18NCD6 is a nickel-chromium-molybdenum case-hardening steel from the AFNOR system, equivalent to Wnr. 1.6587 / DIN 17CrNiMo6 / BS 820A16. It's one of the heaviest-duty carburizing grades in common use, combining substantial nickel (roughly 1.4-1.7%) for core toughness with chromium and molybdenum for deep hardenability and resistance to temper embrittlement. That combination lets 18NCD6 carburize to a hard, wear-resistant case while retaining a genuinely tough, high-strength core even in thick gear and shaft sections — a step up in capability from lighter case-hardening grades like 20MnCr5.

18NCD6 is supplied and machined in the annealed or normalized condition, then carburized and case-hardened afterward to develop its working properties. It's the go-to steel for heavily loaded, large-module gears, pinions, and shafts in gearboxes, off-highway equipment, and industrial drivetrains where both a hard wearing surface and a core strong enough to resist bending and shock fatigue are required.

In its annealed or normalized delivery condition (typically around 200-230 HB), 18NCD6 machines like a moderately alloyed low-carbon steel — manageable cutting forces and generally favorable chip formation, though the nickel and chromium content make it a bit gummier and more prone to built-up edge at low speeds than a plain carbon or simple Cr-Mo grade. A sharp, positive-rake edge and adequate cutting speed help avoid that tendency.

Because this steel gets its working hardness from carburizing and case hardening rather than through-hardening, essentially all machining — gear tooth cutting, shaft turning, keyways, bores — is done in the soft annealed state, with the part carburized and hardened only after machining is complete. Any post-hardening work is limited to grinding the hardened case, not conventional cutting.

Chip control is generally good in the annealed condition, and a coated carbide grade with balanced toughness and wear resistance handles the alloy content well. Keeping the tool cutting cleanly with steady feeds — rather than rubbing at low speed — is the main defense against the built-up edge and work hardening that nickel-alloyed steels are somewhat prone to.