ASTM F593 covers the chemical and mechanical requirements for stainless steel bolts, hex cap screws, and studs in inch-series diameters from 1/4" through 1-1/2". It's the spec you're working under any time a drawing calls for a stainless bolt in corrosive service — marine, chemical processing, food and pharmaceutical, or exterior architectural. What makes F593 worth understanding rather than just ordering from is that a single alloy can ship at very different strength levels depending on one line item most buyers skip: the condition.
How F593 Is Organized
F593 sorts fasteners two ways at once. First by alloy group, which sets the corrosion behavior and the base chemistry. Then by condition, which sets the strength. Get the group right and you've matched the environment; get the condition right and you've matched the load. Most spec mistakes come from nailing the first and ignoring the second.
Alloy Groups
The spec defines six alloy groups. In practice, the austenitic grades in Groups 1, 2, and 3 are the overwhelming majority of stainless fasteners you'll actually specify — non-magnetic in the annealed condition and carrying the best general corrosion resistance.
Group 1 — Austenitic (304, 305, 384): General-purpose stainless; the baseline for most non-marine applications.
Group 2 — Austenitic (316, 316L): Marine and chloride-resistant; the molybdenum addition is what separates it from Group 1.
Group 3 — Austenitic (321, 347): Stabilized grades for welding and elevated temperature service.
Group 4 — Ferritic (430): Moderate corrosion resistance; magnetic.
Group 5 — Martensitic (410, 416, 431): Higher strength, moderate corrosion resistance; magnetic.
Group 6 — Precipitation-hardened (17-4 PH / 630): High strength combined with corrosion resistance.
If a drawing just says "stainless bolt," it's almost always Group 2 (316) for anything outdoor or near saltwater, and Group 1 (304) for drier environments.
The 304 vs. 316 Point Everyone Gets Backwards
The instinct is that 316 is "more stainless" than 304 because it resists saltwater better. But look at the chemistry and the reason isn't chromium — it's molybdenum.
Group 1, Type 304: Chromium 18.0–20.0%, Nickel 8.0–10.5%, no molybdenum.
Group 2, Type 316: Chromium 16.0–18.0%, Nickel 10.0–14.0%, Molybdenum 2.00–3.00%.
316 actually carries slightly less chromium than 304. What it adds is 2–3% molybdenum, and that moly is what buys the pitting and crevice-corrosion resistance in chlorides. The rule isn't "more chromium is better." 316's molybdenum is the reason it belongs anywhere near seawater, coastal air, pool chemistry, or a washdown line — and why it costs more. Specify 304 in those environments and you'll see pitting inside a year.
The L Designation — 304L and 316L
The "L" suffix indicates low carbon (0.030% max vs. 0.08% standard). Low-carbon grades resist sensitization after welding by preventing chromium carbide precipitation at grain boundaries — which would otherwise strip the base metal of the chromium doing the corrosion work. Specify the L grade when the fastener will be welded in service or will see corrosive environments after welding. Dual-certified material (304/304L, 316/316L) meets both grades and is standard commercial practice.
Conditions — Where the Strength Actually Comes From
This is the section that separates people who order F593 from people who understand it. The alloy sets corrosion resistance; the condition sets strength. The same 316 bar can ship soft or strong depending on how it was worked.
- Condition A (Solution Annealed) — the default softer state. Maximum corrosion resistance, lowest strength. The right call when the application is corrosion-driven and load is minimal.
- Condition CW (Cold Worked) — headed and rolled from annealed stock, picking up cold work in the process. Moderately higher strength, but the numbers are diameter-dependent — they step down above 5/8" because the cold work doesn't penetrate the full cross-section of larger diameters.
- Condition SH (Strain Hardened) — deliberately worked to a higher, controlled strength level. The one to call out when you need real clamp force from a stainless bolt, and the one spec sheets most often leave off.
- Condition H / HT — for the martensitic (Group 5) and precipitation-hardening (Group 6) grades. Hardened and tempered, or heat-treated to a condition target — 17-4 PH ships in H900, H1025, or H1150 depending on the strength and toughness balance required.
The takeaway: "316 stainless" is not a strength callout. If the joint carries load, the condition has to be on the line item.
