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Eight key points to consider when welding stainless steel: 1. Chromium-containing stainless steel possesses certain resistance to corrosion (oxidizing acids, organic acids, cavitation), as well as heat and wear resistance. It is commonly used in equipment materials for power plants, the chemical industry, petroleum, and other fields. Chromium stainless steel has poor weldability, so attention should be paid to welding procedures, heat treatment conditions, etc. 2. Chromium 13 stainless steel has high post-weld hardening tendency and is prone to cracking. When welding with chromium stainless steel electrodes of the same type (G202, G207), preheating at temperatures above 300°C and post-weld slow cooling at around 700°C are mandatory. If post-weld heat treatment is not possible for the welded parts, chromium-nickel stainless steel electrodes (A107, A207) should be used. 3. Chromium 17 stainless steel: Appropriate amounts of stabilizing elements such as Ti, Nb, and Mo are added to improve corrosion resistance and weldability; its weldability is better than that of chromium 13 stainless steel. When using chromium stainless steel electrodes of the same type (G302, G307), preheating at over 200°C and tempering at around 800°C after welding are required. If the welded parts cannot be heat-treated, chromium-nickel stainless steel electrodes (A107, A207) should be used. 4. When welding chromium-nickel stainless steel, repeated heating leads to the precipitation of carbides, which reduces its corrosion resistance and mechanical properties. 5. Chromium-nickel stainless steel welding electrodes possess excellent corrosion resistance and oxidation resistance, and are widely used in the chemical, fertilizer, petroleum, and medical equipment manufacturing industries. 6. The fluxes for chromium-nickel stainless steel come in titanium-calcium type and low-hydrogen type. The titanocalcium type can be used for both AC and DC welding, but the penetration depth is shallower during AC welding, and redening occurs easily; therefore, a DC power supply should be used whenever possible. Diameters of 4.0 and below can be used for all-position welding, while those of 5.0 and above are suitable for flat welding and fillet welding. 7. Welding electrodes should be kept dry during use; titanium-calcium type electrodes need to be dried at 150°C for 1 hour, while low-hydrogen type electrodes should be dried at 200–250°C for 1 hour. Drying should not be repeated multiple times, as this can cause the coating on the electrodes to crack and peel off. This is done to prevent the coating from getting contaminated with oil or other impurities, which could increase the carbon content in the weld and affect the quality of the welded parts. 8. To prevent interpass corrosion caused by heating, the welding current should not be too high; it should be about 20% lower than that used with carbon steel electrodes. The arc length should also be kept short, rapid cooling between passes is necessary, and narrow weld beads are preferred. 9 major issues in welding stainless steel 1. What are stainless steel and acid-resistant steel? Answer: The content of the main element \"chromium\" in metal materials (with other elements such as nickel and molybdenum also added) enables the steel to be in a passivated state, granting it rust-resistant properties. Acid-resistant steel refers to steel that is resistant to corrosion in strongly corrosive media such as acids, alkalis, and salts. 2. What is austenitic stainless steel? What are the commonly used grades? Answer: Austenitic stainless steels are the most widely used and come in the greatest variety. For example: <1>18–8 series: 0Cr19Ni9 (304), 0Cr18Ni8 (308); <2>18–12 series: 00Cr18Ni12Mo2Ti (316L); <3>25–13 series: 0Cr25Ni13 (309); <4>25–20 series: 0Cr25Ni20, etc. 3. Why is welding stainless steel considered to be a task with certain difficulties? Answer: The main process challenges are: <1> Stainless steel is highly sensitive to heat; if it remains in the temperature range of 450–850°C for an extended period, the corrosion resistance of the welds and heat-affected areas suffers significantly. 〈2〉 Thermal cracking is likely to occur. 〈3〉Poor protection, severe high-temperature oxidation. 〈4〉It has a high linear expansion coefficient, resulting in significant welding deformation. 4. Why are effective process measures necessary when welding austenitic stainless steel? Answer: Common process measures include: <1> The welding materials must be carefully selected based on the chemical composition of the base material. 〈2〉Low current., fast welding ； Low linear energy, reducing heat input. 〈3〉Thin-diameter welding wires and electrodes, no oscillation, multi-layer multi-pass welding. 〈4〉Forceful cooling of the weld and heat-affected zone to reduce the residence time at 450–850°C. <5> Argon shielding on the back side of TIG welds. 〈6〉Welds in contact with corrosive media are welded last. 〈7〉Passivation treatment of welds and heat-affected zones. 5. Why are 25–13 series wires and electrodes chosen for welding austenitic stainless steels to carbon steel and low-alloy steel (intermetallic welding)? Answer: When welding dissimilar steel joints that consist of austenitic stainless steel joined to carbon steel or low-alloy steel, the weld metal must be produced using wires from the 25–13 series (309, 309L) as well as electrodes such as Ö312 and Ö307. If other stainless steel welding materials are used, martensite formation at the fusion line on the carbon steel or low-alloy steel side will lead to cold cracks. 6. Why is a shielding gas mixture of 98% Ar + 2% O2 used for solid stainless steel welding wires? Answer: When using solid stainless steel welding wire in MIG welding with pure argon gas shielding, the surface tension of the molten pool is high, resulting in poor weld formation and a \"humpbacked\" shape for the weld. Adding 1–2% oxygen reduces the surface tension of the molten pool, resulting in a smooth and aesthetically pleasing weld shape. 7. Why does the surface of MIG welds made with solid stainless steel wire turn black? Answer: The MIG welding speed using solid stainless steel wire is relatively fast (30–60 cm/min). By this time, the shielding gas nozzle has already reached the front end of the molten pool. The weld remains at a red-hot temperature; it gets oxidized by air, resulting in the formation of oxides on its surface and a blackened appearance of the weld. The pickling and passivation method can remove the black scale and restore the original surface color of stainless steel. 8. Why does solid stainless steel welding wire require a pulsed power supply in order to achieve a jet transition and spatter-free welding? Answer: When using solid stainless steel wire for MIG welding, with a wire diameter of φ1.2, flow transition can be achieved only when the current I is ≥ 260–280A ； If the value is below this, the droplet transition is of the short-circuit type, resulting in significant spattering; such a condition generally makes it unsuitable for use. Only by using a MIG power supply with pulses, where the pulse current is greater than 300A, can a pulsed droplet transfer be achieved at welding currents of 80–260A, resulting in spatter-free welding. 9. Why is CO2 gas used for shielding cored stainless steel welding wires? No power supply with pulses needed? Answer: The commonly used cored stainless steel welding wires at present (such as 308, 309, etc.) have flux formulations designed to facilitate welding electro-chemical reactions under CO2 gas shielding; therefore, they cannot be used for MAG or MIG welding ； Pulsed arc welding power sources cannot be used.