ASTM A194 Grade 7 vs Grade 7M Nuts: Sour Service Comparison
ASTM A194/A194M-20a Grades 7 and 7M are made from the same chromium-molybdenum alloy steel: Table 1 lists them together (C 0.38 to 0.48 %, Cr 0.80 to 1.10 %, Mo 0.15 to 0.25 %; footnote G gives 4140, 4142, 4145 and their H versions as typical steels). They differ in tempering temperature, hardness, proof load and testing. Grade 7M is tempered higher and held to a lower hardness for applications that limit bolting hardness, such as sour service.
Grade 7 vs Grade 7M at a Glance
- Heat treatment (6.4): both are liquid quenched. Grade 7 is tempered at 1100 °F [595 °C] minimum; Grade 7M at 1150 °F [620 °C] minimum, with a final stress relief at or above that temperature after all forming, machining and tapping.
- Hardness (Table 2): Grade 7, 248 to 327 HB or 24 to 35 HRC. Grade 7M, 159 to 235 HB or 84 to 99 HRB.
- Proof stress (Tables 3 to 6): Grade 7 is in the 175 000 psi [1205 MPa] heavy hex group, Grade 7M in the 150 000 psi [1035 MPa] group. For a 1 in. heavy hex nut that is 106 000 lbf against 90 900 lbf (Table 3).
- Testing (8.1.2, 8.1.3): Grade 7 is sampled by lot size; every Grade 7M nut is hardness tested per ASTM A962/A962M or F606/F606M.
- Marking (Table 7): 7 or 7B; 7M or 7MB (the underline once required for 7M is no longer required but is permitted). With Supplementary Requirement S3: 7L or 7BL, and 7ML or 7MLB.
- Certification (12.1): the certificate states the minimum tempering temperature for both grades.
Grade 7 vs Grade 7M Comparison Table
| Property | Grade 7 | Grade 7M |
|---|---|---|
| Material | AISI 4140 Cr-Mo Q&T | AISI 4140 Cr-Mo Q&T |
| UNS | G41400 | G41400 |
| Hardness | 248-321 HBW (24-35 HRC) | 235 HBW max (22 HRC max) |
| NACE MR0175 / ISO 15156 | No | Yes (sour-service rated) |
| Proof Load (1-inch) | 165,000 lbf min | 120,000 lbf min (reduced for hardness limit) |
| Companion stud bolt (A193) | B7 | B7M |
| Typical service | Standard refinery/petrochem up to 900°F | Sour-gas (H2S) refinery/upstream service |
This table was published before the October 2026 review and is flagged for technical correction. Where it differs from the clause-referenced values above (hardness, proof load, service notes), the values above from ASTM A194/A194M-20a govern.
Sour Service
ISO 15156-2:2020 Table A.4 lists ASTM A194 Grades 2HM and 7M as acceptable nuts for ASTM A193 Grade B7M and ASTM A320 Grade L7M bolts. For carbon and low-alloy steels in general, clause A.2.1.2 sets 22 HRC maximum, with less than 1 % nickel, no free-machining steels and listed heat-treatment conditions. Grade 7 is not in Table A.4; its hardness range reaches 35 HRC.
Low-Temperature Service
ASTM A320/A320M-21 clause 10.1 calls for A194 Grade 4 or Grade 7 nuts on L7, L7A, L7B, L7C, L43, L1 and L70 to L73 bolting, and Grade 7M nuts at 235 HBW maximum on L7M bolting. Nut material is subject to the A320 impact requirements, and nuts from impact-tested material are marked L (A320 13.2). Within A194, Supplementary Requirement S3 applies the A320 Grade L7 Charpy test to Grade 7 nuts and the Grade L7M test to Grade 7M nuts, and adds L to the marking; impact testing is not required below 5/8 in. [16 mm].
Grade pages: Grade 7 · Grade 7M · Grade 2HM · Grade 2H. More: all A194 grades · ASTM A194 PDF · A194 hub.