Guide
Does 18-8 stainless steel meet a drawing that calls for 304?
18-8 stainless steel is a trade name for a family of austenitic chromium-nickel stainless steels, named for their nominal 18 chromium and 8 nickel content, that includes 302, 303, 304, 304L and 305 and, in the fastener trade, higher-nickel cold-heading grades such as 384. It is not a grade designation in any ASTM bar specification or in ISO 3506, so a part or certificate marked only “18-8” does not prove that it meets a drawing that calls for 304, and it may in fact be free-machining 303.
By Saola Manufacturing engineering team · Published · Updated
Many pages that compare 18-8 and 304 come from hardware sellers and stop at “304 is a type of 18-8”. That is true, but it does not answer the question a design engineer or a buyer actually has: the drawing says 304, the shipment says 18-8, so do the parts conform? This guide answers that from the documents. It shows which grades sit inside the 18-8 label, compares their chemistry limits with 304 and 303, explains why fastener standards let the maker pick the alloy, and gives a decision rule for accepting or rejecting an “18-8” certificate on machined parts and on hardware.
On this page
What does “18-8” mean on a quote, a fastener box or a certificate?
“18-8” names a composition family, not a specification: it tells you the metal is an austenitic chromium-nickel stainless from the 300 series, but not which grade, which standard or which condition.
The name is a holdover from before the numbered grades were widely used, when the nominal chromium and nickel content was the easiest way to describe austenitic stainless. The family has since been split into numbered grades, and today the requirements live in product standards keyed to a UNS number: 304 is UNS S30400 and, for bar, its chemistry and mechanical properties are set by ASTM A276/A276M. Free-machining 303 (UNS S30300) is a different grade in a different specification, ASTM A582/A582M, with sulfur added on purpose. Both are “18-8”.
In practice the label shows up in three places. On fastener packaging and catalog listings it is the normal way to describe 300-series screws, nuts and washers, because the fastener standards themselves group several alloys together. On quotes for machined parts it often means the supplier intends to use whatever 300-series bar is on the shelf. On a certificate it is a red flag, because a mill test report normally names the grade and the standard the heat was tested against; a document that says only “18-8” is usually a distributor or supplier statement, not a record of a tested heat.
How do the 18-8 grades compare with the 304 and 303 limits?
The 18-8 grades overlap heavily in chromium and nickel, but they differ in carbon, nickel range and, above all, sulfur, so one grade can sit outside another’s limits while carrying the same 18-8 label.
The table below sets the published limits side by side. Read it the way an inspector reads a certificate: a heat conforms to 304 only if every reported element falls inside the 304 column. A 303 heat fails that test on sulfur alone: 303 carries a 0.15 % sulfur minimum per ASTM A582/A582M, while 304 is held to a 0.030 % maximum per ASTM A276/A276M; the 303 phosphorus ceiling is also far looser. A 302 heat can fail on carbon, since 302 may run up to 0.15 % against 0.08 % for 304, both per ASTM A276/A276M. For 305, the nickel range per ASTM A276/A276M is 11.0–13.0 %, which touches the 304 range only at its 11.0 % ceiling, and a chromium minimum lower than 304’s. None of these grades is a bad material; they are simply not 304.
| Element | 302 (S30200) | 303 (S30300) | 304 (S30400) | 304L (S30403) | 305 (S30500) |
|---|---|---|---|---|---|
| Carbon (C), % | 0.15 | 0.15 | 0.08 | 0.030 | 0.12 |
| Manganese (Mn), % | 2.00 | 2.00 | 2.00 | 2.00 | 2.00 |
| Phosphorus (P), % | 0.045 | 0.20 | 0.045 | 0.045 | 0.045 |
| Sulfur (S), % | 0.030 | 0.15 min | 0.030 | 0.030 | 0.030 |
| Silicon (Si), % | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 |
| Chromium (Cr), % | 17.0–19.0 | 17.0–19.0 | 18.0–20.0 | 18.0–20.0 | 17.0–19.0 |
| Nickel (Ni), % | 8.0–10.0 | 8.0–10.0 | 8.0–11.0 | 8.0–12.0 | 11.0–13.0 |
| Meets a 304 callout? | Not as a grade: carbon limit 0.15 vs 0.08 | No: sulfur minimum is above the 304 maximum | Yes | Only if the heat is also reported as 304 | Not as a grade: nickel normally above 11.0, chromium minimum lower |
Two notes on reading the table. First, check the current edition of the specification your drawing cites before you rely on a limit, because ASTM revises these tables and footnotes between editions, and product specifications do not all agree: per ASTM F593 (2024), for example, 304 fastener stock is capped at 10.5 % nickel rather than the 11.0 % in ASTM A276/A276M. Second, 304L is the one 18-8 grade that routinely satisfies a 304 callout, because mills commonly melt a single heat to meet both grades and report it as 304/304L; the certificate must say so explicitly.
