ASTM A387 Grade 11 Class 1 is a chromium-molybdenum (Cr-Mo) alloy steel plate designed for pressure vessel components that operate at elevated temperatures. The material belongs to the ASTM A387 specification, which covers alloy steel plates for welded boilers and pressure vessels in high-temperature service. Grade 11 defines the chemical composition, nominally 1.25 percent chromium and 0.5 percent molybdenum, while Class 1 defines the heat treatment condition in which the plates are supplied.
Chemical Composition and Alloying Roles
The heat analysis of ASTM A387 Grade 11 typically falls within the following limits: carbon 0.05-0.17%, manganese 0.40-0.65%, silicon 0.50-0.80%, phosphorus 0.025% max, sulfur 0.025% max, chromium 1.00-1.50%, and molybdenum 0.45-0.65%. Chromium improves oxidation resistance and scaling resistance at high temperature, while molybdenum strengthens the steel against creep deformation under sustained loading. Together these elements give the plate its characteristic ability to retain mechanical strength well above the limits of plain carbon steel.
Mechanical Properties and Heat Treatment
For Class 1 plates, the specified tensile strength is 75 to 100 ksi (515 to 690 MPa), with a minimum yield strength of about 43 ksi (310 MPa) measured at 0.2 percent offset. Minimum elongation is typically 18 percent in 8 inches (200 mm) and 22 percent in 2 inches (50 mm). Class 1 plates are supplied in the annealed or the normalized and tempered condition. Annealing softens the plate and improves ductility for forming, while normalizing refines the grain structure and tempering balances strength with toughness. Because Class 1 material is commonly selected when the fabricator will perform post-weld heat treatment (PWHT), the as-supplied condition is designed to remain stable through subsequent thermal cycles.
Fabrication and Welding Considerations
Grade 11 Class 1 is weldable by conventional processes, but the hardenability of the Cr-Mo chemistry demands controlled procedures. Preheating in the range of 150-200 degrees C (300-400 degrees F) is normally required, and low-hydrogen consumables should be used to avoid hydrogen-induced cracking. Interpass temperature should be monitored and kept within the limits of the welding procedure specification. Post-weld heat treatment is generally mandatory for pressure vessel service; typical PWHT temperatures fall in the range of 595-720 degrees C (1100-1325 degrees F), which tempers the weld and heat-affected zone, restores toughness, and relieves residual stresses. Hot forming and cold forming both require attention to temperature limits so that the final mechanical properties remain compliant with the specification.
Typical Applications
The material is widely specified for critical high-temperature equipment. In petroleum refineries it is used for hydrocracking reactors, catalytic units, separators, and high-temperature process vessels. In chemical and petrochemical plants it serves in hydrogen service, where its resistance to high-temperature hydrogen attack is important, as well as in high-pressure reactors. In power generation it appears in boiler components, steam drums, and heat exchangers that operate under sustained elevated temperatures and pressures. Designers select the grade when carbon steel no longer provides sufficient strength or oxidation resistance at the operating temperature.
Frequently Asked Questions
What is ASTM A387 Grade 11 Class 1?
ASTM A387 is the standard specification for chromium-molybdenum alloy steel plates used in welded boilers and pressure vessels at elevated temperatures. Grade 11 refers to the 1.25Cr-0.5Mo composition, and Class 1 refers to the annealed or normalized and tempered supply condition.
What is the difference between Class 1 and Class 2?
Class 2 plates are supplied in the normalized and tempered condition with higher minimum strength, while Class 1 plates are supplied annealed or normalized and tempered at lower strength levels. Class 1 is often chosen when the fabricator will perform post-weld heat treatment after forming and welding.
What is the equivalent grade in the ASME system?
ASME SA387 Grade 11 Class 1 is the direct equivalent of ASTM A387 Grade 11 Class 1. The two documents carry identical requirements, and the ASME version is recognized for Boiler and Pressure Vessel Code applications.
Is Grade 11 Class 1 suitable for low-temperature service?
No. The grade is optimized for elevated-temperature service. At very low temperatures its toughness can be insufficient, and the specification does not require low-temperature Charpy impact testing. For low-temperature pressure vessels, other grades are more appropriate.
Why is post-weld heat treatment required?
PWHT tempers the hard weld and heat-affected zone produced during welding, restores toughness, reduces residual stress, and improves resistance to hydrogen attack and stress corrosion in high-temperature service.
Can A387 Grade 11 Class 1 plates be hot formed?
Yes, hot forming is common for dished heads and shells. The forming temperature should be controlled so that the final condition remains compliant, and PWHT after forming is typically required to restore the specified properties.