Advanced forging processes now yield parts with directional strength and high impact resistance.
Thomas F. Schwingbeck Jr.
Scot Forge Co.
Spring Grove, Ill.
Forging is one of the oldest mechanical metalworking processes. It forms and shapes metals through the use of hammering and pressing. A cast ingot (or a "cogged" billet that has already been forged from a cast ingot) is heated to its plastic deformation temperature, then upset or "kneaded" between dies to the desired shape and size.
Recent advances in forging technology, including better equipment and the use of computers and electronic controls, have greatly increased the ability to produce superior quality metal parts in a wide array of sizes, shapes, materials, and finishes. Several well-known forging processes include impression or closed die, cold forging, and extrusion. There are also open-die and seamless-rolled-ring forging processes, each offering their own advantages while serving in diverse applications ranging from high-quality driveshafts, gear rims, pinions, and couplings to eye bolts, nozzles, hydraulic cylinders, and suspension components.
Open-die forging shapes heated metal parts between a top die attached to a ram and a bottom die attached to a hammer anvil or press bed. Metal parts are worked above their recrystallization temperatures (1,900 to 2,400°F for steel) and gradually shaped into the desired profile through skillful hammering or pressing operations.
Unlike impression or closed-die forging that confines the metal in dies, open-die forging uses flat dies that do not completely confine or restrain the metal. However, round swaging dies, V-dies, mandrels, pins, and loose tools are also used depending on the desired part profile and its size.
The open-die forging process is often associated with larger, simpler-shaped parts such as bars, blanks, rings, hollows, or spindles. But it can be considered the ultimate option for custom-designed metal components. It produces highstrength, long-life parts optimized for both mechanical properties and structural integrity in sizes ranging from a few pounds to hundreds of tons. In addition, advanced forge shops now offer shapes previously thought to be impractical with open die forging process.
SEAMLESS ROLLED RING
Seamless forged rings are often made on rolling mills using a process called ring rolling. These mills vary in size to produce rings with outside diameters of just a few inches to over 300 in., weighing anywhere from 1 to over 300,000 lb.
The process starts with a circular metal preform that has been previously upset using the open-die forging process. The preform is pierced to form a hollow "donut," heated above the alloy's recrystallization temperature, and then placed over the idler or mandrel roll. The idler roll moves under pressure toward a drive roll that continuously rotates the preform to reduce wall thickness and thereby increase both the inside and outside diameters of the resulting ring.
Seamless rings can range in shape from flat, washerlike parts to tall cylinders, with heights spanning <1 in. to >9 ft. Wall thickness-to-height ratios of rings typically range from 1:16 to 16:1, although greater proportions are possible with special processing. The simplest and most commonly used shape is a rectangular cross-section ring. But shaped tooling can produce seamless rolled rings in complex, custom shapes with contours on the inside and/or outside diameters.
Open-die and rolled-ring forging takes advantage of metal plasticity at high temperatures to deliver significant advantages in economics, manufacturing, and part quality when compared to alternative metalworking processes.
Directional strength: The process of mechanically deforming heated metal under tightly controlled conditions produces predictable and desirable grain flow properties. It also refines the grain microstructure and thus improves mechanical properties and metallurgical soundness of the part. These qualities translate into superior metallurgical and mechanical capabilities by giving the final part more directional strength.
Structural and impact strength: Forging gives structural integrity that is unmatched by other metalworking processes. By consolidating the ingot center, forging eliminates internal voids and porosity. The process also breaks up and eliminates the dendritic microstructure that is inherent in the original cast ingot. Predictable structural integrity reduces the need for part inspection, simplifies heat treating and machining, and ensures the part will perform well in the field.
Parts can also be forged to meet virtually any impact requirement. Proper orientation of grain flow assures maximum impact strength and fatigue resistance. The high-strength properties of the forging process can serve to reduce sectional thickness and overall weight without compromising final part integrity.
Size and shapes: Limited only to the largest ingot that can be cast, open-die forged parts can weigh a single pound or over 1,000,000 lb. In addition to commonly purchased open-die parts, forgings are often specified for their soundness in place of rolled bars or castings, or for those parts that are too large to produce by any other metalworking method.
Parts range from simple bar, shaft, and ring configurations to specialized shapes. These include multiple OD/ID hollows, single and double hubs similar to closed-die designs. Custom shapes are also available by combining forging with secondary processes such as torch cutting, sawing, and machining.
Metallurgical spectrum: Forgings come from almost all ferrous and nonferrous metals. The forging process itself can be adjusted through the selection of alloys, temperatures, working methods, and postforming techniques, to yield virtually any metallurgical property.
Part count and materials savings: Open-die and rolled-ring forgings are custom made one at a time. Designers, therefore, have the option of purchasing one, a dozen, or hundreds of parts for prototyping or low-volume production. Open-die forgings impart similar grain flow orientation, deformation, and other beneficial properties similar to those formed in closed-die forgings. So the open-die process is often used to proofout a design that will ultimately go into closed-die production, but without the high costs and long lead times associated with closed-die tooling and setups.
Forging can measurably reduce material costs because it needs less starting metal to produce many part shapes. For example, with a torch-cut part, all corner stock and the full center slug are lost. There is a lot of excess material that ends up as scrap. With forgings, parts are shaped to size with minimal waste.
Forging can also yield advantages in machining, lead time, and tool life. Savings come from forging to a closertofinish size than is feasible with alternate metal sources such as plate or bar. There's less machining needed to finish the part, which shortens lead time and reduces wear and tear on equipment.
Reduced rejection rates: Fewer rejects are the result of parts consolidation and the elimination of welding operations, coupled with higher quality materials boasting improved structural integrity. Forging also lets the same part be made from many different-sized ingots or billets. This, in turn, allows for a wider variety of inventoried grades. The resulting flexibility means that forged parts of virtually any grade can be manufactured more quickly and economically.
Secondary finishing on forgings also adds value and oftentimes reduces overall costs. Heat-treating, machining, sawing, torch-cutting, and testing options let forgings be produced to virtually any desired shape, size, or property.
Open-die forging process
Steps to produce a typical spindle-shaped part.
The seamless rolled-ring forging process
PRODUCING A RING PREFORM BY THE OPEN-DIE FORGING PROCESS:
Almost all metals can be forged. This gives a wide range of physical and mechanical properties spanning the entire spectrum of ferrous and nonferrous metallurgy:
- Tool steels
- Custom grades
WHEN COMPARED TO...
OPEN-DIE AND ROLLED-RING FORGED-METAL PARTS DELIVER ...
|MACHINED BAR|| |
|CENTRIFUGAL CASTINGS|| |
|TORCH CUT PLATE|| |
|CLOSED-DIE/IMPRESSION-DIE FORGING|| |