In This Article
- 1.Casting Process - How Cast Valves Are Made
- 2.Forging Process - How Forged Valves Are Made
- 3.Mechanical Property Comparison
- 4.When to Specify Cast vs Forged
When specifying an industrial valve, one of the most fundamental decisions is whether to require a cast body or a forged body. Both manufacturing processes produce valves that meet ASME B16.34 pressure-temperature requirements when properly designed and tested, but the two processes have different strengths, weaknesses, and appropriate size ranges. Understanding the engineering difference helps procurement engineers and plant engineers write better valve specifications.
Casting Process - How Cast Valves Are Made
Cast valve bodies are manufactured by pouring molten metal into a sand mould (sand casting) or a wax pattern mould (investment casting, also called lost-wax casting). The molten metal solidifies around the mould core to form the valve body shape. Investment casting is used for smaller, more complex shapes (small ball valve bodies, globe valve bodies) and produces better surface finish and dimensional accuracy than sand casting. Sand casting is used for larger valve bodies (gate valves DN100+, globe valves DN80+, butterfly valve bodies). The cast microstructure has a dendritic (tree-like) grain structure that can include porosity (gas voids), inclusion (sand or slag particles), and shrinkage cavities if not properly designed and heat-treated.
Forging Process - How Forged Valves Are Made
Forged valve bodies are manufactured by pressing or hammering a heated metal billet (typically between 900-1200°C for steel) into a die set that defines the valve shape. The forging process closes any internal voids, breaks up the cast grain structure, and aligns the grain flow in the direction of the applied force. The result is a denser, tougher metal with higher yield strength and impact toughness than an equivalent cast part. Forged valve bodies are the standard for smaller sizes (DN15-DN50, occasionally to DN100 for high-pressure service) where the billet cost is manageable, and for high-pressure/high-temperature service where maximum material integrity is required.
Mechanical Property Comparison
| Property | Cast Body (A216 WCB) | Forged Body (A105) |
|---|---|---|
| Yield Strength (min) | 250 MPa (36 ksi) | 250 MPa (36 ksi) |
| Tensile Strength (min) | 485 MPa (70 ksi) | 485 MPa (70 ksi) |
| Impact Toughness | Lower (cast grain) | Higher (worked grain) |
| Porosity Risk | Present in castings | None in forgings |
| Max UT inspection | Required for high integrity | Less critical - denser |
| NACE MR0175 hardness | 22 HRC maximum | 22 HRC maximum |
| Typical size range | DN50 to DN600+ | DN15 to DN100 (typical) |
| Lead time | 10-16 weeks (heavy castings) | 6-10 weeks typically |
| Cost relative | Lower unit cost at large size | Lower cost at small size |
When to Specify Cast vs Forged
Cast valve bodies: DN80 and above for most valve types; all butterfly and strainer bodies; large-bore ball valves (DN150+) in standard service; where complex internal geometry is required (globe valve body with contoured flow port); where cost is the primary factor at large sizes. Forged valve bodies: DN50 and below as standard; DN15-DN80 for high-pressure service (Class 1500, 2500); socket weld and threaded end connections (all socket weld valves are forged - you cannot socket-weld a cast fitting); NACE sour service where maximum material integrity is required; critical service where API 598 100% testing and PMI are specified; hydrogen service (ASME B31.12) where hydrogen embrittlement of cast structures is a concern.
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