SA387 Grade 12 Class 2 is a chromium-molybdenum alloy steel plate specified under the ASME SA387 standard, primarily designed for elevated-temperature service in pressure vessel and boiler applications. With a nominal composition containing about 1% chromium and 0.5% molybdenum, the grade offers good creep resistance, high-temperature strength, and oxidation resistance. Class 2 indicates a normalized and tempered heat treatment, which provides improved toughness and ductility compared with Class 1.
What Is SA387 Grade 12 Class 2?
SA387 covers chromium-molybdenum alloy steel plates for pressure vessels, and Grade 12 is the 1% chromium, 0.5% molybdenum composition with a chromium range of 0.80-1.15% and molybdenum of 0.45-0.60% by heat analysis. Class 2 plates are supplied in the normalized and tempered condition, giving a minimum yield strength of 275 MPa and a tensile strength range of 450-585 MPa. The equivalent ASTM designation is A387 Grade 12 Class 2. The grade is commonly used in refineries, petrochemical plants, power generation facilities, and other industrial systems where reliable performance under high temperature and pressure is required.
Chemical Composition
By heat analysis, the composition of SA387 Grade 12 is carbon of 0.05-0.17%, manganese of 0.40-0.65%, phosphorus of 0.035% maximum, sulfur of 0.035% maximum, and silicon of 0.15-0.40%. The chromium content is 0.80-1.15% and the molybdenum content is 0.45-0.60%. Product analysis limits are slightly wider. The chromium provides oxidation and corrosion resistance, and the molybdenum provides creep strength at elevated temperatures, which together make the grade suitable for sustained high-temperature service.
Mechanical Properties and Heat Treatment
For Class 2 plates, the tensile strength is 65-85 ksi (450-585 MPa) and the minimum yield strength is 40 ksi (275 MPa), with a minimum elongation of 22% in a 2-inch gauge length and 19% in an 8-inch gauge length. To achieve these Class 2 properties, the steel must undergo thermal treatment consisting of normalization and tempering or quenching and tempering, with a minimum tempering temperature of 1150 degrees Fahrenheit for Grade 12. Plates are typically produced by hot rolling with computer-controlled processing to meet the specified requirements, and standard testing includes ultrasonic testing, radiography, magnetic particle testing, mechanical testing including impact tests, and positive material identification.
Welding and Fabrication
The material has good weldability, which is essential for fabricating complex pressure vessels. Common processes include shielded metal arc welding, gas tungsten arc welding, and submerged arc welding, with compatible filler metals such as E8018-B2 electrodes and ER80S-B2 filler wire. Preheating before welding is recommended to avoid cold cracking, with typical preheat temperatures of 150-200 degrees Celsius, followed by post-weld heat treatment to stabilize the structure. Cutting is recommended by plasma, water-jet, or flame methods, with preheating often necessary for flame or plasma cutting to prevent cracking.
Applications
In the oil and gas and petrochemical industry, the grade is used extensively for separator vessels, refinery equipment, and storage tanks for high-pressure gases, and its oxidation resistance makes it a standard for sour service applications involving hydrogen sulfide. In power generation, it is used for industrial boilers, boiler drums, and heat recovery steam generators, as well as high-temperature ducting. In heat transfer equipment, it is used for heat exchangers, condensers, and superheaters because of its thermal stability. In chemical processing, it is used for reactors and vessels handling corrosive media at high temperatures. Beyond large vessels, the grade is processed into steam pipelines, high-temperature ducting, flanges, valves, pipe clamps, and high-temperature storage tanks.
Frequently Asked Questions
What is the main material category of SA387 Grade 12 Class 2? It is a low-alloy chromium-molybdenum steel, designed for high-temperature and pressure applications, offering excellent creep resistance and structural stability in harsh industrial environments.
Which standard specifies SA387 Grade 12 Class 2? It is specified by ASTM A387 and ASME SA387, which cover alloy steel plates for welded boilers and pressure vessels, defining the chemical composition and mechanical properties.
What is the typical composition of SA387 Grade 12? The carbon content is 0.05-0.17%, chromium is 0.80-1.15%, and molybdenum is 0.45-0.60% by heat analysis, with silicon of 0.15-0.40% and manganese of 0.40-0.65%.
What are the mechanical properties of Class 2? The tensile strength is 65-85 ksi (450-585 MPa), the minimum yield strength is 40 ksi (275 MPa), and the minimum elongation is 22% in a 2-inch gauge length.
What temperature range is SA387 Grade 12 Class 2 suitable for? It is designed for elevated-temperature service, commonly up to about 900 degrees Fahrenheit (482 degrees Celsius), with the governing design code consulted for allowable stresses at the specific design temperature.
Is SA387 Grade 12 Class 2 weldable? Yes, it is readily weldable with proper procedures. Preheating of 150-200 degrees Celsius and post-weld heat treatment are recommended to avoid cold cracking and relieve residual stresses.