How Carbon Steel Is Classified

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.

What Changes as Carbon Goes Up

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.

Picking the Right Tooling for the Grade

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.