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What is high-strength bolt spheroidizing annealing?

Update:19-11-2020
Summary:

When countersunk head screws and hexagon socket head bo […]

When countersunk head screws and hexagon socket head bolts are produced by cold heading, the original structure of the steel will directly affect the forming ability during cold heading. In the process of cold heading, the plastic deformation of the local area can reach 60%-80%. For this reason, the steel must have good plasticity. When the chemical composition of the steel is constant, the metallographic structure is the key factor that determines the plasticity. It is generally believed that the coarse flaky pearlite is not conducive to cold heading, while the fine spherical pearlite can significantly improve the plastic deformation ability of the steel. For medium-carbon steel and medium-carbon alloy steel with a large amount of high-strength bolts, spheroidizing (softening) annealing is carried out before cold heading in order to obtain uniform and fine spheroidized pearlite to better meet actual production needs. For the softening annealing of medium carbon steel wire rod, the heating temperature is mostly selected at the upper and lower critical points of the steel. The heating temperature is generally not too high, otherwise it will produce tertiary cementite precipitation along the grain boundary, causing cold heading cracking. Medium-carbon alloy steel wire rods are annealed by isothermal spheroidization. After heating in AC1+ (20-30%), the furnace is cooled to slightly lower than Ar1, and the temperature is about 700 degrees Celsius for a period of time, and then the furnace is cooled to about 500 degrees Celsius and air-cooled. The metallographic structure of the steel changes from coarse to fine, from flake to spherical, and the cold heading cracking rate will be greatly reduced. The general softening annealing temperature of 35\45\ML35\SWRCH35K steel is 715-735 degrees Celsius; while the general heating temperature of SCM435\40Cr\SCR435 steel spheroidizing annealing is 740-770 degrees Celsius, and the isothermal temperature is 680-700 degrees Celsius.

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