There are many aspects to forging metal. However the most important thing to know is the difference between hot forging and cold forging. Below we will describe the basic components about each method and try to shed some light on the advantages and disadvantages of each.
Getting the proper Temperature
For the hot forging process to occur the metal must be brought to a temperature where it is both malleable and also retains the same molecular structure. At temperatures ranging from 660 to 2200 degrees Fahrenheit each different type of metal will have a workable temperature. For metals like aluminum alloys the temperature must be between 680 and 1148. For forging copper alloys such as bronze and brass it increases to between 1292 1472 degrees. The temperature that a metal forges at is often very close it its melting point. An experience meatal smith will know the temperature just by sight as the colour will change dramatically as it reaches different degrees. One key difference between hot and cold forging aside from the difference in temperature is that the hot forging method goes through a process that is called “quenching” and “annealing”. The term quenching is when the metal’s temperature is brought down rapidly by submerging it in water or a special chemical. This in turn hardens the metal. The process of annealing is when the temperature is brought down slowly which preserves it malleable characteristics. Some type of electric or gas kiln is used for this.
Deformation is the name given to the process when you manipulate its original shape and turn it into something usable for other things. When metal is going under the process small crystals will be produced when the metal cools. After the heating and cooling of metal occurs and many crystals are being developed they will start to be displaced by new ones. This process is called recrystallization. Recrystallization is very important to forging as it makes the metal lose its strength so it can be shaped into form. During the hot forging process the metals temperature must be above the point of recrystallization as to prevent a condition known as strain hardening.
The word cold forging gives away what it is about in the title as most of the time the process is done at or below room temperature. Most jewelry metals are cold forged into a wide variety of artistic results. However the most common product that is cold forged are industrial items like carbon alloy sheets. The main difference between hot and cold forging is that hot forging takes place above the metals point of recrystallization and cold forging takes place below. Metal that are suitable candidates for cold forging are typically softer ones. Harder metals need the softening treatment of hot forging or they would largely be unworkable. Cold process forging is cheaper as there is much less involved with the process. It also produces a product that requires less finishing. Even though the cold process is largely ‘cold” the interesting thing is that it is also tempered in that the metal is heated slightly and then it is hammer by a mechanical hammer. Often times this occurs when the metal is pushed through a die.
Pros and Cons
Both methods have distinct advantages and disadvantages as cold forging costs less however the end result of the hot forging process is a better and stronger product. Cold forged materials are also more resistant to contaminate and environmental effects. One disadvantage to cold forging is that the process may create residual stress on the metal which may lead to problems later.