In past issues we’ve featured forging companies serving the gear-manufacturing industry in our profile and Q&A sections, and talking with them piqued our curiosity. We wanted to learn more, so we went straight to the source and contacted the Forging Industry Association, which agreed to allow us to publish a bit of the information they’ve compiled in this issue of the magazine. Here’s some of what we’ve learned, which we hope you’ll find useful.
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 — but not always — performed hot by preheating the metal to a desired temperature before it is worked. It is important to note that the forging process is entirely different from the casting (or foundry) process, as metal used to make forged parts is never melted and poured, as it is in the casting process.
Forging Methods
Impression-Die Forging pounds or presses metal between two dies (called tooling) that contain a precut profile of the desired part. Parts from a few ounces to 60,000 lbs. can be made using this process. Some of the smaller parts are actually forged cold.
Commonly referred to as closed-die forging, impression-die forging of steel, aluminum, titanium, and other alloys can produce an almost limitless variety of 3-D shapes that range in weight from mere ounces up to more than 25 tons. Impression-die forgings are routinely produced on hydraulic presses, mechanical presses, and hammers, with capacities up to 50,000 tons, 20,000 tons, and 50,000 lbs., respectively.
As the name implies, two or more dies containing impressions of the part shape are brought together as forging stock undergoes plastic deformation. Because metal flow is restricted by the die contours, this process can yield more complex shapes and closer tolerances than open-die forging processes. Additional flexibility in forming both symmetrical and non-symmetrical shapes comes from various preforming operations (sometimes bending) prior to forging in finisher dies.
Part geometries range from some of the easiest to forge simple spherical shapes, block-like rectangular solids, and disc-like configurations to the most intricate components with thin and long sections that incorporate thin webs and relatively high vertical projections like ribs and bosses. Although many parts are generally symmetrical, others incorporate all sorts of design elements — flanges, protrusions, holes, cavities, pockets, etc. — that combine to make the forging very non-symmetrical. In addition, parts can be bent or curved in one or several planes, whether they are basically longitudinal, equidimensional, or flat.
Most engineering metals and alloys can be forged via conventional impression-die processes, among them: carbon and alloy steels, tool steels, and stainless, aluminum and copper alloys, and certain titanium alloys. Strain-rate and temperature-sensitive materials (magnesium, highly alloyed nickel-based superalloys, refractory alloys, and some titanium alloys) may require more sophisticated forging processes and/or special equipment for forging in impression dies.
Cold Forging: Most forging is done as hot work, at temperatures up to 2,300 degrees F. However, a variation of impression die forging is cold forging. Cold forging encompasses many processes — bending, cold drawing, cold heading, coining, extrusions, and more — to yield a diverse range of part shapes. The temperature of metals being cold forged may range from room temperature to several hundred degrees. These shapes include various shaft-like components, cup-shaped geometries, hollow parts with stems and shafts, all kinds of upset (headed) and bent configurations, as well as combinations.
Most recently, parts with radial flow like round configurations with center flanges, rectangular parts, and non-axisymmetric parts with three- and six-fold symmetry have been produced by warm extrusion. With cold forging of steel rod, wire, or bar, shaft-like parts with three-plane bends and headed design features are not uncommon.
Typical parts are most cost-effective in the range of 10 lbs. or less; symmetrical parts up to 7 lbs. readily lend themselves to automated processing. Material options range from lower-alloy and carbon steels to 300 and 400 series stainless, selected aluminum alloys, brass, and bronze.
There are times when warm forging practices are selected over cold forging, especially for higher carbon grades of steel or where in-process anneals can be eliminated. Often chosen for integral design features such as built-in flanges and bosses, cold forgings are frequently used in automotive steering and suspension parts, antilock-braking systems, hardware, defense components, and other applications where high strength, close tolerances, and volume production make them an economical choice.
In the process, a chemically lubricated bar slug is forced into a closed die under extreme pressure. The unheated metal thus flows into the desired shape. Forward extrusion involves steel flow in the direction of the ram force. It is used when the diameter of the bar is to be decreased and the length increased. Backward extrusion, where the metal flows opposite to the ram force, generates hollow parts. In upsetting, the metal flows at right angles to the ram force, increasing diameter and reducing length.
Open-Die Forging is performed between flat dies with no precut profiles in the dies. Movement of the work piece is the key to this method. Larger parts over 200,000 lbs. and 80 feet in length can be hammered or pressed into shape this way.
