Forging
Forging deforms solid metal under compressive load in a hammer, press or dies, so the grain flow follows the part contour and fatigue strength rises.
- Part
- Forming
- Variants
- 2
- Revised
- 2026-08-11
At a glance
- Family
- Metal
- Typical tolerances
- Commercial impression-die forgings hold roughly ±0.030 in (±0.8 mm) on small parts, widening with size, plus separate allowances for die wear, mismatch, and die closure; machine anything needing better than about ±0.010 in
- Surface finish
- Roughly 125–500 µin Ra (3.2–12.5 µm) as forged after descaling; critical surfaces are machined
- Typical volumes
- 1–100 pieces open die; 1,000–500,000+ per year impression die
- Lead time
- 8–16 weeks to sink an impression die and produce first articles; 3–8 weeks per production run. Open-die work in 2–6 weeks with no tooling
- Materials
- Metal
What it is
Forging shapes solid metal under compressive load — in a hammer, a press, or between rotating dies — rather than by melting or cutting it. The metal is worked while hot enough to flow plastically, typically 2,100–2,300°F (1,150–1,260°C) for carbon steel and 700–850°F (370–455°C) for aluminum, and the grain structure deforms with the shape instead of being cut through.
That grain flow is the reason forging exists. A forged crankshaft, connecting rod, hook, or landing gear component has continuous fiber following its contour, giving fatigue and impact performance that a casting or a part machined from bar cannot match at the same weight. Porosity is closed rather than created.
Forging covers a huge range: open-die work on one-off shafts and rings weighing tons, impression-die production of automotive and hand-tool components at hundreds of thousands per year, and cold heading of fasteners at hundreds of parts per minute.
How it works
Three families cover most production.
Open-die forging works a heated billet between flat or simply contoured dies, with the operator repositioning it between blows. It produces shafts, blocks, discs, and — with a mandrel — rolled rings, from a few pounds to hundreds of tons. There is no part-specific tooling, so it suits one-offs and very large parts.
Impression-die (closed-die) forging squeezes the billet into a cavity cut into matched die halves. Excess metal escapes as flash around the parting line, which is what generates the back pressure that fills the cavity corners. Flash is trimmed afterward and recycled.
Cold forging and heading work the metal below its recrystallization temperature, gaining excellent surface finish and tight tolerance at the cost of much higher forces. This is how bolts, rivets, and small precision parts are made at rate.
The hot impression-die cycle:
- Cut and heat. Bar or billet is sheared to weight and heated — induction for speed, gas furnace for large sections. Getting the temperature right matters at both ends: too cold and the die loads spike and the part cracks, too hot and the grain coarsens or the metal burns.
- Preform. Fullering and edging operations redistribute the mass along the billet's length so the finish impression only has to do local work.
- Blocker. An intermediate cavity brings the part near shape with generous radii.
- Finisher. The final impression forms the part and squeezes flash out around the parting line.
- Trim. Flash is sheared off in a trim die while the part is still hot.
- Heat treatment. Normalize, quench and temper, or solution and age to reach the specified properties. See metal melting points for how forging temperatures sit relative to melting.
- Finishing. Shot blast to remove scale, then straighten and machine the critical features.
A forging is almost never a finished part: it is a near-net shape with the metallurgy already right, delivered to machining with stock only where it is needed.
Design guidelines
Draft
Impression-die forgings need substantial draft — commonly 3–7° for steel and 1–3° for aluminum, which flows more readily and releases better. Internal surfaces (pockets shrinking onto a die projection) need more draft than external ones. Precision and cold forgings can approach zero draft at higher tooling cost.
Radii, generously
Corner radius should be at least 0.125 in (3 mm) and fillet radii larger still, growing with the depth of the cavity being filled. Sharp corners in a die cavity are unfillable — metal simply will not flow into them — and they crack the die. Where a forging keeps rejecting for underfill, an undersized fillet is the usual cause.
Parting line
Put the parting line on a single plane through the largest cross-section wherever possible. A flat parting line is the cheapest die to sink and the easiest to trim. Expect a small mismatch across it and never place a tight dimension so it depends on die-half alignment.
Webs and ribs
Thin webs chill against the die and resist filling. Keep web thickness at a workable minimum — thicker than a casting would need in the same place — and keep rib height-to-width ratios modest. Deep, narrow ribs are the most expensive feature to fill and the fastest way to wear out a die.
