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Flange Nuts
A comprehensive engineering reference for EPC contractors, structural engineers, procurement heads and TPI agencies covering flange nut types, integrated washer-face advantages, bearing stress calculations, dimensional standards, property classes, material grades, surface treatments and full project documentation requirements.
Flange Nut Types, Engineering Function
& Integrated Washer-Face Principles
Plain Flange · Serrated Flange · Heavy Hex Flange · Thin Flange
Definition and Engineering Function
A flange nut is a hex nut with an integral circular flange projecting radially from the base of the hex body, acting as a built-in washer. The flange distributes the bolt clamping load over a bearing area substantially larger than the hex nut face alone, reducing the contact stress at the nut-to-substrate interface. This makes flange nuts the preferred fastener in applications where: (1) a separate washer cannot be used (inaccessible installation locations, high-volume automated assembly); (2) the substrate material has limited compressive strength (aluminium, timber, FRP, plastic, thin sheet metal); (3) anti-vibration properties are required through the serrated flange variant; or (4) rapid assembly is required without a separate washer component.
The flange nut's engineering advantage over a standard hex nut + washer combination is primarily assembly consistency and joint repeatability: the flange is concentric with the thread bore, cannot be inadvertently omitted, cannot rotate or shift under installation torque, and eliminates washer-to-nut tolerance stack-up in critical joint stiffness calculations. In high-volume automotive and machinery assembly, the elimination of a separate washer component reduces assembly time, inventory carrying cost and the risk of missing components in the joint.
For an M12 standard hex nut (ISO 4032), the bearing area under the nut face is approximately 113 mm² (annular ring between bolt hole and hex body across-flats circle). For an M12 DIN 6923 flange nut, the bearing area under the flange is approximately 314 mm² — a 178% increase. This nearly three-fold reduction in contact stress is the primary reason flange nuts are specified on aluminium castings, sheet metal, FRP panels and polymer substrates, where a standard hex nut without a washer would embed under preload and cause joint preload loss over time.
Flange Nut Types — Complete Engineering Descriptions
Flange Nut vs Hex Nut + Washer — Engineering Comparison
| Type | Standard | Flange Face | Locking | Re-usable | Primary Application |
|---|---|---|---|---|---|
| Plain Hex Flange Nut | DIN 6923 / ISO 4161 | Smooth | None | Yes | Aluminium, soft substrate, general industrial |
| Serrated Flange Nut | DIN 6923 / ISO 4161 | Serrated (radial) | Yes | Once per surface | Automotive BiW, vibrating machinery, structural |
| Heavy Hex Flange Nut | ASME B18.2.2 / A194 | Smooth or serrated | Optional | Yes (plain) | ASME piping, pressure vessels, B7 stud bolts |
| Thin / Low Flange Nut | Custom / DIN var. | Smooth or serrated | Optional | Yes (plain) | Panel assemblies, limited axial clearance |
| Extra-Large Flange Nut | Custom / structural | Smooth | None | Yes | FRP, insulation, soft composite, timber |
| Nyloc Flange Lock Nut | Custom / DIN 985+ | Smooth | Nylon insert | 3–5 times | Vibration + soft substrate, automotive, machinery |
Dimensional Data, Governing Standards
& Bearing Area Reference
Flange Diameter · Nut Height · Width Across Flats · Bearing Area
Metric Flange Nut Dimensions — DIN 6923 / ISO 4161
| Thread | Pitch (mm) | W/F s (mm) | W/C e (mm) | Nut Height m (mm) | Flange Dia. D (mm) | Flange Thick. c (mm) | Bearing Area (mm²) |
|---|---|---|---|---|---|---|---|
| M5 | 0.80 | 8.0 | 9.24 | 5.0 | 11.8 | 1.0 | 72 |
| M6 | 1.00 | 10.0 | 11.55 | 6.0 | 14.2 | 1.2 | 103 |
| M8 | 1.25 | 13.0 | 15.01 | 8.0 | 17.9 | 1.5 | 167 |
| M10 | 1.50 | 15.0 | 17.32 | 10.0 | 21.8 | 1.8 | 244 |
| M12 | 1.75 | 18.0 | 20.78 | 12.0 | 26.0 | 2.0 | 334 |
| M16 | 2.00 | 24.0 | 27.71 | 16.0 | 34.5 | 2.5 | 588 |
| M20 | 2.50 | 29.0 | 33.53 | 20.0 | 42.0 | 3.0 | 866 |
| M24 | 3.00 | 36.0 | 41.57 | 24.0 | 50.5 | 3.5 | 1260 |
DIN 6923:1983 / ISO 4161:1998. W/F = width across flats; W/C = width across corners. Bearing area = π/4 × (D² − d_h²) where D = flange OD, d_h = bolt hole diameter (nominal thread + 0.5 mm). Property class 8 and 10. Thread tolerance class 6H. Serrated variant has identical external dimensions — specify "serrated" or "plain" on order.
