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 diffused hydrogen in the weld metal and avoids hydrogen-induced cracks. At the same time, it also reduces the hardening degree of the weld seam and the heat-affected zone and improves the crack resistance of the welded joint.
(2) Preheating can reduce welding stress. Uniform local preheating or overall preheating can reduce the temperature difference between the welded parts in the welding area (also known as temperature gradient). 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 avoid welding cracks.
(3) Preheating can reduce the degree of restraint of welded structures, especially for reducing the degree of restraint of fillet joints. With the increase of preheating temperature, the incidence of cracks decreases.
The selection of preheating temperature and interlayer temperature is not only related to the chemical composition of steel and electrode, but also related to the rigidity of the welding structure, welding method, ambient temperature, etc., and should be determined after comprehensive consideration of these factors. In addition, the uniformity of the preheating temperature in the thickness direction of the steel plate and the uniformity in the weld area 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 uneven, instead of reducing the welding stress, it will increase the welding stress.
There are three purposes of post-weld heat treatment: eliminate hydrogen, eliminate welding stress, improve weld structure and comprehensive 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 below 100°C. The general specification is to heat to 200~350°C and keep warm for 2-6 hours. The main function of post-weld hydrogen elimination treatment is to accelerate the escape of hydrogen in the weld seam and heat-affected zone, and it is extremely effective in preventing welding cracks in low-alloy steel welding.
During the welding process, due to the non-uniformity of heating and cooling, and the constraints of the components themselves or external constraints, welding stress will always be generated in the components 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, produce plastic deformation, and even lead to the destruction of the component in severe cases.
Stress relief heat treatment is to reduce the yield strength of the welded workpiece at high temperature to achieve the purpose of relaxing the welding stress. There are two commonly used methods: one is the overall high temperature tempering, that is, put the weldment as a whole into the heating furnace, slowly heat it to a certain temperature, then keep it warm for a period of time, and finally cool it 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 cooling slowly to reduce the peak value of welding stress, so that the stress distribution is relatively gentle, and the purpose of partially eliminating welding stress is achieved.
After some alloy steel materials are welded, hardened structures will appear in the welded joints, which will deteriorate the mechanical properties of the materials. In addition, this hardened structure may lead to joint damage under the action of welding stress and hydrogen. After heat treatment, the metallographic structure of the joint is improved, which improves the plasticity and toughness of the welded joint, thereby improving the comprehensive mechanical properties of the welded joint.
Hydrogen elimination treatment is to keep warm for a period of time within the heating temperature range of 300-400 degrees. The purpose is to accelerate the escape of hydrogen in welded joints, and the effect of hydrogen removal treatment is better than that of low temperature afterheating. Post-welding and post-welding heat treatment, post-welding post-heating and dehydrogenation treatment are one of the effective measures to prevent welding cold cracks. For thick-walled pressure vessels and other important product components with a thickness of For hydrogen-induced cracks caused by the accumulation of hydrogen in multi-pass multi-layer welding of thick plates, 2 to 3 intermediate hydrogen elimination treatments should be carried out.
Consideration of Heat Treatment in Pressure Vessel Design
Consideration of heat treatment in pressure vessel design 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. Here we focus on discussing the relevant issues of post-weld heat treatment, which is widely used in pressure vessel design.
1. Do austenitic stainless steel pressure vessels need post-weld heat treatment? Post-weld heat treatment is to use the reduction of the yield limit of metal materials at high temperatures to cause plastic rheology in places with high stress, so as to achieve the purpose of eliminating welding residual stress. At the same time, it can improve the plasticity and toughness of welded joints and heat-affected zones, and 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 the body-centered cubic crystal structure. 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 surfaces 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 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, it is not necessary to put forward post-weld heat treatment requirements for pressure vessels made of austenitic stainless steel. As for corrosion due to use, and 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 feasible heat treatment plan and ensure that the comprehensive performance of the pressure vessel 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 unfeasible to simply make an analogy to the situation of carbon steel and low alloy steel to put forward heat treatment requirements for austenitic stainless steel. In the current standard, the requirements for post-weld heat treatment for pressure vessels made of austenitic stainless steel are rather ambiguous. In GB150-89 “Steel Pressure Vessels”, 10.4.1.3 stipulates: “Unless otherwise specified in the drawing, the cold-formed austenitic stainless steel head may not undergo heat treatment.” As for whether to carry out heat treatment in other cases, it may be different due to the understanding of different people. It is stipulated in 10.4.1 of GB150-1998 “Steel Pressure Vessels”: Vessels and their 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 for liquefied petroleum gas and liquid ammonia” and “Containers for extremely toxic or highly hazardous media”. It is only stipulated in 10.4.1.1.f): “Unless otherwise specified in the drawing, the welded joints of austenitic stainless steel may not be heat treated”. Analyzing from the level of standard expression, this requirement should be understood as mainly for the various situations listed in the first item. The above-mentioned second and third items may not necessarily be included. Therefore, it is suggested that “10.4.1.1.f)” should be changed to “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 conduct heat treatment for austenitic stainless steel pressure vessels according to actual conditions. Article 74 of the 99 version of the “Regulations” clearly states: “Austenitic stainless steel or non-ferrous metal pressure vessels generally do not require heat treatment after welding. If there is a special requirement for heat treatment, it should be indicated on the drawing.”
