Related Specifications
Surface Treatments
Certifications
- ISO 9001 - 2015 Certified
- PED 2014/68/EC
- API 6A ; API 20E; API 20F
- NACE MR0175/ISO 15156-2
- NORSOK M-650
- DFAR
- MERKBLATT AD 2000 W2/W7/W10
ASTM A194 Grade 8M Heavy Hex Nuts: Companion Nut for A193 B8M Bolts
Every B8M stud bolt heading to a 316L stainless flange asks the same question on the line item below it: which nut? The right answer is ASTM A194/A194M Grade 8M. Same UNS S31600 base metal as the bolt, same 16 to 18 percent chromium, same 2 to 3 percent molybdenum, same chloride pit resistance. Mismatch the nut grade and the joint corrodes one side first, usually the cheaper side, usually long before the flange leaks somewhere a refinery shutdown can fix it.
This page covers what A194 8M actually is, how it differs from 8MA, how ASME B18.2.2 sizes the heavy hex blank, and how marking lets a third-party inspector read the grade off the chamfer without a hardness file.
For the bolt half of the assembly, start at the parent page B8M Class 2 stud bolts and the B8M Class 1 stud bolts route. Material-data backstop is B8M UNS S31600 material data. Flange dimensions live on ASME B16.5 flange dimensions, and sour-service qualification on NACE MR0175 B8M compliance. The site root is astma193b8m.com.
What ASTM A194 Grade 8M actually is
A194/A194M is the umbrella nut specification for high-pressure and high-temperature bolted joints. Inside it, the 8-series covers the austenitic stainless grades. Grade 8M is the molybdenum-bearing one, equivalent to AISI Type 316 and designated UNS S31600. It is the deliberate counterpart to A193 B8M bolting, and the pairing is not a coincidence. Both grades trace to the same 316 master heat practice, the same chemistry envelope, and the same solution-treatment route.
The grade exists because the cheaper austenitic nut, Grade 8 (Type 304, UNS S30400), pits in chloride service. Adding 2 to 3 percent molybdenum widens the passive window, raises pitting resistance, and brings the nut into specification for sour gas, seawater, glycol, amine, and most chlorinated process streams that B8M bolts already qualify for.
Chemistry (A194/A194M Table 1)
| Element | 8M / 8MA limit (%) | Role in the alloy |
|---|---|---|
| Carbon (C) | 0.08 max | Held low to keep chromium carbide off grain boundaries during weld HAZ exposure. |
| Manganese (Mn) | 2.00 max | Austenite stabiliser, deoxidiser at melt. |
| Phosphorus (P) | 0.045 max | Residual; capped for hot ductility. |
| Sulphur (S) | 0.030 max | Residual; capped for hot workability and chloride pit resistance. |
| Silicon (Si) | 1.00 max | Deoxidiser, slight ferrite former. |
| Chromium (Cr) | 16.0 to 18.0 | Passive film former. The reason the alloy is stainless at all. |
| Nickel (Ni) | 10.0 to 14.0 | Austenite former. Keeps the structure FCC and tough at cryogenic temperature. |
| Molybdenum (Mo) | 2.00 to 3.00 | The "M". Lifts pitting resistance equivalent number and pushes chloride pitting threshold well above the Type 304 baseline. |
The chemistry envelope is identical to A193 B8M and identical to ASME SA-194 8M for vessel applications under the BPVC route. That cross-walk is what allows a mill to ship one melt and certify it to all three documents on the same 3.1 inspection certificate.
