Cold-drawn pipe is a hollow steel pipe with round, square and rectangular cross-sections that is produced without joints on the entire steel pipe surface by drawing, extrusion, perforation, etc. During the processing of cold drawn tubes, many steps are taken to obtain such high performance tubes. Satisfactory metallographic structure can be obtained by eliminating work hardening of cold-drawn steel pipes. The process equipment is a bright annealing furnace, which is mainly used for heat treatment of stainless steel products in a protective atmosphere. When the application performance requirements of cold-drawn tubes are different, the requirements for the metallographic structure after bright annealing are different, so the bright heat treatment process will also be different.
After cold working, residual stress will be left in the cold drawn pipe material, which is very detrimental to the stress corrosion cracking resistance of the pipe. Any degree of cold working results in a greatly increased susceptibility of the material to stress corrosion cracking. The degree of cold working can also adversely affect the high temperature durability of austenitic stainless steels. Generally, the higher the operating temperature or the higher the fracture life requirement, the lower the degree of cold working. Another important issue in the heat treatment of cold-rolled strip for cold-drawn pipe is the requirement that the entire strip has a uniform structure in width and length. The muffle type bright annealing furnace adopts a large size muffle tube, which organizes the heating airflow from the outside of the muffle tube evenly, and rotates around the furnace to make the strip steel evenly heated.
It can be seen from the above introduction that the heat treatment process of cold-drawn tubes is still relatively cumbersome. In order to obtain a qualified metallographic structure, the adjustment of the equipment in the cooling section of the bright annealing furnace requires a lot of space. Therefore, the cooling section of the current advanced bright annealing furnace mostly adopts strong convection cooling, and is equipped with three cooling sections, which can adjust the air volume separately. It is divided into three sections along the width direction of the strip, and the cooling rate in the width direction of the strip is adjusted by airflow splitting to control the shape of the strip.
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