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Weld Neck Flanges
A comprehensive engineering reference for EPC contractors, piping engineers, stress analysts and procurement teams — covering weld neck flange geometry, hub taper mechanics, bore schedule matching, ASME B16.5 dimensional data, P-T ratings, NACE compliance, material grades and full project documentation requirements.
Weld Neck Flange Geometry, Hub Taper Mechanics
& Engineering Advantages
Raised Face · RTJ · Flat Face · Bore Schedule
Definition and Engineering Advantages
A weld neck flange (WNF) is a pipe flange characterised by a long tapered hub that transitions from the flange body down to match the pipe wall thickness at the butt weld joint. The flange is welded to the pipe by a full-penetration butt weld — the same weld type used between pipe spools — making the weld neck flange the most structurally integrated and mechanically superior flange type in ASME B16.5 piping systems.
The tapered hub of the weld neck flange is its defining engineering feature. As the flange is pressurised or loaded, the hub distributes stress progressively from the flange body into the pipe wall. Without the hub taper, stress would be concentrated at the abrupt flange-to-pipe transition — the failure point of slip-on and socket weld flanges under cyclic or high-pressure loading. The gradual taper effectively eliminates this stress concentration, making the weld neck flange the preferred choice for high-pressure, high-temperature, cyclic, fatigue-sensitive and safety-critical piping applications.
The tapered hub of a weld neck flange acts as a stress transition zone. At the face of the flange, the cross-sectional area is large (the full flange body). As the hub tapers outward toward the pipe, the section area reduces progressively to match the pipe wall area at the butt weld. This progressive change in section means there is no abrupt stress concentration at the weld — the bending, pressure and thermal stresses in the pipe transition smoothly into the flange body. In contrast, a slip-on flange welded to the pipe with two fillet welds creates a stress riser at each weld toe — making slip-on flanges unsuitable for fatigue, cyclic and high-stress applications. The ASME B31.3 stress intensification factor (SIF) for a weld neck flange butt weld is i = 1.0 — the lowest of all flange types.
Weld Neck Flange Anatomy — Four Key Zones
Face Types — Engineering Descriptions
When to Specify a Weld Neck Flange
The weld neck flange is the correct specification in the following conditions, where other flange types (slip-on, socket weld, blind) are not adequate: (1) High-pressure service — Class 600 and above, where the bolt loads and pressure forces demand the full structural integrity of a butt-welded connection; (2) High-temperature service — above 260°C where creep, thermal expansion and PWHT considerations make the butt weld the only reliable connection method; (3) Cyclic or fatigue service — compressors, pumps, reciprocating machinery, thermal cycling applications where the stress intensification factor of slip-on flanges makes them susceptible to fatigue cracking; (4) Hazardous or toxic fluids — where ASME B31.3 Category M (highly toxic) or D (non-flammable, non-toxic above 345 kPa) requires weld neck flanges; (5) Below-ground or insulated piping — where the joint cannot be inspected in service and must be reliable for the full design life without maintenance.
ASME B16.5 Dimensional Data
& Pressure-Temperature Ratings
Flange OD · Bolt Circle · Bolt Holes · P-T Rating A105
Bore Matching — Critical Specification Requirement
The bore diameter at the weld end of a weld neck flange must be specified to match the inside diameter of the mating pipe. ASME B16.5 does not define the bore diameter — it is a purchaser-specified dimension that must match the applicable pipe schedule per ASME B36.10M (carbon and alloy steel) or B36.19M (stainless and duplex). A bore mismatch between the flange and the pipe creates a step at the weld root that acts as both a flow restriction (increasing turbulence and erosion) and a fatigue crack initiation site under cyclic loading. The acceptable mismatch tolerance per ASME B31.3 is ±1.6 mm for weld end alignment — however best practice is to specify the exact bore to match the pipe schedule and verify by dimensional inspection of each flange.
| NPS | Flange OD (mm) | Bolt Circle (mm) | No. Bolts | Bolt Size | RF Dia. (mm) | Flange Thick. (mm) | Hub OD at Base (mm) |
|---|---|---|---|---|---|---|---|
| ½" | 95.2 | 66.7 | 4 | M16 | 34.9 | 14.2 | 35.1 |
| 1" | 123.8 | 88.9 | 4 | M16 | 50.8 | 17.5 | 51.1 |
| 1½" | 155.6 | 114.3 | 4 | M20 | 73.0 | 19.1 | 73.2 |
| 2" | 165.1 | 127.0 | 8 | M16 | 92.1 | 22.4 | 92.2 |
| 3" | 209.5 | 168.3 | 8 | M20 | 127.0 | 28.4 | 127.3 |
| 4" | 254.0 | 200.0 | 8 | M20 | 157.2 | 31.8 | 157.6 |
| 6" | 317.5 | 269.9 | 12 | M20 | 215.9 | 35.1 | 216.3 |
| 8" | 381.0 | 330.2 | 12 | M24 | 269.9 | 41.1 | 270.5 |
| 10" | 444.5 | 387.3 | 16 | M24 | 323.8 | 47.8 | 324.6 |
| 12" | 520.7 | 450.8 | 16 | M27 | 381.0 | 50.8 | 381.9 |
ASME B16.5-2017. Class 300 RF dimensions. Raised face height 6.4 mm for Class 300 and above. RF finish: 125–250 AARH serrated for spiral-wound gaskets. Bolt sizes given as nearest metric equivalent to ASME bolt designation. For precise ASME bolt sizing (stud bolts per ASME B18.31.2), refer to the ASME B16.5 bolt tables directly. Hub OD at base is the maximum hub diameter at the flange back face — actual taper profile per B16.5 Appendix E.
