How to Prevent Heat Checking in Mold and Die Steel

An aluminum die casting cavity ran 45,000 shots without a dimensional complaint. Then the molder saw a spider-web of fine lines radiating from the gate. Within 8,000 more shots the network deepened, the part started flashing, and the insert needed replacement 6 weeks early. That failure mode is heat checking, and it scraps hot-work tooling faster than wear ever will. Industry surveys of die casting tool failures put thermal fatigue among the top 2 causes of premature die retirement, and one crack network can force $8,000 to $20,000 of insert rework or replacement on a mid-size die.

Heat checking is a mold-design and steel problem long before it is a process problem. The surface temperature swing, the hardness you specify, the cleanliness of the steel bar and the condition left by EDM all decide when the first crack appears. This guide covers the failure mechanics, the prevention levers you control at the tooling stage, and the repair-or-replace decision once a crack network shows up.

The Snapshot

• Heat checking is low-cycle thermal fatigue. Cavity surfaces in aluminum die casting swing 100°C to 200°C per cycle, and each swing plastically strains the top 0.1 mm to 0.5 mm of steel.
• Typical die casting hardness targets are HRC 44 to 48 for H13. Above HRC 52, thermal fatigue crack initiation gets measurably faster.
• Preheating dies to 150°C to 200°C before production cuts the per-cycle temperature swing and commonly extends die life by 2x to 3x in industry field data.
• EDM leaves a recast white layer 0.005 mm to 0.05 mm thick under tensile stress. Remove it by grinding or polishing before the die ever sees molten metal.
• Nitriding adds a 0.05 mm to 0.20 mm hard compound layer that can delay crack initiation by 2x to 3x, but it does not stop cracks once they start.

What Heat Checking Actually Is

Heat checking is a network of shallow, interconnected cracks on the working face of hot-work tooling. The cracks typically start 0.1 mm to 1 mm deep and grow with every thermal cycle. In die casting the pattern appears after 20,000 to 60,000 shots on untreated H13 surfaces, and much earlier if the steel is hard, dirty or EDM-finished.

The mechanism is low-cycle thermal fatigue. Molten aluminum at 660°C to 720°C hits the cavity face and raises the surface layer to 500°C to 600°C within 2 to 5 seconds. The hot surface wants to expand. The cool core beneath, sitting near 150°C to 250°C, holds it back. That constraint puts the surface into compression, often beyond its yield strength at temperature.

Then the cycle reverses. Cooling water and the ejection pause pull the surface back down. The yielded surface now contracts against the same stiff core, and the stress flips to tension. Steel tolerates compression well but cracks under repeated tension. After 10,000 to 50,000 of these reversals, micro cracks nucleate at the weakest surface points and link into the classic craze pattern.

Three conditions must all exist for heat checking to develop: a surface temperature swing above roughly 100°C per cycle, a hardened steel face under constraint, and enough cycles to accumulate plastic strain. Remove any one leg and the failure slows dramatically. Every prevention strategy in this guide attacks one of those 3 legs.

This is also why heat checking is a hot-work problem. Cold-work stamping dies see near-room-temperature cycles, so they fail by galling or chipping instead. Injection molds for commodity resins run 20°C to 80°C cavity faces with small swings. Heat checking becomes the dominant surface failure once mold faces see PPS or PEEK at 320°C to 400°C, aluminum at 660°C, or hot forging stock at 900°C to 1,200°C.

Heat Checking vs Other Mold Cracks

Misdiagnosis wastes repair budget. A quench crack ground out with polishing will reopen in a few thousand shots, while heat checking mistaken for a structural crack can trigger an unnecessary full insert replacement. The 5 crack types below look different, sit at different depths and demand different fixes.

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Table note: Depths and causes are typical industry figures for H13-class hot-work tooling.

The fastest field check is depth and shape. Heat checking is always a network of many shallow cracks, never one deep crack. If a crack runs deeper than about 1 mm or follows a straight path from a sharp corner, you are looking at thermal shock or mechanical fatigue, and the mold-side countermeasures in this guide will not fix it alone.

Where Heat Checking Starts First

Cracks do not appear randomly. They nucleate where the local temperature swing is widest, where the surface is roughest or where stress concentrates. On a die casting die, 80% of first heat checks show up in 4 zones.

