High-temperature piping is unforgiving. A grade that survives five years in one service fails in eighteen months in another, and the difference almost always comes down to the spec sheet written before procurement. When the wrong stainless steel pipe is ordered, the cost shows up later as unplanned shutdowns, weld repairs, or replacement of entire headers.

This guide is built for the people who write those specs. It covers the three most common high-temperature service classes, the standard grades that match them, the dimensional and testing choices that actually matter, and the practical value of bundling the pipe fittings and pipe flanges with the pipe itself from one qualified source.

Start With the Service Envelope, Not the Grade

The most common mistake in stainless pipe procurement is to begin with a grade name ("we want TP304") and try to make it fit the service. It is faster and more reliable to start with four service parameters and work backwards to a grade:

• Peak metal temperature — the design metal temperature, not the fluid temperature. They differ by 20–80 °C in most fired heaters.
• Pressure class — ANSI / ASME Class 150, 300, 600, or higher, which sets the schedule and the flange rating.
• Fluid compatibility — chloride content, sulfur compounds, water chemistry, and trace contaminants.
• Expected life before replacement — 100,000 hours (≈11 years) is the typical design target for refinery and power plant headers.

Once those four are pinned down, the shortlist of grades drops from twenty to three or four, and the decision is mostly mechanical.

The Three Grades That Cover Most High-Temperature Service

Across refineries, ethylene plants, and utility boilers, three austenitic grades carry about 80% of the load in stainless steel pipe orders. Knowing when to pick each is the entire job of the spec writer in most projects.

TP304 / TP304H — The General-Purpose Austenitic

TP304 (UNS S30400) is the baseline 18Cr-8Ni stainless. It handles oxidation up to about 870 °C in intermittent service and 925 °C in continuous service, with adequate strength and good weldability. TP304H is the creep-resistant variant, with carbon pushed into the 0.04–0.10% range, and is the grade of choice for fired heater tubes, ethylene cracking outlet tubes, and boiler superheaters operating above 600 °C.

Use TP304H wherever you have sustained high metal temperature and a clean fluid. Avoid it in chloride-rich service, in polythionic acid environments during downtime, and in the temperature band where stress corrosion cracking is most aggressive.

TP316 / TP316H — When You Need Molybdenum

TP316 adds roughly 2% molybdenum to the 18Cr-8Ni base, which lifts pitting and crevice corrosion resistance noticeably in chloride-bearing fluids. The H variant (TP316H) is used in the same creep-sensitive services as TP304H but where the fluid chemistry would attack a 304-grade surface. In refinery hydroprocessing units, 316H is often the default for reactor effluent and fractionator overhead lines.

TP321 / TP347 — The Stabilized Grades for Weld-Heavy Service

TP321 is titanium-stabilized, TP347 is niobium-stabilized. The point of the stabilizer is to lock up carbon as TiC or NbC, which prevents chromium carbide precipitation at the grain boundaries during welding or long exposure in the 425–860 °C sensitization range. The result is far less intergranular corrosion risk in the heat-affected zone.

Use TP321H or TP347H in heavily welded headers, in piping that sees repeated thermal cycling, and in services where post-weld heat treatment is impractical. They are the grades most commonly chosen for power plant reheater and superheater tubing.

Matching the Standard to the Application

A stainless steel pipe order without a clear standard is a gamble. The standards are not interchangeable — they are written for very different geometries and inspection regimes.

Two practical points to write into the PO. First, name the standard AND the grade together — "ASTM A312 TP304H" is a complete spec, "304 stainless" is a request for clarification. Second, call out the supplementary requirements: solution-anneal condition, hardness limit, grain size for H-grades, intergranular corrosion test, and the type of NDT (eddy current, ultrasonic, or hydrostatic). These are the items that distinguish a serious order from a stock-list re-shipment.

What the Testing Package Should Look Like

A high-temperature stainless order is only as good as its test report. The minimum acceptable package is EN 10204 3.1, with traceability back to the heat. For critical service, ask for 3.2 certification with independent witness.

