This specification covers seamless austenitic steel pipe intended for high-temperature service. Among the grades covered are H grades and nitrogen grades that are specifically intended for high-temperature service.Optional supplementary requirements (S1 through S10) are provided. These supplementary requirements specify additional tests that will be made only when stated in the order, together with the number of such tests required.Grades TP321 and TP321H have lower strength requirements for nominal wall thicknesses greater than 3/8 in. [9.5 mm].