| Thread | Std Hex Nut Bearing Area (mm²) | DIN 6923 Flange Bearing Area (mm²) | Flange vs Hex Nut Advantage | Hex Nut + M Washer (DIN 125) |
|---|---|---|---|---|
| M6 | 43 | 103 | +140% | ~130 mm² |
| M8 | 73 | 167 | +129% | ~212 mm² |
| M10 | 107 | 244 | +128% | ~311 mm² |
| M12 | 147 | 334 | +127% | ~437 mm² |
| M16 | 242 | 588 | +143% | ~706 mm² |
| M20 | 370 | 866 | +134% | ~1060 mm² |
The DIN 6923 flange nut provides approximately 127–143% greater bearing area than a standard hex nut of the same thread size. A hex nut + DIN 125 washer provides even greater bearing area but requires a separate washer component. The flange nut is the optimum when component elimination, assembly consistency and moderate bearing area increase are the design objectives.
| Thread Size | TPI (UNC) | W/F (in) | Nut Height (in) | Flange Dia. (in) | ASTM Grade |
|---|---|---|---|---|---|
| 1/4" | 20 | 7/16 | 17/64 | 0.547 | A563 / A194 2H |
| 5/16" | 18 | 1/2 | 21/64 | 0.656 | A563 / A194 2H |
| 3/8" | 16 | 9/16 | 25/64 | 0.781 | A563 / A194 2H |
| 1/2" | 13 | 3/4 | 33/64 | 1.031 | A563 / A194 2H |
| 5/8" | 11 | 15/16 | 41/64 | 1.281 | A563 / A194 2H |
| 3/4" | 10 | 1-1/8 | 49/64 | 1.531 | A563 / A194 2H |
| 1" | 8 | 1-1/2 | 65/64 | 2.031 | A563 / A194 2H |
ASME B18.2.2-2010. Thread tolerance class 2B per ASME B1.1. A194 Grade 2H for structural/piping applications; Grade 2HM for NACE MR0175 sour service zones.
Property Classes and Proof Load
DIN 6923 flange nuts are manufactured in property class 8 and 10 per ISO 898-2. Class 8 provides 800 MPa proof load stress and is the standard grade for automotive and industrial applications paired with 8.8 bolts. Class 10 provides 1040 MPa proof load and is used with 10.9 bolts in high-strength structural and machinery applications. The flange geometry does not reduce the thread-strip load capacity relative to an equivalent standard hex nut of the same property class — the flange is in pure bearing, not in the load path of the axial bolt preload. For stainless steel flange nuts, property class A2-70 (SS 304) and A4-70 / A4-80 (SS 316) are the standard designations per ISO 3506-2.
