ASTM A387 Grade 11 Class 2 is a high-strength chromium-molybdenum alloy steel plate designed for high-temperature pressure vessels. The chromium and molybdenum contents provide enhanced mechanical properties at elevated temperature, including better creep resistance and corrosion resistance, making the grade suitable for demanding environments in the oil, gas, and chemical processing industries. This article reviews the composition, mechanical properties, service conditions, and applications of the grade.
Chemical Composition
The heat analysis of A387 Grade 11 Class 2 is limited to carbon 0.04 to 0.17%, silicon 0.44 to 0.86%, manganese 0.35 to 0.73%, phosphorus 0.035% maximum, sulfur 0.035% maximum, chromium 0.94 to 1.56%, and molybdenum 0.40 to 0.70%. The chromium content provides oxidation and corrosion resistance at elevated temperature, while molybdenum improves the high-temperature strength and creep resistance of the base metal. The controlled carbon and manganese levels support welding, and the alloy design is intended primarily for use in welded boilers and pressure vessels designed for elevated temperature service.
Mechanical Properties
For plates up to 50 mm thick, the minimum yield strength of A387 Grade 11 Class 2 is 310 MPa and the tensile strength is 515 to 690 MPa, with a minimum elongation of 18%. The Class 2 designation requires a higher tensile strength level than Class 1, which is achieved by enhanced heat treatment, making the grade stronger and suitable for more demanding conditions. The mechanical properties are verified by tensile testing, and the plate is normally supplied in the quenched and tempered or normalized and tempered condition in accordance with the specification.
Heat Treatment and Service Temperature
A387 Grade 11 Class 2 is designed for high-temperature pressure vessel service, commonly in the range of 350 to 480 C. After heat treatment, a mandatory minimum tempering temperature of 620 C is required to achieve the Class 2 mechanical properties, which relieves internal stress and restores toughness while maintaining the specified strength. The steel offers excellent strength and corrosion resistance in elevated-temperature service, and components such as reactors and heat exchangers rely on these properties for long-term operation under sustained temperature and pressure.
Applications
In the oil and gas sector, A387 Grade 11 Class 2 is used for hydrotreating reactors, high-temperature storage vessels, sour gas service equipment, and piping systems. In petrochemical and refining, the grade serves in pressure vessels, heat exchangers, and other components in demanding process environments. Power generation applies it to boilers and components operating under sustained high temperatures and pressures, and chemical processing uses the material where corrosion resistance at elevated temperatures is required. Because the grade is intended for welded pressure equipment, fabrication follows the requirements of the governing code, including preheat and post-weld heat treatment for the chromium-molybdenum alloy system.
Frequently Asked Questions
What temperature range is A387 Grade 11 Class 2 designed for?
The grade is designed for high-temperature pressure vessels, commonly used in the 350 to 480 C range, with a mandatory minimum tempering temperature of 620 C after heat treatment to achieve Class 2 properties.
What is A387 Grade 11 material?
ASTM A387 is the standard specification for pressure vessel plates of alloy steel, chromium-molybdenum, intended primarily for use in welded boilers and pressure vessels designed for elevated temperature service.
How strong is A387 Grade 11 Class 2?
The grade offers a tensile strength of 515 to 690 MPa (75 to 100 ksi) and a minimum yield strength of 310 MPa (43 ksi), making it stronger and suitable for more demanding conditions than Class 1.
What is the difference between Class 1 and Class 2?
Class 2 requires a higher tensile strength than Class 1, which is achieved through enhanced heat treatment. Both classes share a similar chemical composition, but Class 2 is used where higher strength is required.
What are the typical applications of A387 Grade 11 Class 2?
Typical applications include hydrotreating reactors, high-temperature storage vessels, sour gas service equipment, pressure vessels, heat exchangers, boilers, and chemical processing equipment at elevated temperature.
Is A387 Grade 11 Class 2 weldable?
Yes. As a chromium-molybdenum alloy steel, it is weldable with proper procedures, including preheat and post-weld heat treatment, which are essential to avoid hydrogen-induced cracking and to restore toughness in the heat-affected zone.