Specifications
Surface Treatments
Certifications
- ISO 9001 - 2015 Certified
- PED 2014/68/EC
- NACE MR0175/ISO 15156-2
- NORSOK M-650
- DFAR
- MERKBLATT AD 2000 W2/W7/W10
ASTM A194 Nuts: Companion Nut Grades for ASTM A193 Stud Bolts
The ASTM A194/A194M nut specification covering carbon, alloy and stainless steel nuts intended for high-pressure or high-temperature service, paired with ASTM A193 stud bolts on every flange, pressure vessel and heat-exchanger joint.
Every ASTM A193 stud bolt ships with a matching ASTM A194 nut, and selecting the wrong nut grade is the single most common bolted-joint specification mistake in pressure-equipment construction. ASTM A194/A194M-20a covers carbon, alloy and martensitic stainless steel nuts in 1/4 in. through 4 in. (M12 through M100) sizes, plus austenitic stainless steel nuts from 1/4 in. (M12) and above, all intended for high-pressure or high-temperature service. Grade substitutions without purchaser permission are forbidden by the specification. Request a quote for matched A193 stud bolts and A194 nuts.
What ASTM A194/A194M covers
ASTM A194/A194M is the umbrella nut specification for high-pressure or high-temperature bolting. It covers four families: carbon steel (Grade 1, 2, 2H, 2HM), alloy steel (Grade 3, 4, 7, 7M, 16), martensitic stainless (Grade 6, 6F) and the austenitic Grade 8 family (8, 8A, 8C, 8M, 8T, 8N, 8R, 8S and variants). Each grade is fixed by chemistry, hardness, proof load and heat-treatment condition. The standard is referenced by ASME Section II Part A as SA-194.
A193 to A194 Pairing Matrix
The table below summarises standard A193 stud to A194 nut combinations. The stud carries the tensile load; the nut is selected to match stud chemistry, service temperature and corrosion environment.
| ASTM A193 Stud Grade | Stud Material | Recommended ASTM A194 Nut Grade | A194 Nut Material / UNS | Service Notes |
|---|---|---|---|---|
| B5 | 5Cr-1/2Mo (UNS S50100) | 4 | Alloy steel (quenched and tempered) | High-temperature alloy service matched to B5 stud strength. |
| B6 | 13Cr martensitic (UNS S41000) | 6 (or 6F free-machining variant) | 12-13Cr martensitic stainless (S41000 / S41600) | Mated chromium nut for B6 13Cr stud, moderate corrosion service. |
| B7 | AISI 4140 Cr-Mo (UNS G41400) | 2H | Carbon steel, quenched and tempered | Standard refinery and petrochemical pairing. |
| B7M | AISI 4140 Cr-Mo, controlled-hardness | 2HM | Carbon steel, controlled hardness, 235 HBW max | NACE MR0175 sour-service equivalent of B7 plus 2H. |
| B16 | 1Cr-1Mo-1/4V (UNS K14072) | 4 or 7 | Alloy steel, quenched and tempered | Elevated-temperature alloy nut for B16 creep-resistant stud (rated to 593 deg C / 1100 deg F). |
| B8 Class 1 | 304 SS solution treated (UNS S30400) | 8 | 304 austenitic stainless, solution treated | Standard 304 SS pairing for moderate corrosion service. |
| B8 Class 2 | 304 SS strain-hardened | 8A (or 8 per purchaser agreement) | 304 austenitic stainless, solution treated | A194 does not list a strain-hardened nut variant; nuts remain solution treated even when paired with a Class 2 stud. |
| B8M Class 1 | 316 SS solution treated (UNS S31600) | 8M | 316 austenitic stainless, solution treated | Chloride-service pairing. 316 Ni-Mo resists pitting and crevice corrosion. |
| B8M Class 2 | 316 SS strain-hardened | 8MA | 316 austenitic stainless, solution treated | Solution-treated 316 nut paired with the strain-hardened B8M Class 2 stud. |
| B8C Class 1 | 347 SS Nb-stabilised (UNS S34700) | 8C | 347 austenitic stainless with niobium stabilisation | Nb-stabilised pairing for sensitisation-prone service (425 to 815 deg C). |
| B8T Class 1 | 321 SS Ti-stabilised (UNS S32100) | 8T | 321 austenitic stainless with titanium stabilisation | Ti-stabilised alternative to 347 / Grade 8C. |
A194 Grade 2H: the workhorse carbon-steel nut
Grade 2H is the most widely produced A194 nut in the world. Chemistry is plain carbon steel (0.40 percent C min, 1.00 percent Mn max), quenched and tempered to 24 to 35 HRC up to 1-1/2 in. (M36), and 35 HRC max with 95 HRB min over 1-1/2 in. Sample-nut Brinell after heat treatment is 179 min. Grade 2H is the default companion for A193 B7 studs in refinery piping, ASME B16.5 flange bolting, heat-exchanger tube-sheet bolting and pressure-vessel manholes.
A194 Grade 8M: the chloride-service nut for B8M studs
Grade 8M is produced from UNS S31600 (Type 316) austenitic stainless steel in the solution-treated condition: 16.0 to 18.0 percent Cr, 10.0 to 14.0 percent Ni and 2.00 to 3.00 percent Mo. The molybdenum delivers pitting and crevice-corrosion resistance in chloride streams. Hardness is capped at 32 HRC max and 60 HRB min. Grade 8M pairs with A193 B8M Class 1 studs on seawater, brine, chloride-bearing process streams, sour gas where 316 is acceptable, and pulp-and-paper bleach plants. For more aggressive service the B8MLCuN or B8R stud pairs with the 8MLCuN or 8R nut.
