The flange type determines how a carbon steel flange connects to the pipe, how it handles pressure, and how easily it installs. For example, A105 material can be machined into six different types, and each of them is suited to a different service condition. This guide covers all six common carbon steel flange types, how each connects, where it is used, and the pressure range it can withstand.
! Each type is dimensioned to ANSI/ASME B16.5 (NPS ½” through 24″) or ANSI/ASME B16.47 (NPS 26″ through 60″, weld neck and blind only), with waterworks flanges covered by AWWA C207 (up to 300 psi). Most are produced in ASTM A105 forged carbon steel for pressure service.
1. Weld Neck Flanges
Weld neck flanges provide the highest structural integrity. They have a long tapered hub that gradually transitions from the flange thickness down to the pipe wall thickness. The pipe is joined to the hub with a full-penetration butt weld. That tapered hub is the defining feature. It helps distribute stress from the flange into the pipe, reducing stress concentration at the base of the flange.
Common Service Conditions:
- High-pressure and high-temperature process piping
- Cyclic loading and thermal expansion conditions
- Critical service in refineries, chemical plants, and power generation
- All pressure classes from 150 through 2500
2. Slip-On Flanges
Slip-on flanges are the most commonly specified configuration for general-service piping. The pipe slides through the flange bore, and the flange is secured with two fillet welds: one on the inside of the bore and one on the outside face. Because the bore is slightly larger than the pipe outside diameter, alignment is simpler than with weld neck flanges.
Common Service Conditions:
- General-service piping at lower and moderate pressure
- Class 150 through 600 applications
- Water systems, utilities, and non-critical process lines
- Projects where ease of alignment and lower cost are priorities
3. Blind Flanges
Blind flanges are solid discs with no center bore. They bolt onto the end of a pipe, a nozzle, or a vessel opening to close it off completely. In pressure service, a blind flange carries the full system pressure across its entire face, so it is engineered to the same pressure class as the rest of the joint. Blind flanges can be unbolted and removed, they allow access to a piping system without cutting pipe.
Common Service Conditions:
- Terminating the end of a pipe run, nozzle, or vessel
- Isolating a section of piping during maintenance or testing
- Future expansion and tie-in points
- All pressure classes
4. Socket Weld Flanges
Socket weld flanges are used on small-bore piping, typically NPS ½” through 3″. The pipe is inserted into a recessed socket bore and secured with a single fillet weld around the outside. The socket positions the pipe accurately and produces a strong, leak-resistant joint without the need for a full butt weld. A small expansion gap is left at the bottom of the socket to accommodate thermal expansion and prevent weld cracking.
Common Service Conditions:
- Small-bore piping, NPS ½” through 3″
- High-pressure instrumentation and chemical injection lines
- Hydraulic and steam systems in small diameters
- Applications where a smooth bore and strong joint are required
5. Threaded Flanges
Threaded flanges connect to pipe through a tapered thread in the bore, with no welding required. The pipe simply screws into the flange. This makes them well suited to applications where welding is impractical, prohibited, or unsafe, such as flammable-atmosphere environments where hot work is restricted. They are not recommended for systems with thermal cycling or vibration, which can loosen the threaded connection over time.
Common Service Conditions:
- Low-pressure, non-critical service (Class 150 through 300)
- Environments where welding is restricted or impractical
- Utility and low-pressure water or gas lines
- Small to moderate pipe sizes
6. Lap Joint Flanges
Lap joint flanges are used together with a stub end. The stub end is butt-welded to the pipe, and the lap joint flange slips over it and rotates freely. Because the flange is not fixed to the pipe, it can be turned to align bolt holes with the mating flange. The free-rotating design also allows a carbon steel lap joint flange to be paired with a corrosion-resistant stub end.
Common Service Conditions:
- Systems requiring frequent disassembly and reassembly
- Applications where bolt hole alignment is difficult
- Corrosive service paired with alloy or lined stub ends
- Low to moderate pressure
Carbon Steel Flange Face Types
Independent of the flange type, every flange has a face that determines how it seals against its mating component. The three standard face types for carbon steel flanges are:
- Raised Face (RF) — the most common face type for carbon steel flanges, used across all pressure classes from 150 through 2500.
