ASTM A387 Grade 22 is a chromium-molybdenum alloy steel plate designed for welded pressure vessels operating at elevated temperatures. It is also known by its alloy composition of 2.25% chromium and 1% molybdenum, commonly written as 2.25Cr-1Mo. The higher alloy content distinguishes it from the more common Grade 11 material and provides significantly improved performance in severe service environments such as high-temperature, high-pressure, and hydrogen-rich service. This article reviews the properties, heat treatment classes, fabrication requirements, and applications of the grade.
Mechanical Properties by Class
A387 Grade 22 is available in two heat-treated classes. Class 1 plates in the thickness range of 0.185 to 3.60 inches have a minimum yield strength of 30 ksi, a tensile strength of 60 to 85 ksi, a minimum elongation of 18% in 2 inches, and a minimum reduction of area of 40%. Class 2 plates in the same thickness range have a minimum yield strength of 45 ksi, a tensile strength of 75 to 100 ksi, and the same elongation and reduction of area requirements. Class 2 achieves its higher strength through a more severe heat treatment, and the choice between classes depends on the fabrication methodology and whether a full final heat treatment of the vessel is feasible.
Composition and Heat Treatment Classes
The chemistry of A387 Grade 22 is a nominal 2.25% chromium and 1% molybdenum. Chromium enhances oxidation and corrosion resistance, while molybdenum increases high-temperature strength and creep resistance. Class 1 is supplied in the normalized or stress-relieved condition and is intended for fabrications where the completed vessel will undergo a full post-weld heat treatment. Class 2 is supplied in the normalized and tempered or quenched and tempered condition and is used when the final vessel cannot be fully heat-treated, because it arrives with higher, pre-tempered strength.
High-Temperature and Hydrogen Service Properties
A387 Grade 22 has higher design strength, expressed as allowable stress values per ASME codes, at temperatures above approximately 450 C compared with lower-grade alloys. It performs well under long-term stress at high temperatures and offers excellent creep resistance. Its key advantage is superior performance in high-pressure hydrogen service, as indicated by its position on the Nelson Curve of API RP 941; it resists hydrogen embrittlement and decarburization better than lower chromium-molybdenum steels, which makes it the material of choice for reactors in severe hydrogen environments.
Fabrication Considerations
Because of its high hardenability, A387 Grade 22 requires careful preheating, typically 200 to 250 C, before welding. Mandatory post-weld heat treatment is critical to relieve stresses, temper the heat-affected zone, and achieve optimal toughness and corrosion resistance; the PWHT temperature range is typically 690 to 750 C. Weld procedure qualification must be performed for the specific thickness and joint configuration, and the heat-affected zone hardness and toughness should be verified after heat treatment to confirm that the intended properties have been achieved.
Primary Applications
A387 Grade 22 is the workhorse material for the most demanding high-temperature and high-pressure vessels. In petroleum refining, it is used for critical reactors in hydrocracking, hydrotreating, and catalytic reforming units. In power generation, it serves in the high-pressure sections of boilers, steam drums, and heat recovery steam generators. In chemical and petrochemical plants, the grade is used for reactors operating in severe hydrogen and high-temperature environments. It is selected when service conditions, such as high temperature, high pressure, and exposure to hydrogen, exceed the capabilities of standard chromium-molybdenum steels like Grade 11.
Frequently Asked Questions
What is A387 Grade 22 material?
A387 Grade 22 is a chromium-molybdenum alloy steel plate with a nominal composition of 2.25% chromium and 1% molybdenum, designed for welded pressure vessels operating at elevated temperatures.
What is the difference between Class 1 and Class 2?
Class 1 is supplied in the normalized or stress-relieved condition with a minimum yield strength of 30 ksi and is intended for vessels that receive a full post-weld heat treatment. Class 2 is supplied normalized and tempered or quenched and tempered with a minimum yield strength of 45 ksi, for vessels that cannot be fully heat-treated.
What preheat is required for welding A387 Grade 22?
Careful preheating is required due to the high hardenability of the steel, typically 200 to 250 C, followed by mandatory post-weld heat treatment at 690 to 750 C.
Why is A387 Grade 22 used in hydrogen service?
The 2.25Cr-1Mo chemistry resists high-temperature hydrogen attack, hydrogen embrittlement, and decarburization better than lower chromium-molybdenum steels, as indicated by its position on the Nelson Curve in API RP 941.
What are the tensile properties of A387 Grade 22?
Class 1 has a tensile strength of 60 to 85 ksi and a minimum yield strength of 30 ksi. Class 2 has a tensile strength of 75 to 100 ksi and a minimum yield strength of 45 ksi, both with a minimum elongation of 18% in 2 inches.
What are the typical applications of A387 Grade 22?
Typical applications include hydrocracking, hydrotreating, and catalytic reforming reactors in refining; high-pressure boiler sections, steam drums, and HRSGs in power generation; and reactors in severe hydrogen and high-temperature chemical service.