Of all the materials used for high-temperature steam pipe, X20 (12 percent chromium, 1 percent molybdenum, 1/4 percent vanadium) and P91 (9 percent chromium, 1 percent molybdenum, 1/4 percent vanadium) stand out because of their very high creep rupture properties, even at elevated temperatures.
P91, introduced in the 1980s in the U.S., has both very high strength at elevated temperatures and good fabrication properties. These features have made P91 the material of choice for high-temperature steam and other, similar noncorrosive services.
The use of X20 in preference to P22 (2-1/4 percent chromium, 1 percent molybdenum) in the latter application allowed wall thickness reductions of about 50 percent. These reductions resulted in easier handling; less energy needed for preheating, welding, and postweld heat treatment; and faster start-up, load changes, and shutdown of the unit.
Overall savings in the cost of the piping supplies and their fabrication, including welding, was claimed to be about 40 percent.1
In spite of such strong credentials, X20 has not been included in the ASME Boiler & Pressure Vessel Code. This is likely one of the main reasons that the U.S. remains a notable exception in the long list of countries using this material.
P91 is a modified form of P9 (9 percent chromium, 1 percent molybdenum) steel. The steel can have low impurity limits, thanks to the development of processes such as argon-oxygen decarburization (AOD) and electroslag remelting (ESR), which make the steel behave consistently during fabrication and resist the effects of aging. When properly heat-treated as specified in ASTM specification A335, the steel acquires room temperature properties as shown in Figure 1.
The steel has high creep rupture strength because of the precipitation of submicroscopic vanadium and niobium carbonitrides. Low carbon content aids its fabrication characteristics. The material responds well to hot and cold bending, as well as to welding.
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