F91 and F92 Steels: Differences in High-Temperature Performance of Two Heat-Resistant Steels

Exploring the unique strengths, applications, and trade-offs of two critical alloys powering modern industry

In the heart of a power plant, where temperatures soar past 600°C and pressure builds to hundreds of bars, or deep within a petrochemical reactor processing volatile fuels, the difference between a reliable operation and a catastrophic failure often comes down to one thing: the metal in the pipes and tubes. High-temperature, high-pressure environments demand materials that don't just "survive"—they perform consistently, year after year, without succumbing to creep, oxidation, or structural fatigue. This is where heat-resistant alloy steels like F91 and F92 step in, serving as the backbone of industries from power generation to aerospace, and from marine shipbuilding to petrochemical facilities.

You might not see them, but these steels are everywhere: in the heat exchanger tubes that transfer thermal energy in power plants, the pressure tubes that contain high-pressure fluids in pipelines, and the structural components of aerospace engines pushing the boundaries of flight. While both F91 and F92 belong to the family of creep-resistant ferritic-martensitic (CRFM) steels, their subtle differences in composition and performance make them better suited for specific challenges. Let's dive into what sets them apart, and why those differences matter for engineers, plant operators, and anyone relying on the machinery that powers our world.

Meet the Contenders: F91 and F92 in Context

Before we compare their performance, let's ground ourselves in what F91 and F92 actually are. Both emerged from a decades-long quest to improve the efficiency of power plants and industrial processes. In the late 20th century, as global energy demands grew, engineers sought materials that could withstand higher temperatures to boost thermal efficiency—after all, even a 10°C increase in operating temperature can translate to a 1-2% jump in power plant efficiency, reducing fuel consumption and emissions significantly.

F91 (officially designated as ASTM A335 P91) was one of the first "next-generation" CRFM steels. Developed in the 1980s, it built on earlier molybdenum-based alloys by adding vanadium, niobium, and nitrogen to enhance creep strength. Its claim to fame? It offered 20-30% better creep resistance than its predecessor, P22 steel, allowing power plants to operate at higher temperatures (up to ~600°C) without compromising on durability. For years, it became the go-to choice for alloy steel tube applications in fossil fuel power plants, refineries, and pipeline works.

F92 (ASTM A335 P92), introduced in the early 2000s, took F91's formula and refined it. By reducing molybdenum content and adding tungsten, along with tweaks to boron and nitrogen levels, F92 pushed the temperature envelope even further—now capable of handling sustained temperatures up to 650°C. This made it a star in advanced ultra-supercritical (AUSC) power plants, where every degree of heat translates to lower carbon footprints. Today, it's increasingly specified in high-stakes sectors like power plants & aerospace , where material performance directly impacts safety and operational efficiency.

The Recipe Matters: How Composition Shapes Performance

At first glance, F91 and F92 might seem nearly identical—both are 9% chromium steels, after all. But in metallurgy, small tweaks to alloying elements can lead to dramatic differences in behavior, especially under extreme heat. Let's break down their key components and what each brings to the table:

Element	F91 (Typical Range)	F92 (Typical Range)	Role in Performance
Chromium (Cr)	8.0-9.5%	8.5-9.5%	Forms a protective Cr₂O₃ oxide layer, resisting high-temperature oxidation and corrosion.
Molybdenum (Mo)	0.85-1.05%	0.30-0.60%	Strengthens the matrix and enhances creep resistance; reduced in F92 to make room for tungsten.
Tungsten (W)	0.0-0.1%	1.50-2.00%	The "secret sauce" in F92—boosts creep strength at 600-650°C by stabilizing carbides in the microstructure.
Vanadium (V) + Niobium (Nb)	V: 0.15-0.25%; Nb: 0.06-0.10%	V: 0.15-0.25%; Nb: 0.04-0.09%	Form fine carbides (VC, NbC) that pin grain boundaries, slowing creep deformation at high temps.
Boron (B) + Nitrogen (N)	B: ≤0.006%; N: 0.03-0.07%	B: 0.001-0.006%; N: 0.03-0.07%	Boron strengthens grain boundaries; nitrogen enhances carbide formation for creep resistance.

The most striking difference here is the addition of tungsten in F92. Tungsten atoms are larger and slower to diffuse than molybdenum, which means they form more stable carbides at high temperatures. This stability is critical for creep resistance—over time, at extreme heat, weaker carbides can dissolve, allowing the metal to slowly deform (creep). F92's tungsten-rich carbides hold their ground longer, making it the better choice for applications where temperatures hover around 600-650°C.

