Cold drawn seamless steel after annealing are necessary to stress, the residual stress of the materials is to improve the organization of materials, to improve the plasticity, thus preventing the breakage failure occurs seamless purposes. When the steel pipe is heated to a certain temperature (usually in the phase transition temperature of recrystallization temperature or less), holding time, and then slowly cooled, to eliminate the internal stress of precision seamless steel annealing process. In the pressure processing, casting, welding, heat treatment, machining and other processes, the products may produce stress. In most cases, after the end of the process, will be retained within the metal part of the residual stress. Residual stresses can lead to rupture of the workpiece, deformation or dimensional change, residual stress is increased chemical activity of the metal, the residual tensile stress is particularly likely to cause intergranular corrosion cracking. Therefore, the residual stress will affect the use of precision seamless performance or lead to premature failure of the workpiece.
For stress relief annealing, the steel pipe at a certain temperature under the action of local plastic deformation by an internal (temperature when the stress exceeds the yield strength of the material) or partial relaxation process (when the stress is less than the temperature of the yield strength of the material ) so as to eliminate the residual stress relaxation purposes. In stress relief annealing, the workpiece is heated to a lower temperature generally slow (gray cast iron is 500 ~ 550 a??, precision seamless steel pipe for the 500 ~ 650 a??, non-ferrous metal alloy stampings for the recrystallization temperature less), a period of time After slow cooling to prevent new residual stress.
Stress relief annealing and can not completely eliminate the residual stress within the precision seamless, but the majority is eliminated. To completely eliminate residual stress, the need to precision seamless steel pipe is heated to higher temperatures. Under these conditions, it may lead to other organizational changes, the use of performance-threatening precision seamless cold drawn seamless steel pipe deformation is too small to reach the surface finish and dimensional accuracy requirements, but also unable to reach members of the strength index; large deformation, seamless ductility, toughness decreases too much, and that the elongated crystal pulled too much, the formation of fibrous tissue, with obvious anisotropy metals. Cold drawn seamless mild steel pipe in the axial direction, parallel to the elongate direction of the grain, the intensity increased; cold drawn seamless radially perpendicular to the elongated direction of the grain, but lower strength, and maximum stress of the hydraulic cylinder is present in the radial direction of seamless, so much distortion on the performance of fully drawn tubes unfavorable. For steel and fibrous tissue appears dislocations, vacancies and other crystal defects are generally taken annealing or normalizing heat treatment methods to eliminate them. The purpose is to refine the grain annealing to remove tissue defects, reduce hardness, improved ductility, but also facilitate cold. Cold drawn seamless steel pipe production, annealing equipment is indispensable, therefore, to develop an appropriate annealing process is to ensure that cold drawn seamless organization qualified defective product is not a necessary condition for a fracture. If the manufacturers in order to unilaterally reduce production costs, reduce the annealing process is bound to adversely affect the quality of products.
