Quick Answer
CD4MCu is a cast duplex stainless steel — roughly half ferrite, half austenite — that delivers about twice the yield strength of CF8M together with better chloride pitting and stress-corrosion resistance. It is the step buyers take when 316-class castings either corrode (pitting in brines) or wear and flex too much (pump internals). Typical homes: pump casings and impellers in mining, desalination, chemical and marine service.
Composition (ASTM A995 Gr. 1A / A744 CD4MCu type)
| Element | Typical range |
|---|---|
| Chromium | 24.5–26.5% |
| Nickel | 4.75–6.0% |
| Molybdenum | 1.75–2.25% |
| Copper | 2.75–3.25% |
| Carbon | ≤ 0.04% |
High chromium with moderate nickel produces the balanced two-phase structure; copper adds resistance in acidic and brine media.
Properties: The Duplex Advantage
| Property | CD4MCu | CF8M (for contrast) |
|---|---|---|
| Tensile min | 690 MPa (100 ksi) | 485 MPa |
| Yield min | 485 MPa (70 ksi) | 205 MPa |
| Elongation min | 16% | 30% |
| Hardness (typical) | ~250 HBW | ~160 HBW |
| Chloride pitting | Superior | Good |
| Chloride SCC | Highly resistant | Susceptible when hot |
The yield-strength gap is the headline: 485 vs 205 MPa lets designers thin walls, resist pressure pulsation and survive abrasive slurries that peen soft austenitics.
Applications Where CD4MCu Earns Its Cost
- Slurry and process pump casings, impellers and wear parts in mining and FGD systems
- Desalination and seawater handling components
- Valve bodies and internals in chloride-bearing chemical streams
- Marine hardware where 316 pits at crevices
The Specification Pitfalls (Read Before Ordering)
- Solution annealing is critical and less forgiving than for austenitics. Wrong cooling through 700–950°C precipitates sigma phase — a brittle intermetallic that wrecks toughness and corrosion resistance. Require the anneal cycle on the certificate.
- Ferrite balance matters. Ask for a ferrite measurement (point count or feritscope) on qualification castings; 40–60% is the healthy window.
- Welding needs duplex procedure discipline — matched filler, controlled interpass temperature. Casual repair welding is where duplex reputations die.
- Machining is harder than CF8M — plan roughly 30–50% more machining effort; another argument for near-net investment casting with minimal stock.
Where CD4MCu is over-kill and CF8M under-kill, modern 2205-type cast duplex (CD3MN, A995 Gr. 4A) sits between — worth a line in your RFQ if sourcing flexibility helps. Grade-family context is in the cast stainless overview.
FAQ
Is CD4MCu magnetic?
Noticeably, yes — about half the structure is ferrite. Unlike with CF8M’s faint pull, a magnet grips duplex firmly. Normal, not a defect.
CD4MCu vs CD3MN (2205) — which duplex?
CD4MCu: higher chromium and copper, long track record in acids and mining. CD3MN: tougher, easier to weld, broader mill familiarity. Media chemistry and your industry’s habit usually decide.
Can CD4MCu be used at high temperature?
Keep continuous service below roughly 250–300°C — above that, embrittlement mechanisms activate. Duplex is a wet-corrosion family, not a heat-resistant one.