Welding is involved in manufacturing, installation, repair, and modification.BoilerOne of the primary processing methods involves the welding of boiler pressure elements, which directly impacts the safety of the boiler's operation. To ensure the quality of the welding on boiler pressure elements, it is essential to understand the factors that affect welding quality, in order to enhance management and control and prevent the occurrence of quality issues. There are numerous factors influencing welding quality; they must be identified and prioritized based on specific circumstances, with differentiated treatment.
Connector Design
Junction design is one of the prerequisites for ensuring welding quality. For welding joints that require full penetration, particularly corner joints, the design of the bevel angle, chamfer, and gap should facilitate penetration, while minimizing the amount of filler metal used. This approach reduces welding stress and deformation, facilitating the welding process and non-destructive testing of the weld.
Welding Methods
There are numerous types of welding methods, such as manual arc welding, submerged arc welding, tungsten inert gas welding, flux cored arc welding, plasma welding, friction welding, and flash welding, which are commonly used in boiler manufacturing. In boiler installation, manual arc welding, tungsten inert gas welding, and gas welding are frequently employed.
Different welding methods have varying principles, ease of meeting quality requirements, and application characteristics. Manual electric arc welding is simple, adaptable, suitable for various steels and some non-ferrous metals and alloys, and various welding positions, but it requires high operator skills, significant labor intensity, and low production efficiency. Submerged arc welding is suitable for carbon steel, low-alloy steel, and stainless steel. It can use high currents for welding, thus offering high production efficiency and low labor intensity. Due to the larger molten pool, it is generally used for flat welding positions. Tungsten inert gas (TIG) welding is suitable for various materials and positions, with easy-to-achieve high-quality welds. However, it demands high operator skills, is not suitable for windy conditions, is costly, and has low production efficiency. Gas welding is suitable for thin materials and various positions without electricity. However, it has a broader heating area, slightly inferior joint properties, requires high operator skills, and has low production efficiency.
The selection of welding methods should consider the material of the workpiece, welding position, quality requirements, and equipment conditions. A weld seam can be completed using one welding method, or it can be achieved with two or more welding methods.
3. Welding properties of the base material
Boiler pressure elements utilize the metal materials specified in the regulations, generally considering their weldability. However, the weldability varies among different steel grades. There are differences in weldability between imported and domestic steels. Some steels are less prone to cracking after welding, while others exhibit good joint properties. In some cases, the mechanical or corrosion resistance of the joints may degrade after welding. The weldability of the base material can generally be assessed through weldability tests, which typically include welding crack tests, joint mechanical property tests, and joint corrosion tests.
4. Welding Materials
Welding materials not only play a crucial protective role for the molten pool during the welding process but also can alter the chemical composition of the weld and the properties of the joint, thereby directly affecting the welding quality of the joint. The selection of welding materials is of great importance and should be comprehensively considered based on the chemical composition, mechanical properties, crack resistance of the welding joint, whether post-weld heat treatment is required, and conditions such as high temperature and corrosion. After testing, a determination should be made. When welding the same type of steel, it is generally advisable to choose welding materials with metallic properties and chemical composition similar to the base material and good process performance. When welding dissimilar steels, if one of the steels on either side is austenitic stainless steel, it is generally recommended to use welding rods or wires with a higher nickel content.
Welding Environment
During winter welding, if the ambient temperature is below freezing, the cooling rate of the molten pool and the heat-affected zone is too fast, which can easily lead to cracks, especially in alloy steel or thick-walled boiler pressure elements. When the relative humidity of the welding environment is excessively high, it is crucial to dry the welding rods or flux properly to prevent cracks, porosity, white spots, or degradation of the joint's cold bending properties. When welding outdoors, precautions against wind, rain, or snow should be taken.
Welding Position
Ensure that the joints of the boiler's pressure parts to be welded are positioned for ease of welder operation whenever possible, as this can reduce the occurrence of welding defects. Different welding positions require varying welding process parameters.
Welding Process Parameters
The quality of welding joints is closely related to welding process parameters. Different welding methods have varying primary process parameters. For example, in manual arc welding, the main process parameters include welding current, electrode diameter, and welding layers. An excessive welding current can lead to burn-through and edge burning; conversely, a too low welding current can result in slag inclusion and incomplete fusion. The electrode diameter is typically selected based on the thickness of the weldment. To ensure thorough fusion at the root of the weld seam, the diameter of the electrode for the first pass should not exceed 3.2mm. Additionally, enhancing the number of welding layers generally benefits the plasticity and toughness of the welding joint. For submerged arc automatic welding, the primary process parameters, in addition to welding current, wire diameter, and welding layers, include arc voltage, welding speed, wire extension length, and wire or workpiece inclination. The process parameters for welding boiler pressure parts are usually determined by welding process qualification tests.
8. Preheating and Post-Weld Heat Treatment
Preheating can reduce the cooling rate after welding. When welding steels with quench hardening tendencies, preheating is typically used to decrease the degree of hardening and prevent welding cracks. Whether preheating is required and the selection of preheating temperature should be considered comprehensively, including the composition and thickness of the base material, the rigidity of the structure, the joint type, the welding material, the welding method, and the welding environment, and determined through welding crack tests. In addition to flame heating, preheating methods include infrared heating and industrial frequency induction heating, among others.
Post-weld heat treatment, also known as hydrogen removal treatment, involves immediately heating the welded part to 250-350°C, holding it at temperature for 2-6 hours, and then air-cooling it. The purpose is to accelerate the escape of diffused hydrogen from the weld metal, preventing cold cracks from forming at the焊接 joints. If the part cannot be heat-treated promptly after welding but must be dehydrogenated immediately, the part must undergo post-weld heat treatment promptly; otherwise, cracks may form during the storage period before heat treatment.
Requirement for Welders and Welding Operators
Manual welding techniques, such as manual arc welding, manual tungsten inert gas (TIG) welding, and gas welding, greatly depend on the operator's skill and responsibility for the quality of the weld. Mechanized welding methods, like submerged arc welding, flux cored arc welding, and plasma welding, also partly rely on the operator's technical skills for weld quality. Therefore, prior to product welding, the welders and welding operators must undergo qualification recognition. Only those who pass the exam are eligible to perform the respective welding tasks.
In addition to the above factors, the suitability of the welding equipment to the welding method, welding rod, flux, welding current, and the flexibility and convenience of the parameter adjustment device are also crucial. The accuracy of the ammeter and voltmeter readings, as well as the fluctuations of the network voltage, have a significant impact on the quality of the weld.
