Post-weld dehydrogenation and heat treatment

Thread Content

Welding of critical components, welding of alloy steels, and welding of thick parts all require preheating before welding. The main functions of preheating before welding are as follows: (1) Preheating can slow down the cooling rate after welding, which facilitates the escape of diffused hydrogen from the weld metal and helps to prevent hydrogen-induced cracks. It also reduces the hardening degree of the weld and heat-affected zone, thereby improving the crack resistance of the welded joint. 　　 (2) Preheating can reduce welding stress. Uniform local preheating or overall preheating can reduce the temperature difference (also known as the temperature gradient) between the workpieces in the welding area. In this way, welding stress is reduced on one hand, and the welding strain rate is decreased on the other hand, which helps to prevent the formation of welding cracks. 　　 (3) Preheating can reduce the constraint on the welded structure, and this effect is particularly evident in reducing the constraint on fillet joints; as the preheating temperature increases, the crack incidence decreases. 　　 The selection of the preheating temperature and interpass temperature depends not only on the chemical composition of the steel and welding electrodes, but also on factors such as the rigidity of the welded structure, the welding method, and the ambient temperature; these factors should be taken into consideration comprehensively before making a decision. Furthermore, the uniformity of the preheating temperature in the thickness direction of the steel plate and in the weld area has a significant impact on reducing welding stress. The width of local preheating should be determined based on the degree of restraint of the workpiece to be welded; it generally should be three times the wall thickness around the weld area, and must not be less than 150–200 millimeters. If preheating is not uniform, it not only fails to reduce welding stress, but may actually increase it. Post-weld heat treatment serves three purposes: removing hydrogen, eliminating welding stresses, and improving the microstructure and overall properties of the weld. 　　 Post-weld dehydrogenation treatment refers to low-temperature heat treatment carried out after welding is completed, before the weld has cooled below 100°C. The general standard is to heat to 200–350°C and maintain that temperature for 2–6 hours. The main purpose of post-weld degassing treatment is to accelerate the escape of hydrogen from the weld and heat-affected zone, and it is highly effective in preventing welding cracks that occur during the welding of low-alloy steel. 　　 During the welding process, due to uneven heating and cooling, as well as constraints arising from the components themselves or external factors, welding stresses always develop in the components after welding is completed. The presence of welding stress in the components reduces the actual load-bearing capacity of the welded joint area, causes plastic deformation, and in severe cases, can lead to the failure of the components. 　　 Stress-relief heat treatment involves exposing the welded workpiece to high temperatures in order to reduce its yield strength, thereby relieving the welding stresses. There are two common methods: one is overall high-temperature tempering, which involves placing the welded piece as a whole into a heating furnace, slowly heating it to a certain temperature, holding it at that temperature for a while, and then cooling it in air or within the furnace. This method can eliminate 80%-90% of welding stress. Another method is local high-temperature tempering, which involves heating only the weld area and its surrounding region before cooling it slowly, thereby reducing the peak value of welding stresses and smoothing out the stress distribution, thus partially eliminating the welding stresses. 　　 In some alloy steel materials, hardening of the weld joint occurs after welding, which deteriorates the mechanical properties of the material. Furthermore, this hardened structure may lead to joint failure under the influence of welding stresses and hydrogen. If the microstructure of the joint is improved after heat treatment, the plasticity and toughness of the welded joint are enhanced, thereby improving the overall mechanical properties of the welded joint. Dehydrogenation treatment involves maintaining the temperature in the range of 300 to 400 degrees for a certain period of time. The purpose is to accelerate the escape of hydrogen from the weld joint, and the degassing treatment is more effective than low-temperature post-heating. Welding followed by post-weld heat treatment, as well as timely post-weld tempering and dehydrogenation, are among the effective measures to prevent welding cold cracks. For thick-walled pressure vessels with a thickness of over 100 mm and other important structural components, in order to avoid hydrogen-induced cracks resulting from the accumulation of hydrogen during multi-pass, multi-layer welding of thick plates, 2 to 3 intermediate dehydrogenation treatments should be carried out. Considerations of heat treatment in pressure vessel design. As a traditional and effective method for improving and restoring metallic properties, heat treatment has historically been a relatively weak aspect in the design and manufacturing of pressure vessels. Pressure vessels involve four types of heat treatment: post-weld heat treatment (stress-relief heat treatment) ； Heat treatment to improve material properties ； Heat treatment for restoring material properties ； Post-weld hydrogen removal treatment. Here, the issues related to post-weld heat treatment, which is widely used in pressure vessel design, are discussed in detail. 