Pulsed TIG Welding Of Super Ferritic Stainless Steel

Super ferritic stainless steel is a kind of alloy metal designed for environment with chloride which may cause pitting corrosion, crevice corrosion and stress corrosion cracking. It is characterized by good heat transfer performance, high corrosion resistance to chlorine ion and sea creatures and low linear expansion coefficient etc. Now super ferritic stainless steel is successfully employed in manufacturing a variety of heat-exchange equipments using water as heat transfer medium including sea water. However joint embrittlement, corrosion resistance reduction and coarse grain would happen after welding prossessing. The microstructural features, mechanical properties and corrosion resistance have been investigated during pulsed TIG welding of 30Cr-4Mo and 26Cr-3.5Mo ferritic stainless steels with 0.9mm thickness in this paper.Study results indicated that: Two type ferritic stainless steels have similar optimized pulse TIG welding parameters which were gained by single factor experiment. Increase the heat imput such as increase peak current, duty cycle and reduce welding speed will obtain a larger weld bead. Increase frequency will get finer grain in weld metal. The optimized parameters were 120 A peak current, 5~7Hz pulse frequency, 45~55% duty cycle and 6mm/s welding speed.After welding, σ and χ phase were not observed in heat-affected zone. In this area there are a lot of pits which may have separate and gathered carbonitride particles containing Ti and Nb. In 30Cr-4Mo weld metal, σ phase, χ phase and a large number of tiny Ti and Nb carbonitrides were found. Rod like and aggregation of small χ phase particles as well as Ti and Nb carbonitrides were observed in 26Cr-3.5Mo weld metal.Increase welding input of 30Cr-4Mo decreased the tensile strength of weld joint, but augmented the elongation percentage and facture position transferred from base metal or heat-affected zone to heat-affected zone or weld metal. When facture located at base metal the morphology is river pattern whereas it is dimple fracture when located at heat-affected zone or weld metal. As for 26Cr-3.5Mo when heat input is large the tensile strength sharp decreased, as the heat input increased, elongation percentage decreased and the facture location transferred from base metal to weld metal.The weld joints of two type super ferritic stainless steels under optimized parameter processing have superior pitting resistance through the analysis of cyclic anodic polarization.