Carbon steel remains appropriate in many pressure systems

ASTM A105 carbon-steel flanges are widely used because they are available, weldable under established procedures and cost-effective. When design temperature, Druck, toughness and corrosion conditions are within the approved line class, carbon steel can be the technically correct choice.

For low-temperature, sauer, severe cyclic or highly corrosive service, additional requirements or another material specification may be necessary.

Why chrome-moly alloy steel is selected

Grades such as F11, F22 and F91 are evaluated for elevated-temperature strength and oxidation behaviour. Their benefit appears within an engineered alloy system and a suitable fabrication route. They also introduce stricter heat-treatment, consumable and PWHT controls.

Alloy content does not guarantee resistance to wet process corrosion. The corrosion study must address the actual medium, water content, Verunreinigungen, shutdown conditions and external environment.

A practical selection matrix

Questions to resolve before specifying the flange

• What pressure and temperature apply simultaneously?
• Which material group and pressure-temperature table governs?
• Is creep, Oxidation, low-temperature toughness or corrosion the controlling damage mechanism?
• Can the fabricator perform the required welding and heat treatment?
• Are matching pipe, Beschläge, valves and bolting available?
• What inspection and documentation does the owner require?