There are many aspects that go into all kinds of metal work. Forging is interesting as it has two basic methods that are totally different however they deliver nearly the same result. The main idea behind either concept of hot and cold forging is that metal will be deformed to a predetermined shape. Each have their own advantages and disadvantages and we will describe the basic criteria that go into each.
Hot Forging
When something is being hot forged, it must as the name implies be heated significantly. The average temperatures for metals such as steel is up to 1150 degrees Celsius. Metals that have a lower forging temperature like aluminum alloys will be around 700 degrees celsius. When a metal is being hot forged the temperature must be kept above the recrystallization point. As the metal cools down and is re heated again a process of crystallisation occurs when is crystals form on top of previously formed crystal it leads to a condition called strain hardening. If the metal gets too hard then it will be too difficult to form into shape. Sometimes during the forging of certain metals it is necessary to prevent the oxidation that occurs. This is a special treatment for when forging superalloys and takes place in an atmosphere controlled environment such as a vacuum.
Advantages to Hot Forging
Items that have been hot forged have an increased ductility which means that the metal will stretch under tensile stress. This method of forging has a better flexibility for customization and allows for a variety of finishings which can be modified to suit the customer.
Disadvantages to Hot Forging
Some of the aspects where cold forging may have an advantage would be that with hot forging there is a less accurate dimensional tolerance. There is also the concern that during the cooling process there will be a warping effect. The grain of hot forged metals may vary and as mentioned above there may be oxidisation in the piece when it interacts with the atmosphere. Also there is a high amount of water that is required in the hot forging process. In areas that are affected by the global water shortage these forges must be extra careful with waste water disposal from production.
Cold Forging
Unlike Hot forging the cold forging takes place at or below room temperature. Cold forging is done well below the recrystallization point and is the prefered method of forging for typically “softer” metals like aluminum. Cold forging has a definite advantage over hot forging as it is very cost effective and does not require the same considerations and tooling as hot forging. Cold forged metals also require only a small amount of finishing work. Even though the process is cold there is still a treatment of heating the metal to strengthen it which is called tempering. One of the most common kinds of cold forging is called “impression die cast” where in by a piece of metal is hammered through a fixed in place die. Cold forging does also not leave the metal susceptible to environmental factors. One downside to this process is that it requires stronger heavier machinery than the hot forging process.
Advantages of Cold Forging
Cold forging is a very effective means of saving money during production as the rough product does not require that much finishing work to be showfloor ready. The production method also saves a lot of material by being precise. Highly reliable production rates along with a long die life mank cold forging the choice for many production manufactures of metal products.
Disadvantages to cold forging
Cold forging has some good characteristics when it comes to efficiency however the process is a bit lacking in the detail of shapes that can be produced compared to hot forging. A customized metal shape or item may be beyond the abilities of cold forging. The metals are also a lot less ductile which will not be acceptable in certain conditions. Furthermore the grain structure may be compromised due to residual stress.
Conclusion
These methods of hot and cold forging both have their advantages when producing certain products and shapes. The cold method being cheaper when it comes to mass producing the same configuration, however it gives way to the strength and versatility of the hot forging method.