Carbon steel flanges are key components of industrial pipe systems. They form the bolted joints that link pipes, valves, pumps, and tanks to create a sealed, pressure-rated assembly. Used in the oil and gas, petrochemical, power generating, shipbuilding, and water treatment industries, they are valued for their high strength, wide temperature tolerance, and cost effectiveness when compared to stainless or alloy steel alternatives.
This guide teaches engineers, procurement professionals, and project specifiers everything they need to know about carbon steel flanges, including the ASTM material grades, standard types, chemical and mechanical properties, applicable codes, and the key factors that determine which grade and type to specify for a given application.
What is a Carbon Steel Flange?
A carbon steel flange is a forged or cast disc with bolt holes around its circumference that is used to form a detachable, bolted connection between two pieces of a piping system or a pipe and a piece of equipment. The flange faces are sealed with a gasket (flat, raised face, ring type joint, or spiral wound) and secured with bolts and nuts to a predetermined torque.
Carbon steel, as specified by the AISI/SAE standard, is steel in which carbon is the primary alloying ingredient, with up to 2.1% carbon by weight. Low to medium carbon steels (0.10 to 0.35% C) are most commonly used in flange applications, since they provide a reasonable balance of strength, weldability, toughness, and machinability.
The term “CS flanges” or “carbon flanges” is commonly used in procurement and refers to flanges manufactured of carbon or carbon manganese steel, as opposed to stainless steel flanges, alloy steel flanges, or duplex flanges.
Carbon Steel vs Other Flange Materials
| Property | Carbon Steel Flanges | Stainless Steel Carbon Steel Flanges | Alloy Steel Carbon Steel Flanges |
| Cost | Low | High | Medium to High |
| Corrosion Resistance | Moderate (coating required) | Excellent | Good (service dependent) |
| Strength at Ambient Temp | Good | Good | Excellent |
| High Temp Performance | Good (up to 800°F) | Very Good | Excellent (up to 1200°F+) |
| Low Temp Performance | Limited (A350 LF2 needed) | Good | Good (with correct grade) |
| Weldability | Excellent | Good | Moderate |
| Best For | General industrial service | Corrosive environments | High temp or high pressure |
Carbon Steel Flange Grades and ASTM Standards.
The American Society for Testing and Materials (ASTM) publishes the primary material requirements for carbon steel flanges worldwide. The grade specifies the alloy composition, heat treatment, mechanical qualities, and service conditions that the flange may safely withstand.
ASTM A105 and A105N: Standard Grade.
ASTM A105 is the most commonly specified grade for forged carbon steel flanges, fittings, and valves used in pressure systems ranging from ambient to high temperatures. A105N denotes normalized heat treatment (heated above the transformation range and air cooled), which refines grain structure and increases toughness. Normalization is required for flanges larger than 2 inches in ASME B16.5 Class 600 and higher.
Service temperature range: -20°F to 800°F (-29°C to 427°C). Used in refineries, chemical facilities, steam systems, and other industrial piping.
ASTM A350 LF2 for Low Temperature Service
Standard A105 flanges cannot be used in plumbing systems with temperatures below -20°F (-29°C). Due to the steel’s ductile to brittle transition, it may break unexpectedly under cold temperatures. ASTM A350 LF2 is designed for low temperature duty at −50°F (−45°C), requiring Charpy V notch impact testing at −50°F to ensure notch toughness.
Applications include LNG processing plants, cryogenic storage, refrigeration systems, cold climate pipeline installations, and Arctic offshore platforms.
ASTM A694 F52–F80: High Yield Transmission
These grades are used for high-pressure oil and gas pipeline flanges that require strong yield strength. The designation (F52, F60, F65, F70, F80) refers to the minimum yield strength in ksi. A694 grades differ from A105 in that they are property-based rather than composition-based. The maker can vary the alloy additives to get the needed yield.
Applications include high-pressure natural gas transmission (ANSI 600# and above), subsea pipelines, offshore topsides, and large bore pipeline systems.
ASTM A182 F11 and F22, Alloy Steel for High Temperature Service
A182 F11 (1.25% Cr, 0.5% Mo) and F22 (2.25% Cr, 1% Mo) flanges are alloy steel, not pure carbon steel, and are specified for high temperature operation in power generating and refinery applications when carbon steel’s creep resistance is insufficient. They are typically purchased with carbon steel flanges and have the same fitting specifications.
