1.1 This specification covers five grades of bolting with twelve classes of yield strength ranging from 50 to 120 ksi [345 to 827 MPa] for use in high-temperature service such as fasteners for pressure vessel and valve flanges. See Speci This specification covers standards for Grade 660 (Class A, B, C, and D) , Grade 651 (Class A and B), Grade 662 (Class A and B), and Grade 665 (Class A and B) of bolting materials for use in high-temperature service such as fasteners, for pressure vess