API 605 Class 400 Weld Neck Flanges

API 605 Class 400 Weld Neck Flanges are large-bore, high-integrity flanges covering Nominal Pipe Sizes (NPS) 26 through 60, manufactured to the pressure class that sits between Class 300 and Class 600 in the API 605 rating system. API 605 — originally published as the American Petroleum Institute Standard for Large Diameter Carbon Steel Flanges — governed flanges too large for ASME B16.5 (which tops out at NPS 24). The standard has since been withdrawn and its dimensional requirements absorbed into ASME B16.47 Series A, but API 605 remains deeply embedded in older piping specifications, procurement documents, and replacement-part orders for existing plant — making it an active commercial designation. Class 400 carries an approximate working pressure of 990 psi (68 bar) at ambient temperature for carbon steel ASTM A105 material, providing a useful intermediate rating for transmission pipelines, large-bore crude oil headers, and offshore separation systems where Class 300 is marginal and Class 600 is over-specified.

Tesco Steel & Engineering manufactures API 605 Class 400 Weld Neck Flanges in the full NPS 26–60 size range across all three face types — Raised Face (WNRF), Flat Face (WNFF), and Ring Type Joint (WNRTJ) — in carbon steel ASTM A105, low-temperature carbon steel ASTM A350 LF2, alloy steels ASTM A182 F11/F22, stainless steels ASTM A182 F304/F316/F321/F347, duplex UNS S32205, super duplex UNS S32750, and nickel alloys (Monel 400, Inconel 625, Hastelloy C-276). Every flange is CNC-machined to API 605 and ASME B16.47 Series A dimensional tolerances, hydrostatically tested, and supplied with full material test reports (MTRs), dimensional inspection reports, and third-party inspection (TPI) clearance as required. Tesco exports API 605 Class 400 weld neck flanges to oil and gas operators, EPC contractors, and piping distributors in 96 countries.

API 605 Class 400 Weld Neck Flange — Key Specifications

API 605 Class 400 Weld Neck Flange Dimensions

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All dimensions in millimetres (mm) unless otherwise noted. Dimensions per API 605 / ASME B16.47 Series A.

What Is API 605? Relationship to ASME B16.47 Series A

API Standard 605, Large Diameter Carbon Steel Flanges (26 through 60 inch Nominal Pipe Size), was published by the American Petroleum Institute to address flanges that exceeded the NPS 24 upper limit of ASME B16.5. It defined six pressure classes — Class 75, 150, 300, 400, 600, and 900 — with dimensional tables covering NPS 26 through NPS 60.

API 605 was withdrawn as a standalone standard after its bolt patterns and dimensions were incorporated into ASME B16.47 Series A. The two are dimensionally identical: if a project specification calls for API 605 Class 400 flanges, the correct supply standard is ASME B16.47 Series A Class 400, and these flanges are interchangeable with legacy API 605 components.

Important: ASME B16.47 Series A and Series B flanges are NOT interchangeable. Never mix Series A (API 605 pattern) and Series B (MSS SP-44 pattern) flanges in the same joint — the bolt hole patterns and flange ODs differ.

API 605 Class 400 Pressure–Temperature Ratings

Pressure ratings decrease with increasing temperature. The table below shows Class 400 working pressures across the most common material groups per ASME B16.47 (equivalent to API 605).

Values are indicative per ASME B16.47. Consult the applicable code and material group for final design. Duplex 2205 ratings drop significantly above 300°C — consult your materials engineer for elevated-temperature duplex service.

Material Grades Available

API 605 Class 400 Weld Neck Flange — Face Types

Applicable Standards & Codes

Why Specify Weld Neck Flanges for Large-Bore API 605 Service?

Applications of API 605 Class 400 Weld Neck Flanges

API Weld Neck Flange — Full Series

Frequently Asked Questions — API 605 Class 400 Weld Neck Flanges

What is an API 605 Class 400 flange?

An API 605 Class 400 flange is a large-bore pipe flange (NPS 26 to NPS 60) rated to approximately 990 psi working pressure at ambient temperature in carbon steel ASTM A105 material. Class 400 is the intermediate pressure class in the API 605 standard, sitting between Class 300 (~740 psi) and Class 600 (~1480 psi). API 605 has been withdrawn as a standalone standard; its dimensions were absorbed into ASME B16.47 Series A, which is the current equivalent standard.

What is the difference between API 605 and ASME B16.47 Series A?

API 605 and ASME B16.47 Series A are dimensionally identical — same bolt patterns, same flange ODs, same pressure classes, same NPS range (26–60). API 605 was an API-published standard that was withdrawn after its content was incorporated into ASME B16.47 Series A. If a legacy piping spec calls for API 605 Class 400, supply ASME B16.47 Series A Class 400 — the flanges are fully interchangeable.

What is the difference between ASME B16.47 Series A and Series B?

ASME B16.47 Series A (derived from API 605) and Series B (derived from MSS SP-44) are NOT interchangeable. Series A has a larger flange OD and a different bolt hole pattern than Series B for the same NPS and pressure class. Always verify which series is specified before ordering — mixing Series A and Series B flanges in the same bolted joint is a serious engineering error.

What is the working pressure of an API 605 Class 400 flange?

At ambient temperature (–20°F to 100°F), an API 605 Class 400 weld neck flange in ASTM A105 carbon steel is rated to approximately 990 psi (68 bar). This decreases at elevated temperatures — approximately 845 psi at 500°F (260°C) and 740 psi at 700°F (371°C). For stainless steel grades, ratings may be slightly different; always consult the ASME B16.47 pressure-temperature tables for your specific material group.