- Flat Face (FF) — used when connecting to cast iron or non-metallic equipment to prevent flange distortion, in applications up to 300 psi.
- Ring-Type Joint (RTJ) — a metal ring gasket seated in a machined groove, used in high-pressure and high-temperature service.
Face type and surface finish must be compatible with the mating flange and the gasket. See flange facing options and how to choose for full detail.
Carbon Steel Flange Types: Summary Table
| Flange Type | How It Connects | Typical Use | Pressure Suitability |
| Weld Neck (WN) | Butt-welded to pipe via tapered hub | Critical and high-pressure service | High — all pressure classes |
| Slip-On (SO) | Pipe slides through bore; fillet welded inside and out | General service, lower pressure | Low to moderate (Class 150–600) |
| Blind (BL) | Solid disc; no bore | Pipe termination, isolation | All pressure classes |
| Socket Weld (SW) | Pipe inserted into socket and fillet welded | Small-bore, high-pressure lines | High (NPS ½ – 3) |
| Threaded (TH) | Threaded connection; no welding required | Low-pressure, non-critical service | Low (Class 150–300) |
| Lap Joint (LJ) | Used with stub ends; flange rotates freely | Systems requiring frequent disassembly | Low to moderate |
Conclusion
Carbon steel flange types each solve a specific connection problem. Weld neck for critical and high-pressure service, slip-on for general piping, blind for termination, socket weld for small-bore high-pressure lines, threaded for no-weld low-pressure service, and lap joint for systems that come apart often. Matching the type to the service condition is what produces a reliable, code-compliant joint.
API International supplies all six flange types in carbon steel, fully machined to ASTM A105 material requirements and dimensioned per ANSI/ASME B16.5, ANSI/ASME B16.47, and AWWA C207. Explore our full range in the online product catalog, or contact us for custom machining on nonstandard sizes and configurations. Get connected with a dedicated sales representative today, or call us at 503.692.3800.
Related Products
-
AWWA Class F Steel Ring Flanges
This product has multiple variants. The options may be chosen on the product page -
AWWA Class F Steel Blind Flanges
This product has multiple variants. The options may be chosen on the product page -
AWWA Class E Steel Hub Ring Flanges
This product has multiple variants. The options may be chosen on the product page -
AWWA Class D Steel Hub Ring Flanges
This product has multiple variants. The options may be chosen on the product page
Frequently Asked Questions
What is the most common type of carbon steel flange?
Slip-on flanges are the most commonly specified type for general-service piping because they are easy to align and economical to produce. Weld neck flanges are the most common choice for high-pressure and critical service.
What grades of carbon steel are used in flanges?
The most common grades are ASTM A105 (forged, for pressure piping) and ASTM A36 (plate, for waterworks and structural service). Higher-strength applications use ASTM A694 for oil and gas pipelines, and ASTM A516 for pressure vessel connections. See our guide to carbon steel flange grades for a full breakdown.
What is the difference between carbon steel and stainless steel flanges?
Carbon steel flanges contain carbon as the main alloying element and offer strength and weldability at a lower cost, but they require coatings or treatment for corrosion resistance. Stainless steel flanges contain chromium and nickel, giving them built-in corrosion resistance for chemical, marine, and sanitary service. Dimensions are identical for a given standard and class; the difference is material performance and cost.
What standards apply to carbon steel flanges?
Carbon steel flanges are governed by material specifications from ASTM and dimensional standards from ASME and AWWA. ANSI/ASME B16.5 covers all flange types from NPS ½″ through 24″, ANSI/ASME B16.47 covers large-diameter weld neck and blind flanges from NPS 26″ through 60″, and AWWA C207 covers waterworks flanges up to 300 psi.
How do I choose the right flange type?
Match the flange type to the service condition. Use weld neck for high-pressure and critical service, slip-on for general lower-pressure piping, blind for terminating a line, socket weld for small-bore high-pressure lines, threaded where welding is impractical, and lap joint for systems that require frequent disassembly. Pressure, temperature, pipe size, and maintenance needs all factor into the choice.