Dear teachers, is P91 ferritic or martensitic?
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As the title suggests, dear teachers, is P91 ferritic or martensitic? In China, it seems to be commonly referred to as martensitic steel, while according to ASTM and ASME standards it is called ferritic steel – which one is correct?
Composition (wt%): C 0.08–0.12, Mn 0.30–0.60, Si 0.20–0.50, P ≤0.02, S ≤0.01, Cr 8.0–9.5, Mo 0.85–1.05, V 0.18–0.25, Nb 0.06–0.1, N 0.03–0.07, Al ≤0.04, Ni ≤0.4. P91 steel is increasingly used in the superheaters, reheaters, and main steam pipes of power station boilers, thanks to its excellent comprehensive properties such as high-temperature endurance strength, thermal stability, and resistance to high-temperature creep
For ultra-high-pressure units of 125 MW and 200 MW, the main steam pipes are made of 10 CrMo910 steel； From the late 1970s to the 1980s, subcritical 300 MW and 600 MW units were introduced. For their main steam pipes, A335P22 steel was used; these two types of steel have similar properties and both belong to the pearlitic heat-resistant steel category. Their maximum operating temperature is 580–590 °C. At higher temperatures, austenitic heat-resistant steel must be used, as it can withstand temperatures up to 700 °C. However, this type of steel has a high coefficient of thermal expansion, is sensitive to stress corrosion, and its joints made of different steel types have a short lifespan, making it unsuitable for use in high-temperature, high-pressure steam and water pipelines. P91 steel was developed as a new steel for steam and water pipelines to be used in the temperature range of 600–650 °C, filling the gap between pearlitic heat-resistant steels and austenitic heat-resistant steels. It belongs to the category of martensitic heat-resistant steels, with a maximum operating temperature of 650 °C. In fact, it is a modified steel grade formed by adding strengthening elements such as V, Nb, and N to the original 9Cr-1Mo steel.