When the
carbon seamless steel pipe is cold drawn, there are defects such as cracks in the hot rolled tube blank. For example, after the high-precision
cold-drawn tube is made into an oil cylinder, the fracture that occurs during use has almost no plastic deformation, and is generally brittle fracture. Brittle fracture is caused by a variety of reasons. For example, when there are precipitates on the grain boundary, no matter their strength is stronger or weaker than that of the matrix, it is the cause of cracks; the segregation of inclusions on the grain boundary is also the cause of fracture; in addition, even at the intersection far less than the yield limit Under the action of variable load, fatigue fracture will also occur.
Generally speaking, the mechanical performance indicators used in the design of hydraulic (pneumatic) parts assume that the material is uniform, continuous and isotropic. According to this method, the design is considered to be safe, and sometimes accidents may occur. breakage accident. It has been found that during low-stress brittle fracture in high-strength metallic materials, the material structure is far from homogeneous and isotropic. There will be cracks, inclusions, pores and other defects in the structure, and these defects can be regarded as micro-cracks in the material.
In addition, brittle fracture is also related to the service temperature of the component. Through research, it has been found that when the temperature is lower than a certain temperature value, the material will transform into a brittle state, and its impact absorption work will decrease significantly. This phenomenon is called cold brittleness. Therefore, the design should also be based on the working temperature of the component. Choose a material with a suitable cold-brittle transition temperature.
At present, the material of cold-drawn seamless steel pipe (
a53 seamless) in my country is ordinary 45# or 20# steel 27SiMn steel, which improves the metal strength through cold-drawing deformation; however, it is at the expense of sacrificing the plasticity and toughness of the metal. High-precision cold-drawn seamless steel pipe is based on its high dimensional accuracy and high-strength performance. It must ensure that the amount of deformation is within a certain range in order to maximize the performance of the material and reduce the adverse effect on the material.
The deformation of the cold-drawn seamless steel pipe is too small to meet the requirements of surface finish and dimensional accuracy, and the strength index of the component cannot be achieved; if the deformation is too large, the plasticity and toughness of the seamless steel pipe will be reduced too much, and the grains will be pulled too much. Elongated, forming a fibrous structure, the metal will have obvious anisotropy. The axial direction of the cold-drawn seamless steel pipe is parallel to the elongation direction of the grain, and the strength increases; the radial direction of the cold-drawn seamless steel pipe is perpendicular to the elongation direction of the grain, and the strength decreases instead, and the maximum stress of the hydraulic cylinder Existing in the radial direction of the seamless steel pipe, so too large deformation is unfavorable to give full play to the performance of the cold drawn pipe.
For the fiber structure and crystal defects such as dislocations and vacancies in the steel, heat treatment methods such as annealing or normalizing are generally used to eliminate them. The purpose of annealing is to refine grains, eliminate structural defects, reduce hardness, improve plasticity, and facilitate cold working. In the production of cold-drawn seamless steel pipe, annealing equipment is indispensable. Therefore, formulating a suitable annealing process is a necessary condition to ensure that the cold-drawn seamless steel pipe is qualified and the product does not have fracture defects. If the manufacturer reduces the annealing process in order to unilaterally reduce the production cost, it will definitely have an adverse impact on the product quality.
