Analysis of experience in selecting welding materials for pressure vessels

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This post was last edited by sjytangtang on 2019-9-24 at 10:00. Summary: Boiler and pressure vessels have a wide range of applications, and their manufacturing process is relatively complex; it is necessary to strengthen control over the relevant processes to ensure that the quality meets the requirements for actual use. Welding is a crucial step in the manufacturing of boiler and pressure vessels, and it is also a area where safety hazards are likely to arise. It is necessary to pay close attention during this process, select appropriate welding materials, ensure precision in the welding techniques, address any issues that arise promptly, and continuously improve the welding processes to guarantee the safe and stable operation of boilers and pressure vessels. This, in turn, helps to enhance the overall quality of these devices, thereby supporting the better development of the enterprise. Keywords: boiler ； Pressure vessel ； Selection of welding materials ； Empirical analysis shows that with the improvement of technical standards and changes in the socio-economic landscape, the dimensions of boiler and pressure vessels, as well as the specific manufacturing requirements, have undergone significant changes. Welding techniques have also been adjusted and improved to meet the demands of the new era. Due to their large size, boiler pressure vessels require welding at various locations, which makes them prone to safety issues that can lead to serious accidents. Therefore, it is necessary to strengthen control over the welding process, select appropriate welding materials, and ensure the quality of these vessels in order to prevent safety incidents. 1 Basic aspects of welding boiler pressure vessels. There are many factors that influence the welding techniques for boiler pressure vessels. Some high-strength low-alloy steel materials contain large amounts of carbon and manganese; these elements cause hardening after welding, leading to cracks in the steel structure. Although such cracks may not become apparent immediately, they still pose safety risks. During actual welding operations, the location of the weld joint is affected by high temperatures, which causes some elements to remain there; these elements cannot be removed after cooling, leading to issues such as cracks in the weld joint area. Furthermore, the high heat generated by the welding line causes the grain size in the affected areas to increase, reducing their plasticity; softening occurs in some areas as well, which leads to a reduced service life of the pressure vessel. Boiler and pressure vessels are large in size and have thick inner walls; as a result, it is difficult to locate welds and examine the microstructure during actual operation. Traditional welding methods can no longer meet the demands of current production, so it is necessary to incorporate new technologies to achieve more precise, simplified, intelligent, and automated welding processes. This represents the main direction of development for welding technology in the future. 2 Main Methods for Selecting Welding Materials for Boiler Pressure Vessels 2.1 Criteria for Selecting Welding Materials The selection of welding materials must be based on the specific performance requirements of the product, including parameters such as load-bearing capacity, corrosion resistance, temperature tolerance, and strength; these factors are clearly specified in the design drawings. The relevant data on boiler and pressure vessels are primarily contained in the design documents; it is necessary for the relevant personnel to carefully review the data sheets of the products, understand the design requirements, and select appropriate welding materials. 2.2 Method for selecting welding materials: To select welding materials, it is necessary to find out the specific standards applicable to the base material, and to understand its important properties such as low-temperature performance, mechanical properties, high-temperature performance, and chemical composition. By performing calculations, the carbon equivalent can be determined, providing data support for subsequent welding processes. Appropriate welding materials are selected based on factors such as the impact success rate of the materials to be welded, the temperature of the impact tests, yield strength, tensile strength, and chemical composition, in order to meet the specific requirements regarding the performance of the product. When selecting welding materials, it is necessary to carefully examine the different meanings of matching, being similar, and being identical. Matching refers to the use of welding materials specifically designed for the material to be welded, so that after welding its properties are essentially identical to those of the base material; this is the most practical method of selection. \"Similar\" mainly refers to the fact that the mechanical properties and chemical composition of the welding material are similar to those of the base material, but the existing deviations are difficult to control; this also leads to a variety of options when selecting welding materials. It is quite rare to use welding materials that are exactly the same; this is only occasionally done in TIG welding. 