1. Is post-weld heat treatment required for pressure vessels made of austenitic stainless steel? Post-weld heat treatment takes advantage of the reduced yield strength of metal materials at high temperatures, enabling plastic flow in areas with high stress levels; this helps to eliminate residual welding stresses. It also improves the plasticity and toughness of the welded joint and the heat-affected zone, thereby enhancing resistance to stress corrosion. This stress-relief method is widely used in carbon steel and low-alloy steel pressure vessels with a body-centered cubic crystal structure. The crystal structure of austenitic stainless steel is face-centered cubic; since metallic materials with a face-centered cubic structure possess more slip planes than those with a body-centered cubic structure, they exhibit good toughness and strain-hardening properties. Furthermore, in the design of pressure vessels, stainless steel is often chosen for the purposes of preventing corrosion and meeting specific temperature requirements. Additionally, since stainless steel is more expensive than carbon steel and low-alloy steel, its wall thicknesses are not very thick. Therefore, from the standpoint of safety during normal operation, there is no need to require post-weld heat treatment for pressure vessels made of austenitic stainless steel. As for corrosion that occurs due to use, as well as material instability resulting from abnormal operating conditions such as fatigue and impact loads, these are difficult to take into account in conventional design. If such situations exist, it is necessary for the relevant technical personnel (from units involved in design, operation, research, etc.) to conduct thorough research and comparative experiments in order to develop a practical heat treatment plan that ensures the overall performance of the pressure vessel is not affected. Otherwise, if the necessities and possibilities of heat treatment for pressure vessels made of austenitic stainless steel are not given due consideration, simply applying heat treatment requirements for carbon steel and low-alloy steel to austenitic stainless steel by analogy often proves ineffective. In the current standards, the requirements regarding post-weld heat treatment for pressure vessels made of austenitic stainless steel are rather vague. Section 10.4.1.3 of GB150—89 \"Steel Pressure Vessels\" stipulates: \"Unless otherwise specified in the drawings, cold-formed austenitic stainless steel heads need not undergo heat treatment.\" As for whether heat treatment is required in other cases, it may vary depending on different people's understanding. Clause 10.4.1 of GB150—1998 “Steel Pressure Vessels” stipulates that heat treatment shall be carried out if a vessel and its pressure-bearing components meet one of the following conditions. The second and third items among them are: “Containers subject to stress corrosion, such as those used for storing liquefied petroleum gas, liquid ammonia, etc.” and “Containers used for storing media with extremely high or high toxicity.” It is merely stipulated in 10.4.1.1.f) that: “Unless otherwise specified in the drawings, the welded joints of austenitic stainless steel need not be heat-treated.” From the perspective of standard formulation, this requirement should be understood as primarily referring to the various situations listed in the first item. The aforementioned second and third scenarios may not necessarily be included. Therefore, it is recommended that at an appropriate time, “10.4.1.1.f)” be amended in an ‘additional’ manner to use the format of “10.4.1.4”. This allows for a more comprehensive and accurate description of the requirements for post-weld heat treatment of austenitic stainless steel pressure vessels, enabling designers to decide on their own whether heat treatment is necessary for such vessels and how to carry out that treatment, based on the actual circumstances. Article 74 of the 1999 version of the \"Code for Pressure Vessels\" states clearly: \"Welded pressure vessels made of austenitic stainless steel or non-ferrous metals generally do not require heat treatment; if heat treatment is necessary due to special requirements, it shall be specified in the drawings.\" ” 2. Heat treatment of containers made of explosive stainless steel composite plates. Due to its excellent combination of corrosion resistance and mechanical strength, along with its favorable cost-performance ratio, explosive stainless steel composite plate is being used increasingly widely in the pressure vessel industry. However, the heat treatment issues related to this material also deserve attention from pressure vessel designers. For pressure vessel designers, the technical parameter that is usually given considerable attention regarding composite sheets is their bond strength, while the issue of heat treatment for such sheets is often given little consideration, or it is believed that this matter should be addressed by relevant technical standards and manufacturers. The process of explosive machining metal composite sheets is essentially a process of applying energy to the metal surface. Under the action of high-speed pulses, the composite material impacts the substrate at an inclined angle; in the state of metal jet flow, a serrated composite interface is formed between the layered metal and the base metal, enabling atomic-level bonding. The substrate metal after explosive processing has actually undergone a strain-hardening treatment. As a result, the tensile strength σb increases, the plasticity index decreases, and the yield strength value σs is not significant. Whether it is steel from the Q235 series or 16MnR, testing their mechanical properties after explosive processing reveals the aforementioned strain strengthening phenomenon. In this regard, both titanium-steel composite sheets and nickel-steel composite sheets require stress-relief heat treatment after being manufactured by explosive bonding. The 1999 version of the “Compatibility Regulations” also contains clear provisions on this, but no such provisions exist for explosively clad austenitic stainless steel plates. The current relevant technical standards are rather vague regarding whether heat treatment is required for austenitic stainless steel sheets after explosive processing, and how such heat treatment should be carried out. GB8165-87 \"Stainless Steel Clad Plates\" stipulates that: \"As agreed upon by the supplier and the buyer, it may also be delivered in the hot-rolled state or after heat treatment.