Open-die forging can produce forgings from a few pounds up to more than 150 tons. Called ìopen-dieî because the metal is not confined laterally by impression dies during forging, this process progressively works the starting stock into the desired shape, most commonly between flat-faced dies. In practice, open-die forging comprises many process variations, permitting an extremely broad range of shapes and sizes to be produced. In fact, when design criteria dictate optimum structural integrity for a huge metal component, the sheer size capability of open-die forging makes it the clear process choice over non-forging alternatives. At the high end of the size range, open-die forgings are limited only by the size of the starting stock; namely, the largest ingot that can be cast.
Practically all forgeable ferrous and non-ferrous alloys can be open-die forged, including some exotic materials like age-hardening superalloys and corrosion-resistant refractory alloys. Open-die shape capability is indeed wide in latitude. In addition to round, square, rectangular, hexagonal bars, and other basic shapes, open-die processes can produce:
• Step shafts solid shafts (spindles or rotors) whose diameter increases or decreases (steps down) at multiple locations along the longitudinal axis.
• Hollows cylindrical in shape, usually with length much greater than the diameter of the part. Length, wall thickness, ID, and OD can be varied as needed.
• Ring-like parts can resemble washers or approach hollow cylinders in shape, depending on the height/wall thickness ratio.
• Contour-formed metal shells like pressure vessels, which may incorporate extruded nozzles and other design features.
Not unlike successive forging operations in a sequence of dies, multiple open-die forging operations can be combined to produce the required shape. At the same time, these forging methods can be tailored to attain the proper amount of total deformation and optimum grain-flow structure, thereby maximizing property enhancement and ultimate performance for a particular application. Forging an integral gear blank and hub, for example, may entail multiple drawing or solid forging operations, then upsetting. Similarly, blanks for rings may be prepared by upsetting an ingot and then piercing the center prior to forging the ring.
Seamless Rolled Ring Forging is typically performed by punching a hole in a thick, round piece of metal (creating a donut shape) and then rolling and squeezing (or in some cases, pounding) the donut into a thin ring. Ring diameters can be anywhere from a few inches to 30 feet.
Rings forged by the seamless ring rolling process can weigh < 1 lb up to 350,000 lbs., while ODs range from just a few inches up to 30 ft. in diameter. Performance-wise, there is no equal for forged, circular-cross-section rings used in energy generation, mining, aerospace, off-highway equipment, and other critical applications.
Seamless ring configurations can be flat (like a washer) or feature higher vertical walls, approximating a hollow cylindrical section. Heights of rolled rings range from less than an inch up to more than 9 ft. Depending on the equipment utilized, wall-thickness/height ratios of rings typically range from 1:16 up to 16:1, although greater proportions have been achieved with special processing. In fact, seamless tubes up to 48 in. diameter and over 20 ft. long are extruded on 20 to 30,000-ton forging presses.
Even though basic shapes with rectangular cross-sections are the norm, rings featuring complex, functional cross-sections can be forged to meet virtually any design requirements. Aptly named, these contoured rolled rings can be produced in thousands of different shapes with contours on the inside and/or outside diameters. A key advantage to contoured rings is a significant reduction in machining operations. Not surprisingly, custom-contoured rings can result in cost-saving part consolidations. Compared to flat-faced seamless rolled rings, maximum dimensions (face heights and ODs) of contoured rolled rings are somewhat lower, but are still very impressive in size.
High tangential strength and ductility make forged rings well-suited for torque- and pressure-resistant components such as gears, engine bearings for aircraft, wheel bearings, couplings, rotor spacers, sealed discs and cases, flanges, pressure vessels, and valve bodies. Materials include not only carbon and alloy steels, but also non-ferrous alloys of aluminum, copper, and titanium, as well as nickel-base alloys.
Comparisons
How forgings compare to castings:
• Casting cannot obtain the strengthening effects of hot and cold working. Forging surpasses casting in predictable strength properties, producing superior strength that is assured, part to part.
• A casting has neither grain flow nor directional strength, and the process cannot prevent formation of certain metallurgical defects. Preworking forge stock produces a grain flow oriented in directions requiring maximum strength. Dendritic structures alloy segregations and like imperfections are refined in forging.
• Casting defects occur in a variety of forms. Because hot working refines grain pattern and imparts high strength, ductility, and resistance properties, forged products are more reliable. And they are manufactured without the added costs for tighter process controls and inspection that are required for casting.
• Castings require close control of melting and cooling processes because alloy segregation may occur. This results in non-uniform heat treatment response that can affect straightness of finished parts. Forgings respond more predictably to heat treatment and offer better dimensional stability.
• 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 forgings compare to weldments/fabrications:
• 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, and forging’s intrinsic production economics lower labor costs, scrap, rework reductions, and reduced inspection costs.
• Welded structures are not usually 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.
• 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.
