Stainless steel thread-shaped products, due to their unique structure and material properties, are prone to defects such as cracking during welding caused by factors such as thermal stress, microstructural transformation, or impurity segregation. To avoid these problems, a comprehensive approach is needed, encompassing material selection, process control, structural design, and environmental management.
First, material selection is fundamental to preventing welding defects. For stainless steel thread-shaped products, low-carbon welding materials or those containing stabilizing elements (such as titanium and niobium) should be prioritized to reduce intergranular corrosion tendency. For example, 304L or 316L welding materials, by reducing carbon content, can effectively inhibit carbide precipitation at grain boundaries, thereby avoiding stress corrosion cracking caused by sensitization. Simultaneously, it is necessary to ensure that the chemical composition of the welding material matches that of the base metal to avoid segregation or brittle phase formation due to elemental differences. Furthermore, strictly controlling the content of harmful impurities such as sulfur and phosphorus can reduce hot cracking susceptibility and improve weld ductility.
Optimizing the welding process is the core element in preventing cracking. For thread-shaped stainless steel products, low heat input and rapid welding methods, such as pulsed TIG welding, are required to reduce the dwell time of the weld in the high-temperature zone and inhibit the formation of coarse columnar grains. During welding, short arc operation should be maintained to avoid insufficient shielding gas coverage due to excessive arc length, which can lead to oxidation or nitriding. For multi-pass welds, interpass temperature must be strictly controlled to prevent localized overheating that could cause grain coarsening or stress accumulation. Furthermore, the crater must be completely filled when the weld is finished to prevent subsequent propagation of crater cracks.
The impact of structural design on weld quality cannot be ignored. The threaded structure of thread-shaped stainless steel products is prone to welding stress concentration; therefore, the bevel shape and dimensions must be optimized in the design. For example, using U-shaped or V-shaped bevels can reduce the amount of weld metal filler and lower shrinkage stress. Simultaneously, increasing the transition fillet radius or reducing the thread root radius can disperse stress peaks and prevent crack initiation. In addition, for long welds or complex structures, the welding sequence must be rationally planned, employing symmetrical welding or segmented back-welding methods to balance heat input and reduce deformation and residual stress.
Preheating and post-heat treatment are crucial for controlling welding stress. Although austenitic stainless steel has poor thermal conductivity, proper preheating is still necessary for thick-walled or high-rigidity structures to reduce the temperature gradient and slow down the cooling rate, thereby preventing cold cracking. The preheating temperature needs to be adjusted according to the material thickness and composition, and is usually controlled within a certain range. Immediately after welding, solution treatment or stress-relieving heat treatment is required. This involves heating to a specific temperature and rapidly cooling to fully dissolve carbides, eliminate residual welding stress, and improve the weld's crack resistance and corrosion resistance.
Managing the welding environment is equally important. Stainless steel thread-shaped products are sensitive to environmental factors such as humidity and wind speed, and welding must be carried out under dry, windless conditions. High humidity environments can easily cause hydrogen absorption in the weld, leading to hydrogen-induced cracking; therefore, dehumidification measures or the construction of rain shelters are necessary. Furthermore, the welding area must be thoroughly cleaned of oil, rust, and other impurities to prevent porosity or oxidation inclusions caused by hydrogen or oxygen intrusion. For outdoor operations, wind protection measures must be strengthened to prevent the protective gas from being blown away, affecting weld quality. Operating skills and equipment selection directly affect welding stability. Welders must be proficient in short-arc welding and uniform electrode manipulation to avoid defects such as undercut and lack of fusion caused by improper operation. Simultaneously, selecting appropriate welding equipment and parameters, such as using a DC reverse polarity power supply, can improve arc stability and reduce spatter and porosity. For automated welding, it is essential to ensure equipment accuracy and program rationality to avoid welding defects caused by mechanical vibration or parameter fluctuations.
Finally, quality inspection and defect repair are the last line of defense for ensuring welding quality. After welding, visual inspection and non-destructive testing (such as penetrant testing and radiographic testing) are required to promptly detect and repair defects such as cracks and porosity. For existing cracks, they must be completely removed before re-welding to prevent defect propagation. At the same time, a welding process record and traceability system must be established to analyze the causes of defects, continuously optimize process parameters, and form a closed-loop management system.
