ASTM A320 Grade L7 is a material specification for bolts, studs, nuts, and other fasteners designed for high temperature bolting applications. ASTM A320 L7 grades are divided into 4 performance classes, A, B, C, and D, each with different tensile and stress rupture properties. Class 660 fasteners are used to bolt boilers, pressure vessels, pipe flanges and valves for high temperature service.