As I've already stated, the presence of carbon in iron is necessary to make steel. Carbon is essential to the formation of cementite (as well as other carbides), and to the formation of pearlite, spheroidite, bainite, and iron-carbon martensite, with martensite being the hardest of the micro-structures, and the structure sought after by knifemakers. The hardness of steel (or more accurately, the hardenability) is increased by the addition of more carbon, up to about 0.65 percent. Wear resistance can be increased in amounts up to about 1.5 percent. Beyond this amount, increases of carbon reduce toughness and increase brittleness. The steels of interest to knifemakers generally contain between 0.5 and 1.5 percent carbon