Engineering Guide · 7 min read
304 Stainless Steel Fasteners: When to Use Them and What to Specify
304 suits indoor equipment and mild outdoor service. Salt deposits, cleaning chemicals, strength and thread galling determine whether it fits the joint.
The short answer: 304 stainless steel suits indoor equipment and many mild outdoor environments. A correctly identified 304 bolt can still be the wrong purchase if the joint traps salt water, needs more strength, or seizes during tightening.[1][2]
What the name “304” tells you
304, or UNS S30400, is an austenitic chromium-nickel stainless steel, often described as nominally 18% chromium and 8% nickel. Chromium supports the protective surface film.[1]
| Description | What it establishes |
|---|---|
| 304 / S30400 | The alloy identity. Finished-fastener strength is specified separately. |
| 18-8 stainless | A commercial description covering several chromium-nickel alloys; it does not identify 304 specifically.[4] |
| A2-70 | An ISO fastener grade and property class. A2 covers a broader chemistry range than 304; 70 identifies strength.[2] |
| ASTM F593-24 Group 1 | An inch fastener specification and alloy group permitting several alloys, including 304.[3] |
When the drawing requires 304, name that alloy and its material specification, with batch-traceable chemical analysis. If any compliant A2 alloy is acceptable, make that clear.
304L for welded parts
304L’s lower carbon content reduces welding-related carbide precipitation and the risk of intergranular corrosion. In ordinary unwelded service, its corrosion resistance is similar to 304’s.[1][5]
Welding can change the strength gained through cold work. For welded studs, agree on the welding procedure and final mechanical properties.[5][8]
Where 304 fits—and when to reconsider it
304 is widely used in food and beverage equipment as well as general indoor assemblies.[1][5]
A sheltered bracket can expose a bolt to harsher conditions than the panel beside it. Rain may wash the panel while salt and dirt remain under the bracket. Chloride deposits, time spent wet and limited natural washing all affect corrosion.[6]
| At the joint | Purchasing decision |
|---|---|
| Clean indoor service; little salt; practical drainage and cleaning | 304 is a reasonable starting choice. |
| Sea spray, road salt or persistent deposits | Compare 316 or a more resistant alloy for the exposure and joint geometry. |
| Hot washdown, bleach or chloride-bearing process liquids | Provide concentration, temperature, contact time and rinsing details before selecting the alloy. |
| Replacement of a high-strength steel bolt, or sustained hot service | Check the mechanical requirements and service-temperature data. |
Concentrated or hot bleach can attack stainless steel. For washdown equipment, follow the cleaner manufacturer’s concentration, contact-time and rinsing instructions.[7]
Why a 304 bolt can show rust
Iron left on the surface
Carbon-steel dust, shared tooling and contaminated handling equipment can leave iron on stainless steel. That iron can rust even when the alloy is correct. Prevent cross-contamination during storage and fabrication; test a suspect surface before choosing a cleaning treatment.[10]
Corrosion of the stainless itself
Water trapped under a washer, bolt head or gasket can lose oxygen while chlorides concentrate and acidity increases. The protective film may break down inside that gap.[9]
For a returned batch, record where the marks occur and whether cleaning reveals pits or damaged threads. Investigate metal loss before reusing a load-bearing fastener. Changing suppliers will not resolve a joint that repeatedly traps the same corrosive liquid.
If chemical passivation is required, specify the treatment and agreed verification test. ASTM A967/A967M-25 provides treatment options and tests of their effectiveness, particularly for removing contaminant iron.[11][12]
Material and finished-fastener strength
Austenitic stainless gains strength through cold work, so an annealed sheet or bar datasheet does not describe a finished bolt’s strength.[4][5]
For an A2-70 metric fastener with full tensile loadability under ISO 3506-1:2020, the ambient-temperature minima are 700 MPa tensile strength and 450 MPa stress at 0.2% permanent elongation. These concern fracture and permanent deformation, respectively; they are not allowable joint loads. Some head designs have reduced loadability.[2][8]
Replacing an 8.8 or 10.9 steel bolt with 304 requires approval against the original strength and preload requirements.[8]
ASTM F593-24 covers specified inch bolts, hex cap screws and studs. An inch catalog part labeled “A2-70” still needs an agreed mechanical specification.[3][8]
A magnet cannot confirm 304
Cold forming can transform some austenite into magnetic martensite, so a genuine 304 fastener may attract a magnet. The response varies with composition and processing; other alloys may show little attraction.[13][14]
Match the delivered lot to its chemical analysis and finished-fastener mechanical reports. Where alloy mix-ups matter, agree on a positive material identification (PMI) method and sampling plan.[14]
Handheld X-ray fluorescence (XRF) can help distinguish 304-type from molybdenum-bearing 316-type material, but cannot measure carbon. Testing a low-carbon L grade requires a validated carbon-capable method.[14][15]
When stainless threads seize
A 304 nut and bolt can gall: sliding disrupts the protective films, and exposed metal transfers between loaded thread surfaces until they seize. Torque can rise before the joint develops its intended preload.[16]
Clean, undamaged threads, suitable lubrication and controlled tightening reduce galling. Anti-seize lowers friction, so using a dry-thread torque can produce excessive preload. Use data for the supplied lubricant and fastener pair.[16]
Pairing 304 with 316 alone is an unreliable cure because their hardness can overlap. The thread galling guide covers the assembly controls.[16]
A 304 fastener RFQ example
“500 hexagon head screws, ISO 4017:2022, M10 × 1.5 × 40, fully threaded. Source wire: Type 304 / UNS S30400 to ASTM A493-25. Finished screws: A2-70 to ISO 3506-1:2020. Clean indoor equipment, ambient service. Provide batch-traceable chemical and finished-fastener mechanical reports. Quote compatible nuts with their dimensional and mechanical specifications, and propose lubrication with matching tightening guidance.”
