Butterfly Valve Body & Disc Materials: Cast Iron, Ductile Iron, CF8M, Aluminium Bronze & Duplex
Written by
Allen Zhang · Senior Application Engineer, LAUX VALVE
The seat seals the fluid, but the body and disc carry the pressure and meet the media head-on. Choose them wrong and a valve that seals perfectly on day one corrodes, erodes or cracks within a season. The body holds the pressure boundary and bolts to the pipe; the disc sits permanently in the stream and takes the full chemical and abrasive attack of the fluid every second. This guide walks through the mainstream body and disc materials — from grey and ductile iron through carbon steel, CF8M stainless, aluminium bronze to duplex — and gives a decision flow that matches the metal to the fluid, the temperature and the pressure.
Body and disc do different jobs
The body is the pressure-retaining shell: it must withstand the rated pressure and the bolt loads from the flanges, but on a resilient-seated valve the rubber liner usually shields it from the fluid, so a coated cast or ductile iron body is fine for water even though iron would corrode if bare. The disc is the opposite — it is permanently wetted and wiped by the flow, so its alloy must resist the fluid's corrosion and any abrasion directly. That is why a common, economical build pairs a coated ductile-iron body with a stainless or aluminium-bronze disc: cheap metal where it is protected, corrosion-resistant metal where it is exposed.
Material comparison table
| Material | Typical use | Strengths | Avoid | Cost |
|---|---|---|---|---|
| Grey cast iron (GG25) | Low-pressure water bodies | Cheap, good damping, easy to cast | Brittle, low PN, no impact/shock | $ |
| Ductile iron (GGG40/50) | Most water & HVAC bodies | Tough, strong, handles PN16/25 | Bare iron corrodes — needs coating/lining | $ |
| Carbon steel (WCB) | Hydrocarbons, steam, oil & gas | High pressure/temperature, weldable | Rusts in water unless coated/painted | $$ |
| CF8M (cast 316 SS) | Corrosive, food, pharma, chemicals | Broad corrosion resistance, hygienic | Chlorides (pitting) — go duplex for seawater | $$$ |
| Aluminium bronze (disc) | Water, brackish water, HVAC discs | Excellent in water, anti-biofouling | Not for strong acids or high abrasion | $$$ |
| Duplex / Ni-Al bronze | Seawater, desalination, chloride brine | Superb pitting/SCC resistance, high strength | Highest cost, longer lead time | $$$$ |
Choosing the metal: a decision flow
- 1
1. Is the media corrosive or hygienic-critical?
Acids, chemicals, food or pharma contact? Specify a CF8M (cast 316) disc — and a CF8M body if the liner does not fully isolate the shell. For seawater or chloride brine, jump to duplex or Ni-Al bronze.
- 2
2. Is it hydrocarbon, steam or high pressure/temperature?
For oil & gas, steam and Class 300+ service, a carbon-steel (WCB) body with a stainless or alloy disc is the standard, with a fire-safe or metal seat. Iron bodies are usually excluded by the spec here.
- 3
3. Is it abrasive or biofouling-prone water?
Aluminium bronze discs resist erosion and marine biofouling well, making them a favourite for raw, brackish and cooling water. For heavy slurry, consider a hardened or coated disc and a robust ductile-iron or steel body.
- 4
4. Otherwise — coated ductile iron + bronze/SS disc
For ordinary potable water, HVAC and building services, an epoxy-coated ductile-iron body with an EPDM liner and a stainless or aluminium-bronze disc is the proven, economical default — strong, corrosion-protected and a fraction of an all-stainless valve's cost.
Frequently asked questions
What is CF8M and how does it relate to 316 stainless steel?
CF8M is the cast equivalent of 316 stainless steel under ASTM A351 — essentially 316 in cast rather than wrought form, with molybdenum for improved resistance to pitting and chlorides. Because valve bodies and discs are cast, you will see CF8M on the data sheet where a fabricated part would say 316. It gives broad corrosion resistance for chemicals, food and pharma, but like wrought 316 it is still vulnerable to pitting in high-chloride water, where duplex is the better choice.
Can the body and disc be different materials?
Yes — and they very often are. Because the resilient liner usually shields the body from the fluid while the disc is fully wetted, the most cost-effective build uses a cheaper protected body (coated ductile iron) with a corrosion-resistant disc (stainless or aluminium bronze). You pay for premium alloy only where the fluid actually attacks it. Just make sure the shaft, bearings and fasteners are also rated for the media, since they are wetted too — a stainless disc on a valve with a carbon-steel shaft can still fail at the shaft.
Is ductile iron strong enough for a butterfly valve body?
For most water and HVAC duty, yes. Ductile (nodular) iron is far tougher and more impact-resistant than grey cast iron because its graphite is in spheroidal rather than flake form, and it comfortably handles PN10/16/25 water service. It is the workhorse body material for resilient-seated valves up to large diameters. Where it falls short is high temperature, hydrocarbons and fire-safe codes — there a carbon-steel (WCB) or stainless body is required. And remember bare iron corrodes: it must be epoxy-coated or fully lined.
What body and disc do I need for seawater or desalination?
Use duplex / super-duplex stainless or nickel-aluminium bronze for the wetted parts. Seawater is rich in chlorides that pit standard 316/CF8M, especially when warm or stagnant, so the disc, shaft and any wetted body surface should be a chloride-resistant alloy. Duplex offers very high strength plus excellent pitting and stress-corrosion-cracking resistance; nickel-aluminium bronze adds natural biofouling resistance and is long-established in marine cooling water. Match the bolting, stem and seat materials to suit too — the weakest wetted component sets the valve's service life.
References & further reading
- ASTM A351 — Castings, austenitic, for pressure-containing parts (CF8M)
- ASTM A536 — Standard Specification for Ductile Iron Castings
- ASTM A216 — Carbon steel castings for high-temperature service (WCB)
- NACE MR0175 / ISO 15156 — Materials for H2S-containing (sour) service
- NSF/ANSI 61 — Drinking Water System Components (coatings & wetted metals)
