Heat treatment as a traditional and effective method of improving and restoring metal properties in the pressure vessel design, manufacturing and other links has been a relatively weak link. Pressure vessels involve four types of heat treatment: post-weld heat treatment (stress relief heat treatment); improve material properties heat treatment; restore material properties heat treatment; post-weld hydrogen elimination treatment. Here the focus on the pressure vessel design is widely used in the post-weld heat treatment of the relevant issues to be discussed.
1 post-weld heat treatment (stress relief heat treatment) purpose 1. Relaxation of welding involved in stress.
2. Stabilize the shape and size of the structure and reduce distortion.
3. improve the performance of the base material, welding area, including a. improve the plasticity of the weld metal. b. reduce the heat affected zone hardness. c. improve fracture toughness. d. improve fatigue strength. e. restore or improve the yield strength reduced in cold forming.
4. Improve resistance to stress corrosion.
5. Further release of harmful gases, especially hydrogen, from the weld metal to prevent delayed cracking.
2 austenitic stainless steel pressure vessel whether the need for post-weld heat treatment Post-weld heat treatment is the use of metal materials at high temperatures to reduce the yield limit, so that the stress of high places to produce plastic rheology, so as to achieve the purpose of eliminating residual stress in welding, while improving the plasticity and toughness of the welded joint and heat-affected zone, to improve the ability to resist stress corrosion. This method of stress relief is widely used in carbon steel and low-alloy steel pressure vessels with a body-centred cubic crystal structure. Austenitic stainless steel crystal structure is face-centred cubic, due to the face-centred cubic crystal structure of the metal material than the body-centred cubic has more slip surface, thus showing good toughness and strain strengthening properties. In addition, in the design of pressure vessels, the choice of stainless steel is often to prevent corrosion and meet the special requirements of the temperature for these two purposes, coupled with stainless steel and carbon steel and low alloy steel compared to the expensive, so its wall thickness are not very thick. Therefore, from the normal operation of the security considerations, there is no need for austenitic stainless steel pressure vessels to put forward the requirements of post-weld heat treatment. As for the use of corrosion, material instability, such as: fatigue, shock load and other abnormal operating conditions brought about by the deterioration of the situation, in the conventional design is difficult to consider. If these conditions exist, the relevant scientific and technical personnel (such as: design, use, scientific research and other relevant units) after in-depth research, comparison experiments, come up with a practical heat treatment programme and ensure that the comprehensive use of pressure vessels performance is not affected. Otherwise, if not fully considered heat treatment for austenitic stainless steel pressure vessel needs and possibilities, simply analogous to the case of carbon steel and low-alloy steel and austenitic stainless steel heat treatment requirements, is often not feasible.
In the current standard, the austenitic stainless steel pressure vessel whether the post-weld heat treatment is not clear requirements. In GB150.4 "pressure vessel" section 8.2.4: "When the need for austenitic stainless steel, austenitic-ferritic stainless steel for post-weld heat treatment, according to the design documents". In GB150.4 "Pressure Vessels", Article 8.2.5 states: "Unless otherwise specified in the design documents, welded joints of austenitic stainless steel, austenitic-ferritic stainless steel may not be heat treated".
TSG21-2016 "tolerance" of Article 3.2.11 provides: "austenitic stainless steel and non-ferrous metal pressure vessels after welding generally do not require post-weld heat treatment, such as special requirements for heat treatment, should be specified in the design drawings. "
3 Exploded stainless steel composite steel plate heat treatment of the vessel Exploded stainless steel composite steel plate because of its superior corrosion resistance and mechanical strength of the perfect combination and its reasonable cost performance, so the pressure vessel industry is increasingly widely used, but the heat treatment of this material should also draw the attention of pressure vessel designers.
Pressure vessel designers for the composite plate usually pay more attention to the technical indicators is its bonding rate, and for the composite plate heat treatment issues are often considered very little or that this issue should be considered by the relevant technical standards and manufacturing plants.
Explosive processing of metal composite plate process, is essentially the process of applying energy to the metal surface. In the role of high-speed pulse, the composite material to the base material tilt collision, in the state of the metal jet, compound metal and base metal between the formation of jagged composite interface, to achieve the combination of inter-atomic.
After the explosion process of the base metal, in fact, is subjected to a strain strengthening process. The result is a rise in tensile strength σb, plasticity indicators decline, yield strength value σs is not obvious. Whether Q235 series of steel or Q345R, after the explosion process and then test its mechanical properties indicators, all show the above strain strengthening phenomenon.
NB/T47002.1-2009 "pressure vessel with explosion welding composite plate" Part I: stainless steel - steel composite plate provisions for: "composite steel plate should be heat treatment, leveling, cutting edge (or cutting) supply, composite plate heat treatment state shall comply with the provisions of GB150 or JB4732 in the corresponding base material. According to the requirements of the demand side, and specified in the contract, the surface of the composite material can be sandblasted, polished or pickled.
4 Can the overall heat treatment of the equipment be replaced by other means Due to the constraints of the manufacturing plant, and economic interests, many people have explored other ways to replace the overall heat treatment of the pressure vessel, although these explorations are useful and valuable, but currently can not replace the overall heat treatment of the pressure vessel. In the standards and regulations currently in force, the requirements for integral heat treatment have not been relaxed. In place of the overall heat treatment of various programmes are more typical: local heat treatment, hammering method to eliminate residual stress in welding, explosion method to eliminate residual stress in welding and vibration method, hot water bath method, etc.
Local heat treatment: In GB150.4 "Pressure Vessel" 8.2.6.5, it is stipulated that "B, C, D, E welded joints, spherical head and cylinder connection joints and defective weld repair parts are allowed to use local heat treatment methods." This provision means that local heat treatment methods are not permitted for Class A welds on the cylinder, i.e.: local heat treatment methods are not permitted for the whole unit, one reason being that the weld residual stresses cannot be symmetrically eliminated.
Hammering method for the elimination of weld residual stresses: i.e. by manual hammering, a layer of compressive stress is iteratively applied to the surface of the welded joint, thus partially counteracting the detrimental effect of residual tensile stresses. In principle, this method will have some inhibiting effect on the prevention of stress corrosion cracking. However, due to the lack of quantitative indicators and strict operating procedures in practice, coupled with insufficient verification of comparative use, it has not been adopted by current standards.
Explosion method to eliminate residual stress in welding: is the explosive special made of tape, the inner wall of the equipment stick in the welded joint surface, the mechanism and hammering method to eliminate residual stress in welding the same. It is said that this method can make up for the hammering method to eliminate welding residual stress some shortcomings, however, there are units in the same conditions of the two LPG
Storage Tanks were used on the overall heat treatment and the explosion method to eliminate welding residual stress to compare the test, a year after opening the tank inspection found that the former welded joints intact, and by the explosion method to eliminate welding residual stress tank welding joints are many cracks. In this way, there was a popular explosion method to eliminate residual stress welding method is silent.
There are some other methods of eliminating residual stress in welding, for various reasons are not accepted by the pressure vessel industry.
In short, the pressure vessel after welding the whole heat treatment (including the furnace segment heat treatment) although with energy consumption, long cycle deficiencies, and in practice because of the pressure vessel structure and other factors face a variety of difficulties, but it is still the only pressure vessel industry is currently accepted by all parties to eliminate residual stress welding methods.
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