How to Machine Mold Steel: Soft Cut, Heat Treat, Hard Finish

A through-hardened core that grew 0.3 mm in the furnace and left only 0.15 mm of grinding stock is a scrapped insert. On a 16 cavity automotive connector tool that single miss moves the schedule three weeks and buys new steel. This guide shows the machining sequence that prevents it: what to cut soft, what to leave for hard finishing, and how much stock each step must carry.

The route follows standard practice at a precision mold shop: rough soft, stress relieve, finish soft near net, heat treat, then grind, EDM or hard mill to ±0.005 mm.

The Snapshot

• Cut soft at 28 to 34 HRC. Pre-hardened and annealed tool steel machines 3 to 5 times faster than hardened steel and saves carbide. Rough and semi-finish everything the furnace can move later.
• Expect 0.1 to 0.4 percent growth. Through-hardening a 1.2344-class block at 58 to 62 HRC shifts dimensions 0.1 to 0.4 percent, less with vacuum quench. Dimensions before the furnace are estimates.
• Leave 0.2 to 0.5 mm per ground surface. Flat grinding stock of 0.3 to 0.8 mm on parting faces covers treatment movement plus decarb removal while leaving steel to square the block.
• Hard finishing sets the true tolerance. Grind, jig grind and EDM hold ±0.002 mm at DieStrike; cavity dimensions land at ±0.005 mm after the furnace, not before it.

Why the Sequence Decides Accuracy

A mold insert is machined twice: once to shape it, once to size it. The first pass happens in soft steel where cutters are cheap and metal moves fast. The second happens in hardened steel, where only grinders, EDM sinks and jig grinders hold the drawing. The furnace sits between them and changes everything it touches.

Two physical effects drive the sequence. First, phase transformation: austenite converts to martensite during quench, the crystal structure expands, and a through-hardened 1.2344-class block grows 0.1 to 0.4 percent in its longest dimensions. Second, residual stress: roughing cuts lock tensile stress into the surface, and heating to 840 to 1,030 C releases it unevenly, bowing the part.

A shop that machines to final size before heat treat fights both effects and loses. A 200 mm core that grows 0.2 percent has moved 0.4 mm, which is 80 times the ±0.005 mm tolerance a cavity is held to. Dimensional control is only possible in the last operation before assembly.

The Standard Route: Soft Cut, Treat, Hard Finish

The production route for a through-hardened cavity insert has five stations, and each one leaves work for the next.

1. Rough soft. Pre-hardened 1.2344 or annealed P20-class stock at 28 to 34 HRC goes onto a mill. Face, drill, bore and pocket to shape, leaving 0.5 to 1.0 mm on surfaces that will be ground after treatment. Roughing removes 60 to 80 percent of the stock in one setup.

2. Stress relieve. A soak at 600 to 650 C, two hours at temperature for a 100 mm section, slow cool, releases the stress the roughing cuts put in. Skipping it is how a block that measured flat at 0.02 mm bows 0.15 mm overnight on the bench.

3. Finish soft near net. Semi-finish the form: cavities to within 0.2 to 0.5 mm of final, and all wire EDM through-features sunk while the steel is still cuttable. Standard components such as lifters and sleeves are made in parallel here, which is why stocked hardware ships in 3 to 7 days while a treated tool runs 2 to 5 weeks.

4. Heat treat. Austenitize, quench and double temper to band: 48 to 52 HRC for hot-work cavity duty, 58 to 62 HRC for wear-critical cores, punches and shearing edges. DieStrike caps cavity heat treatment at HRC 62. Vacuum quench is the default for dimension-critical parts because it moves the block less than oil quench.

5. Hard finish. Square the blocks on the grinder, jig grind the ±0.002 mm bores, EDM or hard mill the cavity form, polish or texture, and inspect on the CMM. Every number on the drawing is produced in this station and nowhere else.

How Heat Treat Shifts Dimensions

Dimensional change at hardening is not random, within limits. Martensite is roughly 4 percent less dense than austenite, so the part expands as it transforms, and the direction depends on when the outer shell transforms relative to the core.

