Finished Hex Nuts in 18-8 stainless steel are designed for use with stainless steel bolts or screws with a specified minimum tensile strength equal or less than the specified proof stress of the mating nut. 18-8 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 18% and nickel content of 8-10%. Hardness of Rockwell B95 - C32 for diameters 1/4 through 5/8. Rockwell B80 - C32 for diameters of 3/4 through 1 inch. Proof load of 100,000 psi for diameters 1/4 through 5/8 and 85,000 psi for diameters 3/4 through 1 inch.Star Stainless Screw Co.