Heat Checking refers to an intricate network of tiny cracks on die-cast surfaces. It begins as tiny pits to grow large and shape to cause internal casting cracks. These cracks eventually become deep enough to initiate visible surface cracks.
Heat checking is one of the most frequent adversaries to fail die-cast parts, especially steel. However, several aluminum alloys also exert this defect to a considerable extent. Since it more or less affects the entire cross-section, preventive measures are mandatory.
This article briefly focuses on the causes, effects, and countermeasures of die-casting heat checking.
Causes of Heat Checking
The primary reason for heat checking concerns thermal fatigue, resulting from the repeated heating and cooling cycle. Molten metal alloys heat the die, whereas the mold lubricant cools down the casting. This alternate hoop during casting initiates the overheating of the system.
The mechanism is quite complex as it involves compressive stress temper resistance, soaking time, and yield strength. In simple, overheating ultimately forces the thermal stresses to exceed their acceptable limit. This causes the heat-check-triggering fatigue to make the die-cast vulnerable.
Nonetheless, Al alloy properties play a major role in controlling most of the reasons behind overheating. Modern die-casting processes are considerably short-timed, contributing to more thermal stresses. So, improper or unfavorable casting arrangements can further affect the thermal hoop.
Improper or imbalanced carburization can cause premature heat checks in die casts. It’s mandatory to avoid heavy/excessive carburization as well as decarburization at all times. This may worsen the casting vulnerability to thermal stresses by deducting the metal fracture’s toughness.
Effects of Heat Checks in Die Casting
Metal parts with deep defects are incredibly difficult or almost impossible to treat or remove. And the problem keeps worsening with ongoing and pending growth. Its effects can be summarized as follows –
- Decrease integrity, workability, strength, ductility, and toughness with a network of cracks.
- Increase in production costs due to the necessity of more re-sinking or remaking of metal casts.
Preventing Heat Checking
Avoiding this ever-worsening issue requires high processing precision with maximum casting balance. The most convenient ways to confront heat checks include –
- Balanced Quenching Rate –Quenching refers to the rapid cooling process of die casting. Low-speed quenching enables excellent dimensional stability against compromised microstructure, whereas high-speed quenching promotes lifespan. A balanced one is desirable to keep up with the cost.
- Standard Temperatures –High yield strength results in more resistance to softening, reducing heat cracks. But it incorporates grain growth to reduce material ductility and toughness. Different alloys require distinctive casting temps, so proper temperature control plays an important role in the casting process.
- Recommended Hardness –A higher hardness means more resistance to thermal fatigue. However excessive hardness contributes to heavier cracks and total failure. Therefore, 50 HRC (Rockwell Hardness) is the maximum recommended limit for aluminum alloys. And the limit gets reduced to 46 HRC for molten brass.
- Proper Cast Tempering –After cooling down the die, conducting two or three tempers is essential. A satisfactory structure, as well as the desired hardness, results from these operations. A subtle range (50°C – 70°C or 120°F – 160°F) is necessary for the first one. The third one is primarily done on casting dies.
- Advanced Thermal Process –Complex thermomechanical processes like two-step shot-peening can help with the heat-checking tendency. It concerns a specified heat treatment using nitrides and subsequent application of shot-peening. However, this advanced process may provide a good outcome for steel alloys only.
Heat checking is one of the prime defects that require optimum balance and precision. Early prevention is most likely how to deal with heat checks, as it can severely affect the functionality of castings. The exact countermeasure will also vary based on the chosen alloy and the casting process.