Chromium-Molybdenum Fittings for Refinery and Steam Service Above the Carbon Steel Range

What is ASTM A234 WP11?

WP11 is a ferritic low-alloy steel with 1.00-1.50% chromium and 0.44-0.65% molybdenum. Carbon is held to 0.05-0.15%, which is deliberately narrower than WPB on both ends: enough carbon to form the carbides that carry creep load, not so much that toughness or weldability suffers. Silicon runs 0.50-1.00% for oxidation resistance.

The grade is common in refinery hydrotreating, catalytic reforming, and high-pressure steam piping, and in heat recovery steam generators where the design temperature sits between the carbon steel and 2¼Cr limits. Fittings are hot formed and then normalised and tempered, so the microstructure is ferrite with spheroidised carbides rather than the banded as-rolled structure of carbon steel.

WP11 is not a substitute for WP22 on creep grounds. Above about 540°C in steam service, or where hydrogen partial pressure is high, 2¼Cr-1Mo or 9Cr-1Mo-V is the correct choice. The two grades are visually similar after sandblasting, so PMI on every fitting is standard practice on refinery turnarounds.

Chemical Composition (Weight %)

A234 chemical requirements for WP11. Phosphorus and sulfur are capped at 0.030% each, tighter than the carbon grades because the fitting will run hot for decades and residuals govern temper embrittlement.

A234 applies a residual sum limit of Cu + Ni + Cr + Mo ≤ 1.00% to the carbon grades; for WP11 the chromium and molybdenum are intentional, so the requirement is read as a cap on the residual elements only. Product analysis tolerances of A960 apply.

Mechanical Properties

Tensile requirements of A234 for WP11, in the normalised and tempered condition in which the fittings are supplied.

The 585 MPa upper limit matters: heats that test harder than the range are rejected even though the yield is acceptable, because excess strength in a Cr-Mo fitting usually means an incomplete temper. Hardness is not mandated by A234 for WP11, but 190 HBW is a common purchase-order ceiling and 22 HRC applies where NACE MR0175 is invoked.

Physical Properties

Thermal expansion is higher than carbon steel, which matters at dissimilar joints to A106 pipe: the weld sees an expansion mismatch of about 7% at 500°C, and that is one reason PWHT and a tapered transition are specified.

Heat Treatment, Welding & PWHT

Normalising and tempering

A234 requires heat treatment for WP11. The fitting is normalised at 900-960°C, air cooled, then tempered at 650-730°C. Normalising from the wrong temperature leaves delta ferrite or coarse grains, and a temper below 650°C leaves hardness above the range. Both show up on the mill certificate, so read the cycle, not just the result.

Post-weld heat treatment

B31.3 classifies WP11 as P-No. 4. PWHT is mandatory for all thicknesses in most specifications and runs at 704-760°C, with a minimum hold of one hour per 25 mm. Cooling below 315°C should be controlled. Skipping PWHT leaves a hard, brittle heat-affected zone that will crack in thermal cycling.

Welding

Preheat at 150-200°C and maintain interpass below 300°C. Filler metal is ER80S-B2 for GTAW and E8018-B2 for SMAW, matched to the 1¼Cr-½Mo deposit. Do not use carbon steel filler: the joint then has a soft, low-creep-strength band that becomes the life-limiting detail of the line.

Dissimilar joints

When WP11 meets A106 Gr B, the joint is usually buttered or made with ER80S-B2 and PWHT is governed by the Cr-Mo side. The transition is placed where the design temperature is lowest, and the weld is examined by radiography after PWHT, not before.

Service Limits & Corrosion Behaviour

Temperature envelope

ASME B31.3 Table A-1 lists A234 WP11 to 649°C. In practice the grade is specified for steam and hydrocarbon service to about 540°C; above that, creep rupture data favour WP22. Between 540 and 593°C the choice depends on wall thickness and design life, not on the code ceiling.

Oxidation and scaling

The 1.25% chromium raises resistance to steam oxidation compared with carbon steel. Scaling becomes significant above about 565°C, which is one reason not to run WP11 at the top of its code range in steam.

Hydrogen service

WP11 appears on the Nelson curves for hydrotreating and hydrodesulfurisation. The allowable hydrogen partial pressure falls as temperature rises; the curve, not the fitting standard, sets the grade. Above the WP11 line the specification moves to WP22 or WP91.

Temper embrittlement

The 1¼Cr grade is less sensitive than 2¼Cr, but long service in the 370-565°C band still embrittles the steel. Impact testing after step cooling is available on request for critical service, and weld repair of aged fittings should be approached with care.

ASTM A234 WP11 vs Similar Grades — Selection Guide

WP11 and WP22 are often confused at the receiving inspection because both are chrome-moly. Positive material identification with an alloy analyser is the only reliable field check; hardness alone will not separate them.

Applicable Standards

HT PIPE Supply — ASTM A234 WP11 Product Forms

Each heat is supplied with the normalising and tempering cycle on the certificate, hardness results, and Charpy data where specified. PMI is performed on 100% of fittings for refinery orders.

Project Experience — ASTM A234 WP11

WP11 fittings are supplied into hydrotreating, steam and HRSG service where the design temperature sits just above the carbon steel range.

Our team reviews the design temperature, hydrogen partial pressure and PWHT window before confirming the grade, because the fitting standard alone does not answer those questions.

FAQ — ASTM A234 WP11