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A major differentiating factor in forging is the temperature of the billets at the start of the process. In the case of hot forging, the billets are heated to a temperature at which recrystallization processes occur during forging. Thus no strain hardening takes place in the material during forging, providing it with almost unlimited formability.
Materials made of steel are usually heated up to a starting temperature of approx. 1,200 °C. Hirschvogel carries out closed-die forging in which the dies generate the desired part contour in several stages.
Cold forging is a process in which the billets are not preheated, but are forged at room temperature. Due to the fact that shrinkage and scaling do not occur, cold forged components have greater shape and dimensional accuracy than comparable hot forged parts. However, the choice of materials and geometries is more limited than with hot forging. Cold forging is thus suitable for transmission shafts with minimum machining allowances.
In warm forging, steel is processed at a temperature of between 700 °C and 950 °C. The flow stress is lower than in cold forging; scale and distortion are lower than in hot forging. Thus warm forging allows a wider range of shaping options at a greater level of precision.
It is the combination of the various forging processes, in particular, which allows specific benefits to be drawn from each technology. It is thus possible, when producing a component, to combine the good formability achieved in hot forging with the high level of precision that cold forging brings.
The hot forging of aluminum is basically the same as the closed-die forging of steel. It takes place at temperatures of around 500 ° C. Sawn billets from extruded or continuously cast aluminum profiles are mostly heated in synchronized continuous furnaces. Following any initial forming to achieve material distribution, the parts are forged in a pre- and finished die. The flash is then removed. Forged aluminum achieves maximum strengths with very good toughness values, and is regarded as a premium material for tapping optimum lightweight potential.