Heavy-Duty Chrome-Moly Fittings for Hydroprocessing and High-Pressure Steam

What is ASTM A234 WP22?

WP22 carries 1.90-2.60% chromium and 0.87-1.13% molybdenum. The combination gives a ferritic steel that keeps its strength at temperatures where carbon steel and even WP11 have lost most of their allowable stress. Carbon is 0.05-0.15% and silicon is capped at 0.50%, a tighter silicon limit than WP11 because the thicker sections used in this grade need the toughness.

Reactor effluent lines, hydrocracker charge and inter-bed piping, hot high-pressure separators and main steam pipe all specify WP22. Fittings are normalised at 900-960°C and tempered at 680-750°C. The tempering temperature is not a formality: it sets both the strength and the resistance to temper embrittlement that governs long-term toughness.

The grade has one behaviour that separates it from WP11 in service. Held for years in the 350-550°C range, it embrittles as phosphorus, tin, antimony and arsenic segregate to grain boundaries. Modern purchase specifications control this through the J-factor and X-bar, and heavy-wall fittings for critical service are often ordered with step-cooling tests. A WP22 fitting that passes its as-supplied Charpy test can still fail after 100,000 hours if the chemistry was not controlled.

Chemical Composition (Weight %)

A234 chemical requirements for WP22. The residual elements at the bottom of the table are the ones that matter for temper embrittlement; they are not incidental.

For critical service, project specifications add a J-factor limit of 100 or lower, calculated from Si, Mn, P, Sn, As and Sb, and an X-bar limit of 15 ppm. These are not A234 requirements, so they must be quoted at inquiry stage or the mill will ship to the base standard.

Mechanical Properties

A234 tensile requirements for WP22. Values assume the normalised and tempered condition in which the fitting leaves the works.

The 585 MPa ceiling protects against an under-tempered fitting. Note that the aged condition shows higher strength and lower elongation, which is the mechanical signature of temper embrittlement; Charpy energy falls much faster than tensile properties, which is why impact testing rather than tension testing is used to monitor it.

Physical Properties

Low thermal conductivity and high expansion compared with carbon steel make WP22 spools more sensitive to thermal gradients. Welding procedures for thick sections normally include a soak band of at least four times the wall thickness each side of the weld.

Heat Treatment, Welding & PWHT

Normalising and tempering

Normalise at 900-960°C, air cool, then temper at 680-750°C. Below 680°C the fitting will exceed the tensile ceiling; above 750°C the strength drops below the floor. The window is narrow by design and the certificate should show both temperatures, not just the final result.

Post-weld heat treatment

WP22 is P-No. 5A in ASME B31.3. PWHT is mandatory at 704-760°C with a one hour per 25 mm hold. Thick-section welds need a controlled heating rate below 400°C and slow cooling to avoid cracking; a 25 mm soak band either side is the minimum, and four times the thickness is better practice in heavy wall.

Welding

Preheat at 200-250°C and hold interpass below 300°C. Filler metal is ER90S-B3 or E9015-B3, or ER80S-B3 where the design allows lower strength. Carbon steel filler is not acceptable in pressure service: the deposit has no creep strength at the operating temperature and becomes the weak link.

Temper embrittlement control

Heavy-wall fittings for hydrocrackers are frequently ordered with a step-cooling test after PWHT, simulating long service exposure, followed by Charpy testing at the design minimum metal temperature. Where the specification invokes it, the test is destructive and drawn from a prolongation or a sacrificial fitting.

Service Limits & Corrosion Behaviour

Temperature envelope

B31.3 lists WP22 to 649°C, and the allowable stress stays useful to about 600°C. Above 600°C creep rupture data favour 9Cr grades, so WP22 in main steam is normally limited to 565-600°C depending on pressure and design life.

Hydrogen service

WP22 sits well inside the Nelson curves for hydrotreating and hydrocracking, which is the main reason it dominates those units. Above the curve for 2¼Cr, the specification moves to 3Cr or 9Cr-1Mo-V, and the decision is made on hydrogen partial pressure rather than temperature alone.

Oxidation and scaling

The 2.25% chromium gives good resistance to steam oxidation to about 600°C, better than WP11 and adequate for main steam service without internal coating.

Creep and thermal fatigue

WP22 has lower thermal expansion than austenitic stainless and better creep strength than carbon steel, which makes it the default for thick-wall, high-pressure, cyclic service. Weld joints remain the critical detail, and Type IV cracking in the fine-grained heat-affected zone is the usual failure location in long service.

ASTM A234 WP22 vs Similar Grades — Selection Guide

The WP22 and WP91 decision is rarely made on metal temperature alone. Wall thickness drives it as much as temperature, because the creep life advantage of 9Cr grows quickly with section size.

Applicable Standards

HT PIPE Supply — ASTM A234 WP22 Product Forms

Heavy-wall WP22 fittings can be supplied with step-cooling and Charpy testing, J-factor calculation, and full PWHT simulation on request. Lead time is longer than for standard stock because the tests are destructive.

Project Experience — ASTM A234 WP22

WP22 is supplied into hydrocracking, hydrotreating and main steam service where wall thickness and design life both favour 2¼Cr-1Mo.

Fittings are supplied with heat treatment cycles, hardness surveys and Charpy results at the design minimum metal temperature, with third-party inspection available.

FAQ — ASTM A234 WP22