Service temperature range: up to 1100°F (593°C). Used for steam turbine pipework, boiler headers, and catalytic reformer units.
Chemical Composition of A105 Carbon Steel Pipe Flanges.
| Element | Composition,% |
|---|---|
| Carbon (C) | 0.35max |
| Manganese (Mn) | 0.60–1.05 |
| Phosphorus (P) | 0.035max |
| Sulfur (S) | 0.040max |
| Silicon (Si) | 0.10–0.35 |
| Copper (Cu) | 0.40max |
| Nickel (Ni) | 0.40max |
| Chromium (Cr) | 0.30max |
| Molybdenum (Mo) | 0.12max |
| Vanadium (V) | 0.08max |
Mechanical Properties of ASTM A105
| Mechanical Properties | Requirements |
|---|---|
| Tensile strength,min,psi [MPa] | 70000 [485] |
| Yield strength,min,psi [MPa] | 36000 [250] |
| Elongation, min, % | 30 |
| Reductionofarea,min,% | 30 |
| Hardness,HB,max | 187 |
Flange Types and Connection Methods
The type of flange dictates how it joins to the pipe as well as the joint’s structural qualities. ASME B16.5 specifies the following standard types:
Weld Neck Flanges (WNRF)
A long tapered hub transitions to the thickness of the pipe wall. The hub is butt-welded to the pipe, resulting in a smooth bore for unrestricted flow and spreading mechanical and thermal load across the weld region. The ideal solution for critical, high-pressure, high-temperature, or cyclic loading applications.
Slip-on Flange (SORF)
Slips over the pipe and is fillet welded on both the interior and exterior surfaces. Installation is easier than with a weld neck since no bore machining is required, but it is significantly weaker under bending loads. Used in utility pipelines, water systems, and low to medium pressure service.
Blind Flanges (BLRF)
A solid disk with no bore is affixed to the end of a pipe, vessel nozzle, or valve to close off the system. Blind flanges are the most stressed flange type under internal pressure since the entire face is loaded, which is why they are usually the thickest flanges in their pressure class.
Socket Weld Flanges (SWRF)
The pipe end fits into a counter bore socket in the flange and is secured with a single fillet weld on the outside. Used in high-pressure applications for tiny bore pipework (usually 2 inches or less). Not recommended for cryogenic, high-temperature cyclic use, or applications with crevice corrosion issues.
Threaded Flange
Pipe threads are used to connect it to the pipe, eliminating the need for welding. The internal thread corresponds to conventional pipe threads (NPT or BSP). Suitable for low-pressure, non-hazardous, non-cyclical applications. The threaded joint’s vulnerability to leaking during temperature cycling limits its use in critical applications.
Lap Joint Flange
Used in conjunction with a stub-end fitting. The flange is flexible and free to spin, making bolt hole alignment easy. Common in systems that require frequent disassembly for inspection, as well as carbon steel systems with stainless or alloy stub ends to decrease expense.
Ring-Type Joint Flange (RTJ)
The face has a machined groove that accommodates a metal ring gasket (of oval or octagonal cross section). The ring deforms under bolt load, forming a metal-to-metal seal that outperforms spiral wrapped or flat gaskets. It is designed for high pressure, high temperature, and sour service (H2S-containing) applications and is standard in oil and gas wellhead and Christmas tree assemblies.
Forged vs Cast Carbon Steel Flanges
Most carbon steel flanges for pressure pipelines are forged rather than cast. Forging matches the grain structure of the steel with the geometry of the flange, resulting in higher mechanical qualities than casting. The distinctions are crucial in practice:
| Characteristic | Forged Carbon Steel | Cast Carbon Steel (Cast Iron) |
|---|---|---|
| Grain Structure | Oriented, refined by forging process | Random, may contain voids or inclusions |
| Tensile Strength | Higher: typically 70,000+ psi | Lower: typically 30,000 to 40,000 psi |
| Pressure Rating | Suitable for Class 150 to 2500 | Typically Class 125 or 250 only |
| Weldability | Good (low carbon content, A105) | Poor: cast iron is not weldable |
| Cost | Higher unit cost, lower lifecycle cost | Lower unit cost |
| Applications | All pressure piping: oil, gas, chemical, power | Low pressure non critical service, drainage |
For pressure systems, ASME B31.3 (Process Piping) and B31.1 (Power Piping) specify the use of forged flanges. Cast iron flanges are not authorized in hydrocarbon or steam applications.
