Carbon-Moly Steel for Pressure Vessels Materials

Carbon-moly steel is similar to carbon steel but with 0.5% molybdenum added. The molybdenum improves the steel’s high temperature strength and graphitization resistance. The corrosion resistance is the same as for carbon steel. A discussion of limitations of carbon-moly steels follows:

Brittle Fracture. Unless made to fine-grain practice and normalized, carbon-moly steels may have poor toughness (increased susceptibility to brittle fracture).

Hydrogen Attack. Experience has indicated that carbon-moly steel cannot be relied upon to resist hydrogen attack. For new construction, carbon-moly should not be specified for hydrogen attack resistance. Instead, 1¼ Cr—1½ Mo should be
specified. Refer to the Corrosion Prevention and Metallurgy Manual and API RP 941 for detailed information.

Graphitization. Like carbon steel carbon-moly will graphitize, but carbon-moly is resistant to a maximum service temperature of 850°F.

Stress Corrosion Cracking. Same as for carbon steel.

Sulfide Stress Cracking. Same as for carbon steel.

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