Our stud bolt and heavy hex nut articles both ended on the same point: ASME standards give bolting its shape, ASTM gives it its strength. This piece covers the two ASTM specifications that do most of that work in flange and pressure service — A193 for the bolting, A194 for the nuts — and how their grades pair up on a compliant purchase order.
ASTM A193: High-Temperature and High-Pressure Bolting
A193 covers alloy steel and stainless steel bolting material for high-temperature or high-pressure service — studs, stud bolts, and bolts destined for flanges, valves, pressure vessels, and exchangers. Its grades fall into two families:
Ferritic alloy steel (heat treated):
- B7 — quenched and tempered chromium-molybdenum steel; the default grade of the pressure industry, combining high strength with service capability into the low-400s °C range
- B7M — B7 with controlled (reduced) hardness and 100% hardness testing, specified for sour service environments under NACE MR0175 / ISO 15156
- B16 — chromium-molybdenum-vanadium steel for elevated temperatures beyond B7's comfort zone
Austenitic stainless:
- B8 (304) and B8M (316) — corrosion-resistant and suitable from cryogenic through high-temperature service
- B8T (321) and B8C (347) — stabilized grades for temperature cycling
- Each is available as Class 1 (carbide solution treated) or Class 2 (strain hardened for higher strength, with size-dependent properties)
One boundary worth knowing: A193 is the high-temperature specification. For low-temperature service with impact testing requirements, the sister spec ASTM A320 (e.g., grade L7) takes over.
ASTM A194: The Nuts That Match
A194 covers carbon, alloy, and stainless steel nuts for high-pressure or high-temperature service — overwhelmingly in the heavy hex pattern dimensioned per ASME B18.2.2. Its grade numbering deliberately mirrors the bolting it serves, which makes pairing straightforward:
| Bolting (A193) | Nut (A194) | Typical Service |
|---|---|---|
| B7 | 2H | General refinery, pipeline, pressure service |
| B7M | 2HM | Sour service (controlled hardness both sides) |
| B16 | 7 (or 4) | Elevated temperature |
| B8 Cl.1/Cl.2 | 8 | Stainless, corrosive / cryogenic |
| B8M Cl.1/Cl.2 | 8M | 316 stainless, marine and chemical |
The pairing is a requirement, not a convention. The nut must develop the full proof load of the stud — a soft or mismatched nut caps the strength of the entire assembly. Sour-service joints are the strictest case: B7M studs demand 2HM nuts, because a standard 2H nut reintroduces the very hardness the specification exists to control.
Traceability: Where These Specs Earn Their Keep
A193/A194 product lives or dies on documentation. Expect and specify:
- Grade marking on one stud end and on the nut face, plus the manufacturer's identifier
- Heat number traceability with mill test reports (EN 10204 3.1 certificates are the customary commercial ask)
- Charpy or hardness testing where the grade or service demands it (B7M's 100% hardness testing is a defining requirement, not an optional extra)
For buyers, the practical rule: if the certificate can't trace the lot to a heat and a grade marking, it isn't A193/A194 product regardless of what the box says.
The Bottom Line
A193 and A194 are two halves of one system: matched bolting and nuts for the pressure industries, graded for temperature, corrosion, and sour-service regimes. Specify them as pairs — B7/2H, B7M/2HM, B8M/8M — insist on marking and heat traceability, and remember the boundaries: A320 below ambient extremes, B16 above B7's temperature ceiling. Get the pairing right and the paperwork right, and the metallurgy takes care of itself.