SAE/AISI 1040, cross-referenced to DIN C40, is a plain medium-carbon steel with a carbon content of roughly 0.37–0.44% and 0.60–0.90% manganese. That places it right in the heart of the medium-carbon band, a step up from lower-carbon grades like 1035 in terms of achievable strength and hardness, while still relying on carbon alone — rather than chromium, nickel, or molybdenum — to deliver its mechanical properties.

This combination of moderate strength, decent ductility, and workable machinability makes 1040 a common choice for shafts, bolts, gears, axles, and other general-purpose mechanical and structural parts that need more muscle than a low-carbon steel can offer but don't call for the added cost and complexity of an alloy grade. Because the carbon content is high enough to respond well to heat treatment, 1040 is frequently supplied normalized for machining and then quenched and tempered afterward when a finished part needs greater surface hardness or wear resistance.

At roughly 0.40% carbon, 1040 sits comfortably in the medium-carbon range where the material still cuts cleanly with standard carbide tooling but generates noticeably more cutting force and heat than a lower-carbon grade. In the normalized or annealed condition, chip formation is generally well-behaved, and moderate cutting speeds combined with a properly matched insert grade will deliver good tool life and predictable surface finish.

Because 1040 has enough carbon to respond meaningfully to heat treatment, hardness increases quickly once the material is quenched and tempered. That jump in hardness raises cutting forces and accelerates flank wear, so a cutting speed and grade dialed in for soft, normalized stock will underperform — or wear out fast — against tempered material. It pays to confirm the actual condition of the workpiece before committing to a cutting program.

This grade also has some tendency to work-harden if the tool is allowed to rub rather than cut, particularly on light finishing passes or interrupted cuts. Keeping the feed rate adequate, the cutting edge sharp, and the workholding rigid all help avoid burnishing the surface and keep tool wear predictable through a run.