ASTM A387 Grade 11 Class 2
UNS K11789
Chromium-molybdenum low-alloy pressure-vessel steel plate for elevated-temperature service · ASTM A387/A387M-25 — Pressure Vessel Plates, Alloy Steel, Chromium-Molybdenum · ASTM A20/A20M — General Requirements for Steel Plates for Pressure Vessels · ASME SA-387/SA-387M — code-adopted specification in BPVC Section II, Part A
ASTM A387 Grade 11 Class 2 is the higher-strength class of 1¼Cr-½Mo alloy-steel plate intended primarily for welded boilers and pressure vessels in elevated-temperature service. Grade 11 defines the chemistry, while Class 2 defines the room-temperature tensile level and effectively requires a strengthened heat-treated condition rather than annealing. Proper specification requires more than the grade name: the governing ASTM or ASME edition, heat-treatment route, plate dimensions, required toughness and ultrasonic examination, anticipated fabrication heat cycles, construction code, and service environment must all be established.
- 1.25% Cr–0.5% MoNominal alloy system · Chromium and molybdenum provide elevated-temperature strength and improved oxidation resistance relative to ordinary carbon steel; this is not stainless steel.
- 515–690 MPa (75–100 ksi)Class 2 tensile strength · Room-temperature specification requirement; Class 2 is not applicable to annealed plate.
- 310 MPa (45 ksi)Minimum yield strength · Determined at 0.2% offset.
- 620°C (1150°F)Minimum tempering temperature · ASTM minimum for Grade 11; actual producer heat treatment may be higher.
- Plate onlyBase specification scope · Pipe, tube, forgings, castings and fittings use separate specifications even when informally described as Grade 11 or P11-family materials.
- Not automatically defined by the basic grade designationImpact and UT requirements · Required test temperature, acceptance energy, specimen orientation and ultrasonic examination standard must be invoked through the order, construction code or a
Overview
- Designation system
- ASTM grade and tensile-strength class; UNS K11789 identifies the Grade 11 chemistry
- Product forms
- Hot-rolled pressure-vessel plate
- Condition
- Normalized and tempered (N), Accelerated cooled from the austenitizing temperature and tempered (Q), when permitted by the purchaser
- Density
- 7.85 g/cm³ (Representative engineering value for low-alloy steel; density is not an ASTM A387 acceptance requirement.)
How the designation works
A387/A387M is a material specification for chromium-molybdenum alloy-steel pressure-vessel plate. “Grade 11” identifies the 1¼Cr-½Mo chemical family. “Class 2” identifies the higher room-temperature tensile-strength level. Class 1 and Class 2 therefore share the same Grade 11 chemistry but are not interchangeable strength conditions.
ASTM A387/A387M governs commercial material supplied to the ASTM specification. For construction under the ASME Boiler and Pressure Vessel Code, material is normally specified as SA-387/SA-387M Grade 11 Class 2 under the applicable BPVC edition. ASTM and ASME requirements are closely related, but the purchase order and material certification must identify the specification and edition required by the construction code. An ASTM certificate should not be relabeled as ASME material without satisfying the applicable Code rules.
ASTM A20/A20M supplies the common rules for ordering, manufacture, testing, dimensional tolerances, surface quality, repair, marking and certification. Where A387 and A20 conflict, A387 controls.
Specified chemical composition
| Element | Heat analysis | Product analysis |
|---|---|---|
| Carbon | 0.05–0.17 | 0.04–0.17 |
| Manganese | 0.40–0.65 | 0.35–0.73 |
| Phosphorus | 0.025 max | 0.025 max |
| Sulfur | 0.025 max | 0.025 max |
| Silicon | 0.50–0.80 | 0.44–0.86 |
| Chromium | 1.00–1.50 | 0.94–1.56 |
| Molybdenum | 0.45–0.65 | 0.40–0.70 |
Heat analysis and product analysis limits are intentionally different. Class 1 and Class 2 use the same Grade 11 chemistry.
Chromium improves hardenability and resistance to oxidation and scaling, while molybdenum contributes elevated-temperature strength and suppresses softening. The relatively broad carbon range permits substantial variation in weldability and hardenability between heats. Actual heat chemistry, plate thickness and cooling history therefore matter when establishing welding procedures and predicting heat-affected-zone hardness.
Do not use a generic supplier composition table as the purchase requirement. Confirm the limits against the specifically invoked edition of ASTM A387/A387M or ASME SA-387/SA-387M.