Does an 18-8 certificate satisfy a drawing that calls for 304?
No, not on its own. A drawing that calls for 304 is satisfied only by a document that names 304 or UNS S30400, cites the specification on the drawing, ties the result to a heat number and reports chemistry inside the 304 limits.
The reason is simple: the drawing requirement is a specification, and 18-8 is not one. A certificate that says “stainless steel 18-8” cannot be checked against anything, so it cannot demonstrate conformance even when the bar underneath is perfectly good 304. A receiving inspector who accepts it is making an engineering judgment that belongs to the design authority.
What a conforming document looks like is covered in detail in the mill test report guide. For a 304 callout the minimum is: grade 304 or S30400; the bar specification, normally ASTM A276/A276M, or ASTM A479/A479M for boiler and pressure-vessel service; the product form and size; the condition, usually annealed; the heat number; the full heat chemistry; and the mechanical test results the specification requires. A multi-grade certificate listing 304/304L, and sometimes the JIS and EN equivalents as well, is normal and acceptable, provided 304 and the specification on your drawing are among the listed grades. The SUS304 vs 304 guide covers what to do when the bar was made to JIS rather than ASTM.
The same logic runs in the other direction. If the drawing says 303 because the part needs free-machining stock, the certificate must name 303 and ASTM A582/A582M. An “18-8” label is no better evidence of 303 than of 304.
Which grades can hide behind an 18-8 bolt, screw or nut?
Under the common US bolt standard, an austenitic “Group 1” fastener may be any of several 18-8 type alloys at the maker’s choice, so an 18-8 bolt is not guaranteed to be 304 unless the order names 304.
ASTM F593, the specification for stainless bolts, hex cap screws and studs, sorts its alloys into groups rather than single grades. In the 2024 edition, Group 1 lists 304, 305, 384 and 304L along with the cold-heading variants 18-9LW, 302HQ and 304J3, and the standard leaves the choice of alloy within a group to the fastener manufacturer unless the inquiry and order specify one. Free-machining 303 is not one of those base alloys; the specification lets it be furnished, along with 303Se and XM1, only when the purchaser approves. The high-nickel cold-heading grades such as 384 exist because they work-harden less in heading; fastener wire of this kind is covered by ASTM A493. The companion nut specification, ASTM F594, uses a matching alloy-group scheme, and F593 requires nuts of the same group unless otherwise specified. So a box of F593 Group 1 cap screws labeled “18-8” is fully conforming to F593 even if no screw in it is 304.
Metric hardware follows ISO 3506-1, which sorts austenitic fastener steels into lettered groups rather than AISI grades. As the British Stainless Steel Association summarises in its note on A1 to A5 fastener grades, A2 is where 304-type steels sit, A4 is the molybdenum-bearing 316 type, and A1 is the free-machining group with 0.15–0.35 % sulfur, where 303 belongs. The A2 composition window is wide, with nickel from 8 to 19 % per ISO 3506-1 (2020), so a head marking such as A2-70 fixes a chemistry band and a strength class, but it no more pins down 304 than an “18-8” label does.
For most fasteners this flexibility is harmless, which is why the standards allow it. It matters when the drawing or a customer specification names 304 for a reason: a downstream weld, a food-contact or medical requirement written against a grade, or a corrosion environment where the sulfur-bearing 303 type would pit. In those cases the purchase order must name the alloy, for example “ASTM F593, Group 1, alloy 304 only, no substitution”, and require a certificate that reports it.
When should you reject an “18-8” certificate on machined parts or fasteners?
Reject or hold the lot when the drawing names a grade and the document names only a family, when the chemistry shows a free-machining sulfur level against a 304 callout, or when nothing ties the document to a heat number.
The decision table below is a rule a receiving inspector can apply without making an engineering call. A hold is not a rejection of the material; it is a request for the document that proves conformance, or for a written deviation from the design authority. The certificate of conformance guide explains how a supplier’s own statement relates to the mill’s record.
