Forging is the term for shaping metal by using localized compressive forces. Cold forging is done at room temperature or near room temperature. Hot forging is done at a high temperature, which makes metal easier to shape and less likely to fracture. Warm forging is done at intermediate temperature between room temperature and hot forging temperatures. Forged parts can range in weight from less than a kilogram to 170 metric tons. Forged parts usually require further processing to achieve a finished part.
A forging press, often just called a press, is used for press forging. There are two main types: mechanical and hydraulic presses. Mechanical presses function by using cams, cranks or toggles to produce a preset (a predetermined force at a certain location in the stroke) and reproducible stroke. Due to the nature of this type of system difference forces are available at different stroke positions. Mechanical presses are faster than their hydraulic counterparts (up to 50 strokes per minute). Their capacities range from 3 to 160 MN (300 to 18,000 tons). Hydraulic presses use fluid pressure and a piston to generate force. The advantages of a hydraulic press over a mechanical press are its flexibility and greater capacity. The disadvantages are that it is slower, larger, and more costly to operate.
The roll forging, upsetting, and automatic hot forging processes all use specialized machinery.
The dimensional tolerances of a steel part produced using the impression-die forging method are outlined in the table below. It should be noted that the dimensions across the paring plane are affected by the closure of the dies, and are therefore dependent die wear and the thickness of the final flash. Dimensions that are completely contained within a single die segment or half can be maintained at a significantly greater level of accuracy.
|Dimensional tolerances for impression-die forgings|
|Mass [kg (lbs)]||Minus tolerance [mm (in.)]||Plus tolerance [mm (in.)]|
|0.45 (1)||0.15 (0.006)||0.48 (0.018)|
|0.91 (2)||0.20 (0.008)||0.61 (0.024)|
|2.27 (5)||0.25 (0.010)||0.76 (0.030)|
|4.54 (10)||0.28 (0.011)||0.84 (0.033)|
|9.07 (20)||0.33 (0.013)||0.99 (0.039)|
|22.68 (50)||0.48 (0.019)||1.45 (0.057)|
|45.36 (100)||0.74 (0.029)||2.21 (0.087|