Common Procurement Mistakes When Buying ASTM A335 Alloy Steel Pipe
Sourcing high-temperature alloy steel pipe for process plant and power station work is not as straightforward as ordering standard carbon steel. The grade system within ASTM A335 is wide enough that mix-ups happen at the procurement stage, and some of those mix-ups only surface during installation — or worse, during in-service inspection years later. After working through a few of these situations, here’s where I’ve seen things go wrong and what to check before the order goes in.
Treating P11, P22, and P91 as Interchangeable Upgrades
The most common assumption that causes problems is that P11, P22, and P91 form a straightforward upgrade ladder — that P22 is simply better than P11, and P91 is better still. From a creep resistance standpoint at extreme temperatures, this is roughly true. But the grades have different allowable stresses at different temperatures, different welding requirements, different PWHT requirements, and different long-term behavior under cyclic thermal loading. Substituting upward is not automatically conservative.
P91 in particular has strict requirements around heat treatment — both the normalization and tempering done at the mill and the post-weld heat treatment done in fabrication. A P91 weld that receives PWHT at too low a temperature, or for insufficient time, can end up with a heat-affected zone that has significantly lower creep strength than the parent pipe. This isn’t a theoretical concern; it’s a documented failure mode. Upgrading from P22 to P91 because P91 was available and P22 wasn’t, without reviewing the fabrication implications, has caused real problems on real projects.
When the design specifies a grade, that grade was chosen for reasons that go beyond the operating temperature. Check with the piping engineer before substituting, even within the same standard.
Not Verifying the Heat Treatment Condition in the MTC
ASTM A335 requires that pipe be supplied in either the normalized, normalized-and-tempered, or annealed condition depending on the grade. P91, for example, must be supplied normalized and tempered; the normalization temperature, tempering temperature, and time at temperature all affect the final microstructure and mechanical properties.
A mill test certificate that shows the chemical composition and room-temperature tensile test results is not sufficient confirmation that the heat treatment was performed correctly. The MTC should also state the heat treatment condition and, for P91, the specific temperatures used. If this information is absent or incomplete, request it directly from the mill. For P91 purchased through distributors — as opposed to direct mill orders — there’s an additional verification step: confirm that the pipe has not been re-heat-treated after the original mill processing, as improper field re-heat-treatment to correct dimensional issues can degrade the material properties.
Confusing Grade Markings
ASTM A335 pipe is marked on the outside surface with the grade designation — P11, P22, P91, and so on. In a warehouse or on site, these markings can be worn, partially obscured by dirt or rust, or abbreviated in ways that create ambiguity. P1 (which exists) and P11 look similar when the second digit is dirty or the paint is worn. P22 and P2 are both valid grades with significantly different compositions.
On a project I was involved with, a batch of P11 pipe was delivered with worn surface markings. The procurement team read them as P1 — a grade with no chromium content — and nearly installed carbon-equivalent pipe in a service requiring the chromium content for oxidation resistance. The issue was caught during QC review of the MTCs before installation.
The practical answer is to verify grade through the MTC chemical composition — not the surface marking alone — before accepting a delivery. The chromium and molybdenum content in the MTC should match the compositional requirements for the specified grade. P11 requires 1.00–1.50% Cr and 0.44–0.65% Mo. P22 requires 1.90–2.60% Cr and 0.87–1.13% Mo. P91 requires 8.00–9.50% Cr and 0.85–1.05% Mo, along with controlled additions of vanadium, niobium, and nitrogen. These numbers are specific enough that any significant deviation immediately flags a wrong grade.
Ordering Without Specifying Supplementary Requirements
ASTM A335 includes several supplementary requirements that are not mandatory unless the purchaser specifies them in the order. For high-temperature services where long-term creep life is the primary concern, some of these are worth specifying — particularly for P91.
Hardness testing (Supplementary Requirement S5) verifies that the tempered martensitic microstructure of P91 is within the required range. The ASTM standard allows a hardness range of 197 to 265 HB for P91; material outside this range — either too soft (under-tempered, residual austenite) or too hard (over-tempered, wrong microstructure) — has reduced creep strength. Specifying hardness testing and requiring the results on the MTC is a straightforward check that catches heat treatment problems that wouldn’t show up in the standard tensile test.
Grain size certification is sometimes specified for P11 and P22 in services where notch toughness at lower temperatures is also a concern, though this is less common than hardness specification for P91.
For ASTM A335 alloy steel pipe in critical services — main steam lines, high-pressure process heater outlet headers — the additional cost of specifying supplementary requirements is small relative to the verification value they provide.
Buying from Distributors Without Confirming Heat Number Traceability
For mill-direct orders, heat number traceability from the order to the MTC to the marked pipe is generally reliable. For purchases through distributors — particularly for smaller quantities where the distributor draws from stock — the traceability chain is more likely to have gaps.
The minimum verification is that the heat numbers marked on the pipe correspond to MTCs in the package, and that those MTCs show the specified grade and composition. For P91 specifically, the industry guidance (including EPRI and the Pipe Fabrication Institute) recommends additional verification: chemical analysis of the actual delivered pipe, not just review of the MTC. PMI (positive material identification) with an XRF analyzer can quickly confirm that the chromium and molybdenum content of the physical pipe matches the MTC values. This is worth doing on critical P91 orders, especially when the source is a distributor rather than the producing mill.
The reason PMI has become a standard practice for P91 in power plant work is that cases of incorrect material supplied with correct-looking documentation have occurred — not as deliberate fraud in all cases, but as inventory errors in mixed-grade stock. PMI before installation is a low-cost check that closes this gap.
What to Build Into the Purchase Order
Before the order is placed, the purchase order for ASTM A335 alloy steel pipe for high-temperature service should specify:
- The exact grade (P11, P22, P91 — not just “A335 alloy steel pipe”)
- The required heat treatment condition
- Any supplementary requirements (S5 hardness testing for P91 is the most common)
- MTC requirements, including that heat treatment data be provided
- PMI requirements at delivery if the source is a distributor or if the application is critical
These are procurement decisions, not engineering decisions in the technical sense — they don’t change the pipe design, just what verification you receive with the delivery. Getting them right at the order stage avoids the much more expensive process of dealing with non-conforming material after it’s on site.
