What Is ASTM A194? Nut Specifications
ASTM A194 is the standard specification for carbon, alloy, and stainless steel nuts used with high-temperature and high-pressure stud bolts in flanged piping connections. Every stud bolt grade has a specific matching A194 nut grade. Using the wrong nut grade compromises the flanged joint and violates ASME B16.5 requirements.
| Attribute | Detail |
|---|---|
| Full title | Standard Specification for Carbon Steel, Alloy Steel, and Stainless Steel Nuts for Bolts for High Pressure or High Temperature Service, or Both |
| Nut type | Heavy hex per ASME B18.2.2 |
| Thread | UNC (Unified National Coarse) per ASME B1.1 |
| Key grades | 2H, 2HM, 4, 7, 7M, 8, 8M |
| Referenced by | ASME B16.5, ASME B31.3, ASME B31.1 |
| Companion spec | ASTM A193 (studs), ASTM A320 (low-temp studs) |
ASTM A194 Grades
| Grade | Material | Stud Bolt Match | Service |
|---|---|---|---|
| 2H | Carbon steel, quenched & tempered | A193 B7 | General high-temp service (up to 450 degC) |
| 2HM | Carbon steel, restricted hardness (22 HRC max) | A193 B7M | Sour service per NACE MR0175 |
| 4 | Alloy steel (Cr-Mo), quenched & tempered | A320 L7 | Low-temperature service (to -100 degC) |
| 7 | Alloy steel (AISI 4140), quenched & tempered | A320 L7, L7M | Low-temperature service (to -100 degC) |
| 7M | Same as 7, restricted hardness | A320 L7M | Low-temp + sour service |
| 8 | SS 304 (18Cr-8Ni), austenitic | A193 B8 | Corrosion-resistant service |
| 8M | SS 316 (16Cr-12Ni-2Mo), austenitic | A193 B8M | Marine, chloride, chemical service |
| 8T | SS 321, austenitic | A193 B8T | High-temp stainless service |
| 8F | SS 303, free-machining austenitic | A193 B8F | Non-critical, easy machining |
Mechanical Properties
| Grade | Hardness (HB) | Proof Load Stress (ksi) | Temperature Range |
|---|---|---|---|
| 2H | 248-352 | 175 (for 1” and under) | -29 degC to 450 degC |
| 2HM | 159-237 | 150 (for 1” and under) | -29 degC to 450 degC |
| 4 | 248-352 | 175 (for 1” and under) | -100 degC to 450 degC |
| 7 | 248-352 | 175 (for 1” and under) | -100 degC to 450 degC |
| 8 | 241 max | - (no proof load specified) | -254 degC to 540 degC |
| 8M | 241 max | - (no proof load specified) | -254 degC to 540 degC |
Grade 2H is by far the most commonly purchased nut in oil and gas piping, matching the ubiquitous A193 B7 stud bolt.
Nut-to-Bolt Pairing Rules
The nut must be at least as strong as the bolt to prevent thread stripping before bolt yield. ASME B16.5 Table 1 specifies the required pairings.
| Stud Specification | Stud Grade | Required Nut Grade |
|---|---|---|
| ASTM A193 | B7 | A194 2H |
| ASTM A193 | B7M | A194 2HM |
| ASTM A193 | B16 | A194 4 or 7 |
| ASTM A193 | B8 Class 1 | A194 8 |
| ASTM A193 | B8M Class 1 | A194 8M |
| ASTM A320 | L7 | A194 4 or 7 |
| ASTM A320 | L7M | A194 7M |
Marking Requirements
ASTM A194 requires each nut to be marked with:
- The manufacturer’s identification symbol
- The grade designation (2H, 8, 8M, etc.)
- Heat or lot number traceability
These markings are stamped or forged into one face of the nut. On mill test certificates (EN 10204 3.1), the chemical composition, hardness, and proof load test results are documented per heat.
Hex nut dimensions (width across flats, thickness) are per ASME B18.2.2. Bolt torque values for each flange size and class are in the ASME B16.5 bolt chart.
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