SA387 Grade 11 Class 1 is an ASME and ASTM alloy steel plate specification for chromium-molybdenum steel used in high-temperature pressure vessels and boilers, featuring around 1.25% chromium and 0.5% molybdenum for enhanced strength, heat resistance, and oxidation resistance. Class 1 indicates a specific lower tensile strength level compared with Class 2, reflecting the annealed supply condition that favors forming and subsequent heat treatment in fabrication.
What Is SA387 Grade 11 Class 1?
SA387 covers chromium-molybdenum alloy steel plates for welded boilers and pressure vessels operating at elevated temperatures, and Grade 11 is the nominally 1.25% chromium, 0.5% molybdenum composition. The corresponding ASTM designation is A387 Grade 11 Class 1. Class 1 plates are supplied in the annealed condition, which provides maximum ductility and uniformity for severe forming operations, while Class 2 is supplied normalized and tempered with higher as-supplied strength. The chemical composition of the two classes is identical; the difference is created by the mill heat treatment.
Chemical Composition
The composition of SA387 Grade 11 is carbon of 0.04-0.17%, manganese of 0.35-0.73%, phosphorus of 0.035% maximum, and sulfur of 0.035% maximum. The silicon content is 0.44-0.86%, which is higher than in many pressure vessel steels and supports deoxidation and high-temperature strength. The chromium content is 0.94-1.56% and the molybdenum content is 0.40-0.70%. This combination provides oxidation resistance, creep strength, and resistance to hydrogen attack at elevated temperatures.
Mechanical Properties
For Class 1 plates, the tensile strength is 60-85 ksi (415-585 MPa) and the minimum yield strength is 30 ksi (205 MPa), with a minimum elongation of 22% in a 2-inch gauge length and 19% in an 8-inch gauge length. The annealed condition gives the material lower strength but higher ductility than Class 2, which suits deep forming operations. In finished pressure vessels, the final properties are established by the post-weld heat treatment, so the as-supplied Class 1 condition is a starting point for fabrication rather than the final service condition.
Applications
SA387 Grade 11 Class 1 is widely used in the fabrication of pressure vessels for oil refineries, chemical plants, and power generation facilities, where its creep resistance and high-temperature strength suit corrosive fluids and gases under elevated pressures and temperatures. It is used for high-temperature tubing for boilers, heat recovery systems, and petrochemical processing units; flanges for high-pressure and high-temperature piping systems such as refinery headers and power plant steam lines; process piping for the oil, gas, and chemical industries; shell-and-tube heat exchangers, reboilers, and condensers; high-temperature ducting in power plants, incinerators, and industrial furnaces; heavy-duty pipe clamps and supports; and valve bodies, bonnets, and trim parts for critical control and isolation valves.
Heat Treatment and Fabrication
Class 1 plates are normally supplied in the annealed condition, and heat treatment during fabrication may include normalization followed by tempering, or in some cases quenching and tempering, to refine the microstructure and optimize mechanical performance for the final application. Normalizing is typically performed by heating above the critical temperature, around 890-920 degrees Celsius, followed by air cooling. Welding requires qualified procedures with preheating and interpass temperature control, and post-weld heat treatment is mandatory after welding to temper the weld zone and relieve residual stresses. The grade is more cost-effective than stainless steel for Cr-Mo compatible high-temperature environments, while stainless steel is preferred where strong corrosion resistance is the governing requirement.
Frequently Asked Questions
What is the main material category of SA387 Grade 11 Class 1? It is a low-alloy chromium-molybdenum steel plate, primarily used for high-temperature and high-pressure applications such as boilers and pressure vessels.
What core mechanical properties does SA387 Grade 11 Class 1 possess? It has good high-temperature strength, toughness, creep resistance, and weldability. Class 1 provides a tensile strength of 60-85 ksi (415-585 MPa) and a minimum yield strength of 30 ksi (205 MPa) in the annealed condition.
Which heat treatment processes are used for SA387 Grade 11 Class 1? Class 1 is normally supplied annealed. During fabrication, normalization plus tempering, or quenching plus tempering in some cases, may be applied to refine the microstructure and optimize mechanical performance.
What is the critical temperature range for normalizing this grade? Normalizing is typically performed by heating above the critical temperature, around 890-920 degrees Celsius, followed by air cooling to refine the grain structure.
Which standard governs the production of SA387 Grade 11 Class 1? It complies with ASME SA-387 and ASTM A387, the specifications for pressure vessel plates for high-temperature service.
How does this grade compare with stainless steel in cost and application? It is more cost-effective than stainless steel and is suitable for Cr-Mo compatible high-temperature environments, while stainless steel is preferred where strong corrosion resistance is the primary requirement.