low-temperature pressure vessels
low-temperature pressure vessels
1 Key points for material control of low-temperature pressure vessels
The quality of low-temperature pressure vessels firstly depends on the quality of low-temperature steel. Low-temperature steels are roughly divided into three categories according to the operating temperature: when the temperature is above -40℃, low-carbon (less than 0.25% carbon content) carbon-manganese steel is used; when -40～-196℃, medium-nickel steel and chromium-nickel steel are mostly used For austenitic steel, chromium-nickel austenitic steel is mostly used at -196～-273℃.
Commonly used steels include 16MnDR, 15MnNiDR, 09Mn2VDR, 09MnNiDR, 06MnNbDR, CF-62, etc., as well as nickel-based low-temperature steel 1.5Ni, 2.5Ni, 3.5Ni, 5Ni, 9Ni steel, etc.
The main failure mode of steel at low temperature is brittle fracture. When the temperature of steel is lower than the brittle transition temperature (NDTT), it may cause brittle fracture under low stress when there are sufficiently sharp notches or defects. This kind of fracture happens suddenly and can lead to catastrophic consequences. The impact value Akv of steel at low temperature reflects the plastic deformation ability at the tip of the steel notch and the sensitivity to crack propagation at low temperature, that is, low temperature toughness.
Material procurement should first select qualified suppliers who have passed the internal management review of the enterprise. At the same time, in order to obtain good cold and hot processing performance and low temperature toughness, the selected low temperature steel should be selected in the procurement process, chemical composition, internal structure of the steel, heat treatment status, etc. All aspects should be strictly regulated and required to ensure the quality of low-temperature steel.
1.1 Inspection of low temperature materials
The re-inspection of the low-temperature steel after entering the factory is of great significance to ensure the quality of the material and thus the quality of the low-temperature pressure vessel at the source. The low-temperature impact value of steel for low-temperature pressure vessels must be re-examined before processing and manufacturing. Re-inspection of the whole project shall be carried out for the steel materials used for low-temperature three-class pressure vessels and spherical tanks. That is, the chemical composition of the material, the mechanical properties at room temperature, the low temperature impact value and the ultrasonic inspection of the steel are re-inspected.
The steel reinspection is carried out according to accurate blanking to ensure the accurate shape of the cylinder. For the batch of heads and spherical shell plates, each batch shall be re-inspected by the same brand, the same furnace number, the same size, energy, low temperature impact value and steel ultrasonic testing. The re-inspection of steel is carried out in batches, and each batch is composed of steels of the same brand, the same retort number, the same specification and size, and the same heat treatment system.
Welding rods for low-temperature pressure vessels should be low-hydrogen alkaline electrodes with similar chemical composition and mechanical properties as the base material. Submerged arc welding flux should be alkaline or neutral flux, and its low-temperature impact value should not be less than the standard and base material requirements. All welding rods used for low-temperature steel should be retested for the moisture content of the coating or the diffusion hydrogen content of the deposited metal according to the approval.
1.2 Management of cryogenic materials
Establishing a strict system for the distribution, recycling and on-site management of low-temperature steel materials is an important quality assurance method for the manufacture of low-temperature pressure vessels. Especially in the manufacturing site, because low-temperature steel and ordinary steel are easy to confuse, if not strictly managed, it will leave a big potential accident. Low-temperature steel and welding rods should be managed by dedicated personnel and warehouses. After technical clarification, construction personnel and related management personnel should be familiar with the identification of low-temperature steel and welding rods to prevent confusion with other steels.
The entry and exit of materials should be recorded in the account, and the remaining materials should be marked and transplanted in time. The surface quality of low-temperature steel is required to be high, and the surface should be protected and marked with color code during storage and transportation of low-temperature steel. Unloading and cutting should be carried out under the supervision of the material management personnel, and the color code transplantation should be carried out in time. The low-temperature steel sheet is not allowed to be stamped for identification. Steel plates and semi-finished products are stacked on shelves according to batch numbers and specifications, prefabricated, and processed and formed materials are stored in mold brackets. It is strictly forbidden to place low-temperature steel materials, especially welding materials, directly on the ground. The distance between the bracket and the ground and wall should not be less than 300mm.