σ_bearing = F_preload / A_flange // Bearing stress on substrate [MPa]
σ_bearing ≤ f_c,perp / γ_M // Must not exceed substrate allowable compressive stress
// WORKED EXAMPLE: M12 DIN 6923 flange nut on 6061-T6 aluminium (f_c = 276 MPa), proofload = 47,200 N
A_flange = π/4 × (26.0² − 13.0²) = π/4 × (676 − 169) = 398 mm²
σ_bearing = 47,200 / 398 = 118.6 MPa // Below aluminium f_c allowable → flange nut acceptable without additional washer
Material Grades, Mechanical Properties
& Surface Treatments
Zinc · HDG · Dacromet · Passivation · NACE MR0175
| Grade | Standard | Proof Load (MPa) | Hardness | Temp (°C) | Corrosion | Key Application |
|---|---|---|---|---|---|---|
| CS Class 8 | ISO 898-2 / DIN 6923 | 800 | 200–302 HV | −50 to +300 | Low | General industrial, automotive, structural |
| CS Class 10 | ISO 898-2 | 1040 | ≥272 HV | −50 to +250 | Low | High-strength structural, 10.9 bolt pairing |
| A194 Grade 2H | ASTM A194 | 1207 (175 ksi) | 235–300 HB | −29 to +399 | Low | ASME code piping & pressure vessel flange joints |
| A194 Grade 2HM | ASTM A194 | 1207 (175 ksi) | ≤235 HB (NACE) | −29 to +399 | Low | NACE MR0175 sour service, H₂S zones |
| SS 304 (A2-70) | ISO 3506-2 | 700 | ≤320 HV | −196 to +650 | High | Food, marine-adjacent, outdoor, soft substrate |
| SS 316L (A4-70) | ISO 3506-2 | 700 | ≤320 HV | −196 to +650 | Very High | Offshore, chloride environments, chemical plant |
| Duplex 2205 | ASTM A182 F51 | — | ≤310 HB | −50 to +300 | Very High | Offshore structural, sour service |
| Super Duplex | ASTM A182 F53 | — | ≤310 HB | −50 to +280 | Extreme | High-chloride subsea, severe sour service |
| Brass (CuZn37) | BS 2872 / ASTM B16 | — | ~120 HV | −50 to +180 | Good | Plumbing, marine, decorative, electrical panels |
| Inconel 625 | ASTM B564 | — | ≤241 HB | −200 to +980 | Extreme | High-temp, chemical process, turbine flanges |
Material Considerations for Soft and Dissimilar Substrates
Flange nuts are frequently specified on aluminium alloy substrates (6061, 6082, 7075) where a standard hex nut without a washer would embed into the aluminium bearing surface under preload, progressively losing joint clamping over time. A stainless steel 304 or 316L plain flange nut is typically specified on aluminium — not the serrated variant, as the serrations would gouge deeply into the softer aluminium surface on first tightening and may not provide adequate anti-rotation resistance thereafter due to the softness of the indented material. For dissimilar-metal corrosion prevention in wet environments, an isolating washer (nylon or PTFE) between the flange face and the aluminium substrate may be specified to break the galvanic couple.
Flange nuts in H≶S service zones must comply with NACE MR0175 / ISO 15156 maximum hardness of 22 HRC (237 HB) through the full cross-section including the flange. The flange section is thinner than the hex body and can develop higher hardness in some manufacturing processes (cold-forging of the flange). NACE A194 Grade 2HM flange nuts must be supplied with full cross-section hardness survey including a minimum of 9-point mapping on the flange face in addition to the hex body cross-section. Duplex 2205 and super duplex flange nuts are specified for subsea and severe sour service applications where both corrosion resistance and NACE compliance are required.
| Finish | Standard | Thickness (µm) | Salt Spray (hrs) | Max Temp (°C) | Notes & Application |
|---|---|---|---|---|---|
| Plain / Bare | — | — | <24 | — | Indoor dry; VCI for export |
| Black oxide + oil | MIL-DTL-13924 | 0.5–2 | 24–48 | 150 | Indoor machinery; consistent K-factor |
| Zinc electroplate SC3 | ASTM B633 SC3 | 13 min | 200 | 150 | Standard outdoor industrial; verify serration sharpness post-plate |
| Yellow chromate + ZP | ASTM B633 + ISO 4520 | 13–20 | 240 | 150 | Improved salt spray vs. clear ZP; general outdoor |
| Hot-dip galvanise | ASTM A153 Class C | 45–86 | 500+ | 200 | Outdoor structural; re-tap bore post-HDG; serrations obscured |
| Zinc-nickel alloy | ASTM B841 | 8–15 | 500–1000 | 200 | Offshore-adjacent, marine coastal; maintains serration geometry |
| Dacromet / Geomet | ISO 10683 | 8–12 | 720+ | 300 | High-strength; no H₂ embrittlement risk; low K-factor variation |
| SS 316L passivation | ASTM A380 | N/A | 1000+ | 870 | Offshore, food, pharma, chemical plant SS flange nuts |
| PTFE / Xylan | Whitford spec. | 15–30 | 400+ | 230 | Low friction (K=0.08–0.12); anti-galling SS-to-SS assemblies |
For serrated flange nuts: thick coatings (hot-dip galvanise 45–86 µm, heavy electroplate >20 µm) can fill the serration gullets and reduce locking effectiveness. Thin coatings (Dacromet 8–12 µm, zinc-nickel 8–15 µm) are preferred on serrated flange nuts to preserve serration geometry. Specify "serration geometry preserved post-coating" on purchase orders for serrated variants.
