In its simplest form, steel is an alloy of Iron and Carbon. It can be hardened if it contains sufficient, but not too much, Carbon. Theoretically, this range is from 0.008%C to 2.08%C (by weight) but for our purposes 0.5% to 1.2%C represents a steel that is practical to use for knife making.
When plain carbon steels (alloys primarily of iron and carbon only – they all contain a little silicon and manganese for other reasons) are hardened by heat treatment they rely on Iron Carbide to impart hardness to the structure. Carbon contents of up to 0.8%C improve the hardenability of the steel structure and the higher the Carbon content the higher the hardness of the steel will be after hardening. Once the alloy contains more than 0.8% Carbon free carbides are also formed in the steel structure and these will, in addition to the blade hardness, help to keep the cutting edge sharper for longer.
There are also other elements that will form harder (and some very much harder) carbides that iron will. On a Knoop hardness scale some of these carbides are over twice as hard as Iron Carbide eg chromium, molybdenum, silicon and vanadium carbides. These other alloying elements can also toughen the hardened steel if added in the correct proportions and a blade with an improved toughness and a greater ability to retain an edge is produced. However, (there’s always a however in Metallurgy – it’s a science of compromise!) add too much and the steel will be impossible to use as it will crack when being forged and/or heat treated.
Carbon steels in general use for knife making in South Africa mostly fall into the Alloy Carbon Steel category. Examples are AISI 5160, En45, Bohler K600 & AISI O1 (gauge plate & Bohler K460 are the same as O1). Plain carbon steels would be the likes of AISI 1055, 1070 & 1080. AISI D2 is a high alloy tool steel that’s not quite stainless – I’ll discuss this steel in a later article on Stainless Steels.
At present, only one of the knives I make comprise of Carbon Steel – the Bulldog Bush WP – the image at the top. Because it is carbon steel I can “selectively harden” the blade so that only the cutting edge is hard. The spine of the blade remains springy and tough and can thus cope with the chopping, prying etc that it will see in Bushcraft use. Carbon Steel is also thicker (6mm instead of 4mm) to give it more weight for these activities.