| Temperature | Class 150 (bar) | Class 300 (bar) | Class 600 (bar) | Class 900 (bar) | Class 1500 (bar) | Class 2500 (bar) |
|---|---|---|---|---|---|---|
| −29 to +38°C | 19.6 | 51.1 | 102.1 | 153.2 | 255.3 | 425.4 |
| 100°C | 17.7 | 46.6 | 93.2 | 139.8 | 233.1 | 388.4 |
| 200°C | 13.8 | 43.8 | 87.6 | 131.4 | 219.0 | 365.1 |
| 300°C | 10.2 | 39.8 | 79.7 | 119.5 | 199.1 | 331.9 |
| 400°C | 6.6 | 31.0 | 62.0 | 93.0 | 155.0 | 258.3 |
| 450°C | 5.2 | 20.7 | 41.4 | 62.1 | 103.4 | 172.4 |
| 500°C | — | 13.8 | 27.6 | 41.4 | 68.9 | 114.9 |
ASME B16.5-2017 Table 2-1.1 (Material Group 1.1, A105). For alloy steel F11/F22/F91, stainless F304L/F316L and duplex F51, use the applicable ASME B16.5 material group tables — ratings differ significantly. All values in bar gauge. The pressure class designation (150, 300, etc.) is not the maximum allowable working pressure — it is a designation; the actual MAWP depends on the material group and temperature per the B16.5 tables.
A widely misunderstood aspect of ASME B16.5 flanges is the pressure class numbering. Class 150 does not mean a maximum allowable working pressure of 150 psi or 150 bar. The class number is a designation only — the actual MAWP of a Class 150 A105 carbon steel flange at ambient temperature is 19.6 bar (285 psi). At 400°C, it drops to only 6.6 bar (95 psi). When specifying flanges for any elevated temperature service, always look up the actual P-T rating from ASME B16.5 Table 2 for the specific material group — do not rely on the class number alone as a pressure limit.
Material Grades, Mechanical Properties
& NACE and Coating Requirements
Inconel 625 · Hastelloy C276 · NACE MR0175 · PWHT
| Material | ASTM Grade | Yield (MPa) | UTS (MPa) | Temp Range (°C) | Corrosion | Key Application |
|---|---|---|---|---|---|---|
| Carbon Steel | A105 N | ≥250 | ≥485 | −29 to +538 | Low | Standard process piping, utilities, general service |
| LTCS (−46°C) | A350 LF2 Cl.1 | ≥260 | ≥485 | −46 to +343 | Low | LNG, cryogenic, low-temperature process |
| LTCS (−59°C) | A350 LF3 | ≥260 | ≥485 | −59 to +343 | Low | Very low temperature, ethylene plant |
| 1.25Cr-0.5Mo | A182 F11 Cl.2 | ≥310 | ≥515 | −29 to +593 | Moderate | High-temp refinery, hydrogen service |
| 2.25Cr-1Mo | A182 F22 Cl.3 | ≥310 | ≥515 | −29 to +621 | Moderate | High-temp/pressure refinery, H₂ resistant |
| 9Cr-1Mo-V | A182 F91 | ≥585 | ≥760 | −29 to +649 | Moderate | Ultra-high-temp power gen., USC steam |
| SS 304/304L | A182 F304/F304L | ≥205 | ≥515 | −196 to +816 | High | Chemical, pharma, food, cryogenic |
| SS 316/316L | A182 F316/F316L | ≥205 | ≥515 | −196 to +816 | Very High | Offshore, chloride, chemical, pharma |
| SS 321 | A182 F321 | ≥205 | ≥515 | −196 to +816 | High | High-temp SS, sensitisation resistance |
| Duplex 2205 | A182 F51 | ≥450 | ≥620 | −50 to +315 | Very High | Offshore sour, seawater, chloride |
| Super Duplex | A182 F53/F55 | ≥550 | ≥750 | −50 to +300 | Extreme | Subsea, severe sour, high-chloride |
| Inconel 625 | A182 F625 | ≥414 | ≥827 | −196 to +980 | Extreme | High-temp corrosive, acid, CPI |
| Incoloy 825 | B564 N08825 | ≥241 | ≥586 | −196 to +540 | Extreme | Sulphuric/phosphoric acid, sour gas |
| Titanium Gr.2 | B381 F-2 | ≥275 | ≥345 | −196 to +315 | Extreme | Seawater, wet chlorine, desalination |
NACE MR0175 Compliance
Weld neck flanges in sour H≶S service must comply with NACE MR0175 / ISO 15156 hardness limits throughout the full cross-section. For carbon steel A105, the NACE maximum is 22 HRC (237 HB) — A105 in normalised condition typically complies, but every heat must be hardness-verified on the flange cross-section. For duplex 2205 (F51) and super duplex (F53), the ferrite/austenite microstructure must be verified by ferrite number measurement. Hardness mapping must include a minimum of five measurement points across the flange cross-section including the hub taper zone. All NACE hardness results are reported in the EN 10204 3.2 MTC.
