While the hot, glowing material is closely associated with the forging process this is not always the case. Rob Mayer discusses the differences between forging at cold, warm, and hot temperatures and how each of these affect the finished product. By forging at different temperatures there are unique advantages that can be achieved while forging. Working with a forging expert who understands these differences ensures a quality product.
Well, forging of course is deformation in a solid state. Cold forging would be doing it at room temperature or near to room temperature. The purpose of doing that is to save energy for one thing. Not to have to heat the materiel. Also, the materials that you can cold forge will then maintain those kind of dimensions once they are removed from the die. They are not going to heat up so much that will be subject to shrink; that's the primary reason to do it. High volume is typical for cold forging operations.
The advantage of cold forging is, again, it's a savings of energy, so it's very efficient for high volume production. You're going to get more accurate dimensions for those materials that can be cold forged because they are not going to shrink in dimension as you would get if you had something heated to warm or a hot forging temperature.
Warm forging is forging at an intermediate temperature between what you might consider room temperature or cold forging, and hot forging, what we think of as traditional forging temperatures. Typically the idea is to stay below, in steel materials anyway, what's considered the scaling temperature where oxides would begin to form on the outside surface of the part. So, again, you're maintaining much closer tolerances because you're not having to deal with scale. You're not dealing with full shrink from hot forging temperatures, so you're getting closer to net shape that way. That means there's potentially no or less finishing.
Now with hot forging every material is going to have a different temperature range for what's considered hot. The idea that you've put the material into a thermal state is that you're able to deform it most easily, most readily. So, there's certain kinds of materials like very high nickel chrome, super alloys, that really you cannot form in any other way. You really have to take them to these higher temperatures to form them. So, that's the kind of thing that you see in hot forging.
Now the other thing is that in comparison to all of these different processes– hot, warm, cold– there are different metallurgical characteristics that one will achieve by starting with those different temperatures. In certain cases, one must start with hot forging temperatures in order to develop the correct microstructure.
The advantage of hot forging is going to be simply the ability to form certain kinds of materials that cannot be formed any other way. Even then if you were trying to forge them, and you might be able to forge them successfully at slightly lower temperatures, you might also destroy the tooling because the material is so literally at that temperature stronger than the tool you're trying to use. So, to preserve tooling is another reason to extend temperature up higher so that you can actually get productivity out of the dies.