Welding of important components, welding of alloy steel 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 is conducive to the escape of diffusible hydrogen in the weld metal and avoids hydrogen-induced cracks. At the same time, the degree of hardening of the weld and the heat-affected zone is reduced, and the crack resistance of the welded joint is improved.
(2) Preheating can reduce welding stress. Uniform local preheating or overall preheating can reduce the temperature difference (also known as temperature gradient) between the workpieces to be welded in the welding area. In this way, on the one hand, the welding stress is reduced, and on the other hand, the welding strain rate is reduced, which is beneficial to avoiding welding cracks.
(3) Preheating can reduce the restraint of the welded structure, especially the restraint of the fillet joint. With the increase in the preheating temperature, the incidence of cracks decreases.
The selection of preheating temperature and interpass temperature is not only related to the chemical composition of the steel and electrode but also related to the rigidity of the welded structure, welding method, ambient temperature, etc., which should be determined after comprehensive consideration of these factors. In addition, the uniformity of the preheating temperature in the thickness direction of the steel sheet and the uniformity in the weld zone have an important influence on reducing the welding stress. The width of local preheating should be determined according to the restraint of the workpiece to be welded. Generally, it should be three times the wall thickness around the weld area, and should not be less than 150-200 mm. If the preheating is not uniform, instead of reducing the welding stress, it will increase the welding stress.
There are three purposes of post-weld heat treatment: eliminating hydrogen, eliminating welding stress, improving weld structure and overall performance.
Post-weld dehydrogenation treatment refers to the low-temperature heat treatment performed after the welding is completed and the weld has not been cooled to below 100 °C. The general specification is to heat to 200~350℃ and keep it for 2-6 hours. The main function of post-weld hydrogen elimination treatment is to accelerate the escape of hydrogen in the weld and heat-affected zone, which is extremely effective in preventing welding cracks during the welding of low-alloy steels.
During the welding process, due to the non-uniformity of heating and cooling, and the restraint or external restraint of the component itself, welding stress will always be generated in the component after the welding work is completed. The existence of welding stress in the component will reduce the actual bearing capacity of the welded joint area, cause plastic deformation, and even lead to the damage of the component in severe cases.
Stress relief heat treatment is to reduce the yield strength of the welded workpiece at high temperatures to achieve the purpose of relaxing the welding stress. There are two commonly used methods: one is the overall high-temperature tempering, that is, the whole weldment is put into the heating furnace, slowly heated to a certain temperature, then kept for a period of time, and finally cooled in the air or in the furnace. In this way, 80%-90% of welding stress can be eliminated. Another method is local high-temperature tempering, that is, only heating the weld and its surrounding area, and then slowly cooling, reducing the peak value of the welding stress, making the stress distribution relatively flat, and partially eliminating the welding stress.
After some alloy steel materials are welded, their welded joints will appear hardened structure, which will deteriorate the mechanical properties of the material. In addition, this hardened structure may lead to the destruction of the joint under the action of welding stress and hydrogen. After heat treatment, the metallographic structure of the joint is improved, the plasticity and toughness of the welded joint are improved, and the comprehensive mechanical properties of the welded joint are improved.
Dehydrogenation treatment is to keep warm for a period of time within the heating temperature range of 300 to 400 degrees. The purpose is to accelerate the escape of hydrogen in the welded joint, and the effect of dehydrogenation treatment is better than that of low-temperature post-heating. Post-welding and post-weld heat treatment, timely post-heating and dehydrogenation treatment after welding is one of the effective measures to prevent cold cracks in welding. Hydrogen-induced cracks caused by the accumulation of hydrogen in multi-pass and multi-layer welding of thick plates should be treated with 2 to 3 intermediate hydrogen removal treatments.
Consideration of Heat Treatment in Pressure Vessel Design
Consideration of Heat Treatment in Pressure Vessel Design Heat treatment, as a traditional and effective method to improve and restore metal properties, has always been a relatively weak link in the design and manufacture of pressure vessels. Pressure vessels involve four kinds of heat treatment: 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. The focus here is to discuss issues related to post-weld heat treatment, which is widely used in the design of pressure vessels.