PWHT Requirements — Alloy Steel Flanges
Alloy steel flanges in F11, F22 and F91 require post-weld heat treatment (PWHT) after butt welding to the pipe. For F91 (9Cr-1Mo-V), PWHT temperature control is critical — the 750–775°C normalise-and-temper cycle must be strictly controlled. PWHT below this range produces a non-compliant microstructure with inadequate creep resistance. Hardness verification post-PWHT is mandatory for F22 and F91 in NACE sour service.
Inspection, QC Protocols, Applications
& Export Documentation
Petrochemical · Offshore · Power · Refinery · Cryogenic
Inspection and Quality Control
Every weld neck flange must be dimensionally inspected to confirm compliance with ASME B16.5 for the specified NPS, pressure class and face type. Critical dimensions include: flange OD, bolt circle diameter, bolt hole diameter and number, raised face diameter, raised face height, flange body thickness, hub OD at base, bore diameter at weld end (to match specified pipe schedule), and bore perpendicularity to the flange face. The bore diameter at the weld end is a purchaser-specified dimension that must be confirmed against the applicable pipe schedule — this is the most commonly missed dimensional check in flange inspection.
The raised face finish must be verified using a surface profilometer — 125–250 AARH (125–250 Ra micro-inch, equivalent to 3.2–6.3 µm Ra) serrated (phonographic) finish for spiral-wound gaskets is standard per ASME B16.5. A finish that is too smooth (below 125 AARH) does not provide adequate gasket seating stress in the spiral-wound gasket seating radial serrations; a finish that is too rough may cause the soft gasket layers to shear off rather than compress. For RTJ grooves, groove surface finish Ra ≤ 1.6 µm and groove hardness greater than the ring gasket hardness must be verified.
NDE requirements for weld neck flanges vary with the service class and material. Standard minimum NDE: (1) Magnetic particle testing (MT) per ASME V Article 7 for the raised face and hub surfaces — verifying freedom from surface cracks and laps; (2) Liquid penetrant testing (PT) for austenitic stainless, duplex and non-ferromagnetic materials where MT is not applicable. For severe service, heavy wall or high-alloy flanges, ultrasonic testing (UT) per ASME V Article 5 is added for volumetric examination of the hub and flange body. For safety-critical applications (Class 900 and above, or any Category M fluid service per ASME B31.3), radiographic examination of the butt weld after installation is mandatory.
Applications by Industry
Weld neck flanges are the standard flange type for all high-pressure, high-temperature and hydrogen service piping in refineries and petrochemical plants. A105 normalised for carbon steel utility piping; A182 F11 and F22 for hydrocracker, desulphuriser and reformer hydrogen service; A182 F304L and F316L for chemical and corrosive service; duplex F51 for sour service with chloride — the entire pressure-containing piping system in a refinery or chemical plant uses weld neck flanges at virtually every flange connection above Class 300 and in any hazardous fluid service.
All high-integrity flange connections on offshore platforms — process piping, wellhead Christmas tree pipework, riser flanges, subsea production headers — are weld neck flanges in duplex 2205 (F51), super duplex (F53), SS 316L or Inconel 625 depending on the service. NACE MR0175 compliance, EN 10204 3.2 MTC with TPI (third-party inspection) countersignature, PMI, full cross-section hardness mapping, ferrite content measurement (duplex grades), Charpy impact testing, UT/MT NDE, and PWHT certificates (alloy steel) are standard offshore project documentation requirements for every flange.