A194 nuts under NACE MR0175 sour service
Hydrogen-sulfide service overrides every other selection. NACE MR0175 / ISO 15156-3 caps nut hardness at 22 HRC (235 HBW) to prevent sulfide stress cracking. Standard A194 Grade 2H is NOT NACE compliant because its hardness ceiling reaches 35 HRC. The compliant variant is A194 Grade 2HM, with the same chemistry as 2H but controlled-hardness heat treatment to 159 to 235 HBW (84 to 99 HRB). The matching stud is A193 B7M. The 2HM nut is functionally weaker than 2H, so joint design must verify that the 2HM proof load satisfies the bolted-joint analysis. The austenitic Grade 8 family is generally NACE acceptable in the solution-treated condition.
Hardness, surface finish and marking requirements
ASTM A194 Table 2 fixes the hardness range for every grade and is the principal acceptance criterion at incoming inspection. Carbon and alloy grades are quenched and tempered; austenitic grades are solution treated. Surface finish per A194 Section 11 calls for nuts free of fins, loose scale and sharp edges. Marking is mandatory: every nut 1/4 in. (M12) and larger carries the manufacturer identification symbol plus the A194 grade symbol on one face (for example 2H, 2HM, 7, 8, 8M). Missing or incorrect marking is a non-conformance and grounds for lot rejection.
How to specify A194 nuts on a purchase order
A complete A194 nut line on a PO or material requisition should contain quantity, type, thread size, grade, dimensional standard, finish, marking, MTC and any NACE call-out. Example:
QTY 200 NOS, HEAVY HEX NUT, 1-INCH-8UNC, ASTM A194 GRADE 2H, DIMENSIONS PER ASME B18.2.2, HOT-DIP GALVANISED PER ASTM F2329, TB MARKING + 2H GRADE STAMP, MTC EN 10204 3.1, PMI ON 100 PERCENT OF NUTS, NACE MR0175 NOT REQUIRED.
Swap 2H with 2HM for sour service, 8 for 304 SS, 8M for 316 SS, 8C for 347, 8T for 321. Swap EN 10204 3.1 with 3.2 for nuclear, sour, duplex or thick-wall critical service. Galvanising applies to carbon and alloy nuts only; the A194 8 family is supplied bare.
Related ASTM A193 stud bolt pages
- A193 B7 stud bolts (pairs with A194 2H)
- A193 B7M stud bolts (pairs with A194 2HM)
- A193 B8 Class 1 stud bolts (pairs with A194 8)
- A193 B8M Class 1 stud bolts (pairs with A194 8M)
- A193 B8C Class 1 stud bolts (pairs with A194 8C)
- A193 B8T Class 1 stud bolts (pairs with A194 8T)
- A193 B16 stud bolts (pairs with A194 4 or 7)
- NACE MR0175 sour-service A193 stud bolts
- ASME B16.5 flange bolt chart with A193 + A194 selections
References
- ASTM A194/A194M-20a, Standard Specification for Carbon Steel, Alloy Steel, and Stainless Steel Nuts for Bolts for High Pressure or High Temperature Service, or Both.
- ASTM A193/A193M, Standard Specification for Alloy-Steel and Stainless Steel Bolting for High Temperature or High Pressure Service.
- NACE MR0175 / ISO 15156-3, Petroleum and natural gas industries - Materials for use in H2S-containing environments in oil and gas production - Part 3: Cracking-resistant CRAs and other alloys.
- ASME B18.2.2, Nuts for General Applications: Machine Screw Nuts, Hex, Square, Hex Flange, and Coupling Nuts (Inch Series) - dimensional reference for A194 nut dimensions.
- ASME Boiler and Pressure Vessel Code Section II Part A, SA-194 / SA-194M.
Frequently Asked Questions
- What is the difference between A194 Grade 2H and Grade 2HM?
- Both share the same plain-carbon-steel chemistry. Grade 2H is quenched and tempered to 24 to 35 HRC. Grade 2HM is controlled-hardness heat treated to 159 to 235 HBW (84 to 99 HRB), capped at 22 HRC equivalent for NACE MR0175 sour-service compliance. Use 2H for general refinery service; use 2HM whenever H2S partial pressure exceeds the NACE screening threshold.
- Which A194 nut grade pairs with A193 B7M stud bolts?
- A193 B7M studs pair with A194 Grade 2HM nuts. Both are NACE MR0175 controlled-hardness versions held at 22 HRC equivalent (235 HBW max) to prevent sulfide stress cracking. Specifying B7M with standard 2H nuts is a non-conformance — the nut must also be the 2HM variant for end-to-end NACE compliance.
- Is A194 Grade 8M nut chloride-resistant?
- Yes. Grade 8M is produced from UNS S31600 (Type 316) austenitic stainless steel with 16.0 to 18.0% Cr, 10.0 to 14.0% Ni and 2.00 to 3.00% Mo. The molybdenum delivers pitting and crevice-corrosion resistance in seawater, brine, chloride process streams and sour gas. Pair with A193 B8M Class 1 or Class 2 studs. For more aggressive chloride service, step up to Grade 8MLCuN (Alloy 254) or 8R (Nitronic 50).
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