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I. Properties of stainless steel Stainless steel includes types that are resistant to atmospheric pollution and those that are resistant to acids. Based on their primary microstructural state, they are classified into three main categories: martensite, ferrite, and austenite. Among these, austenitic stainless steels are 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 also widely employed (all stainless steels mentioned here refer to austenitic stainless steel). Taking ICrl8Ni9Ti as an example, it belongs to the 18-8 type of chromium-nickel austenitic stainless steel. It possesses good corrosion and heat resistance, with a usable temperature range of 600–700°C; it also has high oxidation resistance at temperatures of 700–900°C. It has good plasticity, but is sensitive to work hardening, and its machinability is very poor. II. Analysis of Stainless Steel Corrosion Types The main quality issues that occur during the welding of austenitic stainless steel are intergranular corrosion and stress corrosion cracking; corrosion fatigue, weld corrosion, pitting, and hydrogen embrittlement can also occur to varying degrees. In most cases, corrosion of stainless steel is the result of the combined action of multiple types of corrosion. 1. Intergranular corrosion: Austenitic stainless steels are prone to grain separation and intergranular corrosion at temperatures between 450 and 850°C. This type of corrosion leads to a significant reduction in the mechanical properties of the material. Since intergranular corrosion occurs suddenly, it can cause sudden failure of equipment, making it a serious hazard. As the material user, an effective way to prevent intergranular corrosion is to reduce its carbon content; this can be achieved by heating the material to 1100°C for solution treatment, which not only improves its corrosion resistance but also softens it. 2. Stress corrosion cracking: A type of fracture failure that occurs in metal materials under the combined effect of tensile stress and chemical corrosion. The cracks are small; sometimes there is only one crack, but they often branch out. The sources of stress include applied stresses (operational stresses and thermal stresses during equipment operation), residual stresses (fixed residual stresses from welding, cold working, and equipment installation), as well as stress caused by corrosion products. For stress corrosion cracking, the stresses remaining from welding and processing are the most important. The surface condition of the material also has an impact on stress corrosion cracking. Weld thickening (resulting from repeated welding) and welding spatter often serve as indirect causes of stress corrosion cracking; therefore, these should be removed, with a smoother surface being preferable. 3. Corrosion fatigue: Corrosion fatigue occurs due to the action of corrosive agents, which reduces the fatigue resistance of metal materials. Its characteristic is the presence of corrosion products over large areas, along with roughness in smaller areas. Corrosion fatigue can involve multiple cracks, which usually originate from a deep pitting area. 4. Weld corrosion Weld corrosion is divided into heat-affected zone corrosion and kerf corrosion. Corrosion occurs in the heat-affected zones on either side of the welds in stainless steel welded parts. This corrosion is caused by the fact that during welding, the material is within a sensitive temperature range (450–850°C), which leads to intergranular corrosion. The characteristic of edge (blade) corrosion is the preferential corrosion of metal in a very narrow area right adjacent to the weld fusion line, whereas heat-affected zone corrosion refers to the corrosion of the base metal areas that do not melt during cutting or welding, under the influence of heat; this type of corrosion occurs at a distance from the weld. Generally, the corrosion resistance of stainless steel welds is inferior to that of the base metal. 5. Pitting Pitting is a type of corrosion that occurs in deep areas on specific small regions of the metal surface. In most cases, pitting is relatively minor, but cold working increases the tendency for pitting to occur. 6. Hydrogen embrittlement: Hydrogen ions in the solution are reduced to hydrogen at the cathodic region of the crack. Under stress, this hydrogen diffuses into the metal, causing it to become brittle and making it easier for the crack to propagate. As more hydrogen is generated and diffuses to the crack tip, the crack continues to extend forward. III. Welding process measures It can be seen from the analysis of the types of corrosion in stainless steel that, in addition to being sensitive to temperature (intergranular corrosion, weld corrosion), stress corrosion cracking is a very important cause of corrosion in stainless steel. Therefore, when welding stainless steel, appropriate welding sequences and directions, as well as other technical measures, must be adopted to eliminate and reduce residual stresses and corrosion. When welding long welds or large structural components, the welding sequence should proceed from the center toward the ends or around the periphery to distribute stress. For joints with intersecting welds on the welding plane, the welding sequence should be such that defects and excessive stress do not occur at the intersection points. When welding joint plates, short welds offset from each other should be welded first, followed by long straight welds. Weld joints with a large shrinkage should be welded first, as when such joints shrink, they encounter less resistance and thus experience lower stress. If a structure contains