Key points to note for low-temperature pressure vessels

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1 Key points for material control of cryogenic pressure vessels: The quality of cryogenic pressure vessels depends first and foremost on the quality of the steel materials used in such applications. Steel for low temperatures is generally divided into three categories based on the operating temperature: for temperatures above -40°C, low-carbon (with carbon content of less than 0.25%) carbon-manganese steel is commonly used ； -At temperatures ranging from 40 to -196°C, medium-nickel steel and chromium-nickel austenitic steel are commonly used; at temperatures from -196 to -273°C, chromium-nickel austenitic steel is primarily utilized. Commonly used steel grades include 16MnDR, 15MnNiDR, 09Mn2VDR, 09MnNiDR, 06MnNbDR, CF-62, etc., as well as nickel-based low-temperature steels such as 1.5Ni, 2.5Ni, 3.5Ni, 5Ni, and 9Ni steels. The main mode of failure for steel at low temperatures is brittle fracture. When the temperature of steel is below its brittle transition temperature (NDTT), the presence of sufficiently sharp notches or defects can lead to brittle fracture under low stress. This type of fracture failure occurs suddenly and can lead to catastrophic consequences. The impact value Akv of steel at low temperatures reflects its plastic deformation capacity at the notch tip under low temperature conditions, as well as its sensitivity to crack propagation – that is, its low-temperature toughness. When purchasing materials, suppliers that have passed the company’s internal management reviews should be chosen first. To achieve good cold and hot working properties as well as low-temperature toughness, strict specifications and requirements must be set for the selected low-temperature steel in terms of manufacturing methods, chemical composition, internal structure of the steel, and heat treatment conditions, in order to ensure its quality. 1.1 Inspection of low-temperature materials: Re-inspection of low-temperature steel materials upon arrival at the factory is of great significance for ensuring the quality of these materials, and thus for guaranteeing the quality of low-temperature pressure vessels from the outset. The low-temperature impact value of the steel used for low-temperature pressure vessels must be retested prior to processing and manufacturing. Full re-inspection of all parameters is also required for the steel used in low-temperature category III pressure vessels and spherical tanks. That is, retesting of the chemical composition of the material, its mechanical properties at room temperature, its impact resistance at low temperatures, as well as ultrasonic testing of the steel. Re-inspection of steel materials is carried out to ensure precise cutting, thus guaranteeing accurate cylinder formation. For batches of end plates and shell plates, each batch must consist of materials of the same grade, from the same furnace batch, with the same specifications and dimensions, as well as having the same low-temperature impact values; in addition, ultrasonic testing of the steel is required for re-inspection. Re-inspection of steel is carried out in batches, with each batch consisting of steel of the same grade, same furnace number, same size specifications, and same heat treatment process. For welding electrodes used in low-temperature pressure vessels, low-hydrogen basic electrodes with chemical compositions and mechanical properties similar to those of the base material should be selected; for submerged arc welding, basic or neutral fluxes should be used, and their low-temperature impact values must be at least as high as the requirements specified by the standards and the base material. All electrodes used for low-temperature steel shall have their flux moisture content or the diffused hydrogen content in the deposited metal tested as approved. 1.2 Management of cryogenic materials: Establishing strict systems for the issuance, recovery, and on-site management of cryogenic steel materials is an important quality assurance measure for the manufacturing of cryogenic pressure vessels. Especially in manufacturing sites, low-temperature steel and ordinary steel are easily confused; without strict management, this can pose a significant risk of accidents. Low-temperature steels and welding electrodes must be managed by designated personnel in a dedicated storage area. After receiving technical instructions, construction workers and relevant management personnel should be familiar with the markings on low-temperature steels and welding electrodes to prevent them from being confused with other types of steel. Records must be kept for materials entering and leaving the warehouse, and any remaining materials should have their labels updated promptly. Low-temperature steel requires high standards for surface quality; its surface must be protected during storage and transportation, and it should be labeled using color codes. Blanking and cutting shall be carried out under the supervision of material management personnel, and color coding transfer shall be performed promptly. It is not permitted to stamp steel marks on the surface of low-temperature steels for identification purposes. Steel plates and semi-finished products are stored on shelves classified by batch number and specifications, while prefabricated and processed materials are kept using formwork supports. It is strictly prohibited to place low-temperature steel materials, especially welding materials, directly on the ground. The distance between the bracket and the ground as well as the wall should not be less than 300 mm. The setup of the welding rod storage area complies with relevant regulations for welding materials; the temperature inside the storage area must not be lower than 10°C, and the relative humidity must not exceed 60%, with proper records to be kept. Before use, the welding electrodes must be dried at a specified temperature for 2 hours; after drying, they should be placed in a constant-temperature drying oven (100–150°C). 