Annealing of heat treatment for precision parts processing
The main purpose of processing and annealing for precision parts is to soften hard steel for machining or cold working.
Usually, the steel is heated very slowly above the critical temperature, and maintained at this temperature until the workpiece is heated uniformly, and then cooled slowly at a controlled rate, so that the temperature of the surface and interior of the workpiece is approximately the same. This process is called complete annealing because it removes all traces of the previous structure, refines the grains and softens the metal. Annealing also releases the internal stress previously in the metal. The annealing temperature of a given steel depends on its composition; For carbon steel, it can be easily obtained from the local equilibrium diagram of iron carbon alloy. After reaching the annealing temperature, the steel shall be kept at this temperature until it is heated uniformly. Generally, the heating time is about 45 minutes per inch (25 mm) based on the maximum section thickness of the workpiece. For maximum softness and ductility, the cooling rate should be very slow, such as allowing parts to cool down with the furnace. The higher the carbon content, the slower the cooling rate must be. The heating rate should also be coordinated with the size and uniformity of the cross section, so that the whole part can be heated as evenly as possible.