CF8M vs 316 and CF3M vs 316L: Cast Stainless Grades Explained
Drawings often say "316 stainless" for a part that will be investment cast. Foundries pour cast grades, so engineers ask which cast grade matches 316 or 316L, why a cast 316 part can attract a magnet, and how to write the callout so the material certificate matches the drawing. A common thread on engineering forums is exactly this mix-up, and it even appears in supplier catalogues. The short version: CF8M is the cast counterpart of 316, and CF3M is the cast counterpart of 316L.
Questions and answers
Is CF8M the same as 316 or 316L?
CF8M corresponds to 316; CF3M corresponds to 316L. The "8" and "3" refer to the maximum carbon content: 0.08% for CF8M (like 316) and 0.03% for CF3M (like 316L). Both contain about 2–3% molybdenum for chloride and seawater resistance.
The grades are close but not identical. Cast grades are balanced with somewhat more chromium and less nickel than wrought 316/316L, which gives them a small amount of ferrite in an austenitic structure. That improves castability and resistance to hot cracking.
Which standards and EN numbers apply?
In ASTM, CF8M and CF3M are covered by A351 (pressure-containing parts), A743 (general corrosion-resistant castings) and A744 (severe service). The usual European equivalents are 1.4408 (GX5CrNiMo19-11-2) for CF8M and 1.4409 (GX2CrNiMo19-11-2) for CF3M.
Write the standard and grade on the drawing, for example "ASTM A351 CF8M" for a valve or pump body, or "ASTM A743 CF3M" for general hardware, and ask for an EN 10204 3.1 certificate showing the chemical analysis of each heat.
Why is my cast 316 part slightly magnetic?
Because of the delta ferrite that cast austenitic grades normally contain. Wrought 316 is close to non-magnetic in the annealed state; CF8M and CF3M castings typically contain a few percent up to around 20% ferrite, so a strong magnet will often stick weakly.
This is normal and does not mean the material is wrong. If low magnetic permeability matters (for example near sensors or in some medical or marine equipment), state a maximum ferrite or permeability on the drawing so the chemistry can be balanced for it before pouring.
Do CF8M and CF3M castings need heat treatment?
Yes, solution annealing is required by the ASTM specifications. Castings are heated to at least about 1040 °C and quenched, which dissolves chromium carbides and restores corrosion resistance. Ask for the heat-treatment record if your industry requires it.
CF8M or CF3M: which should I specify?
Choose CF3M if the part will be welded after casting or used in aggressive chemical service; CF8M is the common, slightly stronger choice otherwise. Low carbon reduces the risk of intergranular corrosion in heat-affected zones. For marine hardware, railing fittings and mirror-polished parts, CF8M is widely used; for parts welded into tanks, pipework or frames, CF3M is the safer default.
Can CF8M and CF3M be mirror polished and passivated?
Yes. Both polish well, and passivation (for example to ASTM A967) or electropolishing is a common final step for marine, food and architectural parts. A good polish depends more on casting quality (porosity, inclusions) than on the choice between CF8M and CF3M.
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