What exactly is the difference between hot forging and cold forging? Are there advantages to one over the other? In this article, we explain both the differences and the advantages of each method.
When it comes to metal forging, there are basically two kinds: hot forging, and cold forging. While Weldaloy only works with hot forging, it’s our desire to have this article be an excellent one-stop guide to both kinds.
Heating to the Proper Temperature
In order to properly hot forge metal, it must be heated significantly. Average temperatures can range from the low 660s Fahrenheit to nearly 2200, depending on the metal. For example, aluminum alloys can be successfully hot forged anywhere from 680 to 1148 degrees Fahrenheit. Copper alloys, like brass and bronze, hot forge at much higher temperatures—about 1292 to 1472 degrees Fahrenheit. Steel has the highest hot forging temperatures, as it is workable up to 2102 degrees Fahrenheit.
Oftentimes, the temperature at which a metal forges and at which it melts can be quite close; experienced metallurgists can tell the difference often by mere sight (the metal’s color changes quite rapidly). Hot forging also uses the quenching and annealing process more frequently than cold forging, given the heat difference. (Quenching is the process by which a metal is cooled rapidly in order to harden it, using water or sometimes chemicals; annealing is when the heated metal is allowed to cool slowly, so that it softens and can be manipulated more easily by the metalworker.)
The Process of Deformation
Both hot and cold forging are done in order to deform metal—to cause it to lose its initial shape and form (and to an extent its chemical composition) in order to shape it and make it useable for other things. During deformation, grains or crystallites in the metal are produced as the metal cools, and recrystallization is when the deformed grains are displaced by newly-formed ones which eventually consume and replace the deformed ones. This is called recrystallization; this process is vital to forging, as it allows the metal to reduce in strength and hardness in order to be shaped and molded. During hot forging, the metal’s temperature must be higher than the metal’s point of recrystallization—this is necessary to prevent the metal from strain hardening, a kind of strengthening of the metal through deformation. (The metal can’t be worked if it is too hard.)
Hot Forging vs. Cold Forging
Ideally, hot forging is best suited to metals which possess a high degree of formability (the ability to be deformed without being damaged, which differs depending on the specific metal). However, there are multiple things to consider when determining whether to use hot or cold forging in your metallurgy work.
Some of the unique results and considerations involved in hot forging include things like the ability to produce discrete pieces more easily, low to medium accuracy, the formation of scale (a mind of oxidation) on the metal, low metal stress levels and lower work hardening temperature, grain homogenization, higher ductility levels, and the elimination of chemical incongruities in the metal.
But there are drawbacks as well. The metal can warp during the cooling process. Some metals become very brittle or break if they go too long without being annealed. Some metals have less precise tolerances for the process. The grain structure in the metals can vary. Additionally, the surrounding atmosphere can possibly create negative reactions on the metal as it is being forged. Still, hot forging is better suited to things like aerospace products and airplane parts, as the softness produced by the metal’s heat allows it to be more easily shaped and intricately molded. (Some jewelry-related metals, like copper, are also cold-forged.)
Cold forging is a bit of a misnomer. It is only “cold” in relationship to the very high degrees used for hot forging. In reality, the temperatures for cold forging are at or near room temperature. While many jewelry metals, including gold, brass, silver, and copper, can be cold forged to produce beautiful and artistic results, most commonly cold-forged items are limited to steel products (like carbon alloy sheets). Recall our discussion of recrystallization above.
Hot forging happens above a metal’s recrystallization point. Cold forging, then, takes place below it. Because of this lower temperature, generally cold forging is best done on metals that are already softer and do not require the softening process of hot forging. Because there is less involved in the process, cold forging often tends to be less expensive than hot, and requires little finishing work.
Interestingly, after cold forging is completed, a heating process called “tempering” can be done to strengthen the metal. Quite commonly, cold forging is done by placing the metal into a die, and a hammer will then force the metal into it, causing it to take on the die’s shape. The hammer can either be mechanized or operated manually, and depending on the desired finished product, the hammer can actually be brought down repeatedly in a very rapid sequence.
As with hot forging, cold forging offers a number of distinct advantages, weaknesses, and things to consider. Because cold forging is less involved and does not require the same “finishing” processes as hot-forged metals (like spraying on thermal coatings or other things), these aspects can usually be dispensed with entirely and thus can save money.
Cold-forged metals are less vulnerable to contaminants and environmental stressors, and often the metals’ surfaces feature a better finish than hot forged products. Additionally, cold forged metals impart directional properties more easily, are improved in interchangeability and reproducibility, and have increased dimensional control. They are also better at handling higher die and stress loads, and can easily produce net-shape parts (or close to them).
However, these same things can also be disadvantages to choosing cold metal forging. Moreover, any scale on the metal must be removed before cold forging, and resultant metals are far less ductile than hot forged ones. The process may create residual stress on the metal, and because both more powerful, much heavier equipment and stronger tooling are necessary, some consumers may find that cold forging may create more problems than it solves—or at least is better suited to a more specialized and limited array of both metals and metallurgists.
The capability to transform hard metals into both things of beauty and utility is both a gift and a talent, and both hot and cold forging will be mainstays of that creative process for years to come.