Gate and runner faces. Molten metal enters at 2 m/s to 5 m/s and dumps its heat into a small area. Gate regions run 50°C to 100°C hotter than the general cavity face, so they accumulate fatigue cycles roughly 2x faster. Expect the first craze lines within 5 mm to 10 mm of the gate land.

Thin cores and pins under 6 mm diameter. A thin core heats almost to melt temperature in seconds because it has little mass. Its swing can exceed 250°C per cycle. Cores under 4 mm in zinc and aluminum dies are the classic first casualties, and designers buy life by specifying H13 or SKD61 cores with generous fillets instead of sharp steps.

Inside corner radii below 1 mm. A radius under 1 mm creates a local stress multiplier of 2x to 3x on top of the thermal strain. Cracks start at the radius and then link into the network. Increasing a 0.5 mm radius to 2 mm can double the local fatigue life at essentially zero part cost.

Weld repair zones and EDM surfaces. Deposited weld metal has a different expansion response than the parent steel, and its heat-affected zone is often harder than HRC 52. EDM skins carry the tensile white layer described below. Any die face with these features should be assumed to crack 30% to 50% earlier than polished, as-hardened steel.

Knowing these zones changes how you inspect. A 10x loupe pass over gates, thin cores and radii every 5,000 shots catches craze lines while they are still under 0.1 mm deep and polishable. Waiting for visible flash means the cracks have passed 0.5 mm and the repair bill grows accordingly.

The Thermal Cycle Math Behind Cracking

Thermal fatigue life follows the Coffin-Manson relationship: life drops steeply as strain range rises. Strain range scales with the surface temperature swing, roughly 1.3 x 10⁻⁵ per °C of swing for steel. A swing of 150°C produces about 0.2% plastic strain per cycle at the surface, and H13 at HRC 46 tolerates on the order of 10,000 to 30,000 cycles at that strain level.

That math explains why cycle time matters more than most molders realize. A die casting cell running a 35-second cycle performs about 100 shots per hour. Over 2 shifts and 300 working days that is 480,000 cycles per year. A die that survives 30,000 swings before heavy heat checking lasts barely 2 months of that schedule. Cutting the swing from 150°C to 100°C can push the same die past 100,000 cycles, which is the difference between quarterly and annual insert maintenance.

Two numbers you can actually control. The swing is set by peak surface temperature minus minimum surface temperature. Peak temperature follows melt temperature and gate design. Minimum temperature follows die preheat and coolant set point. Aluminum die casting best practice holds the cavity face between 180°C and 280°C during production, which caps the swing near 250°C instead of 400°C on a cold-started die.

The second lever is cycle time floor. Every second the melt sits against the steel adds heat input. A molder chasing a 3-second cycle saving on a 40-second process raises peak surface temperature by an estimated 20°C to 40°C because the die has less time to dump heat between shots. Tool life math says that trade costs more in insert maintenance than the 7% output gain returns.

High-temperature injection molding follows the same rules with milder numbers. A PPS job at 320°C melt with a 90°C die face swings about 230°C at the gate region, versus under 60°C for an ABS cavity at 50°C. That is why PPS and PEEK tooling heat checks in the hundreds of thousands of shots while commodity molds run 1 million+ shots without a craze line.

White Layer and Temper Softening

Two surface conditions quietly pre-load a die for heat checking before it ever runs: EDM white layer and temper softening from grinding or welding heat. Both create a thin skin with the wrong stress state, and both are invisible to the naked eye.

White layer. Electrical discharge machining melts and re-quenches the surface, leaving an untempered recast layer 0.005 mm to 0.05 mm thick. It is hard (HRC 65+ on H13), brittle, and locked in tensile residual stress with micro cracks already nucleated inside it. Industry practice for dies that will see thermal cycling is to remove the entire recast layer plus 0.01 mm to 0.02 mm of the heat-affected zone beneath it, by grinding, stone polishing or abrasive blasting. Leaving white layer on a gate face is equivalent to starting production with a pre-cracked surface.

Temper softening. H13 hardened to HRC 48 was tempered near 590°C to 620°C. Any later operation that reheats the surface above roughly 550°C locally over-tempers it. Aggressive grinding with a dull wheel, or welding without preheat, can push a surface patch to HRC 38 to 42. Soft spots yield plastically at lower thermal stress, become crack nucleation sites, and show up later as craze lines that cluster in exactly the zones someone ground or welded.

