Creep-Strength Chrome-Moly Fittings for Main Steam and Advanced Power Plant

What is ASTM A234 WP91?

WP91 is a martensitic 9Cr steel with tightly controlled chemistry. Carbon is 0.08-0.12%, chromium 8.00-9.50%, molybdenum 0.85-1.05%. The strengthening comes from vanadium at 0.18-0.25%, niobium at 0.06-0.10% and nitrogen at 0.030-0.070%, which together form fine MX precipitates that pin the microstructure at temperature. Remove any one of those three and the creep strength collapses back toward WP9.

Because the chemistry window is narrow, the grade depends on the mill as much as on the fitting shop. Phosphorus is capped at 0.020% and sulfur at 0.010%, and nickel, aluminium, titanium and zirconium are all limited to prevent delta ferrite and coarse inclusions. The narrow limits are what make WP91 expensive relative to WP22, and also what make substitution impossible once a line is designed to it.

Heat treatment is not optional and not forgiving. Normalise at 1040-1080°C, cool in air to form martensite, then temper at 730-790°C. Missing the normalising temperature leaves delta ferrite; tempering above 790°C destroys the precipitate structure and drops creep strength permanently. A WP91 fitting cannot be re-treated without risking the whole heat, so the certificate is as important as the test result.

Chemical Composition (Weight %)

A234 chemical requirements for WP91. Most of these limits exist for metallurgical reasons rather than for strength, and a single out-of-range element can make the fitting unusable in creep service.

Additional limits in A234 and A182 for WP91: Ti ≤ 0.01%, Zr ≤ 0.01%, and the sum of Al + Ti + Zr ≤ 0.05%. These are not housekeeping limits. Titanium and zirconium tie up nitrogen and remove the precipitate strengthening the grade depends on.

Mechanical Properties

A234 tensile requirements for WP91 in the normalised and tempered condition. Both the floor and the ceiling are higher than the other chrome-moly grades.

Hardness above 250 HBW is a rejection criterion, not a warning: it usually means the temper was incomplete, and an under-tempered WP91 fitting will crack in service. Because of this, 100% hardness testing of WP91 fittings is normal practice, not sampling.

Physical Properties

Low expansion and low conductivity compared with carbon steel make WP91 attractive for thick-wall main steam, and also make the welding thermal cycle harder to control. Preheat uniformity across a heavy section matters more than the nominal preheat figure.

Heat Treatment, Welding & PWHT

Normalising

1040-1080°C, held long enough to dissolve the carbides, then air cooled. The cooling must produce martensite throughout the section. Slow cooling of a heavy wall can leave ferrite, which has no creep strength and is not recoverable by tempering.

Tempering and PWHT

Temper or post-weld heat treat at 730-790°C. The window is narrow: below 730°C the hardness stays above 250 HBW, above 790°C the MX precipitates coarsen and the creep strength is lost for the life of the component. Heating and cooling rates above 315°C are controlled, typically to 150°C/h or less for heavy sections.

Welding

Preheat 200-250°C, interpass below 300°C, no preheating above 300°C. Filler metal is ER90S-B9 for GTAW and E9015-B9 for SMAW. Stringer beads, controlled heat input and no weaving: the weld metal and the fine-grained heat-affected zone are where Type IV cracking starts.

Type IV cracking

In long-term service, WP91 welds fail in the fine-grained heat-affected zone, where the microstructure is weaker than both the weld metal and the parent. The risk rises with thick sections and high restraint. It is managed by weld geometry, heat input control and by not over-tempering during PWHT; it cannot be eliminated by chemistry alone.

Service Limits & Corrosion Behaviour

Temperature envelope

B31.3 lists WP91 to 649°C and the grade is designed for 600-620°C main steam. Above 620°C the creep strength advantage narrows and grade 92 or austenitic options are considered.

Creep strength

At 600°C and 100,000 hours, WP91 supports roughly twice the stress of WP9 for the same rupture life. That is why a WP91 main steam line can be significantly thinner than the WP9 or WP22 equivalent, and why the whole-line economics favour it despite the higher metal price.

Oxidation and steam resistance

9% chromium gives good resistance to steam oxidation to about 620°C. Above that, internal oxide scale becomes a concern and higher chromium grades or coatings are used.

Hydrogen and sour service

WP91 is used in hydrogen service at temperatures above the WP22 range. It is not a wet H₂S material by default; where sour service applies, hardness control to 22 HRC conflicts with the 250 HBW allowance and the requirement must be resolved at design stage.

ASTM A234 WP91 vs Similar Grades — Selection Guide

Grade 91 is not a drop-in upgrade for grade 22. The welding procedure, PWHT window, hardness acceptance and NDE are all different, and a fabricator qualified on P22 is not automatically qualified on P91.

Applicable Standards

HT PIPE Supply — ASTM A234 WP91 Product Forms

WP91 fittings are supplied with 100% hardness testing, the full normalising and tempering cycle, and product analysis including V, Nb, N, Al, Ti and Zr. Third-party witness of hardness and PMI is available on request.

Project Experience — ASTM A234 WP91

WP91 is supplied into supercritical and ultra-supercritical main steam, hot reheat, and retrofit projects replacing WP22 or WP9 lines.

Every WP91 heat is hardness surveyed at the body and both weld ends, and certificates carry the complete chemical analysis rather than only the elements A234 requires.

FAQ — ASTM A234 WP91