Research on the defects of stainless steel pressure vessel head and handling measures
The article introduces the basic situation of stainless steel pressure vessel head, points out the main characteristics of stainless steel head defects when they occur in terms of processing technology, materials used and defective parts, and deeply analyzes the main causes and treatment methods of stainless steel pressure vessel head defects, puts forward effective strategies to prevent problems of stainless steel pressure vessel head, and puts forward some modification opinions on the existing pressure vessel technical standards in order to promote the improvement of stainless steel pressure vessel head quality and provide reference for pressure vessel manufacturing enterprises.
The quality of pressure vessel head is an important part directly related to the safety and service life of pressure vessels, and the natural material characteristics of stainless steel with corrosion resistance, high temperature resistance and low temperature resistance, coupled with good mechanical and technological performance, are widely used in the production of pressure vessel heads . However, in recent years, stainless steel pressure vessel head defects have emerged from time to time, causing considerable trouble to pressure vessels and related manufacturing companies and users, so it is necessary to study the stainless steel pressure vessel head defects and handling measures.
1. Basic information of stainless steel pressure vessel head
Pressure vessel head (hereinafter referred to as “head”) is the main pressure-bearing element of pressure vessel. Its quality affects the safety of the pressure vessel, therefore, it is very important for the pressure vessel neck or head manufacturers to carry out the necessary quality control . Stainless steel has the characteristics of corrosion resistance, high temperature resistance and low temperature resistance, which is highly favored by the pressure vessel manufacturing industry and is widely recognized within the industry.
2. Main characteristics of stainless steel pressure vessel head defect problems
2.1 In the processing process
In the processing process, the stainless steel pressure vessel heads with defects are all cold formed at room temperature, although they differ in specific types, for example, some are cold stamping and some are cold spinning, but they all belong to the category of cold forming . By cold forming, we mean the plastic deformation process of the workpiece material below the crystallization temperature. The scope of cold forming is relatively wide, some engineering plastic deformation processing at ambient temperature, and some literature “warm forming” are also the scope of cold forming.
2.2 In the use of materials
Defects in the stainless steel pressure vessel, the materials used are sub-stable austenitic stainless steel, including 304, 304L or 321 stainless steel. Austenitic stainless steel is a class of stainless steel whose matrix is dominated by austenitic tissue, non-magnetic, and strengthened mainly by cold working . Due to its own stability is relatively strong and widely used, under the range of austenitic stainless steel, there are many types, according to the stability of the level, it can be specifically can be divided into sub-stable austenitic stainless steel and steady state austenitic stainless steel. Sub-stable austenitic stainless steel in the plastic deformation of part of the austenitic organization is more easily transformed into martensitic organization. Compared with the steady-state austenitic stainless steel, the performance is relatively backward. The stainless steel materials with defects are sub-stable austenitic stainless steel.
2.3 On the defective parts
Head defects are mostly located in the straight edge section, transition zone or splice weld heat affected zone, the tissue structure of this part of the region is more unstable, belonging to the austenite and martensite coexistence group . Among them, the area with the greatest force and deformation in the head is the straight edge section. Next is the transition zone, and the least stressed and deformed is the top curved surface. In the heat-affected zone of the splice weld, because of the most affected, the solid base material organization and properties on both sides of this part of the weld change significantly, so it is often prone to some defect problems.
3. Main causes of stainless steel pressure vessel head defects and treatment methods
3.1 Head defects caused by cold deformation strengthening
Austenitic stainless steel strengthening treatment can only be carried out in the form of cold processing, the so-called cold deformation strengthening is to make the metal plastic deformation with the help of low temperature . In the process of low-temperature deformation, as the deformation increases, the hardness and strength of the metal increases, and the plasticity and toughness decrease. In the process of cold processing of austenitic stainless steel pressure vessel head, the material will also produce the corresponding plastic deformation, and the material crystal will also produce the corresponding slip and twinning, which makes the dislocation defects of the lattice further Zengqing, and with the increasing amount of deformation, the dislocation density is also gradually strengthened. Due to the interaction between dislocations, the hardness and strength of the material is increased, while the plasticity and toughness of the material is reduced, but when the plastic deformation reaches a certain degree, the head cracking defect will be produced.
3.2 Head defects caused by martensitic phase transformation
Martensitic phase transformation is a first-level, nucleation-length phase transformation that replaces atoms without diffusion shear (collaborative movement of atoms along the phase interface) and changes their shape. According to the martensitic phase transformation theory, it is known that when the temperature is above the martensite point, and below the critical temperature of deformation-induced martensitic phase transformation, the plastic deformation of austenitic stainless steel will produce martensitic phase transformation. The result is that some face-centered cubic austenitic tissues are transformed into strain-induced martensitic tissues. Martensitic tissues can be further divided into two types: α’ phase martensite, and ε phase martensite. The process temperature of austenitic stainless steel is under the condition temperature of martensite phase transformation, so in the production, especially the application of sub-stable austenitic stainless steel material for production, part of the austenitic tissue is easy to turn into martensite, and with the increasing amount of deformation, the corresponding amount of martensite transformation also gradually increased. The martensite tissue is harder and more brittle than the austenite tissue, and the two different characteristics of the tissue work, so that the austenitic stainless steel head material after cold forming is very easy to form microscopic cracks, resulting in head defects.
