Open Die Forging

Open die forging plastically deforms heated metal between flat or simple dies to create large, high-integrity preforms with flexible geometry but looser tolerances.

Overview

Open die forging (open die) shapes hot metal by repeated press or hammer blows between flat or simply contoured dies, without enclosing the workpiece in a cavity. It’s used to break down ingot/billet into bars, blocks, shafts, and stepped preforms, improving grain flow and reducing internal defects for high mechanical properties.

Choose it for large parts, low-to-medium quantities, and when you need soundness and toughness more than near-net shape. Typical results are forged “blanks” that go to heat treat and finish machining.

Tradeoffs: geometry is limited compared to die-sink forging; dimensional variation and draft/straightness are larger; machining allowance is required. Lead time is often faster than closed-die tooling, but piece price can be higher for small parts due to touch labor and multiple heats/press sequences.

Common Materials

  • 4140 steel
  • 4340 steel
  • 17-4 PH stainless
  • Inconel 718
  • Ti-6Al-4V
  • Aluminum 7075

Tolerances

±0.030" to ±0.125" (as-forged), with machining stock typically added

Applications

  • Large drive shafts
  • Turbine and generator rotor shafts
  • Crankshaft preforms
  • Heavy-duty valve bodies and blocks
  • Tool steel die blocks
  • Pressure vessel nozzles and hubs

When to Choose Open Die Forging

Pick open die forging when the part is large, the volume is low to medium, and you want high mechanical integrity and directional grain flow. It fits parts that can be made as simple sections (round, rectangular, stepped) with machining stock and post-forge heat treat. It’s also a good choice when you need flexibility to iterate dimensions without buying dedicated die cavities.

vs Closed Die Forging

Choose open die forging when part size is large, geometry can be a preform, and you want to avoid the cost and lead time of impression-die tooling. Open die is better for low volumes and for making forge blanks that will be machined to final features.

vs Cold Forging

Choose open die forging when the material or section size requires hot working to achieve deformation without cracking and when very high forming loads would make cold forging impractical. Open die also handles much larger cross-sections and allows more flexibility in preform shape at the expense of tighter cold-forged tolerances.

vs Ring Rolling

Choose open die forging when the part is not a continuous ring or when you need solid hubs/shafts/blocks rather than a rolled ring. Open die is also useful to create the initial preform that may later be pierced and ring-rolled if the design evolves into a ring geometry.

vs Upset Forging

Choose open die forging when you need distributed deformation along length (drawing out, stepping, cogging) rather than localized thickening at an end or specific station. Open die handles longer parts and larger diameters with fewer constraints on how the material is supported during forming.

Design Considerations

  • Provide a clear forging envelope (max OD, length, and target preform shapes) rather than final machined features
  • Specify machining allowance and critical datum locations so the shop can plan stock distribution and straightening
  • Avoid thin webs, deep pockets, and sharp inside corners; plan those as machined features after forging
  • Call out grain-flow or orientation requirements (e.g., axial grain in shafts) and any ultrasonic inspection levels up front
  • Define straightness/runout requirements separately from size; open die often needs post-forge straightening
  • List heat treat condition, surface condition (as-forged vs machined), and any NDE requirements on the RFQ to prevent quote gaps