40CMD8 is a French AFNOR-designated (standard NF A35-590) medium-carbon, chromium-manganese-molybdenum low-alloy steel. Reading the designation: "40" indicates roughly 0.40% carbon, and "C-M-D" identifies chromium, manganese, and molybdenum as the alloying elements in decreasing order of content, with chromium — the first-named element — landing around 2% per the trailing coefficient. Verified mill data for this grade confirms a chromium-manganese-molybdenum chemistry; it does not carry a vanadium addition, despite that sometimes being assumed by association with other Cr-Mo-V tool and mold steels.

With roughly 2% chromium, a comparatively high manganese content (over 1%), and a modest molybdenum addition, 40CMD8 combines good hardenability with reasonable toughness at medium-to-high strength levels. It is typically supplied and machined in an annealed or pre-hardened-and-tempered condition and used for medium-stress mechanical components and general-purpose tooling applications where a Cr-Mo alloy steel's strength is needed but the cost of a more heavily alloyed grade isn't justified.

No independently verifiable Wnr./DIN cross-reference was located for this specific AFNOR grade. 40CMD8 is documented under French standard NF A35-590; confirm equivalency against mill certification or AFNOR documentation for critical applications rather than assuming a cross-reference to similarly-named Cr-Mo or Cr-Mo-V grades from other systems.

40CMD8's roughly 2% chromium content and elevated manganese level put its machining behavior closer to a heavier-alloyed Cr-Mo engineering steel than to a standard 4140-class grade. The higher chromium promotes more fine carbide formation, which increases abrasive flank wear, while the high manganese content — present mainly for hardenability — adds toughness that resists clean chip shearing compared with a lower-manganese alloy steel. Cutting forces and heat generation both run somewhat higher than on a standard Cr-Mo grade with less total alloy content.

Chip formation is generally manageable with the right chipbreaker geometry, especially in the annealed or moderately tempered condition this material is typically supplied in. As alloy content and hardness climb toward the upper end of its pre-hardened range, expect shorter, more abrasive chips and correspondingly faster flank wear, which is why a coated carbide grade with good hot hardness outperforms uncoated tooling on anything beyond light finishing passes.

Rigid setups and a wear-resistant coated grade matched to alloy steel work make the biggest practical difference in tool life here. Because this grade is often supplied pre-hardened for tooling and mold-adjacent applications, confirming actual supply hardness before selecting speeds and feeds is worthwhile — the ranges below assume a nominal, moderately hardened condition.