Methods of heat treatment for containers

This is too general: high-pressure, medium-pressure storage reaction vessels, horizontal vessels for storing mixed liquefied petroleum gas, mobile pressure vessels, overall heat treatment

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 to restore material properties ； Post-weld dehydrogenation 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 crystal structure have 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 perspective of safety in 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, use, research, etc.) to conduct thorough studies 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 requirements and possibilities of heat treatment for austenitic stainless steel pressure vessels are not fully considered, and heat treatment requirements for austenitic stainless steel are simply derived by analogy with carbon steel and low-alloy steel, it often does not work. 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 may not require heat treatment.\" As for whether heat treatment is applied in other cases, it may vary depending on different people's understandings. Section 10.4.1 of GB150—1998 \"Steel Pressure Vessels\" stipulates that vessels and their pressure-bearing components shall undergo heat treatment if they 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 only specified in 10.4.1.1.f) that: “Unless otherwise specified in the drawing, welded joints of austenitic stainless steel may not require heat treatment.” From the perspective of standard phrasing, this requirement should be understood as primarily referring to the various situations listed in the first item. The second and third situations mentioned above may not necessarily be included. Therefore, it is recommended that at an appropriate time, “10.4.1.1.f)” be revised 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 exploded stainless steel composite plates. Due to their excellent combination of corrosion resistance and mechanical strength, along with favorable cost-performance ratios, exploded stainless steel composite plates are 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 base metal that has undergone explosive processing has, in fact, been subjected to a strain-hardening treatment process. 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, when their mechanical properties are tested after explosive processing, the aforementioned strain strengthening phenomenon is observed. In this regard, both titanium-steel composite sheets and nickel-steel composite sheets require stress-relief heat treatment after being produced by explosive bonding. The 1999 version of the “Code for Acceptance” 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 Composite Sheets\" 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 steel plates for pressure vessels\" stipulates that: \"Composite steel 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 a 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 zones 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℃ ± 25℃, 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 need to be fully aware of this, pay due attention, 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 explosion-composite 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 likelihood of sensitization. The heat treatment requirements for the composite panel 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 required corrosion resistance. 3. Can other methods be used as a substitute for the overall heat treatment of equipment? Due to limitations imposed by the manufacturing facilities and considerations related to economic benefits, many people have explored alternative methods to replace the overall heat treatment of pressure vessels. Although these attempts are useful and valuable, they cannot yet serve as substitutes for 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: for welding joints of categories B, C, and D, welding joints of category A where a spherical head is connected to a cylinder, as well as areas that have been repaired due to defects, local heat treatment methods may be employed. ”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 stress 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: A specially designed tape is used to be 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 remove 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. Thus, the once-popular method of using explosions to eliminate residual welding stresses faded away 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 currently accepted across all sectors in the pressure vessel industry for eliminating welding residual stresses.