T91 vs T92 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
T91 and T92 are two closely related ferritic-martensitic chromium-molybdenum (and tungsten-modified) steels widely used in high-temperature power-generation and petrochemical equipment. Engineers, procurement managers, and manufacturing planners frequently face a selection dilemma between them driven by trade-offs among creep strength, weldability, oxidation/corrosion resistance, and material cost. Typical decision contexts include selecting pipe or tube material for advanced steam conditions, choosing forging or fitting materials for elevated-temperature service, and balancing life‑cycle cost against fabrication difficulty.
The fundamental difference between the two is alloying strategy: T92 (also referenced as P92 in some standards) substitutes significant tungsten and adjusts molybdenum and microalloying levels to increase creep strength and microstructural stability at higher temperatures, whereas T91 relies more on molybdenum with a slightly simpler composition. That alloying shift leads to distinct hardenability, tempering behavior, and application envelopes that make these grades commonly compared in component design and materials procurement.
1. Standards and Designations
- Common ASTM/ASME designations:
- T91: ASTM A387 Grade 91 (plate), A335 Grade P91 (seamless pipe), A213 TP91 (tubing) — commonly referenced as Grade 91 / P91.
- T92: ASTM A387 Grade 92, A335 Grade P92, A213 T92 — commonly referenced as Grade 92 / P92.
- European and other standards:
- EN: Equivalent 9Cr steels appear under EN designations (but direct one-to-one EN equivalents are limited).
- JIS/GB: Local designations exist for 9Cr steels based on similar chemistries (often used in Asia).
- Steel classification: Both are alloy steels within the ferritic-martensitic class; they are not stainless or tool steels and are generally considered high‑strength, creep‑resisting alloy steels (HSLA/heat‑resisting family).
2. Chemical Composition and Alloying Strategy
The overview table shows typical composition ranges for each grade (weight %). Exact limits depend on the specific standard/specification and product form.
| Element | T91 (typical range, wt%) | T92 (typical range, wt%) |
|---|---|---|
| C | 0.08–0.12 | 0.08–0.12 |
| Mn | 0.30–0.60 | 0.30–0.60 |
| Si | 0.20–0.70 | 0.20–0.50 |
| P | ≤0.02 | ≤0.02 |
| S | ≤0.01 | ≤0.01 |
| Cr | 8.5–9.5 | 8.5–9.5 |
| Ni | ≤0.30 | ≤0.30 |
| Mo | 0.85–1.05 | 0.45–0.65 |
| W | — (trace) | 1.8–2.5 |
| V | 0.18–0.25 | 0.18–0.25 |
| Nb (Cb) | 0.06–0.12 | 0.06–0.12 |
| Ti | trace | trace |
| B | trace* | trace* |
| N | ~0.03–0.06 | ~0.03–0.06 |
*Boron (B) and nitrogen (N) are control elements; boron is used in very small ppm levels to influence hardenability and creep.
How the alloying affects properties: - Chromium (Cr) provides oxidation resistance and strengthens the ferritic matrix. - Molybdenum (Mo) increases strength and creep resistance by solid-solution strengthening and carbide formation; T91 has higher Mo than T92. - Tungsten (W) in T92 substitutes partly for Mo, increasing high-temperature strength and stabilizing carbides at higher service temperatures. - Vanadium (V) and niobium (Nb) form stable carbides/nitrides that refine grain size and improve creep strength; they also affect weldability and HAZ behavior. - Carbon controls hardness/toughness balance and martensite formation; levels are kept modest to balance weldability with strength.
3. Microstructure and Heat Treatment Response
Typical microstructures - In the normalized-and-tempered condition both grades develop a tempered martensitic microstructure with a high density of fine carbides and carbonitrides (V- and Nb-rich precipitates). The tempered martensite lath structure provides the combination of strength and toughness required for elevated-temperature service. - T92 tends to develop finer, more stable carbide distributions at high temperature due to tungsten’s carbide stabilization effect; this contributes to improved creep resistance and temper resistance at the upper end of design temperatures.
Heat treatment response - Normalizing: Both grades are typically normalized (air cooling) from temperatures in the range of ~980–1050 °C (process parameters per standard) to refine prior austenite grain size. - Quenching & tempering: Tempering after normalization at temperatures typically between 700–760 °C produces tempered martensite. Higher tempering reduces hardness and increases toughness but can reduce creep strength. - Thermo-mechanical routes: Controlled rolling and thermomechanical processing (for tubes/plates) refine grain size and dislocation density; T92 benefits particularly from careful control to obtain optimum precipitate distributions because its W content affects precipitation kinetics.
4. Mechanical Properties
Mechanical properties depend strongly on heat-treatment and product form. The table below gives representative typical ranges for normalized-and-tempered conditions commonly used in power-plant components.
| Property (typical, N&T condition) | T91 | T92 |
|---|---|---|
| Yield strength (0.2% offset) | ~350–450 MPa (typical minimums per spec ~415 MPa) | ~400–550 MPa (higher upper ranges in many cases) |
| Tensile strength | ~560–700 MPa | ~600–750 MPa |
| Elongation (A%) | ~18–25% | ~15–22% |
| Impact toughness (Charpy-V, room temp) | moderate; depends on notch & heat treatment (e.g., tens of J to >40 J) | comparable to slightly lower in some conditions due to higher hardenability; depends on tempering |
| Hardness (HRC/HB) | moderate (tempered) | slightly higher tendency at same tempering due to W |
Interpretation: - T92 is generally engineered to provide higher creep strength and better retention of strength at elevated temperatures than T91, at the expense of somewhat higher hardenability and, in some cases, slightly reduced room-temperature toughness or ductility when compared at equivalent tempering conditions. - For components designed for the same tempering regime, T92 often shows higher tensile and creep strength, while T91 can offer somewhat better ductility and easier processing in some fabrication scenarios.