A serrated flange nut must not be re-used on the same bearing surface impression. On first tightening, the serrations displace the substrate material and create a matching indentation pattern. On re-tightening, the serrations seat back into the existing impressions — they do not re-cut the material — and provide substantially reduced locking resistance (estimated 30–60% of original). For any application where re-torquing or joint disassembly is anticipated during service, specify the plain flange nut variant. For safety-critical applications (structural, pressure vessel, offshore), single-use is mandatory: replace the serrated flange nut with a new unit at every re-assembly.
Inspection, QC Protocols, Applications
& Export Documentation
Automotive · Piping · Structural · Offshore · Soft Substrates
Inspection and Quality Control
For flange nuts, flange diameter is the most operationally significant dimension beyond thread gauging — it directly determines the bearing area and consequently the maximum permissible bearing stress on the substrate. Flange diameter is measured with calibrated ring gauges or a CMM, and must be verified against the DIN 6923 or ASME B18.2.2 tolerance on each batch. Flange flatness and concentricity with the thread bore are also verified: an eccentric flange shifts the centroid of bearing force off the bolt axis, creating an unintended bending moment in the bolt that is not accounted for in standard torque-tension calculations. Thread gauging (Go/No-Go per ISO 1502 or ASME B1.2) is performed on each batch.
For serrated flange nuts, serration sharpness, depth and geometry are inspected on each production batch using profile measurement (profilometer or optical comparator). Serrations that are too shallow (worn tooling) will not penetrate the mating substrate surface and provide no locking action. Serrations that are too deep may cause excessive substrate damage, surface cracking in brittle coatings, or galvanic crevice corrosion under the indented material in outdoor service. Surface coating thickness after electroplating or Dacromet treatment is verified on the flange face specifically — thick coating deposits in the serration gullets are identified and the batch rejected if the coating fills more than 30% of the serration depth.
Type 3.1: Minimum for all EPC, OEM and structural project supply. Must state material standard, grade, heat/lot number, chemical analysis, mechanical test results (proof load, hardness) and heat treatment condition. Flange diameter must be stated as a mandatory measured characteristic in the dimensional inspection record where bearing area is critical to the joint design. Type 3.2: Co-validated by purchaser-nominated TPI body. Mandatory for offshore, NACE, pressure vessel and structural safety-critical applications. For NACE A194 2HM flange nuts, full cross-section hardness mapping including the flange face is required as part of the 3.2 certificate documentation.
Applications by Industry
Serrated hex flange nuts in M6–M12 property class 8 or 10, carbon steel with zinc electroplate, are the highest-volume fastener in automotive body-in-white (BiW) and chassis fabrication. They are installed robotically in automated assembly cells at rates of hundreds of units per minute. The integrated flange eliminates a separate washer, reduces component count by 50% in every bolted joint, and the serrations provide the vibration locking required for automotive NVH (noise, vibration, harshness) performance targets without an additional lock washer component. Serrated flange nuts are specified per VDA 235-101 (German automotive industry standard for fastener locking performance) and IATF 16949 quality management.
Plain hex flange nuts per ASME B18.2.2 or DIN 6923 in carbon steel with hot-dip galvanise (ASTM A153) or Dacromet are used in structural connections, column base plates, portal frame rafter connections and steel-to-steel bolted joints where the increased bearing area reduces the local surface stress on the steel face, particularly important for ASTM A36 or S235 structural steel connections where flange contact stresses from small-face hex nuts can cause local yielding at design load.
Heavy hex flange nuts per ASTM A194 Grade 2H (or 2HM for sour service) are specified in ASME B31.3 process piping and ASME VIII pressure vessel bolted flange joints where the integrated flange face distributes bolt load over a larger bearing area on the raised face gasket or ring joint groove landing — critical for maintaining gasket seating stress uniformity across the flange circumference. EN 10204 3.2 MTC with TPI countersignature, PMI and full cross-section hardness survey are standard offshore project requirements.