The welding rod storehouse is set up in accordance with the relevant welding material management regulations. The temperature in the storehouse shall not be lower than 10℃, and the relative humidity shall not be higher than 60%, and make a record. The electrode should be dried at the specified temperature for 2 hours before use, and placed in a constant temperature drying oven (100～150℃) after drying.
2 Control points of manufacturing and installation process
In addition to the quality of the steel itself, the internal stress concentration caused by manufacturing and installation defects is also an important cause of low-temperature brittle fracture. Especially at low temperatures, the large peak stress at the stress concentration is superimposed on the overall film stress and bending stress of the equipment, so that the low-temperature pressure vessel can reach a high stress level locally, while the plastic deformation ability of the steel decreases at low temperature, which is self-limiting The conditions disappear, causing the steel to suddenly brittle fracture.
In addition, during the manufacturing process, when the deformation rate of steel in the cold state is too large, the strength and hardness will increase, while the plasticity and toughness will decrease, and the cold work hardening phenomenon will increase if the brittle transition temperature rises. If not eliminated, it will increase Risk of brittle failure at low temperature. Therefore, measures should be taken to reduce the internal stress level and cold work hardening during the manufacturing and installation of low-temperature pressure vessels.
(1) When the steel plate is blanked, it must be accurately blanked in strict accordance with the calculated layout size to ensure the accurate shape of the cylinder. The head and the spherical shell plate are blanked twice to ensure accuracy. Appropriate molds should be used for clamping during transportation to prevent deformation. Before assembly and installation on site, the size should be re-checked, and the re-checked plates that exceed the standard should be re-pressed or rounded to ensure the quality of the assembly. If the deviation exceeds the standard and the pairing is forced, a large assembly stress will be generated, which is very unfavorable for the low-temperature pressure vessel. Therefore, the pairing and assembly of the shells must not be forced.
(2) In order to avoid cold work hardening of steel, when the parts of low-temperature pressure vessels are formed or corrected at room temperature, the cold plastic deformation rate of the steel plate should be controlled ≤ 2%, and it is not allowed to use a hammer to form or correct the shape. When the ambient temperature is lower than -10°C, cold deformation processing is not allowed. For the container cylinder, the minimum bending radius should be controlled and multiple roll forming should be carried out. For the spherical shell plate, suitable molds should be used for multiple multi-point compression forming. For the elliptical head, the most serious cold work hardening is the most deformed transition zone and straight edge part. In order to eliminate cold work hardening, before pressing Or the slab is annealed in the middle to soften the structure and prevent fracture. After cold pressing, the same heat treatment as the original base metal should be carried out to restore low temperature toughness.
(3) When pairing, the deviation of each tube section should be evenly distributed, and must not be concentrated to a certain side or a certain section, so as to avoid excessive shape mutation and reduce the stress concentration level. For butt joints of unequal thickness, the thick plate should be thinned and smoothly transitioned to be flush with the thin plate. For low-temperature spherical tanks, the piece-by-piece bulk method should be used to assemble piece-by-piece to reduce uneven deviation and shape mutations, and ensure that the correct and wrong variables, angularity, roundness and other indicators meet the requirements.
(4) The surface of the container shell and pressure components shall not be marked with steel stamps, only paint is allowed.
3 Key points of welding process quality control
The welding quality of cryogenic pressure vessels is another important factor affecting the manufacturing quality of cryogenic pressure vessels. In addition to preventing welding cracks in the welding of low-temperature steel, the key is to ensure the low-temperature toughness of the weld and the heat-affected zone, which is a major part of the quality control of the low-temperature steel welding process.
