steel tube grade: T91, T23, T22, T12, T11, T5, T2, STBA 22/24/ ... . Seamless Ferritic and Austenitic Alloy Steel Tube for Super heater and Heat Exchanger (1)Steel Grade: T91, T23, T22, T12, T11, T5, T2 ... )Application: This product applies to high temperature service such as for super-heaters and reheaters. And the features are precise dimension.