Quick
Hot working is the deformation of an alloy at high temperatures without strengthening.
Details
Hot working (also called hot forming) involves deformation at temperatures above the recrystallization temperature. The recrystallization temperature for a given metal is about one-half of its melting point on the absolute scale. In practice, hot working is usually carried out at temperatures somewhat above 0.5Tm where Tm is the melting point. The work metal continues to soften as temperature is increased beyond 0.5Tm, thus enhancing the advantage of hot working above this level. However, the deformation process itself generates heat, which increases work temperatures in localized regions of the part. This can cause melting in these regions, which is highly undesirable. Also, scale on the work surface is accelerated at higher temperatures. Accordingly, hot working temperatures are usually maintained within the range 0.5Tm to 0.75Tm.
Advantages
The most significant advantage of hot working is the capability to produce substantial plastic deformation of the metal—far more than is possible with cold working or warm working. The principal reason for this is that the flow curve of the hot-worked metal has a strength coefficient that is substantially less than at room temperature, the strain hardening exponent is zero (at least theoretically), and the ductility of the metal is significantly increased. All of this results in the following advantages relative to cold working:
- the shape of the workpart can be significantly altered;
- lower forces and power are required to deform the metal;
- metals that usually fracture in cold working can be hot formed;
- strength properties are generally isotropic because of the absence of the oriented grain structure typically created in cold working; and
- no strengthening of the part occurs from work hardening.
This last advantage may seem inconsistent, since strengthening of the metal is often considered an advantage for cold working. However, there are applications in which it is undesirable for the metal to be work hardened because it reduces ductility, for example, if the part is to be subsequently processed by cold forming.
Disadvantages
Disadvantages of hot working include:
- lower dimensioal accuracy;
- higher total energy required (due to the thermal energy to heat the workpiece);
- work surface oxidation (scale);
- poorer surface finish; and
- shorter tool life.
Recrystallization of the metal in hot working involves atomic diffusion, which is a time-dependent process. Metal forming operations are often performed at high speeds that do not allow sufficient time for complete recrystallization of the grain structure during the deformation cycle itself. However, because of the high temperatures, recrystallization eventually does occur. It may occur immediately following the forming process or later, as the workpiece cools. Even though recrystallization may occur after the actual deformation, its eventual occurrence, and the substantial softening of the metal at high temperatures, are the features that distinguish hot working from warm working or cold working.