Heat Checking In H13 Die Casting Dies - China

Every aluminium die casting die eventually heat checks.

The useful questions are not whether the fine crack network will appear, but where it will start, how quickly it will grow, and whether it will stay a cosmetic surface condition or become a leak, a casting defect and an unplanned die change.

All three are answerable, because heat checking is a strain-controlled fatigue process with identifiable initiation sites and identifiable material and heat-treatment variables behind them.

This article works from the mechanism outward: the stress cycle the cavity surface sees, the features that concentrate damage, and the steel and heat-treatment decisions that determine how much strain the surface absorbs before a crack opens.

Heat checking is thermal fatigue, not a generic crack Heat checking appears as a network of fine cracks, typically 0.1–1 mm deep in its early stage, running roughly perpendicular to the cavity surface in a mud-crack or "elephant skin" pattern.

It is a low-cycle, strain-controlled fatigue phenomenon driven by the temperature cycle, not by the pressure of the melt.

That definition matters because several other die failure modes look similar in a photograph and have completely different remedies.

A gross crack — one or two long cracks that run through a wall or open a leak — is usually a toughness or design problem: an over-hard die, a sharp internal corner, a cold die hit by the first shot, or a thermal shock after a stoppage.

Washout at the gate is mechanical erosion by melt moving at 30–60 m/s.

Soldering is a chemical and metallurgical adhesion problem between the melt and the die surface.

Pitting is corrosion, often from release agent or cooling water.

Heat checking is none of these; it is the accumulated plastic strain of the surface layer, and the only durable remedies are the ones that reduce that strain or delay its consequences.

Heat checking is also progressive.

A check that is 0.2 mm deep after 20, 000 shots may be 1 mm deep and branching after 100, 000 - Cracks link, small pieces of the surface may spall, and once a crack reaches a cooling bore or a sealing face the die is out of service.

The practical objective is therefore not to prevent the first crack forever, but to keep the crack network shallow and uniform for as many cycles as possible.

The stress cycle, step by step Heating: the surface is pushed into compression When the shot enters, the cavity surface is heated from its bulk temperature of roughly 250–350°C to 400–520°C within milliseconds to a few seconds.

Gate areas and thin sections run hotter, and hot spots can exceed 550°C.

The heated skin wants to expand — the coefficient of thermal expansion of H13 is about 12 × 10⁻⁶ per kelvin — but it is constrained by the colder steel behind it.

For a fully constrained surface the elastic thermal stress is approximately EαΔT/(1−ν); with E of the order of 200 GPa and a surface-to-bulk difference of 300–400 K, that is a stress of the order of 1, 000–1, 400 MPa, and it is compressive.

H13 at 500–600°C has a 0.2% proof strength of roughly 700–1, 100 MPa depending on hardness and temperature.

The thermal stress therefore exceeds the elevated-temperature yield strength, and the outermost few millimetres yield in compression on every shot.

The depth of the yielding layer is set by how far heat penetrates during the contact time, which is why short shots with a high surface temperature and long shots with a deep gradient produce different damage patterns.

Cooling: the same layer is pulled into tension

Company Contact:

• Posted By: Hubei Keyspark Special Steel Co., Ltd.

• Phone: +8615024056480

• Address: 30 Huahu Avenue, Huangshigang District, Huangshi , Hubei , China

• Email:

• Website: https://keysparksteel.com

• Business Description: Hubei Keyspark Special Steel Co., Ltd. is an integrated special steel manufacturer in Huangshi, Hubei, China.
  
  Melting, ladle refining, vacuum degassing, ESR, forging and heat treatment are all in house.
  
  Products: plastic mold steel (1.2311/P20, 1.2312, 1.2316, 1.2738), cold work steel (1.2083, 1.2379/D2, DC53), hot work steel (1.2344/H13, 1.2714), high speed steel, alloy and stainless steel.
  
  Supplied with mill test certificates worldwide.