Why are sand casting molds prone to thermal fatigue cracking when repeatedly exposed to high-temperature molten metal?
Publish Time: 2025-10-27
In foundry production, sand casting is widely used in the machinery, automotive, and energy sectors due to its advantages, including low cost, strong adaptability, and ability to produce complex castings. As the key container for holding and shaping the molten metal, sand casting molds are typically made from a mixture of quartz sand, binders, and other additives, offering a certain level of high-temperature resistance and structural strength. However, in actual production, especially during repeated batch casting, sand molds often crack, peel, or even collapse after repeated use. Thermal fatigue cracking is one of the main causes of mold failure.1. Severe thermal stress is the root cause of crackingWhen high-temperature molten metal is rapidly injected into the sand mold cavity, the mold's inner wall heats from room temperature to hundreds or even thousands of degrees Celsius in a very short period of time, generating a severe temperature gradient. Due to the poor thermal conductivity of the sand mold material, heat cannot be quickly transferred to the interior, causing rapid expansion of the surface while the interior remains relatively cool, limiting expansion. This uneven thermal expansion generates significant thermal stress within the material. When thermal stress exceeds the tensile strength of the sand mold material, microcracks form on the surface. As the number of pours increases, this cycle of "thermal expansion-cooling-reheating" repeats, causing the microcracks to gradually expand and connect, ultimately forming distinct thermal fatigue cracks.2. The inherent brittleness and low toughness of the material exacerbate the risk of cracking.Sand molds are primarily composed of quartz sand particles and a binder, resulting in a porous and brittle structure. While the binder provides sufficient strength at room temperature, at high temperatures, especially near or above its softening point, the binder decomposes, carbonizes, or loses its adhesive properties, significantly reducing the bond between the sand particles. Furthermore, quartz sand undergoes a crystal phase transition at 573°C, accompanied by a sudden change in volume of approximately 0.8%. This process, during repeated heating and cooling, induces "phase change stress," further exacerbating the formation and propagation of microcracks within the sand mold. Because sand molds lack the plastic deformation capacity of metal materials and cannot relieve internal stress through "stress relaxation," cracks can easily develop into macroscopic cracks.3. Mechanical erosion and chemical attack by molten metal accelerate fatigue.High-temperature molten metal not only causes thermal shock but also intense mechanical erosion. Especially around the gate, ingates, and thick areas of the casting, the rapid molten metal flow and high impact force continuously erodes the sand mold surface, rapidly propagating existing microcracks and causing sand grains to flake off, resulting in "sand washout" defects.4. Improper process design amplifies thermal fatigue effects.The structural design of the sand mold and casting process parameters also directly affect its thermal fatigue performance. For example, sudden changes in mold wall thickness, sharp corner transitions, and improper cooling channel design can cause localized stress concentrations, which can become the starting point for crack initiation. Furthermore, process factors such as excessively high pouring temperatures, excessively fast pouring speeds, and insufficient cooling cycles can exacerbate the intensity of thermal shock and shorten the life of the sand mold. Failure to maintain stable process control in mass production further increases the risk of thermal fatigue cracking.In summary, thermal fatigue cracking of sand casting molds caused by repeated exposure to high-temperature molten metal is the result of multiple factors, including thermal stress accumulation, material brittleness, mechanical erosion, chemical attack, and process design. Although sand molds are designed for single or limited use, improving their thermal fatigue resistance is crucial in modern casting production, which strives for high quality, high efficiency, and low cost.
