Alloy Steel · Power

ASTM A234 WP91 Fittings for HRSGs in the Gulf: Hardness, PMI and Heat-Treatment Records to Ask For

Published 16 October 2026 · Arshya Pipe Fittings Pvt Ltd Technical Desk

Heat recovery steam generators in combined-cycle plants across the Gulf commonly use Grade 91 for their hottest high-pressure and reheat piping. ASTM A234 WP91 fittings in those lines look like any other alloy fitting, but their creep strength depends almost entirely on getting the heat treatment right. A certificate that only says "WP91, N&T" is not enough. This guide sets out the records to ask for, and what each one tells you.

Why Grade 91 needs more than a material certificate

WP91 is a modified 9Cr-1Mo steel with small additions of vanadium, niobium and nitrogen. It belongs to the family of creep strength-enhanced ferritic (CSEF) steels. Its high allowable stress at temperature comes from a tempered martensite structure with fine, stable precipitates. That structure is created by the heat treatment, not by the chemistry alone.

If normalising or tempering goes wrong, a fitting can pass a room-temperature tensile test and still have reduced creep strength in service. This is not something a site inspector can see. It shows up later as cracking or creep damage, often at welds and in the most highly stressed fittings. The power industry has published a good deal of experience on this. The practical answer for a buyer is to ask for records that show the heat treatment was correct, and to check them before the fittings are welded in.

HRSGs add one more factor: combined-cycle plants that start, stop or follow load put their hottest headers and piping through thermal cycles, so components with marginal properties have less room for error.

1. Heat-treatment records

ASTM A234 requires WP91 fittings to be normalised and tempered as the final heat treatment. In the 2018 edition, both WP91 Type 1 and Type 2 are normalised at 1040 °C (1900 °F) minimum and 1080 °C (1975 °F) maximum, and tempered at 730 to 800 °C (1350 to 1470 °F). Liquid quenching and tempering is allowed only with the purchaser's approval, and Supplementary Requirement S1 covers a lower temper by the fitting maker with the final temper done by the purchaser. Confirm the requirements in the edition your specification calls up. Ask for:

• The time-temperature chart for each normalising and tempering cycle, with the fittings in that load identified by heat number;
• Thermocouple positions, so you know the chart represents the parts and not just the furnace atmosphere;
• Hold times related to the section thickness, and the cooling method after normalising;
• Evidence that the furnace and instruments are calibrated.

One point catches buyers out. A fitting hot-formed from P91 pipe or plate loses the original heat treatment of the starting material. The finished fitting must be normalised and tempered again. The record you need is the one for the fitting, not the mill certificate for the pipe it was made from.

2. Hardness

Hardness is the quickest practical check that the heat treatment did what it should. ASTM A234 requires every WP91 fitting to be hardness tested and to fall within 190 to 250 HBW, and since the 2018 edition this applies to both Type 1 and Type 2. Type 2 differs in chemistry, not in hardness or heat treatment: it has tighter limits on manganese, sulphur, silicon, nickel and copper, a minimum nitrogen-to-aluminium ratio of 4.0, and limits on trace elements such as boron, tin, antimony, arsenic and tungsten. State on the order which type you need; the marking is WP91T1 or WP91T2. Some project specifications set tighter hardness limits of their own. Both ends of the band matter:

• Low hardness can point to over-tempering, an incorrect normalising temperature or excessive ferrite, all of which reduce creep strength.
• High hardness can point to incomplete tempering, leaving a brittle structure.

Ask for a reading on every piece, with the location of each reading. Portable hardness testers are sensitive to surface preparation, so ask that readings be taken on a ground surface below any decarburised layer, and that the method and conversion used are stated on the report. If your inspector re-checks at receipt, use the same method so the numbers can be compared.

3. Positive material identification (PMI)

Grade 91 is easily confused with other chromium-molybdenum grades on a busy fabrication floor. PMI on every piece prevents mix-ups with:

• WP22 (2¼Cr-1Mo): much lower chromium;
• WP9 (plain 9Cr-1Mo): similar chromium and molybdenum, but no deliberate vanadium or niobium;
• WP92: contains tungsten and less molybdenum.

Handheld XRF confirms chromium, molybdenum and tungsten well, and many instruments can detect vanadium and niobium at the levels found in Grade 91. Check this with your PMI provider. XRF cannot measure carbon or nitrogen, so those rely on the heat analysis.

4. Chemistry and mechanical tests on the certificate

The EN 10204 3.1 or 3.2 certificate should show the full heat analysis, including vanadium, niobium, nitrogen and the residual elements your specification limits, such as aluminium and nickel. Mechanical results should meet the A234 requirements for WP91 (tensile strength 620 to 840 MPa, minimum yield 415 MPa) and show the heat-treatment condition. If the order calls for metallography, for example a replica or a sample section to confirm a tempered martensite structure, it must be on the purchase order before manufacture.

Records checklist

Storage and site welding in the Gulf

Coastal sites in the Gulf combine heat, humidity and airborne salt. Keep bevel protectors on until fit-up, store fittings off the ground and under cover, and keep WP91 segregated from lower-alloy fittings so PMI at site remains a confirmation rather than a sorting exercise. Welding Grade 91 needs a matching filler, controlled preheat and interpass temperature, and post-weld heat treatment within a narrow window below the lower critical temperature. These are fabricator responsibilities, but the fitting records above are what your welding engineer will rely on when something needs investigating later.

WP91 from Arshya

Our WP91 elbows, tees, reducers and caps are made at the group manufacturing facility, Arshya Engineers & Fabrications Pvt Ltd, Village Dundsa, Palwal, Haryana, with heat treatment carried out in-house. Each WP91 fitting is supplied with heat-treatment and hardness records, and PMI is part of every order. Certificates are EN 10204 3.1 as standard, or 3.2 witnessed by TÜV, Lloyd's Register, Bureau Veritas, DNV or SGS. The facility holds ISO 9001, PED 2014/68/EU, ASME U-Stamp and IBR approval; download the certificates from our quality page. See the WP91 grade page for chemistry and sizes, and our UAE page for shipping to Gulf ports.

Frequently asked questions