Forging
Also: hot forging, closed die forging, impression die forging, open die forging, drop forging, press forging, ring rolling, seamless rolled rings, upset forging, cold forging, forged parts, forgings
Forging squeezes hot metal between dies into near-net blanks with aligned grain flow: the strongest, toughest parts in steel, aluminum, titanium or nickel.
237 forging companies in the Noramark directory
Forging shapes metal by compressing it between dies, usually hot: steel at about 1,100-1,250 °C (2,000-2,300 °F), aluminum at about 400-480 °C (750-900 °F), titanium and nickel alloys in narrow windows at specialist forges. Hammers and presses do the work, and four methods cover most of it. Open-die forging squeezes a heated billet between flat or simple dies while a manipulator turns it, drawing out shafts, blocks, discs and hollow sleeves with no part-specific tooling. Closed-die (impression-die) forging presses the metal into a cavity machined into two die halves; the excess escapes as flash at the parting line and is trimmed off. Ring rolling pierces a forged donut and rolls it between a driven roll and an idler to grow its diameter, which makes seamless rings for bearings, flanges and gear blanks. Upset forging gathers the end of a bar into a head or flange, which is how bolts, valve stems and axle shafts get their heads.
The reason to forge is grain flow. The rolling and forging of a billet stretch its grain structure and inclusions into fibers, and a forging bends those fibers around the contour of the part instead of cutting across them as machining from bar does. The result is higher fatigue strength and impact toughness in the directions that carry load, and a fully dense part with no casting porosity. That is why crankshafts, connecting rods, gears, axles, landing gear, pressure flanges, lifting hooks and hand tools are forged, and why a forging from a certified billet is the most reliable form of a given alloy.
Closed-die tooling costs thousands to tens of thousands of dollars and takes weeks, so it pays from a few hundred pieces up. Every face parallel to the die motion needs draft, corners need generous radii, and the hot surface carries scale and, on steel, a decarburized skin, so forgings come with machining allowance on every functional surface and are machined to finish. Open-die forgings and rolled rings need no dedicated tooling but are rough shapes that are machined all over; they suit large parts in small numbers. Precision and near-net forging cut the machining at a higher die cost, and cold forging and cold heading make small, accurate parts such as bolts and fittings at very high volume.
At a glance
| Typical tolerances | Closed-die steel forgings to commercial practice: about ±0.8 mm (±1/32") on lengths and widths up to 150 mm (6"), growing with size; thickness across the parting line about +1.5/-0.5 mm (+0.060/-0.020"); die mismatch 0.4-0.8 mm (0.015-0.030"); flash trimmed to within 0.8-1.5 mm (1/32-1/16"). Precision forgings hold roughly half that. Open-die forgings and rolled rings run ±3 mm (±1/8") or more and are machined all over. EN 10243-1 and the Forging Industry Association tolerance tables are the usual references. Machined features ±0.025-0.05 mm (±0.001-0.002") after machining. |
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| Size limits | Closed-die forgings at most job forges run from about 50 g to 25 kg (0.1-50 lb); large press shops forge several hundred kilograms, and the biggest aerospace presses make fittings and bulkheads over 1,000 kg. Open-die forgings run from about 10 kg (20 lb) to well over 50 tonnes, with shafts of 10 m (30 ft) and more at heavy forges. Seamless rolled rings from about 150 mm (6") to 6 m (20 ft) or more in diameter. Upsetters take bar to about 230 mm (9") diameter. Minimum web thickness in closed-die steel forgings is about 3-6 mm (1/8-1/4"), rising with plan area. |
| Surface finish | Hot forgings are shot blasted to remove scale, leaving about 3.2-6.3 µm Ra (125-250 µin) on closed-die parts and rougher surfaces on open-die work. Steel forgings carry a decarburized skin of roughly 0.25-0.75 mm (0.010-0.030") that must be machined off wherever hardness or fatigue strength matters. Cold-forged and coined surfaces reach 0.8-1.6 µm Ra (32-63 µin). Functional surfaces are machined; the rest is left as forged and painted, oiled, plated or powder coated. |
| Lead time | Closed-die tooling 6-12 weeks to first articles, then 4-8 weeks for production lots; open-die forgings and rolled rings 3-8 weeks with no tooling, longer when the mill must melt a special heat. Aerospace forgings with ultrasonic inspection and certification run 12-30 weeks. Heat treatment and machining add 1-3 weeks. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Carbon and alloy steels are the bulk of forging: 1045 for shafts and general parts, 4140, 4340 and 4130 for high-strength quenched-and-tempered parts, 8620 for carburized gears and 52100 for bearing rings. Stainless 304, 316 and 2205 duplex (ASTM A182 F304, F316, F51) for flanges and valve bodies, and 17-4 PH, 15-5 PH and 410 for aerospace, pump and valve parts. Aluminum 6061 and 2014 for general and automotive forgings, 7075 and 7050 for airframe fittings (ASTM B247). Titanium Ti-6Al-4V and nickel superalloys such as 718 forge on heated dies at a few specialist forges. H13 and other tool steels are forged into die blocks. Copper, C377 forging brass and beryllium copper forge well. Cast iron cannot be forged, leaded free-machining grades such as 12L14 and C360 are hot-short, and cast-only alloys such as C954 have forgeable wrought relatives instead.
