CF8 vs CF8M: SS304 and SS316 Cast Valve Grades Compared

Technical guide · Supreme Valves India · Updated 2026-10-24

"SS304 or SS316?" is one of the most common material questions on valve enquiries. For cast valve bodies the answer is a choice between ASTM A351 grades CF8 and CF8M, or their low-carbon versions CF3 and CF3M. The difference comes down to molybdenum, and it matters most where chlorides are present.

Grade names and equivalents

In the grade name, C means corrosion-resistant service, F indicates the iron-chromium-nickel family (the "18-8" type), the number is the maximum carbon in hundredths of a percent, and M means molybdenum is added.

Why molybdenum matters

Molybdenum improves resistance to pitting and crevice corrosion from chlorides, and to some reducing acids. A common way to compare pitting resistance is the pitting resistance equivalent number (PREN = %Cr + 3.3 × %Mo + 16 × %N). Type 304 sits at about 18 to 20; type 316 at about 23 to 26. That gap is why CF8M is the default for:

• Brackish water, cooling water with chlorides, and coastal or marine atmospheres.
• Many chemical and pharmaceutical services, including dilute acids where 304 is marginal.
• Food and beverage plants using chloride-containing cleaning solutions.

CF8 is adequate for clean water, steam condensate, air, nitrogen, many organic chemicals, nitric acid service and general atmospheric use, at a lower cost.

Where neither is enough

• Seawater: both grades suffer crevice corrosion in stagnant or warm seawater. Use nickel aluminium bronze, duplex or super duplex.
• Hot chlorides above about 60 °C: austenitic stainless steels are prone to chloride stress corrosion cracking. Duplex grades resist it much better.
• Concentrated sulfuric or hydrochloric acid: consider Alloy 20 (CN7M), Hastelloy or lined valves.

Low carbon grades: CF3 and CF3M

When austenitic stainless steel is held between roughly 425 and 815 °C, for example in a weld heat-affected zone, chromium carbides can form at grain boundaries. The chromium-depleted zones next to them then corrode preferentially, which is called sensitisation or intergranular corrosion. Castings are solution annealed after casting, so the risk comes mainly from field welding (butt weld and socket weld valves) and long service in that temperature range. CF3 and CF3M limit carbon to 0.03 percent to avoid this. Specify them for weld-end valves in corrosive service unless the valve will be solution annealed after welding.

Ferrite and magnetism

Cast CF8 and CF8M usually contain some delta ferrite, which improves strength and reduces hot cracking in the casting. Ferrite is magnetic, so stainless valve bodies may attract a magnet weakly. That is normal. For cryogenic or specific corrosion services, project specifications sometimes limit ferrite content; state the limit in the enquiry if one applies.

Pressure ratings

ASME B16.34 places CF3 and CF8 in material group 2.1 and CF3M and CF8M in group 2.2. Ratings at ambient temperature are close; at elevated temperatures CF8M generally holds its rating slightly better. The low-carbon grades have somewhat lower allowable stresses at temperature. Check the B16.34 table for the actual design temperature.

Trim and other parts

A CF8M body doesn't automatically mean every wetted part is 316. Stems may be 316, 410, 17-4PH or another grade depending on design, and seats may be PTFE, RPTFE or hard-faced metal. If all wetted parts must be 316, say so in the enquiry.

Quick selection guide

• Clean water, air, steam condensate, nitrogen, general utilities: CF8.
• Chlorides present, coastal sites, chemical, pharma and food plants: CF8M.
• Butt weld or socket weld ends in corrosive service: CF3M (or CF3).
• Seawater, hot chlorides, sour or high-chloride produced water: duplex, super duplex or NAB.

Send the medium, concentration, temperature and chloride content with your enquiry and we will confirm the grade.

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