Verification is cheap. A portable hardness check across weld repairs and ground faces, plus a hardness survey every 25 mm on the cavity face, catches soft spots in minutes. Many tool shops also run a light nital etch on suspect areas: over-tempered zones darken visibly. Budget this check into every repair, and specify maximum stock removal per pass (under 0.01 mm final passes with a fresh wheel) in your grinding instructions.

For a deeper look at how the base hardness itself is produced, see our guide on how to specify heat treatment for mold steel. Temper softening and quench cracks are both downstream of that specification.

Steel Cleanliness and Cr/Mo Balance

Two identical H13 bars from different mills can differ in heat check life by 2x or more. The difference is cleanliness and how the alloy is balanced for hot strength versus toughness.

Cleanliness first. Thermal fatigue cracks nucleate at non-metallic inclusions: sulfides, oxides and alumina stringers left from melting. Conventional air-melted H13 carries inclusion ratings that permit crack initiation at moderate strain. Electroslag remelted (ESR) H13 refines the structure and cuts inclusion content dramatically. For aluminum die casting dies targeting 80,000+ shots, ESR or premium-grade H13 is standard practice in Europe and Japan, and it typically buys 30% to 100% more life before heavy crazing. Expect a material premium of roughly 20% to 40% over air-melted bar.

The Cr/Mo/V balance. Chromium (4.75% to 5.25% in H13) provides temper resistance so the steel holds hardness at 500°C to 600°C. Molybdenum (1.1% to 1.75%) adds hot strength and reduces temper embrittlement. Vanadium (0.8% to 1.2%) forms hard carbides for wear, but more vanadium means larger primary carbides and slightly lower toughness. Heat checking resistance favors the tough side of this balance: a die that sees 250°C swings per cycle benefits more from higher toughness than from 1 extra HRC point.

That logic drives grade selection. H11 (lower vanadium, higher toughness) is chosen over H13 for severe thermal shock service like hot forging. 1.2343 ESR sits between the two. For the hottest faces, copper-bearing grades like H19 or high-tungsten grades trade away some thermal conductivity for hot strength. The comparison table in our H13 vs H11 vs 1.2367 guide walks through the trade-offs with numbers.

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Table note: Hardness ranges and life multiples are typical industry figures, not guarantees; actual life depends on process temperature swing.

Hardness Targets That Resist Heat Checking

Harder is not better in thermal fatigue. It is one of the most common specification errors in hot-work tooling, and it is easy to fix.

Above about HRC 52, H13's toughness falls off and its yield strain shrinks. The surface layer reaches plastic strain at a smaller temperature swing, so cracks initiate earlier. Below HRC 40, the face deforms, washes out at gates and loses dimensional control. The industry sweet spot for aluminum die casting is HRC 44 to 48, with most European die shops specifying 46 to 48 HRC for cavity inserts and 42 to 46 HRC for shock-loaded cores.

Uniformity matters as much as the number. A die tempered unevenly can sit at HRC 47 on one face and HRC 51 near a thin section. The harder zone cracks first and looks like random heat checking to an untrained eye. Specify a hardness survey across all working faces at tool buy-off, with a tolerance band of ±2 HRC, and reject or re-temper any block outside it. This check takes under an hour on a $30,000 insert and is standard practice at IATF 16949 shops like DieStrike, where cavity faces are qualified to HRC 62 for cold-work tooling and to the hot-work band your die design needs.

For hot forging dies the target drops further, to HRC 38 to 45, because the billet contact reaches 900°C to 1,200°C and toughness rules. For high-temperature injection molds running PPS or PEEK, keep the same HRC 48 to 52 band used for hardened stainless cavities but watch gate areas, where local swings can still exceed 200°C. The right number depends on the service, and the wrong number by 4 HRC points can halve your tool life.

Preheat and Cooling Rhythm Control

Process-side control of the temperature swing is the single biggest life lever in aluminum die casting, and die design enables or blocks it. This section is the boundary where mold engineering hands the problem to the molder, and where you write the handoff requirements into the tool.

