Stainless steel welding process

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1. Characteristics of stainless steel Stainless steel includes air pollution-resistant stainless steel and acid-resistant steel. According to the main organizational state, it is divided into three categories: martensite, ferrite and austenite. Among them, austenitic stainless steel is the most widely used, accounting for 70% to 80% of the total. Among the stainless steel equipment commonly used in chemical plants, austenitic stainless steel is commonly used (the following stainless steel refers to austenitic stainless steel). Take ICrl8Ni9Ti as an example, it belongs to type 18-8 chromium-nickel austenitic stainless steel. It has good corrosion resistance and heat resistance, the operating temperature can reach 600~700℃, and has high oxidation resistance of 700~900℃. It has good plasticity, but is sensitive to work hardening and has poor cutting performance. 2. Analysis of Stainless Steel Corrosion Types The main quality problems that occur in austenitic stainless steel welding are intergranular corrosion and stress corrosion cracking. Corrosion fatigue, weld corrosion, pitting corrosion and hydrogen embrittlement may also occur to varying degrees. In most cases, corrosion of stainless steel is the result of the coexistence of multiple corrosion types. 1. Intergranular corrosion: When austenitic stainless steel is between 450 and 850°C, crystal grains are prone to precipitate and intergranular corrosion occurs. This corrosion will cause a significant reduction in the mechanical properties of the material. Since intergranular corrosion is not easy to occur, it often causes sudden damage to equipment, so it is very harmful. As a material user, the effective way to prevent intergranular corrosion is to reduce its carbon content. To reduce the carbon content, the material can be heated to 1100°C for solid solution treatment, which can also improve the corrosion resistance of the material and soften it. 2． Stress corrosion cracking refers to the fracture damage of metal materials caused by the combined action of tensile stress and chemical corrosion. The cracks are small, sometimes only one, and usually branched. The sources of stress include external stress (working stress and thermal stress during equipment operation), residual stress (fixed residual stress during welding, cold working and equipment installation), and corrosion product stress. For stress corrosion cracking, residual stress from welding and machining is most important. Material surface conditions also have an impact on stress corrosion cracking. Weld thickening (repeated repair welding) or welding spatter often become indirect causes of stress corrosion cracking, so they should be polished off, and the smoother the better. 3． Corrosion fatigue Corrosion fatigue is the decline in the fatigue resistance of metal materials due to the action of corrosive media. Its cross-sectional characteristics are corrosion products in large areas and roughness in small areas. Corrosion fatigue can have multiple cracks, often originating from a deep pitting area. 4． Weld corrosion Weld corrosion is divided into heat affected zone corrosion and blade (knife edge) corrosion. Corrosion occurs in the heat-affected zone on both sides of the weld of stainless steel weldments. The reason for this corrosion is that the welding process happens to be in the sensitive temperature range (450~850°C), resulting in intergranular corrosion. Blade-shaped (knife-edge) corrosion is characterized by preferential corrosion of metal in a very narrow area close to the fusion line of the weld, while heat-affected zone corrosion is corrosion of the basic metal area that does not melt during cutting or welding under the action of heat. Its location is still some distance away from the weld. Generally, the corrosion resistance of stainless steel welds is worse than that of the base metal. 5. Pitting Corrosion Pitting corrosion is concentrated in relatively deep corrosion on individual small areas of the metal surface. In most cases, the pitting corrosion is relatively small, and cold working will increase the tendency of pitting corrosion. 6. The hydrogen ions in the hydrogen embrittlement solution are reduced to hydrogen in the cathode area of ​​the crack, and under the action of stress, diffuse into the interior of the metal, making the metal there embrittled and the crack prone to expand. As hydrogen continues to be generated and diffuses to the crack tip, the crack continues to develop. 3. Welding process measures From the analysis of stainless steel corrosion types, it can be seen that in addition to being sensitive to temperature (intergranular corrosion, weld corrosion), stress corrosion cracking is a very important cause of stainless steel corrosion. Therefore, welding of stainless steel must take reasonable welding sequence and direction and other process measures to eliminate and reduce residual stress and corrosion. When welding long welds or large structural parts, the welding sequence should be from the middle to both ends or around to disperse the stress. For joints with cross welds on the welding plane, the welding sequence should ensure that defects and excessive stress are not likely to occur at the intersection. When welding panels, short staggered welds should be welded first, and then long straight welds should be welded. Weld the weld with the largest shrinkage first, because when the weld welded first shrinks, it will