ASTM A387 Grade 5 Class 2 is a chromium-molybdenum alloy steel plate designed for elevated-temperature service. It belongs to the A387 family of pressure vessel plates for welded boilers and pressure vessels, and its chemistry, nominally 5 percent chromium with 0.5 percent molybdenum, provides good strength and resistance to creep and oxidation at high temperatures. The grade is typically used in the fabrication of boilers, pressure vessels, and other equipment that must withstand harsh thermal conditions, and it is produced with controlled chemistry and heat treatment to ensure consistent mechanical properties.
What Is A387 Grade 5 Class 2?
A387 Grade 5 is the member of the A387 family with the highest chromium content among the common grades, at approximately 5 percent, combined with about 0.5 percent molybdenum. The high chromium level gives the material excellent resistance to oxidation and corrosion, including resistance to hydrogen sulfide attack, which makes it valuable in sour service environments in the oil and gas industry.
Class 2 is the higher-strength class of the grade, requiring higher minimum tensile and yield strength than Class 1, achieved through the appropriate heat treatment. The plate is normally supplied in the normalized and tempered condition, and it is often specified with impact testing at low temperatures as a supplementary requirement.
Chemical Composition
The composition of A387 Grade 5 is controlled to a tight specification. Typical limits include:
Carbon: maximum about 0.15 percent, supporting weldability.
Manganese: about 0.30 to 0.60 percent.
Silicon: about 0.50 percent maximum.
Phosphorus and sulfur: each limited to a maximum of about 0.035 percent.
Chromium: 4.00 to 6.00 percent, the element responsible for the outstanding oxidation and corrosion resistance of the grade.
Molybdenum: 0.45 to 0.65 percent, providing elevated-temperature strength and creep resistance.
Because of the chromium content, welding requires particular care to avoid hydrogen-induced cracking, and preheating is essential.
Mechanical Properties
The mechanical requirements of A387 Grade 5 are defined for each class:
Class 1: tensile strength 415 to 585 MPa, minimum yield strength about 205 MPa, minimum elongation 18 percent in 50 mm, with a minimum reduction of area of about 40 percent.
Class 2: tensile strength 515 to 690 MPa, minimum yield strength about 310 MPa, minimum elongation 18 percent in 50 mm, with a minimum reduction of area of about 40 percent.
Class 2 material is therefore significantly stronger than Class 1, and it is selected where the design requires higher allowable stress at elevated temperature. Impact testing is not mandatory in the base specification but can be specified as a supplementary requirement, for example at minus 52 degrees Celsius, for service where fracture toughness is critical.
Processing, Heat Treatment, and Welding
The mechanical properties of Class 2 are achieved through controlled thermal cycles:
Normalizing: the plates are heated to an austenitizing temperature and cooled in air to refine the grain structure.
Tempering: the normalized plate is reheated to a minimum tempering temperature of approximately 705 degrees Celsius to relieve internal stresses and achieve the specified tensile strength.
Accelerated cooling: where specified, liquid quenching followed by tempering is used to enhance toughness in thicker plates.
In fabrication, the plates can be hot rolled with controlled edge preparation, cold or hot formed into shells and heads, and cut by oxy-fuel or plasma processes. Because the chromium-bearing heat-affected zone can harden, preheating before cutting is often recommended to prevent edge cracking. Post-weld heat treatment is mandatory for most pressure vessel applications to restore ductility and reduce hardness in the heat-affected zone, and simulated post-weld heat treatment of test coupons is used to verify the final properties.
Welding of A387 Grade 5 requires disciplined procedure control. Preheating is essential to minimize the risk of hydrogen-induced cracking, and post-weld heat treatment is a mandatory thermal cycle for pressure vessel construction. The material is examined by non-destructive testing including ultrasonic testing for internal flaws and magnetic particle inspection for surface cracks, and Charpy V-notch impact testing is performed where toughness verification is required. Positive material identification is commonly used to confirm the chromium and molybdenum content before shipment.
Typical Applications
A387 Grade 5 Class 2 is used in equipment exposed to corrosive media and high operating heat:
Oil, gas, and petrochemical refining: hydrocracking units, separators and reactors, and sour service environments handling hydrogen sulfide, where its resistance to corrosion and cracking is essential.
Power generation: boiler drums, superheater tubes, steam generators, and high-temperature ducting carrying high-pressure steam.
Chemical processing and synthesis: pressure vessels, heat exchangers, and synthesis towers for ammonia and urea production where high pressure and chemical durability are required.
Specialized industrial machinery: furnace liners and structural components, high-temperature valves, pipe clamps, and flanges, and marine applications where additional protective measures are applied.
The grade is commonly used at service temperatures up to approximately 900 degrees Fahrenheit (about 480 degrees Celsius), with the exact allowable values defined by the governing design code, and its chromium-molybdenum composition provides good creep resistance and structural stability at these elevated temperatures.
Frequently Asked Questions
What is the minimum yield strength of ASTM A387 Grade 5 Class 2? The minimum yield strength is about 310 MPa (45 ksi), which ensures the material can withstand significant internal pressures and loading without excessive deformation in high-temperature service.
How does ASTM A387 Grade 5 Class 2 compare to ASTM A285 Grade C? 387 Grade 5 Class 2 is a chromium-molybdenum alloy steel offering superior high-temperature strength compared with A285 Grade C, a carbon steel. Grade 5 Class 2 is used in more demanding thermal environments, while A285 Grade C is suitable for lower-temperature pressure vessel applications.
What heat treatment is required for ASTM A387 Grade 5 Class 2? It is usually supplied in the normalized and tempered condition. Normalizing refines the grain structure, while tempering reduces hardness and improves toughness so that the material meets the required mechanical properties.
What is the maximum temperature at which ASTM A387 Grade 5 Class 2 can operate? It is commonly used in applications up to approximately 900 degrees Fahrenheit. Its chromium-molybdenum composition provides good creep resistance and structural stability at these temperatures, making it suitable for refinery and petrochemical equipment.
How does ASTM A387 Grade 5 Class 2 compare to ASTM A387 Grade 9? Grade 5 Class 2 has lower chromium content than Grade 9, resulting in lower high-temperature strength and creep resistance. Grade 9, with higher chromium, is better suited for more severe elevated-temperature applications, while Grade 5 Class 2 is used in moderately high-temperature pressure vessel components.
What is the difference between ASTM A387 Grade 5 Class 2 and ASTM A516 Grade 70? 387 Grade 5 Class 2 is a chromium-molybdenum alloy steel designed for high-temperature service, while A516 Grade 70 is a carbon steel for low-to-moderate temperature pressure vessels. Grade 5 Class 2 offers better creep resistance, while A516 Grade 70 provides good toughness at lower temperatures.
What is the difference between ASTM A387 and ASME SA387 Grade 5 Class 2? They are essentially the same material. SA387 is the ASME designation adopted from ASTM A387 for use in boiler and pressure vessel codes, ensuring compliance with ASME standards for construction and certification.