding to form pits,accelerate the corrosion of the joints under stress and corrosive media conditions,making them less resistant to stress corrosion than BM.The high heat input of K-PAW results in a large HAZ range,and the two-phase ratio is severely unbalanced.Therefore,the rotating arc NG-GTAW process is adopted,which can achieve multi-layer single pass welding under narrow gap conditions and significantly reduce heat input.The medium-thick plate SDSS 2507 was welded and analyzed for microstructure,the ratio of the two phases in the joints was close to 1:1 and the average grain size was approximately 7.33 μm.At lower welding heat input,the HAZ range is only 150 μm and the proportion of the two phases is relatively balanced.The rotary arc prevents localized overheating of the joints reducing the precipitation of large amounts of harmful intermetallic compounds and by replacing the normal V-bevel with a narrow gap,the filler metal is significantly reduced resulting in higher welding efficiency.The higher Ni content and lower heat input in the rotary arc NG-GTAW process results in a joints with significantly better mechanical properties than the base material.At the same time,the smaller grain size and higher Ni content in the weld increases the stability of the steel and improves the pitting resistance and stress corrosion cracking of the joints.The results indicate that both methods are suitable for welding SDSS 2507 medium thick plates.A comprehensive comparison between K-PAW and NG-GTAW welding methods reveals that K-PAW has a higher welding efficiency,and the two-phase ratio and comprehensive performance of the joints meet industrial usage standards;The welding quality of NG-GTAW is higher,with a two-phase ratio close to 1:1,and the joints has excellent microstructure and comprehensive properties. |
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