Carbon Steel Flanges

The Most Widely Used Flange Material for General and High-Temperature Service

What Are Carbon Steel Flanges?

Carbon steel flanges are the most widely used flange type in industrial piping systems. ASTM A105 is the most common forging specification for carbon steel flanges, covering flanged fittings for ambient and high-temperature service in pressure systems. These flanges are suitable for temperatures ranging from -20°F to approximately 1000°F, making them versatile for a broad range of applications.

Carbon steel gets its name from the relatively low alloy content — the primary strengthening element is carbon rather than chromium, molybdenum, or nickel. This makes carbon steel flanges cost-effective while still providing excellent mechanical properties for general service.

Key ASTM Specifications

  • ASTM A105 — The standard specification for carbon steel forgings for piping applications. Covers high-temperature service and is by far the most commonly specified carbon steel flange material.
  • ASTM A350 LF2 — Carbon steel forgings requiring Charpy V-notch impact testing for low-temperature service down to -50°F. Used when the operating temperature is below the -20°F lower limit of A105.
  • ASTM A694 (F52–F70) — High-yield carbon and microalloy steel forgings for transmission and distribution piping. Grades F52 through F70 provide higher strength for pipeline applications.

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Chemical Composition

ASTM A105 limits carbon content to a maximum of 0.35%, with manganese ranging from 0.60% to 1.05% and silicon from 0.10% to 0.35%. The relatively low alloy content — with no significant additions of chromium, molybdenum, or nickel — is what distinguishes carbon steel from alloy steel and stainless steel.

This composition provides a good balance of strength, weldability, and machinability at a lower cost compared to alloyed materials.

Common Applications

  • Refineries and petrochemical plants
  • Power generation facilities
  • General industrial piping systems
  • Oil and gas production and transmission
  • Water and steam service
  • HVAC and building services

Available Flange Types

Carbon steel flanges are available in all standard types defined by ASME B16.5, including weld neck, slip-on, blind, socket weld, threaded, and lap joint configurations. They are manufactured in all standard pressure classes from Class 150 through Class 2500.

Temperature Considerations

ASTM A105 flanges are not suitable for service below -20°F without supplementary impact testing. For low-temperature applications, ASTM A350 LF2 should be specified instead, as it includes mandatory Charpy impact testing to ensure adequate toughness at temperatures down to -50°F.

At the upper end, carbon steel flanges begin to lose strength above approximately 800°F, and creep becomes a consideration. For sustained high-temperature service above 900–1000°F, chrome-moly alloy steel flanges are generally preferred.

Related References