Mechanical Properties — Groups 1 and 2
The actual minimums for austenitic 304 and 316:
Condition A, all diameters 1/4"–1-1/2": 75 ksi tensile / 30 ksi yield
Condition CW, 1/4"–5/8": 100 ksi tensile / 65 ksi yield
Condition CW, over 5/8"–3/4": 85 ksi tensile / 45 ksi yield
Condition SH, 1/4"–5/8": 120 ksi tensile / 95 ksi yield
Condition SH, over 5/8"–3/4": 110 ksi tensile / 75 ksi yield
Condition SH, over 3/4"–1-1/2": 100 ksi tensile / 60 ksi yield
Two things worth reading off this table. First, annealed Condition A yields at just 30 ksi — fine for corrosion-driven, low-load work, but not structural clamp force. Second, notice the CW step-down at 5/8": tensile drops from 100 to 85 ksi and yield from 65 to 45 ksi the moment you cross that diameter. If you need consistent strength across a range of sizes, SH holds up far better than CW.
How F593 Compares to Other Standards
F593 vs. A193 B8 / B8M — B8 (304) and B8M (316) are the pressure and piping-code cousins to F593 Group 1 and Group 2. Same base alloys, different spec intent. If you're bolting flanges in an ASME-code piping system, the drawing usually calls B8/B8M rather than F593. B8M Class 2 is the strain-hardened, high-strength version — directly analogous to F593 Condition SH.
F593 vs. ISO 3506 — ISO 3506 is the metric stainless fastener standard. Property class A2-70 is roughly 304, and A4-80 is roughly 316 strain-hardened to about 116 ksi tensile. If you're sourcing replacement hardware for European-built equipment, expect the drawing to call A4-80, not F593.
F593 bolts pair with F594 nuts — always match the nut alloy group to the bolt alloy group. Group 2 bolt, Group 2 nut. For exterior and marine service, use stainless washers throughout — mixing carbon-steel washers with stainless bolts in wet environments creates galvanic corrosion that attacks the carbon-steel washer and streaks rust onto the stainless assembly.
Selecting the Right F593 Fastener
- Match the environment first. Chlorides, saltwater, pool chemistry, or washdown → Group 2 (316) for the molybdenum. Drier or indoor corrosive service → Group 1 (304).
- Then match the load. No real clamp demand → Condition A is fine and cheapest. Meaningful clamp force → specify Condition CW, or Condition SH if you need strength that holds across larger diameters.
- Put the condition on the line item. "316 stainless" tells the supplier the alloy, not the strength. Add the condition or you're leaving the mechanical properties to chance.
- Match the nut. F594, same group as the bolt.
A Galling Note That Isn't in the Spec
One field reality F593 won't warn you about: stainless nuts on stainless bolts gall. The threads cold-weld under torque, seize, and the joint is ruined — irreversible and common. Use a nickel-based anti-seize on any stainless assembly you intend to torque. For applications where galling is a chronic problem, Nitronic 60 hardware is inherently galling-resistant and worth specifying in place of standard 316. This trips up crews used to plated carbon steel who don't expect it.
Frequently Asked Questions
Can F593 fasteners be used in marine environments?
Yes — specifically Group 2 (316/316L), whose molybdenum content gives it the pitting resistance that 304 lacks in saltwater and coastal air.
What nuts pair with F593 bolts?
ASTM F594 stainless nuts, matched to the same alloy group as the bolt.
What do the conditions (A, CW, SH) actually mean?
They set strength, not corrosion resistance. A is annealed and softest; CW is cold-worked and stronger in small diameters but steps down above 5/8"; SH is strain-hardened for the highest and most consistent strength across all sizes. Same alloy, very different mechanical properties.
How do I identify the alloy group of a fastener I already have?
It's called out in the order documentation and usually in the head marking. If you can't confirm it and the application matters, a handheld XRF (PMI) reads the alloy composition — though note it won't tell you the condition, only the chemistry.
The Bottom Line
F593 is straightforward once you separate its two jobs: the alloy group handles corrosion, the condition handles strength. Match the group to the environment, put the condition on the line so you actually get the mechanical properties you're counting on, pair with the right F594 nut, and anti-seize anything you plan to torque.
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