Chapter 16: Forging Processes

A critical element of closed-die forging is the formation of flash, a fin of excess metal that acts as a safety valve and regulates cavity pressure to ensure complete die filling. The summary details the mechanics of forging equipment, distinguishing between energy-restricted machines like gravity and power drop hammers, stroke-restricted mechanical presses, and load-restricted hydraulic presses, while also introducing high-energy-rate forging (HERF) machines. Theoretical concepts are explored in depth, specifically plane-strain forging conditions and the creation of the friction hill, where pressure rises exponentially from the edge to the center of the workpiece due to frictional shear stress at the die interfaces. The text differentiates between sliding friction (governed by Coulomb’s law) and sticking friction, where the surface shear stress reaches the shear yield limit. Key design considerations are highlighted, including the shape classification of forgings, the necessity of preform design to distribute mass correctly, and the increasing role of CAD/CAM systems in optimizing die geometry and minimizing material waste. The chapter also addresses forging defects such as laps, cold shuts, and internal cracking caused by secondary tensile stresses, as well as the importance of controlling fiber structure (flow lines) for mechanical properties. Finally, the summary covers specialized topics like powder metallurgy (P/M) forging, which involves the densification of porous sintered preforms, and the management of residual stresses in large steel forgings to prevent thermal flaking.