How to control the quality of brewery equipment – discussion of issues related to post-weld heat treatment in pressure vessles
As a traditional and effective method to improve and restore metal properties, heat treatment has always been a relatively weak link in the design and manufacturing of pressure vessels. (brewery equipment including brewhouse, fermenters, bright beet tanks) Pressure vessels involve four kinds of heat treatment: post-weld heat treatment (stress relief heat treatment); heat treatment for improving material properties; heat treatment for restoring material properties; post-weld hydrogen elimination treatment. The focus here is to discuss the issues related to post-weld heat treatment which is widely used in the design of pressure vessels.
1. Relaxation welding participates in stress.
2. Stabilize the shape and size of the structure and reduce distortion.
3. Improve the performance of the base metal and the weld zone, including a. Improve the plasticity of the weld metal. b. Reduce the hardness of the heat-affected zone. c. Improve fracture toughness. d. Improve fatigue strength. e. Restore or increase the yield strength reduced during cold forming.
4. Improve the ability to resist stress corrosion.
5. Further release the harmful gases in the weld metal, especially hydrogen, to prevent the occurrence of delayed cracks.
Post-weld heat treatment is to use the reduction of the yield limit of metal materials at high temperatures to produce plastic rheology where the stress is high, so as to eliminate the welding residual stress, and at the same time, it can improve the plasticity and toughness of the welded joint and the heat-affected zone, and improve the resistance. The ability to stress corrosion. This stress relief method is widely used in pressure vessels made of carbon steel and low alloy steel with a body-centered cubic crystal structure. The crystal structure of austenitic stainless steel is face-centered cubic. Because metal materials with a face-centered cubic crystal structure have more sliding surfaces than body-centered cubic, they exhibit good toughness and strain strengthening properties. In addition, in the design of pressure vessels, stainless steel is often used for the two purposes of anti-corrosion and meeting the special requirements of temperature. In addition, stainless steel is more expensive than carbon steel and low-alloy steel, so its wall thickness will not be very high. thick. Therefore, considering the safety of normal operation, there is no need to put forward post-weld heat treatment requirements for pressure vessels made of austenitic stainless steel. As for corrosion due to use, material instability, such as fatigue, impact load and other abnormal operating conditions, it is difficult to consider in conventional design. If these conditions exist, relevant scientific and technical personnel (such as design, use, scientific research and other relevant units) need to conduct in-depth research and comparative experiments to come up with a practical heat treatment plan and ensure that the comprehensive performance of the pressure vessel is not affected. Otherwise, if the need and possibility of heat treatment for austenitic stainless steel pressure vessels is not fully considered, it is often not feasible to simply compare the conditions of carbon steel and low-alloy steel and put forward heat treatment requirements for austenitic stainless steel.
In the current standard, there is no clear requirement on whether the pressure vessel made of austenitic stainless steel is subjected to post-weld heat treatment. It is stipulated in Article 8.2.4 of GB150.4 “Pressure Vessel”: “When post-weld heat treatment of austenitic stainless steel and austenitic-ferritic stainless steel is required, it shall be specified in the design document.” It is stipulated in Article 8.2.5 of GB150.4 “Pressure Vessel”: “Unless otherwise specified in the design documents, the welded joints of austenitic stainless steel and austenitic-ferritic stainless steel may not be heat treated.”
Article 3.2.11 of TSG21-2016 “Capacity Regulations” stipulates: “Austenitic stainless steel and non-ferrous metal pressure vessels are generally not required to be subjected to post-weld heat treatment after welding. If heat treatment is required for special requirements, it shall be stated on the design drawing. Specify.”
Explosive stainless steel clad steel plate is widely used in the pressure vessel industry due to its excellent corrosion resistance and mechanical strength, and its reasonable cost performance. However, the heat treatment of this material should also cause pressure vessel designers Notice.
Pressure vessel designers usually pay more attention to the technical index of the composite plate is its bonding rate, and the heat treatment of the composite plate is often considered very little or thinks that this problem should be considered by the relevant technical standards and manufacturers.
The process of explosive processing of metal composite panels is essentially a process of applying energy to the metal surface. Under the action of high-speed pulse, the composite material obliquely collides with the base material. In the state of the metal jet, a zigzag composite interface is formed between the composite metal and the base metal to achieve the bond between atoms.
The base metal after explosive processing has actually undergone a strain-strengthening process. As a result, the tensile strength σb increases, the plasticity index decreases, and the yield strength value σs is not obvious. Whether it is Q235 series steel or Q345R, after explosive processing and then testing its mechanical performance indicators, it shows the above-mentioned strain strengthening phenomenon.
Therefore, the heat treatment of austenitic stainless steel plate after explosive processing is stipulated in the current relevant technical standards. NB/T47002.1-2009 “Explosive Welded Clad Plate for Pressure Vessels” Part 1: Stainless steel-steel clad plate stipulates: “The clad plate shall be supplied by heat treatment, leveling, trimming (or cutting), and heat treatment of the clad plate The state should meet the requirements of the corresponding base material in GB150 or JB4732. According to the requirements of the buyer and specified in the contract, the surface of the cladding material can be processed by sandblasting, polishing or pickling.
Due to the constraints of the manufacturing plant and the consideration of economic interests, many people have explored other ways to replace the overall heat treatment of pressure vessels. Although these explorations are beneficial and valuable, they cannot yet replace the overall heat treatment of pressure vessels. In the current effective standards and regulations, the requirements for overall heat treatment have not been relaxed. Among the various alternatives for overall heat treatment are: local heat treatment, hammering method to eliminate welding residual stress, explosion method to eliminate welding residual stress and vibration method, hot water bath method, etc.
Local heat treatment: It is stipulated in GB150.4 “Pressure Vessel” 18.104.22.168: “B, C, D, E types of welded joints, spherical head and cylinder connection joints and defective welded parts, local heat treatment methods are allowed.” This provision means that the type A weld on the cylinder is not allowed to use local heat treatment methods, that is, the entire equipment is not allowed to use local heat treatment methods, one of the reasons is that the welding residual stress cannot be symmetrically eliminated.
Hammering method eliminates welding residual stress: that is, through manual hammering, a lamination stress is superimposed on the surface of the welded joint, thereby partially offsetting the adverse effects of residual tensile stress. In principle, this method will have a certain inhibitory effect on preventing stress corrosion cracking. However, due to the lack of quantitative indicators and stricter operating procedures in the practical operation process, and the insufficient verification work for comparison, it has not been adopted by the current standards.
Explosion method to eliminate welding residual stress: the explosive is specially made into a tape shape, and the inner wall of the equipment is stuck on the surface of the welding joint. The mechanism is the same as that of the hammer method to eliminate welding residual stress. It is said that this method can make up for some of the shortcomings of the hammering method to eliminate welding residual stress. However, some units used integral heat treatment and explosion method to eliminate welding residual stress on two liquefied petroleum gas storage tanks with the same conditions for comparative tests. Years later, the tank opening inspection revealed that the welded joints of the former were intact, while many cracks appeared in the welded joints of the welding residual stress tank after the explosion method was used. In this way, the once-popular explosive method to eliminate welding residual stress is silent.
There are other methods to eliminate welding residual stress, which have not been accepted by the pressure vessel industry due to various reasons.
In short, although the overall heat treatment of pressure vessels (including section heat treatment in the furnace) has the disadvantages of high energy consumption and long cycle, and faces various difficulties in actual operation due to factors such as pressure vessel structure, it is still the current pressure vessel industry. The only method of eliminating welding residual stress that is acceptable to all parties.
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