What drives the cost
- Die cost for closed-die work: a few thousand dollars for a simple single-impression die, $50,000 or more for large, multi-impression or aerospace dies
- Input weight: the billet pays for flash, scale loss and machining stock as well as the finished part
- Alloy and forging temperature: titanium and nickel alloys need heated dies, narrow temperature windows and more press passes, and cost many times steel
- Quantity, which amortizes the dies and setup; closed-die forging rarely pays under a few hundred pieces
- Tolerance and draft: precision and low-draft forgings need more die work and tighter process control
- Heat treatment, testing and certification: mechanical tests, grain-flow macroetch, ultrasonic inspection, ASTM or AMS certs
- Machining after forging, set by the machining allowance and the datums the forging provides
When to use it
- Parts under fatigue, impact or high stress: crankshafts, connecting rods, gears, axles, landing gear, lifting hooks, hand tools
- Pressure-containing parts that must be free of porosity: flanges, valve bodies, fittings, pressure-vessel nozzles
- Medium and high volumes of a shape that would waste most of a bar if machined from solid
- Large shafts, blocks, discs and rings (open-die and ring rolling) in quantities of one to a few hundred
- Expensive alloys (titanium, nickel, 17-4 PH) where a near-net forging saves material over machining from billet
When not to
- Prototypes and small quantities of a closed-die shape: machine from bar or plate until the design settles
- Complex internal cavities, cored passages and thin, intricate walls: casting or machining
- Cast iron, leaded free-machining grades and cast-only alloys, which cannot be forged
- Parts machined all over anyway whose loads do not need grain flow: a part machined from bar skips the die
- Long parts with a constant cross-section: extrusion or roll forming
Design tips
- Give faces parallel to the die motion draft: about 5-7° outside and 7-10° inside on steel hammer forgings, 3-5° on press forgings with ejectors, and near zero only on precision aluminum forgings at a higher die cost.
- Put the parting line at the largest cross-section, on a flat plane if possible, and away from surfaces that must seal or look good.
- Use generous radii: corner radii of 2-3 mm (3/32-1/8") and fillet radii of 6 mm (1/4") or more on small steel forgings; sharp inside corners fold into laps and wear the dies.
- Add machining allowance of about 1.5-3 mm (1/16-1/8") per surface on small closed-die forgings and 5 mm (3/16") or more on open-die and large parts, on top of the forging tolerance.
- Keep ribs no taller than about six times their width and webs no thinner than the forger minimum for the plan area; ask the forger early.
- Show the required grain-flow direction on the drawing for fatigue-critical parts, and ask for a macroetch section on first articles.
- Call out the forging spec and material (ASTM A668, A105, A182, B247, or the AMS for aerospace), the heat treatment, and inspection such as ultrasonic testing.
- Send the finish-machined drawing and let the forger design the forging, or supply a forging drawing with draft, radii and allowance already on it.
Frequently asked questions
- Is forging stronger than casting?
- For the same alloy and heat treatment, yes, in the ways that usually matter. A forging is fully dense, with no shrinkage or gas porosity, and its grain follows the part contour, so fatigue strength and impact toughness are higher and more consistent. Castings win on complex shapes, internal passages and cost at low volume.
- What is the difference between open die and closed die forging?
- Open-die forging works a billet between flat or simple dies with no part-specific tooling, making shafts, blocks, discs and rings that are machined all over; it suits large parts and small quantities. Closed-die forging presses metal into a machined impression, giving a near-net shape with flash to trim; it needs a die costing thousands of dollars and pays at hundreds of pieces and up.
- How much does a forging die cost?
- A simple single-impression closed die for a small steel part is typically a few thousand to $15,000; multi-impression dies with blocker and finisher stages, and large or aerospace dies, run $25,000-100,000 and more. Open-die forging and ring rolling need no part-specific die, which is why they are used for one-offs and large parts.
- What is grain flow in forging?
- The fiber-like direction of the grain and inclusions in wrought metal. Forging bends that direction to follow the outline of the part, so loads run along the fibers rather than across them. It is revealed by cutting and etching a section (a macroetch), and fatigue-critical drawings often specify it.
- Forged vs machined from bar stock: which is better?
- A forging is stronger in fatigue and impact where the grain follows the contour, and it wastes less metal at volume. Machining from bar needs no die and is faster to first parts, and bar is already wrought, so for moderate loads and small quantities it is often good enough. Many parts start as bar-machined prototypes and move to forgings in production.
