Our A193/A194 article covered stainless bolting for pressure service — but most stainless fasteners never see a flange. Architectural fixings, marine hardware, food processing equipment, and general machinery run on commercial stainless bolts, and in the inch world those are governed by ASTM F593. It's a compact specification, but its alloy groups, conditions, and marking suffixes confuse more RFQs than almost any other stainless standard.
What ASTM F593 Covers
F593 is the specification for stainless steel bolts, hex cap screws, and studs in inch sizes, nominally 1/4" through 1-1/2" diameter. Its companion for nuts is ASTM F594 — the two are specified together, just as A193 pairs with A194.
The specification is organised along two axes: what alloy the fastener is made from, and what condition (processing route) it's supplied in. Both must appear in a complete callout.
Axis One: Alloy Groups
F593 sorts stainless alloys into numbered groups. The two that dominate commercial supply:
- Alloy Group 1 — the 18-8 family (304, 304L, 305 and close relatives): the general-purpose default
- Alloy Group 2 — 316/316L: molybdenum-bearing, for marine, chloride, and chemical exposure
Further groups cover stabilized austenitics (321/347), ferritic and martensitic grades (430, 410, 416, 431), and precipitation-hardening alloys — available, but specialty territory compared with Groups 1 and 2.
Axis Two: Conditions
Condition describes how the fastener was processed, which directly sets its strength:
| Condition | Meaning | Practical Note |
|---|---|---|
| A | Machined from annealed stock | Lowest strength; softest, most corrosion-stable |
| CW | Headed/rolled and cold worked | The commercial standard; strength varies by diameter |
| SH | Strain hardened | Highest austenitic strength; limited sizes |
The subtlety buyers miss: CW strength is size-dependent. Smaller diameters pick up more cold work, so a 1/2" CW bolt carries a higher minimum tensile requirement than a 1-1/4" one made the same way. The specification splits CW into diameter bands for exactly this reason — never assume one tensile figure across a size range.
The Marking Suffixes: F593C, F593G, and Friends
Heads are marked "F593" plus a letter that encodes the alloy-group-and-condition combination. The ones seen constantly in the market:
- F593C / F593D — Group 1 (304/18-8), cold worked, smaller / larger diameter bands
- F593G / F593H — Group 2 (316), cold worked, smaller / larger diameter bands
So when a customer sends a photo of a bolt head stamped F593G, they've told you the alloy family, the condition, and the diameter band in a single character. Quoting "304 where 316 was marked" is the fastest way to fail an incoming inspection.
Metric Cross-Reference
F593 is the inch cousin of ISO 3506 — the standard behind the familiar A2-70 and A4-80 metric markings. The rough correspondence: Group 1 ≈ A2, Group 2 ≈ A4, and Condition CW in the smaller diameter band lands near property class 70. Useful for conversation; not a substitution table — thread forms, exact strength minima, and testing regimes differ, so mixed-standard supply needs explicit approval.
The Bottom Line
An F593 callout is complete only when it names alloy group and condition — "F593 316 CW" says something; "stainless bolt" says nothing. Read the head-marking suffix, respect the diameter-dependent strength bands, pair with F594 nuts, and keep ISO 3506 as a reference point rather than a swap. That's the whole standard in four habits.