• Selective heating and non-uniform cooling that occur in welding can yield such undesirable metallurgical properties as inconsistent grain structure. In use, a welded seam may act as a metallurgical notch that can lead to part failure. Forgings have no internal voids that cause unexpected failure under stress or impact.
• Welding and mechanical fastening require careful selection of joining materials, fastening types and sizes, and close monitoring of tightening practice both of which increase production costs. Forging simplifies production and ensures better quality and consistency part after part.
How forgings compare to machined bar/plate:
• 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 in. to parts weighing more than 450,000 lbs.
• Machined bar and plate may be more susceptible to fatigue and stress corrosion because machining cuts material grain pattern. In most cases forging yields a grain structure oriented to the part shape, resulting in optimum strength, ductility, and resistance to impact and fatigue.
• 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 is lost in subsequent machining.
• Forgings, especially near-net shapes, make better use of material and generate little scrap. In high-volume production runs, forgings have the decisive cost advantage.
• 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, machineability, and strength. Often, forgings can be put into service without expensive secondary operations.
How forgings compare to powder metal parts (P/M):
• Low standard mechanical properties (e.g. tensile strength) are typical of P/M parts. The grain flow of a forging ensures strength at critical stress points.
• Costly part-density modification or infiltration is required to prevent P/M defects. Both processes add costs. The grain refinement of forged parts assures metal soundness and absence of defects.
• Special P/M shapes, threads and holes, and precision tolerances may require extensive machining. Secondary forging operations can often be reduced to finish machining, hole drilling, and other simple steps. The inherent soundness of forgings leads to consistent, excellent machined surface finishes.
• P/M shapes are limited to those that can be ejected in the pressing direction. Forging allows part designs that are not restricted to shapes in this direction.
• The starting materials for high-quality P/M parts are usually water atomized, pre-alloyed, and annealed powders that cost significantly more per pound than bar steels.
How forgings compare to reinforced plastics/composite (RP/C):
• New advanced-composite part designs may often require long lead times and substantial development costs. The high production rates possible in forging cannot yet be achieved in reinforced plastics and composites.
• RP/C physical property data are scarce, and data from material suppliers lack consistency. Even advanced aerospace forgings are established products with well-documented physical, mechanical and performance data.
• RP/C service temperatures are limited and effects of temperature are often complex. Forgings maintain performance over a wider temperature range.
• Deterioration and unpredictable service performance can result from damage to continuous, reinforcing RP/C fibers. Forging materials outperform composites in almost all physical and mechanical property areas, especially in impact resistance and compression strength.
Conclusion
“No man is an island,” according to John Donne, and neither is an industry. The more you know about the suppliers you rely on — and the processes, materials, and technologies they utilize — the better you’ll be able to make good decisions for your company’s well being.
Case Study: High Performance Gears
Forging Process: Hot closed impression die
Size, mm (in.): 76 to 432 mm (3 to 17 in.) diameter, face widths to 203 mm (8 in.)
Alloy: 8620 steel
Tensile strength, MPa (psi)1: 635 (92,000)
Yield strength, MPa (psi)1: 355 (52,000)
Hardness, BHN: L 180
Elongation1 2: 2 26.3%
Reduction in Area1 2: 59.7%
Impact Toughness, J (ft-lb)1 2: 99.7 (73.5)
Secondary Operations: Cold draw through finish sizing die and grind
Heat treatment: Normalize before shipment
Alternate process: Forge the blank, rough and finish hob
Annual Production: 5,000 to 7,000
1 standard handbook values
2 as normalized
A family of gears is being produced to near-net shape by hot forging in closed impression dies. Finish grinding allowances range from as little as 0.1 mm (0.004 in.) up to 2 mm (0.080 in.). The process offers two distinct advantages over the alternative of forging the blank and hobbing the teeth: near-net shape forging reduces production costs by requiring only a finish grinding operation, which incurs lower costs both for processing and capital investment than hobbing operations, and; the continuous and uninterrupted grain flow established in forging the teeth virtually eliminates residual stresses in the teeth, resulting in substantially higher gear life.
These advantages required that several critical manufacturing problems be solved, including high scrap rates, lower die life and high costs of grinding. The problems were solved by interaction of the forging engineers with gear designers and quality technicians and grinding wheel suppliers. The high degree of forging precision is achieved by very close control of process variables in all stages.
For example:
• The steel bar stock is turned and polished to improve the surface, and cut to precise lengths to ensure tight control of the volume of steel that is placed into the closed dies.
• Temperature of the forging stock is maintained to within ±14’C (25’F) in an induction heating furnace.
• Special press controls were developed in-house to ensure repeatability of press operations.
In addition to reduced cost, the process offers the potential for reducing the metal content of the gears by designing to the higher tooth strength that is developed by the process.