8M versus 8MA: the heat-treatment fork
Both grades share the chemistry shown above. The condition is what splits them.
| Attribute | Grade 8M | Grade 8MA |
|---|---|---|
| UNS designation | S31600 | S31600 |
| Type | AISI 316 austenitic | AISI 316 austenitic |
| Condition | Carbide-solution-treated; may be machined from bar that received solution treatment before final cutting. | Carbide-solution-treated in the finished condition. The completed nut goes back through solution anneal so machined surfaces are sensitisation-free. |
| Brinell hardness, completed nut (A194 Table 2) | 126 to 300 HBW | 126 to 192 HBW |
| Rockwell hardness, completed nut | HRC 32 max, or HRB 60 min | HRB 60 to 90 |
| Typical pairing | A193 B8M Class 1 or Class 2 studs in routine high-pressure service. | A193 B8MA studs where the assembly will see severe chloride or sour-service corrosion and the entire joint must be in the lowest-residual-stress, fully-annealed condition. |
| Cold work in the finished nut | Permitted residual cold work from machining. | Effectively zero. The final anneal recrystallises any work the threads imparted. |
Practical read: order 8M for general 316 service. Order 8MA when the bolt is B8MA, when the joint is in seawater splash zone, or when the project specification calls for fully-annealed nuts to keep stress-corrosion-cracking risk minimal. Mixing 8M nuts on a B8MA stud is allowed by the chemistry, but defeats the purpose of paying for the annealed bolt.
Dimensional spec: ASME B18.2.2 heavy hex
A194 specifies the metal. ASME B18.2.2 specifies the geometry. A heavy hex nut differs from a finished hex nut in two ways: width across flats is one nominal size larger, and thickness is approximately equal to the bolt diameter rather than 7/8 of it. That extra meat is what lets the nut transmit the full proof load of an A193 B8M Class 2 stud without thread strip.
The most-asked sizes for B16.5 flange service:
| Nominal size (in.) | Threads per inch | Width across flats F (in.) | Thickness H (in.) | Typical flange class served |
|---|---|---|---|---|
| 1/2 | 13 UNC | 7/8 | 31/64 | NPS 1/2 to 2, Class 150 to 300 |
| 5/8 | 11 UNC | 1-1/16 | 39/64 | NPS 2-1/2 to 4, Class 150 to 600 |
| 3/4 | 10 UNC | 1-1/4 | 47/64 | NPS 4 to 8, Class 300 to 900 |
| 7/8 | 9 UNC | 1-7/16 | 55/64 | NPS 6 to 12, Class 600 to 1500 |
| 1 | 8 UN | 1-5/8 | 63/64 | NPS 8 to 16, Class 600 to 2500 |
| 1-1/8 | 8 UN | 1-13/16 | 1-7/64 | NPS 12 to 24, Class 900 to 2500 |
| 1-1/4 | 8 UN | 2 | 1-7/32 | NPS 14 to 24, Class 1500 to 2500 |
| 1-1/2 | 8 UN | 2-3/8 | 1-15/32 | NPS 18 to 36, Class 1500 to 2500 |
Sizes 1 inch and above run the 8UN (eight-pitch) thread series rather than UNC, matching the A193 B8M stud thread side for side. Metric counterparts run M12 through M100 and follow ASME B18.2.4.6M or DIN EN ISO 4775 for the heavy pattern.
Mechanical and proof-load behaviour
A194 nuts are qualified by hardness range and by proof load on a threaded mandrel rather than by tensile coupon. The proof load value for an 8M nut is set equal to the minimum specified tensile strength of the matched bolt grade multiplied by the stress area of the threaded section. For B8M Class 2 studs up to 3/4-in. diameter, that puts the nut proof load at 110 ksi nominal, which is why a heavy hex 8M nut on a fully strain-hardened B8M Class 2 stud yields the stud before stripping the nut threads, and that is the design intent. The nut should always be the survivor.
Hardness range (126 to 300 HBW for 8M) is the routine acceptance test. Anything inside that band has passed solution treatment, has acceptable residual carbide, and will not gall on tightening provided the assembler uses a clean dry-film anti-seize. Galling resistance is the single most common field complaint on 316 nut/bolt joints, and the specification permits but does not enforce the lubricant; that selection sits with the procurement engineer.
