What Is A387 Grade 22 Class 2 Steel?
ASTM A387 / ASME SA387 Grade 22 Class 2 is a chromium-molybdenum alloy steel plate with a nominal composition of 2.25% chromium and 1.0% molybdenum, specified for welded pressure vessels and boilers operating at elevated temperature and pressure. It is the higher-strength variant of Grade 22: Class 2 requires mandatory normalizing and tempering, which raises the tensile and yield strength well above Class 1 while keeping adequate toughness. The grade is identified internationally by UNS number K21590, and its SA387 designation is the ASME Boiler and Pressure Vessel Code adoption of the same ASTM A387 specification, so the two are interchangeable for code work.
Chemical Composition
The heat-analysis limits for Grade 22 are shown below; product analysis tolerances apply to the finished plate.
| Element | C | Mn | Si | P | S | Cr | Mo |
|---|---|---|---|---|---|---|---|
| Range / max (%) | 0.05–0.15 | 0.30–0.60 | 0.50 max | 0.035 max | 0.035 max | 2.00–2.50 | 0.90–1.10 |
Chromium forms a dense protective oxide film that gives the steel its oxidation and corrosion resistance in hot, sour and hydrogen-bearing environments. Molybdenum is the element that matters most for creep: it raises the high-temperature strength and resistance to hydrogen attack in the 400–600 °C range, which is the reason the grade holds its structural integrity over decades of service.
Mechanical Properties and Heat Treatment
Room-temperature minimum values for Class 2 plate, with Class 1 shown for comparison:
| Property | Class 2 | Class 1 (reference) |
|---|---|---|
| Tensile strength (MPa / ksi) | 515–690 / 75–100 | 415–585 / 60–85 |
| Yield strength, min (MPa / ksi) | 310 / 45 | 205 / 30 |
| Elongation in 50 mm, min | 18% | 18% |
| Reduction of area, min | 45% | 40% |
Class 2 is supplied normalized at approximately 890–950 °C with air cooling, then tempered at approximately 650–720 °C with air cooling. This thermal cycle balances strength against toughness and stabilises the microstructure for long-term elevated-temperature service. Charpy V-notch impact testing is frequently specified as a supplementary or project requirement; acceptance temperature and energy are defined in the purchase order.
Global Equivalent Grades
Internationally recognized equivalents, commonly cited for substitution studies, include EN 10028-2 / DIN 17155 10CrMo9-10 (1.7380), GB/T 713 12Cr2Mo1R, JIS G3103 SCMV4, the French AFNOR 10CD9-10, the Chile GOST 10KH2M and the Swedish SS 2218. When substituting, the delivery condition matters: Grade 22 Class 2 is normalized and tempered, and only equivalents supplied in the same condition can be treated as directly comparable.
Typical Applications
The grade is used for critical high-temperature and high-pressure equipment: hydrocracking reactors, catalytic reformers, hot separators, coke drums and hydrogen service vessels in refineries; boiler drums, superheater headers and industrial heat exchangers in power generation; high-pressure separators and LPG storage tanks in oil and gas; and ammonia converters, sulfur recovery units and high-temperature heat exchangers in the chemical industry. Thickness ranges of 6–300 mm with widths up to 4500 mm cover the bulk of these fabrications.
Welding and Fabrication Guidelines
Grade 22 plates are welded with low-hydrogen processes. Typical practice: preheat at 150–250 °C, with higher preheat for plates above 25 mm thickness; matching filler metals such as AWS E9018-B3 for SMAW and EB3/FB3 for SAW, which deposit the 2.25Cr–1Mo chemistry; and post-weld heat treatment at 680–710 °C for about one hour per 25 mm of thickness, with a minimum hold of two hours, to relieve residual stress and restore toughness in the heat-affected zone. Thermal cutting with laser or plasma should be carried out with preheat at 150–200 °C to avoid edge cracking.
Frequently Asked Questions
What is the maximum service temperature for A387 Grade 22 Class 2?
Peak temperature applications approach 600 °C (1112 °F); for long-term continuous operation, design temperatures are generally kept at or below about 550 °C, subject to the governing design code and the thickness-based allowable stress curves.
Can A387 Grade 22 Class 2 replace 15CrMoR?
No. 15CrMoR (GB/T 713) is a 1Cr–0.5Mo class material comparable to Grade 11, with significantly lower high-temperature strength and creep resistance than the 2.25Cr–1Mo Grade 22. Where Grade 22 Class 2 properties are required, a lower-chromium grade is not a valid substitute.
Is 10CrMo9-10 the same as Grade 22 Class 2?
EN 10028-2 10CrMo9-10 is the most frequently cited European equivalent grade for 2.25Cr–1Mo plate. It is close in chemistry and strength, but the delivery condition and acceptance limits should be checked against both specifications for the project.
Why is Class 2 preferred over Class 1?
Class 2 delivers roughly 50% higher minimum yield strength (310 MPa versus 205 MPa) through mandatory normalizing and tempering, which allows thinner vessel walls at the same design pressure and reduces weight and welding volume.
Is impact testing required for Grade 22 Class 2?
The standard itself does not mandate Charpy testing by default; impact testing at an agreed temperature (for example −52 °C or higher, per project requirements) is typically added as a supplementary or contractual requirement for cryogenic-adjacent or cyclic service.
What filler metal is used for welding Grade 22?
Matching 2.25Cr–1Mo consumables are used: AWS E9018-B3 for SMAW and EB3/FB3 for SAW. Lower-chromium electrodes such as E8018-B2 belong to the 1.25Cr–0.5Mo family and are not matching for this grade.