2 Key points for control during manufacturing and installation processes: In addition to the quality of the steel itself, internal stress concentration resulting from manufacturing and installation defects is also an important cause of low-temperature brittle fracture in pressure vessels. Especially at low temperatures, the high peak stresses at stress concentration areas combine with the overall film stress and bending stress of the equipment, resulting in very high stress levels in certain areas of the low-temperature pressure vessels. At these temperatures, the plastic deformation capacity of steel decreases, and the self-limiting properties disappear, which leads to sudden brittle fracture of the steel. Furthermore, during the manufacturing process, when the deformation rate of steel under cold conditions is too high, cold work hardening occurs, resulting in increased strength and hardness, decreased plasticity and toughness, and an elevated brittle transition temperature; if this phenomenon is not eliminated, it increases the risk of brittle failure at low temperatures. Therefore, during the manufacturing and installation of low-temperature pressure vessels, measures should be taken to reduce internal stress levels and cold work hardening. (1) When cutting the steel plates, it is necessary to proceed with precise cutting in strict accordance with the calculated layout dimensions to ensure accurate cylinder formation. Secondary cutting is performed on the end caps and shell plates to ensure accuracy. Appropriate fixtures should be used for clamping during transportation to prevent deformation. Before on-site assembly, the dimensions should be checked again; any plates that exceed the specified limits must be reshaped or rounded to ensure the quality of the assembly. If forced alignment is carried out when the deviation exceeds the limit, significant assembly stress will be generated, which is very detrimental to low-temperature pressure vessels. Therefore, the housings must not be forced together during assembly. (2) To avoid cold hardening of the steel, when forming or straightening the components of low-temperature pressure vessels at room temperature, the cold plastic deformation rate of the steel sheet should be controlled to ≤2%, and hammering for forming or straightening is not permitted. Cold deformation processing must not be carried out when the ambient temperature is below -10°C. For the container cylinder, the minimum bending radius should be controlled, and multiple rolling formings should be carried out. For spherical shell plates, appropriate dies should be used to carry out multiple pressing operations at various points in order to achieve the desired shape. For elliptical end caps, the areas with the most severe cold work hardening are the transition zones and the straight edges where deformation is greatest. To eliminate cold work hardening, the sheet metal should be annealed before or during pressing in order to soften the material and prevent fractures. After cold pressing, the same heat treatment as that of the original base material should also be carried out to restore low-temperature toughness. (3) During alignment, the deviations of each tube section should be distributed evenly; they must not be concentrated on one side or in one section, in order to avoid excessive shape changes and reduce stress concentration. For butt joints with unequal thicknesses, the thicker plate should be thinned and smoothly transitioned to be level with the thinner plate. For low-temperature spherical tanks, the segmented bulk loading method should be used for assembly piece by piece, in order to reduce unevenness and sudden shape changes, and to ensure that parameters such as alignment errors, edge angles, and roundness meet the required standards. (4) No steel stamps shall be used to apply any markings on the container shell or the surfaces of pressure-bearing components; only paint may be used for marking. 3 Key Points for Quality Control of Welding Processes The welding quality of low-temperature pressure vessels is another important factor that affects the manufacturing quality of such vessels. In the welding of low-temperature steel, in addition to preventing welding cracks, it is crucial to ensure the low-temperature toughness of the weld and the heat-affected zone; this constitutes a key aspect of quality control in the welding process for low-temperature steel. 