Stainless steel pipe is widely used in chemical industry, construction, food and other fields due to its excellent corrosion resistance and mechanical strength. However, in the welding process, intergranular corrosion and stress cracking are common and difficult problems. Intergranular corrosion will weaken the corrosion resistance of the pipe, and stress cracking will directly threaten the safety of the structure. Therefore, optimizing the welding process to avoid these two problems is crucial to ensure the welding quality and engineering reliability of stainless steel pipe.
Selecting suitable welding materials is the basis for preventing intergranular corrosion and stress cracking. For stainless steel pipe welding, welding materials with low carbon content (C≤0.03%) are preferred, such as 308L, 316L and other types of stainless steel welding wire. Low carbon content can reduce the precipitation of chromium carbide during welding, avoid chromium depletion at grain boundaries, and thus reduce the risk of intergranular corrosion. In addition, stabilizing elements such as niobium (Nb) and titanium (Ti) in welding materials can preferentially combine with carbon to form stable carbides, further preventing the formation of chromium carbide at grain boundaries. In terms of stress cracking, choosing welding materials with similar composition and good toughness to the parent material can ensure that the performance of the weld and the parent material match and reduce stress concentration caused by performance differences.
Welding process parameters have a direct impact on intergranular corrosion and stress cracking. The use of low current and fast welding can reduce welding heat input, shorten the residence time of the weld joint in the sensitization temperature range (450-850℃), reduce the precipitation of chromium carbide, and inhibit intergranular corrosion. For example, when welding thin-walled stainless steel pipe, controlling the welding current to 80-100A and the welding speed to 150-200mm/min can effectively avoid intergranular corrosion in the heat-affected zone. At the same time, reasonably control the interlayer temperature to avoid welding the next layer before the previous layer of weld is cooled during multi-layer welding, and prevent local overheating from causing coarse grains and stress accumulation. In addition, the use of symmetrical welding, segmented skip welding and other methods can disperse welding stress, reduce stress concentration, and reduce the possibility of stress cracking.
Taking protective measures during welding can effectively reduce the intrusion of harmful elements and improve welding quality. When using argon shielded welding (TIG or MIG), ensure that the purity of argon is above 99.99%, and set the gas flow rate reasonably, generally controlled at 8-15L/min, to ensure that the welding area is in inert gas protection, prevent impurities such as oxygen and nitrogen from reacting with alloy elements in stainless steel, and avoid weld metal embrittlement and intergranular corrosion. For root welding, back argon filling protection can be used to prevent oxidation of the back of the weld and ensure the quality of the full section of the weld. In addition, before welding, thoroughly clean the oil, rust and other impurities on the surface of the pipe and welding material to avoid component segregation and local corrosion caused by impurities.
Post-weld heat treatment is an effective means to eliminate welding residual stress and improve organizational properties. For austenitic stainless steel pipe, solid solution treatment or stabilization treatment is often used. Solution treatment is to heat the weld joint to 1050-1100℃, keep it warm for a certain period of time, and then cool it quickly to dissolve the chromium carbide in austenite and restore the corrosion resistance of the grain boundary; stabilization treatment is to heat it to 850-950℃, keep it warm, and then cool it slowly to make stabilizing elements such as niobium and titanium fully combine with carbon to form stable carbides to avoid intergranular corrosion. At the same time, stress relief annealing (generally heated to 550-650℃) can effectively eliminate welding residual stress and reduce the risk of stress cracking. However, different types of stainless steel pipe need to choose appropriate heat treatment processes and parameters according to their composition and performance requirements.
The operating skills of welders have a significant impact on welding quality. Mastering welding techniques, maintaining a uniform welding speed and a stable arc length can ensure good weld formation, reduce defects such as pores and slag inclusions, and reduce stress concentration and corrosion risks. In addition, a complete quality inspection system is indispensable. After welding is completed, visual inspection, non-destructive testing (such as X-ray flaw detection, ultrasonic flaw detection) and other methods are used to strictly inspect the appearance and internal quality of the weld to promptly discover and repair welding defects. Through hardness testing, intergranular corrosion testing and other means, verify whether the performance of the welded joint meets the requirements to ensure the reliable quality of stainless steel pipe welding.
To avoid intergranular corrosion and stress cracking during stainless steel pipe welding, comprehensive measures are needed from multiple aspects such as welding material selection, process parameter control, process protection, post-weld heat treatment, operating skills and quality inspection. By reasonably optimizing the welding process, the corrosion resistance and structural strength of the stainless steel pipe welded joint can be effectively improved, its safety and stability in various engineering applications can be guaranteed, and the material advantages of stainless steel pipe can be fully utilized.
