According to the forging temperature, forging can be classified into hot forging, cold forging and warm forging. Most metal components intended to be forged are performed under hot forging.
Hot forging is the most widely used forging process. In hot forging process, forging is carried out at a temperature above the recrystallization temperature of the metal which means at the temperature at which the new grains are formed in the metal. This kind of extreme heat is necessary in avoiding strain hardening of the metal during deformation.
In real conditions during industrial manufacturing, friction plays a part in the process. Friction forces at the die-work interface oppose the spreading of the material near the surfaces, while the material in the centre can expand more easily. The result is to create a barrel shape to the part. This effect is called barreling in metal forging terms. Barreling is generally undesirable and can be controlled by the use of effective lubrication. Another consideration, during hot forging manufacturing process, which usually acts to increase the barreling effect, is the heat transfer between the hot metal and the cooler die. The metal nearer to the die surfaces cool faster than the metal towards the centre of the part. The cooler material is more resistant to deformation and expands less than the hotter material in the centre, also causing a barreling effect.
The Advantages of Hot Forging are:
High strain rates and hence easy flow of the metal.
Recrystallization and recovery are possible.
Forces required are less.
Disadvantages of Hot Forging are:
Lubrication at high temperatures is difficult.
Oxidation and scaling occur on the work piece.
Poor surface finish.
Lesser precise tolerances.
Possible warping of the material during the cooling process.