Stainless Steel Casting Grades Explained: CF8, CF8M, CF3M & Duplex

CF8, CF8M, CF3, CF3M, CF8C and duplex CD3MN — the six stainless steel casting grades behind most industrial valve and pump bodies. Here is what each is, how it maps to the wrought grade on your drawing, and how to choose without over- or under-specifying.

Why Cast Grades Have Different Names

If you are used to specifying 304 or 316 on a drawing, the CF8 and CF8M notes on a casting quote can look like a different alloy. They are not — they are the cast versions of the same families under a separate designation system. Cast stainless grades are standardized under ASTM A351 (pressure-service valves, flanges and fittings), ASTM A743 and ASTM A744. The system exists because a casting cannot use the same chemistry as wrought bar: foundries add more silicon for fluidity and must control ferrite content to avoid hot cracking.

casting material selection." data-zh="一旦了解编码规则，读牌号名称就很简单。首字母 C 表示该合金用于耐腐蚀用途；第二个字母 F 表示铬和镍含量高于简单的 CF 牌号；后面的数字与碳和镍含量相关，用于定位该牌号在族内的位置。变形牌号使用不同逻辑——三位数 AISI 编号——这正是对照表重要的原因。关于材料选择与工艺如何相互作用，请参阅我们的铸造材料选择指南。">Reading the name is simple once you know the code. The first letter C means the alloy is intended for corrosion-resistant service; the second letter F signals higher chromium and nickel than the simple CF grades; the digit that follows ties to carbon and nickel content and places the grade in the family. Wrought grades use different logic — a three-digit AISI number — which is why a cross-reference table matters. For how material choice interacts with process, see our guide to casting material selection.

The Six Grades at a Glance

The table below is the cross-reference you will use most often, mapping each cast grade to its wrought equivalent and summarizing elements, carbon, corrosion, temperature range and typical applications.

CF8 — the 304 Equivalent

CF8 is the workhorse of cast stainless. With 18–21% chromium and 8–11% nickel it forms a passive chromium-oxide film that protects the surface in oxidizing media such as fresh water, many food fluids and mildly corrosive streams. It is widely available, machines well, and is the cheapest austenitic cast grade.

Its limitation is chloride and welding. With no molybdenum, CF8 can pit and crevice-corrode in chloride fluids where CF8M would not. The 0.08% carbon ceiling also makes it prone to sensitization: welded or held between roughly 425°C and 815°C, chromium carbides precipitate at grain boundaries and leave chromium-depleted zones that corrode first. That is precisely why the low-carbon grades exist.

CF8M — the 316 Equivalent

CF8M adds 2–3% molybdenum to the CF8 base, and that single addition transforms its chloride behaviour. Molybdenum stabilizes the passive film so pitting and crevice corrosion need a much higher chloride concentration or temperature to start. This is why CF8M — not CF8 — is standard for seawater, brackish water, many chemical streams, pulp and paper liquor, and equipment cleaned with chloride sanitizers.

CF8M keeps the same 0.08% carbon as CF8, so it shares the same sensitization concern in welded, non-annealed service. Its PREN (≈ %Cr + 3.3×%Mo + 16×%N) is about 25 — well above CF8 but below duplex. Mechanically it is close to CF8; the molybdenum premium shows up mainly in price. If chlorides are present, CF8M is the minimum sensible grade.

CF3 & CF3M — the 304L and 316L Equivalents

CF3 and CF3M are the low-carbon versions of CF8 and CF8M, capped at 0.03% carbon instead of 0.08%. That is the whole design intent: with so little carbon, chromium carbides cannot form at grain boundaries during welding, so the part stays resistant to intergranular corrosion even without a post-weld anneal. Welders prefer these grades because they remove the heat-treatment step from the schedule.

The trade-off is a small loss of strength — low-carbon austenitic grades have slightly lower minimum tensile and yield strengths, which rarely matters for pressure castings sized by corrosion allowance. The rule is simple: if a part is welded and cannot be annealed afterward, specify CF3 or CF3M instead of CF8 or CF8M. CF3M answers the common case of a welded, chloride-bearing 316-type part.

CF8C — the 347 Equivalent

CF8C solves sensitization from the other direction. Instead of lowering carbon, it adds niobium — at least eight times the carbon content — a strong carbide former that grabs the carbon first, forming stable niobium carbides and leaving chromium in the matrix. The result is a stabilized austenitic stainless that resists sensitization during welding and performs well in continuous high-temperature service.