Practical bands: through-hardening a 1.2344 or H13-class block at 58 to 62 HRC moves long dimensions 0.1 to 0.4 percent. Vacuum quench with high-pressure gas lands near 0.1 to 0.2 percent because cooling is symmetric. Oil quench on an unsupported plate can exceed 0.4 percent on thin, asymmetric parts.

Thickness matters more than hardness: a 30 mm plate moves less than a 200 mm block with one thin rib. Bores shrink 0.05 to 0.10 mm on quench, so finish reaming belongs after the furnace. On a 150 mm cavity block, a 0.2 percent growth budget is 0.30 mm, which is why the stock table below carries 0.3 to 0.8 mm on the flats.

Machining Allowances After Heat Treat

Allowance rules convert the movement bands into shop numbers. Leave too little steel and the grinder runs out of stock on the high corner of a warped face. Leave too much and hard-machining time and polishing hours grow on every part.

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Table note: stock bands are shop rules of thumb for through-hardening on 1.2344 and H13-class steels at 58 to 62 HRC. Vacuum quench allows the low end; unsupported oil quench demands the high end.

The decarb layer eats this stock silently. Treatment in air without protective atmosphere pulls carbon from every exposed face and leaves a soft skin 0.2 to 0.5 mm deep that must be ground off from the allowance, not in addition to it. A 0.3 mm allowance with a 0.35 mm decarb layer exposes soft steel across the whole shut-off land, and a flash edge built on soft steel fails within 100,000 shots.

EDM allowances differ from cutter allowances. A sinker electrode removes 0.2 mm of hardened stock in one or two passes with no cutting force, in under an hour on a normal electrode. Corner radii below R0.3 mm and ribs deeper than 4 to 1 belong to the EDM path, because a 60 HRC cutter cannot reach them without chatter or breakage.

Controlling Distortion Through the Furnace

Distortion is a design problem before it is a furnace problem. The treatment house can only manage what the geometry lets it manage.

Symmetric sections. Keep wall thickness even. A core with one 40 mm boss and one 8 mm wall bows toward the heavy side because the thick section transforms last. Mirror unavoidable bosses with relief pockets on the opposite face.

Through holes before treatment. Blind holes trap quench media and cool unevenly; the drilled bottom is a crack starter. Drill and EDM through-holes, or drill oversize and finish after. Fillets of R1 mm or more at every internal corner are mandatory: that is where a quench crack begins.

Fixturing and press quench. Flat parts go between ceramic or steel plates in the stack, and critical plates are clamped to a flat carrier before heating. Press quenching can pull a thin plate back to 0.05 mm flatness from 0.2 to 0.4 mm of free movement. Specify the as-quenched flatness on the purchase order, or the furnace house stacks six blocks high.

Expected recovery. Even with all three controls, plan 0.1 to 0.3 mm of face movement and budget the grinding stock to absorb it. When distortion exceeds the stock, the block is scrapped: re-tempering to straighten it drops hardness on the map with it.

Hard Finishing: Grind, EDM or Hard Mill

After the furnace, three machine types produce the drawing. Which one touches which surface is the second half of the sequence decision.

Grinding owns flatness and datums: parting faces, block height, shut-off lands, and every dimension a ±0.002 mm tolerance rides on. A surface grinder takes 0.01 mm finishing passes on 62 HRC steel and leaves Ra 0.2 µm or better, which is why flash lands are ground and never milled.

EDM owns geometry the wheel cannot reach: sharp internal corners, deep ribs, lettering and texturing zones. Wire EDM after treatment re-splits through-features to ±0.002 mm without touching hardness. Electrodes are cut from the pre-treatment 3D model, so the growth compensation lives in the electrode while the block never needs guessing.

Hard milling owns the open cavity area. Rigid 5-axis machines with high-pressure coolant mill 58 to 62 HRC at 0.1 to 0.3 mm stepovers and reach near Ra 0.4 µm, removing most of the polishing hours EDM leaves behind. The trade-off is a rigid setup: a spindle that flexes 0.01 mm puts that error straight into the cavity. The full matrix sits in the hard milling vs EDM guide.

