The stainless steel weld tube, abbreviated as welded pipe, is a steel pipe welded by rolling steel or steel strip through a unit and mold.The welded steel pipe has a simple manufacturing process, high production efficiency, diversified variety and specifications, and less equipment investment, but generally has lower strength than seamless steel pipe. Since the 1930s, with the rapid development of high-quality strip steel continuous rolling production and welding and inspection technology, the quality of welds has been continuously improved, the variety and specifications of welded steel pipes have increased day by day, and they have replaced seamless steel pipes in more and more fields, especially in heat exchange equipment tubes, decorative pipes, and low and medium pressure fluid pipes.
The material and mechanical properties of stainless steel weld tube fittings will change during production due to rolling, stretching, and high temperature during welding, which seriously affects the quality of the pipe. These changes mainly include: work hardening, that is, regular arrangement of one or several rows of atoms in the crystal, distortion of the lattice, and formation of a stress concentration zone. This phenomenon is also called dislocation. Due to the uneven welding stress and the formation of ferrite in the original single austenite structure, carbides and other uneven structures, in order to improve production efficiency, ensure the surface of the welded pipe is bright, eliminate residual stress, and obtain a single austenite structure to produce high-quality welded pipes, the online bright solution treatment method must be implemented.
Liquid ammonia will undergo the following decomposition reaction above 400℃ when used as a protective atmosphere. Nitrogen is a neutral gas that can reduce certain metals under certain conditions. Using the decomposition gas of liquid ammonia as a protective atmosphere can make the surface of stainless steel weld tube fittings white and smooth, so it is a more economical and ideal protective gas.
The solid solution treatment temperature is one of the more important parameters in the solid-liquid treatment process. If it is too high or too low, it will directly affect the quality of the welded pipe. When the temperature is too high, the welded pipe structure will become coarse and its properties will decrease. If the temperature is too low, the solid solution will not be complete and the stress relief will not be complete. Therefore, the temperature of 1080℃±10℃ should be selected, and proper insulation should be carried out to fully dissolve the carbides in the steel.
Austenitic stainless steel will become brittle in the temperature range of 650℃-850℃, usually in the form of needle-shaped or plate-shaped crystals in the austenite grains or grain boundaries, greatly reducing the plasticity, toughness, and strength. In the temperature range of 450℃-850℃, it is also the intergranular corrosion sensitive zone of austenitic stainless steel. The essence is that in the metal grains in the temperature zone, the carbon atoms that are over-saturated and solid-solved gradually diffuse to the grain edges, form carbides with the chromium atoms in the edge layer, and precipitate along the grain boundary, resulting in a loss of corrosion resistance due to the chromium depletion at the grain edges. When the austenitic steel is heated to 1080℃±10℃ and the carbides in the steel are fully dissolved, it should be rapidly cooled to avoid staying in the temperature range of 450℃-850℃, which can prevent intergranular corrosion and not allow carbides to precipitate along the austenitic grain boundary, thereby obtaining the uniform saturated single austenite structure we need.