Although the casting defects produced in the casting process of steel castings are similar to those produced by ingot casting, they are still casting defects. The common casting defects include pores, inclusions, shrinkage cavities, looseness and cracks.
(1) Pores (bubbles): pores (bubbles) are cavities formed due to excessive gas content in liquid metal, wet model and poor air permeability. The pores in castings can be divided into single dispersed pores and dense pores.
(2) Inclusions: inclusions are divided into non-metallic inclusions and metallic inclusions. Nonmetallic inclusions are the products formed by the chemical reaction between metal and gas during smelting or the inclusions formed by the mixing of refractory and molding sand into liquid steel during pouring. Metal inclusions are inclusions formed when dissimilar metals occasionally fall into liquid steel and fail to dissolve.
(3) Shrinkage cavity: shrinkage cavity is a casting defect formed because the volume shrinkage of liquid metal cannot be supplemented during cooling and solidification. Shrinkage cavities are mostly located near the pouring riser and at the position with the largest section or at the sudden change of section.
(4) Looseness: due to poor melting, improper mold shape and other reasons, fine grain boundary cracks or fine voids are generated in the middle of the steel casting wall thickness, resulting in loose structure. The bonding between these grains is quite weak (cloud shadow is formed on the radiographic negative film).
(5) Crack: crack refers to the casting defect formed by excessive impurities with low melting point and excessive internal stress (thermal stress and structural stress) during the cooling process of liquid steel. At the sudden change of casting section size, the stress concentration is serious and cracks are easy to appear.
To sum up, the remarkable feature of casting defects in steel castings is complex shape; The casting defects of steel castings are mainly fatigue cracks, including mechanical fatigue cracks and thermal fatigue cracks.