[Haichuan Chemical Installation Technology] Key Points and On-site Control for Welding P91 Steel Pipes

Thread Content

Key points of welding construction and on-site control for P91 steel pipes: P91 steel is a typical martensitic heat-resistant steel. Thanks to its excellent high-temperature endurance strength, structural stability, and creep resistance, it has become the preferred material for key components such as superheaters, reheaters, and main steam pipes in power plant boilers. However, due to the properties of the material, its welding process requires high standards in terms of techniques, operations, and temperature control; strict oversight throughout the entire process is necessary to ensure the quality of the joints and the long-term safe operation of the pipeline. Below, based on actual on-site conditions, the key control points for P91 pipeline welding are systematically explained. I. Pre-welding preparations (1) Requirements for workers: Welders who are to perform welding on P91 steel must hold the appropriate qualifications for this type of work, and they must have received specialized training on the relevant welding techniques. They need to be familiar with the welding properties of this material, as well as the requirements regarding temperature control and key operating procedures. Only after passing the necessary assessments can they start working. (II) Selection and management of welding materials: Currently, imported welding materials such as Berle are widely used in engineering projects. Domestic P91/P92 welding materials, which have been under development since 2005, have been successfully applied in numerous projects, offering stable and reliable performance. For some large-scale projects, imported welding materials are given priority for risk control reasons; this is part of the project management strategy, and it is sufficient to strictly follow the established plan on site. Welding wire: ER90S-B9 is primarily used; its composition must match that of the base material. The surface should be clean, free from rust, oil, and contamination, and it should be used within its shelf life. Rare earth-modified welding wires for special operating conditions must be used in accordance with specific manufacturing procedures. Welding rod: Use the E9018-B9 low-hydrogen welding rod. Before use, it should be dried as specified, with the temperature generally maintained at 350–400°C for 1–2 hours. After drying, it should be placed in a storage container for immediate use, to prevent moisture absorption and an increase in hydrogen content. (III) Pipe and groove pretreatment: First, inspect the appearance, dimensions, and surface quality of the pipes to ensure that there are no defects such as cracks, thick coatings, or damage that exceed the specified limits. The groove is usually V-shaped or U-shaped, and can be machined or cut with plasma. After processing, the groove as well as any oxide scale, rust, or oil on its sides must be thoroughly removed until a uniform metallic shine is achieved. II. Key Controls in the Welding Process (1) Preheating before welding: P91 steel has a high alloy content and a strong tendency to harden; it is also sensitive to cold cracks. Preheating is the first step in preventing cracks. Preheating temperature: 200–300°C, to be accurately monitored using a thermocouple or infrared thermometer. Preheating range: **≥100 mm** on each side of the weld to ensure uniform temperature in the area. On-site reminder: P91 is highly sensitive to preheating; insufficient temperature can easily cause cracking. Based on field experience, it is recommended to prefer electric heating and use flame heating with caution. In actual projects, a reasonable choice can be made based on the pipe diameter, wall thickness, and site conditions, but it is essential to ensure uniform heating. (II) Interlayer temperature control: The interlayer temperature should generally not exceed 350°C. Excessively high temperatures can cause the grains in the weld and heat-affected zone to become coarse, resulting in a decrease in toughness. Both manual welding and submerged arc welding require strict monitoring: submerged arc welding involves high heat input and slow heat dissipation, making continuous overheating likely to occur ； To speed up the process, manual welders tend to increase the current, which can also lead to overheating. After each weld is completed, the temperature must be measured; welding the next layer can only proceed once it falls within the allowable range. (III) Common welding parameters and processes: TIG welding for root pass, SMAW welding for fill and cap pass. Adhere to the principle of multi-layer and multi-pass welding, control the thickness of each pass as well as the heat input, to avoid excessive thickness in any single pass. Based on the wall thickness and position, appropriately select current, voltage, welding speed, and electrode diameter to ensure good fusion. (IV) Key points for on-site operation: Arc initiation must be carried out within the groove; it is prohibited to initiate the arc arbitrarily on the surface of the base metal, as this can cause arc pits. The arc crater must be filled when ending the arc to avoid cracks in it. Maintain a proper angle for the welding torch/welding rod, and use an appropriate welding technique to ensure good shape and fusion quality. III. Post-weld treatment (key step): The post-weld treatment of P91 directly determines the quality of the joint. It is recommended to proceed with temperature control and heat treatment without delaying for intermediate inspections after welding is completed: cool the joint slowly to 80–100°C and maintain this temperature for about 1 hour, after which heat treatment can be initiated. Within the specified temperature range, keeping the temperature as high as possible and ensuring sufficient holding time facilitates hydrogen diffusion and stress relief. (1) Post-heating: Immediate post-heating is carried out after hydrogen elimination welding, at a temperature of 300–350°C, for 2–4 hours, to facilitate the escape of hydrogen and significantly reduce the risk of hydrogen-induced cracks. (II) Post-weld heat treatment temperature: 730–780°C. Insulation time: Approximately 1 hour for every 25 mm of wall thickness, with a total insulation time of no less than 1 hour. The heating rate should be ≤220°C/h, and the cooling rate must be kept within 80–100°C/h to avoid the generation of new stresses. After completion, the effect can be verified through hardness testing to ensure that the structure and properties meet the required standards. IV. Welding quality inspection – Visual inspection: Check the weld shape, surface defects (cracks, pores, slag inclusions, undercuts, etc.), and dimensional dimensions. Non-destructive testing: RT, UT, and other testing methods are carried out in accordance with specifications and design requirements to detect defects such as internal lack of fusion and incomplete welding. Mechanical property tests: Tensile, impact, and other tests are conducted as required to verify the strength and toughness of the joint. Conclusion P91 steel pipeline welding is a high-precision, high-risk task. Only through meticulous control of every aspect of the process – from the personnel involved, welding materials, groove preparation, preheating, layer temperature, welding procedures, post-heating, heat treatment to inspection – can welding quality be consistently ensured, thereby guaranteeing the long-term safe operation of the pipelines under high-temperature and high-pressure conditions.