The cause of defects in the production process of pump and valve castings

2018-10-31

1.Porosity


This is a small cavity formed by the gas that does not escape during the solidification process of the metal, which has a smooth inner wall, contains a gas, and has a high reflectance to the ultrasonic wave, but because it is substantially spherical or ellipsoidal, It is also a point defect that affects its reflection amplitude. The pores in the ingot are crushed into area-type defects after forging or rolling to facilitate the detection by ultrasonic testing.背景.png

2. Shrinkage and looseness


When the casting or the ingot is cooled and solidified, the volume is contracted, and a void-like defect is formed in the final solidified portion because the liquid metal is not replenished. Large and concentrated cavities are called shrinkage cavities, and small and scattered cavities are called loose. They are generally located in the last solidified part of the ingot or casting center. The inner wall is rough and surrounded by many impurities and fine pores. Due to the law of thermal expansion and contraction, shrinkage cavities are inevitable, but have different shapes, sizes and positions depending on the processing method, and become defects when they extend to the casting or ingot body. If the steel ingot is cut into the forging after the blank forging, it will become a residual shrinkage hole (recession of the shrinkage hole, residual shrinkage pipe).


3. Slag inclusion


The slag in the smelting process or the refractory material on the furnace body is peeled off into the liquid metal, and is entangled in the casting or the steel ingot body during casting to form a slag inclusion defect. The slag inclusions usually do not exist singly, tend to be dense or dispersed at different depths, and they are similar to volumetric defects but tend to have a certain degree of linearity.


4. Mixed


The reaction product (such as oxides, sulfides, etc.) in the smelting process - non-metallic inclusions, or the addition of some components of the metal component is not completely melted and remains to form metal inclusions, such as high-density, high-melting component - tungsten , molybdenum, etc.


5. Segregation


Segregation in castings or ingots mainly refers to the segregation of components formed during the smelting process or in the melting process of metals due to uneven distribution of components. The mechanical properties of the regions where segregation exists are different from the mechanical properties of the entire metal matrix, and the difference exceeds the allowable standard. The scope becomes a defect.


6. Casting crack


The crack in the casting is mainly caused by the shrinkage stress of the metal during solidification and solidification exceeding the ultimate strength of the material. It is related to the shape design and casting process of the casting, and also to the cracking sensitivity caused by the high content of some impurities in the metal material. Sexually relevant (for example, hot brittle when the sulfur content is high, cold brittleness when the phosphorus content is high, etc.). Axial intergranular cracks are also generated in the ingot, and if it cannot be forged in the subsequent blank forging, it will remain in the forging as an internal crack of the forging.


7. Cold partition


This is a delamination defect unique to castings. It is mainly related to the casting process design of castings. It is caused by splashing, tumbling, pouring, or two (or more) strands from different directions when pouring liquid metal. The metal flow encounters and the like, because the semi-solid film formed by the cooling of the liquid metal surface remains in the body of the casting to form a diaphragm-shaped area-type defect.


8. Flip


This is when the steel ingot is poured from the ladle to the ingot during steelmaking. Because of the interruption of the pouring, the pause, etc., the surface of the first poured liquid metal is rapidly cooled in the air to form an oxide film, and the newly poured liquid metal will continue to be poured. Breaking through a layered (area type) defect formed by turning into a steel ingot, which cannot be forged and removed in the subsequent ingot forging.


9.Anisotropy


When the casting or ingot is cooled and solidified, the cooling rate from the surface to the center is different, and thus different crystal structures are formed, which exhibits anisotropy of mechanical properties and also causes anisotropy of acoustic properties, that is, from the center to The surface has different sound speeds and sound attenuation. The existence of such anisotropy adversely affects the size and location of defects in the ultrasonic testing of castings.