Equivalents and cross-walks
The most useful identity to commit to memory is ASTM A194 Grade 8M equals ASME SA-194 Grade 8M equals AISI Type 316 equals UNS S31600 equals EN 1.4401 equals JIS SUS 316. Every one of those names points at the same melt envelope inside a fraction of a percent. The only deliberate gap is between 8M and 8MA: same metal, different heat-treatment paperwork.
| Reference system | Designation equivalent to A194 8M |
|---|---|
| ASME BPVC Section II Part A | SA-194 Grade 8M |
| AISI / SAE | Type 316 |
| UNS | S31600 |
| EN 10088 / EN ISO 3506 | 1.4401 / A4 group base |
| JIS | SUS 316 |
| DIN (legacy) | X5CrNiMo17-12-2 |
| BS EN 10269 bolting | 1.4401 condition AT |
Inside Europe the typical purchase order calls A4-70 or A4-80 for the assembly, where A4 corresponds to the 316 chemistry family and 70 / 80 corresponds to the tensile property class. A4-80 maps closely to A194 8M / A193 B8M Class 2 combined behaviour, while A4-70 matches A194 8MA / A193 B8MA. The mapping is approximate because EN classifies by tensile, ASTM classifies by hardness and condition, but a buyer who needs ISO 3506 paperwork on an A194 8M nut will get it from any reputable mill at no premium.
Markings on the chamfer
A194 Section 15 requires three pieces of information stamped on one face of every nut at or above the 1/4-in. nominal size: the grade symbol, the manufacturer's identification mark, and (for metric units of ordering) the suffix "M". The grade symbol for this page is the literal text 8M or 8MA. Marks are usually applied to the bearing face or to the chamfer, raised or depressed, in characters large enough to be read without magnification.
An inspector reading a delivery on a flange spread will look for, in order:
- The grade symbol. Either 8M or 8MA. Nothing else qualifies as a 316 austenitic nut against this specification.
- The manufacturer's identifier. This is the heat traceability anchor and ties back to the 3.1 inspection certificate that travelled with the lot.
- The "M" suffix where the nut was ordered to metric dimensions. Absence of this suffix means inch-series, which matters when matching to a metric stud.
Sour-service nuts qualified to NACE MR0175 / ISO 15156 carry the additional marking allowed by Section 15.4 of MR0175, usually an "NL" or "HIC" stamp at the discretion of the manufacturer, but that mark is not mandated by A194 itself.
Why the pairing matters for warranty
Galvanic mismatch on a 316 flange is real. Pair a carbon-steel A194 Grade 2H nut with a B8M stud and the bolt becomes the anode in seawater within weeks. Pair a Grade 8 (304) nut with a B8M stud and the nut pits first under chloride because it lacks the molybdenum. The only combination that holds both sides of the joint inside the same corrosion envelope is A194 8M (or 8MA) with A193 B8M (or B8MA). Project specifications written by oil majors enforce this pairing in their bolt schedules precisely because the field-failure history on mixed-grade joints is well documented.
For TorqBolt customers, the standard quotation against any B8M stud line ships A194 8M heavy hex nuts at one nut per stud end, both ends. 8MA substitution is available on request when the assembly drawing or the project specification explicitly calls for the annealed condition. Both routes are certified to 3.1 inspection certificate per EN 10204, with chemistry, hardness, and proof load traceable to the parent heat. Enquiries to info@torqbolt.com with the stud diameter, length, and flange class get a same-shift quotation.
Related references
Spec hub: ASTM A193 / SA-193 B8M overview (covers NORSOK M-650 qualification, EN 10204 inspection, F436 washers, B16.5 flange pairing, and the full applications register). Compatible bolt grades: B8M Class 1 (NACE-qualified), B8M Class 2 (strain-hardened, non-sour). Compliance route: NACE MR0175 / ISO 15156-3 sour service. Material backstop: UNS S31600 datasheet. Flange geometry: ASME B16.5 dimensions.
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