Plain stainless steel 316L flange nuts on aluminium extrusion modular framing systems (Bosch Rexroth, Item, Misumi) provide the large bearing area needed to clamp components to the extrusion without over-stressing the softer aluminium profile face. The concentric flange of DIN 6923 is dimensionally matched to the standard T-slot extrusion profiles, making it a direct-fit component in these systems without requiring a separate washer.
Offshore and petrochemical structural framing, equipment base plate anchor bolting and cable tray connections use stainless steel 316L or duplex 2205 plain flange nuts with passivation (ASTM A380). In sour service H≶S zones, NACE A194 2HM heavy hex flange nuts with EN 10204 3.2 MTC, full cross-section hardness survey and PMI are project-standard requirements. Flange diameter must be confirmed against the substrate (flange face, base plate, structural section) bearing allowable as part of the bolting calculation per ASME B31.3 design code.
Export Packaging and Preservation
- Flange nuts packed in heat-sealed polypropylene bags, batch-labelled with PO number, type (DIN 6923 plain / serrated / ASME B18.2.2), grade, thread size, heat/lot number and quantity
- VCI (Volatile Corrosion Inhibitor) poly liner in each bag for plain and black-oxide-finished carbon steel flange nuts for sea freight or long-term storage
- Serrated flange nuts must not be packed flange-to-flange in bulk without separator layers — serration-to-serration abrasion damages serration tips and reduces locking effectiveness; use tissue paper or foam separators
- Bags packed in double-wall corrugated cartons with foam or kraft paper void fill
- Cartons secured on ISPM-15 heat-treated timber pallets with stretch wrap and minimum two steel strapping bands per pallet layer
- MTC, flange diameter inspection certificate, thread gauge certificate and all project documents in waterproof sealed envelope attached to crate exterior
- Outer crate labels: PO number, item tag, material grade, type (plain/serrated), standard, heat number, quantity, gross weight, country of origin
| # | Document | Standard / Reference | Minimum Requirement |
|---|---|---|---|
| 01 | Material Test Certificate (MTC) | EN 10204 3.1 / 3.2 | 3.2 for offshore / NACE / pressure vessel applications |
| 02 | Dimensional Inspection Report | DIN 6923 / ISO 4161 / B18.2.2 | AQL 1.0 per ISO 2859; flange diameter as mandatory measured characteristic |
| 03 | Thread Gauge Certificate | ISO 1502 / ASME B1.2 | Go/No-Go per heat lot for project supply |
| 04 | Serration Geometry Report | DIN 6923 / project spec | Profilometer trace — mandatory for serrated flange nuts in automotive / structural |
| 05 | Proof Load Test Report | ISO 898-2 / ASTM A194 | Per heat/lot — mandatory for all structural and code-compliance grades |
| 06 | Hardness Test Report | ISO 6507 / ISO 6506 | Cross-section mapping incl. flange face for NACE 2HM flange nuts |
| 07 | PMI Report (XRF / OES) | Project specification | 100% of SS, duplex, alloy and exotic grade flange nuts |
| 08 | Chemical Analysis Report | ISO 898 / ASTM A194 | Included in MTC — verify against grade limits |
| 09 | Heat Treatment Certificate | ISO 898-2 / ASTM A194 | Required for Q&T grades (Class 10, A194 2H/2HM) |
| 10 | Surface Coating Certificate | ASTM B633 / ISO 10683 / A380 | Required for all coated finishes; serration depth post-coat verification |
| 11 | ISO 9001 Manufacturer Certificate | ISO 9001:2015 | Current; scope must include flange nut manufacture |
| 12 | ISPM-15 Phytosanitary Certificate | IPPC / FAO | All wood packing for international export |
RR Hydraulics manufactures and exports flange nuts in all types — DIN 6923 plain and serrated, heavy hex (ASME B18.2.2), thin flange, extra-large flange and Nyloc flange — in carbon steel, stainless steel 304/316L, duplex & super duplex, brass and Inconel. Full EN 10204 3.1/3.2 MTC documentation, ISO 9001:2015 QMS, TPI witness by BV/DNV/Lloyds/SGS/TÜV accommodated. M5–M24 metric and 1/4"–1" inch. NACE MR0175-compliant A194 2HM flange nuts with full cross-section + flange face hardness mapping. 48-hour express dispatch on in-stock sizes.