1. Cold cracking casting defect
Cold cracking casting defect refers to the crack caused by the casting stress greater than the alloy strength limit when the casting solidifies and cools to the elastic state. Therefore, cold cracking casting defects usually appear in the tensile part of the casting, especially in the area of stress concentration. Cold cracking casting defects usually have the characteristics of transgranular expansion to the whole section, and the shape is continuous straight line or smooth curve. Cold cracking casting defect lines, clean fracture, showing metallic luster or slight oxidation color.
Factors such as large hanging difference of casting wall thickness, excessive harmful impurity elements (such as carbon, chromium and nickel in steel, phosphorus and sulfur in cast iron), early subsequent unpacking and too fast cooling all affect the tendency of cold cracking and casting defects. Therefore, the fundamental way to control cold cracking casting defects is to control the casting stress produced in the process of casting production and minimize it. The specific prevention measures include: improving the structural design of castings and reducing the shrinkage resistance of castings; Improve the quality of alloy smelting and control the content of harmful elements; The cooling time of the casting in the mold shall be prolonged as far as possible to prevent large internal stress from being generated early when unpacking.
2. Hot cracking casting defect
In the late stage of solidification or shortly after solidification, the cracks caused by solid shrinkage due to low strength and plasticity are called hot cracking casting defects. The shape of hot cracking casting defect mouth is tortuous and irregular, and the surface is seriously oxidized. Microscopic observation shows that the crack originates from the grain boundary and passes along the grain boundary. According to the location of cracks on castings, hot cracking casting defects can be divided into internal cracking and external cracking. The outer crack is tear like, with wide surface and narrow inside; The internal cracking section is usually accompanied by dendrite, which usually occurs at the last solidification part of the casting.
In the temperature range where hot cracking casting defects may form, excessive linear shrinkage of alloy, excessive low melting point phase elements in alloy composition (such as phosphorus and sulfur in cast iron) and unreasonable casting structure and casting process may lead to hot cracking casting defects. Due to many factors affecting the tendency of hot cracking casting defects, corresponding preventive measures should be taken in combination with the actual production situation. Firstly, the sudden change of wall thickness should be avoided as much as possible in the structural design of castings, and attention should be paid to reducing the obstruction of shrinkage; In terms of melting process, it should be ensured that the alloy composition is close to the eutectic composition as far as possible without affecting the service performance of castings, and the molten steel should be vacuum treated to reduce the gas content in the steel and improve the crack resistance; When designing the molding process, an appropriate amount of wood chips can be added to the clay sand to improve the collapsibility and concession of molding sand and core sand; In addition, it is also an effective measure to prevent hot cracking casting defects by setting cutting bars in the parts prone to hot cracking casting defects.