Machining Prehardened Mold Steel: Milling P20-Family Blocks at 30–36 HRC

The workflow premise

Prehardened steels — 1.2311, 1.2312, 1.2738 and their P20 relatives — are delivered at roughly 290–340 HB (about 30–36 HRC) precisely so the mold maker can machine the cavity and put the tool into service with no further heat treatment and no hardening distortion to correct. The price of that convenience is that every operation happens in moderately hard steel, and the strategies differ from soft-steel machining.

Milling strategy

• Tooling: fine-grain coated carbide is the baseline — TiAlN/AlTiN-type coatings tolerate the heat of hard milling; ball-nose and corner-radius (bull-nose) end mills dominate cavity work, in the shortest lengths the geometry allows.
• High-speed machining (HSM) approach: relatively small radial engagements and shallow axial steps at high spindle speed and feed keep chip thickness consistent and cutting forces low, which protects both tool life and surface finish. Large step-overs with a buried tool overheat edges quickly in this hardness range.
• Trochoidal/adaptive roughing paths maintain constant tool engagement in pockets and slots — the engagement spikes of conventional offset roughing are the usual cause of sudden end-mill failure in prehardened stock.
• Climb milling is preferred: it directs heat into the chip and avoids the rubbing entry of conventional milling, which work-hardens and dulls edges.

Semi-finishing and finishing

Because there is no post-hardening grinding stage to rescue the cavity, the milling itself must deliver near-final accuracy and finish. Rest-machining passes clean the material the rougher could not reach; finishing runs with small step-overs (fractions of a millimetre on contoured surfaces) leave a surface that needs only stoning and polishing. Leave uniform stock from each stage — uneven allowance shows up as chatter marks and polish variation later.

Where EDM takes over

Some cavity features simply cannot be milled in hardened steel:

• Deep, narrow ribs and slots, sharp internal corners (a milled corner cannot be sharper than the tool radius), and fine engraving go to sink EDM.
• Wire EDM handles through-profiles, ejector and leader-pin holes in finished plates, and parting of inserts.

EDM on prehardened steel needs no softening or re-hardening, which is exactly why the prehardened concept pairs so well with it — but budget for removing or accounting for the thin recast layer on polished or high-fatigue surfaces.

Practical notes

• Minimize setups and re-clamping: distortion-free delivery condition is worth little if the block is sprung in the vise. Support large plates fully; skim stressed faces symmetrically, since removing material unevenly from a rolled plate can release residual stress and bow the block slightly even without heat treatment.
• 1.2312 (the sulphur-bearing free-cutting variant) machines noticeably faster and is ideal for holder blocks and non-polished parts; keep it out of mirror-polished cavities, where its inclusions cap the achievable finish.