PROCESS ENGINEERING · August 10, 2026 · 9 MIN READ

Hot Forging Process Parameters: How to Calculate Tonnage, Blow Energy and Stage Count

Required press tonnage for closed-die hot forging is estimated as F = K × σf × A, where A is the projected area including flash, σf is the flow stress at the finishing temperature, and K is a constraint factor between 3 and 6 — with K near 3 for simple upsetting and near 6 for thin webs with a wide flash land. A 1.8 kg connecting rod with 9,500 mm² projected area in 42CrMo at 1,000 °C lands at roughly 2,300–2,800 t, which is why it is normally run on a 2,500 t press with 20–30 % margin.

WHERE THIS APPLIES AT FORGEPRESS CHINA

ForgePress China sizes the press from the projected area including flash, and returns a tonnage and stage-count calculation with every quotation, for the HKM3 line from 1,000 t to 5,000 t. Capacity: HKM3 hot-die forging line 1,000–5,000 t, part weight 0.1–500 kg, maximum envelope 1,200 × 800 mm, as-forged to ISO 8062 class C with critical dimensions machined to ±0.2 mm, process control to ISO 9001, first article 4–8 weeks from die design release.

KEY NUMBERS

F = K × σf × A, with K = 3–6 depending on flash ratio and web thinness.
Flow stress of 42CrMo at 1,000 °C is roughly 110–130 MPa; at 900 °C it is 150–180 MPa.
Add 20–30 % tonnage margin for flash land variation, temperature scatter and off-centre loading.
Stage count follows reduction ratio: no single closed-die stage should reduce area by more than about 60 %.

Step 1 — projected area, including the flash

The projected area is measured on the parting plane and must include the flash land, because the flash is what generates the back-pressure that fills the impression. A common error is to use the part's plan area and then wonder why the last corner does not fill. For a part with plan area 8,200 mm² and a 25 mm flash land around a 900 mm perimeter, the flash adds 22,500 mm² — nearly three times the part area.

Measure the area from the die layout, not the finished drawing, because the die carries shrinkage allowance and the flash land is a die feature.

Step 2 — flow stress at the finishing temperature

Flow stress is not the room-temperature yield strength. At 1,000 °C, 42CrMo is around 110–130 MPa; at 900 °C it climbs to 150–180 MPa; at 800 °C it can exceed 220 MPa. Using the room-temperature value of 1,000 MPa overestimates by a factor of eight, and using a handbook value at the wrong temperature underestimates by 40 %.

If you do not have flow-stress curves for the exact grade, an acceptable approximation for carbon and low-alloy steel is σf ≈ 1.6 × UTS at 20 °C × (1 − 0.0011 × (T − 20)), where T is in °C, valid between 800 and 1,200 °C. It is a fit, not a law, but it lands within about 15 % across the common grades.

Step 3 — constraint factor and flash ratio

Geometry	Flash ratio (land width / web thickness)	K
Simple upsetting, no flash	—	1.0–1.15
Open-die forging	—	1.5–2.5
Closed die, thick web (t > 12 mm), narrow land	< 1.5	3.0–4.0
Closed die, medium web (6–12 mm)	1.5–3	4.0–5.0
Closed die, thin web (t < 6 mm), wide land	> 3	5.0–6.5

Constraint factor K for hot closed-die forging. Higher K means the flash is doing more of the work of building cavity pressure.

Step 4 — worked example

Part: connecting rod, 1.8 kg, 42CrMo, finish temperature 980 °C, projected area with flash 9,500 mm². Flow stress at 980 °C ≈ 125 MPa. Web thickness 9 mm with a 20 mm flash land, so flash ratio ≈ 2.2 and K ≈ 4.5. Then F = 4.5 × 125 MPa × 9,500 mm² = 5.34 MN ≈ 545 t.

That number is the force at the instant of final filling, and it is not the press rating you need. A mechanical press delivers its rated tonnage near bottom dead centre only, and the load builds over the last 3–5 mm of stroke. Applying a 1.3 margin gives 709 t, so a 800 t press is the minimum and a 1,000 t press is the sensible choice once off-centre loading and die wear are included.

Note how far the naive estimate is from reality if you skip the flash: part-only area 8,200 mm² with K = 3 gives 307 t — less than half the correct figure. This is the single most common reason a quoted press is undersized.

Step 5 — stage count from reduction ratio

A single closed-die impression should not reduce cross-sectional area by more than about 60 % in one blow; beyond that, folding and laps become likely. For a part that starts from a 60 mm round billet and ends with a 22 mm web, the area ratio is 7.4:1, which needs three stages, not two.

The usual sequence is: roll or upset to gather metal, blocker (preform) to distribute it, then finisher to size it. Each stage is a separate impression and a separate tonnage calculation — the blocker often needs more force than the finisher because it moves more metal.

Upset / gather: area reduction up to 50 %, low tonnage, controls billet mass distribution.
Blocker: area reduction 40–60 %, highest tonnage of the sequence, sets fibre flow.
Finisher: area reduction 10–25 %, defines tolerance and surface, needs the flash land to build pressure.
Trimming: 8–12 % of forging tonnage, shear rather than press load.

Frequently asked questions

What tonnage press do I need for a 2 kg forging?

There is no answer from mass alone. A 2 kg part with 6,000 mm² projected area in carbon steel needs roughly 500–700 t; the same mass spread into a thin, wide shape with 15,000 mm² projected area needs 1,600–2,200 t. Projected area and web thickness drive tonnage, mass does not.

Why does the calculation not match the press nameplate?

A mechanical press delivers rated tonnage only near bottom dead centre, and die height, ram adjustment and clutch slip all reduce what actually reaches the die. A 1,000 t nameplate press may deliver 700 t at the point where a deep impression needs it. Ask for the load-stroke curve, not the nameplate.

How much margin should I add?

Twenty to thirty percent on the calculated peak force covers flash land variation, billet temperature scatter of ±20 °C, and off-centre loading from imperfect billet placement. Below 20 % you will see short fills on cold mornings; above 40 % you are paying for capacity you will never use.