Preheat to 150°C to 200°C minimum. A cold die hit by 700°C aluminum sees a surface swing above 500°C per cycle and can crack in the first 100 shots. Die preheaters, torch preheat on small dies or built-in cartridge heaters bring the mass up before first shot. Industry field data consistently shows 2x to 3x die life extension from disciplined preheat. Specify heater pockets and thermocouple locations in the die design so preheat is measured, not guessed.

Hold production temperature with balanced cooling. Once running, the die should stay in a 180°C to 280°C window on cavity faces. Cooling channels sized for steady-state heat removal, not for fastest possible quench, keep the swing small. Channels placed 15 mm to 25 mm from the face with turbulent flow (Reynolds number above 10,000) remove heat evenly. Channels that are too close create cold spots that raise local stress and start cracks of their own.

Respect the interruption rules. Most thermal shock cracks happen at process upsets, not in steady running. Three rules belong in every die's operating sheet: never spray coolant on a hot die face during a stoppage, never restart after a long pause without re-verifying die temperature above 150°C, and ramp production speed up over the first 20 to 30 shots after any pause longer than 15 minutes. A 30-second mistake at 2 a.m. can crack an insert that had 50,000 shots of healthy life left.

Build the swing budget into the die. On high-volume dies, thermal expansion of 0.10 mm to 0.15 mm on hot cores is normal, so shut-off dimensions must be drawn at operating temperature, not cold. A die that only holds its dimensions cold invites the molder to run it colder, which raises the swing and accelerates heat checking. Design for the hot state and both problems shrink together.

Can Nitriding Delay Heat Checking

Yes, but only the first half of the failure. Nitriding delays crack initiation. It does nothing for crack growth, and a nitrided die with a deep crack network is just as dead as an untreated one.

Gas or plasma nitriding at 480°C to 530°C diffuses nitrogen into the H13 surface, building a compound layer 0.05 mm to 0.20 mm thick at 1,000 to 1,200 HV with compressive residual stress. That compression has to be overcome by thermal tension before a crack can open, which typically multiplies initiation life by 2x to 3x in die casting service. Field data from European aluminum die casters routinely reports nitrided inserts reaching 80,000 to 120,000 shots before the first craze line, versus 30,000 to 50,000 on polished untreated steel.

The caveats matter as much as the benefit. Nitriding temperature sits near the tempering temperature of H13, so a poorly controlled cycle can soften the core below HRC 44. The compound layer is brittle: on gate areas with metal velocities above 3 m/s, erosion can spall it off in patches, and spalled edges become stress risers. And once you polish a nitrided die to remove heat checks, you grind away the very layer that was protecting it. Plan for one polishing life of 0.02 mm to 0.03 mm maximum, then re-nitride or replace.

PVD coatings like TiAlN behave differently. At 0.002 mm to 0.005 mm thick they sit above the cracks rather than resisting them, so on thermal-fatigue-dominated faces they add little. Their strength is erosion and soldering resistance. For the full selection logic, our nitriding vs PVD coatings guide compares both processes zone by zone.

Polish Out or Replace the Insert

Once heat checking shows, every repair decision trades stock removal against dimensional budget. Use these field-tested thresholds instead of judgment calls.

Polish it out. If cracks are under 0.1 mm deep and the zone has dimensional allowance, polishing removes them cleanly. Expect to remove 1.5x to 2x the crack depth: 0.15 mm to 0.20 mm of stock for a 0.1 mm network. After polishing, re-check hardness of the exposed layer. If the die was nitrided, budget a re-nitride cycle of 20 to 40 hours, or accept that the polished zone will re-crack 2x to 3x faster than its neighbors.

Re-nitride after moderate polishing. Networks between 0.1 mm and 0.3 mm deep on non-critical dimensions usually justify polish plus re-nitride. Total stock loss of 0.3 mm to 0.5 mm starts to open shut-offs and clearances, so map every polished zone on the die drawing and track cumulative removal. Most dies tolerate 2 to 3 polish cycles before dimensions run out.

Replace the insert. Swap the insert when any of these appear: cracks deeper than 0.5 mm, crack networks that cross a parting line or shut-off, dimensional stock below 0.1 mm remaining on a critical feature, or a polished face that re-cracks within 5,000 shots (a sign the swing problem was never fixed). On a $8,000 to $20,000 insert in a 60,000-shot-per-year program, replacing at 0.5 mm crack depth is almost always cheaper than repeated welding, which risks new distortion and hardness mismatch.

