ASTM A387 Grade 11 Class 2 is a high-quality chromium-molybdenum (Cr-Mo) alloy steel plate, specified by ASTM A387 and its ASME counterpart SA387, designed for pressure vessels and boilers operating at high temperatures. The grade offers excellent creep strength, oxidation resistance, and weldability, which is why it is widely used in refinery, petrochemical, and power generation equipment. The Class 2 designation refers to the normalized and tempered condition, which provides higher yield and tensile strength than the annealed Class 1 condition.
What Is A387 Grade 11 Class 2?
A387 Grade 11 is a chromium-molybdenum alloy steel with a nominal composition of about 1.25% chromium and 0.5% molybdenum, and Class 2 is supplied in the normalized and tempered condition. Normalizing refines the grain structure, and tempering restores toughness and ductility, producing a plate with stable mechanical properties suited to high-temperature, high-pressure service. The corresponding ASME designation is SA387 Grade 11 Class 2, which is accepted for pressure equipment built to the ASME Boiler and Pressure Vessel Code. The grade is available in thicknesses commonly up to about 200 mm, with mechanical requirements defined by thickness in the specification.
Chemical Composition
The composition of A387 Grade 11 Class 2 is controlled to provide hardenability, oxidation resistance, and weldability. Typical limits are carbon of 0.04-0.17%, silicon of 0.44-0.86%, manganese of 0.35-0.73%, phosphorus of 0.035% maximum, and sulfur of 0.035% maximum. The chromium content is 0.94-1.56% and the molybdenum content is 0.40-0.70%, which together give the steel its elevated-temperature strength and resistance to oxidation and hydrogen attack. The balanced composition supports welding with appropriate preheat and post-weld heat treatment.
Mechanical Properties
For A387 Grade 11 Class 2 plates up to 50 mm thick, the minimum yield strength is 310 MPa (45 ksi) and the tensile strength range is 515-690 MPa (75-100 ksi), with elongation defined by the specification for the applicable thickness and gauge length. For thicker plates, the requirements follow the thickness tables in the standard. Compared with Class 1, Class 2 offers higher yield and tensile strength, greater hardness, and improved stability after heat treatment. These properties allow thinner sections or higher design pressures in the as-supplied condition, reducing material weight and fabrication cost.
Key Advantages
A387 Grade 11 Class 2 maintains strength and integrity at elevated temperatures, resisting creep and thermal stress. The chromium content provides excellent resistance to oxidation and corrosion in harsh environments, and the alloy composition offers better resistance to hydrogen-induced damage than carbon steels. The material provides a higher safety margin for long-term, continuous operation in extreme conditions, and its good weldability allows fabrication of complex pressure vessel components. In high-temperature hydrogen service, the grade resists high-temperature hydrogen attack and helps prevent sulfide stress cracking when hardness and fabrication are properly controlled.
Applications
In the oil and gas and petrochemical industry, the material is used for hydrotreating and hydrocracking reactors, fractionating columns, gas-liquid separators, and storage vessels, where it resists hydrogen sulfide corrosion and high-temperature hydrogen attack under pressures up to about 20 MPa and temperatures exceeding 400 degrees Celsius. In the power generation industry, it is critical for boiler systems, applied to boiler drums, headers, and pressure parts such as main steam pipes operating under conditions around 540 degrees Celsius, resisting creep deformation, thermal cycling, and oxidation. In general industry, it suits heat exchangers, high-temperature process piping, and welded boilers across the chemical, pharmaceutical, and metallurgical sectors.
Frequently Asked Questions
What is A387 Grade 11 Class 2 equivalent to? ASTM A387 Grade 11 Class 2 is equivalent to ASME SA387 Grade 11 Class 2, which carries identical chemical and mechanical requirements and is accepted under the ASME Boiler and Pressure Vessel Code.
What material is A387 Grade 11 Class 2? It is a chromium-molybdenum alloy steel plate with a nominal composition of about 1.25% chromium and 0.5% molybdenum, designed for high-temperature and high-pressure applications such as pressure vessels, boilers, and heat exchangers.
What is the difference between Class 2 and Class 1? Class 1 is supplied in the annealed condition with lower strength for severe forming applications, while Class 2 is normalized and tempered, offering higher tensile and yield strength and improved stability after heat treatment.
Can A387 Grade 11 Class 2 be welded? Yes, it can be welded effectively for fabricating complex pressure vessel components. Welding requires appropriate preheat, interpass temperature control, and mandatory post-weld heat treatment to temper the weld zone and restore properties.
What service temperature range is suitable for this grade? The grade is designed for high-temperature service, with common applications around 400-540 degrees Celsius. The governing design code must be consulted for allowable stresses at the specific design temperature.
Why is chromium-molybdenum steel resistant to hydrogen attack? The chromium and molybdenum additions stabilize carbides and strengthen the steel matrix, reducing the decarburization and fissuring caused by high-temperature hydrogen in refinery service, which carbon steels cannot withstand.