both butt welds and fillet welds, the butt welds should be welded first, followed by the fillet welds. Using a lower welding energy can reduce the thermocompressible deformation in the welding heat-affected zone, thereby lowering stress. Use a hammer to strike the weld and its surrounding area evenly in order to stretch it, thereby reducing internal stress. The counter-deformation method can be employed when welding closed circular welds or other welds with high rigidity and few degrees of freedom; by using this method, the degrees of freedom of the weld are increased, thereby reducing stress. Welds should be avoided as much as possible in areas with maximum stress and stress concentrations, as well as on the surfaces that have been machined. A high density of welds or their intersection can cause the metal to overheat, enlarge the heat-affected zone, and degrade its structure; therefore, the distance between two welds should generally be ≥100 mm. IV. Preparations before welding If it is a repair weld on an existing device, clean the leaking area of the device with clean water, paying special attention to removing any corrosive substances. Before performing patch welding, the weld at the leak site (the accumulated weld beads) must be removed with tools and polished smooth. For patch welding container defects, when the patch welding length is ≥ 100 mm, the size of the reinforcement plate used for patch welding (to create a patch) should be > 100 mm × 100 mm. When performing patch welding at the intersections of multiple welds, the size of the reinforcement plate (scar) should be increased appropriately to avoid welding at those intersection points. Before performing patch welding on defects such as cracks or high material brittleness, gently tap the cracked area with a hammer to help relieve residual stresses and detect the trend of crack progression. Then, drill stop-crack holes with a diameter of 5–8 mm at positions 10–50 mm away from each end of the crack in the direction of its length (including any branchings), with a depth equal to that of the groove grinding. Before patch welding, prepare bevels according to the material conditions (weld seam). The surface of equipment defects should first be cleaned of dirt using ** and alcohol; in special cases (such as high-concentration alkalis), hydrochloric acid can also be used to prepare an acidic solution at a concentration of 5% to 15% for cleaning, after which it should be rinsed thoroughly with plenty of water. It should be noted that austenitic stainless steel is quite sensitive to pickling and prone to pitting corrosion; therefore, pickling should be used with caution. V. Selection of electrodes and welding machines Selection of electrodes. When welding stainless steel, repeated heating leads to the formation of carbides, which reduces its corrosion resistance and mechanical properties. Therefore, when welding, it is necessary to select the appropriate welding electrode based on factors such as the chemical composition of the workpiece, the type of environment in which it will operate, and the working temperature. For 18-8 chromium-nickel stainless steel, when the working temperature is below 300°C and it is used for general structural welding, the A102 welding electrode can be used. For patch welding after various defects occur, welding rod A122, which offers good crack resistance and corrosion resistance, can be used. To prevent porosity during welding, the electrodes must be dried; for calcium-tin type electrodes, a temperature of 150°C is used for drying for 1–2 hours. Selection of welding machine. Since the penetration depth in AC welding is relatively shallow and the electrode tends to become red-hot, it is advisable to use a DC welder with reverse polarity whenever possible. VI. Welding Processes At present, the most commonly used methods for welding austenitic stainless steel are manual arc welding and hydrogen arc welding; manual arc welding is generally used for repair welding. Here, only the welding process of manual arc welding will be discussed. 1. Preheating before welding: Preheating before welding helps to reduce the temperature difference between the weld metal and the heat-affected zone. It also slows down the cooling rate after welding, thereby reducing welding stresses. The preheating temperature is generally maintained between 250 and 425°C. 2. Welding process: Chromium-nickel austenitic stainless steels have good weldability. To prevent intergranular corrosion that may occur if the welded joint remains in the hazardous temperature range of 450–850°C for too long, and to avoid thermal cracking due to overheating of the joint, fast welding speeds and narrow weld beads should be used during welding. During welding, arcing should not be initiated randomly on the welded piece; the ground wire must make tight contact with the welded piece (workpiece) to avoid damaging its surface and affecting its corrosion resistance. It is best not to move the welding rod laterally; the length of the weld formed in one pass should not exceed 3 times the length of the path taken by the welding rod. The movement of the rod must be steady, the arc should not be too long, and the arc pit should be filled when ending the weld. The welding current should be about 20% lower than that used for welding low-carbon steel; it is generally calculated as 25–35 times the current required for electrode feeding. During multi-layer welding, after each layer is welded, the slag must be thoroughly removed, and the weld must be carefully inspected to ensure there are no defects; furthermore, it is necessary to wait until the preceding and following welds have cooled down (