receive less resistance and the corresponding stress will be smaller. For example, when there are both butt welds and fillet welds on the structure, the butt welds should be welded first and the fillet welds last. Using smaller welding energy can reduce the thermal compression plastic deformation in the welding heating zone, thereby reducing stress. Use a hand hammer to hammer the weld and its surroundings evenly to extend it, thereby reducing internal stress. When welding closed ring welds or other welds with greater rigidity and smaller degrees of freedom, the anti-deformation method can be used to increase the degree of freedom of the weld and thereby reduce stress. Welds should be avoided at locations with maximum stress and stress concentration, and should also be kept away from machined surfaces. Dense or intersecting welds will cause the metal to overheat, enlarge the heat-affected zone, and worsen the structure. Therefore, the distance between two welds is generally ≥100mm. 4. Preparation before welding If it is supplementary welding (repair welding) of online equipment, clean the leakage of the equipment with clean water, and pay special attention to the cleaning of corrosive media. Before repair welding, the leaking weld seam (accumulated weld flash) must be removed with tools and smoothed. For repair welding of container defects, the length of the repair welding shall be ≥100mm, and the size of the repair welding (scarring) using reinforced plates shall be >100mm×100mm. If it is repair welding at the intersection of multiple welds, the size of the reinforcing plate (scar) should be appropriately enlarged to avoid welding at the intersection of welds. Before repairing welding of defects such as cracks that have occurred or high brittleness of the material, use a hand hammer to gently hammer the crack area to eliminate residual stress and promptly detect the crack development trend. Then drill anti-crack holes with a diameter of 5 to 8 mm 10 to 50 mm away from each endpoint in the length direction of the crack (including crack branches), and the depth is the same as the groove grinding depth. Before repair welding, bevel according to the material condition (weld seam). The defective surface of the equipment should be cleaned and decontaminated with acetone and alcohol first. In special cases (such as high-concentration alkali), hydrochloric acid can also be prepared into an acidic solution of 5% to 15% for cleaning, and then washed with a large amount of water. It should be noted that austenitic stainless steel is more sensitive to pickling and is prone to pitting corrosion. Therefore, pickling should be used with caution. 5. Selection of welding rods and welding machines. Selection of welding rods. When welding stainless steel, due to repeated heating, carbides will precipitate, which will reduce the corrosion resistance and mechanical properties. Therefore, welding electrodes should be reasonably selected according to the chemical composition of the workpiece, type of medium, and operating temperature. For 18-8 type chromium-nickel stainless steel, the operating temperature is lower than 300°C. For general structural welding, A102 electrodes can be used. For repair welding after various defects occur, A122 electrodes with better crack and corrosion resistance can be used. In order to prevent the formation of pores during welding, the welding rod needs to be dried. For calcium calcium welding rods, use a temperature of 150°C and bake for 1 to 2 hours. Welding machine selection. Because the penetration depth of AC welding is shallow and the electrode is prone to redness, it is recommended to use a DC welding machine for reverse connection if possible. 6. Welding process At present, when welding austenitic stainless steel, the most commonly used are manual arc welding and hydrogen arc welding. Repair welding generally uses manual arc welding. Here, only the welding process of manual arc welding is introduced. 1. Preheating before welding Preheating before welding can reduce the temperature difference between the metal in the weld and the heat affected zone, and can also slow down the cooling rate after welding, thereby reducing the welding stress. Generally, the preheating temperature is controlled at 250~425℃. 2. Chromium-nickel austenitic stainless steel has good weldability during the welding process. In order to prevent the welded joint from staying too long in the dangerous temperature range (450~850°C) to cause intergranular corrosion, and to prevent the joint from overheating and causing thermal cracks, fast welding and narrow welding passes should be used during welding. During the welding process, arcs must not be struck casually on the weldment. The ground wire and the weldment (workpiece) should be in close contact to avoid operating the surface of the weldment and affecting the corrosion performance. It is best not to swing the welding rod laterally. The weld formed at one time should not exceed 3 times the length of the welding rod. The rod should be transported steadily. The arc should not be too long. The arc crater should be filled when the arc is closed. The welding current should be about 20% lower than when welding low carbon steel, generally calculated as 25 to 35 times that of the electrode wire. When multi-layer welding, the slag must be thoroughly removed after each layer of welding, and the welds must be carefully inspected to ensure there are no defects, and wait until the front and rear welds have cooled down (