2. Heat treatment of explosion stainless steel clad steel plate container Explosive stainless steel clad steel plate is more and more widely used in the pressure vessel industry because of its perfect combination of superior corrosion resistance and mechanical strength and reasonable cost performance. Heat treatment issues should also be brought to the attention of pressure vessel designers. Pressure vessel designers usually pay more attention to the technical index of the composite plate is its bonding ratio, but they often think little about the heat treatment of the composite plate or think that this issue 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 pulses, the composite material tilts and collides with the base material. In the state of a metal jet, a jagged composite interface is formed between the clad metal and the base metal to achieve the bonding between atoms. The base metal after explosive processing has actually undergone a strain-strengthening process. As a result, the tensile strength σb increases, the plastic index decreases, and the yield strength σs is not obvious. Whether it is Q235 series steel or 16MnR, the above-mentioned strain strengthening phenomenon is shown after the mechanical performance index is tested after explosive processing. In this regard, both titanium-steel clad plates and nickel-steel clad plates require that the clad plates should be subjected to stress-relief heat treatment after explosive cladding. The 99 version of the “Content Regulations” also has clear regulations on this, but there is no such regulation for explosive composite austenitic stainless steel plates. In the current relevant technical standards, the expression of whether and how to heat treat the austenitic stainless steel plate after explosive processing is relatively ambiguous. 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.” Leveling, trimming or cutting supply. According to the requirements of the buyer, the composite surface can be pickled, passivated or polished, and can also be supplied in a heat-treated state.” There is no mention here of how to do the heat treatment. The main reason for this situation is still the aforementioned problem of the sensitized area of intergranular corrosion in austenitic stainless steel. GB8547-87 “Titanium-Steel Composite Plate” stipulates that the heat treatment system for stress relief heat treatment of titanium-steel composite plate is: 540°C ± 25°C, heat preservation for 3 hours. And this temperature is just within the sensitization temperature range of austenitic stainless steel (400°C–850°C). Therefore, it is difficult to give clear regulations on the heat treatment of explosive-clad austenitic stainless steel plates. In this regard, our pressure vessel designers must have a clear understanding, give full attention, and take corresponding measures. First of all, 1Cr18Ni9Ti should not be used as stainless steel for composite materials. The reason is that compared with low-carbon austenitic stainless steel 0Cr18Ni9, its carbon content is higher and it is more prone to sensitization, which reduces its ability to resist intergranular corrosion. In addition, when the pressure vessel shell and head made of explosive composite austenitic stainless steel plate are used in relatively harsh conditions, such as: high pressure, pressure fluctuations, and extremely hazardous media, such as 00Cr17Ni14Mo2 should be used Ultra-low carbon austenitic stainless steels minimize the possibility of sensitization. The heat treatment requirements of the composite board should be clearly stated, and the heat treatment system should be determined through consultation with relevant parties, so as to achieve the purpose that the base material has a certain plastic reserve and the composite material has satisfactory corrosion resistance.
3. Can other methods be used to replace the overall heat treatment of the equipment 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 the pressure vessel. Although these explorations are beneficial and valuable, but at present, It is not a substitute for the overall heat treatment of the pressure vessel. In currently valid standards and regulations, the overall heat treatment requirements have not been relaxed. Among the various alternatives to the overall heat treatment, the 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. Local 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 where the spherical head is connected to the cylinder, and defective welding repair parts are allowed to use partial heat treatment. Heat treatment method.” This regulation means that the A-type 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 eliminated symmetrically. The hammering method eliminates welding residual stress: that is, through artificial hammering, a layer of compressive 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, due to the lack of quantitative indicators and strict operating procedures in the actual operation process, and insufficient verification work for comparative use, 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, which sticks to the surface of the welded joint on the inner wall of the equipment. The mechanism is the same as that of the hammering method to eliminate welding residual stress. It is said that this method can make up for some shortcomings of the hammering method to eliminate welding residual stress. However, some units conducted comparative tests on two liquefied petroleum gas storage tanks with the same conditions by using integral heat treatment and explosion methods to eliminate welding residual stress. After the tank was opened and inspected, it was found that the welded joints of the former were intact as before, while many cracks appeared in the welded joints of the storage tank whose welding residual stress was eliminated by the explosion method. In this way, the once-popular method of eliminating welding residual stress by explosion method is silent. There are other methods of relieving welding residual stress, which has not been accepted by the pressure vessel industry for various reasons. In short, although the overall post-welding heat treatment of pressure vessels (including segmental heat treatment in the furnace) has the disadvantages of large energy consumption and long cycle, and faces various difficulties in actual operation due to factors such as the structure of pressure vessels, it is still an important part of the current pressure vessel industry. The only method accepted by all parties to eliminate welding residual stress.