Drill bits composed of chrome vanadium, cobalt, or titanium carbide are recommended for hard metals such as stainless steel. The HSS label denotes that the drill bit is made of a tougher metal grade. Metal drill bits with sizes ranging from 1 to 13 mm are commonly available. Plastic drill bits are also available for softer materials like wood and foam. Because chrome vanadium is more resistant to thermal stress, it can be used for prolonged periods without sharpening. However, like other carbide-based drills, chrome vanadium bits will need to be replaced periodically due to wear. Drilling into harder materials may require drilling down to a size smaller than what is currently in the stock set of bits. In this case, one must buy extra bits in sets so that there is not a gap in the cutting surface when changing out bits. Hardness ranges from 60 HRC for carbon steel to over 90 HRC for stainless steel. The higher the number, the harder the metal. Stainless steel has a resistance to corrosion about three times that of carbon steel. It is common practice to coat some types of metal tools to improve their performance. Vanadium is used because it holds its hardness at lower temperatures than other metals. Also, vanadium does not rust. Stainless steel should never be drilled with carbon steel bits as this will cause the steel to fail prematurely.