Ball (softened) annealed countersunk head screw, hex socket head cap bolts cold forging process, the original organization of steel will directly affect the forming ability of cold heading. Cold heading process in the local area of ??plastic deformation up to 60% -80%, for which the requirements of steel must have good conductility. When the chemical composition of the steel is constant, the microstructure is the key factor that determines the plasticity. Generally, it is considered that the coarse lamellar pearlite is not conducive to cold heading and the fine spherical pearlite can significantly improve the plastic deformation ability of the steel. Medium-carbon steels and medium-carbon alloy steels with high amount of high-strength fasteners are used for balling (softening) annealing before cold forging so as to obtain even and fine spherical pearlite to better meet practical production needs. For mild steel rod softening annealing, the heating temperature and more choice at the critical point of the steel up and down insulation, heating temperature generally can not be too high, otherwise it will produce three cementite precipitation along the grain boundary, resulting in cold heading cracking, and for Medium-carbon alloy steel wire rod isothermal annealing, AC1 + (20-30%) heating, the furnace cooled slightly lower than Ar1, the temperature is about 700 degrees Celsius for a period of time, and then cooled to about 500 degrees Celsius baked air-cooled. Metallurgical structure of the steel from coarse to thin, spherical change from the sheet, cold heading cracking rate will be greatly reduced. 35 45 ML35 SWRCH35K steel softening annealing temperature is generally 715-735 degrees Celsius.
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