Also known as carbon tool steel, or M2, high-carbon steel refers to any steel with a carbon content of over 0.6%. Any lower than that, then it falls into the medium- or low-carbon category. If it’s higher than 1.0%, it belongs to the ultra-high-carbon steel group. This type of steel has a very simple composition: in addition to the 0.60–1.00% carbon, it’s made up of just 98.1–99.1% iron, and 0.30–0.90% manganese.
Like other steel types, high-carbon steel is made by melting either recycled steel or pig iron to remove impurities, and then adding the alloying elements before casting or extruding. Molten steel, by nature, dissolves carbon at high temperatures, but will typically release it if it is allowed to cool slowly. So as not to allow the carbon to escape the metal’s lattice structure, the metal is quenched (rapidly cooled) to increase the cooling rate from about 200°C to 1000°C per minute. This retains all the internal stresses in the microstructure—although “internal stress” sounds like a bad thing, it’s not—in this case, it improves the steel’s strength and hardness.
Carbon steel is commonly used to make a wide variety of appliances, toys, tools (i.e. knives, swords, shears, etc.), gear wheels, chains, and hardware like nails, fasteners, drill bits, and springs (pictured below).
Although it’s sometimes confused with stainless steel because they both have a high carbon content, these two steels are quite different. Stainless steel has at least 10.5% chromium in it, while high-carbon steel can have just a little, or none at all. The high chromium in stainless steel makes it corrosion-resistant, and this is perhaps the main differentiating factor as high-carbon steel can rust a lot more easily. While a high carbon content offers this steel an array of benefits (it also has more tensile strength than stainless steel), it also gives it low ductility and malleability, and makes it hard to weld, cut, and form.
