Materials Glossary
Carbon is the single most influential element in shop-floor metallurgy. A few tenths of a percent, added to iron, is the difference between a soft bar of mild steel and a hardened blank that can hold a cutting edge.
Steel is simply iron with a small, carefully controlled dose of carbon mixed in — typically somewhere between 0.03% and 3% by weight, though most working steels fall in the 0.1%–1% range. That small percentage has an outsized effect: it is the main lever that controls how hard, strong, and ductile a piece of steel will be. The classification system used across the industry sorts plain carbon steels into four bands by carbon content: low carbon steel (under 0.25% C), medium carbon steel (0.25%–0.55% C), high carbon steel (over 0.55% C), and tool carbon steel (over 0.8% C). As carbon content climbs through these bands, tensile strength and hardness go up while ductility — the material's ability to bend or stretch before breaking — goes down. That trade-off is the reason a low-carbon steel bracket can be bent cold in a press brake without cracking, while a high-carbon tool steel blank needs to be heat treated with real care.
Cast iron is iron, carbon, and silicon, with carbon content running roughly 1.7% to 3% — well above what steel can hold while staying weldable and ductile. That extra carbon exists either as free graphite or as iron carbide (Fe₃C, also called cementite), and it's what gives gray, white, and malleable cast iron their characteristic hardness, brittleness, and excellent machinability. On the other end of the scale, most stainless steel grades intentionally keep carbon very low, typically 0.03%–0.1%, because carbon can tie up the chromium that gives stainless its corrosion resistance. Grades under 0.035% carbon are labeled "L" for low-carbon; grades above 0.08% are labeled "H."
Carbon isn't only the additive that shapes steel — it's also, quite literally, why carbide cutting tools exist. Tungsten carbide, the workhorse material behind almost every insert and end mill in this shop, is built from roughly equal parts tungsten and carbon atoms bonded together into an extremely hard compound. Even polycrystalline diamond (PCD), the hardest cutting material in common use, is a carbon-atom structure — sintered diamond particles fused with a metal binder under intense heat and pressure. From the steel being cut to the tool doing the cutting, carbon is at the center of the whole process.