Open Die Forging

Open die forging is a hot forming process in which a heated workpiece is shaped by compressive blows or squeezes between tools that do not enclose it. The tools are typically flat dies, V-dies or simple radiused swages. Instead of a cavity defining the shape, the geometry is created by how the operator positions, rotates and advances the workpiece between successive blows.

Because the metal is never confined, it is free to flow sideways — hence the alternative name free forging. The same pair of flat dies can make a 60 mm shaft in the morning and a 600 mm disc in the afternoon, which is exactly why the process has no dedicated tooling cost and why single pieces are economically sensible.

Open die versus closed die in one paragraph

Closed die forging pushes metal into a shaped cavity: it produces complex, repeatable, near-net parts in volume, but needs a die that must be paid for and amortised. Open die forging produces simpler shapes with larger machining allowances, but needs no dedicated die and can start the day material is available. The two are complementary, and many large closed die forgings begin life as an open die preform. See closed die versus open die for a full comparison.

Why engineers specify open die forgings

• No tooling cost — viable from a single piece, ideal for spares, prototypes and low volumes.
• Internal soundness — heavy working closes porosity and breaks down the coarse cast structure of the original steel.
• Size flexibility — lengths and diameters are limited by equipment reach, not by a die cavity.
• Speed — production can start as soon as material is ready, with no die to design or cut.
• Directional properties — grain is aligned along the axis of shafts and bars, which is where most shaft loads act.

Every open die forging, however complex it looks, is built from a small set of basic operations applied in sequence.

Upsetting and why it is used even on shafts

Upsetting compresses a billet along its length. It is obviously used to make discs, but it is also used on shafts: upsetting and then drawing out again gives the metal more total working, improving soundness and transverse properties beyond what simply drawing a bar down would achieve.

Drawing out and bite ratio

When a bar is drawn out, the dies take a series of overlapping bites along its length, with the bar rotated between passes. The width of each bite relative to the bar thickness matters: bites that are too narrow deform only the surface and can fold material over, while sensible bite ratios drive deformation into the core. This is a large part of what separates a skilled open die forge from an average one.

Piercing and mandrel forging

Piercing creates a through hole in an upset preform. The pierced piece can then be expanded over a mandrel to make a ring, or drawn over one to make a sleeve. For rings in production quantities, ring rolling is usually more efficient — see open die forging versus ring rolling.

Most open die work we do is for replacement spares, low-volume machinery components and preforms, in quantities from one piece to a couple of hundred. Where the same part is needed in thousands, a closed die normally becomes cheaper per piece, and we will say so.

Steel starts life as a casting — an ingot or continuously cast bloom — with a coarse dendritic structure, some chemical segregation and, potentially, small internal voids. Forging breaks down that structure, welds shut internal voids under compressive stress and refines grain. How much this happens depends on how much the section is reduced, which is expressed as the [forging ratio](/forging-glossary/forging-ratio).

Calculating forging ratio

For drawing out, the forging ratio is the starting cross-sectional area divided by the final cross-sectional area. For example, a 180 mm square billet has an area of 32,400 mm². Drawn down to a 110 mm diameter round, the area becomes about 9,503 mm². The forging ratio is 32,400 ÷ 9,503 ≈ 3.4:1.

Specifications should state whether the ratio is calculated from the original ingot or from the billet as received, because the two give very different numbers. A ratio quoted from the ingot includes all the reduction done at the steel mill.

Proving soundness

Internal soundness cannot be judged from the outside. Critical open die forgings are examined by ultrasonic testing, commonly to ASTM A388 or, for customers working to German practice, SEP 1921, with the acceptance class taken from the customer's specification. Ultrasonic examination is normally carried out after heat treatment and before final machining, once the surface has been rough machined to allow good coupling.

Open die forgings are not near-net. The surface carries scale and small irregularities from the tooling, and dimensions are controlled by manipulation rather than by a cavity. A machining allowance is therefore added to every surface, and it is larger than on closed die parts.

Worked example: estimating forged weight

Take a finished shaft of 100 mm diameter and 500 mm length in carbon steel.