3.1 Measures to prevent cold cracks in low temperature steel
The cause of cold cracks in low-temperature steel is the result of the combined effect of stress, hardened structure and diffusive hydrogen content of the weld metal. Impurities in low-temperature steel materials, oil stains in the welding area, rust, and water vapor in the atmosphere decompose hydrogen atoms into the molten pool under the action of the high temperature of the arc, and diffuse and precipitate in the supersaturated state during the cooling process of the weld. The heat-affected zone near the fusion line is prone to cold cracks under the joint action of welding stress and hardened structure. In addition, some process defects such as undercut, incomplete penetration, etc. also contribute to cold cracks. In our factory's manufacturing, the following measures have been taken to prevent cold cracks, and good results have been received:
(1) Reduce the source of hydrogen. Choose low-hydrogen electrode, even ultra-low hydrogen electrode. Secondly, the electrode must be thoroughly dried, put in a portable heat preservation tube, and taken as needed. The temperature in the tube is kept at 100～150℃. The electrode is placed in the air for 4 hours and must be re-dried, and the re-drying shall not exceed one time. Before welding, the vicinity of the welding groove must be polished clean with a grinder, rust, oil, water vapor and other dirt must be thoroughly removed, and welding can be performed after passing the inspection.
(2) Select appropriate welding boundary line energy. If the heat input is too small, the heat-affected zone will easily appear hardened structure. The heat input is large, which is beneficial to eliminate cold cracks, but it is easy to form overheated structure and affect the low temperature toughness, so the welding heat input should be appropriate.
(3) Strictly inspect the quality of the team pairing process. Welding is not allowed when defects such as the amount of misalignment and angularity exceed the standard. At the same time, it is strictly prohibited to compulsory assembly pairing to reduce assembly stress. In addition, the welding sequence should be arranged reasonably to minimize the restraint and stress generated during welding.
(4) Preheat before welding and slow cooling after welding. Avoid hardened structure and reduce welding stress, and timely post-weld heat treatment. If the post-welding stress relief heat treatment cannot be performed in time, the hydrogen elimination treatment should be performed immediately after welding, and the temperature should be kept at 300～350℃ for 2～6h. However, attention should be paid to avoiding the tempering brittleness temperature range for some steel grades with high tempering brittleness tendency.
(5) It is strictly forbidden to spot welding and arc starting at the non-welded part of the shell. Because the cooling rate of spot welding and arc ignition is lower than the normal welding cooling rate, cold cracks are more likely to occur. Moreover, some arc ignition scars are not easy to find, and they are more likely to leave safety hazards. For arc craters, welding scars, mechanical damage, etc., it should be polished and cleaned, and the grinding part and the base material should be smoothly transitioned, and the grinding slope should be at least 1:3, and then weld repaired and polished to be flush with the base material, and 100% magnetic powder Or color inspection.
3.2 Measures to prevent hot cracks in low temperature steel
The occurrence of thermal cracks is related to stress, impurities and chemical composition. The harmful elements sulfur, phosphorus and other elements that are easy to form a low melting point eutectic in the weld work together to produce serious segregation, thereby forming hot cracks. Especially the welding of 9Ni steel, using austenite filler material different from the base material, is more prone to hot cracks. In addition, the shape of the weld pool formed during welding is related to thermal cracks. If the molten pool is deep and narrow, the segregation is mostly concentrated in the middle of the weld, which is easy to form hot cracks. The molten pool is shallow, wide, and round, and the thermal crack resistance is good. To prevent thermal cracks, we take the following measures to ensure welding quality:
(1) Considering the dilution effect of the base metal on the weld metal, the alloy content in the electrode to increase low temperature toughness and crack prevention should be higher than the base metal, and the sulfur and phosphorus content should also be lower. In addition, increasing the alkalinity in the welding rod and flux can improve the degree of segregation in the weld and improve the crack resistance.
(2) Properly preheat and formulate a reasonable welding sequence to reduce the stiffness of the welded joint and reduce the welding stress.
(3) Use correct welding parameters. Keep a circular, while shallow and wide welding pool.
(4) Once cracks appear, use carbon arc gouging or a grinding wheel to thoroughly polish and clean the cracks, and no subsequent weld bead melting can be used to eliminate the cracks.
4 Key points of overall post-weld heat treatment quality control
Welding is the most direct thermal processing process that generates residual stress. Welding residual stress is mainly caused by welding thermal stress and structural restraint stress during the welding process. In addition, the weld metal and heat-affected zone base material undergoes phase transformation during the welding process to produce phase transformation stress, as well as additional stress generated during processing, forming and assembly, and the welding residual stress is superimposed, so that the pressure vessel stress The state is more complicated. The existence of various internal stresses increases the risk of brittle fracture at low temperatures.
