Guide
Is CF8M Really the Same as 316 for Cast Marine Hardware?
CF8M is the cast stainless grade that corresponds to wrought 316, and it is not the same material: castings are bought to a casting specification, carry a chemistry adjusted for pouring, contain some ferrite, and are supplied heat treated. For a cast cleat, hinge or fitting that is machined afterwards, treat “316” and “CF8M” as two different lines on the PO, and say which one the drawing allows.
By Saola Manufacturing engineering team · Published · Updated
This guide is for buyers and engineers who hold a 316 machined-part drawing and have been quoted a cast CF8M blank, or the reverse. It covers what the two names mean, what changes in the part when the blank is cast, why a magnet behaves differently, and the wording to put on the RFQ. The ASTM specifications named here are sold documents, so only their public scopes are used; limits and grade tables must come from your own copy.
On this page
Is CF8M the same material as 316?
No. CF8M is the casting equivalent of 316, meaning the same alloy family and the same corrosion philosophy, but a different product made to a different specification with a different chemistry window and a different microstructure.
HG Houston, a stainless casting supplier, describes CF-8M as “the casting equivalent of 316 stainless steel” and says its chemistry has been adjusted, particularly silicon, to improve casting properties such as fluidity (H&G Houston technical FAQ). The same page ties its stronger magnetic response to a higher ferrite content than 316 and says that ferrite should not adversely affect corrosion resistance. That is the whole relationship in two sentences: same family, adjusted chemistry, some ferrite, no corrosion penalty claimed.
The Australian Stainless Steel Development Association makes the general point that castings have subtly different compositions than their “equivalent” wrought alloys, and that cast austenitic alloys typically contain a few percent ferrite (ASSDA, magnetic effects of stainless steels). The quotes around “equivalent” are theirs, and they are a good reminder for a drawing: equivalence between a cast and a wrought designation is a statement about service, not an identity of composition.
The low-carbon version follows the same logic. CF3M is usually treated as the cast counterpart of 316L; confirm the pairing and the carbon limit with the foundry and the specification text before relying on it for a welded part.
What changes when the blank is cast instead of cut from bar?
The governing specification, the heat treatment route, the microstructure and the soundness risk all change. The machining stays similar, but the blank arrives with a casting certificate, a different magnetic response and a different place for defects to hide.
ASTM A351/A351M-24e1 covers austenitic steel castings for valves, flanges, fittings and other pressure-containing parts, and its abstract requires that all castings receive heat treatment followed by a water quench or other rapid cool. ASTM A276/A276M-25 covers hot- and cold-finished stainless bars, supplied in conditions such as annealed. The two documents are not interchangeable even when the grade names look alike. Neither public page lists the CF8M or 316 chemistry tables, so take those from your copy.
The HG Houston FAQ refers to ASTM A743 for its stainless castings, so that is the second casting specification a customer may name. A351 is aimed at pressure-containing parts, which makes it the natural citation for a seacock body; for a cleat or a hinge a customer may call A743 instead. Ask which one their drawing or their end-product standard calls, and do not substitute one for the other without approval.
Soundness is the other difference. A bar is wrought and dense through its section. A casting can carry shrinkage porosity or inclusions that only show when machining opens a surface, which is why a machined cast part needs a defined acceptance rule. The guides on shrinkage defects and casting crack disposition cover how to write one.
| Item | 316 bar machined from stock | CF8M cast blank, then machined |
|---|---|---|
| Governing specification | ASTM A276/A276M for stainless bars and shapes | ASTM A351/A351M for pressure-containing austenitic castings, or A743 where the customer names it |
| Chemistry window | Wrought composition range | Casting composition, with silicon adjusted for fluidity, so not identical to wrought 316 |
| Heat treatment | Supplied in a stated condition such as annealed (Condition A) | All castings heat treated and rapidly cooled, so state solution annealed on the PO |
| Ferrite | Annealed wrought 316 is generally non-magnetic | A few percent ferrite is typical, so a weak magnetic response is expected |
| Soundness risk | Dense wrought section | Shrinkage porosity or inclusions can appear when machining opens a surface |
| Certificate | Mill certificate against the heat | Foundry certificate against the heat and heat treatment lot; EN 10204 applies to castings as well as bars |
Why does a magnet stick to cast 316-type hardware?
Cast austenitic stainless typically contains a few percent ferrite, and ferrite is magnetic, so a weak pull on a CF8M part is expected. It does not by itself show a wrong grade or a corrosion problem.
ASSDA explains that cast austenitic alloys typically contain a few percent ferrite, which makes them weakly magnetic, while annealed wrought austenitic grades are generally non-magnetic (ASSDA FAQ). The same page notes that weld metal in austenitic steel can contain a deliberate low ferrite content to prevent hot cracking, and that poor heat treatment or high heat input welding can cause sensitization that forms magnetic martensite around chromium carbides. So magnetism is a clue, not a verdict.
For incoming inspection this means a magnet is the wrong acceptance test for a cast blank on a 316 drawing, and a good screen for the opposite mistake. If the question is the alloy, ask for positive material identification and see the PMI guide for what a handheld analyzer can and cannot show; it identifies alloying elements, not ferrite content or corrosion performance.
