SAE/AISI 1039, equivalent to DIN 40Mn4, is a medium-carbon manganese steel carrying roughly 0.37–0.44% carbon along with an elevated 0.70–1.00% manganese content. That manganese addition is higher than what you'll find in a plain carbon grade like 1035, and it does real work here: it improves hardenability and adds strength without requiring the chromium or molybdenum you'd find in a true alloy steel, keeping 1039 in the low-cost, easy-to-source carbon steel family while pushing its mechanical properties a step further.

The combined carbon and manganese content puts 1039 solidly in the medium-carbon range, giving it a good working balance of strength, ductility, and machinability that suits shafts, axles, spindles, gears, and other structural or mechanical components where moderate strength and wear resistance matter. Like other steels in this family, it can be supplied normalized or annealed for general machining, or quenched and tempered afterward to reach higher strength and hardness levels for finished parts.

1039's carbon content sits toward the upper end of the medium-carbon band, and the added manganese pushes strength up further, so this grade cuts a bit harder than a leaner steel like 1035 even before any heat treatment. In the normalized or annealed condition it's still reasonably free-cutting with standard carbide tooling, but chip formation and tool wear are a little more demanding, and cutting speeds typically need to sit a step below what you'd run on a lower-carbon, lower-manganese grade.

Quenching and tempering raises hardness quickly on 1039, just as it does on other steels in this carbon range, and that hardness increase drives up cutting forces and flank wear while generating more heat at the edge. Confirming the actual condition of the stock — normalized versus heat-treated — before selecting a speed and grade avoids the common mistake of running tempered material at parameters meant for soft stock.

Because the manganese content also raises the material's tendency to work-harden under light or interrupted cuts, keeping a steady, adequate feed rate and sharp cutting edges matters more here than on lower-alloy carbon steels. Rigid workholding and a coolant strategy suited to the operation round out the practical basics for consistent tool life on this grade.