T91 and P91 are two widely used high-temperature, high-strength martensitic stainless steels derived from the 9Cr-1Mo-V alloy system. They are categorized differently due to variations in manufacturing standards and product forms, yet both play critical roles in energy and chemical industries. Below is a detailed analysis of their distinctions and applications.

1. Key Differences Between T91 and P91

• Product Form and Standards:

T91: Classified as "Tube" or "Tubing" under the ASME (American Society of Mechanical Engineers) standards. It is typically used for boiler tubes, superheater tubes, and reheater tubes.

P91: Classified as "Pipe" under ASME standards. It is primarily employed in main steam lines, large-diameter high-pressure piping, and headers.

• Manufacturing Requirements:

T91 demands higher dimensional accuracy, surface quality, and bending performance to accommodate complex boiler tube arrangements.

P91 focuses more on thick-wall pressure-bearing capacity and weldability, making it suitable for large structural components.

• Performance Emphasis:

While both alloys share similar chemical compositions, their microstructures and heat treatment processes differ slightly. T91 prioritizes high-temperature creep strength and resistance to flue gas corrosion, whereas P91 emphasizes overall toughness and long-term service stability.

2. Application Fields

• Power Generation Industry:

T91 is mainly used in high-temperature components of supercritical and ultra-supercritical power plant boilers, such as superheater and reheater tubes, capable of operating long-term at temperatures up to 620°C.

P91 is applied in main steam pipelines, headers, and high-temperature high-pressure valves, offering excellent pressure-bearing capacity for critical transmission systems in power plants.

• Petrochemical Industry:

Both alloys are utilized in high-temperature high-pressure reactors, heat exchangers, and cracking units, demonstrating resistance to corrosive environments like hydrogen sulfide.

• Other Industrial Sectors:

These include auxiliary systems in nuclear power plants and high-temperature heat treatment equipment requiring heat and pressure resistance.

3. Selection Considerations

The choice between T91 and P91 depends on specific working conditions: T91 is preferred for small-diameter, thin-walled piping requiring bending, while P91 is more suitable for large-diameter, thick-walled straight pipes or structural components. Both alloys require strict control over welding and heat treatment processes to maximize their high-temperature performance advantages.