1. Pores

This is a small cavity formed inside the metal during the solidification process where gas that cannot escape remains. Its inner wall is smooth and contains gas, which has a high reflectivity for ultrasonic waves. However, because it is basically spherical or ellipsoidal, that is, a point like defect, it affects the amplitude of its reflection wave. The pores in the steel ingot are flattened into area type defects after forging or rolling, which is conducive to being detected by ultrasonic testing.

2. Shrinkage and looseness

When castings or steel ingots are cooled and solidified, their volume shrinks, and in the final solidified part, hollow defects may form due to the lack of liquid metal replenishment. Large and concentrated voids are called shrinkage cavities, while small and dispersed voids are called looseness. They are generally located in the last solidified part of the steel ingot or casting center, with rough inner walls and many impurities and small pores around them. Due to the law of thermal expansion and contraction, shrinkage cavities are inevitable, but they have different shapes, sizes, and positions depending on the processing method. When they extend to the casting or steel ingot body, they become defects. If the shrinkage cavity of the steel ingot is not completely removed during the forging process, it becomes residual shrinkage cavity (shrinkage cavity residue, residual shrinkage tube) and is carried into the forging.

3. Slag inclusion

During the smelting process, the slag or refractory material on the furnace body peels off and enters the liquid metal, which is then rolled into the casting or steel ingot body during pouring, forming slag inclusion defects. Slag inclusions usually do not exist singly, but rather in a dense state or dispersed at different depths. They resemble volumetric defects but often have a certain degree of linearity.

4. Inclusions

The reaction products (such as oxides, sulfides, etc.) generated during the smelting process - non-metallic inclusions, or metal inclusions formed by the residual addition of certain components in the metal composition that have not melted, such as high-density, high melting point components - tungsten, molybdenum, etc.

5. Segregation

Segregation in castings or steel ingots mainly refers to the compositional segregation formed during the smelting process or the melting process of metals due to uneven distribution of components. The mechanical properties of areas with segregation are different from those of the entire metal matrix, and if the difference exceeds the allowable standard range, it becomes a defect.

6. Casting cracks

Cracks in castings are mainly caused by the shrinkage stress during metal cooling and solidification exceeding the ultimate strength of the material. It is related to the shape design and casting process of the casting, as well as the cracking sensitivity caused by high impurity content in the metal material (such as thermal brittleness when sulfur content is high, cold brittleness when phosphorus content is high, etc.). Axial intergranular cracks can also occur in steel ingots, and if they cannot be forged during subsequent forging, they will remain in the forging and become internal cracks.

7. Cold insulation

This is a delamination defect in castings, mainly related to the casting process design of castings. It is a diaphragm like area type defect formed by the semi-solid film formed by the cooling of the liquid metal surface during the pouring of liquid metal due to splashing, flipping, pouring interruption, or the encounter of two (or more) metal streams from different directions.

8. Peel over

This is a layered (area type) defect formed when pouring steel ingots from the ladle to the ingot mold during steelmaking. Due to interruptions, pauses, and other reasons, the surface of the liquid metal poured first rapidly cools in the air to form an oxide film. When continuing to pour, the newly poured liquid metal breaks through and flips into the ingot body, forming a layered (area type) defect that cannot be eliminated during subsequent ingot forging.

9. Anisotropy

When castings or steel ingots are cooled and solidified, the cooling rate from the surface to the center is different, resulting in different crystalline structures, manifested as anisotropy of mechanical properties, and also leading to anisotropy of acoustic properties, that is, different sound velocities and attenuation from the center to the surface. The existence of this anisotropy will have a negative impact on the evaluation of defect size and location during ultrasonic testing of castings.