CF8C is the grade for castings above about 425°C — exhaust parts, furnace and heat-treat fixtures, refinery valves, and steam service where CF8 or CF8M would sensitize and lose toughness. Its range extends to roughly 600°C. Below that, CF8M or CF3M does the same job for less, so CF8C earns its place only for genuinely elevated thermal duty.

Duplex & Super Duplex — CD3MN and CD3MWCuN

Duplex cast grades are the step change. CD3MN (cast 2205) has a microstructure roughly half ferrite and half austenite, produced by balanced chromium-nickel-molybdenum chemistry with deliberate nitrogen. It gives two things austenitic stainless cannot offer at once: about double the yield strength of CF8M, and strong resistance to chloride stress-corrosion cracking — the mode that cracks ordinary 304 and 316 in warm chloride service even when they resist pitting.

CD3MN carries a PREN of at least 30 and suits offshore and marine hardware, seawater pumps, desalination components and chemical valves. When chlorides, temperature or pressure climb further, super duplex CD3MWCuN (PREN above 40) takes over. The catch is heat treatment: the ferrite-austenite balance is only correct after a proper solution anneal and rapid quench, so supplier qualification matters most here.

Relative Cost and Machinability

Grade selection is a cost decision as well as a metallurgy one. The table below uses CF8 as the baseline of 1.0 and shows how each step up the corrosion ladder affects material cost and machinability. These are planning multipliers, not quotes.

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How to Specify the Right Grade

Work through these questions in order. The first answer that rules a grade in or out usually decides the specification, and you should stop as soon as a grade satisfies every requirement — over-specifying wastes money and can add lead time.

• Media or fluid — What flows through or around the part? Fresh water, food, seawater, acid, caustic, hydrocarbon or sour gas each sets a different baseline. Identify chlorides first; they drive everything else.
• Temperature — What are the continuous and peak temperatures? Below about 425°C austenitic grades are fine; sustained higher temperatures favour CF8C; duplex should not be used above roughly 300°C.
• Chloride level — Is the chloride concentration high enough to pit or crack 304? Warm chlorides rule out CF8 and often rule in CF8M, CF3M or duplex.
• Welding — Will the part be welded, and can it be solution annealed afterward? If not, low-carbon CF3/CF3M is the answer; if it also sees high temperature, consider CF8C.
• Pressure — What is the design pressure and which code applies? Pressure castings are typically ordered to ASTM A351 with ASME B16.34 ratings, which also fix required heat treatment and testing.
• Regulatory or NACE — Is the service sour (H2S-bearing)? Sour oil and gas requires grades qualified to NACE MR0175 / ISO 15156, with hardness limits, often duplex or super duplex.

Common Mistakes

Most stainless grade problems trace back to a few recurring errors. Watch for these on your next casting order.

• Specifying 304 (CF8) in a chloride environment. CF8 has no molybdenum, so warm chloride fluids pit it and can eventually crack it. With chlorides present, specify CF8M or better — never CF8.
• Ignoring low-carbon grades on welded parts. Using CF8 or CF8M where a part is welded and cannot be solution annealed invites intergranular corrosion. Use CF3 or CF3M instead.
• Substituting the grade without written approval. A foundry that 'upgrades' or 'downgrades' to manage availability changes corrosion performance and can violate the specification or code. Every substitution needs the buyer's written approval.
• Skipping PMI verification. Certificates can be transcribed from a template onto the wrong heat. On-site PMI on finished castings confirms which elements are actually present — chromium, molybdenum and nickel first.
• Confusing cast and wrought grades. CF8 and 304 are not the same specification, and neither are CF8M and 316. Specify the cast grade on the casting drawing, and the wrought grade only on the machining drawing.
• casting defects and quality control for the tests that catch a bad ferrite balance." data-zh="将双相钢当作即插即用的升级。双相钢需要受控的固溶退火和淬火，且不能在约 300°C 以上使用。关于能发现铁素体平衡不良的试验，请参阅我们关于铸件缺陷与质量控制的指南。">Treating duplex as a drop-in upgrade. Duplex needs a controlled solution anneal and quench and cannot be used above roughly 300°C. See our guide to casting defects and quality control for the tests that catch a bad ferrite balance.

Conclusion

CF8, CF8M, CF3, CF3M, CF8C and duplex CD3MN cover the overwhelming majority of stainless steel castings in industrial service, and the logic linking them is consistent: chromium and molybdenum give corrosion resistance, carbon control or niobium stabilization prevents welding-related sensitization, and a ferrite-austenite balance delivers duplex strength and chloride stress-corrosion resistance. Match the grade to the fluid, temperature, chloride level, welding plan, pressure code and any sour-service regulation — then verify that the grade you specified is the grade that was actually poured.