What should be paid attention to during the welding process of heat-resistant steel pipes

Basic Characteristics of Heat-Resistant Steel Pipe Welding
Heat-resistant steel pipes are widely used in high-temperature environments such as boilers, heat exchangers, and chemical equipment. Their welding requirements differ from those of ordinary carbon steel. Heat-resistant steel pipes typically contain high proportions of alloying elements such as chromium, nickel, and molybdenum. These elements enhance their strength and oxidation resistance at high temperatures, but they also affect their hot crack susceptibility, microstructure stability, and process window during welding. Therefore, during welding, a thorough understanding of the material's microstructure and stress dynamics is essential to avoid joint failure or degradation of heat resistance due to improper handling.

Pre-Welding Preparation and Preheating Requirements
Before welding heat-resistant steel pipes, thorough cleaning is essential to ensure the weld groove and surrounding areas are free of oil, rust, and impurities to prevent the formation of pores and slag inclusions during welding. Preheating is particularly important for welding heat-resistant steels, especially those containing high alloying elements. Preheating slows the cooling rate, reduces the tendency of hardening in the weld area, and minimizes the risk of cracking. Depending on the steel grade and wall thickness, the preheat temperature generally ranges from 150°C to 350°C. Before welding, a temperature measurement tool must be used to confirm that the preheat temperature meets the specified standards.

Welding Material Selection and Matching
Selecting the right welding material is crucial to ensuring welding quality. Heat-resistant steel pipe welding typically uses electrodes or wires that match the base material's composition to maintain joint strength and corrosion resistance in high-temperature environments. For example, Cr-Mo heat-resistant steel requires low-hydrogen electrodes, while austenitic heat-resistant steel requires heat-resistant consumables containing nickel and chromium. The diffusible hydrogen content of the consumables must also be considered. Low-hydrogen electrodes should be dried before use to prevent hydrogen-induced cracking.

Control of Welding Process Parameters
The heat input of heat-resistant steel pipe welding must be properly controlled. Excessive heat input can lead to coarsening of the grains and increased weld stress, thereby reducing the toughness and heat resistance of the joint. Excessive heat input can result in incomplete fusion or weld defects. Multi-pass welding is typically used to minimize the thermal effects of a single weld. Parameters such as welding current, voltage, and speed must be adjusted according to the pipe's material, thickness, and welding position to avoid stress concentration and potential cracking.

Deformation and Stress Control During Welding
Heat-resistant steel generates significant welding stress during welding. Improper control can lead to deformation or even cracks. Therefore, symmetrical welding or skip welding is often used to distribute stress during welding. Appropriate fixtures and fixtures are also used to prevent excessive displacement of the welded components. For thicker steel pipes, localized heating can be applied in sections during welding to alleviate stress concentration and ensure dimensional stability after welding.

The Importance of Post-Weld Heat Treatment
Heat-resistant steel pipe welded joints generate significant residual stress during cooling, and some steel grades can even develop a brittle microstructure, which is detrimental to pipes subjected to long-term high-temperature operation. Therefore, post-weld heat treatment is essential. Tempering or normalizing can reduce residual stress, refine the microstructure, and improve the overall performance of the joint. The heat treatment temperature and holding time should be tailored to the pipe material and wall thickness to ensure good high-temperature performance of the welded joint.

Inspecting and Repairing Welding Defects
After welding, the weld quality should be thoroughly evaluated through visual inspection, radiographic testing, ultrasonic testing, and other methods to ensure the absence of defects such as cracks, pores, and slag inclusions. Once defects are discovered, they should be repaired according to process requirements. Preheating and post-heat treatment should also be performed when repairing welded areas to prevent new defects. All repair processes should be documented for subsequent quality traceability.

Comparison of Welding Precautions for Different Types of Heat-Resistant Steel Pipes
Welding precautions vary depending on the type of heat-resistant steel. For example, preheating, welding material selection, and post-weld treatment are different for martensitic, austenitic, and duplex heat-resistant steels. The following table compares key precautions for welding several common types of heat-resistant steel:

Welding Environment and Safety Measures
Heat-resistant steel welding typically occurs under high-temperature and high-intensity working conditions, so safety precautions must be taken. Good ventilation should be maintained at the construction site to prevent welding fumes and harmful gases from affecting operators. Welders should wear protective clothing, masks, and high-temperature-resistant gloves to avoid injuries from welding spatter. Appropriate firefighting equipment should also be available to prevent fires caused by welding sparks.