Many piping system fabricators and contractors simply order stainless fasteners as “304″ or “18-8″ or “316″. Most fastener distributors will sell you an 18-8 fastener instead of 304. In reality, however, 18-8 stainless can be several types of fasteners, including 302, 302HQ, 303, 304, 305, 384, and XM7. 304 is always an 18-8, but 18-8 is not always 304! In many applications, there is no functional difference. Technical Threads, however, supplies both 18-8 and F593 304 fasteners for discerning applications.
However, ASTM specs F593 (bolts, studs and hex cap screws) and F594 (corresponding nuts) are gaining in popularity in systems that require more stringent limitations on the fasteners. The tensile strength requires are generally higher than 18-8/316 “commerical” grades. In addition, the tensile strength has tigher limitations in F593/F594. Also, the chemical makeups of F593/F594 are more specific than many commercial grades. Technical Threads stocks both!
The spec sizing only covers .25″ through 1.5″ (A193/194 usually is used in larger systems). A variety of stainless alloys are covered under F593/F594. The most common are 304 (UNS S30400), 304L (30403), 316 (S31600) and 316L (S31603).
Technical Threads stocks and supplies F593/F594 fasteners, as well as 18-8 and 316 fasteners for commerical systems, and A193/A194 B8/B8M and Grade 8/8M for more demanding applications.