ASTM F593
Group 2
AISI 316 / 316L Bolts
Why Molybdenum Matters Here
Group 2 builds directly on Group 1’s 18-8 austenitic base — chromium and nickel remain the foundation, but Group 2 deliberately adds roughly 2–3% molybdenum, a specific choice made to solve a corrosion mechanism that chromium’s passive oxide film alone doesn’t fully resist: localized pitting and crevice corrosion driven by chloride ions.
Chlorides — present in seawater, de-icing salts, and many industrial chemical streams — can locally break down the chromium oxide passive film at microscopic weak points, initiating small pits that, in crevices or stagnant areas like under washers or within thread engagement, can progress into meaningful metal loss even while the surrounding surface remains passively protected. Molybdenum genuinely strengthens the passive film’s resistance to this specific chloride-driven breakdown, measurably raising the chloride concentration a fastener can tolerate before pitting initiates.
That’s precisely why Group 2 (316/316L) is the standard default for marine hardware, coastal construction, and any chemical process involving chloride-bearing streams — Group 1 (304/304L) remains entirely adequate for general atmospheric and mild chemical service, but chloride exposure is the specific trigger that justifies stepping up to Group 2’s added cost. The 316L variant adds the same low-carbon, weld-decay protection that 304L provides within Group 1, for weld-adjacent areas in this same chloride-resistant base.
Akbar Fasteners Pvt. Ltd. exports to over 50 countries, and our ASTM F593 Group 2 fastener range is manufactured and verified by chemical composition and mechanical testing under our ISO 9001:2015 quality system.
A Note on Standard Scope
ASTM F593 covers externally threaded stainless fasteners including bolts, hex cap screws, and studs. If your requirement is for matching nuts, ASTM F594 is the companion nut specification typically paired with F593 — please let us know if you’d like both quoted together for a matched Group 2 assembly.
Where This Material Genuinely Applies
Genuine Chloride & Pitting Resistance
Molybdenum content measurably raises the chloride concentration a fastener can tolerate.
Correct Default for Marine & Coastal Service
The standard grade specified wherever chloride exposure is a genuine operating condition.
316L Weld-Decay Protection
Low carbon content genuinely reduces sensitization risk near welded assemblies, same as 304L.
Verified Mechanical Properties
Batch testing confirms genuine alloy content and mechanical properties for every lot.
Group 2 (316) vs Group 1 (304) — Quick Reference
Confirming whether chloride exposure is a genuine operating condition before paying for the added molybdenum. This table is the quick reference we use with buyers before quoting.
| Aspect | Group 2 (316/316L) | Group 1 (304/304L) |
|---|---|---|
| Molybdenum Content | ~2 – 3% | None |
| Chloride/Pitting Resistance | Genuinely superior | Good, general exposure only |
| Cost | Higher, for the added resistance | More economical |
| Best Suited For | Marine, coastal, chloride-heavy chemical service | General atmospheric, food, mild chemical |
Group 2 (316/316L) Specification Details
Nominal Composition
- Chromium 16.0 – 18.0%
- Nickel 10.0 – 14.0%
- Molybdenum 2.0 – 3.0%
- Carbon (316L) ≤0.03%
Typical Mechanical Range
- Condition Cold Worked / Annealed
- Chloride/Pitting Resistance Genuinely Superior to Group 1
- Weld-Decay Resistance 316L Enhanced
- Fastener Standard ASTM F593 Group 2
Fastener Types We Manufacture
- Hex Bolts & Cap Screws Standard & Custom Sizes
- Studs Standard & Custom Sizes
- Matching Nuts ASTM F594, On Request
- Washers Matching Material
Application Areas
Wherever chloride exposure — marine, coastal, or chemical — genuinely drives the material selection beyond Group 1’s capability.
Marine & Offshore Hardware
spray exposure fastening
Coastal Architectural & Structural
infrastructure fastening
Chemical & Pharmaceutical Processing
stream equipment fastening
Desalination & Water Treatment
water equipment fastening
Standard Export Documentation
Prepared as standard practice for every ASTM F593 Group 2 fastener shipment.
Mill Test Certificate (MTC)
Heat number, chemical composition, and mechanical property verification for every lot.
Chemical Composition Report
Confirms actual chromium, nickel, molybdenum, and carbon content fall within the specified 316/316L range.
Certificate of Conformance
Confirms the shipment matches the purchase order grade/size specification in full.
Certificate of Origin (COO)
Confirms India as country of manufacture for destination customs purposes.
NABL Accredited Test Reports
Independent mechanical and dimensional testing available on request.
Commercial Invoice & Packing List
Itemized value, Incoterm, and detailed carton/weight breakdown for customs clearance.
Internationally Recognised Standards Coverage
Related Fastener Grades & Materials
ASTM F593 Group 1 (304/304L)
Our general-purpose grade, sufficient where chloride exposure is not a driving concern.
View Group 1 →ASTM A194 Grade 8M Nuts
Our 316-equivalent stainless nut grade, the common counterpart to Group 2 bolting.
View Grade 8M Nuts →UNS C71500 Cupro-Nickel 70/30
Our copper-nickel alloy, an alternative non-ferrous option for high-velocity seawater erosion service.
View UNS C71500 →Stainless Steel Fasteners Manufacturer India
Our broader fastener manufacturing capability across all major carbon, alloy, stainless, and non-ferrous grades.
View SS Manufacturer Guide →Marine, Coastal,
or Chloride Exposure?
Tell us your application and chloride exposure level — we’ll confirm whether 316 or 316L fits, and quote with full documentation, matched nuts included on request. Akbar Fasteners Pvt. Ltd. responds within 24 hours.
Sales: +91 724 0000 733 · Support: +91 800 9315 333 · sales@akbarfasteners.in