ASTM A194 supplementary requirement S7 / S7M, companion to ASTM A193 S6 stud bolts
Ferralium 255 heavy hex nuts ship as the standard companion to ASTM A193 supplementary S6 stud bolts in the same heat. Where the bolt is a super duplex grade, the nut must match: a carbon-steel A194 grade 2H nut on a super duplex stud creates a galvanic couple in seawater and a Cr-depleted contact zone in sour service. ASTM A194 supplementary requirement S7 (inch) and S7M (metric) cover super duplex nuts to UNS S32550, S32750, S32760 with mechanical properties parallelling A193 S6: hardness ≤ 32 HRC for NACE MR0175 / ISO 15156-3 sour-service, ferrite content 35 to 55%, full PREN ≥ 40 chemistry. Heavy series dimensional per ASME B18.2.2. Per-heat EN 10204 type 3.2 MTC. RFQ: info@torqbolt.com / WhatsApp +91-22-66157017.
| Standard | Region | Size range | Across-flats |
|---|---|---|---|
| ASME B18.2.2 heavy hex | US, ASME flanges | 1/2" to 4" | Larger by 1/8" over regular hex |
| ISO 4032 / DIN 934 | EU, regular hex (companion only for low-class joints) | M6 to M64 | Standard A/F per ISO |
| ISO 4775 / DIN 6915 | EU, structural high-strength heavy hex | M12 to M36 | EU heavy series |
| Property | Requirement |
|---|---|
| Proof load, min | Equal to bolt yield × stress area (per A194 paragraph 9) |
| Hardness, max | 32 HRC (NACE) |
| Ferrite content | 35 to 55% (ASTM E562) |
| Solution-anneal | 1080 °C ± 20, water quench |
| Marking | Manufacturer ID, S7 (or S7M), heat number |
Mixed-metallurgy bolted joints in seawater or sour service exhibit galvanic corrosion at the bolt-nut thread interface, with the lower-PREN component sacrificing. A carbon steel A194 grade 2H nut on a super duplex S6 bolt is a textbook example: the nut corrodes preferentially, loses preload, and the joint leaks. NACE MR0175 paragraph 8 and NORSOK M-630 explicitly require matched-grade fastener pairs. Standard pairings: A193 B7 with A194 grade 2H (carbon), A193 B8M with A194 grade 8MA (austenitic), A193 S1 with A194 S3 (duplex 2205), A193 S6 with A194 S7 (Ferralium 255 / 2507 / Zeron 100 super duplex).
