What are the common materials used for manufacturing pressure tubes in boiler systems?

In a boiler system, the pressure tube is the component that actually carries the working fluid while it absorbs heat from the fire side. Because the tube wall simultaneously handles internal pressure, elevated temperature, and—depending on the fuel and feedwater—corrosion or oxidation, material selection is the single decision that defines the safety margin, the design life, and ultimately the total cost of the boiler. The materials most commonly used for manufacturing pressure tubes in boiler service fall into four broad families: carbon steel, low-alloy (Cr-Mo) steel, high-alloy martensitic steel, austenitic stainless steel, and—to a more limited extent—nickel and copper-nickel alloys.

1. Carbon Steel — The Workhorse for Low- and Medium-Temperature Sections

Plain carbon steel is the default choice wherever the metal temperature stays at or below roughly 450–500 °C, which is typical for water walls, economizer coils, and the cooler passes of the evaporator. Two ASTM grades dominate the global supply chain:

• ASTM A192 / ASME SA192 — seamless carbon steel tube for high-pressure service, widely used in water walls and economizers of subcritical and conventional power-plant boilers.
• ASTM A210 / ASME SA210 (Grades A-1 and C) — medium-carbon seamless tube with higher tensile strength, used where A192's minimum 325 MPa is not enough for the required wall-thickness calculation.

For projects that follow Chinese, Japanese, or Russian national standards, equivalent grades such as GB/T 5310 (20G), JIS G3461 (STB340/STB410), and GOST 8732 serve the same role. At EZ Steel Industrial, these pressure tubes are produced as seamless pipe and supplied with full mill test certificates, hydrostatic testing, and ultrasonic or eddy-current inspection on request.

2. Low-Alloy Cr-Mo Steel — The Mainstream Choice for 500–600 °C Service

Once the metal temperature crosses 500 °C, plain carbon steel is no longer adequate because long-term creep and steam-side oxidation start to govern the design. Low-alloy chromium-molybdenum steels are introduced in this range, and the relevant grades are covered by ASTM A213 (for tubes) and ASTM A335 (for pipes):

• T11 / P11 — 1.25Cr-0.5Mo, used in the cooler half of the 500–600 °C band.
• T22 / P22 — 2.25Cr-1Mo, the most widely specified alloy for superheaters, reheaters, and main steam lines in 540–580 °C service. It is often the baseline alloy for new thermal-power projects.
• T23 / P23 — a modified 2.25Cr alloy with W, V, Nb additions, used where higher creep strength is required at similar temperatures.

ASTM A335 alloy steel pipes in P5, P9, P11, P22, and P91 grades, as well as the corresponding A213 tube grades, are part of EZ Steel's regular production portfolio. These tubes are typically delivered in the normalized-and-tempered condition, with optional ultrasonic, magnetic-particle, and PMI checks for high-pressure service.

3. High-Alloy Martensitic Steel — T91 and T92 for Supercritical Units

For supercritical and ultra-supercritical boilers where the final-stage superheater and reheater metal temperatures reach or exceed 600 °C, the 9–12 % chromium family of creep-strength-enhanced ferritic (CSEF) steels has become the industry standard:

• T91 / P91 (ASTM A213 / A335) — 9Cr-1Mo-V-Nb, the baseline material for supercritical units. It delivers roughly twice the allowable stress of T22 at 600 °C and has decades of in-service track record.
• T92 / P92 — 9Cr-0.5Mo-1.8W-V-Nb, used in the most demanding ultra-supercritical sections where extra creep margin is needed.

These grades require strict control of heat treatment (normalizing and tempering) and are sensitive to fabrication. Procurement should always be accompanied by full traceability, witnessed mechanical testing, and—preferably—an independent review of the heat-treatment records.

