Discussion on post weld heat treatment
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 manufacture of pressure vessels. Pressure vessels involve four heat treatments: post weld heat treatment (stress relief heat treatment); Heat treatment to improve material properties; Heat treatment to restore material properties; Post weld hydrogen elimination treatment. This paper focuses on the relevant problems of post weld heat treatment widely used in the design of pressure vessels.
Purpose of post weld heat treatment (stress relief heat treatment)
1. Relax welding stress.
2. Stabilize the shape and size of the structure and reduce distortion.
3. Improve the performance of base metal and welding area, including
- a. Improve the plasticity of weld metal.
- b. Reduce the hardness of heat affected zone.
- c. Improve fracture toughness.
- d. Improve fatigue strength.
- e. Restore or increase the reduced yield strength in cold forming.
4. Improve the ability to resist stress corrosion.
5. Further release harmful gases, especially hydrogen, in the weld metal to prevent the occurrence of delayed cracks.
Does austenitic stainless steel pressure vessel need post weld heat treatment
Post weld heat treatment uses the reduction of yield limit of metal materials at high temperature to produce plastic rheology in places with high stress, so as to eliminate welding residual stress. At the same time, it can improve the plasticity and toughness of welded joints and heat affected zone and improve the ability to resist stress corrosion. This stress relief method is widely used in carbon steel and low alloy steel pressure vessels with body centered cubic crystal structure. The crystal structure of austenitic stainless steel is face centered cubic. Because the metal material with face centered cubic crystal structure has more slip surfaces than body centered cubic, it shows good toughness and strain strengthening properties. In addition, in the design of pressure vessels, stainless steel is often selected for the purposes of anti-corrosion and meeting the special requirements of temperature. In addition, stainless steel is expensive compared with carbon steel and low alloy steel, so its wall thickness will not be very thick. Therefore, considering the safety of normal operation, it is not necessary to put forward the requirements of post weld heat treatment for austenitic stainless steel pressure vessels. As for the deterioration caused by the use of corrosion and material instability, such as fatigue, impact load and other abnormal operating conditions, it is difficult to consider in the conventional design. If these conditions exist, relevant scientific and technological personnel (such as design, use, scientific research and other relevant units) need to come up with a practical heat treatment scheme after in-depth research and comparative experiments, so as to ensure that the comprehensive service performance of pressure vessels will not be affected. Otherwise, if the need and possibility of heat treatment for austenitic stainless steel pressure vessels are not fully considered, it is often impractical to put forward heat treatment requirements for austenitic stainless steel simply by analogy with the situation of carbon steel and low alloy steel.
In the current standard, there are no clear requirements on whether austenitic stainless steel pressure vessels are subject to post weld heat treatment. It is specified in article 8.2.4 of gb150.4 pressure vessels that “when post weld heat treatment is required for austenitic stainless steel and austenitic ferritic stainless steel, it shall be in accordance with the provisions of the design documents”. Article 8.2.5 of gb150.4 pressure vessels stipulates that “unless otherwise specified in the design documents, the welded joints of austenitic stainless steel and austenitic ferritic stainless steel may not be subject to heat treatment”.
Article 3.2.11 of tsg21-2016 “volume specification” stipulates: “post weld heat treatment is generally not required for austenitic stainless steel and non-ferrous metal pressure vessels after welding. If heat treatment is required for special requirements, it shall be indicated on the design drawing.”
Heat treatment of explosion stainless steel clad steel vessel
Explosive stainless steel composite plate is more and more widely used in pressure vessel industry because of its excellent corrosion resistance, perfect combination of mechanical strength and reasonable cost performance. However, the heat treatment of this material should also attract the attention of pressure vessel designers.
Pressure vessel designers usually pay more attention to the technical index of composite plate is its bonding rate, but they often consider little about the heat treatment of composite plate, or think that this problem should be considered by relevant technical standards and manufacturers.
The process of explosive machining metal composite plate is essentially the process of applying energy to the metal surface. Under the action of high-speed pulse, the composites collide obliquely towards the substrate. In the state of metal jet, a sawtooth composite interface is formed between the clad metal and the base metal to achieve the bonding between atoms.
After explosive processing, the base metal is actually subjected to a strain strengthening process. The result is tensile strength σ B increases, the plasticity index decreases, and the yield strength value increases σ S is not obvious. Both Q235 series steel and Q345R show the above strain strengthening phenomenon after explosive processing and then testing their mechanical properties.
Therefore, the heat treatment of austenitic stainless steel plate after explosive processing is specified in the current relevant technical standards. Nb/T 47002.1-2009 explosive welded composite plates for pressure vessels, part I: stainless steel composite plates stipulates that “composite steel plates shall be subject to heat treatment, leveling and trimming (or cutting) For supply, the heat treatment state of the composite plate shall comply with the provisions of the corresponding base material in GB150 or JB4732. According to the requirements of the demander and indicated in the contract, the surface of the cladding material can be treated by sand blasting, polishing or pickling.
Can other methods be used to replace the overall heat treatment of the equipment
Due to the limitations of the manufacturer’s conditions 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 can not replace the overall heat treatment of pressure vessels at present. In the current effective standards and regulations, the requirements for overall heat treatment have not been relaxed. In various alternative overall heat treatment Typical schemes include: local heat treatment, hammer 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 specified in 18.104.22.168 of gb150.4 pressure vessels that “local heat treatment method is allowed for class B, C, D and e welded joints, spherical head and cylinder connection joints and defective weld repair parts.” This provision means that local heat treatment is not allowed for class a welds on the cylinder, that is, local heat treatment is not allowed for the whole equipment, one of the reasons is that the welding residual stress cannot be eliminated symmetrically.
Elimination of welding residual stress by hammering method: that is, a layer of compressive stress is superimposed on the surface of the welded joint by manual hammering, so as to partially offset the adverse effect of residual tensile stress. In principle, this method will inhibit the prevention of stress corrosion cracking. However, due to the lack of quantitative indicators and strict operating procedures in the process of practical operation, and the comparative use The verification work is not enough and has not been adopted by the current standards.
Elimination of welding residual stress by explosion method: the explosive is specially made into a rubber strip and adhered to the surface of the welded joint on the inner wall of the equipment. Its mechanism is the same as that of hammering method. It is said that this method can make up for some shortcomings of hammering method in eliminating welding residual stress. However, some units adopt integral heat treatment and heat treatment on two LPG storage tanks with the same conditions A comparative test was conducted to eliminate welding residual stress by explosion method. One year later, the tank opening inspection found that the welded joint of the former was intact, while the welded joint of the storage tank eliminated welding residual stress by explosion method appeared many cracks. In this way, the once popular method of eliminating welding residual stress by explosion method was silent.
There are other methods to eliminate welding residual stress, which have not been accepted by the pressure vessel industry for various reasons.
In a word, although the overall post welding heat treatment of pressure vessel (including sectional heat treatment in furnace) has the shortcomings of high energy consumption and long cycle, and faces various difficulties due to pressure vessel structure and other factors in practical operation, it is still the only method to eliminate welding residual stress that can be accepted by all aspects in the pressure vessel industry.
Source: China Flanges Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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