What Is ASTM A387 Vessel Plate?
ASTM A387 is the standard specification for chromium-molybdenum alloy steel plates intended mainly for welded boilers and pressure vessels that operate at elevated temperature. Chromium improves oxidation and corrosion resistance in hot service, while molybdenum raises creep strength and keeps tensile properties stable during long exposure. Because mechanical behaviour depends on both grade and class, a complete order description must state the grade and class, for example ASTM A387 Grade 11 Class 2, together with the required heat treatment condition and plate thickness range. A designation that quotes only the specification number, such as A387 vessel plate, is incomplete for engineering and procurement purposes.
Grades and Chemical Composition
The table below summarises the commonly ordered grades and their typical specified ranges. Values refer to the ladle analysis limits of the specification; the applicable class determines the exact carbon and silicon requirement.
| Grade | Type | Carbon | Manganese | Silicon | Chromium | Molybdenum |
|---|---|---|---|---|---|---|
| A387 Gr 11 | 1.25Cr-0.5Mo | 0.05-0.17% | 0.40-0.65% | 0.50-0.80% | 1.00-1.50% | 0.45-0.65% |
| A387 Gr 12 | 1Cr-0.5Mo | 0.05-0.17% | 0.40-0.65% | 0.15-0.40% | 0.80-1.20% | 0.45-0.60% |
| A387 Gr 22 | 2.25Cr-1Mo | 0.05-0.15% | 0.30-0.60% | 0.50% max | 2.00-2.50% | 0.90-1.10% |
| A387 Gr 5 | 5Cr-0.5Mo | 0.15% max | 0.30-0.60% | 0.50% max | 4.00-6.00% | 0.45-0.65% |
| A387 Gr 9 | 9Cr-1Mo | 0.15% max | 0.30-0.60% | 1.00% max | 8.00-10.00% | 0.90-1.10% |
Phosphorus and sulfur are normally controlled to 0.035% maximum or lower, and residual element limits apply for hydrogen service. Thicker plates and plates intended for wet hydrogen sulfide service are usually ordered with supplementary requirements for hardness, ultrasonic examination and restricted chemistry.
Mechanical Properties
Plates are supplied in two strength classes. Class 1 is supplied at a lower minimum tensile strength for easier forming, while Class 2 is the more common choice for pressure equipment. Minimum values in the normalised and tempered condition are summarised below.
| Grade and class | Yield strength, min | Tensile strength | Elongation, min |
|---|---|---|---|
| A387 Gr 11 Class 2 | 310 MPa | 515-690 MPa | 20% in 200 mm |
| A387 Gr 12 Class 2 | 275 MPa | 485-620 MPa | 20% in 200 mm |
| A387 Gr 22 Class 2 | 310 MPa | 515-690 MPa | 18% in 200 mm |
| A387 Gr 5 | 310 MPa | 515-690 MPa | 18% in 200 mm |
| A387 Gr 9 | 310 MPa | 515-690 MPa | 18% in 200 mm |
Behaviour at Elevated Temperature
Heat resistance is the main reason for specifying an A387 plate. The chromium content forms a protective oxide layer that slows scaling, and the molybdenum addition raises the temperature at which the steel begins to creep under sustained load. As a practical guideline, 1Cr and 1.25Cr grades such as Gr 12 and Gr 11 are common for service temperatures up to roughly 540 Celsius, 2.25Cr-1Mo Gr 22 is widely used up to about 580 Celsius, and the higher-chromium Gr 5 and Gr 9 plate grades are selected where oxidation resistance and creep strength must be retained at higher temperatures. Actual allowable stresses must always be taken from the applicable pressure vessel design code rather than from room-temperature tensile data.
Welding, Heat Treatment and Fabrication
Preheating is required, typically from 150 Celsius for thin 1.25Cr sections up to 200-250 Celsius for thick Gr 22 and 5Cr-9Cr plates.
Post-weld heat treatment is mandatory for pressure service to temper the hard, brittle heat-affected zone and to restore toughness.
Low-hydrogen consumables are essential, and filler metal chemistry should match the base metal chromium and molybdenum content to preserve high-temperature strength.
Heat input should be controlled, and interpass temperature limited, to avoid excessive hardness and cracking.
Cold forming should respect a minimum bend radius that increases with plate thickness, and flame-cut edges of thick plate should be machined or ground before welding.
Hardness surveys of the weld and heat-affected zone are commonly requested as part of the fabrication quality record.
Application Areas
A387 plate is fabricated into reactors, hydrocracking and hydrotreating vessels, heat exchangers, steam drums, boiler furnace walls, tube sheets, headers, coal gasification equipment, and other hot-wall pressure components in refineries, petrochemical plants and power stations. Grade selection is driven by design temperature, hydrogen partial pressure, plate thickness and the welding and heat treatment capability of the fabricator, so the plate grade and the welding procedure are normally qualified together.
Ordering and Verification
A complete purchase description should include the specification, grade and class, plate thickness and size, delivery condition, and any supplementary requirements for ultrasonic testing, restricted chemistry or hardness limits. Each plate is normally supplied with a mill test certificate that reports heat number, chemical analysis, tensile results and heat treatment, and plates should carry legible markings that allow the certificate to be traced back to the individual plate.
Frequently Asked Questions
Q: Is an A387 plate the same as a boiler quality carbon steel plate?
No. A carbon-manganese boiler plate relies on strength and toughness alone, while an A387 plate is alloyed with chromium and molybdenum specifically to resist creep and oxidation at high temperature.
Q: Why must the class be stated on the order?
Class 1 and Class 2 differ in minimum tensile strength and allowable carbon and silicon ranges, and the class also affects the qualified welding procedure and post-weld heat treatment cycle.
Q: Should Gr 11 or Gr 22 be chosen for a heat exchanger?
Gr 11 is normally adequate up to about 540 Celsius, while Gr 22 is preferred for higher design temperatures or thicker sections where creep strength governs the wall thickness.
Q: Can A387 plate be welded without post-weld heat treatment?
For pressure service, post-weld heat treatment is normally required to temper the heat-affected zone; only very thin sections covered by a qualified procedure may be exempted.
Q: How is high-temperature strength verified?
Room-temperature tensile testing confirms the specified minimum properties, while long-term performance is based on published creep and stress rupture data used in the pressure vessel design code.
Q: What thickness range is available?
A387 plate is commonly rolled from about 6 mm up to 150 mm or more, and the required thickness is determined by the design pressure, temperature and corrosion allowance.