During the welding process, uneven localized heating and cooling of socket-welded stainless steel pipe fittings can generate residual stresses in the weld and heat-affected zone (HAZ). These residual stresses not only affect the dimensional stability of the fittings but can also reduce their fatigue strength, corrosion resistance, and overall load-bearing capacity, potentially leading to brittle fracture or stress corrosion cracking. Therefore, effectively eliminating residual stresses in socket-welded stainless steel pipe fittings is crucial for ensuring their long-term safe operation.
Welding residual stresses primarily arise from the uneven temperature field during welding. After melting at high temperatures, the weld shrinks during cooling, constrained by the surrounding cooler metal. This causes localized plastic deformation, which in turn generates self-equilibrium internal stresses within the fitting. Structural features of socket-welded stainless steel pipe fittings, such as the spigot-and-socket connection between the fitting and the pipeline, can exacerbate stress concentrations, particularly at the weld root or at points of geometric change. If not promptly eliminated, these stresses can compound with external loads during service, leading to structural failure.
Heat treatment is a classic method for eliminating residual stresses in socket-welded stainless steel pipe fittings. By heating the pipe fitting to an appropriate temperature and holding it for a specified period, creep or phase transformation occurs in the material, thereby relaxing internal stress. For austenitic stainless steel pipe fittings, solution treatment simultaneously eliminates residual stress and restores corrosion resistance. For martensitic or ferritic stainless steel pipe fittings, annealing effectively reduces hardness and improves toughness. The key to heat treatment lies in controlling the heating rate, holding time, and cooling method to avoid oxidation or grain coarsening caused by excessive temperatures, or the induction of new stresses due to excessive cooling.
Vibrational stress aging (VSA) uses an exciter to apply alternating stress to the pipe fitting. This stress, combined with residual stress, reaches the material's yield strength, inducing localized plastic deformation and gradually releasing internal stress. This method offers the advantages of simplicity, low cost, and high efficiency, making it particularly suitable for socket welding stainless steel pipe fittings with large or complex structures. VSA does not alter the material structure and can avoid oxidation or deformation issues that may be associated with heat treatment. However, careful selection of the excitation frequency and amplitude is crucial to ensure effective stress relief.
The hammering method uses water hammer or specialized tools to strike the weld and heat-affected zone, causing plastic expansion on the metal surface and thus offsetting some of the residual tensile stress. This method is suitable for thin-walled pipes or areas with localized stress concentrations, but the force and uniformity of the hammering must be controlled to avoid surface damage or degradation of the parent material due to excessive impact. After hammering, residual compressive stress forms on the pipe surface, helping to improve fatigue resistance.
The explosive treatment method uses the shock wave generated by the detonation of special explosives to induce instantaneous plastic deformation in the pipe, thereby eliminating residual stress. This method is suitable for long, slender structures or socket welding stainless steel pipe fittings involving dissimilar steels. It can treat large areas simultaneously and achieve significant results. However, the explosive dosage, placement of the explosives, and safety distance must be strictly controlled to avoid macroscopic deformation or damage to the pipe due to improper operation.
The ultrasonic impact method uses high-frequency vibration to impact the weld surface, causing localized plastic deformation and forming a residual compressive stress layer. This method is suitable for thin-walled pipes or applications requiring surface strengthening, significantly improving fatigue strength, suppressing weld cracking, and reducing stress concentration. Ultrasonic impact treatment offers no thermal impact, no pollution, and flexible operation, but its treatment depth is limited and its effectiveness in eliminating residual stress in thick plate pipe fittings is less effective.
In practical applications, eliminating residual stress in socket-welded stainless steel pipe fittings requires comprehensive consideration of the pipe material, structural characteristics, operating environment, and cost. For example, heat treatment remains the preferred method for high-temperature and high-pressure pipes; vibration aging treatment is more economical for large storage tanks or offshore platforms; and ultrasonic impact treatment offers a balanced approach to efficiency and surface quality for precision instruments or food-grade pipes. By choosing the appropriate elimination method, the service life of socket-welded stainless steel pipe fittings can be effectively extended, ensuring their safe and reliable operation.
