1.1 This specification 2 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. Note 1: This specification was originally developed for use for piping in the central stations of electric-power generating thermal plants. However, its use is not restricted to such applications and it may be used in other applications for which the attributes of the materials, as defined by this specification, are appropriate. 1.2 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. 1.3 Grades TP321 and TP321H have lower strength requirements for nominal wall thicknesses greater than 3/8 in. [9.5 mm]. 1.4 The values stated in either inch-pound units or SI units are to be regarded separately as standard. Within the text, the SI units are shown in brackets. The values stated in each system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the two systems may result in non-conformance with the standard. The inch-pound units shall apply unless the "M" designation of this specification is specified in the order. Note 2: The dimensionless designators NPS (nominal pipe size) and DN (Diametre Nominel) have been substituted in this standard for such traditional terms as "nominal diameter," "size," and "nominal size."