This is a small cavity formed by the gas which can not escape during the solidification of metal. The inner wall of the cavity is smooth and contains gas. It has a high reflectivity to the ultrasonic wave. However, because it is basically spherical or ellipsoidal, that is, point defect, it affects the reflected wave amplitude. After forging or rolling, the pores in ingots are flattened into area defects, which is beneficial to be found by ultrasonic testing.
Casting shrinkage and porosity
When the casting or ingot is cooled and solidified, the volume should be shrunk, and the cavity like defects will be formed in the last solidified part due to the lack of liquid metal supplement. Large and concentrated cavities are called shrinkage cavities, while small and dispersed cavities are called porosity. They are generally located in the last solidified part of the ingot or casting center, with rough inner wall and many impurities and small pores around. Due to the law of hot expansion and cold shrinkage, shrinkage cavity is inevitable, but it has different shape, size and position with different processing methods, and it becomes defect when it extends to casting or ingot body. If the shrinkage cavity of ingot is not cut off and brought into the forging, it will become residual shrinkage cavity (residual shrinkage cavity, residual shrinkage tube). A kind of
If the mold design of the casting is improper and the pouring process is improper, it will also produce looseness in the contact part of the casting and the mold. The black part in the fracture picture is the loose part, which appears black because the workpiece has been annealed, which causes the loose part to be oxidized and infiltrated into the engine oil. W18 steel casting – used as milling cutter teeth, the porosity found by ultrasonic longitudinal wave vertical incidence multiple bottom wave attenuation method.
During smelting, slag or refractory on the furnace body peels off and enters the liquid metal, which is involved in the casting or ingot body during pouring, forming slag inclusion defect. Slag inclusions usually do not exist alone, often in a dense state or dispersed at different depths. They are similar to volumetric defects but often have a certain degree of linearity.
The reaction products (such as oxides, sulfides, etc.) in the smelting process are non-metallic inclusions, or the additives of some components in the metal components are not completely melted and remain to form metal inclusions, such as high-density, high melting point components – tungsten, molybdenum, etc., as well as pure titanium segregation in titanium alloy bars.
The segregation in casting or ingot mainly refers to the composition segregation formed due to the uneven composition distribution in the smelting process or metal melting process. The mechanical properties of the area with segregation are different from the mechanical properties of the whole metal matrix. If the difference exceeds the allowable standard, it will become a defect
The cracks in castings are mainly caused by the shrinkage stress of the metal during cooling and solidification exceeding the ultimate strength of the material, which is related to the shape design andof the castings, as well as the cracking sensitivity caused by the high content of some impurities in the metal materials (such as hot brittleness when the sulfur content is high, cold brittleness when the phosphorus content is high, etc.). Axial intergranular cracks will also occur in ingots. If they cannot be forged in subsequent open forging, they will remain in the forging and become internal cracks of the forging.
Casting cold insulation
This is a special layered defect in castings, which is mainly related to the casting process design of castings. It is a kind of diaphragm shaped area formed when pouring liquid metal due to splashing, wave turning, pouring interruption, or the meeting of two (or more) streams of metal from different directions, because the semi-solid film formed by the surface cooling of liquid metal stays in the casting body Type defects.
This is a kind of layered (area type) defect formed when pouring ingot from ladle to ingot mold in steelmaking, because of the interruption and pause of pouring, the surface of the first poured liquid metal cools rapidly in the air to form oxide film, and the newly poured liquid metal breaks through and turns into the ingot body during the continuous pouring, which can not be eliminated by forging in the subsequent ingot open forging.
When the casting or ingot is cooled and solidified, the cooling speed from the surface to the center is different, so different crystal structures will be formed, which is manifested as the anisotropy of mechanical properties, and also leads to the anisotropy of acoustic properties, that is, there are different sound velocities and sound attenuation from the center to the surface. The existence of the problem will have a bad effect on the size and position of the defects in the ultrasonic testing of castings.
In conclusion, most of the defects in the casting are volume type (except for cracks, laps and cold shuts). The orientation of defects is not obvious, but it is mainly related to the direction of cold shrinkage stress. In addition, because the grains of castings are generally coarse and anisotropic, there are many difficulties in ultrasonic testing, which must be considered in combination with casting technology, specific casting materials, casting shape and surface state.