18-8 stainless steel hex coupling nuts are made from one of the following austenitic alloys: 303, 303Se, 304, XM7, all of which are characterized as having a chromium content of 17-19% and nickel content of 8-10%. Dimensional specifications are governed by ASME B18.2.2. There are no grade markings on 18-8 stainless hex coupling nuts. Coupling nuts, also known as extension nuts, are used for joining two male threads, usually threaded rods, in a variety of applications. 300 series stainless steel fasteners are commonly used in applications that require general atmospheric corrosion resistance, such as chemical and food processing equipment. Stainless steel has a higher corrosion resistance than carbon steel.