3.3 The influence of corrosive environment and the use of open plate under the head defects
Martensite corrosion potential is lower than austenite corrosion potential. And when the two exist together in the corrosive environment, martensite will act as the anode, austenite as the cathode, between the two can further form corrosion micro-electrical couples. This corrosion micro-electric couple pair is very easy to cause pitting and stress corrosion, making the austenitic stainless steel more susceptible to internal effects, enhance the susceptibility to galvanic coupling corrosion, pitting and stress corrosion.
Open plate is a steel plate made of rolled steel strip after uncoiling, leveling and shearing in room temperature environment. In the process of using open plate, pressure vessel and head manufacturing enterprises can freely rule on the length of choice, thus saving material and expenses, and thus widely favored by people. However, in the process of using open plate, it needs to be repeatedly cold processed and deformed after solid solution treatment, such as steel strip coiling and flattening operation, which makes its performance in mechanical properties less outstanding than rolled steel plate. As far as the use of stainless steel pressure vessel head is concerned, the head formed with open plate is particularly defective compared with the head formed with rolled steel plate. The reason for this phenomenon is that after forming with open plate, the elongation after break is reduced and the material ductility is decreased. However, in the actual production process, some enterprises or production personnel do not pay enough attention to the hardness index in the mechanical properties of the re-testing results, even if the hardness index exceeds the standard requirement value is also ignored, so the austenitic stainless steel open plate with high hardness, when applied to the stainless steel pressure vessel head, it is very easy to cause head defects.
3.4 Stainless steel pressure vessel head defects of the main processing methods
When the stainless steel pressure vessel head defects, the head needs to be remade, some enterprises in order to avoid cracking again, will be remade stainless steel head in the completion of cold forming treatment, immediately for the restoration of material properties heat treatment. That is to say, the solution treatment of stainless steel. By heating the austenitic stainless steel to the appropriate temperature and holding, the excess phase is fully dissolved in the solid solution, and rapid cooling, thus obtaining a single austenitic organization. It should be noted here that when making, some test pieces need to be prepared, and the feasibility of the process is judged by testing and evaluation of the test pieces. And for some of the higher requirements, the need for corrosion-resistant pressure vessel head, before the heat treatment should remove dirt and harmful impurities, after the heat treatment should be pickling, passivation treatment. 4.
4. Effective strategies to prevent problems of stainless steel pressure vessel heads
4.1 On the use of materials
According to the analysis above, it can be known that the steady state austenitic stainless steel has stronger stability than sub-steady state austenitic stainless steel, therefore, when cold forming should be as far as possible to choose better performance and stability of the steady state austenitic stainless steel. And be careful to use open plate. When using the open plate, it should be noted that the mechanical properties must be rechecked, and if any of the mechanical properties of the index does not match, the cold forming head cannot continue to be made.
4.2 In the process of production
For steelmaking, steel strip flattening and head manufacturing enterprises, they should improve their respective steel strip coiling, steel strip flattening and head cold forming processing technology, so that the process of making stainless steel pressure vessel heads can reduce martensite transformation as much as possible, improve the quality of stainless steel pressure vessel head making and better guarantee the safety of stainless steel pressure vessels. In the case of conditions permitting, the temperature forming process can be applied.
4.3 In the forming temperature control
In the cold forming process of the head, the temperature can be higher than the room temperature, but not more than 150 ℃, research has proved that by enhancing the temperature in the forming process, can greatly reduce the number of austenite transformation into martensite phase, which can reduce the magnitude of plasticity drop caused by deformation. In addition, for the stainless steel pressure vessel head in the defective parts are often straight edge section, transition zone, splice weld and weld heat-affected zone and other parts of the situation, in the production, should pay extra attention to these parts, and carefully measured, when found in which the organizational changes are large, to restore the performance of its material heat treatment.
5. Suggestions on the modification of pressure vessel technical standards
In the prevention of stainless steel pressure vessel head problems in the effective strategy, which is very important to the organization of the change in the timely measurement. Through timely and accurate measurement, it is possible to determine whether the need for heat treatment to restore material properties. This is also a new requirement according to TSG21-2016 “Fixed Pressure Vessel Safety Supervision Regulations (with Revision Notes)”, which came into effect in October 2016, which states that under certain circumstances, restoration of material properties heat treatment is required for pressurized components. This new requirement really captures the essence of the impact of cold forming on material properties, but in the implementation process, the operational level is relatively empty, without achieving specific, practical requirements, although compared with the original regulations have great progress, but there are still certain defects. And austenitic stainless steel cold mold recovery characteristics in addition to the consideration of material heat treatment, but also take into account the corresponding material deformation, climate change, external factors, deformation rate, plastic deformation, etc., want to achieve the control of deformation, need to further enhance the standard, and to restore the material properties of heat treatment in the standard to further clarify the conditions. For example, when the design temperature is controlled below -100℃ or above 675℃, the original deformation control value is reduced from 15% to 10%, and the organization changes of key parts such as straight edge section, transition zone, splice welding seam and weld heat affected zone are finely specified to improve the standard of quality control, better promote the pressure vessel and head manufacturing enterprises to improve the production quality and minimize or avoid the defects of stainless steel pressure vessel head.
The production quality of stainless steel pressure vessel head is closely related to the safety of pressure vessels, which not only involves the sales and economic profit of related enterprises, but also is an important issue related to safety. For the pressure vessel and head manufacturing enterprises, we should improve the quality meeting, strengthen quality control, continuously improve the process and technology innovation, find and deal with the defects of stainless steel pressure vessel head in time, face up to and pay attention to the problem, and continuously improve the safety performance of the products.
Author: Zhu Zhenmin
Source: China Head Manufacturer – Yaang Pipe Industry (www.steeljrv.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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