40CrMnNiMo8 (Wnr. 1.2738, full DIN designation 40CrMnNiMo8-6-4) is a pre-hardened plastic mold steel, not a general structural alloy steel — despite the name pattern reading like a typical Cr-Mn-Ni-Mo engineering steel. It's the European equivalent of AISI P20+Ni, an enhanced version of standard P20 mold steel with roughly 1% nickel added specifically to improve through-hardness uniformity in large mold blocks. Where plain P20 (1.2311) can show a hardness gradient from surface to core in thick sections, the nickel addition in 1.2738 keeps hardness far more consistent across large cross-sections, which is exactly what's needed in big injection mold and die-casting tooling.

This grade is supplied pre-hardened and tempered to roughly 280-320 HB (about 32-36 HRC) directly from the mill, so shops receive it ready to machine into the finished mold cavity without a separate hardening cycle. It's prized for excellent machinability at that hardness, outstanding polishability for high-finish mold surfaces, and good weldability for repair work, making it a mainstay for plastic injection molds, die-casting dies, and other tooling requiring large, dimensionally stable blocks up to roughly 400 mm in section.

Because 1.2738 arrives pre-hardened to roughly 32-36 HRC rather than as soft annealed bar, every cut a mold shop makes on it happens at that hardness — there's no separate soft-machining stage followed by heat treatment the way there is with most alloy steels. That pre-hardened condition, combined with meaningful Cr and Mn content, puts cutting forces and flank wear noticeably above a typical annealed low-alloy steel, though still well below a fully hardened tool steel.

The upside is that 1.2738 is specifically formulated for machinability at this hardness — it's one of the easier-cutting pre-hardened mold steels precisely because mold shops need to cut large cavity geometries and fine surface detail without excessive tool wear or heat-related dimensional drift. Chip formation is generally well-behaved and predictable at this hardness range, and the material takes a good surface finish directly off the tool, which matters for mold work where post-machining polishing time is a real cost driver.

Coated carbide grades with good hot hardness handle the moderate cutting temperatures well, and rigid setups with consistent feed engagement minimize the risk of chatter marks telegraphing into a polished mold surface. Since much of mold work involves finishing passes and fine detail rather than heavy stock removal, edge sharpness and consistency across a long production run often matter more than raw wear resistance.