ASTM A387/A387M Grade 91 Class 2

Overview

Designation system

ASTM grade, type and tensile-strength class designation

Product forms

Plate for welded boilers, pressure vessels, headers and other elevated-temperature pressure components

Condition

Normalized and tempered, Accelerated cooled from the austenitizing temperature and tempered, Stress-relieved or annealed plate ordered for subsequent heat treatment by the purchaser

What the designation means

ASTM A387/A387M covers chromium-molybdenum alloy steel plate intended primarily for welded boilers and pressure vessels in elevated-temperature service. Grade 91 is the modified 9Cr-1Mo composition strengthened by controlled additions of vanadium, niobium and nitrogen. These additions distinguish it from Grade 9 and produce substantially higher creep strength when the required microstructure has been established and preserved.

The unqualified designation “ASTM A387 Grade 91 Class 2” is ambiguous under current editions. The purchase order, material certificate and plate marking should establish Type 1 or Type 2. Grade 91 has no Class 1 option.

ASTM A20/A20M supplies the general pressure-vessel-plate requirements covering matters such as ordering information, testing, dimensional tolerances, workmanship, certification and marking. If A387 and A20 requirements conflict, A387 governs. For construction under the ASME Boiler and Pressure Vessel Code, the material should be specified as ASME SA-387/SA-387M Grade 91 Class 2 with the applicable type rather than relying on an ASTM designation alone.

Type 1 and Type 2 chemistry

Both types use UNS K90901 and share the same principal 9Cr-1Mo-V-Nb alloy concept. Type 2 narrows manganese, silicon, chromium, molybdenum, nickel and nitrogen controls and imposes tighter limits on sulfur and residual elements. These restrictions were introduced because relatively small composition differences can materially affect long-term creep behavior. Composition values are mass percent.

Positive material identification based only on chromium and molybdenum cannot reliably distinguish Grade 91 from all related 9Cr materials or distinguish Type 1 from Type 2. Verification of carbon, nitrogen, boron and the low-level residual restrictions requires suitable laboratory analysis and review of the traceable material test report.

Heat treatment and required room-temperature properties

The intended structure is tempered martensite. Grade 91 plate is thermally treated either by normalizing and tempering or by accelerated cooling from the austenitizing temperature followed by tempering. The specified austenitizing range is 1900–1975°F [1040–1080°C], followed by tempering at 1350–1470°F [730–800°C]. The heat-treatment route must be recorded and correlated with the tested plate or representative coupons.

Plate may be ordered without the final treatment required to achieve specification properties and supplied in an annealed or stress-relieved condition. In that case, the purchaser assumes responsibility for completing the required heat treatment and demonstrating compliance. Such material must not be treated as finished Grade 91 Class 2 plate merely because its chemistry is acceptable.

Why processing control is unusually important

Grade 91 obtains its creep strength from a tempered martensitic lath structure stabilized by chromium-rich carbides and fine vanadium-niobium carbonitrides. Austenitizing, cooling and tempering determine the prior-austenite grain structure, martensitic transformation, precipitate population, hardness and toughness. Improper heat treatment can produce untempered martensite, overtempered material, retained ferrite or other structures that pass a limited set of room-temperature checks yet perform poorly in long-duration service.

Weld heat-affected zones contain several microstructural regions created by different peak temperatures. In creep service, the fine-grained or intercritical region can become the life-limiting location associated with Type IV cracking. Consequently, component integrity cannot be assessed from base-metal tensile strength alone; weld design, procedure qualification, thermal history, inspection and service assessment are all significant.

ASTM A387 does not provide a universal maximum service temperature, allowable stress table or design life. These come from the governing construction code and its adopted material data, such as the applicable ASME BPVC section and Section II, Part D.

Welding, repair and forming

Grade 91 is weldable, but it is not tolerant of casual fabrication practice. Welding produces hard martensitic weld metal and heat-affected zones, making low-hydrogen controls, controlled preheat and interpass temperature, suitable 9Cr-1Mo-V consumables, controlled cooling and postweld heat treatment central parts of the welding procedure. Exact parameters must come from the applicable construction code and a qualified WPS; they should not be inferred from the plate specification alone.

Postweld heat treatment tempers newly formed martensite, reduces hardness and residual stress, and improves toughness. The welding sequence must account for completion of the martensitic transformation before tempering where required by the qualified procedure. Excessive or repeated subcritical thermal exposure can progressively overtemper the base metal and should be included in the planned heat-treatment history and simulated-PWHT testing where applicable.

Hot or cold forming can alter the qualified microstructure. Forming temperature, strain, section thickness and subsequent heat treatment therefore need an approved procedure. Severe hot forming generally requires restoration of the normalized-and-tempered condition; cold forming may also trigger reheat-treatment requirements under the construction code or project specification. The final thermal cycle must be considered before ordering plate and qualifying test coupons.

Testing and purchase specification

The base A387 designation establishes chemistry, heat treatment and room-temperature tensile requirements. It does not automatically impose every examination or toughness requirement needed for a finished pressure component. Impact testing, simulated fabrication heat treatment, ultrasonic examination, through-thickness properties and additional chemical restrictions must be invoked when required by the construction code, design conditions or project specification.

Charpy impact testing is not an automatic base requirement of the A387 Grade 91 designation. If toughness is important at startup, shutdown, hydrotest or minimum design metal temperature, the applicable requirements must be established through the construction code and purchase specification.

Use and substitution boundaries

The material is principally selected where a combination of high-temperature strength, creep resistance and improved steam-side oxidation resistance is required. It is not a stainless steel and should not be presumed resistant to wet corrosion, chlorides, acids, caustic environments, sour service or other process-specific damage mechanisms without a separate corrosion and environmental assessment.

The Grade 91 alloy concept appears in plate, pipe, tube, forging, fitting and casting specifications, but the resulting designations are not interchangeable. Each product specification controls manufacturing route, heat treatment, dimensions, testing and acceptance properties. Likewise, a cross-standard grade with similar nominal chemistry is not an automatic substitute for A387 or SA-387 plate. Substitution requires a documented comparison of the complete specification, type, product form, condition, code approval, allowable stresses, toughness, welding qualification and certification.

Related designations

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