◆1. For the production of countersunk screws and internal hexagonal cylindrical head bolts using the cold heading process, the original structure of the steel directly affects the formability during cold heading.
◆2. The plastic deformation in local areas during the cold heading process can reach 60%-80%, which requires the steel to have good plasticity. When the chemical composition of the steel is fixed, the metallographic structure becomes the key factor determining the quality of plasticity. It is generally believed that coarse lamellar pearlite is not conducive to cold heading forming, while fine spherical pearlite can significantly improve the ability of steel to undergo plastic deformation.
◆3. For medium carbon steels and medium carbon alloy steels, which are used extensively in high-strength fasteners, a spheroidizing (softening) annealing process is conducted before cold heading to obtain a uniform and fine spherical pearlite structure, better meeting the actual production needs.
◆4. For the softening annealing of medium carbon steel wire rods, the heating temperature is often chosen to maintain around the critical point of the steel. The heating temperature should not be too high; otherwise, tertiary cementite will precipitate along the grain boundaries, causing cold heading cracks.
◆5. For medium carbon alloy steel wire rods, an isothermal spheroidizing annealing process is used. After heating to AC1+(20-30%) and then furnace cooling to just below Ar1, the temperature is maintained at about 700 degrees Celsius for a period of time, followed by furnace cooling to around 500 degrees Celsius and then air cooling. The metallographic structure of the steel changes from coarse to fine, from lamellar to spherical, greatly reducing the cold heading crack rate. The softening annealing temperature for steels such as 35\45\ML35\SWRCH35K generally ranges from 715-735 degrees Celsius.
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