Welding is the last resort. Laser or TIG welding of heat-checked zones requires preheat to 250°C to 350°C, matching filler (typically H13-class wire at HRC 45 to 50 as-deposited) and stress relief after. Welded zones rarely match parent-steel fatigue life, so weld only where geometry forbids replacement.

Prevention Checklist by Mold Type

The same physics, different numbers. Here is how to set priorities for the three services that see heat checking most.

Aluminum die casting dies. Specify ESR H13 at HRC 46 to 48 for cavities and cores. Remove all EDM white layer. Nitride gates and cores to 0.08 mm to 0.15 mm. Write preheat to 150°C to 200°C and a 180°C to 280°C production window into the die operating sheet. Inspect gate zones with a 10x loupe every 5,000 shots. Target life: 80,000 to 120,000 shots before the first major polish.

High-temperature injection molds (PPS, PPA, PEEK, PEI). Resin at 320°C to 400°C against a 80°C to 150°C die gives swings of 170°C to 320°C at gates. Use hardened stainless (HRC 48 to 52) or hot-work steel inserts at the gate zone only, keep cooling channels 15 mm+ from the face, and avoid aggressive cycle-time compression that starves cooling time. Expect gate-zone inspection every 100,000 shots.

Hot forging dies. Billet contact at 900°C to 1,200°C demands the toughness side: H11 or H13 at HRC 38 to 45, generous 3 mm to 6 mm fillets, and mandatory preheat to 250°C to 350°C between hits on press-side die heating. Forging dies heat check by design; the goal is a predictable, polishable network, not zero cracks.

Across all three, the tooling-side fundamentals are identical: clean steel, correct hardness with a ±2 HRC survey, no white layer on working faces, radii above 1 mm on hot zones, and cooling designed for a steady temperature window. DieStrike builds these checks into every die casting mold project from DFM review (feedback within 24 hours) through final hardness certification, and holds cavity geometry to ±0.002 mm on component work so repair stock budgets stay predictable.

FAQ: Heat Checking in Mold Steel

Q1. How many shots before heat checking appears on an H13 die?

On untreated air-melted H13 at HRC 48 in aluminum die casting, expect the first craze lines between 20,000 and 60,000 shots. ESR steel plus nitriding plus disciplined preheat pushes that to 80,000 to 120,000 shots in typical field data.

Q2. Does higher hardness always resist heat checking better?

No. Above HRC 52, toughness drops and cracks initiate sooner. The proven window for aluminum die casting is HRC 44 to 48, and for hot forging HRC 38 to 45. Specify a ±2 HRC uniformity survey, because a locally hard zone cracks first.

Q3. Can I just polish out heat checks and keep running?

Yes, if cracks are under 0.1 mm deep and you remove 1.5x to 2x the crack depth (0.15 mm to 0.20 mm of stock). Track cumulative removal on the die drawing; most inserts tolerate 2 to 3 polish cycles before dimensions run out. Replace the insert once cracks pass 0.5 mm.

Q4. Does nitriding stop heat checking permanently?

No. Nitriding delays crack initiation by 2x to 3x through a compressive 0.05 mm to 0.20 mm compound layer, but it does not slow crack growth once cracks start. Polishing a nitrided face removes the protective layer, so plan re-nitriding after each major polish.

Q5. Why do cracks always start near the gate?

Gate faces run 50°C to 100°C hotter than the general cavity because melt enters at 2 m/s to 5 m/s and dumps heat locally. The larger temperature swing produces larger plastic strain, so gates accumulate fatigue damage about 2x faster than the rest of the die.

The Bottom Line

Heat checking is thermal fatigue, and its 3 drivers are surface temperature swing, surface condition and steel quality. Control all three: preheat to 150°C to 200°C, specify ESR H13 at HRC 44 to 48, strip EDM white layer, nitride hot zones and inspect gates every 5,000 shots. DieStrike, an IATF 16949 mold maker with 120+ machines, builds these requirements into die design from day one. Bring us your thermal fatigue problem through the contact page and get DFM feedback within 24 hours.