D4 and D6 are two closely related high-carbon, high-chromium cold-work tool steels from the AISI D-series. Both start from essentially the same base chemistry as D3 — roughly 2.0-2.4% carbon and 11-13.5% chromium — and then add a single secondary alloying element to improve on D3's main weakness, which is oil-hardening and the distortion risk that comes with it. D4 adds molybdenum (0.70-1.20%), while D6 adds tungsten (1.00-1.60%). Both additions raise hardenability enough that D4 and D6 harden more predictably in air or a light oil quench than plain D3, and both promote a somewhat finer secondary carbide structure that adds a measure of toughness without giving up much of the wear resistance the D-series is known for.

Because D4 and D6 share so much of their composition and behavior, they are frequently specified interchangeably by tool designers and are grouped together here as a single reference. Typical service hardness runs 58-64 HRC for both grades. They are used for the same demanding wear applications as other D-series steels — blanking and trimming dies, deep-drawing dies, thread-rolling and forming dies, and cold-extrusion tooling — chosen over D2 or D3 when a shop wants slightly more secondary-hardening response or marginally better toughness at the same carbide-driven wear resistance.

D4 and D6 behave very similarly from a machining standpoint, since both carry the same heavy load of primary chromium carbides that make every D-series steel abrasive to cut. D4's molybdenum addition refines the carbide structure slightly compared with plain D3, which translates to marginally more consistent tool wear rather than a dramatic machinability improvement. D6's tungsten addition works in the opposite direction — tungsten carbides are extremely hard and tend to be somewhat coarser, so D6 is generally regarded as at least as abrasive as D3, if not slightly more so, despite its improved hardenability and service toughness.

In the annealed condition, both grades cut with standard carbide tooling, but expect elevated cutting forces and accelerated flank wear compared with lower-alloy tool steels or engineering steels of similar hardness. Chip formation is short and segmented for both, which aids evacuation but subjects the cutting edge to repeated small impacts rather than a smooth continuous cut. Because both D4 and D6 harden more reliably than D3, more finish machining tends to happen after heat treatment at 58-64 HRC, which raises the importance of rigid setups, well-supported edge geometry, and conservative feeds to prevent chipping.

Coated carbide grades with strong abrasion resistance and good edge toughness are the practical choice for both grades. Favoring a tougher edge preparation over an aggressive sharp one, and keeping speeds moderate, generally produces more predictable tool life than pushing for maximum material removal rate.