What is Forging?
Forging is a manufacturing process where metal is pressed, pounded or squeezed under great pressure into high-strength parts known as forgings. The process is normally performed by preheating the metal to a specific temperature before it is worked, usually around 75% of its melting temperature. It is important to note that the forging process is entirely different from the casting process, as metal used to make forged parts is never melted and poured like at a foundry (factory for making metal castings).
The forging process creates parts that are stronger than those manufactured by any other metalworking process. Forging takes advantage of the metal’s natural grain flow, shaping the grain flow to conform to the contours of each part’s unique geometry. This grain flow contouring is lost when cutting through the grain by machining it and is also lost when casting parts. Forging offers a single piece versus a welded unit, as the weld quality can be hard to replicate without additional inspection.
Closed-die forging uses custom created tooling or impression dies for production. These dies allow for superior consistency and repeatability of the final product as the process can be replicated for each piece as they are forged. This provides uniform part dimensions and quality for the entire run of a product. The production run can then achieve similar physical characteristics with further processing such as heat treating.
Forgings can be nearly any shape, which reduces the need for joining multiple pieces. Reducing the joint can improve the overall strength of the unit as the forging does not need to be welded or otherwise fastened together.
Heat-treated forgings are able to be used almost immediately after cooling, inspection and cleaning, but many are then cut, and or machined. The pieces forged surfaces are suitable for plating, polishing, painting or other protective coatings.
How Do Forgings Compare to Castings?
Forgings are stronger. Castings do not have strengthening benefits yielded by hot and cold forgings. Forging surpasses casting in predictable strength properties and produces superior simultaneously more ductile and resistant pieces with uniform quality assured across the production run.
Forging refines defects from cast ingots or continuous cast bar. A casting has neither grain flow nor directional strength and the casting process cannot prevent formation of certain metallurgical defects. Pre-working forge stock produces a grain flow oriented in directions requiring maximum strength. Dendritic structures, alloy segregations, and similar imperfections are also refined in forging.
Forgings are consistently more reliable and often less costly over time compared to castings. Casting defects occur in a variety of forms. Because hot working refines grain patterns and imparts high strength, ductility, and resistance to each forged piece they are also more durable. Also, they are manufactured without the added costs for tighter process controls and inspection that are required for castings.
Forgings also offer better response to heat treatment. Castings require close control of melting and cooling processes because alloy segregation may occur. This results in a non-uniform heat treatment response that can affect the straightness of finished parts. Forgings respond more predictably to heat treatment and offer better dimensional stability.
Forgings’ flexible, cost-effective production also more easily adapts to demand. Some castings, such as special performance castings, require expensive materials and process controls, and longer lead times. Open-die and ring rolling are examples of forging processes that adapt to various production run lengths and enable shortened lead times.
How Do Forgings Compare to Weldments/Fabrications?
Forgings offer production economies and material savings. Welded fabrications are more costly in high volume production runs. In fact, fabricated parts are a traditional source of forging conversions as production volume increases. Initial tooling costs for forging can be absorbed by production volume and material savings. Forgings’ production economics lower labor costs, scrap and rework reductions through reduced inspection costs.
Forgings are stronger. Welded structures are not generally free of porosity. Any strength benefit gained from welding or fastening standard rolled products can be lost by poor welding or joining practice. The grain orientation achieved in forging makes stronger parts.
Forgings also offer cost-effective designs. A multiple-component welded assembly cannot match the cost-savings gained from a properly designed, one-piece forging. Such part consolidations can result in considerable cost savings. In addition, weldments require costly inspection procedures, especially for highly stressed components. Forgings do not.
Forgings offer more consistent, better metallurgical properties. Selective heating and non-uniform cooling that occur in welding can yield undesirable metallurgical properties such as inconsistent grain structure. When in use, a welded seam may act as a notch that can contribute to part failure. Forgings have no internal voids that might cause unexpected failure under stress or impact.
Forgings offer simplified production. Welding and mechanical fastening require careful selection of joining materials, fastening types and sizes, and close monitoring of tightening practices both of which increase production costs. Forging simplifies production and ensures better quality and consistency.
How Do Forgings Compare to Machined Bar/Plate?
Forgings offer a broader size range of desired material grades. Sizes and shapes of products made from steel bar and plate are limited to the dimensions in which these materials are supplied. Often, forging may be the only metalworking process available with certain grades in desired sizes. Forgings can be economically produced in a wide range of sizes, from parts whose largest dimension is less than 1 inch, to parts weighing more than 450,000 lbs.
Forgings are grain oriented to shape for greater strength. Machined bar and plate may be more susceptible to fatigue and stress corrosion because machining cuts into material grain patterns. In most cases, forging yields a grain structure oriented to the parts’ external contours, resulting in optimum strength, ductility and resistance to impact and fatigue.
Forgings make better, more economic use of materials. Flame cutting plate is a wasteful process, one of several fabricating steps that consumes more material than needed to make such parts as rings or hubs. Even more material is lost in subsequent machining.
Forgings yield lower scrap and increase efficiency of production. Forgings, especially near-net shapes pieces, make better use of material and generate little scrap. In high-volume production runs, forgings have a decisive cost advantage.
Forgings require fewer secondary operations. As supplied, some grades of bar and plate require additional operations such as turning, grinding, and polishing to remove surface irregularities and achieve desired finish, dimensional accuracy, machinability, and strength. Often, forgings can be put into service without expensive secondary operations.