Heat treatment and the meaning of Class 2
| Route | ASTM marking | Important qualification |
|---|---|---|
| Normalized and tempered | N | Conventional Class 2 delivery route. |
| Accelerated cooled from the austenitizing temperature, then tempered | Q | Air-blast or liquid-quench cooling requires purchaser permission. |
| Annealed | A | Not applicable to Class 2 tensile requirements; annealed Grade 11 is a Class 1 condition. |
Grade 11 must be tempered at not less than 620°C (1150°F).
The specification does not prescribe a single austenitizing temperature or a single resulting microstructure for Grade 11. Normalizing commonly produces a ferritic-bainitic or predominantly bainitic structure depending on chemistry, thickness and cooling rate; accelerated cooling produces a more strongly transformed structure. Tempering reduces hardness and residual stress while establishing the Class 2 strength-ductility balance.
Plate thickness is not capped by a fixed number in A387. The practical maximum is the thickness at which the selected chemistry and heat-treatment route can still satisfy all specified mechanical requirements. Through-thickness cooling becomes less effective as thickness increases, so heavy plate may require accelerated cooling, tighter chemistry or additional project controls.
Room-temperature mechanical requirements
| Property | Requirement | Test qualification |
|---|---|---|
| Tensile strength | 515–690 MPa (75–100 ksi) | Room-temperature tension test |
| Yield strength | 310 MPa (45 ksi) minimum | 0.2% offset |
| Elongation in 200 mm (8 in.) | 18% minimum | Subject to applicable A20/A20M elongation adjustments for thin plate |
| Elongation in 50 mm (2 in.) | 22% minimum | Subject to the applicable specimen and A20/A20M provisions |
| Reduction of area | No Grade 11 requirement in the basic table | May be introduced by another specification or project requirement |
The inch-pound and SI systems are separately standard systems. Values should not be mixed when determining conformance.
A387 does not establish a general hardness limit, mandatory Charpy energy, nil-ductility transition temperature, creep-rupture requirement or elevated-temperature tensile value for the basic Grade 11 Class 2 designation. Such controls must come from the construction code, purchaser specification or explicitly invoked supplementary requirements.
Elevated-temperature and service behavior
Grade 11 is selected where carbon steel does not provide adequate elevated-temperature strength or oxidation resistance, but A387 itself remains primarily a material-supply specification. It does not assign allowable design stress, a universal maximum service temperature or component life. ASME construction uses the applicable Section II, Part D stress tables together with the vessel or boiler construction rules, design life, weld-joint factors and service-specific degradation assessment.
Long exposure in the creep range can change carbide distribution and reduce strength and toughness. Chromium-molybdenum steels can also suffer temper embrittlement under unfavorable chemistry and thermal exposure. These mechanisms are not represented by the room-temperature tensile certificate alone; long-term operating temperature, previous heat treatment, shutdown behavior and service history must be considered when assessing existing equipment.
The alloy is not inherently corrosion resistant. Its chromium content is far below stainless-steel levels, and corrosion, sulfidation, wet-H₂S damage, amine cracking or other process-environment risks require a separate materials review. Corrosion-resistant weld overlay or clad construction may be selected where process-side corrosion demands it, but the cladding system introduces separate fabrication and inspection requirements.
For high-temperature, high-pressure hydrogen service, selection must be checked against the current API RP 941 guidance and the complete operating envelope, including hydrogen partial pressure. API RP 934-C provides additional materials and fabrication guidance specifically for heavy-wall 1¼Cr-½Mo vessels in high-pressure hydrogen service at or below its stated temperature scope. Compliance with A387 alone does not establish resistance to high-temperature hydrogen attack.
Welding and fabrication consequences
Grade 11 is fusion weldable, but it is a hardenable low-alloy steel rather than a carbon-steel plate that can be fabricated casually. Welding procedures must control hydrogen, preheat, interpass temperature, heat input, consumable conditioning, joint restraint and cooling rate. Required values depend on thickness, actual chemistry, process, consumable, restraint, toughness requirements and construction code; they should be established by a qualified WPS rather than taken from a generic grade chart.
Postweld heat treatment is normally a central part of pressure-equipment fabrication with this alloy, but the required temperature, holding time, heating and cooling rates, exemptions and local-heating rules belong to the governing construction code and project specification. Excessive accumulated tempering exposure can reduce tensile strength, while inadequate treatment can leave excessive hardness and residual stress. The plate purchase specification should therefore account for all anticipated shop, repair and future field-PWHT cycles.
