Chromium-molybdenum-tungsten alloy boiler tubes and pipes are seamless steel products used in boiler heating surfaces and high-temperature piping systems. This material family includes tube grades such as T23, T92, T911 and T122, together with pipe grades such as P92.
Chromium, molybdenum and tungsten contribute to the steels’ elevated-temperature properties. Controlled additions of other elements and the specified heat treatment also influence strength and creep resistance. The family covers different alloy compositions, so each grade must be selected for the intended service conditions.
Product Specifications
| Item | Description |
|---|---|
| Product | Seamless Cr-Mo-W alloy steel tubes and pipes |
| Main tube grades | T23, T92, T911 and T122 |
| Main pipe grade | P92 |
| Tube standards | ASTM A213/A213M or ASME SA-213/SA-213M, subject to grade coverage in the specified edition |
| Pipe standards | ASTM A335/A335M or ASME SA-335/SA-335M |
| Manufacturing route | Hot-finished or cold-finished, as permitted by the product specification |
| Delivery condition | Heat-treated according to the selected grade and standard |
| Principal applications | Superheater tubes, reheater tubes and high-temperature steam piping |
For tubes, specify outside diameter, wall thickness basis and length. For pipes, specify outside diameter or nominal pipe size, wall thickness or schedule, and length.
Dimensional combinations and tolerances must be confirmed for the particular grade and product specification.
Grades and Product Forms
| Grade | Product form | Material characteristics |
|---|---|---|
| T23 | Seamless tubes | Approximately 2.25% chromium steel with tungsten and controlled alloy additions |
| T92 | Seamless tubes | Approximately 9% chromium steel containing tungsten and molybdenum |
| T911 | Seamless tubes | Approximately 9% chromium steel with a different molybdenum-tungsten balance from T92 |
| T122 | Seamless tubes | Higher-chromium steel, approximately 11–12% Cr, containing molybdenum and tungsten |
| P92 | Seamless pipes | A 9Cr-class alloy steel pipe grade for high-temperature service |
T23 is a low-alloy steel, while the higher-chromium grades have different compositions and heat-treatment requirements. These products should not be specified as interchangeable members of a single grade.
Chemical Composition
The table below compares selected elements listed for the tube grades in the Baosteel boiler tube handbook under ASME SA-213.
| Tube grade | C, % | Cr, % | Mo, % | W, % |
|---|---|---|---|---|
| T23 | 0.04–0.10 | 1.90–2.60 | 0.05–0.30 | 1.45–1.75 |
| T92 | 0.07–0.13 | 8.50–9.50 | 0.30–0.60 | 1.50–2.00 |
| T911 | 0.09–0.13 | 8.50–10.50 | 0.90–1.10 | 0.90–1.10 |
| T122 | 0.07–0.14 | 10.00–12.50 | 0.25–0.60 | 1.50–2.50 |
All percentages are by mass. Other controlled elements, such as vanadium, niobium, nitrogen and boron, depend on the grade. Complete chemical requirements must be taken from the specified product standard.
Mechanical Properties
| Tube grade | Minimum tensile strength, MPa | Minimum yield strength, MPa |
|---|---|---|
| T23 | 510 | 400 |
| T92 | 620 | 440 |
| T911 | 620 | 440 |
| T122 | 620 | 400 |
Data basis for both tables: Baosteel Boiler Tube & Pipe handbook, printed in June 2007. These are historical tube-grade reference values. Final requirements follow the standard edition and purchase specification stated in the order. P92 pipe composition and mechanical properties must be verified separately under ASTM A335 or ASME SA-335.
Room-temperature strength values do not establish a service-temperature limit. High-temperature design requires the applicable allowable stresses, creep considerations and expected operating life.
Material Characteristics
Elevated-Temperature Strength
Molybdenum and tungsten contribute to elevated-temperature strength. The performance of each grade also depends on its other alloying elements, microstructure and heat-treatment condition.
Creep properties should be assessed using the design data applicable to the selected material.
Oxidation Behavior
Chromium contributes to oxidation resistance. The chromium contents of T23, T92, T911 and T122 differ substantially, so oxidation behavior should be evaluated for the particular grade, temperature and steam or combustion-gas environment.
Heat Treatment and Fabrication
Heat treatment establishes the specified microstructure and mechanical properties. Normalizing and tempering conditions must be selected for the particular grade and product standard.
Welding and bending require suitable fabrication procedures. Where applicable, the qualified welding procedure should define preheat, interpass temperature, consumables and post-weld heat treatment. Fabrication heat cycles can affect long-term material performance.
Applications
Superheater Tubes
Selected Cr-Mo-W grades are used in superheater sections where the boiler design requires suitable elevated-temperature strength.
Selection depends on tube metal temperature, steam pressure, allowable stress and exposure to combustion gases.
Reheater Tubes
Reheater components operate under elevated-temperature conditions and may experience repeated thermal cycling.
The selected grade must satisfy the project’s strength, fabrication and service-life requirements.
Steam Pipeline Pipes
P92 pipes are used in high-temperature steam piping where the design specifies this material. Pipe wall thickness, fittings, welds and heat treatment must be evaluated under the governing piping code and project specification.
T92 tubes and P92 pipes have separate product specifications. An order must identify the required product form and standard even when the materials belong to the same alloy family.
Inspection and Quality Documentation
Inspection requirements follow the selected product standard and purchase specification. Depending on the order, they may include:
- Chemical analysis and tensile testing.
- Hardness testing and heat-treatment records.
- Dimensional and surface inspection.
- Hydrostatic testing and/or specified nondestructive examination.
- Technological tests applicable to the product form and size.
- Additional material verification, impact testing or metallographic examination when specified.
Inspection documents should identify the product standard, grade, heat number, dimensions, delivery condition and applicable test results. Traceability should be maintained through cutting and fabrication.
Ordering Information
For a quotation, specify:
- Product form: seamless tubes or seamless pipes.
- Grade: T23, T92, T911, T122 or P92.
- Standard: the required ASTM or ASME standard and edition.
- Dimensions: tube outside diameter and wall thickness, or pipe size and wall thickness/schedule.
- Length and quantity: required lengths and total order quantity.
- Delivery condition: heat treatment and surface condition.
- Application: superheater, reheater or steam piping.
- Inspection: certificate requirements and additional testing.
- Fabrication: drawings and requirements for bending or further processing.
For a proposed grade substitution, provide the design conditions and obtain approval under the applicable project requirements.