ASTM A387 Grade 11 Class 1 is a chromium-molybdenum alloy steel plate used for high-temperature pressure vessels and boilers. It contains roughly 1.25 percent chromium and 0.5 percent molybdenum, which give the steel superior strength, creep resistance, and oxidation resistance in demanding environments such as oil and gas, refining, and power generation. The plate is typically supplied in the normalized and tempered condition, and the Class 1 designation defines specific tensile strength levels that distinguish it from Class 2. Purity control and flaw detection are emphasized because the material is intended for critical welded pressure equipment operating at elevated temperature. This article reviews its composition, properties, equivalents, and applications.
What Is A387 Grade 11 Class 1?
A387 Grade 11 Class 1 is covered by ASTM A387/A387M, the standard specification for chromium-molybdenum pressure vessel plates intended primarily for welded boilers and pressure vessels designed for elevated temperature service. The Grade 11 chemistry is a 1.25Cr-0.5Mo type alloy, and Class 1 defines a specific tensile strength level that is lower than Class 2. The plate is normally supplied normalized and tempered, which develops a stable microstructure with good creep resistance. The specification emphasizes purity and flaw detection because the material is used for demanding applications where internal soundness is critical.
Chemical Composition
The composition of A387 Grade 11 Class 1 is controlled within narrow ranges. Carbon ranges from 0.04 to 0.17 percent, manganese from 0.35 to 0.73 percent, and silicon from 0.44 to 0.86 percent, with phosphorus and sulfur each limited to 0.035 percent maximum. Chromium is specified at 0.94 to 1.56 percent and molybdenum at 0.40 to 0.70 percent, which are the elements that provide high-temperature strength and oxidation resistance. This balanced composition keeps the steel weldable while supporting the creep resistance needed in boiler and pressure vessel service.
Mechanical Properties
For plates up to 50 mm thick, the minimum yield strength is 240 MPa and the tensile strength range is 415 to 585 MPa, with a minimum elongation of 22 percent. For plates over 50 mm up to 200 mm thick, the yield strength remains at 240 MPa and the tensile range stays at 415 to 585 MPa, while the minimum elongation reduces to 19 percent. These properties are developed by normalizing and tempering, with a minimum tempering temperature for SA 387 Grade 11 of about 1150 degrees Fahrenheit, roughly 620 degrees Celsius. The combination of moderate strength and good ductility supports forming and welding of vessel components.
Equivalent Grades
The direct equivalent of ASTM A387 Grade 11 is ASME SA387 Grade 11, which is the code adoption of the same specification; both cover Class 1 and Class 2 conditions. The material is recognized across Belgium, Germany, France, Italy, Sweden, India, Japan, and the United Kingdom under equivalent national designations, including the British standard designation 621 gr. A or B. Although the equivalents share the same composition basis, each must be verified against its own standard before substitution in code work.
Applications
A387 Grade 11 Class 1 is used in a wide range of high-temperature equipment: boilers and heat exchangers, high-pressure vessels, oil and gas industry components, chemical refinery equipment, and pipeline systems. It also serves water pipelines, nuclear power plant components, and food processing and dairy installations where hygienic, reliable fabrication is required. The grade is particularly valuable in paper and pulp companies, where process equipment must withstand elevated temperature and corrosive media. In every case, the plate is welded into pressure-retaining components that operate under sustained thermal and mechanical stress.
Frequently Asked Questions
1. What is ASTM A387 Grade 11 equivalent to?
The equivalent is ASME SA387 Grade 11, which covers the same Class 1 and Class 2 conditions with identical chemistry and property requirements.
2. What is the yield strength of A387 Grade 11 Class 1?
The minimum yield strength is 240 MPa for plates up to 200 mm thick, with a tensile strength range of 415 to 585 MPa.
3. What tempering temperature applies to SA 387 Grade 11?
The minimum tempering temperature is about 1150 degrees Fahrenheit, roughly 620 degrees Celsius, which develops the required microstructure and toughness.
4. What is the difference between Class 1 and Class 2?
Class 2 has higher tensile strength requirements than Class 1, achieved through more demanding heat treatment and testing.
5. Where is A387 Grade 11 Class 1 used?
It is used for boilers, heat exchangers, high-pressure vessels, refinery and pipeline equipment, nuclear power components, and food processing installations.
6. Can A387 Grade 11 Class 1 be welded?
Yes, it has good weldability with standard procedures, and preheat together with post-weld heat treatment is applied as required by the fabrication code.