Materials Glossary
Carbon steel is iron alloyed with carbon and little else. Where that carbon percentage lands — low, medium, or high — decides almost everything about how the material cuts, bends, and holds up: its strength, its ductility, and how hard it will be on your tooling.
Carbon steel is grouped into three bands by carbon content: low-carbon steel runs about 0.05%–0.30% carbon, medium-carbon steel runs 0.30%–0.60%, and high-carbon steel runs 0.60% up to about 1.5%. In the United States, the AISI/SAE four-digit numbering system reflects this directly: for plain carbon steels, the first two digits are typically "10," and the last two digits approximate the carbon content in hundredths of a percent. A 1045 steel, for example, contains roughly 0.45% carbon — the number on the mill certificate is telling you the recipe.
Carbon content is the primary lever for mechanical properties in these steels. As a rule of thumb, tensile strength increases by roughly 100 MPa for every 0.1% increase in carbon in pearlitic steels, while ductility and weldability begin dropping off noticeably once carbon passes about 0.3%. In practical terms: low-carbon (mild) steel is the most machinable and ductile of the group, with relatively low hardness — and a normalizing heat treatment can improve its machinability further. Medium-carbon steel is stronger while still reasonably ductile and machinable, which makes it the workhorse choice for shafts, gears, and structural machined components. High-carbon steel trades away ductility for hardness and wear resistance; it's tougher to machine, more prone to cracking under stress, and demands more disciplined tooling and technique.
That progression from low to high carbon is also a progression in machining difficulty. Mild steel forgives a lot of tooling and technique choices. High-carbon and tool-grade carbon steels do not — they call for rigid setups, sharp, wear-resistant cutting edges, and the kind of premium carbide tooling built specifically to handle harder, less forgiving material without chipping or dulling prematurely.