Strength When It Counts: Mechanical Performance at Room and High Temperatures

While composition tells part of the story, real-world performance hinges on mechanical properties—how these steels behave under stress, whether at room temperature or in the fiery conditions of a power plant boiler. Let's start with the basics: strength and ductility at ambient temperatures.

At room temperature, both F91 and F92 are impressively strong. F91 typically boasts a tensile strength of 620-795 MPa and a yield strength of ≥415 MPa, with 20-25% elongation (a measure of ductility). F92, slightly leaner in molybdenum but richer in tungsten, comes in with similar room-temperature tensile strength (620-795 MPa) but a marginally higher yield strength (≥440 MPa). These numbers mean both can handle heavy structural loads—think structure works in industrial facilities or pressure vessel components—without permanent deformation.

But the real test is at high temperatures. Let's take 600°C, a common operating range for supercritical power plants. Here, F91's tensile strength drops to ~450 MPa, and its yield strength to ~350 MPa. F92, thanks to its tungsten carbides, retains more strength: ~480 MPa tensile and ~380 MPa yield. Jump to 650°C, and the gap widens. F91's strength falls further, while F92 still holds ~420 MPa tensile strength—making it viable for AUSC plants pushing the limits of heat efficiency.

Ductility, or the ability to bend without breaking, is another key factor. Both steels maintain good ductility at high temperatures (15-20% elongation at 600°C), but F92's microstructure, refined by its alloying tweaks, often shows better toughness after long-term exposure to heat. This is crucial for components like heat exchanger tubes , which undergo thermal cycling (heating and cooling) that can lead to brittle fracture if toughness is lost.

Surviving the Heat: Creep Resistance, Oxidation, and Thermal Stability

For high-temperature alloys, "strength" isn't just about withstanding a one-time load—it's about enduring . Creep resistance, oxidation resistance, and thermal stability are the trio that defines long-term performance, especially in components designed to last 20-30 years.

Creep Resistance: The Battle Against Time and Temperature

Creep is the silent enemy of high-temperature metals. It's the slow, permanent deformation that occurs when a material is subjected to constant stress at elevated temperatures—think of a metal pipe gradually bending under its own weight in a hot boiler. For power plant operators, creep failure can mean unplanned shutdowns, costly repairs, or worse. This is where F91 and F92's differences are most pronounced.

Creep resistance is often measured by "creep rupture strength"—the stress a material can withstand for a specific time (usually 100,000 hours, or ~11.5 years) before breaking. At 600°C, F91 has a creep rupture strength of ~100 MPa, which is impressive. But F92? It hits ~130 MPa at the same temperature. At 650°C, F91 drops to ~60 MPa, while F92 still delivers ~90 MPa. That's a 50% improvement at the higher end of the temperature spectrum—why AUSC plants, which aim for 620-650°C operation, now specify F92 for critical pressure tubes and headers.

Oxidation Resistance: Keeping the Metal Protected

At high temperatures, air and moisture can turn steel into rust in no time—unless the metal forms a protective oxide layer. Both F91 and F92 rely on chromium to create this shield: a thin, adherent layer of Cr₂O₃ that blocks further oxygen penetration. Their similar chromium content (8.5-9.5%) means they perform comparably here, with oxidation rates of ~0.1-0.2 mm/year at 600°C in air. However, in more aggressive environments—like the sulfur-rich gases in petrochemical facilities—F92's slightly higher chromium and boron levels can offer a tiny edge in corrosion resistance, though both are outperformed by stainless steels in extreme cases.

Thermal Stability: Avoiding Microstructural Degradation

Over decades of exposure to high heat, even the best alloys can degrade. Carbides may coarsen, grain boundaries may weaken, and strength can fade. F91, while robust, can show signs of carbide coarsening after 100,000 hours at 600°C, leading to a gradual ｄｒｏｐ in creep strength. F92's tungsten carbides are more stable, resisting coarsening longer. This stability means F92 components can maintain their design strength for the full lifecycle of a power plant, reducing the need for premature replacements—a critical factor in industries where downtime is expensive.

Where They Shine: Real-World Applications of F91 and F92

Understanding the science is one thing, but seeing how these steels perform in the field brings their value to life. Let's explore where F91 and F92 are making the biggest impact today.

