Casting shrinkage occurs during cooling after solidification, and some alloys also experience phase changes that may cause expansion or contraction in the solid state. If a part of the casting undergoes dimensional change and is not free to deform, stress, deformation, or cracking can occur. The volume change of molten metal during solidification and cooling is constrained by external factors or internal structure, leading to casting stress. This stress is a primary cause of defects like deformation and cracks in castings.
Casting stress can be classified into three types based on its formation: thermal stress, solid-state phase change stress, and mechanical constraint stress. Thermal stress arises due to uneven cooling rates in different sections of the casting, causing differential shrinkage and mutual restriction. Solid-state phase change stress occurs when different parts of the alloy reach their phase transition temperatures at different times, leading to stress. Mechanical constraint stress results from the resistance of molds, cores, or other components to the casting’s shrinkage.
Casting stress can also be categorized by its duration: temporary stress, which disappears when the cause is removed, and residual stress, which remains after the cause is gone. Residual stress is particularly harmful as it can lead to long-term deformation or failure of the casting. For example, if residual stress combines with working stress, it may exceed the alloy’s strength limit and cause fracture. Castings with residual stress may deform over time or lose precision after machining, making them unsuitable for use.
Several factors influence casting stress, including the elastic modulus of the metal, the alloy's linear shrinkage coefficient, thermal conductivity, and phase transformation effects. Metals with higher elasticity tend to have greater residual stress. For instance, cast steel and ductile iron exhibit higher residual stress than gray cast iron. The thermal conductivity of the alloy affects temperature differences within the casting, with lower conductivity leading to higher stress.
The casting process, mold properties, pouring conditions, and casting design all play critical roles in managing stress. To reduce stress, it’s essential to minimize temperature differences during cooling, improve mold concession, and reduce mechanical constraints. Specific measures include selecting alloys with lower elastic modulus and shrinkage, using cold iron in thick sections, optimizing mold design, and ensuring uniform temperature distribution.
Residual stress can be eliminated through artificial aging, natural aging, or resonance aging. Artificial aging involves heating the casting to an elasto-plastic state to relieve stress. Natural aging relies on long-term exposure to ambient conditions, though it is time-consuming. Resonance aging uses vibration to induce plastic deformation and reduce residual stress efficiently.
Hot cracking occurs when stress exceeds the alloy’s strength during solidification, often at the final stages. It can manifest as surface or internal cracks and severely affects mechanical properties. Factors influencing hot cracking include alloy composition, mold concession, and casting design. Preventive measures involve adjusting alloy composition, improving mold properties, optimizing pouring conditions, and designing castings to avoid stress concentrations.
Cold cracking happens when stress exceeds the alloy’s strength at lower temperatures, typically after solidification. It often occurs in areas of stress concentration or near defects. Prevention involves reducing stress through proper cooling, increasing mold stiffness, controlling boxing time, and implementing anti-deformation techniques.
In summary, understanding and managing casting stress is crucial for producing high-quality castings. By addressing the root causes and applying appropriate preventive measures, defects such as deformation, cracking, and residual stress can be minimized, ensuring reliable and durable cast products.
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