Surface Treatments
Certifications
- ISO 9001:2015
- NORSOK M-650
- NACE MR0175
- ISO 15156-3
- PED 2014/68/EC
- EN 10204 type 3.2
- AD 2000 W2/W7/W10
Ferralium 255 Heavy Hex Nuts (UNS S32550)
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.
Heavy Hex Dimensional
| 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 |
Mechanical and Chemistry Per ASTM A194 S7
| 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 |
Why the Nut Grade Must Match the Bolt
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).
Related Products
- Ferralium 255 stud bolts per ASTM A193 S6 (the matched bolt)
- Ferralium 255 hex bolts for class 150 / 300 service
- Ferralium 255 heavy hex bolts for through-bolt service
- Ferralium 255 flat washers per ASTM F436
- NACE MR0175 / ISO 15156-3 compliance
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