2.3 Special circumstances regarding a company’s choice of welding materials: Some smaller manufacturing enterprises deal with a variety of metal materials, but the production volumes are low. Choosing welding materials according to standard matching criteria would increase the amount of welding materials required, make procurement more difficult, and raise inspection costs significantly. To meet the needs of welding processes, such enterprises introduce welding materials of different specifications and types. Inspecting each batch increases the company’s costs and also complicates management. Therefore, it is necessary for relevant personnel to have a thorough understanding of the properties of welding materials as well as those of the materials to be welded, in order to avoid the difficulties posed by multiple purchases and the need for separate management of different batches. 3 Substitution and Selection of Welding Materials 3.1 Selection of Low-temperature Steels and Low-alloy Welding Materials When equipment operates at temperatures below -20°C, severe brittleness problems can occur. It is possible to ensure the equipment’s performance at low temperatures by controlling the metal structure and the composition of the alloys; common grades used include 10MnDG, 09MnNiDR, 15MnNiDR, and so on. When selecting welding materials, it is necessary to thoroughly study their performance and grades at low temperatures to ensure their impact resistance and toughness. The lower the temperature at which low-temperature steel can be used, the lower its strength at normal temperatures, and the substitution process is relatively complex. First, it is necessary to clearly determine the lowest temperature at which the material to be welded can be used, in order to select the appropriate welding material. During product design, the yield temperature at room temperature is 6 times the stress level, and there is a certain amount of thickness allowance; therefore, materials with good low-temperature impact properties but a lower strength grade can be used as substitutes. 3.2 Selection of the acidity or alkalinity of welding electrodes Welding electrodes are widely used in various welded structures; they possess strong crack resistance and high impact toughness, as well as a low content of diffused hydrogen in the deposited metal. However, it is necessary to thoroughly clean them before welding to prevent defects caused by oil, rust, or water, and they should also be properly dried. Welding is a somewhat difficult task that requires high skills from the technicians. During actual welding, acidic electrodes with good process performance can be used to ensure satisfactory welding results. Boiler and pressure vessel applications place high demands on the strength, impact toughness, and ductility of welds. There is not much difference between electrodes of acidic or basic types when welding the same metal; however, impact toughness varies depending on the specific temperature. Alkaline electrodes ensure that the deposited metal has sufficient impact absorption capacity at -30°C, while acidic electrodes ensure this capability at 0°C. Therefore, for products with a working temperature above 0°C, acidic welding electrodes should be used to ensure that the overall quality of the boiler pressure vessels meets the required standards. 3.3 Selection of welding materials for austenitic stainless steel: Stainless steel is primarily used in high-temperature, low-temperature, and corrosive environments. It can be divided into duplex stainless steel, martensitic stainless steel, ferritic stainless steel, and austenitic stainless steel; among boiler and pressure vessel applications, austenitic stainless steel is the most common. Due to the small variation in strength among austenitic stainless steel materials, ultra-low carbon stainless steel welding materials are generally used for welding. Before selecting welding materials, it is necessary to have a precise understanding of the actual operating conditions of the product. Materials with high requirements for corrosion resistance and low-temperature performance can use austenitic stainless steel welding materials, whereas austenitic stainless steel should not be used in high-temperature environments. 3.4 Selection of welding materials for low-alloy high-strength steel and carbon steel: Low-alloy high-strength steel and carbon steel are primarily used in load-bearing structures where there is no significant oxidation or corrosion; they can be used in temperature ranges from -20°C to 425°C. These are welding materials with a wide range of applications. When selecting such materials, it is possible to use those with higher strength grades, without the need to conduct additional process evaluations. During the actual selection process, it is necessary to control the carbon equivalent and take into account the requirements related to stress corrosion. At the same time, the effects of heat treatment must be taken into account; boiler pressure vessels generally undergo stress-relief heat treatment, and when the temperature reaches a certain level, the overall strength of the welds decreases. Furthermore, the strength of the weld also decreases after undergoing multiple heat cycles. 3.5 Selection of welding materials for low-alloy heat-resistant steel: Low-alloy heat-resistant steel possesses the ability to resist creep at high temperatures as well as oxidation resistance. Common grades include 15CrMo, 12CrlMoV, 12CrMo, and 15Mo; among these, 12CrlMoV is the most widely used. In practice, technicians need to refer to relevant technical standards in order to determine the allowable stress and maximum operating temperature for different materials under various temperature conditions, thereby ensuring that the chosen materials are appropriate. 4 Conclusion The welding quality of boiler pressure vessels directly determines their service life and safety. Therefore, technicians should conduct a thorough analysis of the actual conditions, select appropriate welding materials, ensure high welding quality, prevent various types of accidents, and guarantee safe production. This article is from the Design Institute website www.shejiyuan.com