The stress relief heat treatment of low temperature pressure vessel after welding not only eliminates the welding residual stress, but also removes the hydrogen in the weld metal, softens the structure of the heat affected zone and the processing deformation zone, and improves its low temperature toughness, which is to ensure the quality of the low temperature pressure vessel Important means. The post-weld stress relief heat treatment methods of low-temperature pressure vessels mainly include: local heat treatment, overall heat treatment in the furnace, segmented heat treatment in the furnace, and overall internal heat treatment. Among them, the overall heat treatment has the best effect, and the residual stress relief rate can reach more than 90%.
In order to ensure the manufacturing quality of post-weld heat treatment, there are many factors to consider when formulating the process parameters of post-weld heat treatment, such as the material of the workpiece under pressure and its original structure, the purpose of the heat treatment, the heating method (heating speed), the size of the workpiece and the cooling method Wait. Due to the difference of various factors, the formulated heat treatment process parameters are not the same, but the determination conditions are the same.
4. 1 Determining conditions of holding temperature
(L) For quenched and tempered high-strength low-temperature steel, if the heating temperature after welding exceeds the original tempering temperature, the quenching and tempering effect will be lost and the strength and toughness will be reduced. Especially nickel-based low-temperature steels are prone to temper brittleness, which reduces low-temperature toughness. For low-temperature steels containing more alloying elements, post-weld heat treatment tends to produce temper brittleness (ie, reheat embrittlement), thereby reducing the toughness of the weld and the heat-affected zone.
(2) The conditions for determining the holding temperature range are below the phase transition point and above the recrystallization temperature. The lattice distortion and hardened structure can be eliminated through recovery and recrystallization, the residual stress is fully relaxed and released, and at the same time, the base material and the welding zone will not cause adverse consequences such as reheat embrittlement. For quenched and tempered or normalized + tempered steel, the holding temperature should be about 30℃ below the tempering temperature, so as not to damage its performance and reduce toughness and strength.
4.2 Determination of holding time
(1) A certain holding time can fully relax the residual stress of the weld and the base metal, improve the performance of the welding zone and at the same time eliminate the residual stress without generating new temperature stress, which is necessary.
(2) When the heating temperature used is lower than the required heating temperature, in the current various specifications, the method of extending the holding time is adopted to achieve the effect of eliminating stress and to make up for the lack of temperature. However, experimental studies have shown that the stress drop is more significant at the beginning of the heat preservation, and then the stress drop tends to be slow. If the holding time is too long, it will coarsen the weld metal grains and increase the thickness of the decarburized layer, resulting in a decrease in low temperature toughness .
(3) Generally speaking, the holding time cannot be shorter than the minimum holding time, and as long as the specified holding time and temperature difference requirements are reached, the residual stress can be well eliminated without generating large temperature difference stress. It is also unreasonable to extend the heat preservation time excessively in order to obtain the smallest heat preservation temperature difference.
4.3 Conditions for determining heating and cooling rates
The heating and cooling rate should be reduced with the increase of the alloying elements and thickness in the steel and the increase of the structure complexity to avoid the generation of new temperature stress. According to my country's standard regulations, the heating rate is ≤200℃/h, and the cooling rate is ≤260℃/h. For large-size structures, complex structures, etc., in order to avoid the heating temperature difference, a smaller heating and cooling rate should be adopted.
Generally, the minimum temperature can be 50℃/h for carbon steel and 20℃/h for alloy structural steel. At the same time, measures such as reinforcement or reinforcement support can be taken to prevent deformation. In addition, for some steel grades with a high tendency to temper brittleness, the residence time in the temper brittleness temperature range should be minimized, and the temperature rise and fall speed should not be too small.
4.4 Determining conditions of entering and exiting furnace temperature
The temperature of the heated part when it is put into the furnace or out of the furnace is generally specified below 400 ℃. For some ultra-thick or complex structures, the furnace and furnace temperature below 200°C is sometimes used. It should be based on the shape and size characteristics of the heated parts and the on-site environmental conditions so as to no longer generate large residual stresses, without deformation and cracks.
