Scope
ASTM F593 covers stainless steel bolts, hex cap screws, and studs in inch-series diameters from 1/4" through 1-1/2". The spec groups fasteners by alloy family, then by condition (solution annealed, cold worked, or strain hardened) to define strength levels.
Alloy groups
| Group | Alloy family | Common AISI/UNS | Typical use |
|---|---|---|---|
| Group 1 | Austenitic | 304, 305, 384 | General-purpose stainless; the baseline |
| Group 2 | Austenitic | 316, 316L | Marine and chloride-resistant |
| Group 3 | Austenitic | 321, 347 | Stabilized for welding and elevated-temp |
| Group 4 | Ferritic | 430 | Moderate corrosion; magnetic |
| Group 5 | Martensitic | 410, 416, 431 | Higher strength; moderate corrosion; magnetic |
| Group 6 | Precipitation-hardened | 17-4 PH | High strength + corrosion resistance |
Groups 1, 2, and 3 are the austenitic grades — non-magnetic in the annealed condition, excellent corrosion resistance, the dominant commercial stainless fasteners.
Conditions
Strength comes from the condition of the metal at manufacture:
- Condition A (Solution Annealed) — the default softer state; maximum corrosion resistance, lowest strength
- Condition CW (Cold Worked) — cold-finished headed fasteners; moderately higher strength
- Condition SH (Strain Hardened) — worked to higher strength through cold deformation
- Condition H (Hardened) — martensitic grades; Q&T to a hardness target
- Condition HT (Heat Treated) — precipitation-hardened grades like 17-4 PH (conditions H900, H1025, H1150)
Mechanical properties — common groups
Group 1 / Group 2 Condition A (Solution Annealed)
| Diameter | Min. Tensile | Min. Yield |
|---|---|---|
| 1/4" through 1-1/2" | 75 ksi | 30 ksi |
Group 1 / Group 2 Condition CW
| Diameter | Min. Tensile | Min. Yield |
|---|---|---|
| 1/4" through 5/8" | 100 ksi | 65 ksi |
| Over 5/8" through 3/4" | 85 ksi | 45 ksi |
Group 1 / Group 2 Condition SH
| Diameter | Min. Tensile | Min. Yield |
|---|---|---|
| 1/4" through 5/8" | 120 ksi | 95 ksi |
| Over 5/8" through 3/4" | 110 ksi | 75 ksi |
| Over 3/4" through 1-1/2" | 100 ksi | 60 ksi |
Group 1 vs Group 2 — the practical question
This is the decision 90% of stainless specifiers are actually making:
- Group 1 (304/304L) — the standard austenitic. Excellent corrosion resistance in most environments. Not recommended for chloride-heavy service (seawater, coastal air, pool chemistry, many food-processing washdowns).
- Group 2 (316/316L) — adds 2–3% molybdenum. The molybdenum addition dramatically improves resistance to chloride pitting and crevice corrosion. Specify Group 2 for: - Marine and coastal applications - Pool, spa, and water-treatment equipment - Food and pharmaceutical washdown - Chemical process service - Anywhere exterior and subject to salt spray or road salt
Group 2 costs roughly 20–30% more than Group 1. For corrosion-critical applications, it is almost always the right call.
The L designation (304L, 316L)
The "L" suffix indicates low carbon (0.030% max vs 0.08% standard). Low-carbon grades resist sensitization and intergranular corrosion after welding by preventing chromium carbide precipitation at grain boundaries. Specify the L grade when:
- The fastener will be welded in service
- The fastener will see service in corrosive environments after welding
- A customer spec explicitly calls for low-carbon
Dual-certified material (304/304L, 316/316L) meets both grades and is common commercial practice.
Recommended nuts and washers
F593 bolts pair with F594 nuts in matching alloy group:
| F593 bolt | F594 nut | Washer |
|---|---|---|
| Group 1 | Group 1 | Stainless matching group or F844 plain |
| Group 2 | Group 2 | Stainless Group 2 preferred; plain acceptable when not immersed |
| Group 6 (17-4 PH) | Group 6 | Stainless matching group |
For exterior and marine service, use stainless washers throughout. Mixing carbon-steel washers with stainless bolts in wet environments can create galvanic corrosion that attacks the carbon-steel washer and streaks rust onto the stainless parts.
Galling — a real issue with austenitic stainless
Austenitic stainless bolts and nuts gall readily — the threads cold-weld to each other during tightening, seizing the assembly and sometimes shearing threads off. The risk rises with:
- Speed of installation (faster = more heat = more galling)
- Bolt length and thread engagement
- Dry threads (no lubricant)
- High torque targets
Mitigation in order of effectiveness: specify an anti-seize lubricant (nickel-based, molybdenum disulfide); use Nitronic 60 hardware in place of 316 when galling is chronic (Nitronic 60 is inherently galling-resistant); slow the installation tool speed.
Applications
- Marine hardware (Group 2)
- Architectural facades, railings, signage (Group 1 interior, Group 2 exterior)
- Food, beverage, and pharmaceutical equipment (Group 1 or 2)
- Swimming pool and water-treatment hardware (Group 2)
- Chemical processing equipment
- Outdoor structural hardware where carbon steel would corrode
- Electronic and semiconductor equipment (Group 1 or 2 for cleanliness)
Marking
Required markings include the manufacturer's identification and the alloy group number (F593 Group 1, Group 2, etc.) along with condition (A, CW, SH). Manufacturers often stamp a cleaner identifier like "18-8" (Group 1) or "316" (Group 2) as the primary visible mark.
Related specifications
- F594 — Stainless nuts (companion specification)
- F738M — Metric stainless bolts (the metric counterpart)
- A193 Grades B8, B8M — Stainless bolting for pressure/temp service (overlapping material, different spec intent)
- F836M — Metric stainless nuts
- F1554 — Carbon and alloy anchor bolts (when stainless isn't required)
Documentation
California Fastener F593 orders ship with mill certificates showing alloy group, condition, heat number, chemistry (including "L" verification when applicable), and mechanical properties. Dual-certified material (304/304L or 316/316L) is clearly identified on the cert.