• Hydrostatic test — mandatory for pressure service unless explicitly replaced by NDT.
• Eddy current or ultrasonic test — required for detection of longitudinal and transverse defects, especially in welded tube.
• Flattening, flare, and reverse-bend tests — proves ductility at the weld and the parent tube.
• Grain size verification — required for H-grades; ASTM grain size number is usually specified.
• Intergranular corrosion test (ASTM A262 Practice A or E) — required for austenitic grades in sensitization-prone service.

Hardness limits matter too. A pipe that comes in at 95 HRB has been work-hardened somewhere along the line, often by an aggressive straightening operation. That residual hardness is a creep-life and SCC-life problem waiting to happen in a high-temperature header.

The Fittings and Flanges Are Part of the Same Problem

Specifying the pipe correctly but ordering the pipe fittings and pipe flanges separately is one of the most common ways a project loses control of its high-temperature train. A 304H pipe welded to a 304L fitting is a metallurgical mismatch that most quality plans will not catch at goods-in, but which shows up as a creep failure in the HAZ of the fitting five years into service.

Three rules keep the system consistent:

• Match the grade letter-for-letter. If the pipe is TP304H, the butt-weld fittings should be WP304H per ASME B16.9, not WP304.
• Match the standard. A 304H pipe to A312 should be flanged with A182 F304H weld neck flanges, not A182 F304.
• Match the MTC chain. All components should arrive with EN 10204 3.1 certificates in a consistent format, traceable to the same QA system.

This is the practical reason to source the pipe, the fittings, and the flanges from a single supplier. It removes the most common cause of metallurgical mismatch at the joint, and it cuts the QA reconciliation workload at goods-in by a wide margin.

Bundling the High-Temperature Package

In a typical high-temperature train, the bill of materials breaks down roughly as: 70% pipe, 20% fittings, 10% flanges and bolting. The temptation is to run three separate RFQs because three separate suppliers each offer the lowest unit price on their piece. In practice, that decision almost always costs more than it saves.

A bundled package, sourced against a single specification, gives the project three concrete benefits:

• Material compatibility is guaranteed by the supplier's QA system rather than relying on the buyer's incoming inspection to catch a mismatch.
• Logistics collapses to one delivery sequence rather than three, which materially shortens site erection time on tight outages.
• One point of accountability for any deviation or non-conformance, rather than three suppliers each pointing at the others.

For projects above a few hundred metres of pipe, the bundled approach usually wins on total installed cost even if the unit prices are slightly higher on individual line items.

Common Sourcing Mistakes

A short list of the recurring problems in stainless steel pipe procurement that can be avoided with a tighter spec:

• Ordering "TP304" when the service is sustained above 600 °C. That is a TP304H job, and the L-grade version is a creep-life disaster waiting to happen.
• Writing "ASTM A312" without specifying seamless vs. welded vs. cold-worked. The three are very different in delivery, lead time, and price.
• Forgetting the supplementary requirements — solution annealed, hardness limit, grain size, IGC test — and ending up with stock material that does not meet the design intent.
• Buying fittings and flanges from a different supplier and discovering, at goods-in, that the carbon is outside spec for H-grade service.
• Specifying the schedule without confirming the bore and outside diameter tolerances. Off-tolerance pipe does not align with off-the-shelf fittings and creates field rework.
• Underestimating lead time for H-grade or heavy-wall pipe, which is rarely in stock and is typically produced against the order book.

A Worked Example: Refinery Reactor Effluent Line

A typical reactor effluent line in a hydroprocessing unit might look like this on paper:

That single paragraph covers roughly 80% of the procurement decisions in that line. The remaining 20% is variation on the same theme for the larger and smaller branches of the train.

Final Checklist Before Releasing the PO

• Service parameters (temperature, pressure, fluid, design life) are stated on the spec sheet, not just the grade.
• Standard, grade, condition (annealed / cold-worked), and schedule are all written out.
• Supplementary requirements — hardness, grain size, IGC test, NDT type — are explicit.
• MTC requirement is EN 10204 3.1, with 3.2 reserved for critical service.
• Pipe fittings and pipe flanges are bundled in the same RFQ, with matching grade and standard.
• Delivery split is aligned with site erection sequence, not a single bulk ｄｒｏｐ.
• Lead time is confirmed for the H-grade or heavy-wall sizes, which are rarely in stock.