318 Duplex Steel Welding Procedures and Precautions

The information provided by the original poster is not clear; are these the performance characteristics of SS31803 duplex stainless steel? (1) An excellent duplex stainless steel achieved through proper control of chemical composition and heat treatment processes – offering the good toughness and weldability of austenite, as well as high strength and resistance to chloride stress corrosion associated with ferrite. ⑵ There are four main categories of stainless steel: a. Ferritic stainless steel, b. Austenitic stainless steel, c. Martensitic stainless steel, and d. Duplex stainless steel, with duplex stainless steel being an important sub-category among them. ⑶ Its resistance to stress corrosion is far superior to that of 18-8 austenitic stainless steels, and it also exhibits good resistance to pitting corrosion, crevice corrosion, and intergranular corrosion. The strength of duplex steels is approximately twice that of conventional austenitic stainless steels, yet their Ni content is only half that of the latter. ⑷ The welding of duplex steel materials, due to its dual-phase structure of ferrite and austenite, exhibits the welding characteristics of both ferritic stainless steels and austenitic stainless steels. Inappropriate welding materials and processes can lead to a deterioration in the properties of the welded joints made from duplex steel. Welding characteristics of duplex stainless steel ⑴ Main problems in welding duplex steel and solutions to them. Existing problems, consequences they cause, and solutions: Welding leads to an increase in the recrystallization temperature, which results in the growth of ferrite grains; this causes a sharp decline in the material’s plasticity and toughness. a. Use low current and fast welding speeds to reduce the energy input and thus minimize the size of the heat-affected zone. b. Strictly control the inter-layer temperature to <100°C; continuous welding of multiple layers is strictly prohibited to reduce the time that the weld structure spends in the medium-temperature zone. Prolonged exposure at 450℃～850℃ leads to the precipitation of the σ phase along grain boundaries. Welded joints exhibit a tendency to be brittle, and their resistance to local corrosion decreases. Prolonged exposure at 350℃ to 525℃ will result in 475 brittleness. Welded joints are highly brittle, with deteriorated mechanical properties. ⑵ Tungsten inert gas welding: a. Before welding, it is necessary to pay attention to cleaning the weld groove, and wipe the welding area with acetone. b. Use a low current for fast welding; the heat input should be controlled between 0.2 and 2.5 KJ/mm (the parameters will remain within this range as long as the welding procedures are followed strictly). The interlayer temperature should not exceed 100°C. For welds with multiple layers, the arc energy used for the filler passes must not be higher than that used during the root pass. The purpose is to achieve a balance between the austenite content, which is highest in the first weld pass, and the ferrite content, while minimizing secondary crystallization or secondary austenitization due to reheating in subsequent weld passes. The heat-affected zone of the outermost weld, lacking the heat treatment effect of the outer weld, finds it difficult to achieve a balanced duplex microstructure; therefore, when formulating the welding procedure, efforts should be made to position the final weld on the surface that is not in contact with the working medium. Arc starting for circumferential welds should be carried out within the weld bead, while arc starting and ending for longitudinal welds must be done on the arc starting plate and the arc stopping plate. c. Avoid excessive lateral movement of the welding wire and a too wide molten pool, in order to prevent high currents and high residual stresses. d. Regardless of the type of joint used, backside protection with shielding gas is necessary when performing tungsten inert gas welding. Since the density of the shielding gas is lower than that of air, it should enter from the bottom and exit from the top. e. When using TIG welding, start gas supply before initiating the arc, and continue supplying gas for at least 5 seconds after the arc is extinguished. f. When using TIG welding, tack welding should be performed at appropriate lengths and intervals. The root weld pass should not start from the tack weld. To avoid cracks in the root pass caused by the tack weld, the welder should stop welding at the root before the tack weld, and resume welding at the root only after that tack weld has been completely polished off. g. During tack welding and root welding, its back side must be fully protected by argon gas. h. When using tungsten inert gas welding, the welding torch should be kept perpendicular to the workpiece, in order to minimize the entry of air into the shielding gas. i. Welding metals, welding wires, and electrodes should always be kept dry and stored in covered containers. j. Repair welding is not allowed. ⑶ Welding with electrode arc welding: a. Before welding, it is necessary to pay attention to cleaning the weld groove, and wipe the welding area with acetone. b. Use a low current for fast welding; the heat input should be controlled between 0.2 and 2.5 KJ/mm (the parameters will remain within this range as long as the welding procedures are followed strictly). The interlayer temperature should not exceed 100°C. For welds with multiple layers, the arc energy used for the filler passes must not be higher than that used during the root pass. The heat-affected zone of the outermost weld, lacking the heat treatment effect of the outer weld, finds it difficult to achieve a balanced duplex microstructure; therefore, when formulating the welding procedure, efforts should be made to position the final weld on the surface that is not in contact with the working medium. Arc starting for circumferential welds should be carried out within the weld bead, while arc starting and ending for longitudinal welds must be done on the arc starting plate and the arc stopping plate. c. Avoid excessive lateral movement of the welding rod and a too wide molten pool, in order to prevent high current levels and high residual stresses. d. After completing the manual arc welding, wait until the weld has cooled down before removing the flux, so that the weld can cool under the protection of the flux and be prevented from oxidizing. e. When using shielded metal arc welding, spot welding should be carried out at appropriate lengths and intervals. The root weld pass should not start from the tack weld. To avoid cracks in the root pass caused by the tack weld, the welder should stop welding at the root before the tack weld, and resume welding at the root only after that tack weld has been completely polished off. f. Welding rods must always be kept dry and stored in insulated containers; they must not be left lying around carelessly. g. Repair welding is not allowed.

The analysis is excellent; as an inert shielding gas, argon plus nitrogen can be used, with a purity of 99.9999%. :handshake