Terms and Definitions
Definitions and terms of custom industrial forging for finished metal component parts, for original equipment manufacturers (OEM)
Industrial forging creates parts that are stronger than either casting or machining. The metal, having been hammered or pressed into a shape, creates a grain flow while dispersing inclusions and closing voids from prior processing steps. Deformation energy can drive dynamic recrystallization, often achieved in subsequent heat treatment that follows forging, resulting in a stronger, tougher part.
Cold Forging -- room temperature forging
Cold forging typically means a process performed at ambient temperature. Significantly greater force is usually required and die design and materials must accommodate those requirements. Lubrication of dies or pre-lubrication of blanks to be forged is critical. Since there is less shrink factor to accommodate and no oxidative scale from high temperature, finished forgings can often be net shape or nearly net shape. Not all alloys can sustain deformation at ambient temperature without the formation of internal defects.
Warm Forging – the “in between” process
At a temperature between ambient and traditional high forging temperatures, the yield strength of many alloys decreases sufficiently to allow significant deformation to be accomplished without creating defects. Ideally, the temperature will be below the point where surface oxides form that will cause undesirable surface conditions on the finished forging. Dimensions are typically near net shape as die and part shrink factors, even at temperatures considered “warm”, typically result in less precision than cold forging. Additionally, alloys that are warm forged frequently require additional heat treatment to achieve the required mechanical properties. These repeated thermal cycles cause dimensional changes that require some machining step to achieve finish part requirements.
Hot Forging -- high temperature
Hot forging is done at temperatures where the mechanical properties of the alloy have changed generally allowing deformation, sometimes severe deformation, to occur without damage to the required properties using minimum force. Many structural alloys cannot be cold or warm forged. These are typically materials required in the most severe service conditions such as forgings to create components used in high temperature service. Some geometrically complex components require deformation that, if not done hot, would require many more process steps. Some alloys require deformation energy at high temperature to develop optimum properties. Surface condition and resulting dimensions are typically furthest from finish requirements and additional machining steps are expected.
Open Die Forging
Open die forging application requires the use of a hammer or hydraulic press impacting or pressing the metal. In open die forging shaping of the metal is done with tooling that has no cavities or produces relatively simple rough shapes. This process is ideal for short runs and items with very large surface or plan area.
Impression Die or Closed Die Forging
Hammer impression die or closed die forging uses a die mounted on the hammer ram as well as the anvil. The metal is placed in the die on the anvil and struck with the die on the ram causing the metal to fill the die. This may require a series of progressive die cavities or even a series of dies requiring reheating between forging steps to accomplish a finished component part, but forgings become more economical for longer runs. High velocity of the ram defines this and similar forging processes as being performed on “impact” type machines.
In Press Forging, the metal is forced into a desired shape in a process with lower ram velocities. This can provide some preferential compression of metal through the cross section of the forging and can achieve superior results where an alloy is strain rate sensitive. Press forging may be more demanding of die materials due to longer die contact times. One beneficial tradeoff is that forging accomplished on mechanical presses may be more easily automated.
The upset forging machine performs work with a series of multiple horizontal dies that move in several directions to both clamp a workpiece in place as well as perform the forging operation. This process typically forms a forged end on a rod or bar shape, shortening bar length while increasing diameter. The result is often a mass on the end of the rod that is termed a “head”, such as the “head” of a bolt, therefore the process also goes by other names, such as “heading”. This operation can be performed in a series of progressive steps to produce near net and complex features.
Definitions and terms for custom industrial forging:
For more information about common forging terminology, visit the Forging Industry Association (FIA) website.
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