1. Does the austenitic stainless steel pressure vessel need post-weld heat treatment? The post-weld heat treatment is to use the reduction of the yield limit of the metal material at a high temperature to generate plastic flow in the place where the stress is high, so as to achieve the purpose of eliminating welding residual stress, and at the same time 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, low alloy steel pressure vessels with body-centered cubic crystal structures. The crystal structure of austenitic stainless steel is face-centered cubic. Since the metal material of the face-centered cubic crystal structure has more slip planes than the body-centered cubic, it exhibits good toughness and strain strengthening properties. In addition, in the design of pressure vessels, stainless steel is often selected for the two 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 high. thick. Therefore, considering the safety of normal operation, there is no need for post-weld heat treatment requirements for austenitic stainless steel pressure vessels. As for corrosion due to use, material instability, such as deterioration caused by abnormal operating conditions such as fatigue, impact load, etc., is difficult to consider in conventional design. If these situations 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 and come up with a feasible heat treatment plan to ensure that the comprehensive service performance of the pressure vessel is not affected. Otherwise, if the need and possibility of heat treatment for austenitic stainless steel pressure vessels are not fully considered, it is often unfeasible to simply make heat treatment requirements for austenitic stainless steel by analogy with carbon steel and low alloy steel. In the current standard, the requirements for post-weld heat treatment of austenitic stainless steel pressure vessels are rather vague. It is stipulated in 10.4.1.3 of GB150-89 “Steel Pressure Vessels”: “Unless otherwise specified in the drawings, cold-formed austenitic stainless steel heads may not be heat treated”. As for whether heat treatment is performed in other cases, it may vary according to the understanding of different people. It is stipulated in 10.4.1 of GB150-1998 “Steel Pressure Vessels”: Vessels and pressure components that meet one of the following conditions should be heat treated. The second and third items are: “Containers with stress corrosion, such as containers containing liquefied petroleum gas, liquid ammonia, etc.” and “Containers containing extremely or highly toxic media”. It is only stipulated in 10.4.1.1.f): “Unless otherwise specified in the drawings, the welded joints of austenitic stainless steel may not be heat treated”. From the level of standard expression, this requirement should be understood mainly for the various situations listed in the first item. The above-mentioned second and third situations may not necessarily be included. Therefore, it is suggested that “10.4.1.1.f)” should be replaced by “10.4.1.4” in the form of “addiction” when appropriate. In this way, the requirements for post-weld heat treatment of austenitic stainless steel pressure vessels can be expressed more comprehensively and accurately, so that designers can decide whether and how to heat treatment for austenitic stainless steel pressure vessels according to the actual situation. Article 74 of the 99th edition of “Capacity Regulations” clearly states: “Austenitic stainless steel or non-ferrous metal pressure vessels generally do not require heat treatment after welding. If heat treatment is required for special requirements, it should be indicated on the drawing.”
