Hot Cracks
Hot cracks occur in castings during the solidification process or shortly after solidification when the casting is still in a state of low strength and plasticity, due to the obstruction of solid-state shrinkage. Hot cracks are one of the common casting defects in the production of cast steel, malleable cast iron, and certain light alloy castings. Hot cracks initiate at the grain boundaries and propagate along them, with uneven thickness, and tortuous, and irregular shapes. The surface of the crack appears oxidized, lacking metallic luster. The crack surface of cast steel is approximately black, while aluminum alloys appear dark gray. External cracks are visible to the naked eye and can be distinguished from cold cracks based on their shape and fracture features.
Hot cracks can be divided into external cracks and internal cracks. External cracks, visible on the casting surface, are called external cracks. External cracks often occur at the corners of the casting, where there are abrupt changes in cross-sectional thickness or where local solidification is slow, leading to stress concentration. Their characteristic is a wide surface and narrow interior, with a tearing appearance. Sometimes the fracture penetrates the entire casting cross-section. Another characteristic of hot cracks is that the cracks are distributed along the grain boundaries. Internal cracks generally occur in the last solidified parts of the casting, with very irregular shapes, and the fracture surface often accompanies dendritic crystals. Under normal circumstances, internal cracks do not extend to the casting surface.
Cold Cracks
Cold cracks occur when the casting cools down to an elastic state after solidification, due to local casting stress exceeding the alloy's ultimate strength. Cold cracks always occur in areas that are subjected to tensile stress during the cooling process, especially in areas where tensile stress is concentrated. Cold cracks differ from hot cracks in that they often penetrate through the entire cross-section, with a uniform width and elongated straight or broken line shape. The fracture surface of cold cracks is clean with metallic luster or slightly oxidized, and the crack path is smooth, not occurring along grain boundaries. This is significantly different from hot cracks. Cold cracks can be visually inspected, and they can be distinguished from hot cracks based on their macroscopic appearance and the microscopic characteristics of transgranular propagation.
When the casting stress inside the casting exceeds the strength limit of the metal, the casting will produce cold cracks. Therefore, any factors that increase the casting stress and reduce the metal strength can promote the formation of cold cracks in the casting.
ABOUT KEVA
Keva Casting is a professional casting company that focuses on Construction, Machinery, Automobile, Hardware, Pipeline and many other industries.
We provide top quality products with the best service based on our complete production chain including our own mold design, advanced production techniques, and full management of the supply chain. Keva Casting has two factories located in Zhucheng, China's Shandong province.
We use "shell molding", which is a new technique that has been developed in recent years.This new technique has simplified production procedures and reduced air pollution compared with traditional sand casting and investment casting. Shell making and core making are faster to ensure production in enormous quantities. The dimensions and surface finishing are satisfactory for all kinds of industrial requirements. Besides supplying our own formwork & scaffolding systems, our casting products also supply OEM services in various industries.