Forging Press

How a forging press works

A forging press is a vertical hydraulic ram that drives an upper die down onto a lower die with a slug of red-hot metal — the billet — pinned between them. The ram moves slowly: a few centimetres per second on a 50,000-tonne press, hours of total stroke in extreme cases. Pressure builds gradually as the billet spreads sideways, fills the die cavity, and squeezes flash out the parting line. The slow stroke is what distinguishes a press from a hammer; a hammer dumps kinetic energy in milliseconds, a press feeds force as the metal asks for it.

Inside the metal, the deformation does two important things. First, it folds and stretches the original casting grains into long flowing fibres that follow the contour of the part — what metallurgists call grain flow. A crankshaft journal forged with grain running along the journal axis carries fatigue load along the strongest direction of the metal, the way a wooden axe handle uses straight-grained ash. Second, the compression closes shrinkage porosity left over from the upstream ingot. A forged turbine disk has zero porosity at the part scale, which is why nobody flies on a cast jet engine compressor.

BEFORE FORGING                    AFTER FORGING
   billet                          finished part
   ┌────────┐                    ╱──────────────╲
   │ . . .  │                   │ ═══════════════ │  ← grain flow
   │  . .   │   ─── ram ───►    │ ═══════════════ │     follows
   │ . . .  │      squeeze       │ ═══════════════ │     contour
   │  . .   │                   │ ═══════════════ │
   └────────┘                    ╲──────────────╱
   random                         aligned fibres,
   grain, voids                   no porosity

Press tonnage: how big a press does this part need?

The fundamental forging equation is

F = K · A · σ_flow

  F        = required press force (N)
  A        = projected area of the part on the parting plane (mm²)
  σ_flow   = flow stress of the metal at forging temperature (MPa)
  K        = friction & shape factor (dimensionless, 3 to 10)

Worked example. An aerospace bracket forged from 6061 aluminum at 400 °C has a projected area of 200 cm² (20,000 mm²). The flow stress at that temperature is roughly 80 MPa. For a closed-die forging with deep ribs, K is around 8. Plugging in:

F = 8 × 20,000 × 80 = 12,800,000 N = 12.8 MN ≈ 1,300 tonnes

So a 1,500-tonne press is the right tool. The same calculation for the same part shape in titanium (σ_flow ≈ 250 MPa at 950 °C, K ≈ 9) gives 45 MN — a 4,500-tonne machine. This is why titanium parts are forged on much larger presses than aluminum equivalents.

The shape factor K rises sharply with cavity complexity. A simple pancake upset is K ≈ 1.2; a closed-die part with thin ribs and bosses is K ≈ 8 to 10; a precision near-net forging with very thin flash is K ≈ 12. Engineers add 20 to 30 percent margin to account for die wear and inconsistent billet temperature.

Forging variants compared

Variant	Die	Tolerance	Volume	Best for
Open-die	Flat or simple shaped platens	±5 mm	1 to 100	Large shafts, rolled rings, custom blooms
Closed-die (impression)	Matched cavities, full part shape	±0.5 mm	1,000 to 1,000,000	Crankshafts, connecting rods, turbine disks
Upset forging	Header machine, axial squeeze	±0.3 mm	10,000 to 10,000,000	Bolt heads, valve stems, ball studs
Drop forging	Closed die, hammer-driven	±0.8 mm	500 to 500,000	Hand tools, automotive forgings
Cogging	Open-die incremental bites	±10 mm	1 to 50	Marine shafts, rotor billets, ingot break-down
Roll forging	Two grooved rolls, no platen	±1 mm	1,000 to 100,000	Tapered axles, leaf-spring blanks, preforms

Open-die is the workhorse for large one-off parts: a 200-tonne marine propeller shaft is cogged from an ingot under a 12,000-tonne press over the course of a working day, the operator rotating and indexing between bites. Closed-die dominates anywhere volume justifies a $50,000 to $500,000 die set — automotive, aerospace, hand tools, hardware.

