Key points to note in low-temperature pressure vessel technology

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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. Stainless steels for low temperatures are generally divided into three categories based on the operating temperature: at temperatures above -40°C, low-carbon (with carbon content less than 0.25%) carbon-manganese steels are commonly used; at temperatures between -40°C and -196°C, medium-nickel steels and chromium-nickel austenitic steels are typically used; and at temperatures between -196°C and -273°C, chromium-nickel austenitic steels are preferred. 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), it may suffer from brittle fracture under low stress in the presence of sufficiently sharp notches or defects. 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, it is necessary to choose suppliers that have passed the company’s internal management reviews. 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 items is also required for the steel materials used in low-temperature Class 3 pressure vessels and spherical tanks. That is, retesting of the chemical composition of the material, its mechanical properties at room temperature, its impact strength 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 each batch of heads and spherical shell plates, tests must be conducted on the same grade, heat number, specifications and dimensions; furthermore, low-temperature impact tests and re-inspection via ultrasonic testing of the steel must also be carried out. Re-inspection of steel is carried out in batches, with each batch consisting of steel of the same grade, same furnace number, same specifications and dimensions, 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 fluxes, 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 with each other; without strict management, this can pose significant risks of accidents. Low-temperature steel and welding electrodes should be stored in dedicated warehouses managed by designated personnel. After receiving technical instructions, construction workers and relevant supervisors must be familiar with the markings on these low-temperature steel products and welding electrodes to prevent them from being mixed up 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 regarding surface quality; its surface must be protected during storage and transportation, and it should be labeled using color codes. Material cutting and trimming should be carried out under the supervision of material management personnel, and color coding should be applied promptly; stamping marks are not allowed on low-temperature steel surfaces as identifiers. Steel plates and semi-finished products are stored on shelves sorted by batch number and specifications, while pre-fabricated 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 rod must be dried at the specified temperature for 2 hours; after drying, it is placed in a constant-temperature drying oven (at 100–150°C). 2 Key points for control during manufacturing and installation processes: Apart from 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, reduced 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 steel plates, cuts must be made precisely according to the calculated layout dimensions to ensure accurate formation of the cylinder. 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, dimensional checks should be carried out; plates that exceed the specified dimensions must be reshaped or rounded again 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 is not allowed for forming or straightening. 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 forming processes should be carried out. For spherical shell plates, appropriate molds should be used to carry out pressing in multiple stages and 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 the deformation is greatest. To eliminate cold work hardening, the plate blanks should be annealed before or during pressing in order to soften the material and prevent cracking. 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 particular 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 the pressure-bearing components; only paint is permitted 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 weld seam in a supersaturated state. Under the combined effect of welding stresses and hardened microstructures, cold cracks are very likely to form. In addition, certain process defects such as undercutting and lack of penetration also contribute to the formation of cold cracks. In the manufacturing process at our factory, the following measures have been taken to prevent the formation of cold cracks, with good results: (1) Reducing sources of hydrogen. Low-hydrogen electrodes, or even ultra-low-hydrogen electrodes, should be used. Additionally, the electrodes must be thoroughly dried and stored in portable insulated containers for easy access as needed, with the temperature inside the containers 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 clean. (2) Select an appropriate weld boundary line energy. If the linear 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 rigorously check the quality of each process; welding shall not be carried out if defects such as misalignment and angular deviation exceed the specified limits. Additionally, forced assembly and fitting are strictly prohibited to minimize assembly stresses. Additionally, the welding sequence should be arranged properly to minimize the constriction and stress generated during welding. (4) Preheating before welding and slow cooling after welding. Avoid hardening of the structure and reduce welding stress, while carrying 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 must be performed immediately after welding at 300–350°C for 2–6 hours. However, it should be noted that when processing 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 areas of the housing are strictly prohibited. Since the cooling rate at spot welding and arc initiation 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. Undercuts, weld beads, mechanical damage, etc., should be ground away completely. The repaired area should blend smoothly with the base metal; the grinding slope should be 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 Measures to prevent hot cracks in low-temperature steel: The occurrence of hot cracks is related to stress, impurities, and chemical composition. The harmful elements in the weld, such as sulfur and phosphorus, along with other elements that tend to form low-melting eutectics, act together to cause severe segregation, thereby leading to thermal cracks. Especially in the welding of 9Ni steel, the use of an austenitic filler material different from the base metal increases the likelihood of heat cracks. 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 cracks, we take the following measures to ensure welding quality: (1) Considering the dilution effect of the base material on the weld metal, the alloy content in the electrode that enhances low-temperature toughness and crack resistance should be higher than that in the base material, while the sulfur and phosphorus contents should be even lower. Furthermore, increasing the alkalinity in the electrodes and fluxes can improve the degree of segregation in the welds and enhance their 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; it is not permissible to use subsequent weld passes to melt the crack away. 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 phase transformation stresses resulting from phase changes that occur in the weld metal and the base material in the welding process, along with the additional stresses generated during 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 the 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, overall heat treatment in a furnace, staged heat treatment in a furnace, and overall 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: 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 holding 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 recovery 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 way 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 speaking, 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 differential 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 increases, as well as with increased thickness and greater structural complexity, 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. Additionally, for certain steel grades with a high tendency to temper brittleness, the time spent in the temperature range where temper brittleness occurs should be minimized, and the rates of heating and cooling should also 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.