Based on the method of billet movement, forging can be divided into free forging, upsetting, extrusion, die forging, closed die forging, and closed upsetting. Closed die forging and closed upsetting are two advanced processes in die forging. Because there are no flashes, material utilization is high, and complex forgings can be finished in one or several processes with relatively low load requirements. However, strict control of the billet volume and the relative position of the forging die is necessary.
Cold upsetting is a minimally invasive metal pressure processing process performed at room temperature. It utilizes the plastic deformation of the metal to redistribute and transfer volume. Its characteristics include: improved mechanical properties of parts; high material utilization (up to 85% or more); high production efficiency; and good surface roughness and high precision. This process has strict requirements for raw materials, including chemical composition, mechanical properties, hardness (generally HB110~170), dimensional accuracy, surface quality (no scratches, folds, or other defects), and decarburized layer thickness.
The process design mainly includes: calculating the billet length based on the principle of constant volume; determining the degree of deformation and the number of upsetting operations based on the billet's length-to-diameter ratio (e.g., a length-to-diameter ratio ≤ 2.5 can be upset in one operation, while 2.5~4.5 requires two operations); and developing specific processing plans (such as selecting no-cutting or minimal-cutting processes and equipment).
The basic laws of plastic deformation followed by the upsetting process are: the shear stress law (plastic deformation requires shear stress to reach a critical value), the law of constant volume (the volume before and after deformation is equal), and the law of minimum resistance (metal particles flow in the direction of least resistance).
