The Cause of Heat Checking in Die Steel

The greatest single cause of failure in aluminum die casting dies is heat checking, which is the formation of a network of small cracks in the die surface.
The cause of heat checking in aluminum die casting and die cast dies is the forcing of molten metal under pressure into intimate contact with the die, thus causing the die surface to become very hot in relation to the adjacent metal. The temperature of the steel die surface closely approaches the temperature of the cast metal. The time required to heat the surface of the cavity steel from normal temperature to cast-metal temperature is less than 1/1000 of a second.The actual increase in temperature of the die surface for aluminum die cast metals is 600 to 1000 degrees F.

This increase in temperature is accompanied by a corresponding thermal expansion. When the coefficient of expansion of the die steel is multiplied by the several hundred degrees of temperature differential between the surface of the die cavity and the inner metal adjacent to it, the resultant expansion is greater than the elastic deformation the steel can withstand. Therefore, there is some plastic deformation on the surface of the die. When the heat of the surface is removed by conduction through the die and the surface cools to normal die temperature, it contracts; and because of this plastic flow that occurred on heating, the surface is in tension on cooling. After a sufficient number of cycles in the die casting process, cracking results.

Doehler, H.H. (1951) Die Casting. New York, New York. McGraw-Hill Book Company Inc.