Selecting a carbon steel flange grade is one of the most important decisions in industrial piping procurement. The grade determines whether a flange can handle the system’s pressure, how it performs at operating temperature, and how it aligns with specific industry dimensional standards. This guide covers the four most common carbon steel flange grades in industrial use: ASTM A36, ASTM A105, ASTM A694, and ASTM A516.
What Determines a Carbon Steel Flange Grade?
Carbon steel is defined primarily by its carbon content, which ranges from 0.05% to 2.1% by weight. That percentage governs tensile strength, hardness, ductility, and weldability. Grades in the mild/low carbon range (up to 0.30%) weld easily and are highly ductile, but have lower strength limits. Grades in the medium carbon range (0.30–0.60%) offer higher strength and perform under sustained pressure and elevated temperature.
Beyond carbon content, grade selection is driven by three additional factors:
- Product form. Forged flanges have denser grain structure and higher consistent strength than plate-cut flanges. ASTM A105 and A694 are forged grades; A36 and A516 are plate grades.
- Service conditions. Operating pressure, temperature range, and the type of media in the line all affect which grade is appropriate.
- Code compliance. ASME B31.3 (process piping) and B31.1 (power piping) standards specify which grades qualify for pressure service.
Types of Common Carbon Steels Used for Flanges
| ASTM Grade | Product Form | Carbon % | Primary Service | Key Standard |
| A36 | Plate / structural | 0.29% max (varies by thickness) | Low-temp, non-pressure, structural | ASTM A36/A36M |
| A105 | Forging | 0.35% max | Ambient to high-temp pressure piping | ASTM A105/A105M |
| A694 | Forging | Varies by grade (F42–F70) | High-pressure oil and gas pipelines | ASTM A694/A694M |
| A516 | Plate | 0.31% max (varies by grade and thickness) | Pressure vessels, moderate temperature | ASTM A516/A516M |
ASTM A36 — Mild Carbon Steel Plate Flanges
ASTM A36 is the most widely produced structural steel grade in the United States. In flange applications, A36 is used as a plate material, not a forging. Plate flanges cut from A36 stock serve non-pressure and low-pressure systems (up to 300psi) where ASME piping code compliance is not required. The grade does not qualify under ANSI/ASME B16.5 for pressure-rated service, but it is widely used and practically suited for waterworks, HVAC, and large-diameter structural connections.
Key mechanical properties of A36 carbon steel:
- Yield strength: 36,000 psi (250 MPa) minimum
- Tensile strength: 58,000–80,000 psi (400–550 MPa)
- Elongation: 20% minimum (in 8 inches)
- Carbon content: ≤ 0.25% (for plates up to ¾ inch thick)
Where A36 Flanges Are Used
AWWA C207 governs steel pipe flanges in waterworks service up to 300 psi. A36 plate flanges are the standard material for the following applications: municipal water mains, treatment plants, and irrigation infrastructure.
For AWWA configurations above 24 inches where forged blanks are unavailable off the shelf, custom-made flanges cut from A36 plate are the standard fabrication approach. Lead times stay short because A36 is one of the most stocked plate grades in domestic steel distribution.
Other common applications:
- HVAC and industrial ventilation — large duct connections and air handling equipment
- Structural and agricultural connections — irrigation mainlines, pipe supports, and non-pressure structural pipe frames
For government infrastructure projects, domestically sourced A36 satisfies Buy American and BABA requirements. See API International’s domestic flange options for compliant supply.
What A36 Is Not Suited For
Because A36 is primarily a structural specification, it is not suited for high-pressure or high-temperature process piping. While piping codes like ASME B31.3 technically permit A36 under highly restricted, low-pressure conditions, it lacks the pressure-temperature ratings required to qualify under standard ASME B16.5 flange tables. If your project is governed by a piping code, A105 forged flanges are the required specification. Substituting A36 in a pressure application is a specification error that will be identified during code review or third-party inspection.
ASTM A105 — Medium Carbon Forged Flanges
ASTM A105 is the standard specification for carbon steel forgings used in pressure piping. It covers flanges, fittings, valves, and similar parts which are made for ambient and elevated temperature service. A105 is the material referenced in ANSI/ASME B16.5 and ANSI/ASME B16.47 standards which govern the majority of pressure-rated carbon steel pipe flanges produced worldwide.
The forging process gives A105 a key structural advantage over plate-cut materials. When steel is forged, the steel aligns and compresses the material’s grain structure to follow the contour of the component. This continuous, directional grain flow produces a stronger, tougher, and more uniform material capable of handling severe pressure cycles far better than standard plate stock.