\" ”GB4733-94 \"Explosive-resistant stainless steel composite plates for pressure vessels\" stipulates that: \"Composite plates shall be supplied after heat treatment, leveling, trimming, or cutting.\" At the request of the customer, the composite surface can be treated with acid washing, passivation, or polishing, or it can also be supplied in its heat-treated state. It does not mention how to carry out heat treatment here. The main reason for this situation remains the aforementioned problem of sensitized areas in austenitic stainless steels that lead to intergranular corrosion. GB8547-87 \"Titanium-steel composite plates\" specifies that the stress-relief heat treatment for such plates should be carried out at 540°C ± 25°C, with a holding time of 3 hours. This temperature falls precisely within the sensitization range of austenitic stainless steels (400°C–850°C). Therefore, it is quite difficult to establish clear regulations regarding the heat treatment of explosively clad austenitic stainless steel plates. In this regard, our pressure vessel designers must be fully aware of this, pay due attention to it, and take appropriate measures. Firstly, 1Cr18Ni9Ti should not be used for stainless steel in composite materials, as its carbon content is higher compared to that of the low-carbon austenitic stainless steel 0Cr18Ni9; this higher carbon content makes it more prone to sensitization, thereby reducing its resistance to intergranular corrosion. Furthermore, when the pressure vessel shells and heads manufactured from explosively clad austenitic stainless steel plates are used under harsh conditions, such as high pressures, pressure fluctuations, or when containing extremely hazardous media, ultra-low carbon austenitic stainless steels like 00Cr17Ni14Mo2 should be selected to minimize the risk of sensitization. The heat treatment requirements for the composite plate should be clearly specified, and its heat treatment regime should be determined through consultation with relevant parties, in order to ensure that the base material possesses a sufficient amount of plasticity and that the composite material has the desired corrosion resistance. 3. Can alternative methods be used to replace the overall heat treatment of equipment? Due to limitations imposed by the manufacturer’s conditions as well as economic considerations, many people have explored alternative methods to substitute for the overall heat treatment of pressure vessels. Although these efforts are useful and valuable, they cannot yet replace the traditional overall heat treatment process for pressure vessels. Among the currently valid standards and regulations, the requirements for overall heat treatment have not been relaxed. Typical alternatives to overall heat treatment include: local heat treatment, the hammering method for eliminating welding residual stresses, the explosion method for eliminating welding residual stresses and the vibration method, as well as the hot water bath method. Local heat treatment: Section 10.4.5.3 of GB150—1998 \"Steel Pressure Vessels\" stipulates that: local heat treatment methods are permitted for Class B, C, and D weld joints, Class A weld joints where spherical heads are connected to cylinders, as well as areas that have been repaired due to defects. ”This regulation means that local heat treatment is not permitted for Class A welds on the cylinder; in other words, local heat treatment is not allowed for the entire equipment, one of the reasons being that the residual welding stresses cannot be eliminated symmetrically. Eliminating welding residual stresses by hammering: This involves applying manual hammering to create a layer of compressive stress on the surface of the welded joint, thereby partially counteracting the adverse effects of residual tensile stress. In principle, this method does have a certain inhibitory effect on preventing stress corrosion cracking. However, it has not been adopted by current standards due to the lack of quantitative indicators and strict operating procedures in practical application, as well as insufficient verification work using comparative methods. Eliminating welding residual stress by explosion method: This involves using **specially designed tape-like materials that are attached to the surface of the welded joint on the inner wall of the equipment; the mechanism is the same as that used in the hammering method to eliminate welding residual stress. It is said that this method can compensate for some of the shortcomings of the hammering method in eliminating welding residual stresses. However, certain organizations conducted comparative tests on two liquefied petroleum gas storage tanks under identical conditions, using either overall heat treatment or the explosion method to eliminate welding residual stresses. After one year, inspections of the tanks revealed that the weld joints in those treated with overall heat treatment were still in perfect condition, whereas the weld joints in the tanks treated with the explosion method showed many cracks. In this way, the once-popular method of using explosions to eliminate residual welding stresses also disappeared quietly. There are also other methods for eliminating residual welding stresses, but they have not been adopted by the pressure vessel industry for various reasons. In summary, although post-weld overall heat treatment of pressure vessels (including staged heat treatment in a furnace) has the disadvantages of high energy consumption and long processing times, and faces various challenges in practical application due to factors such as the structure of the pressure vessels, it remains the only method acceptable to all parties in the pressure vessel industry for eliminating welding residual stresses.