| What the document shows | Machined part, drawing says 304 | Fastener, order says ASTM F593 Group 1 only | Fastener, order says alloy 304 |
|---|---|---|---|
| “18-8” only, no grade, no heat number | Hold: request mill test report | Accept if F593 marking and test report are present | Hold: request alloy and heat |
| Grade 304 or 304/304L, specification and heat listed | Accept after chemistry check | Accept | Accept |
| Grade 303 or sulfur at or above the 303 minimum | Reject unless a written deviation exists | Reject unless purchaser approved 303 | Reject |
| Grade 302 or 305 | Reject unless a written deviation exists | 305: accept (a Group 1 alloy). 302: hold, it is not a listed Group 1 alloy | Reject |
| 200-series grade (manganese replacing nickel) | Reject | Reject | Reject |
Two traps deserve a mention. The first is the magnet test. The British Stainless Steel Association’s note on magnetic properties of austenitic stainless steels gives a relative permeability of roughly 1.003 to 1.05 for fully annealed austenitic grades, and ties any higher reading to ferrite or martensite whose amount depends on chemistry, cold work and heat treatment. Every 300-series grade discussed here is austenitic, and cold-headed or thread-rolled hardware is cold worked by definition, so a hand magnet measures processing history as much as alloy. It gives no reliable way to tell 303 from 304. Only a chemistry result does that, from the certificate or from positive material identification on a sample. The second is the passivation bath. Because of its sulfide inclusions, free-machining 303 is usually passivated with a different bath or pre-treatment than 304, and parts that turn out to be 303 can come out stained or etched from a process chosen for 304; the passivation guide covers the choice.
How should a drawing and RFQ word the callout so “18-8” cannot be substituted?
Name the grade by UNS number and specification, state whether free-machining grades are acceptable, and name the document you will accept as proof.
A material note that leaves no room for an 18-8 substitution reads like this: “MATERIAL: STAINLESS STEEL UNS S30400 PER ASTM A276/A276M, ANNEALED. 304/304L DUAL GRADE ACCEPTABLE. NO FREE-MACHINING GRADES. MILL TEST REPORT TRACEABLE BY HEAT NUMBER REQUIRED.” If the part is a good candidate for 303, say so on purpose instead: “UNS S30300 PER ASTM A582/A582M OR UNS S30400 PER ASTM A276/A276M”, and add the passivation specification that suits both. The 303 vs 304 guide covers when the free-machining grade is the better choice for a turned or milled part. For purchased hardware, put the alloy on the purchase order line, not only on the assembly drawing, because the fastener standard otherwise lets the maker choose.
When we review a drawing for CNC turning or CNC milling, the material note is one of the first things we check against the bar certificate, and a quote that would ship “18-8” against a 304 callout is a question we ask before ordering stock, not after. Send the drawing with the material note written as above, and the reply will say plainly whether the material and the paperwork you ask for are a fit. The RFQ package guide lists the rest of what to include, and when the drawing is ready you can send it for a quote.
Frequently asked
Is 18-8 stainless the same as 304?
304 is one member of the 18-8 family, and the most common one, but 18-8 also covers 302, 303, 304L, 305 and other alloys. So the label tells you the family; only the grade name and the certificate tell you whether it is 304.
Is 18-8 stainless the same as A2 stainless?
Close, but not exactly. A2 is the ISO 3506 steel group for general-purpose austenitic fasteners, where 304-type steels belong; free-machining 303-type steel is grouped as A1 in that system. A US “18-8” label does not say which of the two you have.
Is 18-8 stainless as corrosion resistant as 316?
No. 316 adds molybdenum, which improves resistance to chlorides and pitting, and it sits in a separate group (ASTM F593 Group 2, ISO 3506 A4). The 304 vs 316 guide covers when the upgrade is worth specifying.
Why is my 18-8 screw slightly magnetic?
Heading and thread rolling cold work the steel, and cold work can turn some of the austenite into martensite, which responds to a magnet. A slight pull from a cold-formed screw is expected for 300-series hardware, so on its own it tells you nothing about which grade the screw is.
Can 18-8 fasteners be used in a 304 assembly?
Usually yes, from a corrosion point of view, because the alloys are close enough that galvanic effects are negligible. The exceptions are wet chloride service, where a 303 fastener may pit before the 304 part because free-machining grades have lower pitting and crevice resistance (see the BSSA note on free-machining grades), and any specification that names the fastener grade, which then has to be met as written.
Sources
- ASTM A276/A276M Standard Specification for Stainless Steel Bars and Shapes, ASTM International, 2025
- ASTM A582/A582M Standard Specification for Free-Machining Stainless Steel Bars, ASTM International, 2022
- ASTM F593 Standard Specification for Stainless Steel Bolts, Hex Cap Screws, and Studs, ASTM International, 2024
- ASTM A493 Standard Specification for Stainless Steel Wire and Wire Rods for Cold Heading and Cold Forging, ASTM International, 2025
- ISO 3506-1:2020 Fasteners: Mechanical properties of corrosion-resistant stainless steel fasteners, Part 1: Bolts, screws and studs, ISO, 2020 (BSI Knowledge listing)
- Stainless steel fasteners to BS EN ISO 3506, grades A1, A2, A3, A4 and A5, British Stainless Steel Association, accessed 2026
- Magnetic properties of austenitic stainless steels, British Stainless Steel Association, accessed 2026
- Free machining stainless steel grades, British Stainless Steel Association, accessed 2026