Key mechanical properties of A105 carbon steel:
- Yield strength: 36,000 psi (250 MPa) minimum
- Tensile strength: 70,000 psi (485 MPa) minimum
- Elongation: 22% minimum
- Carbon content: 0.35% maximum
- Service Temperature: Ambient to elevated temperature (≤ 800°F / 425°C)
- Product Form: Forgings
Pressure-Temperature Ratings (Class 150 per ANSI/ASME B16.5, Group 1.1)
Pressure ratings for A105 flanges decrease as operating temperature increases. The table below reflects Class 150 ratings for Carbon Steel Group 1.1, which includes A105. Higher pressure classes (such as Class 300, 600, or 900) follow their own distinct, independent derating curves specified within ASME B16.5.
| Temperature (°F) | Max Allowable Pressure (psi) |
|---|---|
| -20 to 100 | 285 |
| 200 | 260 |
| 300 | 230 |
| 400 | 200 |
| 500 | 170 |
| 600 | 140 |
| 700 | 110 |
| 800 | 80 |
Pressure Classes and Dimensional Standards
Standard | Size Range | Pressure Classes |
NPS ½” through NPS 24″ | 150, 300, 400, 600, 900, 1500, 2500 | |
ANSI/ASME B16.47 Series A | NPS 26″ through NPS 60″ | 150, 300, 400, 600, 900 |
ANSI/ASME B16.47 Series B | NPS 26″ through NPS 60″ | 75, 150, 300, 400, 600, 900 |
AWWA C207 | NPS 4″ through NPS 144″ | Up to 300 psi working pressure |
Where A105 Flanges Are Used
Weld neck flanges and slip-on flanges in A105 are standard across process piping, refineries, power generation, and chemical facilities. Common applications include:
- Process piping systems governed by ASME B31.3
- Power piping systems governed by ASME B31.1
- Oil and gas surface facilities and refineries
- Chemical processing plants and petrochemical facilities
- Steam lines and elevated-temperature service up to 800°F
- Municipal water infrastructure requiring ASME-rated components
ASTM A694 — High-Pressure Pipeline Flanges
ASTM A694 covers carbon and alloy steel forgings for pipe flanges, fittings, valves, and parts used in high-pressure oil and gas transmission service. This is the specification for flanges in high-pressure pipeline systems. A694 is a grade-based specification. Each grade designation corresponds to a minimum yield strength in ksi — F42 through F70. This allows engineers to match the flange material to the specific design pressure of the pipeline system.
A694 Grade Designations and Mechanical Properties
Grade | Min Yield Strength | Min Tensile Strength | Typical Application |
F42 | 42,000 psi (290 MPa) | 60,000 psi (415 MPa) | Moderate-pressure gas transmission |
F46 | 46,000 psi (315 MPa) | 60,000 psi (415 MPa) | Gas transmission pipelines |
F52 | 52,000 psi (360 MPa) | 66,000 psi (455 MPa) | High-pressure gas and liquid lines |
F56 | 56,000 psi (385 MPa) | 71,000 psi (490 MPa) | High-pressure transmission service |
F60 | 60,000 psi (415 MPa) | 75,000 psi (515 MPa) | High-pressure oil and gas pipelines |
F65 | 65,000 psi (450 MPa) | 77,000 psi (530 MPa) | High-strength pipeline service |
F70 | 70,000 psi (485 MPa) | 82,000 psi (565 MPa) | Highest strength grade in A694 series |
Key Characteristics
- Impact testing. While standard ASTM A694 does not mandate baseline Charpy V-notch testing, it can be ordered with supplementary impact requirements (or per MSS SP-75) to ensure low-temperature notch toughness in cold-weather pipeline networks.
- Higher yield strength. Each A694 grade designation reflects its minimum yield in ksi. F60 and F65 are the most commonly specified grades in modern high-pressure gas pipeline systems.
- Dimensional compliance. A694 flanges are dimensioned per ASME B16.5 and B16.47. They are a material upgrade, not a dimensional change.
- Marking requirements. Flanges must be marked with the ASTM designation, grade (e.g., A694 F60), heat number, and manufacturer identification.
ASTM A516 — Pressure Vessel Plate Flanges
ASTM A516 is a specification for carbon steel plate intended for pressure vessels that operate at moderate and low temperatures. Its primary applications are pressure vessel shells, heads, and nozzle reinforcement pads, but it is also used to fabricate custom plate flanges for large-diameter pressure vessel nozzles and non-standard connections. A516 comes in four grades (55, 60, 65, and 70) where the number denotes the minimum tensile strength in ksi. Grade 70 is the most widely specified.
