Measures of Preventing Welded Pipe Cold Crack

Pipe welded joints rigid, multi-layered welding, welding metal bulky, so that welded joints in high-stress state, such as the weld metal has a higher yield point, plastic deformation is difficult to relax through the welded joints of high Stress is easy to weld cracks in the heat affected zone of white areas or areas produce martensite, the formation of HAZ cold cracking. Prevent cold cracking high-pressure carbon steel pipe is the most effective way to butt up to 550 ~ 700 ℃ pieces of the overall warm-up, followed by the use of heterogeneous weld welding materials. Brittle fracture is caused by a variety of reasons. Such as grain boundary precipitates on time, regardless of their strength than the matrix strength is strong or weak, are all causes of cracks; grain boundary segregation of inclusions on the fracture is also the reason; In addition, even if far less than the yield limit in the cross variable load, can also cause fatigue fracture phenomenon.

In general, hydraulic (pneumatic) components used in the design of mechanical properties, it is assumed that material is uniform, continuous, isotropic, according to the method of analysis of this design that is safe, sometimes accidents breakage. The study found that in high-strength metals that occur in low-stress brittle fracture process, the materials the organization is far from uniform, isotropic. Organization will have cracks, there will be inclusions, porosity and other defects, which can be seen as material in the micro-cracks. In addition, the brittle fracture with the use of component temperature. People through the study found that when the temperature is below a certain temperature, the material will be transformed into brittle state, the impact absorbed energy decreased, a phenomenon known as cold brittle, so the design must work under the component temperature select a suitable cold-brittle transition temperature of the material. Cold drawn seamless steel pipe large deformation is too small to achieve the surface finish and dimensional accuracy requirements, they can not achieve the strength index components; deformation is too large, high-pressure plastic pipe, reducing too much toughness, and grain is pulled excessively slender, forming a fibrous tissue, the metal will have obvious anisotropy. Shandong cold drawn seamless steel pipe plant in the axial, elongated parallel to the grain direction, intensity increased; high-pressure pipe radial direction perpendicular to the elongated grain, but lower strength, and hydraulic oil cylinder is the largest stress exists in the radial seamless, so the deformation is too large for the full performance of cold drawn tubes negative. For steel and fibrous tissue appear dislocations, vacancies and other crystal defects, generally taken to annealing or normalizing heat treatment methods to eliminate them. The purpose of annealing is grain refinement, elimination of tissue defects, reduce the hardness and improve ductility of carbon steel pipe.