Generation And Elimination Of Cold-rolled Seamless Steel Pipe Fracture

When the seamless steel pipe is cold drawn, due to defects such as cracks in the hot rolled billet or high-precision cold drawn tubes being formed into the cylinder, almost no plastic deformation occurs during the fracture occurring during use, and is generally brittle fracture. Brittle fracture is caused by a variety of causes. For example, when there are precipitates on the grain boundary, whether the strength is stronger or weaker than the strength of the matrix, it is the cause of cracking; the segregation of inclusions on the grain boundary is also the cause of the fracture; in addition, even at a distance far less than the yield limit Under the action of variable load, fatigue fracture will also occur.

In general, the mechanical properties of hydraulic (pneumatic) parts used in design are assumed to be uniform, continuous, and isotropic. According to this method, the design is considered safe and sometimes accidents occur. Break accident. It has been found that in the process of low-stress brittle fractures occurring in high-strength metal materials, the material structure is far from uniform and isotropic. There are cracks in the structure, and there are defects such as inclusions and pores. These defects can be regarded as microcracks in the material.

In addition, brittle fracture is also related to the temperature at which the component is used. It has been found through research that when the temperature is lower than a certain temperature value, the material will change into a brittle state, and its impact absorption work is obviously reduced. This phenomenon is called cold brittleness. Therefore, the design is also based on the operating temperature of the component. Select a material with a suitable cold-brittle transition temperature.

High-precision cold-drawn seamless steel tubes generally improve the properties of the materials by adding alloys to the metal. After cold drawing, stress relief annealing is required to eliminate residual stress of the material, improve the structure of the material, and improve the plasticity of the material. Therefore, the purpose of preventing the fracture failure of the seamless steel pipe is achieved.

At present, most of the cold drawn seamless steel tubes are made of ordinary 45# or 20# steel 27SiMn steel, which is improved by cold drawing deformation; however, it is at the expense of plasticity and toughness of the sacrificial metal. High-precision cold-drawn seamless steel pipe is based on its high dimensional accuracy and high strength performance. It must ensure that the deformation amount is within a certain range to maximize the performance of the material and reduce the material. Negative Effects.

The cold drawn seamless steel pipe is too small to meet the requirements of surface finish and dimensional accuracy, and can not reach the strength index of the component; the deformation is too large, the plasticity and toughness of the steel pipe are reduced too much, and the crystal grains are excessively elongated. The fibrous structure is formed and the metal will have significant anisotropy.

The axial direction of the cold drawn seamless steel pipe is parallel to the elongation direction of the grain, and the strength is increased; the radial direction of the cold drawn seamless steel pipe is perpendicular to the elongation direction of the grain, and the strength is instead decreased, and the maximum stress of the hydraulic cylinder It is present in the radial direction of the seamless steel pipe, so too much deformation is disadvantageous for fully exerting the performance of the cold drawn pipe.

For the fiber structure and the crystal defects such as dislocations and vacancies appearing in the steel, it is generally eliminated by heat treatment such as annealing or normalizing. The purpose of annealing is to refine grains, eliminate tissue defects, reduce hardness, improve plasticity, and facilitate cold working. In the production of cold drawn seamless steel tubes, annealing equipment is indispensable.

Therefore, the development of a suitable annealing process is necessary to ensure that the cold drawn seamless steel tubes are qualified and the products do not have fracture defects. If the manufacturer reduces the production cost and reduces the annealing process in a one-sided manner, it will definitely adversely affect the product quality.