The Steel Forging Process
At Queen City Forging, we use the application of heat and pressure in the forging process to shape and enhance metallurgical properties, ensuring every steel component meets or exceeds design requirements. The forging process ensures durability of safety critical components in the most severe service conditions. The type of forging process used to produce any specific component part will depend upon an evaluation of the costs and benefits best suited to that component including the steel required and the quantity of forgings to be produced
Cutting the raw material
Ordering steel in bar or billet form is the typical first step in the process. Bars or billets are then saw or shear cut to length. Cutting produces individual pieces, sometimes called slugs, next typically entering a heating processes using fuel fired furnaces, electric induction, or electric resistance heating equipment.
In the forging process, the cut slugs are placed between dies and force is applied as the dies are pressed together in a variety of types of forging equipment. This forces the steel slug to flow, while remaining solid, to fill cavities in the dies creating a new shape configuration and enhancing the material properties. In hot forging, slugs are typically heated into a range of temperatures prior to forging in the range of 1750F (950C) to 2300F (1250C). Many steels and steel alloys must be forged at these temperatures to provide most cost effective production and assure alloys are not damaged during the deformation process.
Select alloys and configurations may be forged at lower temperatures in the range of 1350F (750C) to 1750F (950C). The advantage can be to forge the steel at a temperature below which oxides, called scale, form on the surface of the steel due to exposure to atmosphere at temperature. This along with less dimensional change as parts cool from a higher temperature allows closer tolerances to be achieved in the forging process. Typically, significantly higher forging forces and larger forging equipment that can develop greater force will be required.
In this case, cold, typically means room or ambient temperature, 72F (22C). In this case special lubricants, for blanks and dies are often required and the steel grades and configurations are much more limited. Much larger equipment and special die designs may be required. The forces required to deform the raw material result in significant heating from the deformation process and the temperature of the workpiece may reach as high as 350F (175C).
The Properties of Forged Steel
The forging process results in benefits that are impossible to achieve with other production methods.
A properly forged component will not have the inclusions, voids, shrinks or density less than 100% that can be typical of processes such as casting or powder additive processes. Nothing can be entrapped as the product remains in a solid state through the entire production process.
Bar and billet product, the typical input into the forging process, will typically exhibit long, longitudinal grain flow in the length direction due to the way these forms were produced. Forging reorients this grain flow through part contours that can enable enhanced directional properties while also driving recrystallization of the raw material to form smaller grain sizes, a key to improved fatigue properties.
Uniformity and Consistency
Material that has been worked and reworked through deformation processes in a solid state, not only is the material free of voids, it not have the hard spots or internal metallurgical variations that can cause variations in machinability from piece to piece. With consistency of raw material input into each and every forging, heat treat results will be more uniform both metallurgically and dimensionally.
The Forging Advantage
Forging produces product with the highest measured values for impact toughness and fatigue properties. There is a reason all other processes seek to advertise “near wrought properties” or claim comparison to wrought properties under special conditions. Wrought properties means FORGED – The highest quality all others seek to equal.
Types of Forging Processes
Open Die Forging
Open die forge involves placing the workpiece in the work zone of a hammer or press and shaping using simple tools or flat dies – no cavities for a specific shape. This process is used to produce smaller quantities of forgings, often very large workpieces that may weight several tons or thousands of kilograms. The Open Die process produces the most massive forgings of all the forging processes.
Impression Die Forging
Sometimes also referred to as “closed die” or “die” forging, this method uses dies with cavities machined with the intention of shaping the final forging for use as a specific component. This method is used to produce higher volumes of parts with consistent dimensions for additional processing into finished components. Impression die forging produces the largest volume of individual forgings of any of the forging processes.
When a ring shape is required with ultimate strength and durability, a specific forging process, along with specialty forging equipment is used to produce seamless ring shapes. Rings produced with the ring rolling process may be as small as 6 inches (150mm) in diameter or as large as 65 feet (20m) in diameter.