2. Heat treatment of explosive stainless steel clad steel plate containers explosive stainless steel clad steel plates are more and more widely used in the pressure vessel industry because of their excellent corrosion resistance, the perfect combination of mechanical strength and reasonable cost performance. Heat treatment issues should also be brought to the attention of pressure vessel designers. The technical index that pressure vessel designers usually attach importance to for composite panels is its bonding rate, while the heat treatment of composite panels is often considered very little or should be considered by relevant technical standards and manufacturers. The process of blasting metal composite panels is essentially the process of applying energy to the metal surface. Under the action of a high-speed pulse, the composite material collides with the base material obliquely, and in the state of a metal jet, a zigzag composite interface is formed between the clad metal and the base metal to achieve the bonding between atoms. The base metal after explosion processing is actually subjected to a strain strengthening process. As a result, the tensile strength σb increases, the plasticity index decreases, and the yield strength value σs are not obvious. Whether it is Q235 series steel or 16MnR, after explosion processing and then testing its mechanical properties, all show the above strain strengthening phenomenon. In this regard, both the titanium-steel clad plate and the nickel-steel clad plate require that the clad plate be subjected to stress relief heat treatment after explosive compounding. The 99th edition of the “capacity gauge” also has clear regulations on this, but no such regulations are made for the explosive composite austenitic stainless steel plate. In the current relevant technical standards, the question of whether and how to heat treat the austenitic stainless steel plate after explosion processing is relatively vague. GB8165-87 “Stainless Steel Clad Steel Plate” stipulates: “According to the agreement between the supplier and the buyer, it can also be delivered in a hot-rolled state or a heat-treated state.” Supplied for leveling, trimming or cutting. On request, the composite surface can be pickled, passivated or polished, and can also be supplied in a heat-treated state.” There is no mention of how the heat treatment is performed. The main reason for this situation is still the aforementioned problem of sensitized regions where austenitic stainless steel produces intergranular corrosion. GB8547-87 “Titanium-steel clad plate” stipulates that the heat treatment system for stress relief heat treatment of titanium-steel clad plate is: 540 ℃ ± 25 ℃, heat preservation for 3 hours. And this temperature is just in the sensitization temperature range of austenitic stainless steel (400℃–850℃). Therefore, it is difficult to give clear regulations for the heat treatment of explosive composite austenitic stainless steel sheets. In this regard, our pressure vessel designers must have a clear understanding, pay sufficient attention, and take corresponding measures. First of all, 1Cr18Ni9Ti should not be used for clad stainless steel, because compared with low-carbon austenitic stainless steel 0Cr18Ni9, its carbon content is higher, sensitization is more likely to occur, and its resistance to intergranular corrosion is reduced. In addition, when the pressure vessel shell and head made of explosive composite austenitic stainless steel plate are used in harsh conditions, such as: high pressure, pressure fluctuations, and extremely and highly hazardous media, 00Cr17Ni14Mo2 should be used. Ultra-low carbon austenitic stainless steels minimize the possibility of sensitization. The heat treatment requirements for composite panels should be clearly put forward, and the heat treatment system should be determined in consultation with relevant parties, so as to achieve the purpose that the base material has a certain amount of plastic reserve and the composite material has the required corrosion resistance.
3. 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 methods to replace the overall heat treatment of pressure vessels. Although these explorations are beneficial and valuable, at present, It is also not a substitute for the overall heat treatment of pressure vessels. The requirements for integral heat treatment have not been relaxed in currently valid standards and procedures. Among the various alternatives to the overall heat treatment, the more typical ones 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. Partial heat treatment: It is stipulated in 10.4.5.3 of GB150-1998 “Steel Pressure Vessels”: “B, C, D welded joints, A-type welded joints connecting the spherical head and the cylinder and defective welding repair parts are allowed to use partial heat treatment. Heat treatment method.” This regulation means that the local heat treatment method is not allowed for the Class A weld on the cylinder, that is: the whole equipment is not allowed to use the local heat treatment method, one of the reasons is that the welding residual stress cannot be eliminated symmetrically. The hammering method eliminates welding residual stress: that is, through manual hammering, lamination stress is superimposed on the surface of the welded joint, thereby partially offsetting the adverse effect of residual tensile stress. In principle, this method has a certain inhibitory effect on preventing stress corrosion cracking. However, because there are no quantitative indicators and stricter operating procedures in the practical operation process, and the verification work for comparison and use is not enough, it has not been adopted by the current standard. 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 welded 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 have used the overall heat treatment and the explosion method to eliminate welding residual stress on two LPG storage tanks under the same conditions. Years later, the tank opening inspection found that the welded joints of the former were intact, while the welded joints of the storage tank whose residual stress was eliminated by the explosion method showed many cracks. In this way, the once-popular explosion method to eliminate welding residual stress is silent. There are other methods of welding residual stress relief, which for various reasons have not been accepted by the pressure vessel industry. In a word, the overall post-weld heat treatment of pressure vessels (including sub-heat treatment in the furnace) has the disadvantages of high energy consumption and long cycle time, and it faces various difficulties in actual operation due to factors such as the structure of the pressure vessel, but it is still the current pressure vessel industry. The only method of eliminating welding residual stress that is acceptable in all respects.