Selecting the right carbon steel flange begins with understanding your application, including its service conditions, required material grade, and appropriate flange type. Getting one of these wrong may not cause an immediate problem, but issues typically surface later, during pressure testing, code review, or inspection. This guide outlines the remaining requirements for a complete order.
Service Conditions
Operating pressure, temperature, and the media running through the line determine grade, pressure class, and dimensional standard. These variables should be established first, since they rule out entire categories of material and flange type before specific products are compared.
- Pressure. The system’s maximum design pressure must fall within a standard ANSI/ASME pressure class.
- Temperature. Sustained operating temperature determines whether carbon steel applies, or whether an alloy grade is required.
- Media. Water, steam, hydrocarbons, and corrosive process fluids each carry different material requirements. Corrosive or high-hydrogen service can call for an alloy or stainless grade instead of carbon steel.
Applicable Piping Code
The governing piping code determines which material and testing requirements apply to a given system. ASME B31.3 governs process piping, ASME B31.1 governs power piping, and ASME B31.11 governs slurry transportation piping. Each code carries its own material qualification, testing, and documentation requirements, and the code that applies to the system determines much of what follows in the specification.
Code | Scope | Typical Systems |
ASME B31.3 | Process piping | Refineries, chemical plants, pharmaceutical and process piping |
ASME B31.1 | Power piping | Power plant steam lines, boiler external piping, district heating |
ASME B31.11 | Slurry transportation piping | Piping transporting aqueous slurries between plants and terminals |
Material Grade
Grade determines whether the flange is eligible for the intended service at all. ASTM A36 is a plate grade suited to fabricated, medium-pressure applications up to 300 psi, while ASTM A105 is a forged grade for pressure-rated piping under ANSI/ASME code. The two share a similar minimum yield strength but are not interchangeable: substituting A36 where A105 is required is a specification error that surfaces during inspection or code review.
| Property | ASTM A36 | ASTM A105 |
| Product form | Plate / structural shapes | Forgings |
| Max service temperature | Ambient / non-critical | 800°F / 425°C |
| Pressure service | Restricted (AWWA waterworks up to 300 psi) | ASME B31.3 compliant |
| Applicable standards | Structural codes, AWWA C207 | ASME B16.5, B16.47 |
| Typical application | Water systems, HVAC, structural | Process piping, refineries, steam |
For a full comparison of carbon steel grades, see Carbon Steel Flange Grades.
Flange Type and Face Type
Carbon steel flange type and face type together determine fit, sealing, and compatibility with the mating component. Common configurations include weld neck, slip-on, blind, socket weld, threaded, and lap joint flanges, each suited to a different combination of pressure, pipe size, and serviceability. Face type follows a similar logic: raised face is standard for most process piping, flat face is specified when mating to cast iron or non-metallic equipment, and ring-type joint is reserved for high-pressure, high-temperature service.
Dimensional Standard
Dimensional standards define outside diameter, bolt circle, bore, and pressure-temperature ratings, and specifying the wrong one means the flange will not bolt up to its mating component.
Standard | Size Range | Pressure Basis |
NPS ½” through 24″ | Classes 150 through 2500 | |
NPS 26″ through 60″ (weld neck and blind) | Classes 75 through 900 | |
NPS 4″ through 144″ | Classes B (86psi), D (150-175psi), E (275psi), F (300psi) |
Pressure Class
Pressure class is a reference designation rather than a fixed maximum pressure. Under ANSI/ASME B16.5, classes 150 through 2500 are available, and allowable working pressure declines as operating temperature rises. The correct class depends on both the system’s design pressure and its maximum operating temperature, checked together against the B16.5 pressure-temperature tables. See ANSI/ASME flange pressure ratings for the full breakdown by class.
Certified Mill Test Reports
Certified mill test reports (MTRs) document that the material meets the specified ASTM chemistry and mechanical requirements. On code-governed or quality-controlled projects, MTRs are a baseline requirement rather than an optional add-on, and they are most useful when requested at time of order. Retroactive requests after delivery introduce delays and, on some projects, require re-inspection.
Domestic vs. Imported Material
Federal and state infrastructure projects under Buy American, BABA, and AIS provisions require domestically produced steel. Beyond compliance, domestic A105 and A36 flanges also carry practical advantages: full melt-and-manufacture traceability through certified MTRs, and typically shorter lead times than imported material, since domestic mills and forges stock these grades in volume.
Conclusion
A well-specified carbon steel flange holds up under pressure, passes inspection without surprises, and fits the system it was ordered for. API International supplies carbon steel flanges fully machined to ASTM A105 or A36 material requirements, with dimensions and tolerances per ANSI/ASME B16.5, ANSI/ASME B16.47, and AWWA C207. Our sales team specializes in navigating complex flange requirements to ensure a system meets its design specifications. Explore the comprehensive range of flanges in our 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.
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Frequently Asked Questions
What is the difference between ASTM A36 and ASTM A105 for flange orders?
Why are mill test reports (MTRs) required when ordering flanges?
Certified MTRs confirm that the material's chemistry and mechanical properties match the specified ASTM grade. They are required for code-stamped fabrications and most quality-controlled procurement, and are best requested at time of order rather than after delivery.
Does Buy American apply to carbon steel flanges?
On federal and state infrastructure projects under Buy American and BABA provisions, yes. These projects require domestically produced steel, which domestic A36 and A105 flanges satisfy without the import documentation foreign material requires.
Which piping code governs a carbon steel flange order?
That depends on the system. ASME B31.3 governs process piping, ASME B31.1 governs power piping, and ASME B31.11 governs slurry transportation piping, each with its own material and testing requirements.