Load distribution under bolt heads and nut faces in super duplex bolted joints
A flat washer under the head of a Ferralium 255 hex bolt or under the flat face of a heavy hex nut spreads the preload across a larger bearing area, prevents galling of soft flange faces during torque-up, and protects the gasket-side surface from rotation marks. Ferralium 255 (UNS S32550, EN 1.4507) flat washers are the matched grade for ASTM A193 S6 stud bolts and A194 S7 nuts in seawater, sour, and chloride service. Per-heat EN 10204 type 3.2 MTC, hardness ≤ 32 HRC, ferrite content 35 to 55%, NACE MR0175 / ISO 15156-3 Table A.10. RFQ: info@torqbolt.com / WhatsApp +91-22-66157017.
| Type | Standard | Use case |
|---|---|---|
| Type A plain | ASME B18.22.1 | General light-duty bolted joints |
| Type B narrow / regular / wide | ASME B18.22.1 | Specified to suit bolt diameter and bearing area |
| Structural F436 / F436M | ASTM F436 | Heavy hex structural and high-strength flange bolting |
| Form A / B / C / G | ISO 7089, ISO 7090, ISO 7093, ISO 7094 | EU / metric flange bolting, large OD for soft mating face |
| Sealing / bonded | Custom | Pressure-side seal, requires polymer-bonded face |
For ASME B16.5 raised-face flange service, specify Type A or F436 to match the bolt-hole pattern; the washer ID must be 1/16 inch larger than the nominal bolt diameter and the OD must clear the gasket inner edge by at least 1/8 inch. For coated steel flanges with low surface hardness, use the wide series (Type B wide) to spread the bearing stress below the flange yield. The washer thickness is selected to provide a minimum of two thread engagements above the nut face when the joint is fully torqued.
Per-heat EN 10204 type 3.2 MTC: chemistry per ASTM A479, hardness, ferrite-content per ASTM E562, NACE MR0175 conformance with Table A.10 reference. NORSOK M-650 QTR (MDS D55) available on request.
