There are two categories of finishing operations: those that produce minor dimensional corrections, and surface treatment processes.
Coining, sizing and straightening are commonly used to improve the dimensional accuracy of forgings. When tolerances closer than those that can be economically produced in the forging die are specified, coining and sizing operations are often employed. These processes cause plastic deformation of the forging, either by striking or squeezing a defined area. Coining may be performed either hot or cold, and can be accomplished either before, during or after heat treatment. Cold coining is preferred
because material shrinkage is not a factor and the process can produce excellent surface finishes. Special dies can be used to coin irregular surfaces, but flat surfaces, such as bosses, are most suitable and can be sized to very close tolerances.
Forgings may become slightly warped, twisted or bent during trimming, heat treating, cleaning or handling. They can usually be straightened either manually or by using a special purpose fixture and combining the straightening operation with a sizing operation in a coin press. Cylindrical parts such as tubes, axles and various types of shafts are ordinarily straightened in machine rolls when sufficient quantities are involved.
Surface treatment operations may be performed on forgings to remove scale formed during forging and heat treatment, improve corrosion resistance, enhance appearance and improve surface properties. Surface treatment requirements are based on the alloy and forging process employed. For example:
- Cold and warm forging operations are performed on steel alloys below the temperature range at which scale forms and do not require the removal of scale.
- Scale formation of stainless steel alloys is not generally a problem.
- Aluminum alloys can be given a hard anodized surface to improve abrasion and corrosion resistance.
- Forgings made from copper based alloys generally exhibit good surface quality and high corrosion resistance that eliminates the need for most surface treatment operations.
Forgings generally employ essentially the same finishing operations for corrosion protection and enhanced appearance as castings and weldments made from equivalent alloys. When subsequent machining, painting or other coating operations are anticipated, the forgings are usually cleaned by blasting, tumbling, pickling or, (as with aluminum) caustic-nitric etch. Blast or shot cleaning is the most common cleaning method.