Applicable Standards and Codes
Dimensional Standards
- ASME B16.5: Pipe flanges and fittings, NPS 1/2 inch to 24 inch, Classes 150 to 2500
- ASME B16.47: Large diameter steel flanges, NPS 26 inch to 60 inch, Series A (MSS SP 44) and Series B (API 605)
- ASME B16.48: Line blanks, NPS 1/2 inch to 24 inch
- MSS SP 44: Steel pipeline flanges
- DIN 2527, 2566, 2573, 2576, 2631 to 2638: German dimensional standards
- EN 1092-1: European flange standard (PN6 to PN100)
- BS 4504 and BS 10: British standards
- JIS B2220: Japanese Industrial Standard
- GOST 12820 and 33259: Russian standard
Material Standards
- ASTM A105 / ASME SA-105: Carbon steel forgings for piping
- ASTM A350 LF2: Low temperature carbon steel forgings
- ASTM A694: High strength carbon steel forgings for pipeline flanges
- ASTM A182: Alloy and stainless steel pipe flanges and fittings
- ASTM A516: Carbon steel plates for pressure vessels (used in plate flanges)
Corrosion Protection and Coatings
Carbon steel corrodes when exposed to moisture, oxygen, and corrosive process fluids. The following coatings are frequently used to protect flanges during storage, transportation, and in certain service environments:
Anti-rust oil or VCI (Vapour Corrosion Inhibitor) oil: Standard for storage and transportation.
Yellow passivation: A light chromate coating that enhances air corrosion resistance.
Electric zinc coating (galvanizing): White or yellow zinc for medium corrosion conditions.
Hot dip galvanizing: A thicker zinc coating for outdoor or damp applications.
For subterranean pipelines or chemical exposure, apply an epoxy or polyurethane coating.
Alloy cladding or lining: Used where process fluid corrosion needs noble material contact surfaces.
Carbon steel flanges are commonly used with internal lining in process pipework containing corrosive hydrocarbons, acids, or H2S, or they are replaced with stainless steel depending on the design’s corrosion tolerance.
How to Specify a Carbon Steel Flange: Quick Reference
When submitting a material requisition (MR) or purchase order for carbon steel flanges, the following details should be included:
Types include weld neck, slip on, blind, socket weld, threaded, lap joint, or RTJ.
Size: NPS or DN (for example, 4″ or DN100)
Pressure Class: ANSI 150#, 300#, 600#, 900#, 1500#, and 2500#, or DIN PN rating.
Standards: ASME B16.5 or B16.47 Series A or B
Material grade: ASTM A105N, A350 LF2, A694 F65, or equivalent
Face type: raised face (RF), ring type joint (RTJ), or flat face (FF).
Bore: full or reduced bore (check to match pipe schedule).
Heat Treatment: Normalized (N) for A105N, impact tested for A350.
Inspection and testing include PMI, hydrostatic, radiography, and third-party inspection.
Certifications required include EN 10204/3.1, NACE, PED, ATEX, and others as applicable.
Summary: Choosing the Right Carbon Steel Flange Grade
| Service Condition | Recommended Grade | Key Standard |
|---|---|---|
| General industrial, ambient to high temp | ASTM A105N | ASME B16.5 |
| Low temperature (−50°F to −20°F) | ASTM A350 LF2 | ASME B16.5 |
| High pressure oil and gas transmission | ASTM A694 F52 to F80 | ASME B16.47 / API 605 |
| High temperature power generation | ASTM A182 F11 / F22 | ASME B16.5 |
| Very high temperature (above 1100°F) | ASTM A182 F91 (CrMo) | ASME B16.5 |
This blog was created by Newzel Industries, a prominent carbon steel flange manufacturer and supplier in India. Contact our engineering team if you have any technical questions, unique specs, or need bulk supplies.
