Set screws are often used for holding a pulley, gear, or other parts from turning relative to a shaft. The black oxide coating provides some lubricity to the product, meaning that there is less friction generated when using these products. The coating also provides a mild corrosion resistance but should not be relied on in environments where corrosion resistance is a primary concern. Alloy steel is steel that has been alloyed with other materials to improve overall physical properties. Steels are designated by a four-number SAE Steel grade. The first two digits indicate the primary materials used to form the steel. The last two digits identify the percentage of carbon for the alloy (in hundredths). Tensile strength ranges for the alloys typically used in these fasteners range from 170,000 to 180,000 psi (pounds per square inch), making these among the strongest materials. Hex socket drive systems are driven by hex wrenches or power tools with hexagonal bits. Flat point is useful for minimizing marring and making frequent adjustments. A threaded fastener's size name includes information about the major external diameter (in millimeters), followed by the thread pitch (millimeters per thread), which indicates if it is coarse or fine.