This example separates the screw dimensions, source wire and finished mechanical requirements. Add the actual exposure, passivation or welding requirements for the application.[8][17][18]
Explore the stainless steel guide series
Continue with the grade and assembly guides:
- What Is Austenitic Stainless Steel? A Fastener Buyer's Guide — material structure, cold work and grade selection.
- 316 Stainless Steel Fasteners: Applications, Grades and Service Limits — chloride exposure, 316L and finished strength.
- 310S Stainless Steel Fasteners: What to Specify for High-Temperature Service — oxidation, creep and hot-joint conditions.
- 304 vs 316 Stainless Steel Fasteners: What the Data Says — when the exposure justifies choosing 316.
- A2-70 and A4-80: How to Read Stainless Fastener Markings — alloy groups and mechanical property classes.
- Why Stainless Threads Seize: Galling and How to Prevent It — mating threads, lubrication and tightening.
References
- Outokumpu. Core stainless steel range and Core range datasheet. 304/4301 identity, intended environments, and the role of low carbon in 304L.
- British Stainless Steel Association. Stainless steel fasteners. ISO 3506:2020 grade chemistry, property classes and mechanical requirements.
- ASTM International. ASTM F593-24 — Stainless Steel Bolts, Hex Cap Screws, and Studs. Public scope, permitted Group 1 alloys and inch fastener coverage.
- American Iron and Steel Institute / Nickel Institute. Stainless Steel Fasteners: A Systematic Approach to Their Selection, No. 9003. 18-8 terminology and effects of manufacturing on stainless fastener properties. Historical handbook.
- Atlas Steels. Stainless Steel 304, 304L, 304H Grade Data Sheet, revised April 23, 2021. Alloy behavior, applications, welding and oxidation resistance.
- Outokumpu. Stainless steel and atmospheric corrosion: what you need to know. Chloride deposition, wetness, shelter and cleaning.
- worldstainless / International Stainless Steel Forum. Commercial Food Equipment: The Ferritic Solution, p. 12. Cleaning precautions applicable to ferritic and nickel-containing stainless grades.
- ISO. ISO 3506-1:2020 — Bolts, screws and studs with specified grades and property classes. Metric scope, ambient-temperature tests and reduced loadability; confirmed in 2025.
- Australian Stainless Steel Development Association. Crevices and Corrosion. Fastener crevices, stagnant conditions and selection implications.
- British Stainless Steel Association. Iron contamination and rust staining on stainless steel. Contamination prevention, investigation and removal.
- ASTM International. ASTM A967/A967M-25 — Chemical Passivation Treatments for Stainless Steel Parts. Treatment options and alternative tests for treatment effectiveness.
- British Stainless Steel Association. Passivation of stainless steels. Natural passivation, cleaning and the distinction from pickling.
- British Stainless Steel Association. Magnetic Properties of Stainless Steel. Composition, cold work and magnetic response.
- Australian Stainless Steel Development Association. Technical FAQ 4: Testing for Grade Confirmation. Traceability, test selection and limitations of magnetic checks.
- Thermo Fisher Scientific. Analysis of Stainless Steel Testing Using the Niton Apollo Handheld LIBS Analyzer. Carbon measurement and the limits of handheld XRF for low-carbon grade verification.
- Australian Stainless Steel Development Association. Technical FAQ 5: Galling and its Control. Thread seizure, lubrication, torque and limitations of a 304/316 pairing.
- ISO. ISO 4017:2022 — Hexagon head screws, product grades A and B. Product specification used in the example RFQ.
- ASTM International. ASTM A493-25 — Stainless Steel Wire and Wire Rods for Cold Heading and Cold Forging. Source-material specification used in the example RFQ.