What type of material is used for superheater tubes operating at temperatures above 950 °F?

The correct choice for superheater tubes operating at temperatures above 950 °F is indeed B. A 335. This material designation refers to the ASME specification for alloy steel pipes that are specifically designed to withstand high temperatures and pressures, making them well-suited for use in superheaters.

A 335 pipes are typically made from chromium-molybdenum alloy steels, which provide excellent high-temperature strength and oxidation resistance, important properties when dealing with the extreme conditions present in superheater environments. The alloying elements in A 335 help improve the mechanical properties of the steel at elevated temperatures, making it an ideal choice for applications in power plants and industrial boilers.

In contrast, the other options either do not have the same high-temperature resistance or are not specifically designed for superheater applications. For example, A 213 refers to seamless alloy steel and is primarily used for boiler tubes, whereas A 106 is used for standard carbon steel high-temperature service but does not offer the same level of performance as A 335 in really high-temperature environments. Therefore, B. A 335 is the optimal material choice for superheater tubes operating under such demanding conditions.