ASME SA387 Grade 12 Class 2 is a chromium-molybdenum alloy steel plate used for pressure vessels and boilers that operate at elevated temperatures. The grade is often described as 1.25 chromium and 0.5 molybdenum steel, and equivalent designations include BS 1501-620 and DIN 17155 13CrMo44. The Class 2 condition requires normalizing and tempering, which gives the plate enhanced toughness and ductility together with high-temperature strength.
Chemical Requirements
The chemical composition of SA387 Grade 12 is controlled within tight limits. Carbon is 0.05 to 0.17 percent on heat analysis and 0.04 to 0.17 percent on product analysis; manganese is 0.40 to 0.65 percent and 0.35 to 0.73 percent respectively; phosphorus and sulfur are each limited to 0.035 percent; silicon is 0.15 to 0.40 percent and 0.13 to 0.45 percent; chromium is 0.80 to 1.15 percent and 0.74 to 1.21 percent; and molybdenum is 0.45 to 0.60 percent and 0.40 to 0.65 percent. Heat analysis is taken from the ladle during melting, while product analysis verifies the final plate, which is why the two sets of limits differ slightly.
Tensile Requirements
SA387 Grade 12 Class 2 plate requires a tensile strength of 450 to 585 MPa, 65 to 85 ksi, and a minimum yield strength of 275 MPa, 40 ksi at 0.2 percent offset. Minimum elongation is 19 percent in 200 mm gauge length and 22 percent in 50 mm gauge length. These values ensure that the plate can carry design loads at elevated temperature while retaining enough ductility for forming and welding.
Heat Treatment for Class 2
The Class 2 condition is achieved by heat treatment. The plate is heated to the austenitizing temperature and normalized to refine the grain structure, then tempered by reheating to a minimum temperature of about 620 deg C, 1150 deg F, to improve toughness and ductility while maintaining high tensile strength. In some cases accelerated cooling by liquid quenching or air blasting is applied before tempering to enhance the mechanical properties.
Manufacturing and Processing
SA387 plate is typically melted in an electric arc furnace or basic oxygen furnace and refined so that the steel is fully killed and degassed to minimize impurities such as hydrogen and oxygen. The plates are hot rolled to the required thickness and dimensions, then given the normalized and tempered treatment described above. During fabrication, welding should follow an approved procedure, and preheat is generally required for the Cr-Mo composition to avoid hardening in the heat-affected zone.
Typical Applications
Pressure vessel shells, heads, and structural components for elevated-temperature service
Fire-tube and water-tube boilers, where creep resistance and high-temperature strength are required
Refinery reactors, heat exchangers, and fractionation columns
Power plant boiler tubes, headers, and steam piping components
Chemical process vessels and heat exchangers, and oil and gas separators and high-temperature piping
Frequently Asked Questions
What is SA387 Grade 12 Class 2? It is a chromium-molybdenum alloy steel plate for elevated-temperature pressure vessel and boiler service, supplied in the normalized and tempered condition.
What is the tensile strength of SA387 Grade 12 Class 2? The tensile strength is 450 to 585 MPa and the minimum yield strength is 275 MPa.
Why is Class 2 normalized and tempered? Normalizing refines the grain structure and tempering improves toughness and ductility while maintaining high-temperature strength.
What is the chemical composition of SA387 Grade 12? It contains about 0.05 to 0.17 percent carbon, 0.80 to 1.15 percent chromium, and 0.45 to 0.60 percent molybdenum, with controlled manganese, silicon, phosphorus, and sulfur.
Where is SA387 Grade 12 Class 2 used? In pressure vessels, boilers, refinery reactors, heat exchangers, and power plant steam components that operate at elevated temperature.
What is the equivalent grade of SA387 Grade 12? Common equivalents include BS 1501-620 and DIN 17155 13CrMo44.