How does SA387 Grade 22 Class 1 differ from Class 2?
SA387 Grade 22 Class 1
SA387 Grade 22 Class 1 is an annealed 2.25Cr-1Mo alloy steel plate designed for high-temperature pressure vessels, supplied in a softened condition to provide maximum ductility and formability for complex fabrication before final post-weld heat treatment establishes its service strength.
SA387 Grade 22 Class 2
SA387 Grade 22 Class 2 is a normalized and tempered 2.25Cr-1Mo alloy steel plate for welded pressure vessels, offering higher as-delivered strength than Class 1 for more efficient design while maintaining excellent resistance to high-temperature hydrogen attack and creep after mandatory post-weld heat treatment.
The difference between SA387 Grade 22 Class 1 and Class 2 lies entirely in their heat treatment condition and resulting mechanical properties, not chemical composition. Both have identical chemistry (2.25% Cr, 1% Mo), but they are supplied in different states to serve distinct purposes in pressure vessel fabrication.
| Feature | SA387 Grade 22 Class 1 | SA387 Grade 22 Class 2 |
|---|---|---|
| Standard Condition | Annealed (Furnace cooled) | Normalized and Tempered (N&T) |
| Primary Purpose | To provide maximum ductility and formability for severe cold/hot forming during fabrication. | To provide higher as-delivered strength for design efficiency and moderate forming. |
| Microstructure | Soft ferrite-pearlite (coarse, equilibrium structure). | Stronger, tougher bainite/tempered bainite (refined, transformed structure). |
| Tensile Strength | 415 - 585 MPa (60 - 85 ksi) | 485 - 620 MPa (70 - 90 ksi) |
| Yield Strength (Min.) | 205 MPa (30 ksi) | 275 MPa (40 ksi) |
| Key Mechanical Difference | Lower strength, higher ductility. Optimized for forming ability. | ~34% higher yield strength. Optimized for as-supplied strength. |
| Fabrication Impact | Excellent for severe forming (e.g., deep dishing of heads, tight-radius rolling). Less risk of cracking during cold work. | May require more careful forming; better suited for moderate forming and standard fabrication. |
| Post-Weld Heat Treatment (PWHT) | Mandatory. The PWHT tempers the material, raising its final strength toward Class 2 levels. | Mandatory. PWHT must be controlled to avoid over-tempering the base metal. |
| Final In-Service Strength | After PWHT, approaches but does not fully reach Class 2 strength. Design stress values (ASME) are identical for both classes. | Retains higher strength post-PWHT. Design stress values (ASME) are identical for both classes. |
| Common Selection Driver | "Fabricability First" – Chosen when component geometry is complex and requires extensive forming. | "Strength First" – Chosen for standard vessels where higher as-supplied strength allows thinner walls or simpler fabrication. |
The Core Philosophy Behind the Two Classes:
Class 1 (Annealed) is the "Fabricator's Choice": It is supplied soft so it can be easily shaped into complex components (e.g., heavy reactor heads, forged nozzles from plate). The designer sacrifices initial strength for ease of manufacturing, knowing that the final PWHT will develop the required service properties.
Class 2 (N&T) is the "Designer's Choice": It is supplied strong, allowing the engineer to use higher allowable stresses in initial design calculations, potentially saving weight (thinner walls). It is suitable for most standard vessels.
Critical Technical Note: The Convergence After PWHT
While their as-supplied states differ greatly, the final high-temperature performance converges after proper heat treatment. The ASME Boiler and Pressure Vessel Code (Section II, Part D) assigns identical maximum allowable stress values to both Class 1 and Class 2 at all temperatures. This is because:
Both undergo a final PWHT.
The high-temperature creep strength is governed by the alloy composition (2.25Cr-1Mo), which is identical.
How to Choose?
Choose Class 1 if your vessel has complex, heavily formed parts.
Choose Class 2 if you have a standard vessel design where higher as-supplied strength is beneficial and forming is routine.
In essence: Class 1 is optimized for the workshop; Class 2 is optimized for the drawing board. Both, after proper fabrication and heat treatment, perform identically in high-temperature hydrogen service.
What is SA387 Grade 22 Class 1?
SA387 Grade 22 Class 1 is an annealed chromium-molybdenum alloy steel plate designed for high-temperature pressure vessel service, with a composition of 2.25% Cr and 1% Mo to resist oxidation and hydrogen attack in refinery and power plant applications.
What are the mechanical properties of SA387 Grade 22 Class 1?
In its annealed condition, SA387 Grade 22 Class 1 typically has a tensile strength of 415-585 MPa (60-85 ksi), a minimum yield strength of 205 MPa (30 ksi), and high ductility, making it suitable for severe forming before final heat treatment.
Where is SA387 Grade 22 Class 1 commonly used?
It is widely used in hydroprocessing reactors, hydrocrackers, heat exchangers, and high-temperature pressure vessels in the petrochemical and power generation industries, especially where resistance to hydrogen attack and creep is required.
How does SA387 Grade 22 Class 1 differ from Class 2?
Class 1 is supplied in an annealed state for better formability, while Class 2 is normalized and tempered for higher as-delivered strength. Class 1 is chosen when complex fabrication is needed before final PWHT.
What is the post-weld heat treatment (PWHT) for SA387 Grade 22 Class 1?
PWHT is mandatory and typically performed at 1250-1300°F (675-705°C) to relieve welding stresses, prevent hydrogen-induced cracking, and restore corrosion resistance in the weld zone for high-temperature service.
What are the equivalent materials to SA387 Grade 22 Class 1?
Common equivalents include UNS K21590, European 10CrMo9-10 (1.7380), and the generic designation 2.25Cr-1Mo steel. However, exact equivalency depends on heat treatment and certification requirements.
Can SA387 Grade 22 Class 1 be used in hydrogen service?
Yes, its 2.25% chromium and 1% molybdenum content provides excellent resistance to high-temperature hydrogen attack (HTHA), making it a standard choice for hydrogen processing units in refineries.
What are the forming and machining guidelines for SA387 Grade 22 Class 1?
Due to its annealed condition, it offers good cold formability but requires preheating for hot forming and controlled machining to avoid excessive work hardening, with tooling suitable for alloy steels.
What standards certify SA387 Grade 22 Class 1 plates?
Plates are certified to ASME SA387/SA387M and ASTM A387/A387M standards, with mill test reports (MTRs) providing chemical and mechanical properties per EN 10204 3.1 or 3.2.
How does SA387 Grade 22 Class 1 compare to Grade 11 or Grade 5?
Compared to lower-alloy grades like Grade 11 (1.25Cr-0.5Mo) or Grade 5 (5Cr-0.5Mo), Grade 22 offers superior high-temperature strength and hydrogen resistance, but requires stricter welding and heat treatment controls.
Full specification and details are available on request. The above information is provided for guidance purposes only. For specific design requirements please contact our technical sales staff.