1. 01Finished weight. Volume = π × 50² × 500 ≈ 3.93 million mm³ = 3,927 cm³. At 7.85 g/cm³ this is about 30.8 kg.
2. 02Forged size. Adding 10 mm on diameter and 10 mm per end gives a forging of 110 mm diameter and 520 mm length.
3. 03Forged weight. Volume = π × 55² × 520 ≈ 4.94 million mm³ = 4,942 cm³, or about 38.8 kg.
4. 04Billet weight. Allowing roughly 5–8% for scale and cropped ends gives a billet of about 41–42 kg.
5. 05Material utilisation. 30.8 kg finished from about 41.5 kg purchased is roughly 74% — still far better than machining the shaft from oversized bar with a large diameter reduction.

Our forging weight calculator does the volume arithmetic for round and square sections.

Open die forgings are almost always heat treated. The heavier sections involved make the choice of cycle — and the grade's ability to respond through the section — particularly important.

• Normalising refines grain and evens out the variable structure left by forging, giving consistent hardness for machining. It is the standard treatment for carbon steel shafts and blocks.
• Quenching and tempering develops higher strength and toughness in alloy grades such as EN19 and EN24. Section size limits what can be achieved at the core, so grade selection must account for it.
• Annealing gives maximum softness where heavy machining is to follow.
• Stress relieving reduces residual stress before finish machining of long or complex parts.

Furnace charts are recorded for every charge, and hardness is verified before release. See our heat treatment services.

Every heat lot is spectro verified against its mill certificate before forging, so the grade on the certificate is confirmed to be the grade in the part.

Compared with machining a large shaft directly from bar, an open die forging has two advantages beyond material saving: its steel has been worked more thoroughly, and its grain runs along the axis rather than being cut across at every step. See forging versus machining.

Open die forging depends heavily on process discipline — temperature, bite size, reduction per pass and cooling. Most defects trace back to one of those, and most are invisible from the outside, which is why heavy sections are ultrasonically examined.

Internal bursts deserve special mention. When a bar is drawn out with very narrow bites, the dies work the surface but not the centre, and the surface can effectively pull the core apart. The part looks perfect and fails ultrasonic examination — or, worse, fails in service. Correct bite geometry is the prevention; ultrasonic testing is the proof. See ASTM A388.

The examples below are illustrative of the kind of work open die forging handles well, and the decisions involved. They are representative engineering cases rather than specific customer orders.

An urgent replacement mill shaft

A processing plant has a broken stepped drive shaft, no drawing and a stopped line. The broken halves are measured and spectro analysed to confirm the grade — in this kind of case often a chromium-molybdenum steel. The shaft is open die forged with allowance on every diameter, quenched and tempered, ultrasonically examined and machined. With no die to design, the replacement is limited mainly by material availability and machining time rather than tooling. See reverse engineering.

A stainless pump shaft for water service

A pump manufacturer needs twenty shafts in SS 410 for a special order. Twenty pieces would never justify a closed die. Open die forged bars, hardened and tempered, are ground on bearing and seal seats. The forging gives worked, sound material with grain along the shaft axis — better than cutting the shafts from oversized bar. See pump industry forging.

A large gear blank disc

A gearbox maker needs a handful of 550 mm gear blanks for a prototype. The billet is upset to a disc, which drives grain radially outward, then normalised and proof machined. If the design goes into production, the same knowledge carries into a closed die or ring-rolled route. See forged gear blanks.

• Spectro analysis of incoming material against the mill certificate
• Recorded forging sequence and dimensions, supporting the calculated forging ratio
• Furnace chart for each heat treatment charge
• Hardness verification, with a survey across heavy sections where specified
• Ultrasonic examination to ASTM A388 or the customer's standard where required
• Dimensional inspection of forged and machined sizes
• Material test certificates to EN 10204 3.1, with heat number traceability

Third-party inspection agencies and customer inspectors are welcome to witness forging, heat treatment and testing at our Gondal works by appointment.

An open die enquiry needs less information than a closed die one, but a few items make the difference between a quick, accurate quotation and a round of questions.

1. 01Finished drawing, or forged size if you machine yourself
2. 02Material grade and required heat treatment or hardness
3. 03Quantity
4. 04Whether ultrasonic testing is needed, and to which acceptance class
5. 05Any minimum forging ratio requirement, and whether it is calculated from ingot or billet
6. 06Scope: forged and heat treated, proof machined, or finish machined
7. 07Required delivery date, especially for breakdown replacements

If you have only a worn part, our reverse engineering service measures it, identifies the grade and produces a drawing for approval. For urgent work, call Jatin Ramoliya on +91 94274 40621 or Sanjay Lakkad on +91 99136 01800, or request a quote.