High-pressure steam piping in power plants — main steam, hot reheat, cold reheat and feed water lines — uses weld neck flanges in A182 F22 (2.25Cr-1Mo) for subcritical steam, and A182 F91 (9Cr-1Mo-V) for ultra-supercritical steam at temperatures up to 649°C. The F91 flange requires strict PWHT control, hardness verification post-PWHT, and Larson-Miller parameter verification to confirm the heat treatment produced the required creep properties. Fatigue analysis of the flange connection is mandatory per ASME VIII Div.2 for high-cycle thermal transient service.
A350 LF2 (impact tested to −46°C) and A350 LF3 (to −59°C) weld neck flanges are specified for LNG loading arms, cryogenic storage tank connections, ethylene plant cold box piping and CO₂ service. Charpy impact testing at the specified MDMT is mandatory — minimum 27 J (20 ft-lb) at test temperature per the applicable ASME standard. All dimensions and impact test results are reported in the EN 10204 3.2 MTC.
Export Packaging and Preservation
- Weld neck flanges individually wrapped in VCI polyethylene film for carbon steel and alloy steel grades; stainless, duplex and exotic grades in clean poly bags (segregated from carbon steel)
- Raised face protected with cardboard or foam face cover; RTJ groove with conforming plastic insert to prevent groove damage; bore sealed with plastic plug
- Flanges packed on timber cradles in wooden crates with neck supported at two points to prevent bending stress during transport; ISPM-15 heat-treated timber
- Each flange tagged with heat number, material grade, NPS, pressure class, face type, bore, and PO number
- MTC (EN 10204 3.1 or 3.2), dimensional inspection report (bore, OD, bolt circle, face dimensions, AARH), NDE reports (UT/MT/PT), Charpy impact, hardness survey, PMI, ferrite content (duplex), PWHT certificate, and all project documents in waterproof sealed envelope in each crate
- Crates on ISPM-15 pallets with stretch wrap and steel strapping; gross/net weight, project number, tag number and country of origin labelled
| # | Document | Standard / Reference | Minimum Requirement |
|---|---|---|---|
| 01 | Material Test Certificate (MTC) | EN 10204 3.1 / 3.2 | 3.2 (TPI co-signed) for offshore / NACE / alloy / safety-critical |
| 02 | Dimensional Inspection Report | ASME B16.5 | Bore ID, OD, bolt circle, face dia., RF height, flange thickness — all mandatory |
| 03 | Raised Face / RTJ Groove Report | ASME B16.5 / B46.1 | 125–250 AARH for RF; RTJ groove dimensions and ring number confirmed |
| 04 | MT / PT Surface Exam Report | ASME V Art.7 / EN 10228-1 | MT for CS/alloy; PT for SS/duplex/exotic; raised face and hub mandatory |
| 05 | UT Volumetric Exam Report | ASME V Art.5 / EN 10228-3 | Mandatory for Class 900+, alloy steel, duplex and heavy-wall flanges |
| 06 | Hardness Test Report (HB/HRC) | ASTM E10 / E18 | NACE: full cross-section ≤22 HRC; PWHT: range confirmation for F22/F91 |
| 07 | Charpy Impact Test Report | ASTM A370 / EN ISO 148-1 | Mandatory for A350 LF2/LF3; ≥27 J at MDMT; test temp per design |
| 08 | PMI Report (XRF / OES) | Project specification | 100% SS, duplex, alloy steel and all exotic grade flanges |
| 09 | Ferrite Content Report (Duplex) | ASTM E562 / image analysis | Mandatory for duplex 2205 and super duplex; FN 35–65% required |
| 10 | Heat Treatment Certificate | ASME VIII / ASTM A182 | Required for all forged flanges; N, N+T or Q+T as applicable per material |
| 11 | PWHT Certificate (Alloy Steel) | ASME B31.3 / ASME VIII | Required for F11/F22/F91 welds; temperature, hold time and hardness confirmed |
| 12 | ISO 9001 Manufacturer Certificate | ISO 9001:2015 | Current; scope must include pressure flange forging and machining |
| 13 | ISPM-15 Phytosanitary Certificate | IPPC / FAO | All wood packing for international export |
RR Hydraulics manufactures and exports weld neck flanges in all face types (RF, FF, RTJ) and pressure classes (150–2500) per ASME B16.5 and B16.47, in A105, A350 LF2/LF3, A182 F11/F22/F91, A182 F304L/F316L/F321, A182 F51/F53/F55, Inconel 625, Hastelloy C276, Incoloy 825, Monel 400 and Titanium Grade 2. NPS ½"–24". Bore schedule-matched per B36.10/B36.19. Full EN 10204 3.2 MTC, UT/MT/PT NDE, Charpy impact, NACE hardness mapping, PMI, ferrite content (duplex), PWHT certificates and TPI witness by BV/DNV/Lloyds/SGS/TÜV. 48-hour express dispatch on standard in-stock sizes.