DH31 Mold Steel: Hardness, Polishing and Tool Life Guide

What DH31 actually is

DH31 is a Japanese Daido hot-work steel (JIS SKD62 equivalent) chosen for high-temperature die-casting, extrusion and injection moulding tools where thermal fatigue resistance is more important than abrasive wear.

Metallurgically, DH31 is a through-hardened hot-work tool steel — supplied annealed, machined soft, then hardened and tempered to the target hardness. The Through-hardened hot-work steel. It is normally supplied annealed, machined in the soft state, then hardened and tempered to deliver its target hardness. That heat-treatment step adds both schedule and distortion, but it is the reason the grade can hold its hardness at elevated cavity temperatures where a pre-hardened block would soften..

Every mould steel specification sits on three columns: hardness, corrosion resistance and polishability. No single grade leads on all three, so the selection is always a trade. DH31 trades machinability for hardness and hot strength. Reading that trade before the tool is cut is the whole decision.

Hardness, and what it does not buy you

The nominal hardness is 42-52 HRC HRC. The through-hardened hardness is — HRC after heat treatment. The block is supplied annealed, machined soft, then hardened and tempered to that target.

Hardness buys resistance to abrasive wear and to indentation at the parting line. It does not buy toughness. A harder block is more sensitive to a sharp internal corner than a softer one, and an unrepaired EDM recast layer is a crack initiation site that fails after the tool is put into production. Hardness also does not buy corrosion resistance — those are separate properties, and neither substitutes for the other.

The practical failure modes to watch are local, not bulk. The gate, the vent land and any narrow restriction see the highest melt velocity, and that is where a DH31 tool shows its age first. A replaceable insert at those locations is cheaper than re-cutting a cavity, and it is the standard mitigation when the grade may see abrasive resin later.

Corrosion and polishability

Corrosion resistance is Low. The grade has low corrosion resistance — it will rust in a humid tool room, and it is not suitable for PVC or aggressive flame-retardant resins. Discipline rather than a different steel is the mitigation: dry the tool after every run, apply a protective film to the cavity, keep the cooling circuit treated.

Polishability is Very good. Polishability is good but slower than on a pre-hardened block because of the hardness. The work has to be done after heat treatment if the grade is through-hardened.

Surface treatments extend the life of a DH31 cavity without changing the base grade. Nitriding hardens the surface further and improves wear resistance at the gate and vent land; PVD coatings add friction reduction and release. Both are decisions to make before the first trial, because they are harder to correct afterwards.

By production volume

The planning shot-life range is 300 K - 1 M+. Below roughly 100,000 shots, DH31 is usually more steel than the programme needs, and a pre-hardened grade is the lower total cost. Between 100,000 and 500,000 shots it becomes competitive where the resin is abrasive or the part tolerance is tight..

The resin can move the threshold in either direction. A glass-filled tool may justify a premium wear-resistant grade at a modest shot count because the gate would otherwise wash out. An unfilled commodity part may stay in a pre-hardened grade well beyond its nominal planning range if the tool is maintained.

By resin

DH31 is the correct match when one of the resin's properties asks for it. Abrasively filled resins — glass-filled PA, PBT, PPS — drive a wear-resistant grade before volume matters. Unfilled commodity resins such as ABS, PS and PP do not justify the premium, and a pre-hardened grade is the lower cost.. Where the resin is also corrosive, that failure mode trumps volume and changes the steel family before the block grade is settled.

Heat treatment, welding and repair

Supplied annealed, DH31 is hardened and tempered after machining. The cycle has to produce — HRC with the toughness the application needs, and the tool has to allow for the distortion that comes with it. Finish machining and polishing happen after hardening.

Weldability is —. Welding is straightforward with a procedure: preheat the area, match the filler to the base chemistry, control the heat input, and let it cool slowly. The heat-affected zone is softer than the surrounding steel, so a weld on a polished face leaves a faint halo after polishing — plan the repair location before the weld, not after..

Repair by welding is common at the gate and at a damaged parting line. A replaceable gate insert is the better long-term answer when the damage is repeated, because the wear item is then a small block that can be replaced without touching the cavity.

Surface treatment and coatings

Nitriding hardens the surface further and improves wear resistance at the gate and the vent land. PVD coatings reduce friction and release, and they work particularly well on a hardened substrate because the coating has a hard base to support it. Chrome plating improves corrosion resistance and release on cavities that need it, but it is a wear treatment only when the substrate is hard enough.

The trade with every coating is that it adds a process step and makes future correction harder. A nitrided or PVD-coated cavity is more expensive to repair, and any welding or re-polishing after coating means the coating has to be stripped and reapplied. Plan the coating decisions around the final cavity geometry, not around fixing a problem you anticipate.

None of these treatments replace the basic decision. A coating on the wrong steel is a delay, not a fix.

What DH31 costs, and when a cheaper grade is actually cheaper

DH31 costs more than a pre-hardened grade, and the heat-treatment step adds both cost and schedule. On a short campaign that premium is hard to recover. On a long campaign it is recovered through a longer maintenance interval and fewer unplanned stops.. The honest comparison is cost per accepted part over the campaign, including maintenance and downtime.

Where the resin is also abrasive, DH31 may not be the final answer: a wear-resistant coating or a different grade may be cheaper than upgrading the base steel. Where corrosion is also a risk, a plain hardened grade — even harder — is the wrong answer, because corrosion arrives before wear.

Common selection mistakes

• Using DH31 for a failure mode it was not designed for. A hardened grade in a corrosive job, a pre-hardened grade in a glass-filled production tool, or a stainless grade when the only problem is wear — each wastes money and sets the tool up to fail.
• Welding a visible face without planning the repair location. The heat-affected zone polishes differently, and the mark shows under grazing light.
• Expecting every failure mode to be solved by a higher hardness number. Hardness resists wear and indentation, not corrosion, not toughness, not every polish requirement.
• Specifying DH31 without sending the resin and the failure mode. The grade answer changes when the resin is also corrosive, the cavity also needs an optical polish, or the tool also has to survive a million shots.