Carbon Steel for Pressure Vessels Materials
Carbon steel with a 1/8- to 1/4-inch corrosion allowance is the economical material selection for a large percentage of pressure vessels in refinery, chemical plant, and producing applications. Carbon steels for pressure vessels have a nominal composition of iron with about 1% manganese and up to 0.35% carbon. Higher carbon results in poor weldability. In general, carbon steels are readily available and easily fabricated. A discussion of some limitations follows:
Brittle Fracture. Carbon steels may be susceptible to brittle fracture at normal ambient temperatures. Refer to Section 520, “Selection of Materials for Brittle Fracture Prevention.”
Hydrogen Attack. Carbon steel will suffer hydrogen attack at elevated temperature in high pressure hydrogen. Material selection should be based on the “Nelson Curves.” Refer to the Corrosion Prevention and Metallurgy Manual and American Petroleum Institute API RP 941 (available in the Corrosion Prevention and Metallurgy Manual).
Graphitization. Welded carbon steel is limited to 800°F maximum to prevent graphitization. Graphitization is the formation of graphite, primarily in weld heataffected zones, from the decomposition of iron carbides. Graphitized steel can fail under small loads or strains.
Stress Corrosion Cracking (SCC). As-welded or cold-worked carbon steel is susceptible to stress corrosion cracking in caustic, nitrate, carbonate, amine solutions and in anhydrous ammonia. Stress relief is required to prevent failures. More information is given in the Corrosion Prevention and Metallurgy Manual. Consult a materials specialist for specific applications.
Sulfide Stress Cracking (SSC). High strength steel and hard welds in steel in aqueous solutions containing H2S are susceptible to sudden nonductile failures. Controlling maximum strength and hardness is generally sufficient to prevent cracking. The Company’s pressure vessel specifications limit steel strength and weld hardness to prevent cracking. Postweld heat treatment may also be beneficial to prevent cracking. Refer to Section 1000 of the Piping Manual and Section 360 of the Corrosion Prevention and Metallurgy Manual for more detailed discussion of material considerations for H2S services.
Hydrogen-Induced Cracking. Some low strength carbon steels may be susceptible to hydrogen-induced cracking (HIC) in wet services containing H2S. Blistering is one example of this type of cracking. Stress Oriented Hydrogen Induced Cracking (SOHIC) is a specialized type of HIC that has in some cases resulted in through-wall cracks in carbon steel pressure vessels. Refer to the Corrosion Prevention and Metallurgy Manual for additional details. Steel makers offer steels made with very low sulfur contents and calcium treated for inclusion shape control to resist HIC. Standard tests are available to evaluate the HIC resistance of steel plates. Specification of HIC resistant steels is covered by the supplemental requirements of PVM-MS-4749 and PVM-MS-4750. The Company has not traditionally specified these steels. Postweld heat treatment may also be beneficial to prevent cracking.
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