What Is ASTM A387 Grade 11 Class 2
ASTM A387 Grade 11 Class 2 is a chromium-molybdenum alloy steel specifically engineered for weldable pressure vessels and boilers in elevated temperature service. The steel contains approximately 1.25% chromium and 0.50% molybdenum, which provide excellent creep resistance, corrosion resistance, and high-temperature strength. The Class 2 designation indicates enhanced mechanical properties achieved through a normalized and tempered heat treatment, which refines the grain structure and allows the material to handle higher operating stresses than Class 1.
Service Temperature Range
ASTM A387 Grade 11 Class 2 is designed for the medium temperature range, typically 350°C to 480°C (662°F to 896°F). The maximum service temperature is typically up to 593°C (1100°F) for pressure service, and the specified heat treatment involves tempering at a minimum of 620°C (1150°F). The exact allowable temperature for a specific design must be confirmed against the governing pressure vessel code and its allowable stress tables. The material has a density of approximately 7.85 g/cm³.
Chemical Composition and Mechanical Properties
The heat analysis chemical composition of ASTM A387 Grade 11 Class 2 is: carbon 0.05–0.17%, manganese 0.40–0.65%, phosphorus 0.035% maximum, sulfur 0.035% maximum, silicon 0.50–0.80%, chromium 1.00–1.50%, and molybdenum 0.45–0.65%. Product analysis permits slightly wider ranges. Class 1 and Class 2 share the same chemical composition, and their strength profiles differ through heat treatment rather than chemistry.
For Class 2 plates, the tensile strength is 75–100 ksi (515–690 MPa), the minimum yield strength at 0.2% offset is 43 ksi (310 MPa), and the minimum elongation is 18% over 8 inches (200 mm) and 22% over 2 inches (50 mm). Typical plate dimensions range from 5 mm to 150 mm in thickness, 1000 mm to 4500 mm in width, and 3000 mm to 18000 mm in length. The higher strength of Class 2 is achieved through the stricter heat treatment compared with Class 1.
Fabrication and Service Performance
The alloy is highly resistant to high-temperature hydrogen attack, which makes it a staple for hydrocracking and catalytic reforming units in refineries. Because of its air-hardening characteristics, the material requires preheating, typically around 149–200°C, and a minimum post-weld heat treatment temperature of 620°C (1150°F) to ensure joint integrity and stress relief. Suitable welding consumables include low-carbon chromium-molybdenum electrodes such as E7018-B2L, E8018-B2, and E8018-B3, with ER80S-B2 or ER90S-B3 for TIG and MIG processes, following ASME and AWS procedures. The stable chromium oxide layer also provides protection against hydrogen-induced cracking in sour gas environments.
Typical Applications
ASTM A387 Grade 11 Class 2 is used in petrochemical hydrogenation reactors and heat exchangers, power generation high-pressure steam piping and boiler drums, and oil and gas thick-walled vessels operating in corrosive, high-heat environments. Its combination of creep resistance, oxidation resistance, and weldability makes it a preferred material for reactors, hydrocracking units, steam headers, and storage tanks in demanding service.
Frequently Asked Questions
What temperature can ASTM A387 Grade 11 Class 2 withstand? The grade is designed for the medium temperature range, typically 350°C to 480°C (662°F to 896°F), with a maximum service temperature typically up to 593°C (1100°F) for pressure service.
What is ASTM A387 Grade 11 Class 2 equivalent to? It is equivalent to ASME SA387 Grade 11 Class 2, with international counterparts such as EN 10028 13CrMoSi5-5 and British Standard BS 621B offering similar 1.25% chromium, 0.50% molybdenum alloy steel.
What welding rod is used for ASTM A387 Grade 11 Class 2? Use low-carbon chromium-molybdenum electrodes such as E7018-B2L or E8018-B2 for stick welding and ER80S-B2 or ER90S-B3 for TIG and MIG, with preheating around 150–200°C and post-weld heat treatment.
What is the P number of ASTM A387 Grade 11 Class 2? The material belongs to P-Number 5A in ASME Section IX, the group for low-alloy chromium-molybdenum steels used in welding qualification.
What is the difference between Class 2 and Class 1? Both classes share the same composition. Class 2 has higher tensile and yield strength achieved through a stricter normalized and tempered heat treatment, while Class 1 offers slightly higher elongation.
Is ASTM A387 Grade 11 Class 2 carbon steel? No. It is a chromium-molybdenum alloy steel with around 1–1.5% chromium and 0.5% molybdenum, designed for high-temperature pressure vessel service.
