Valve casting is one of the leading products in the cast, the market demand is very large. But due to the production of raw material prices, so that a substantial increase in production costs; production efficiency relative to other castings is lower and higher scrap rate and other factors, a serious impact on the valve casting output. A factory in 2005 the production of more than 2,000 tons of valve casting, integrated waste nearly 170t, scrap rate of more than 8%. We through the plant valve casting defects in the data analysis, to identify the main causes of waste, and its series of improvements, and achieved good results to ensure that the valve casting production pass rate and improve product quality.
Valve casting production process the most likely to nine shortcomings, namely: stomata, slag, inclusions, shrinkage and loose, segregation, cold separation, turning skin, casting cracks, anisotropy, machinery 580 will be with the majority of friends in detail this Nine shortcomings.
1, the stomata: This is the metal solidification process failed to escape the gas left in the metal inside the formation of small holes, the inner wall is smooth, containing gas, the ultrasound has a high reflectivity, but also because it is basically spherical Or ellipsoid, that is, point-like defects, affecting the amplitude of its reflection. The stomata in the ingot is crushed into an area defect by forging or rolling and is advantageous for being found by ultrasonic testing.
2, slag: smelting process in the slag or furnace furnace refractory material flaking into the liquid metal, in the casting was involved in the casting or ingot body, the formation of slag defects. The slag is usually not present in a single, often in a dense state or dispersed at different depths.
3, mixed: the reaction process in the melting process (such as oxides, sulfides, etc.) – non-metallic inclusions, or metal components of some of the ingredients are not completely melted and left to form a metal inclusions, such as high density, high Melting point composition – tungsten, molybdenum and so on.
4, shrinkage and loose: casting or ingot cooling solidification, the volume to shrink, in the final part of the solidification because of the lack of liquid metal can form a hollow-like defects. Large and concentrated cavities are called shrinkage, and small, scattered spaces are called loosened. They are usually located in the final 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 is inevitable, but with the processing technology different processing methods have different shapes, sizes and locations, when it extends to the casting or ingot body becomes defective. In the case of billet forging, the ingot will become the residual shrinkage (shrinkage residual, residual shrinkage) if it is not removed from the forging.
5, segregation: casting or ingot segregation mainly refers to the process of melting or metal melting process, because the uneven distribution of components formed by segregation, segregation exists in the region of its mechanical properties are different from the entire metal matrix mechanics Performance, the difference beyond the allowable standard range becomes a defect.
6, cold separation: This is a unique casting in a layered defects, mainly with the cast casting process design, it is in the casting of liquid metal, due to splash, waves, pouring interrupted, or from different directions of the two (Or more) metal flow encounter and other reasons, because the liquid metal surface formed by the semi-solid film to stay in the casting body to form a diaphragm-like area-type defects.
7, tipping: This is the steel from the ladle into the ingot casting ingot, because pouring interruption, pause and other reasons, the first pouring of liquid metal surface in the air quickly cooled to form an oxide film, continue to pour, the new pouring (Liquid type) defects formed by breaking the liquid metal into the ingot, which can not be wiped out in the subsequent ingot billet forging.
8, casting cracks: cracks in the casting is mainly due to the cooling of the metal when the shrinkage of the stress exceeds the ultimate strength of the material caused by it with the shape of the casting design and casting process, but also with the metal material in some impurities (Eg, when the sulfur content is high, there is hot brittleness, cold brittleness when the phosphorus content is high, etc.). In the ingot will also produce axial intergranular cracks in the subsequent billet forging if not forging, will remain in the forging to become the forging of the internal cracks.
9, anisotropy: casting or ingot cooling solidification, from the surface to the center of the cooling rate is different, which will form a different crystal structure, showing the mechanical properties of anisotropy, also led to the acoustic properties of anisotropy , That is, from the center to the surface of a different sound velocity and sound attenuation. The presence of this anisotropy will adversely affect the size and location of the defect when evaluating the ultrasonic inspection of the casting.
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