If the customer cares about low magnetic response for a functional reason, such as a sensor or compass nearby, that is a separate requirement. Write a permeability limit on the drawing and expect a cast part to need a conversation, because a casting grade cannot be held to the response of annealed bar by default.
What should the PO or RFQ line say for a cast stainless blank?
Name the casting specification and grade, the heat treatment, the certificate type and what the machine shop may and may not change. Something like: “cast to ASTM A351 grade CF8M (or A743 if the customer calls it), solution annealed, EN 10204 type 3.1 certificate”.
The other half of the line is what the drawing replaces. A drawing that says “316” with an A276 bar note has a different meaning from one that says “316 or CF8M”. If either is acceptable, say so, because the supplier will otherwise quote the cheaper route and you will find out at incoming inspection. If only bar is acceptable, say that too: some buyers need wrought material for a specific reason, such as a structural weld or a customer-approved source list.
On certificates, the BSI summary of EN 10204:2004 states that the standard applies to all metallic products, including plates, sheets, bars, forgings and castings. That is why a 3.1 certificate can be asked of a casting. The practical difference is that the document traces a pour and a heat treatment lot, not a bar. See the 3.1 vs 2.2 guide for what each type attests.
Add the finishing steps that follow machining: passivation after machining removes iron smeared from tooling and is placed with a partner plant that Saola manages, with the production site named on the quote. HG Houston notes that where rust on a casting comes from embedded iron contamination, passivating may remove it; if it comes from chloride pitting in salt water, the FAQ treats it as a question of whether the rust is only cosmetic or affects structural integrity before any change of material.
Can a cast CF8M blank replace bar on an existing 316 drawing?
Only through an engineering change the drawing owner approves. The corrosion families match, but the specification, the soundness risk, the magnetic response and the dimensional starting point all differ, so it is a change of material route, not a substitution of equals.
Start with what does not change. BSSA describes 316 as vulnerable to localized crevice and pitting corrosion in seawater and says stagnant conditions must be avoided (BSSA, seawater selection note). Switching to CF8M does not fix a crevice or a stagnant pocket, and it does not make the part worse for being cast; location and design decide marine life more than the 316-or-CF8M name does.
Then list what does change: the foundry must be named on the control plan, the machining allowance must cover casting variation, datums must be set on features the casting can hold (see the guide on datums on cast parts), and the acceptance rule for porosity must be written down. The casting vs machining drawing guide covers how to show the as-cast and finished states on one sheet.
There is also the naming problem. A Wikipedia note on marine grade stainless records that there is no industry-recognized definition of “marine grade”, and calls 316 the preferred steel for marine environments while stating it is not corrosion-proof (Wikipedia, marine grade stainless). So “marine grade 316” on a drawing is not a specification; grade, form and standard are.
How should the RFQ split the casting and the machining?
As two lines: who supplies the cast blank, from which foundry and to which specification, and who machines it. The quote should state the production site and country of origin for each step.
A cast-then-machined fitting has two production sites, and the origin documentation should cover both. The machining side is CNC milling for bores, faces and flange holes on the blank, with CNC turning where the part is round. The casting and machining country of origin guide explains why the origin of a cast-then-machined part needs a sentence of its own.
For an annual program of the size this site serves, the useful RFQ questions are repeatability ones: how the casting is held for the first machining operation, how porosity found in machining is dispositioned, and which certificate comes with each lot. Send the 316 drawing, say whether CF8M is acceptable, and mark the surfaces that are sealing or cosmetic.
Frequently asked
Is CF8M the same as 316L?
No. CF8M corresponds to 316, and the low-carbon cast grade usually paired with 316L is CF3M. Confirm the pairing and the carbon limit in the specification text for a welded part.
Is CF8M magnetic?
Often weakly, because cast austenitic stainless typically contains a few percent ferrite. HG Houston says the higher ferrite should not adversely affect corrosion resistance.
Does CF8M need heat treatment?
Yes. ASTM A351 requires castings to be heat treated and then quenched in water or cooled rapidly by other means, so write the condition on the PO.
Can I get a 3.1 certificate on a casting?
Yes. EN 10204 applies to all metallic products including castings; the certificate traces the pour and heat treatment lot rather than a bar.
Will passivation fix rust on a cast fitting?
Only if the rust comes from embedded iron contamination. If it comes from chloride pitting in salt water, judge first whether it is cosmetic or structural, then review the design and alloy.
Sources
- Stainless castings in salt spray environment, technical FAQ, H&G Houston (undated; accessed October 2026)
- Magnetic effects of stainless steels, technical FAQ, Australian Stainless Steel Development Association (undated; page copyright 2023; accessed October 2026)
- ASTM A351/A351M-24e1, Castings, Austenitic, for Pressure-Containing Parts, ASTM International store page, 2024 edition (updated 2025)
- ASTM A276/A276M-25, Stainless Steel Bars and Shapes, ASTM International store page, 2025 edition
- Selection of 316, 304 and 303 types of stainless steels for seawater applications, British Stainless Steel Association, accessed October 2026 (page copyright 2026)
- Metallic products - Types of inspection documents (EN 10204), BSI Knowledge, BS EN 10204:2004
- Marine grade stainless, Wikipedia, accessed October 2026