Scope
ASTM A194 / A194M covers carbon, alloy, and stainless steel nuts intended for use with bolting for high-pressure, high-temperature, or both services. It is the companion nut specification to A193 and A320 and is structured so that every A193 stud grade has a matching A194 nut grade.
Nuts under A194 are furnished as heavy hex by default. Hex (regular), hex flange, jam, and other patterns are available by supplementary agreement.
Grade map
Carbon and alloy steel grades
| Grade | Material | Typical pairing | Notes |
|---|---|---|---|
| 1 | Carbon steel | A307 bolts | Non-heat-treated, general service |
| 2 | Carbon steel, heat-treated | Low-alloy bolts | Not common |
| 2H | Medium carbon, Q&T | A193 B7, A354, A449 | The industry default |
| 2HM | Medium carbon, Q&T, lower hardness | A193 B7M | Sour service (NACE MR0175) |
| 4 | Alloy steel Cr-Mo, Q&T | A193 B7, B16 | Higher-strength alternative |
| 7 | Alloy steel Cr-Mo, Q&T | A193 B7, B16 | Interchangeable with 2H in most service |
| 7M | Alloy steel, lower hardness | A193 B7M | Sour service; lower hardness version of 7 |
| 16 | Cr-Mo-V, Q&T | A193 B16 | Preferred above 850°F |
| 3 | 5Cr-½Mo | A193 B5 | Uncommon |
Stainless (austenitic) grades
| Grade | Material | Class 1 (SA) | Class 1A (SA, finished) | Class 2 (SH) | Pairing |
|---|---|---|---|---|---|
| 8 | 304 SS | ✓ | ✓ | ✓ | A193 B8 |
| 8M | 316 SS | ✓ | ✓ | ✓ | A193 B8M |
| 8T | 321 SS | ✓ | ✓ | ✓ | A193 B8T |
| 8C | 347 SS | ✓ | ✓ | ✓ | A193 B8C |
| 8F | 303 SS | ✓ | — | — | Free-machining variant |
| 8P, 8N, 8MN, 8MLCuN | Nitrogen / N+Mo | ✓ | ✓ | ✓ | Strength-upgraded variants |
Class 1 = solution-annealed. Class 1A = solution-annealed in the finished condition for maximum corrosion resistance in welded parts. Class 2 = strain-hardened for higher strength.
Key mechanical requirements (selected grades)
| Grade | Proof Load Stress | Max Hardness | Hardness Min |
|---|---|---|---|
| 2H | 175 ksi | 248 HBW / 24 HRC max size-dependent; generally 38 HRC | 24 HRC |
| 2HM | 150 ksi | 235 HBW (22 HRC) | — |
| 4 | 175 ksi | 248 HBW / 38 HRC depending on size | 24 HRC |
| 7 | 175 ksi | 248 HBW / 38 HRC | 24 HRC |
| 7M | 150 ksi | 235 HBW (22 HRC) | — |
| 16 | 175 ksi | 321 HBW / 35 HRC | 26 HRC |
| 8 / 8M Cl. 1 | 80 ksi | — | — |
| 8 / 8M Cl. 2 (≤3/4") | 125 ksi | — | — |
Nuts are acceptance-tested by proof load, hardness, or both depending on grade and diameter. A194 uses the stress-area proof load method: the nut must resist the specified load without stripping or failure.
The 2H vs 7 question
2H and 7 are often interchangeable in refinery and process service. The practical differences:
- 2H is carbon steel, simpler to produce, typically lower cost, and carries decades of field history with B7 studs.
- 7 is alloy steel (chromium-molybdenum), slightly better at sustained high temperature, and required when 2H would not meet toughness after long exposure above roughly 850°F.
For most ambient-to-750°F process bolting, 2H is the correct specification. Above 850°F or where thermal cycling is aggressive, move to Grade 7 or Grade 16.
Sour service (NACE MR0175)
For H₂S-bearing and sour hydrocarbon service, specify 2HM with A193 B7M studs. Both the nut and the stud are limited to lower hardness — typically 22 HRC max — to resist sulfide stress cracking. 100% hardness testing is required on both parts. Grade 7M is the alloy-steel equivalent of 2HM when Grade 7 hardness limits are needed.
Marking requirements
Nuts are marked with the manufacturer's identification symbol and grade. For heavy hex nuts ≥1/4", markings are on one face; smaller sizes may mark on the side. The grade symbol is typically the bare numeral or alphanumeric (2H, 2HM, 7, 7M, 8, 8M).
Overtapping for coatings
When nuts are hot-dip galvanized or zinc-coated, the threads are overtapped to accommodate coating buildup on the mating stud. Overtapping amounts are specified in F2329 (HDG) or F1941 (zinc electroplating) and are communicated on the mill cert. Proof load and hardness still apply post-coating.
Related specifications
- A193 — Companion stud specification
- A320 — Low-temperature stud specification (paired with A194 Grades 4, 7, 7M, 8, 8M)
- A563 — Carbon and alloy steel nuts for general structural applications
- F594 — Stainless steel nuts for general service
- F2329 / F1941 — Zinc coating standards with nut overtap requirements
Documentation
All California Fastener A194 orders ship with mill certificates traceable to heat number. Sour-service orders (2HM, 7M) include NACE MR0175 / ISO 15156 compliance on request.