3.1 Measures to prevent cold cracks in low-temperature steel: Cold cracks in low-temperature steel arise from the combined effect of stress, hardened microstructure, and the hydrogen content in the weld metal. Impurities in low-temperature steel materials, oil and rust in the welding area, as well as water vapor in the atmosphere, decompose under the high temperature of the arc to release hydrogen atoms that enter the molten pool. During the cooling of the weld, these hydrogen atoms diffuse and accumulate in the heat-affected zone near the joint line. Under the combined effect of welding stresses and hardened microstructures, cold cracks are very likely to form. Furthermore, certain process defects such as undercutting and lack of penetration also contribute to the formation of cold cracks. In the manufacturing process at our plant, the following measures were taken to prevent the occurrence of cold cracks, with good results: (1) Reducing the sources of hydrogen. Use low-hydrogen or even ultra-low-hydrogen electrodes. Additionally, the electrodes must be thoroughly dried and stored in a portable insulation container for easy access when needed; the temperature inside the container should be maintained at 100–150°C. If the welding rod is left in the air for 4 hours, it must be dried again, and drying should not be done more than once. The area around the welding groove must be thoroughly cleaned with a grinder before welding; rust, oil, moisture, and other contaminants must be removed completely. Welding can only proceed after the area has been inspected and found to be suitable. (2) Select an appropriate weld line energy. If the wire energy is too low, hardened microstructures are likely to form in the heat-affected zone. A high heat input is beneficial for eliminating cold cracks; however, it tends to produce an overheated microstructure, which adversely affects low-temperature toughness. Therefore, the welding heat input should be kept at an appropriate level. (3) The inspection team must strictly check the quality of the workpieces; welding shall not be carried out if defects such as edge misalignment and corner angles exceed the specified limits. Forced assembly and alignment are also strictly prohibited to reduce assembly stress. Furthermore, the welding sequence should be arranged reasonably to minimize the restraint and stress generated during welding. (4) Preheating before welding and slow cooling after welding. Avoid hardened structures and reduce welding stress, and carry out post-weld heat treatment in a timely manner. If post-weld stress-relief heat treatment cannot be carried out in a timely manner, hydrogen removal treatment should be performed immediately after welding, at 300–350°C, for 2–6 hours. However, it should be noted that when treating certain steel grades with a high tendency to temper brittleness, the temperature range associated with this temper brittleness should be avoided. (5) Spot welding and arc starting at non-welded parts of the housing are strictly prohibited. Since the cooling rate at spot welds and arc starting points is lower than the normal welding cooling rate, cold cracks are more likely to occur. Moreover, some arc-starting scars are difficult to detect, making them more likely to pose safety hazards. Crater marks, weld beads, mechanical damage, etc., should be ground away completely. The repaired area should blend smoothly into the base metal, with a grinding slope of at least 1:3. Subsequently, weld repair should be performed and the area ground flush with the base metal. Finally, 100% magnetic particle or dye penetrant testing must be carried out. 3.2 Preventive measures against hot cracking in low-temperature steels. The occurrence of hot cracks is related to stress, impurities, and chemical composition. The combined effects of harmful elements in the weld, such as sulfur, phosphorus, and other elements prone to forming low-melting-point eutectics, lead to severe segregation, thereby causing hot cracks. In particular, when welding 9Ni steel, using an austenitic filler material different from the base metal makes it more prone to hot cracking. Furthermore, the shape of the weld pool formed during welding is related to hot cracks. When the molten pool is deep and narrow, segregation tends to concentrate in the middle of the weld, making heat cracks more likely to form; whereas when the molten pool is shallow and wide, and circular in shape, its resistance to heat cracks is better. To prevent hot cracking, we take the following measures to ensure welding quality: (1) Considering the dilution effect of the base metal on the weld metal, the content of alloys that enhance low-temperature toughness and crack resistance in the welding rod should be higher than that in the base metal; moreover, the sulfur and phosphorus contents should be even lower. Additionally, increasing the alkalinity in the electrodes and fluxes can improve the degree of segregation in the weld and enhance its crack resistance. (2) Preheat appropriately and establish a reasonable welding sequence to reduce the stiffness of the welded joints and lower welding stresses. (3) Use the correct welding parameters. Maintain a circular, shallow, and wide welding pool. (4) Once a crack appears, it must be completely removed by carbon arc gouging or grinding; the crack cannot be eliminated by melting it with subsequent weld passes. 