18-8 (A2) Stainless Steel Flat Socket Head Cap Screws from Brighton Best. Hardness of RC 38-43, meets or exceeds ASTM F879. Flat Head Socket Cap Screws are used for flush mounting of sections, usually where one part slides over another part or one part is mounted on another. Flat head screws are limited by configuration in their tensile strength capability. Stainless Steel material. Grade 18-8 has the same design advantages as a low carbon bolt, however can be used in environments that require greater atmospheric corrosion resistance.Star Stainless Screw Co.