H-Group Hardened Steel Machining Guide: Hard Milling

Hardened steel is ISO 513's group H (insert colour: grey), driven by hardness state — typically HRC45–65 quenched/heat-treated steel, such as hardened SKD11, DC53 and SKD61, plus bearing steel SUJ2 and powder high-speed steels. It is hard, high-strength, high in cutting force, prone to tool wear and chipping, and runs hot at the tip, though chips are short and brittle. Today it is largely hard-milled directly on quenched parts, partly replacing EDM and grinding. This guide covers tool-selection direction for the hardened-steel types; for the overview see "ISO Workpiece Material Classification."

Hardened-steel types and machinability

The common logic: the harder the steel, the smaller the load per tooth. HRC45–55 is mostly hard-milled with fine-grain carbide + AlCrN/nano coating; for HRC58 and above, especially finishing, CBN (cubic boron nitride) is the first choice. Note: the same steel is group P and easy to cut when annealed, and only moves to group H after hardening (see the dual-listing in the "P-Steel Machining Guide").

A sprayed or fused alloy overlay on the surface is just as hard to cut once it passes HRC50, but it cannot be cut like hardened steel: choose carbide with high thermal conductivity, the opposite of the hardened-steel choice. See Machining Thermal Spray Coatings.

Three hard-milling keys for hardened steel

Tool grade and coating direction

Medium hardness (HRC45–55) mainly uses fine-grain carbide + AlCrN/nano coating; high hardness (HRC58+, especially finishing) prefers CBN for heat and wear resistance and edge retention. Rough out leaving an even stock for finishing; finish with light, fast paths and constant edge engagement. For full grade and coating selection, see "Cutting Tool Material Classification."

Key cutting strategy

Turning: can you still turn after hardening? What hard turning needs

Hardened steel cuts with more force than before hardening, but it has little ductility and hardly sticks. With the right tool and conditions it can be turned to a finish close to grinding.

For the full method see Hard Turning Guide: Replacing Grinding at HRC45–65; for how much it actually saves see How Much Does Hard Turning Actually Save: Twelve Quantified Cases; for regrinding CBN turning tools see Regrinding CBN Turning Tools: Never Along the Rake Face, Flank Meets the Wheel First.

Drilling: why the drill chips so fast once the steel is hardened

When drilling, the whole drill is buried in the hole and its edges are loaded the entire time, and hardened steel is both hard and strong, so the cutting edges and chisel edge chip first. Most of the hard-milling tactics above do not carry over into the hole.

These are typical ranges for rigid parts. As hardness rises further, bring the feed down with it, to roughly 0.025 mm/rev at 55 HRC and 0.02 mm/rev at 64 HRC. Follow the drill catalog in practice; for conversions see Drilling Parameters Guide; for the three carbide drill types see How to Choose a Carbide Drill Bit, and for a precision-drilling example in hard material see 3-Flute Drill Guide. When a hardened part needs a precision hole, the process route is in Machining Route Selection.

Tapping: can you still tap after hardening?

Yes, but with a different tap. Hardened steel is both hard and strong, so wear and thread chipping are far worse than before hardening; above about 45HRC the chips break into powder, and once they pack the flutes the tap breaks.

For using carbide taps see the Carbide Tap Guide; for choosing the holder see Tool Holders for Tapping; for thread milling see the Thread Milling Guide.