Jig grinding keeps the fits: ejector and core-pin bores and guide-post holes at ±0.002 mm. DieStrike holds ±0.005 mm on general mold geometry and ±0.002 mm on EDM and jig-grinding operations, and every post-treatment part returns to the CMM before assembly.

Nitriding and PVD Without Re-Hardening

Surface treatments add wear resistance without a second furnace trip, because their process temperatures sit below the tempering point of hardened steel.

Nitriding diffuses nitrogen at 500 to 550 C, growing a compound layer of 5 to 20 µm over a case 0.1 to 0.5 mm deep and pushing a 48 to 52 HRC hot-work core to HV 1,000 to 1,200 at the surface. The core keeps its hardness and the part moves 0.01 to 0.02 mm, small enough to plan. Plasma nitriding trims the distortion risk further.

PVD runs cooler still: 200 to 350 C for TiN, CrN, TiAlN or DLC layers of 1 to 5 µm. The dimensional change is the coating thickness, so ground faces must start undersize by that amount. A 2 µm film cannot bridge a soft substrate: nitride where the load is high, then PVD on polished faces.

The sequencing consequence matters at quoting time. A nitrided or coated mold can finish its cavity at the 48 to 52 HRC band, where grinding and EDM go faster, and let the surface treatment carry the wear duty. That hybrid still delivers a 1,000,000-shot surface on slides, lifters and shearing edges.

What Cannot Be Fixed After Heat Treat

The furnace is a point of no return for four defect families, and each traces to a decision made before the part went in.

Cracks from sharp corners. A quench crack at an unmachined internal corner or a stress riser above a blind hole cannot be welded into service on a loaded cavity face. Prevention is geometric: R1 mm minimum fillets and through-holes before treatment.

Decarb layers. Soft skin 0.2 to 0.5 mm deep is steel that will never reach its hardness, and stock that cannot swallow it leaves an under-size or soft-faced feature: scrap. The atmosphere requirement goes on the PO, and incoming stock gets checked before the first cut, as covered in the guide to verifying incoming mold steel.

Soft spots from poor quench. Interrupted gas flow or an overloaded batch leaves local spots 5 to 10 HRC below target. Find them with a portable UCI or rebound hardness meter, calibrated on the block's tempered surface: read nine points per critical face against the hardness map the furnace must supply. A soft spot cannot be re-hardened selectively; the block re-enters full treatment or the scrap bin.

Scale and pitting beyond stock. Heavy oxidation from a bad vacuum cycle eats 0.05 to 0.15 mm off polished faces. That sits inside grinding stock on a parting face but outside it on a finished cavity, where a sink pass cannot recover the form without re-cutting electrodes.

Twin-Route Economics: Soft Finish vs Pre-Hard

Two legitimate routes compete for every insert, and the difference is where the cost and the risk land.

Route A, soft finish then treat. Rough and semi-finish at 28 to 34 HRC, where a 12 mm ball nose removes 300 to 500 cubic centimeters per hour, then treat, then hard finish the last 0.2 to 0.5 mm. The route adds one handling cycle and one furnace trip, typically 5 to 10 working days on a 2 to 5 week tool. In exchange it saves hours of EDM and grinding: sinking a hardened cavity from scratch costs 2 to 4 times the electrode and machine time of finishing one cut near net soft.

Route B, pre-hardened all the way. Buy 1.2344 or P20 pre-hard at 30 to 34 HRC, machine to final size, never enter the furnace. No distortion risk, no allowance guessing, and the schedule compresses to machining alone. The cap is wear life: a 32 HRC cavity sees shut-off clearances open after 200,000 to 500,000 shots, where the same geometry through-hard at 58 to 62 HRC holds a million. Grade economics are worked out in the pre-hardened vs through-hardened comparison.

Most production tools run the hybrid: through-hard the cavity and core inserts where flash and wear live, and order bases, clamp plates and backing plates pre-hard. The plates never touch molten polymer, so treating them buys only furnace risk.