4. Austenitic Stainless Steel — High-Temperature Corrosion Resistance

When the operating conditions involve heavy oxidation, high steam-side corrosion, or aggressive flue-gas condensate, austenitic stainless steels are specified. The common grades used in boiler pressure tubes include:

• TP304 / TP304H — general-purpose austenitic steel, suitable up to about 700 °C in non-sensitizing conditions.
• TP321 / TP321H — titanium-stabilized variant with improved resistance to intergranular corrosion after long high-temperature exposure.
• TP347 / TP347H and TP347HFG — niobium-stabilized grades, widely used in final-stage superheaters and reheaters of advanced ultra-supercritical units.
• Super304H and HR3C — high-performance austenitic grades designed for temperatures of 620–680 °C in the most aggressive service.

Austenitic stainless steel tubes are also widely used in marine and offshore boiler plants, where chloride-bearing environments make corrosion the dominant failure mode rather than creep.

5. Nickel and Copper-Nickel Alloys — Special-Service Materials

For boiler systems in contact with seawater, sour hydrocarbons, or highly oxidizing media, conventional iron-based alloys are not sufficient. Two families of non-ferrous alloys are used:

• Nickel alloys such as Monel 400 (B165) and Ni-Cr-Fe alloys (B167, e.g. Inconel 600/690) — used in feedwater heater tubing, nuclear auxiliary systems, and boiler components exposed to caustic or sour service. For demanding nuclear applications, RCC-M Section II grades are also available.
• Copper-nickel alloys (B466, B111, EEMUA 144 90/10 and 70/30) — used in condensers, heat-exchanger tubing, and the feedwater train where biofouling and seawater corrosion must be controlled.

At EZ Steel Industrial, these copper and nickel alloy tubes are produced to ASTM, EEMUA, BS, JIS, and GB/T standards, and they share the same inspection, PMI, and traceability regime as the carbon and stainless lines.

6. How the Material Choice Is Made in Practice

In a real project, the material list is built in three steps:

1. Start from metal temperature. The design heat-balance calculation gives the expected tube-metal temperature for each section. That temperature alone narrows the candidate material family to a small set of grades.
2. Check pressure, life, and corrosion. Within the candidate family, the design pressure fixes the minimum wall thickness (or schedule), the planned service life fixes the allowable stress and creep limit, and the fuel/feedwater chemistry decides whether stainless or nickel alloy must be used.
3. Lock the standard. Each grade is then tied to a specific product standard (ASTM A192 / A210 / A213 / A335, EN 10216-2, GB/T 5310, JIS G3461, GOST 8732, etc.) and to the relevant dimensional and inspection requirements.

A simplified starting point is shown in the table below; it is the same temperature-driven logic used by most EPC and boiler-OEM specifications.

7. Common Pitfalls in Material Selection

A few issues show up repeatedly in procurement and inspection:

• Confusing tube standards with pipe standards. A213 covers tubes, A335 covers pipe. They share grades but are not interchangeable for inspection and dimensional acceptance.
• Ignoring the heat-treatment condition. T22 and T91 in the as-rolled condition do not meet the standard's mechanical properties. Normalizing-and-tempering (or quenching-and-tempering for T91/P91) must be specified.
• Mixing national standards in one order. A "T22 equivalent" may differ in trace-element limits between ASTM, EN, JIS, and GB/T. The certification must clearly state which standard applies.
• Skipping PMI on stainless and alloy deliveries. Positive material identification is a low-cost check that prevents mix-ups between, for example, T11 and T22, or between 304 and 321.

8. Sourcing Pressure Tubes for Boiler Systems

When a project is at the procurement stage, the most efficient way to confirm the right material is to send the heat-balance, the design code (ASME Section I, EN 12952, GB/T 16507, etc.), and the required standard to the supplier up front. EZ Steel Industrial supplies boiler tubes in carbon steel, Cr-Mo alloy, stainless, and copper-nickel grades, produced under ISO 9001 and API 5L/5CT quality systems, with PED-compliant documentation for European projects. Tubes can be ordered to ASTM, EN, JIS, GOST, GB/T, EEMUA, BS, and RCC-M specifications, in standard or custom dimensions, with full traceability, hydrostatic testing, and non-destructive examination.

For enquiries, technical submittals, or sample requests, contact the EZ Steel Industrial team through the contact page with your operating temperature, pressure, fluid medium, and the preferred material standard.