ASME SA387 Grade 12 Class 2 plate is a chromium-molybdenum alloy steel plate covered by the ASME SA-387 specification, intended mainly for welded boilers and pressure vessels that operate at elevated temperatures. The Grade 12 designation signifies a composition dominated by moderate chromium and molybdenum content, approximately 1 percent chromium with 0.5 percent molybdenum, which imparts good high-temperature strength, creep resistance, and oxidation resistance. Class 2 indicates the higher-strength class of the grade, supplied in the normalized and tempered condition, with higher strength and toughness than the lower Class 1 requirements.
What Is SA387 Grade 12 Class 2?
SA387 Grade 12 belongs to the family of chromium-molybdenum pressure vessel plates defined in ASTM A387 and adopted as ASME SA-387 for code construction. The material is designed for elevated-temperature and pressure service, primarily in boiler, pressure vessel, and piping applications where resistance to creep, oxidation, and high-temperature strength are required. The UNS designation of the grade is K11757.
The chemical composition is tightly controlled, with limits on carbon, silicon, manganese, phosphorus, sulfur, chromium, and molybdenum, to ensure consistent properties. Plates are produced in a range of standard thicknesses, widths, and lengths, commonly from about 6 mm up to 200 mm thick depending on the manufacturer, and the heat treatment involves normalizing followed by tempering to refine the microstructure and optimize the balance of strength and toughness.
Chemical Composition
The chemical requirements of SA387 Grade 12 are defined by heat analysis and product analysis. Typical product analysis values include:
Carbon: 0.04 to 0.17 percent.
Manganese: 0.35 to 0.73 percent.
Phosphorus: maximum 0.035 percent.
Sulfur: maximum 0.035 percent.
Silicon: 0.13 to 0.45 percent.
Chromium: 0.74 to 1.21 percent.
Molybdenum: 0.40 to 0.65 percent.
The chromium provides oxidation and corrosion resistance, while the molybdenum raises the elevated-temperature strength and creep resistance of the plate.
Mechanical Properties of Class 2
The mechanical requirements of SA387 Grade 12 Class 2 are defined by the specification. Typical values include:
Tensile strength: 75 to 100 ksi, equal to 515 to 690 MPa.
Minimum yield strength: 45 ksi, equal to about 310 MPa.
Minimum elongation: about 19 percent in 200 mm and 22 percent in 50 mm.
The base specification does not mandate Charpy impact testing for either class. Where fracture toughness is required by the design, impact testing is added as a supplementary requirement, commonly at temperatures such as minus 18 or minus 30 degrees Celsius, with the acceptance values defined in the order. The normalized and tempered condition provides a consistent microstructure that responds well to such requirements.
Heat Treatment, Welding, and Fabrication
The mandatory heat treatment for SA387 Grade 12 Class 2 is normalizing and tempering:
Normalizing: the plate is heated to a temperature above the austenitizing range, typically about 890 to 940 degrees Celsius, and cooled in air. This refines the grain structure, homogenizes the microstructure, and relieves residual stresses from casting or rolling.
Tempering: after normalizing, tempering is performed at a minimum temperature of about 620 degrees Celsius, with sufficient holding time to ensure uniform temperature throughout the plate, followed by cooling in air or water. Tempering reduces internal stresses, improves toughness, and achieves the desired combination of mechanical properties.
For thicker plates or demanding toughness requirements, a sequence of normalizing, rapid cooling such as water quenching, and tempering may be applied to achieve superior toughness and microstructure. These procedures ensure that the material possesses the necessary strength, toughness, and corrosion resistance for service in high-temperature and hydrogenation environments.
SA387 Grade 12 Class 2 is weldable, but proper welding procedures are critical because of the chromium-molybdenum composition. Preheating, typically in the range of 150 to 260 degrees Celsius, is required to prevent cold cracking, and post-weld heat treatment is necessary to relieve residual stresses and restore material toughness. Compatible welding consumables include low-alloy chromium-molybdenum electrodes and filler wires such as the E7018-B2 and ER80S-B2 families for the common welding processes.
The material can be cut, formed, and machined by conventional methods, and for thicker plates additional quality control such as ultrasonic inspection may be required to ensure internal integrity.
Typical Applications
SA387 Grade 12 Class 2 plates are widely used in high-temperature, high-pressure industrial equipment:
Oil and gas industry: sour service environments where resistance to hydrogen sulfide and related forms of corrosion and embrittlement is critical, in both offshore and onshore facilities including platforms, processing plants, reactors, separators, and pressure vessels.
Power generation: steam boilers, heat recovery steam generators, thermal power stations, and turbine-related components, where the material maintains strength and stability at elevated temperatures.
Chemical processing: reactors, pressure vessels, and heat exchangers handling aggressive chemicals under demanding temperature and pressure regimes.
General industrial: tanks, piping for hot liquids, separators, industrial boilers, and various pressure vessels where the combination of strength, toughness, and weldability is valuable.
The material is suitable for continuous service at elevated temperatures up to approximately 593 degrees Celsius, beyond which its creep strength and oxidation resistance may degrade and limit long-term performance; the exact allowable values are defined by the governing design code.
Frequently Asked Questions
What is ASME SA 387 Grade 12 Class 2? It is a low-alloy chromium-molybdenum steel plate specification developed for elevated-temperature and pressure service, primarily used in boiler, pressure vessel, and piping applications where resistance to creep, oxidation, and high-temperature strength are required.
What are the minimum mechanical properties of SA 387 Grade 12 Class 2? For Class 2 material, typical minimum values include a tensile strength of about 515 to 690 MPa and a yield strength of about 310 MPa, with elongation requirements defined by the specification for the relevant gauge length.
What heat treatment is required for SA 387 Grade 12 Class 2? The mandatory heat treatment is normalizing and tempering, with normalizing at about 890 to 940 degrees Celsius followed by air cooling, and tempering at a minimum of about 620 degrees Celsius.
What is the maximum service temperature for SA 387 Grade 12 Class 2? It is suitable for continuous service at elevated temperatures up to approximately 593 degrees Celsius, beyond which creep strength and oxidation resistance may degrade; the exact limits are defined by the design code.
How does SA 387 Grade 12 Class 2 differ from Class 1 of the same grade? The two classes have the same chemical composition but different strength requirements. Class 2 specifies higher minimum tensile and yield strength, achieved through heat treatment, while Class 1 has lower strength requirements. Impact testing is a supplementary requirement in both cases.
Is SA 387 Grade 12 Class 2 weldable? Yes, with proper procedures. Preheating of about 150 to 260 degrees Celsius is required to prevent cold cracking, and post-weld heat treatment is necessary to relieve residual stresses and restore toughness.