A516 Grade Designations and Mechanical Properties
Grade | Min Yield Strength | Tensile Strength Range | Max Carbon (t .51 to 1.50 in.) | Service |
Grade 55 | 30,000 psi | 55,000 – 75,000 psi | 0.20% | Low-temp pressure vessels |
Grade 60 | 32,000 psi | 60,000 – 80,000 psi | 0.23% | Moderate-temp pressure vessels |
Grade 65 | 35,000 psi | 65,000 – 85,000 psi | 0.26% | General pressure vessels |
Grade 70 | 38,000 psi | 70,000 – 90,000 psi | 0.28% | Most widely used grade |
Key Characteristics
- Low-temperature toughness. A516 is designed for improved notch toughness at low temperatures.
- ASME compliance. A516 is a listed material under ASME BPVC Section VIII, Division 1. It is the standard plate material for fabricated pressure vessels under ASME code.
- Flange applications. A516 flanges are used at pressure vessel nozzle connections and heat exchanger flanges where the vessel shell and flange need matching material.
Grade Comparison: Full Summary
| Criteria | A36 | A105 | A694 | A516 Gr. 70 |
|---|---|---|---|---|
| Product form | Plate | Forging | Forging | Plate |
| Carbon content | 0.29% max (varies by thickness) | 0.35% max | Varies by grade | 0.31% max (varies by thickness) |
| Min yield strength | 36 ksi | 36 ksi | 42–70 ksi | 38 ksi |
| Min tensile strength | 58 ksi | 70 ksi | 60–82 ksi | 70 ksi |
| Max service temp | Low-temp only | 800°F (425°C) | Per design | Moderate-temp |
| Pressure rated | AWWA Only (≤ 300 psi) | Yes | Yes | Yes (ASME VIII) |
| ASME B16.5 listed | No | Yes (Group 1.1) | Yes | No |
| Impact testing req. | No | Optional | Optional/Supplementary requirement | Optional/Supplementary requirement |
| Primary application | Water, HVAC, structural | Process piping | Oil & gas pipelines | Pressure vessels |
How to Select the Right Grade
Grade selection is determined directly by service conditions and code requirements. These questions should be reviewed in order before a material is specified:
- Is pressure code compliance required? If the system is governed by ASME B31.3, B31.1, or an equivalent piping code, A105 forged flanges are the minimum requirement. A36 does not qualify.
- What is the operating pressure and temperature? A105 handles up to 800°F and pressure classes through 2500 per ASME B16.5. If yield strength requirements exceed 36 ksi, evaluate A694 grades.
- Is this a pipeline or process system? High-pressure oil and gas transmission pipelines typically require A694. Process piping and facility systems typically require A105.
- Is this a pressure vessel connection? If the flange connects to a pressure vessel, evaluate A516 to match the vessel shell material under ASME Section VIII.
- Is the application non-pressure or waterworks? A36 plate flanges are appropriate for AWWA waterworks service up to 300 psi, HVAC, and structural connections.
- Is low-temperature service a factor? A694 (with impact testing) and A516 are designed for low-temperature toughness. A105 can also be supplied with impact testing when specified.
Conclusion
Carbon steel flange grades are not interchangeable. A36, A105, A694, and A516 each occupy a specific position in the specification landscape, determined by product form, mechanical properties, and the standards they comply with. Getting the grade right at the specification stage prevents material rejections, failed inspections, and costly rework downstream.
API International supplies carbon steel flanges fully machined to meet ASTM A105 material requirements, with dimensions and tolerances per ANSI/ASME B16.5 and ANSI/ASME B16.47. Our sales team specializes in navigating complex flange requirements to ensure your system meets its design specifications. Explore our comprehensive range of flanges in our online product catalog, or contact us for custom options tailored to your unique project needs. Get connected with a dedicated sales representative today, or call us at 503.692.3800.
Frequently Asked Questions
What are carbon steel flanges?
Carbon steel flanges are pipe connection components made from steel with 0.05–2.1% carbon content. They provide strong, durable, and economical joints in industrial piping systems. They are commonly used in power generation, water treatment, chemical processing, construction, and general industrial applications.
What is the difference between carbon steel and stainless steel flanges?
The main difference is corrosion resistance. Stainless steels contain additional elements including chromium, which forms a protective oxide layer that resists rust and chemical attack, making it ideal for corrosive or sanitary environments. Carbon steel, while stronger and more economical, is more prone to rust and usually requires protective coatings for long-term usage.
What grade of steel is used in flanges?
What are carbon steel fittings used for?
Carbon steel flanges and fittings are used to connect, redirect, branch, and terminate piping across a wide range of industrial and utility systems. Common applications include power generation, water and wastewater treatment, HVAC systems, mining, agriculture and irrigation, marine, and lumber and pulp processing.
How to order CS flanges from API International?
To order CS flanges from API International, determine your required pipe size, pressure class, material, and any special standards (such as ANSI/ASME B16.5, or thread style). Contact API International’s experienced team and we will help you select the right flange for your application.