4 Key points for quality control of overall post-weld heat treatment Welding is the thermal processing method that most directly generates residual stresses. Welding residual stress is mainly caused by welding thermal stress and structural restraint stress during the welding process. In addition, the transformation stress generated by phase changes in the weld metal and the base material in the heat-affected zone, along with the additional stresses resulting from processing, shaping, and alignment, combine with the welding residual stresses, thereby complicating the stress state of pressure vessels. The presence of various internal stresses increases the risk of brittle fracture at low temperatures. Performing stress-relief heat treatment on low-temperature pressure vessels after welding not only eliminates residual welding stresses but also removes hydrogen from the weld metal, softens the structure of the heat-affected zone and the work-hardened zone, and improves their low-temperature toughness. This is an important method for ensuring the quality of low-temperature pressure vessels. The main methods for post-weld stress relief heat treatment of low-temperature pressure vessels include: local heat treatment, full furnace heat treatment, segmented furnace heat treatment, and full internal heat treatment. Among them, overall heat treatment yields the best results, with a residual stress reduction rate of over 90%. To ensure the manufacturing quality of post-weld heat treatment, many factors must be taken into account when determining the process parameters for this treatment, including the material of the compressed workpiece and its original microstructural state, the objectives to be achieved through heat treatment, the heating method (heating rate), the dimensions of the workpiece, and the cooling method. Due to differences in various factors, the heat treatment process parameters established vary, but the conditions for determining them remain the same. 4.1 Conditions for determining the heat retention temperature: (l) For high-strength low-temperature steels that have been quenched and tempered, if the heating temperature after welding exceeds the original tempering temperature, the effects of quenching and tempering will be lost, resulting in a decrease in strength and toughness. In particular, nickel-based low-temperature steels are prone to temper brittleness, which reduces their low-temperature toughness. For low-temperature steels containing a high amount of alloying elements, post-weld heat treatment often leads to temper brittleness (i.e., reheat embrittlement), thereby reducing the toughness of the weld and the heat-affected zone. (2) The temperature range for heat retention is determined to be below the phase transition point and above the recrystallization temperature. The lattice distortion and hardened tissue are eliminated through restoration and recrystallization, allowing the residual stresses to be fully relaxed and released, without causing adverse effects such as reheat embrittlement in the base material or the welded area. For quenched and tempered or normalized + tempered steels, the holding temperature should be around 30°C below the tempering temperature, in order to avoid damaging their properties and reducing their toughness and strength. 4.2 Conditions for determining the holding time: (1) A sufficient holding time is necessary to fully relieve the residual stresses in the weld and the base material, thereby improving the properties of the welded area; at the same time, it prevents the creation of new thermal stress gradients while eliminating the residual stresses. (2) When the heating temperature used is lower than the required temperature, various existing standards adopt extending the holding time as a means to eliminate stress and compensate for the insufficient temperature. However, experimental studies show that the stress reduction is more significant at the beginning of the heat treatment period; thereafter, this stress reduction slows down. An excessive heat treatment time can lead to an enlargement of the grain size in the weld metal as well as an increase in the thickness of the decarburized layer, thereby resulting in a decrease in low-temperature toughness. (3) Generally, the holding time should not be shorter than the minimum required time; moreover, as long as the specified holding time and temperature difference requirements are met, residual stresses can be effectively eliminated without the generation of significant temperature-induced stresses. It is also unreasonable to excessively extend the insulation time in order to achieve the smallest temperature difference during insulation. 4.3 Conditions for determining heating and cooling rates The heating and cooling rates should be reduced as the amount of alloying elements in the steel and its thickness increase, as well as as the complexity of its structure increases, in order to avoid the generation of new thermal stress. According to China’s standard specifications, the heating rate shall be ≤ 200°C/h, and the cooling rate shall be ≤ 260°C/h. For large-scale structures and complex structures, etc., to avoid heating temperature differences, smaller heating and cooling rates should be used. Generally, for carbon steel, it can be as low as 50°C/h, while for alloy structural steel, it can be as low as 20°C/h. At the same time, measures such as reinforcement or strengthening of supports can be taken to prevent deformation. Furthermore, for certain steel grades with a high tendency to temper brittleness, the time spent in the temperature range associated with temper brittleness should be minimized, and the rates of heating and cooling should not be too slow. 4.4 Conditions for determining the temperature upon entry and exit from the furnace: The temperature of the item being heated when it enters or exits the furnace is generally specified to be below 400°C. For some extremely thick or complex structures, furnace entry and exit temperatures below 200°C must sometimes be used. It should be done in accordance with the shape and size of the component to be heated, as well as the on-site environmental conditions, following the principles of avoiding excessive residual stress, deformation, and cracks.