Manufacturing requirements for austenitic stainless steel pressure vessels

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Austenitic stainless steel: For the manufacturing of austenitic stainless steel pressure vessels, the specific requirements are as follows. Contamination by iron ions, chloride ions, carbon steel, or low-alloy steel: Austenitic stainless steels possess excellent corrosion resistance, which is attributed to their chromium content; when the mass fraction of chromium reaches 10.5%–12%, a dense, protective passivation layer can form on the surface of the alloy ； Once the passivation film is damaged and finds it difficult to repair itself due to low local chromium content, its corrosion resistance decreases or even disappears. If austenitic stainless steel comes into contact with iron ions, the iron ions will adsorb onto the passive film, forming a galvanic cell that triggers galvanic corrosion. If austenitic stainless steel comes into contact with chloride ions, its passivation film is easily damaged by the highly penetrating effect of these ions. Chloride ions cause the formation of numerous tiny corrosion pits on the surface of austenitic stainless steel, and these pits further reduce the material’s corrosion resistance. The specific requirements for manufacturing enterprises regarding methods to control the contamination by iron ions, chloride ions, as well as carbon steel or low-alloy steel in austenitic stainless steel are as follows. 1) There should be dedicated indoor storage areas for austenitic stainless steel sheets, pipes, heads, components, semi-finished products, and finished products ； Moreover, they must not come into contact with rust, carbon steel, low-alloy steel, etc., when stored. 2) There should be a dedicated manufacturing workshop for austenitic stainless steel pressure vessels ； The manufacturing environment should be kept clean and dry, with dust strictly controlled ； Hardened cement floors are recommended for manufacturing workshops, and floor cleaning should be carried out using cleaning equipment that combines cleaning and drying functions. 3) Mechanical damage to the surface of austenitic stainless steel should be avoided during the manufacturing process ； Before welding or thermal cutting, a splash guard coating should be sprayed or applied to the surface of austenitic stainless steel that may be exposed to splashes. 4) The automatic plasma cutter used for cutting austenitic stainless steel plates should be used exclusively; it should not be used to cut carbon steel or low-alloy steel. The water tanks and internal brackets used in cutting should be made of austenitic stainless steel. 5) The carbon steel pressing rollers of the rolling machine should be surface-treated; for rolling machines designed for rolling austenitic stainless steel sheets on a regular basis, their pressing rollers should have a stainless steel overlay applied to their surfaces ； For rolling machines that occasionally roll austenitic stainless steel sheets, the rust on the surface of the pressure rollers should be removed, and padding (such as aluminum foil) should be used to isolate the pressure rollers from the austenitic stainless steel sheets during rolling. 6) Material marking and weld markings shall be made using chlorine-free markers (steel stamp marking is not allowed) ； When installing the carbon steel clamps of the container, shims (such as aluminum foil) should be used; the carbon steel clamps must not come into direct contact with austenitic stainless steel ； The roller frames and rollers in direct contact with the austenitic stainless steel cylinder should be made of polyurethane material (carbon steel and low-alloy steel materials shall not be used) ； Angle grinders should use special stainless steel grinding wheels (ordinary grinding wheels must not be used) ； For root cleaning of weld beads or repair of welds, an angle grinder should be used for grinding (carbon arc gouging should be avoided to prevent carburization) ； For temporary welds on the base material, as well as for alignment tools and temporary lifting lugs that are in direct contact with the base material, the welding surfaces and contact surfaces should be made of austenitic stainless steel (carbon steel and low-alloy steel shall not be used) ； Lifting rigging should consist of sling rigging or stainless steel chain rigging (carbon steel wire rope rigging should be avoided). Weld shrinkage: The thermophysical properties of austenitic stainless steel are a low thermal conductivity and a high linear expansion coefficient. The thermal conductivity of austenitic stainless steel is approximately 31% that of carbon steel, while its linear expansion coefficient is about 1.46 times that of carbon steel ； At 100°C, the thermal conductivity of S30408/06Cr19Ni10 and carbon steel is 16.3 W/mK and 51.8 W/mK respectively. Between 20°C and 100°C, the linear expansion coefficients of S30408/06Cr19Ni10 and carbon steel are 16.84×10-6/K and 11.53×10-6/K respectively. These thermophysical properties are unfavorable for welding, with the direct consequences being large shrinkage and deformation of the weld seam, as well as high welding stresses. To effectively control these weld defects, manufacturing companies should take the following measures in welding. 1) Select an appropriate welding process. For the welding of austenitic stainless steel pressure vessels, TIG welding, submerged arc welding, and small-hole plasma arc welding processes can generally be used. For the longitudinal seam welding of tube section joints, a submerged arc welder equipped with copper gaskets, a cooling water circulation system, and a clamping device is recommended, as it greatly facilitates heat conduction during welding and helps to control deformation. For the welding of the longitudinal and circumferential seams after the tube section is rolled, if conditions permit, the small-orifice plasma arc welding process, which offers many advantages such as low heat input, low welding stress, minimal welding deformation, as well as high quality and efficiency, is undoubtedly the best choice ； When using the convenient submerged arc welding process, special attention must be paid to controlling the heat input during welding; it is advisable to choose welding parameters with low current and high speed, as well as a multi-pass welding technique. 2) For multi-pass welded joints of thick plates, a reasonable welding sequence should be adopted: first carry out inner groove alignment and tack welding, followed by 1–2 passes of filling welding on the inner groove ； Then, root cleaning of the external groove is carried out, followed by multiple layers of filler welding and finish welding for the external groove ； Finally, complete the remaining filler welding and surfacing welding for the inner groove. 3) Select welding parameters with low current and high speed to reduce the heat input during welding. 4) After welding each pass, it is advisable to use compressed air for forced cooling and keep the interpass temperature below 100°C, in order to reduce the time that the weld seam and heat-affected zone remain within the sensitization temperature range and prevent the formation of chromium-deficient areas prone to intergranular corrosion. 5) Select the appropriate welding material. For low-temperature austenitic stainless steel pressure vessels that require controlled ferrite content (such as distillation columns in air separation equipment), stainless steel welding materials with extremely low carbon content and chromium and nickel levels sufficient to ensure an austenitic structure in the weld metal should be selected (such as ER308L welding material with a 0.03% carbon content, 20.0% chromium content, and 10% nickel content). In summary, pressure vessels are special types of equipment that involve safety concerns and pose a high level of risk. Manufacturers of austenitic stainless steel pressure vessels should be familiar with the manufacturing characteristics of such vessels and adopt effective strategies to ensure their manufacturing quality and operational safety. In particular, at every stage of material storage and manufacturing, efforts should be made to prevent austenitic stainless steel from being contaminated by iron ions, chloride ions, carbon steel, or low-alloy steel ； During the welding process, the most appropriate and qualified welding procedure should be employed to control welding defects in austenitic stainless steel ； During the assembly process, geometric dimension tolerances should be controlled, and forced assembly should be avoided.