The short answer: ASTM A194/A194M covers carbon-steel, alloy-steel and stainless-steel nuts for high-pressure or high-temperature service, or both. Among its common grades, 2H and 2HM are heat-treated carbon steel, Grade 8 is Type 304 stainless, and Grade 8M is Type 316 stainless. The differences affect hardness, proof load, processing and identification—not just the material name.[1] [2] [3]
Start with the A193 and A194 guide for the whole assembly. If you already know the bolt grade, use the separate bolt and nut compatibility guide. This page covers the nut specification itself.
What ASTM A194 specifies
A194 establishes materials, heat treatment, mechanical testing, hardness, proof load, marking and certification. ASTM A962/A962M supplies common requirements invoked by the specification. Complete the grade callout with nut style, dimensions, internal thread and finish; heavy hex inch-series nuts commonly use ASME B18.2.2 dimensions.[1] [2]
Grade 2H and 2HM: heat treatment and hardness control
Grade 2H is quenched-and-tempered carbon steel. Grade 2HM uses the same basic carbon-steel chemistry route with a lower controlled hardness range and different manufacturing and inspection requirements.[1] [3]
For 2HM, liquid quenching is followed by tempering at not less than 1150°F (620°C). After forming, machining and tapping, a final stress-relieving treatment at not less than that temperature is required; the final temper can serve as that treatment. Keeping the hardness low is part of a specified production route, not a reason to use untreated mild steel.
| Grade and nominal diameter | Brinell hardness | Rockwell alternative |
|---|---|---|
| 2H, through 1 1/2 in | 248–327 HBW | 24–35 HRC |
| 2H, over 1 1/2 in | 212–327 HBW | 95 HRB minimum; 35 HRC maximum |
| 2HM | 159–235 HBW | 84–99 HRB |
The 2H minimum changes above 1 1/2 in. For 2HM, every nut receives the required hardness inspection. The base specification permits 100% electromagnetic examination correlated with indentation testing. Individually recorded indentation readings require the appropriate supplementary and documentation requirements; a request for “100% inspection” alone does not define those records.[1] [3]
Selecting 2HM also changes the proof-load basis. It is not a drop-in substitute for 2H, and its designation alone does not establish compliance with every sour-service specification.
Grade 8 and 8M: 304 versus 316 stainless
Grade 8 supplies the Type 304 stainless material route. Grade 8M supplies Type 316, whose molybdenum addition generally improves resistance to pitting and crevice corrosion in many chloride-bearing environments. The ordinary, non-S1 conditions share hardness and proof-stress requirements; the important distinction is chemistry and corrosion suitability.[1] [3]
| Nut grade | Stainless material | Ordinary completed-nut hardness |
|---|---|---|
| Grade 8 | Type 304 | 126–300 HBW; 60 HRB minimum and 32 HRC maximum |
| Grade 8M | Type 316 | 126–300 HBW; 60 HRB minimum and 32 HRC maximum |
ASTM A194 Grade 8 is not SAE J995 Grade 8: the latter belongs to a different high-strength carbon/alloy-steel nut specification. Include the ASTM designation when ordering.
For 8M, the “M” identifies the Type 316 route, not metric dimensions. Temperature, chloride concentration, deposits and crevices still affect suitability; the 304 vs 316 comparison covers those selection factors. Both stainless grades need controlled lubrication and tightening to manage galling.
Proof load: compare the nut style as well as the grade
A nut proof-load test applies a specified axial load through a threaded mandrel. The required load depends on thread stress area, nominal size, pitch and nut style. Proof stress is the stress basis used to determine that test load; it is not the allowable service stress or target joint preload.[1] [3]
| Grade | Heavy hex proof-stress basis, common inch-series sizes |
|---|---|
| 2H | 175 ksi |
| 2HM | 150 ksi |
| 8, ordinary non-S1 condition | 80 ksi |
| 8M, ordinary non-S1 condition | 80 ksi |
For ordinary Grade 8 and 8M standard hex nuts, the basis is 75 ksi rather than the heavy hex value. Use the ordered A194 edition's size and style tables to select an actual proof load. Hardness is a separate acceptance requirement, not a universal substitute for proof-load testing.
When Supplementary Requirement S1 changes acceptance
A194 Supplementary Requirement S1 is an explicitly ordered strain-hardened condition for applicable austenitic stainless nuts. A Class 2 bolt does not automatically require an S1 nut.[1] [4]
For S1 nuts, the ordinary Table 2 hardness limits do not apply. Use the separate, diameter-specific S1 proof-load schedule, confirm that the ordered size is covered, and require the underlined grade symbol. The ordinary 80 ksi heavy hex figures above must not be used to accept an S1 nut.
The A193/A194 compatibility guide applies this distinction to B8 and B8M assemblies.
Grade marking and traceability
The ordinary grade symbol depends on manufacturing route and appears with manufacturer identification.[1] [3]
| Grade | Hot forged or cold punched | Machined from bar |
|---|---|---|
| 2H | 2H | 2HB |
| 2HM | 2HM | 2HMB |
| 8 | 8 | 8B |
| 8M | 8M | 8MB |
Current A194 editions do not require an underline beneath 2HM, although it may be permitted. This differs from the S1 stainless marking rule. A-type stainless conditions such as 8A and 8MA have their own processing and hardness requirements; the ordinary 8/8M table above does not describe them.
Match the grade symbol to the material test report (MTR), heat or lot, heat treatment and ordered condition. A silver surface cannot distinguish certified Grade 8 from Grade 8M.
What to specify when ordering A194 nuts
- ASTM A194/A194M edition and complete grade
- Nut style, dimensional standard, nominal size, pitch and thread fit
- Quantity and plain or fully specified coated condition
- Companion bolting grade and class, design temperature and service environment
- MTR, heat/lot traceability and required inspection records
- S1 or other supplementary requirements when the project calls for them
For example: ASTM A194/A194M-26 Grade 2HM heavy hex nuts to ASME B18.2.2, 1-8 UNC, plain, 240 pcs; service conditions per attached specification; MTR, lot traceability and evidence of 100% hardness inspection required.
For a complete stud-and-nut order, use the assembly examples in the compatibility guide. A194 contains additional grades beyond the four discussed here; select them under the governing specification rather than substituting by appearance or grade-number similarity.
Need ASTM A194 nuts quoted? Send the grade, style, thread, finish, quantity and service specification, including any supplementary testing or individual hardness-record requirements.
References
- ASTM International. ASTM A194/A194M-26. Nut materials, processing, hardness, proof load, marking and supplementary requirements.
- ASTM International. ASTM A962/A962M-26. Common manufacturing, testing and certification requirements.
- ASTM A194 Reference. ASTM A194 Grades Overview. Cross-check of grade materials, hardness and proof-load summaries.
- Portland Bolt. A193 Nut Compatibility. Common pairings and the optional strain-hardened nut condition.