How did adding carbon and quenching make iron tool surfaces harder?

Adding carbon and quenching transforms iron into a harder material by altering its internal microstructure. Pure iron (wrought iron) is too soft to harden significantly through rapid cooling alone. To achieve hardness, iron is first subjected to carburizing, a process where the surface is heated in a carbon-rich environment (such as glowing charcoal). At high temperatures, the iron surface transforms into the austenite phase, which allows carbon to diffuse into the metal.12

Once the surface has absorbed sufficient carbon, the metal is quenched—rapidly cooled by immersion in oil or water. This sudden cooling prevents the carbon-rich surface from transforming into softer structures, instead forcing it into a hard, brittle, and crystalline state known as martensite.312

This technique allows for the creation of tools with a "case" or surface that is exceptionally hard and wear-resistant, while the interior remains composed of lower-carbon iron, which provides the necessary toughness to prevent the tool from shattering during use. This combination of carburization and heat treatment was essential for the development of superior, durable edged tools.21