Casting shrinkage occurs during the cooling process after solidification, and some alloys also undergo phase changes that can result in shrinkage or expansion in the solid state. If a part of the casting experiences dimensional changes and is not free to deform, stress, deformation, or cracking may occur. The volume change of molten metal during solidification and cooling is constrained by external factors or the casting itself. This stress caused by deformation is known as casting stress, which is a primary cause of defects such as deformation and cracking in castings.
There are three main types of casting stress based on their formation causes: thermal stress, solid-state phase transformation stress, and mechanical constraint stress. Thermal stress arises from differences in cooling rates between various sections of the casting, causing uneven shrinkage and internal restraint. Solid-state phase transformation stress occurs when different parts of the alloy reach their phase transition temperatures at different times, leading to uneven phase changes and resulting stress. Mechanical constraint stress is caused by the resistance from molds, cores, or other components during the casting's shrinkage.
Casting stress can also be classified based on its duration: temporary stress and residual stress. Temporary stress disappears when the cause is removed, while residual stress remains even after the cause has been eliminated.
The presence of casting stress poses significant risks to the quality of castings. It is a major cause of deformation and cracking during production, storage, machining, and use. Residual stress can lead to long-term deformation or loss of precision after machining. If internal stress exceeds the yield strength of the alloy, the casting may deform, potentially leading to scrapping due to insufficient machining allowance. In mass production, deformed castings are often discarded during machining, making it crucial to prevent deformation, especially for high-precision castings.
Several factors influence casting stress, including the properties of the metal, casting conditions, and the structure of the casting. Metals with higher elastic modulus tend to have greater residual stress. For example, cast steel, white iron, and ductile iron typically exhibit higher residual stress than gray cast iron. The free linear shrinkage factor of the alloy also affects residual stress. Alloys with higher shrinkage factors tend to have greater residual stress. Additionally, thermal conductivity plays a role in temperature distribution within the casting, affecting the magnitude of residual stress.
Phase transformations also contribute to residual stress through changes in specific volume and temperature distribution. Casting properties, such as heat storage coefficient, influence the cooling rate and thus the stress generated. Metal molds generally result in higher residual stresses compared to sand molds. Pouring temperature, casting structure, and design all play a role in managing casting stress.
To reduce casting stress, it is essential to minimize temperature differences during cooling, improve mold and core concession, and reduce mechanical constraints. Specific measures include selecting alloys with lower elastic modulus and shrinkage, using cold iron to control temperature distribution, optimizing mold design, and adjusting pouring conditions. Improving casting structure by avoiding sharp corners and ensuring uniform wall thickness helps reduce stress concentration and deformation.
Residual stress can be eliminated through methods such as artificial aging, natural aging, and resonance aging. Artificial aging involves heating the casting to an elasto-plastic state and allowing it to cool slowly. Natural aging relies on long-term exposure to environmental conditions to relieve stress, though it is time-consuming. Resonance aging uses vibration to induce plastic deformation and reduce residual stress efficiently.
Hot cracking and cold cracking are two common types of casting defects. Hot cracks occur during the final stages of solidification due to stress exceeding the alloy’s strength at high temperatures. Cold cracks form when residual stress exceeds the alloy’s strength at lower temperatures. Both types of cracks can severely affect the mechanical properties of the casting and must be prevented through proper alloy selection, mold design, and casting process optimization.
Preventing hot and cold cracking requires careful consideration of alloy composition, casting design, and process parameters. Reducing stress concentrations, controlling cooling rates, and improving mold collapsibility are effective strategies. By addressing these factors, the likelihood of cracking can be significantly reduced, ensuring the quality and reliability of castings.
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