This is the metal solidification process can not 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 its reflection amplitude. The stomata in the ingot are crushed into an area defect by forging or rolling and is advantageous for being found by ultrasonic testing.
2. 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 process different methods have different shape, size and location, 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.
The slag in the smelting process or the refractory on the furnace furnace is exfoliated into the liquid metal and is entrained in the casting or ingot body during casting to form a slag defect. The slag is usually not present in a single state, often in a dense state or dispersed at different depths, which resembles volumetric defects but tend to have a certain degree.
4. mixed with
(Such as oxides, sulfides, etc.) in the smelting process – non-metallic inclusions or additives of certain components of the metal component are not completely melted and remain to form metal inclusions such as high density, high melting point components – tungsten , Molybdenum, etc.
The segregation in the casting or ingot mainly refers to the segregation of the components formed during the melting process or the melting of the metal due to the uneven distribution of the components. The mechanical properties of the segregated areas are different from those of the whole metal matrix. The difference is beyond the allowable standard The scope becomes a defect.
6. Casting cracks
Cracks in the casting are mainly due to the fact that the shrinkage stress of the metal cooling and solidification exceeds the ultimate strength of the material. It is related to the shape design and casting process of the casting and the cracking of the metal material. (Such as hot brittleness when the sulfur content is high, cold brittleness when the phosphorus content is high, etc.). In the ingot will also produce axial intergranular cracks in the follow-up billet forging if not forging, will remain in the forging to become the forging of the internal cracks.
7. cold separation
This is a unique delamination defect in the casting, mainly related to the casting process design of the casting, which is due to splashing, tumbling, pouring, or two strands of different directions (or multiple strands) ) Metal flow 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.
This is when the steel from the ladle to 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, in the continued pouring of new liquid metal Breaking through the ingot into the ingot and the formation of a layered (area type) defects, it is in the subsequent steel ingot billet forging can not be forced to eliminate.
When the casting or ingot is cooled and solidified, the cooling rate from the surface to the center is different, resulting in different crystallographic structures, exhibiting anisotropy of mechanical properties, and leading to anisotropy of acoustic properties, The surface has 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.
(1) melting equipment on the iron liquid component of the poor ability to support and the stability of the sand mixer is not good. The composition of the molten iron is limited by the factors such as coke, furnace type, air volume and raw material condition. The resin sand is affected by factors such as temperature, resin and acid addition. Such as sand often do not go through the regeneration and cooling bed, so that the sand temperature is high, seriously affected the sand strength, resulting in casting sand swelling serious, increasing the casting shrinkage, shrinkage defects tend to.
(2) the cavity in the sand and pouring process of the impact of molten iron directly lead to trachoma and sand defects.
(3) molten iron in the melting equipment will always be slag generation, pouring molten iron in the solid and liquid slag together with the molten iron into the cavity to form slag holes.
(4) In the production process, the nitrogen content in the molten iron increases with increasing temperature, and decreases with the increase of the carbon equivalent. When nitrogen and hydrogen are together, it is easy to form pores, which is the main source.
(5) the bottom of the Mold is less rigid, the shape before the placement of uneven and deformation, resulting in sand type surface uneven, when the type of type up and down the gap between the larger, resulting in sub-surface size, poor shape.
(6) 2.2m body at the foot of the sand core in the pouring process drift down, is the foot of the main reason for uneven wall thickness. In the case of
According to the causes of valve casting defects, we mainly from the following aspects to take the improvement measures:
(1) the appropriate increase in iron liquid carbon equivalent, the use of graphitization expansion to enhance the self-filling capacity of the material.
(2) Chung box to ensure the compactness of the sand, to improve the sand type strength, and promote casting from the filling capacity.
(3) before the type of cavity before the scattered sand blowing clean, and carefully check the cavity.
(4) after pouring the site under the pouring of the valve body is not pouring, the gate cup and outlet holes to cover tight, to prevent scattered sand into.
(5) before pouring the solid liquid on the surface of the molten iron clean; to improve the initial pouring temperature of molten iron to reduce the tendency of iron oxide secondary oxidation slag; valve casting as far as possible after the initial pouring in the open, After the use of a large number of thin slag; for 610mm (24in) F body valve, for the runner lap joints, combined with other parameters when pouring, in the import and export are located in the filter, and multi-chip lap filter The mesh is improved in monolithic form to improve the slagging effect of the gating system.
(6) raw materials as far as possible the use of carbon steel, ordinary gray cast iron or ductile iron back to charge; reduce the iron in the alloying elements such as Cr, Mn and other content, to reduce the iron content of their own gas; And in the rainy days or humidity of the larger season, the next core is best to use the torch to the cavity, sand core surface baked again to reduce the amount of sand gas; the use of high temperature pouring small valves, In order to facilitate the exhaust of their own liquid and reduce the slag generation.
(7) pouring 1067mm (42in) F body valve, the need for a fixed mixer in the same floor on the same Chung, the type, and the floor should not have any debris; not allowed to put in other places to reduce It is forbidden to lift the sand and the mold base plate together to prevent deformation of the floor.
(8) pour 2.2n, the valve body, in the foot of the core of the core of the core put the amount of resin sand, and as soon as possible type. In the case of
After taking the above improvement measures, the total production of 2413.78t valve, the internal waste rate of 1.15%, the external waste rate of 1.73%, the comprehensive scrap rate of 2.88%. Compared with the improved scrap rate, the internal waste rate decreased by 2.39 percentage points, the external waste rate decreased by 2.85 percentage points, the comprehensive scrap rate decreased by 5.24 percentage points, the effect is remarkable.
Link to this article：Valve Castings Analysis And Improvement Of Common Defects
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