5. Weldability
Hardenability and weld considerations - Both T91 and T92 require controlled welding procedures: low-hydrogen consumables, preheat, interpass temperature control, and post-weld heat treatment (PWHT) to temper the HAZ and relieve residual stresses. - Because of higher hardenability (W in T92 increases hardenability), T92 may require stricter preheat and PWHT control to avoid HAZ martensite cracking. Microalloying elements (V, Nb) and carbon content also increase susceptibility to hard HAZ and cold cracking if hydrogen is present.
Useful weldability indices (qualitative interpretation) - A commonly used empirical index is the IIW carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Higher $CE_{IIW}$ indicates greater hardenability and greater need for preheat/PWHT. - A more comprehensive index is $P_{cm}$: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$ - $P_{cm}$ helps predict cold-cracking susceptibility; higher values indicate higher risk.
Qualitative outcome: - Both grades require PWHT; T92 often merits higher preheat and careful PWHT schedules because tungsten and the adjusted Mo content slightly raise $CE$ and $P_{cm}$. Welding procedure qualification and hydrogen control are mandatory for pressure‑containing components.
6. Corrosion and Surface Protection
- Neither T91 nor T92 is stainless; they are ferritic steels with moderate high-temperature oxidation resistance due to Cr. For steam and high-temperature oxidizing environments they develop protective oxide layers, but they are susceptible to steam-side oxidation, carburization, and sulfidation depending on service.
- Surface protection strategies include coatings (high-temperature paints, aluminide coatings), internal lining for corrosive media, and routine inspection. For ambient corrosion, standard protective painting or metallizing is typical; galvanizing is not generally used for high-temperature steam-side components.
- PREN (pitting resistance equivalent number) is not applicable for these non‑stainless steels; for stainless grades the formula $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ is used, but it is not relevant for T91/T92.
7. Fabrication, Machinability, and Formability
- Machinability: Both are more difficult to machine than plain carbon steels. T92, with higher tungsten and a tendency to retain hardness, can be slightly harder on tooling and require reduced cutting speeds and robust tooling.
- Formability: Cold forming and bending are limited; components are usually formed in a normalized condition and then tempered. Deep drawing is not typical; hot forming followed by normalization and tempering is preferred for large forgings.
- Finishing: Grinding and polishing are feasible but tooling wears faster than with lower-alloy steels. Heat treatment after forming and welding is essential to restore desired tempered martensitic properties.
8. Typical Applications
| T91 (typical uses) | T92 (typical uses) |
|---|---|
| Boiler tubes, headers, steam piping in conventional/advanced plants operating at moderate-to-high steam temperatures | Superheater and reheater tubes, headers, steam piping, and components for ultra‑supercritical/advanced plants where higher creep strength is needed |
| Headers, elbows, and fittings for plants with design temperatures up to ~600 °C | Components for higher temperature steam conditions (e.g., 600–650 °C range) and longer creep‑life requirements |
| Heat-exchanger tubing, furnace walls for moderate high-temp service | New-build A-USC components, heavy-section cast/forged parts where elevated creep resistance is prioritized |
| Replacement parts in legacy systems where supply, cost, and easier fabrication matter | Critical long-life components where life‑cycle cost justification favors higher material cost |
Selection rationale: - Choose T91 for proven performance in many power-plant applications at slightly lower temperatures, where fabrication simplicity and lower material cost are desirable. - Choose T92 when design temperature, creep life, and long-term strength retention at elevated temperature are the priority, and when procurement/fabrication processes can handle stricter welding and heat‑treatment requirements.
9. Cost and Availability
- Relative cost: T92 is typically more expensive than T91 because of the added tungsten and tighter processing controls. The premium varies by market and product form.
- Availability: T91 has been in service longer and historically has broader availability in pipe, tube, plate, and forgings. T92 availability has increased with demand for advanced steam plants but may still be more constrained in some product sizes and lead times.
- Product forms: Both grades are available as seamless and welded pipes, tubes, plates, forgings, and fittings; availability and lead times should be confirmed with suppliers for critical procurements.
10. Summary and Recommendation
| Criterion | T91 | T92 |
|---|---|---|
| Weldability (procedural difficulty) | Good with qualified low-H procedures; slightly easier than T92 | More demanding due to higher hardenability; stricter preheat/PWHT |
| Strength–Toughness balance | Strong, good toughness in standard tempers | Higher high-temperature strength and creep resistance; marginal trade-off in ductility/toughness if not carefully processed |
| Cost | Lower (generally) | Higher (generally) |
Recommendation: - Choose T91 if you need a well-established, cost-effective 9Cr creep-resistant steel for high-temperature steam service where operating temperatures and required creep life fall within the proven envelope of Grade 91, and when fabrication simplicity and availability are important. - Choose T92 if the design requires superior long-term creep strength and microstructural stability at the upper range of elevated-temperature service (e.g., advanced or ultra-supercritical steam conditions), and if you can accommodate stricter welding, heat-treatment, and procurement requirements.
Final note: Both grades require careful specification of heat treatment, welding procedure qualification, and inspection to achieve reliable, long‑lived performance. For critical high-temperature components, perform component‑level creep testing, supplier capability assessment, and lifecycle cost analysis as part of the selection process.