Real-world specs

• Boeing 787 wing spars. Forged on Alcoa Cleveland's 50,000-tonne press from titanium and aluminum alloys, then machined to net shape. Each spar starts as a 5-tonne billet and finishes as a 600 kg part — a 90 percent buy-to-fly machining loss but with the grain flow needed for a 100,000-cycle airframe life.
• F-22 Raptor titanium bulkheads. The largest single-piece titanium aerospace forgings ever made — 9 metres long, forged at Wyman-Gordon Grafton's 50,000-tonne press at 950 °C with the press loaded for over an hour per part.
• GE9X turbine disks. Inconel 718 nickel superalloy disks forged near isothermally at 1,000 °C between heated dies, then heat-treated for grain size. Each disk holds 100 fan blades at 4,000 rpm and 1,400 °C in service.
• Crankshafts for V8 engines. Forged from medium-carbon steel (1045 or 4140) on a 5,000-tonne press in three blows — flatten, gather, finish — total cycle time around 30 seconds per part.
• Landing gear cylinders. Forged from 300M ultra-high-strength steel; the main landing gear of an Airbus A380 starts as a 4-tonne forging.

Hot, warm, and cold forging

Regime	Temperature (steel)	Force needed	Tolerance	Surface
Hot	1,100 to 1,250 °C (above recrystallization)	1× baseline	±0.5 mm	Scale, oxide, rough
Warm	650 to 950 °C	2× to 3×	±0.2 mm	Cleaner, light scale
Cold	Room temperature	5× to 10×	±0.05 mm	Mirror, work-hardened

Hot forging is the cheap path: low force, complex shapes possible, but post-machining is required because of scale and dimensional drift on cooling. Cold forging produces near-net parts with excellent surface finish and dimensional accuracy — every Phillips-head screw on Earth is cold-headed — but only on small parts and ductile metals. Warm forging is a compromise gaining ground in automotive transmission gears.

Press types

• Hydraulic press. Slow stroke (cm/s), constant force throughout, capable of long dwells. The architecture of every press above 5,000 tonnes. Cycle time is long but force is unlimited — just add more pumps.
• Mechanical (eccentric) press. A flywheel-driven crankshaft converts rotation to ram stroke. Fast (60 to 120 strokes per minute) but force peaks at the bottom of the stroke. Common up to 4,000 tonnes for high-volume small forgings.
• Screw press. A flywheel turns a giant lead screw that drives the ram down; energy is set per blow. Combines hammer-like impact with press-like control; used for coining and small precision forgings.
• Drop hammer. A heavy ram (up to 30 tonnes) lifted by steam, air, or board friction, then released. Energy per blow rather than force. Still preferred for blade preforms and pattern forgings where surface deformation closes lap defects.

Common failure modes

• Lap defects. Metal folds back on itself and the seam is pressed shut without bonding. Caused by oversized billet, asymmetric die fill, or excess flash. Detected by magnetic-particle inspection on ferrous parts; ultrasonic on titanium and nickel.
• Cold shuts. Two flowing fronts meet at temperature too low to fuse. Almost always traced to die temperature below 200 °C or lubricant pooling in cavity bottoms.
• Underfill. Cavity not completely filled — usually because the billet was undersized, the press was under-tonnaged, or flash channels opened too early. Visible as missing geometry at corners.
• Center bursts (chevron cracks). Internal voids that open in the centre of a forging when triaxial tension exceeds the metal's ductility. Common in cold extrusion of brittle materials; mitigated by smaller reduction ratios.
• Die fatigue. The dies themselves fail by thermal fatigue (hot forging) or rolling-contact fatigue (cold). Tool steel H13 is the workhorse for hot dies, hardened to 50 HRC; closed-die life for steel forgings is 5,000 to 20,000 parts.
• Decarburization. Surface carbon burns out of the steel during long heating, leaving a soft skin that has to be machined away. Mitigated by inert atmospheres or short hold times.