Machining stock
Add 0.030–0.125 in (0.75–3 mm) on surfaces that will be machined, and more on large forgings and on surfaces far from the parting line. Enough stock must remain to clean up scale, decarburized skin, and die wear variation. For achievable machined finishes, see the surface finish chart.
Orient the grain flow deliberately
The whole point of forging is that the fiber follows the shape. Orient the part so the principal load path runs along the grain flow rather than across it, and avoid machining operations that cut through the flow lines at a highly stressed section — doing so throws away the advantage that justified forging.
Tolerances
Commercial impression-die tolerances are on the order of ±0.030 in (±0.8 mm) on small parts, widening with size, plus separate allowances for die wear, mismatch, and die closure. Precision forging holds substantially tighter but costs more per part; machine any feature that needs better than about ±0.010 in.
| Feature | Recommended | Limit | Why |
|---|---|---|---|
| Draft, steel | 5–7° | 3° | Hot part grips the die and must release |
| Draft, aluminum | 2–3° | 1° | Aluminum flows and releases more readily |
| Corner radius | 0.19–0.25 in (5–6 mm) | 0.125 in (3 mm) | Metal will not flow into a sharp die corner |
| Fillet radius | Increase with cavity depth | — | Deep cavities need generous fillets to fill |
| Parting line | Flat, through the largest section | Contoured costs more | Cheapest die to sink and trim |
| Machining stock | 0.060 in (1.5 mm) typical | 0.030 in (0.75 mm) | Must clean up scale, decarb, and die wear |
| Ribs | Low height-to-width ratio | — | Deep narrow ribs underfill and wear dies fast |
Cost drivers
Die cost and press time dominate impression-die forging, and both scale with part size and complexity. Dies are sunk from hot-work tool steel and wear out — die life is a real per-part cost, not a fixed one, and it falls sharply with thin ribs, sharp radii, and high forging temperatures. Material cost is billet weight including flash, so flash allowance is money burned every stroke. Heat treatment and machining of the forged part are frequently the largest cost lines of all.
Volume breakpoints: open-die forging is economic at one piece and stays competitive to a few hundred. Impression-die forging typically needs 1,000 or more to justify sinking a die, works well from 5,000 to several hundred thousand, and at the top of that range is usually running on automated hot formers. Below those volumes, machining from bar or investment casting is normally cheaper.
- Open up the radii. Larger fillets fill better, forge with lower loads, and extend die life — three cost reductions from one change.
- Keep the parting line flat. It cuts die sinking, trim die cost, and mismatch scrap.
- Design the machining stock, don't default it. Stock only where a machined surface is needed, at the minimum that reliably cleans up.
- Consider a preform. Better mass distribution before the finisher reduces forging load, flash weight, and die wear at once.
- Match the alloy to forgeability. Plain carbon and low-alloy steels and 6000-series aluminum forge easily; stainless, nickel alloys, and titanium need narrower temperature windows, more force, and cost far more per pound to shape.
Variants
2 named
Drop Forging
Roll Forging
Questions
6 questionsWhy is a forged part stronger than a casting?
Forging deforms the existing grain structure so the fiber follows the part contour, and it closes porosity rather than creating it. The result is better fatigue and impact performance in the direction of the flow lines. A casting solidifies from liquid with no directional fiber and typically some residual porosity.
What draft angle does a forging need?
Commonly 3–7° for steel and 1–3° for aluminum, with more on internal surfaces that shrink onto a die projection than on external ones. Precision and cold forging can approach zero draft, at higher tooling cost and force.
What temperature is steel forged at?
Typically 2,100–2,300°F (1,150–1,260°C) for carbon and low-alloy steels. Aluminum forges at 700–850°F (370–455°C) and copper alloys around 1,300–1,450°F (700–790°C). Forging below the window spikes die loads and cracks the part; above it, the grain coarsens or the metal burns.
What tolerance can forging hold?
Commercial impression-die forgings run about ±0.030 in (±0.8 mm) on small parts, widening with size, plus allowances for die wear, mismatch across the parting line, and die closure. Anything tighter than roughly ±0.010 in should be machined into the forging afterward.
What volume justifies impression-die forging?
Roughly 1,000 pieces as a lower bound for sinking a die, with the process at its best from 5,000 to several hundred thousand per year. Below that, open-die forging, machining from bar, or investment casting is normally cheaper because they carry no die cost.
Forging or machining from billet?
Machining wins for one-offs, complex prismatic geometry, and tight tolerances everywhere. Forging wins once fatigue life matters, once volumes reach the thousands, and on expensive alloys, because it puts material only where the part needs it instead of cutting most of the billet into chips.