Overview of Forging

                                                    OVERVIEW OF FORGING

Forging is a processing method that applies pressure to metal billets using forging machinery, causing them to undergo plastic deformation in order to obtain forgings with specified mechanical properties, shapes, and dimensions. It is a common forming technique in mechanical manufacturing. Through forging, the cast porosity and internal voids in the metal can be eliminated and welded shut; therefore, the mechanical properties of forgings are generally superior to those of castings made from the same material. Important components in machinery that bear heavy loads and operate under severe conditions are mostly produced as forgings, except for those with relatively simple shapes, which may be made from rolled plates, sections, or welded parts.

1. Classification by the Source of the Acting Force During Operation

According to the source of force applied during the process, forging is divided into manual forging and machine forging.

• Manual forging uses simple tools and relies on human power on an anvil. This traditional method has a history of several thousand years and is now gradually being phased out. It is used only for single‑piece or small‑batch production, or for training beginners in basic operational skills.

• Machine forging is the primary method in modern forging production and is carried out on various types of forging equipment. Depending on the equipment and tools used, it can be further divided into open‑die forging, die forging (closed‑die forging), insert‑die forging (loose‑die forging), and special forging.

2. Classification by the Temperature of the Metal During Deformation

Based on the temperature of the billet during processing, forging can be classified into cold forging, warm forging, hot forging, and isothermal forging.

• Cold forging is generally performed at room temperature.

• Hot forging is carried out at temperatures above the recrystallisation temperature of the billet metal.

• Sometimes, forging performed while the material is heated but below the recrystallisation temperature is called warm forging. This classification is not strictly standardised in industrial practice.

For steel, the recrystallisation temperature is approximately 460 °C. However, 800 °C is commonly adopted as the dividing line: above 800 °C is hot forging; between 300 °C and 800 °C is referred to as warm forging or semi‑hot forging.

3. Classification by Forming Method

According to the forming method, forging can be divided into open-die forging, die forging, upsetting, extrusion, drawing, rotary forging, high‑speed forging, and others.

• Open‑die forging (also called free forging) means that the deformation of the metal billet under pressure is basically not constrained externally.

• In contrast, the metal deformation in other forging methods is restricted by the dies, so they are also called closed‑die forging (or closed‑die forming).

Processes such as forming rolling, roll forging, orbital forging (swing forging), and ring rolling involve relative rotational motion between the forming tool and the billet, applying pressure and shaping the billet point‑by‑point and progressively. These methods are therefore collectively referred to as rotary forging.