The controlled cooling of a hot forging has several advantages over the antiquated practice of piling the forgings into a bin until the bin is full, then repeating the same uncontrolled filling of another bin until the forging run is completed. Repeatable cooling for each forging produces a consistent microstructure, hardness and residual-stress state in the parts as well as consistent dimensions at ambient temperature. If the cooling practice is controlled, these characteristics also can be controlled. The result is that finishing operations – such as the machining of the part and minimization of distortion both during cooling and during machining – can be optimized. For certain alloy steels, it is possible to use controlled cooling of the forging to achieve part properties without the need for additional heat treatment.
Acceptable cooling practice requires knowledge of cooling parameters and their effect on the forging as it cools in terms of phase transformations, consequent dimensional changes and stress-state changes. The combination of a computational fluid dynamics (CFD) model and a finite element model (FEM) that includes the metallurgical phase-transformation response offers a powerful tool for designing a well-controlled cooling process for hot forgings.