Can D2 tool steel be wire EDM cut?
Yes. Wire EDM cuts hardened D2 at 58-62 HRC routinely. Harden with a high temper, rough out first, use skim passes to limit recast, then stress relieve.
Wire EDM does not care how hard the steel is, only that it conducts, so hardened D2 is one of the most commonly wire-cut materials: punches, die openings, strippers, form tools and slitter knives are cut at 58-62 HRC after heat treatment. A brass or coated wire, typically 0.25 mm (0.010") in diameter, erodes the profile in deionized water. A rough cut leaves a surface of around 2-3 µm Ra, and two to four skim passes at lower energy bring it to about 0.2-0.5 µm (8-20 µin) Ra with profile tolerances of about +/-0.005 mm (0.0002"). Taper cutting produces die clearance angles in the same setup. Because the part is hardened before cutting, the finished profile does not move in heat treatment, which is the whole point.
The risk with D2 is cracking and movement from the stress locked in by hardening. A hardened block carries residual stress, and when wire EDM removes a large slug or cuts a long slot, the stress redistributes: the part bows, slots close up, and in the worst case the block cracks partway through the cut. D2 is more prone to this than lower-alloy steels, and a block tempered low, around 200 °C (400 °F), is worse than one given the high double temper at about 510-540 °C (950-1000 °F), which relieves more stress at a cost of a few points of hardness. The standard precautions: rough out large openings before hardening, leaving about 1.5-3 mm (0.06-0.12") per side for the wire; drill start holes before hardening; specify the high temper for blocks that will be cut heavily; and on critical parts rough cut the profile, let it relax, then finish cut and skim.
The EDM surface itself needs attention. Each spark melts a thin film that re-solidifies almost instantly, the recast or white layer, typically 10-30 µm (0.0004-0.0012") deep after a rough cut: hard, brittle, in tension and often microcracked, with a rehardened or overtempered zone beneath it. On a punch or die edge this layer chips in service and starts fatigue cracks. Skim passes reduce it to a few microns, and stoning or polishing removes the rest on cutting edges. After heavy wire work, retemper the part about 25-30 °C (50 °F) below its last tempering temperature to relieve EDM stress without losing hardness. Parts that will be nitrided or PVD coated after wire EDM should have the recast removed first, because coatings over recast spall.
Wire EDM for D2 parts: where to send them
What to specify
- Heat treatment before EDM: harden and double temper; for heavily cut blocks, high temper at 510-540 °C (950-1000 °F)
- Work before hardening: start holes drilled and large openings roughed out, leaving 1.5-3 mm (0.06-0.12") per side
- Profile tolerance and finish, for example +/-0.005 mm (0.0002") and 0.4 µm (16 µin) Ra, which sets the number of skim passes
- Recast limit on working edges, for example "5 µm maximum" or "remove recast by skim passes and stoning"
- Die clearance or taper angle per side, and the land length, for punch and die work
- Post-EDM stress relief: retemper about 25-30 °C (50 °F) below the last tempering temperature after heavy cutting
Pitfalls
- Cutting a large slug from a solid hardened block: stress release cracks or bows the part; rough out before hardening
- Low-temper D2 given heavy wire work: more locked-in stress and more cracking; use the high temper
- Leaving rough-cut recast on punch and die edges: the brittle layer chips and shortens tool life
- Thin sections and long slots that move as they are freed: leave tabs, cut in two stages, or finish after the part relaxes
- No heat-treat record sent to the EDM shop: without the hardness and temper they cannot plan the cut sequence
- Nitriding or coating over EDM recast: the coating spalls along with the recast underneath
Frequently asked questions
- Can hardened D2 be wire EDM cut?
- Yes. Hardness does not slow wire EDM, and D2 at 58-62 HRC is cut every day for punches, dies and form tools. The precautions are about residual stress and the recast layer, not hardness.
- Why did my D2 block crack during wire EDM?
- Residual stress from hardening. Removing a large slug redistributes it, and D2, especially with a low temper, can crack or bow. Rough out openings and drill start holes before hardening, use a high double temper, and cut in stages on critical parts.
- What is the recast layer on wire EDM cut D2?
- A thin film of metal melted by the sparks and re-solidified, about 10-30 µm after a rough cut, hard, brittle and often microcracked. Skim passes reduce it to a few microns; stoning or polishing removes it from cutting edges.
- How accurate is wire EDM on D2?
- About +/-0.005 mm (0.0002") on profiles with skim passes on a good machine, and better with extra passes and temperature control. Surface finish runs from about 2-3 µm Ra on a single cut to 0.2-0.5 µm (8-20 µin) Ra after skims.