Hot forming or austenitizing-temperature operations can replace the original mill heat treatment and must be followed by an approved restoration heat treatment. Cold forming may raise hardness and reduce local ductility; whether stress relief or reheat treatment is required depends on strain, thickness and construction-code rules. Flame or induction heating must be controlled to avoid unintended local transformation or overtempering.
Mill repair welding is permitted only with purchaser approval, and repairs must satisfy the construction code specified by the purchaser.
Testing and supplementary controls
| Control | Why it matters | What the order must define |
|---|---|---|
| Simulated PWHT of mechanical-test coupons | Demonstrates properties after anticipated fabrication thermal exposure | Temperature range, holding time, number of cycles and cooling assumptions |
| Charpy V-notch testing | Establishes toughness for the applicable minimum design-metal temperature and thickness | Test temperature, absorbed-energy criteria, orientation, specimen size and testing frequency |
| Ultrasonic examination | Controls internal lamination and discontinuities in heavy or critical plate | ASTM A435/A435M, A577/A577M or A578/A578M; scanning and acceptance level |
| Additional tension tests | Provides greater coverage across plates, heat-treatment lots or locations | Sampling frequency, orientation and test condition |
| Product analysis | Confirms plate chemistry rather than relying only on heat analysis | Sampling frequency and reporting requirements |
| Through-thickness properties | Addresses lamellar-tearing risk in highly restrained joints | Required reduction-of-area criteria and applicable test specification |
| Hardness mapping | Supports weldability, hydrogen-service or fabrication controls | Locations, method, load, acceptance limit and test condition |
These are not automatically imposed merely by writing “A387 Grade 11 Class 2.” Applicable supplementary requirements and acceptance criteria must be stated in the purchase order.
Impact-test results are meaningful only when specimen orientation, size, notch direction, location, test temperature and acceptance criteria are retained. Likewise, an ultrasonic statement such as “UT tested” is incomplete without the examination standard and acceptance class or level.
Specifying and receiving plate
| Item | Required decision |
|---|---|
| Material designation | ASTM A387/A387M or ASME SA-387/SA-387M, Grade 11, Class 2 |
| Edition and code basis | Exact material-specification edition and applicable construction-code edition |
| Units | Inch-pound A387 or SI A387M system; do not combine the systems for acceptance |
| Dimensions | Thickness, width, length, quantity and permitted dimensional tolerances |
| Heat-treatment condition | N or Q; purchaser approval is needed for accelerated cooling |
| Fabrication thermal exposure | Expected hot forming, PWHT temperature and accumulated holding time, including repair cycles |
| Toughness | Test temperature, energy criteria, orientation, specimen size and frequency |
| Internal soundness | Selected UT standard and acceptance level |
| Service-specific controls | Hydrogen service, temper-embrittlement controls, residual-element restrictions, hardness limits, cladding or overlay requirements |
| Documentation | Material test report, heat and product analyses as required, heat-treatment records, mechanical results, supplementary-test results and traceability markings |
At receipt, verify the specification, grade, class, heat-treatment marking, heat number, dimensions and test results against the purchase order. Confirm that the certificate reports the actual delivery condition and that any simulated-PWHT, toughness or ultrasonic results correspond to the ordered plate and required test condition.
Substitution boundaries
A common chromium-molybdenum chemistry does not make products interchangeable. Grade 11 pipe, tube, forgings, fittings and castings are controlled by different product specifications, heat treatments, mechanical requirements, dimensions and quality provisions. They may form part of a compatible welded material system, but they are not substitutes for A387 plate certification.
Cross-standard European or Japanese steels should be treated only as possible engineering counterparts. Substitution requires comparison of product form, complete chemistry, strength after all thermal cycles, toughness, test orientation and frequency, dimensional requirements, allowable-stress listing, construction-code acceptance and service-specific restrictions. A commercial equivalence table is not sufficient authorization.
Sources
- ASTM A387/A387M-25 — Standard Specification for Pressure Vessel Plates, Alloy Steel, Chromium-MolybdenumASTM International
- ASTM A20/A20M — Standard Specification for General Requirements for Steel Plates for Pressure VesselsASTM International
- BPVC Section II, Part A — Ferrous Material SpecificationsASME
- The Grade 11 and 12 Low Alloy Steel HandbookElectric Power Research Institute
- API Standards Digital Catalog — RP 941 and RP 934-CAmerican Petroleum Institute
- API Standards Plan — RP 934-C and RP 941American Petroleum Institute
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