F91: The Reliable Workhorse

F91's sweet spot is in applications operating between 550-600°C, where its balance of strength, cost, and availability shines. You'll find it in:

Power plants : Boiler tubes, superheater/reheater tubes, and steam headers in subcritical and supercritical coal-fired plants.
Petrochemical facilities : Pressure tubes for reactors, heat exchangers, and distillation columns handling high-temperature hydrocarbons.
Pipeline works : Large-diameter alloy steel tube systems transporting hot, high-pressure fluids over long distances.
Marine & ship-building : Engine exhaust systems and heat recovery units, where resistance to thermal cycling is key.

Its longevity in these roles is a testament to its reliability—many plants built in the 1990s still run F91 components with minimal issues, a track record that's hard to ｒｅｐｌａｃｅ.

F92: The Innovator's Choice

F92 is the go-to for cutting-edge, high-temperature applications where efficiency and sustainability are priorities. Its playground includes:

Advanced ultra-supercritical (AUSC) power plants : The next generation of fossil fuel plants, operating at 620-650°C to achieve 45%+ thermal efficiency (vs. 35-40% for older plants).
Power plants & aerospace : Jet engine components and rocket propulsion systems, where lightweight, high-strength materials must withstand extreme heat.
Heat efficiency tubes : Custom heat exchanger tube designs (e.g., U-bend tubes, finned tubes) in waste heat recovery systems, where maximizing heat transfer at 600+°C is critical.
Nuclear power : Secondary loop piping and steam generators, where creep resistance and radiation stability are non-negotiable.

While F92 often costs 10-15% more than F91, its ability to enable higher operating temperatures translates to long-term savings in fuel and maintenance—making it a favorite for new-build projects aiming for net-zero goals.

Head-to-Head: F91 vs. F92 at a Glance

Category	F91 (P91)	F92 (P92)
Key Alloying Elements	Cr (8-9.5%), Mo (0.85-1.05%), V, Nb, N	Cr (8.5-9.5%), W (1.5-2.0%), reduced Mo, V, Nb, N
Max Operating Temp	~600°C	~650°C
100,000-Hour Creep Rupture Strength at 600°C	~100 MPa	~130 MPa
Primary Applications	Subcritical/supercritical power plants, petrochemical reactors, marine exhaust systems	AUSC power plants, aerospace components, advanced heat exchangers
Cost (Relative to F91)	Base cost	10-15% higher
Best For	Proven reliability, moderate temps, cost-sensitive projects	High-efficiency, high-temp applications, long-term sustainability goals

Choosing Between F91 and F92: Practical Considerations

So, when faced with a project, how do you decide whether to go with F91 or F92? It boils down to three key questions:

1. What's the Operating Temperature?

If your system runs below 600°C, F91 is often the smarter choice. It's cheaper, widely available, and has a decades-long track record of reliability. For example, a refinery with heat exchangers operating at 580°C would see no benefit from upgrading to F92—F91 will perform just as well at a lower cost.

But if temps exceed 600°C, F92 is non-negotiable. AUSC power plants targeting 630°C steam temperatures simply can't risk F91's lower creep strength at those levels; the risk of premature failure or costly downtime is too high.

2. What's the Project Lifespan?

F92's higher creep resistance pays off over time. For a new power plant designed to operate for 30+ years, the upfront cost premium (10-15%) is offset by reduced maintenance, fewer replacements, and higher efficiency savings. In contrast, a short-term industrial project (5-10 years) might not justify the extra expense of F92.

3. What's the Supply Chain Availability?

F91 is a mature alloy, so finding wholesale alloy steel tube suppliers or custom fabricators is easier. F92, being newer, has a more limited supply chain—especially for specialized forms like u bend tubes or large-diameter pipes. If your project requires custom shapes or tight delivery timelines, F91 might be the more practical option unless F92's performance benefits are critical.

The Future of High-Temperature Steels

F91 and F92 aren't just metals—they're enablers. They've allowed power plants to squeeze more energy from less fuel, reduced emissions in heavy industry, and opened doors to more efficient aerospace and marine technologies. As the world pushes for net-zero goals, the demand for even higher-performance alloys will grow. Researchers are already experimenting with adding cobalt or rare earth elements to further boost creep strength, aiming for steels that can handle 700°C and beyond.

But for now, F91 and F92 remain the benchmarks. F91, the reliable veteran, continues to power existing infrastructure, while F92, the ambitious innovator, leads the charge toward greener, more efficient industrial processes. Understanding their differences isn't just about materials science—it's about making choices that balance performance, cost, and sustainability in a world that demands more from every component.

In the end, whether you're specifying heat exchanger tubes for a refinery or designing the next generation of power plants, F91 and F92 remind us that even the smallest tweaks in chemistry can lead to giant leaps in what's possible.