Stress relieving is the cheap insurance inside Route A. A 600 to 650 C soak after roughing costs a day and prevents most of the movement the finisher would grind away. The mechanics are in the stress relief guide.

Route Decision by Part Class

The route follows the part class, not the shop's habit.

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Table note: bands reflect standard practice for 1.2344, H13, P20 and 1.2083-class steels. Match the exact grade, hardness target and temper count to the heat treat specification before releasing the PO.

Heat Treat Purchase Order Checklist

Every line below belongs on the PO that goes to the furnace, in writing. A PO that only says "harden to 58 to 62 HRC" authorizes the shop to make no promises about anything else.

• Grade and standard. Name the steel to its number, 1.2344 or H13, with stock hardness and lot number from the incoming cert.
• Atmosphere. Vacuum furnace with high-pressure gas quench for dimension-critical inserts. Salt bath is acceptable only where 0.3 mm of grinding stock exists on every face.
• Hardness band and location. 48 to 52 or 58 to 62 HRC, measured on a ground test tab or scrap pad, never on a finished cavity face.
• Double temper, minimum. Two tempers after quench, three on 58 to 62 HRC die steels, each with soak time recorded. Under-tempered steel looks hard on day one and softens in the first hot month on the press.
• Hardness map required. A per-part map, nine points minimum per critical face, with the furnace cycle record attached, per the receiving rules in the heat treat specification guide.
• Flatness and fixturing. State the as-quenched flatness the stock assumes: for a plate with 0.3 mm stock, ask for 0.15 mm, not "flat".
• Decarb ceiling. Require zero measurable decarb on vacuum cycles, and state the maximum ground stock you carry so the furnace knows your ceiling.

A treated batch that arrives with charts, maps and a stated atmosphere is a known quantity at CMM time. One that arrives with a stamp on a bin card is a survey project.

Machining Mold Steel FAQ

Q1. How much material should I leave for heat treat?

0.2 to 0.5 mm per ground surface, with 0.3 to 0.8 mm on parting faces that must also swallow decarb and warp. Size it from the longest dimension: 0.2 percent growth on a 200 mm block is 0.4 mm of movement.

Q2. Can I machine a mold to final size and then heat treat without re-cutting?

No, not to ±0.005 mm. Through-hardening moves dimensions 0.1 to 0.4 percent and flatness 0.1 to 0.3 mm even with press quench. Final geometry must be produced in hardened steel, after the furnace.

Q3. Why does my hardened part measure softer in one corner?

That is a quench shortfall: the corner cooled too slowly for full martensite, or the batch was overloaded. Verify on a nine-point grid with a portable hardness meter. A 5 to 10 HRC soft spot cannot be fixed selectively; the part re-enters full treatment.

Q4. Should holes be drilled before or after heat treat?

Through-holes before, on the wire EDM or drilled oversize, so nothing traps quench media. Located and reamed holes after, jig-ground or reamed to size, because bores shrink 0.05 to 0.10 mm in the quench.

Q5. Does nitriding replace heat treatment?

No, it completes it. Nitriding at 500 to 550 C hardens only the top 0.1 to 0.5 mm and preserves the core. A nitrided 32 HRC pre-hard block is a wear face on a soft core; a nitrided 50 HRC through-hard block is the production combination.

The Bottom Line

The sequence is the tolerance. Machine to shape at 28 to 34 HRC, relieve the stress, leave 0.2 to 0.5 mm per ground surface, harden to 48 to 52 or 58 to 62 HRC in a stated atmosphere with a hardness map returned, and let grinding, EDM and hard milling produce the final ±0.005 mm. Every shortcut before the furnace is paid for after it, in insert lead time and shut-off accuracy.

DieStrike runs this sequence in-house for production cavities: IATF 16949 tooling on 120+ machines, ±0.005 mm on mold geometry and ±0.002 mm on EDM and jig grinding, heat treatment capped at HRC 62, standard components shipping in 3 to 7 days, and injection mold lead times of 2 to 5 weeks. The route decision for your next insert comes out of the DFM review, not out of habit.