What NPS (pipe size) range does API 605 Class 400 cover?

API 605 — and its equivalent ASME B16.47 Series A — covers Nominal Pipe Sizes NPS 26 through NPS 60. This is specifically the large-bore range that begins where ASME B16.5 ends (at NPS 24). For NPS 24 and below, Class 400 weld neck flanges fall under ASME B16.5, not API 605 / B16.47.

Why are weld neck flanges preferred for Class 400 large-bore service?

Weld neck flanges are preferred for Class 400 large-bore pipelines because their long, tapered hub distributes bending and axial loads gradually from the flange body into the pipe wall — avoiding the stress concentrations inherent in slip-on or socket weld flanges. The single full-penetration butt weld is also fully radiographable (RT) for quality verification, which is mandatory under ASME B31.4 (liquid pipelines) and B31.8 (gas transmission) for high-pressure large-bore lines.

Which materials are available for API 605 Class 400 weld neck flanges?

API 605 Class 400 weld neck flanges are available in: carbon steel (ASTM A105), low-temperature carbon steel (ASTM A350 LF2/LF3), Cr-Mo alloy steel (ASTM A182 F11, F22), stainless steel (ASTM A182 F304, F316, F316L, F321, F347), duplex (F51/UNS S32205), super duplex (F53/UNS S32750), and nickel alloys including Monel 400, Inconel 625, and Hastelloy C-276 per ASTM B564. The material choice depends on the fluid service, temperature, and corrosion environment.

When should I specify RTJ (Ring Type Joint) face for API 605 Class 400 flanges?

Specify RTJ face (WNRTJ) when a metal-to-metal seal is required — typically for high-pressure gas service, high-temperature service above 450°C, cyclic or pulsating pressure systems, or subsea applications where gasket blowout is a critical risk. RTJ uses API ring gaskets (octagonal or oval profile, R or RX type) seated in machined grooves. The RTJ joint is more leak-resistant than a spiral-wound raised face gasket but requires more precise flange alignment and is harder to disassemble in the field.

When should I choose Class 400 instead of Class 300 or Class 600?

Choose Class 400 when your design pressure exceeds the Class 300 rating (~740 psi for A105 at ambient) but doesn't justify the weight and cost penalty of Class 600 (~1480 psi). Class 400 is most commonly used in crude oil and gas pipelines where pump or compressor discharge pressures fall in the 800–950 psi design pressure range. If your design pressure is close to the Class 300 limit, Class 400 provides the necessary safety margin without the significant added weight of Class 600 large-bore flanges.

Can duplex or super duplex be used for API 605 Class 400 flanges?

Yes. Duplex 2205 (ASTM A182 F51, UNS S32205) and Super Duplex 2507 (F53, UNS S32750) are fully available in API 605 / ASME B16.47 Series A Class 400 weld neck flanges for offshore, subsea, and high-chloride service. Duplex F51 has a higher ambient pressure rating (~1170 psi at ambient) than carbon steel A105 (~990 psi) due to its higher yield strength. Important caveat: duplex steel should not be used above ~280–300°C as sigma phase embrittlement becomes a risk.

How does Tesco manufacture API 605 Class 400 weld neck flanges?

Tesco Steel & Engineering manufactures API 605 Class 400 weld neck flanges by forging pre-cut billets on hydraulic presses followed by solution annealing (for stainless and alloy grades), CNC rough turning, bore drilling and facing, CNC finish machining to API 605 / B16.47 Series A tolerances, hydrostatic pressure testing at 1.5× working pressure, and dimensional and visual inspection. All flanges are supplied with EN 10204 3.1 mill test reports, hydrostatic test certificates, heat treatment records, and dimensional inspection reports. Third-party inspection by SGS, Bureau Veritas, or Lloyd's Register is arranged on request.

API 605 Class 400 Weld Neck Flange Price & Inquiry

Pricing for API 605 Class 400 weld neck flanges depends on the NPS size, material grade, face type, and quantity. Carbon steel ASTM A105 flanges in standard sizes (NPS 26–42) are available from stock for fast delivery. Larger sizes (NPS 44–60) and alloy or stainless steel grades are manufactured to order with typical lead times of 3–6 weeks depending on size and material.

To receive a competitive quotation, please share:

• Standard: API 605 / ASME B16.47 Series A, Class 400
• NPS (size) and quantity
• Material grade (A105 / A350 LF2 / A182 F316 / Duplex / other)
• Face type (RF / FF / RTJ)
• Bore size (schedule / wall thickness)
• Inspection and documentation requirements (TPI, 3.1 TC, RT, etc.)

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Global Export: Tesco Steel & Engineering exports API 605 Class 400 Weld Neck Flanges to 96 countries including the USA, UK, Germany, France, Netherlands, Italy, Saudi Arabia, UAE, Qatar, Kuwait, Oman, Bahrain, Iraq, Singapore, Malaysia, Thailand, Indonesia, Vietnam, South Korea, Japan, Australia, Canada, Brazil, Mexico, South Africa, Nigeria, Kenya, Egypt, Turkey, Kazakhstan, Russia, and across the EU, Middle East, Southeast Asia, Africa, and the Americas. All exports are supported by full documentation: EN 10204 3.1 mill test reports, hydrostatic test certificates, dimensional inspection reports, and country-of-origin declarations.