ASTM A387 Grade 22 Class 2

Overview

Designation system

ASTM grade and tensile-strength class; UNS alloy identifier K21590

Product forms

Rolled pressure-vessel plate

Condition

Normalized and tempered, Accelerated cooled from the austenitizing temperature and tempered, when permitted by the purchaser, Plate supplied without final heat treatment for subsequent purchaser heat treatment, subject to the specification's heat, treatment and property requirements

What the designation controls

ASTM A387/A387M covers killed, thermally treated chromium-molybdenum alloy-steel plate intended primarily for welded boilers and pressure vessels operating at elevated temperature. “Grade 22” identifies the nominal 2¼Cr-1Mo chemistry. “Class 2” selects the higher tensile-strength level available for Grade 22. The designation applies to plate; it should not be transferred to pipe, tube, forgings, fittings or castings, which have their own specifications and requirements.

A387/A387M works together with ASTM A20/A20M. A20/A20M supplies the common rules for ordering, dimensions and tolerances, testing, retesting, workmanship, defect repair, marking and certification. Where the two documents conflict, A387/A387M governs. For ASME Code construction, the material is normally specified as SA-387 Grade 22 Class 2 under the applicable edition of ASME BPVC Section II rather than relying only on the ASTM designation.

ASTM and ASME editions are not interchangeable by assumption. The material specification edition accepted by the governing construction code and project documents must be confirmed.

Specified chemical composition

Chromium improves oxidation and scaling resistance and contributes to hardenability and elevated-temperature strength. Molybdenum is particularly important to creep strength. The alloy content also makes the steel more hardenable than carbon pressure-vessel plate, increasing the need for controlled welding heat input, preheat, hydrogen control and postweld heat treatment.

The standard composition does not tightly restrict all residual elements associated with temper embrittlement. For long-term elevated-temperature service, especially heavy-wall hydrogen equipment, supplementary impurity controls may be necessary.

Heat treatment and Class 2 properties

Grade 22 plate must be thermally treated. Permitted routes include annealing, normalizing and tempering, or—with purchaser permission—accelerated cooling from the austenitizing temperature followed by tempering. The minimum tempering temperature for Grade 22 is 675°C (1250°F). However, the Class 2 tensile requirements are explicitly not applicable to annealed material, so Class 2 plate in its final compliant condition is normally normalized and tempered or accelerated-cooled and tempered.

The specification does not impose a universal maximum plate thickness. Maximum producible thickness is limited by the ability of the selected composition, rolling practice and heat treatment to satisfy the required mechanical properties. This is important for heavy-wall reactors: availability at a nominal thickness does not by itself demonstrate through-thickness uniformity, adequate toughness after fabrication heat treatments or suitability for the intended service.

Mechanical properties certified in the mill delivery condition may not represent properties after long or repeated fabrication PWHT cycles. Simulated PWHT testing should be specified when the final vessel will receive significant thermal exposure during fabrication.

Elevated-temperature and hydrogen-service significance

Grade 22 Class 2 is widely associated with refinery reactors, hydroprocessing vessels, high-temperature heat exchangers, boiler components and other welded pressure equipment. Its Cr-Mo alloy system offers better creep strength and oxidation resistance than ordinary carbon or carbon-manganese plate. These characteristics explain the grade's use, but A387/A387M supplies room-temperature acceptance properties rather than a complete elevated-temperature design basis.

Allowable stress, creep-regime limits and permitted design temperature must be taken from the governing construction code for the exact material form, class, thickness and code edition. There is no single defensible “maximum service temperature” inherent in the A387 Grade 22 Class 2 designation.

For high-temperature hydrogen service, material selection must be checked against the applicable edition of API RP 941 using operating temperature and hydrogen partial pressure, together with the required design margin and owner practices. Heavy-wall high-pressure hydrogen vessels may also invoke API RP 934-A, which adds detailed requirements for materials, fabrication, welding, heat treatment, toughness, inspection and documentation. Compliance with A387/A387M alone does not establish fitness for hydrogen service.

Conventional 2¼Cr-1Mo steel can be susceptible to temper embrittlement after long exposure in an embrittling temperature range. Embrittlement raises the ductile-to-brittle transition temperature without necessarily producing an obvious reduction in room-temperature tensile strength. This is particularly significant during startup, shutdown, hydrotest, upset or depressurization, when a thick vessel may be highly stressed at a relatively low metal temperature.

Welding and fabrication

The grade is intended for fusion welding, but it is not a routine carbon-steel fabrication material. Its hardenability creates a risk of hard, crack-sensitive weld metal and heat-affected zones if preheat, interpass temperature, heat input and cooling are uncontrolled. Low-hydrogen consumables and handling practices are fundamental. The welding procedure must be qualified for the applicable construction code, material grouping, thickness, joint configuration, consumable classification and intended heat-treatment cycle.

PWHT is commonly required by pressure-equipment construction codes for 2¼Cr-1Mo weldments. Its functions include tempering hard transformation products and reducing residual stress. The selected cycle must also preserve base-metal and weld-metal strength and toughness. Multiple fabrication PWHT cycles, repair cycles and any future field heat treatment should therefore be included when specifying simulated-PWHT test conditions.

Repair welding of material removed from hydrogen service requires a dedicated engineering assessment. Hydrogen retained in 2¼Cr-1Mo steel can substantially increase cracking susceptibility during welding. Bake-out, preheat, postheat, consumable control, excavation geometry, inspection and PWHT cannot safely be selected from the original material designation alone.

Do not infer a universal preheat or PWHT temperature from the grade name. Required parameters depend on the construction or repair code, thickness, restraint, consumable, hydrogen level, heat input, service history and qualified welding procedure.

Testing and purchase specification

A basic order should identify the governing ASTM or ASME specification and edition, Grade 22, Class 2, unit system, dimensions, required heat-treatment condition, quantity, construction code and certification requirements. For Code work, the purchase description should use the material designation recognized by that Code and should identify any project-specific material specification layered over the base standard.

Inspection documents should be checked for the correct grade and class, heat number and plate identity, actual heat analysis, heat-treatment route and temperatures, tensile results, specimen form, applicable supplementary-test results and traceability after cutting. The plate marking should indicate the delivery heat treatment: A for annealed, N for normalized and tempered, or Q for accelerated-cooled and tempered, as applicable.

If impact toughness, through-thickness quality, hydrogen-service controls or resistance to temper embrittlement are important, a purchase order stating only “A387 Grade 22 Class 2” is incomplete.

Substitution boundaries

Substitution requires more than matching nominal 2¼Cr-1Mo chemistry. The comparison must address product form, governing material specification, strength class, heat treatment, thickness capability, toughness, simulated PWHT, inspection, permissible design stresses and construction-code acceptance. Existing equipment also requires review of service exposure and degradation history.

Grade 22 Class 1 has the same basic alloy system but substantially lower specified room-temperature strength and is not interchangeable with Class 2 without design and code review. The vanadium-modified 2¼Cr-1Mo-¼V family used in some modern heavy-wall reactors is a different material system with different specifications, welding controls, heat treatment and elevated-temperature properties.

Related designations

A related designation or cross-standard counterpart is not a declaration of equivalence; confirm the standard, product form and condition before substituting.