Analysis And Improvement Of Common Defects In Valve Castings

- Jul 31, 2019-

1. Stomata

This is a small void formed by the gas which cannot escape during the solidification of metal. Its inner wall is smooth and contains gas. It has a high reflectivity to the ultrasonic wave. However, because it is basically spherical or ellipsoid, that is to say, it is a dot defect, which affects the reflected wave amplitude. The air holes in ingots are flattened into area defects after forging or rolling, which is conducive to being detected by ultrasonic testing.

2. Shrinkage and Porosity

When a casting or ingot is cooled and solidified, its volume shrinks. The final solidified part will form a void defect because it cannot be supplemented by liquid metal. Large and concentrated voids are called shrinkage voids, while small and dispersed voids are called looseness. They are generally located in the last solidified part of the center of the ingot or castings. Their inner walls are rough, and there are many impurities and small voids around them. Because of the law of thermal expansion and cold contraction, shrinkage holes are inevitable, but they have different shapes, sizes and locations with different processing methods. When they extend to the casting or ingot body, they become defects. Ingot will become residual shrinkage (shrinkage residual, residual shrinkage pipe) if it is not removed and brought into the forging during the ingot forging.

sand test chamber,radiographic test,tensile testing machine

3. Slag inclusion

Slag inclusion defects are formed when slag or refractory material on the furnace body is peeled off into liquid metal during smelting and is involved in the casting or ingot body during pouring. Slag inclusion usually does not exist singly, but tends to be concentrated or dispersed in different depths. It is similar to volume defects but often has certain linearity.

4. Inclusion

The reaction products (such as oxides, sulfides, etc.) in the smelting process - Non-metallic inclusions, or the additives of some components in the metal composition are not fully melted and remain to form metal inclusions, such as high density, high melting point components - tungsten, molybdenum and so on.

5. Segregation

Segregation in castings or ingots mainly refers to the composition segregation formed during smelting or metal melting due to the uneven distribution of components. The mechanical properties of the segregation area are different from those of the whole metal matrix, and the differences beyond the allowable standard range become defects.

6. Casting Cracks

Cracks in castings are mainly caused by shrinkage stress exceeding the ultimate strength of materials during cooling and solidification, which is related to the shape design of castings and casting process, and also to the cracking sensitivity caused by high impurity content in metal materials (e.g., hot brittleness when sculpture content is high, hot brittleness when phosphorus content is high). It has cold brittleness and so on. Axis intergranular cracks also occur in ingots. If they cannot be forged in subsequent open forging, they will remain in the forgings and become internal cracks of the forgings.

7. Cold Separation

This is a unique layered defect in castings, which is mainly related to the casting process design of castings. It is caused by splashing, surge, interruption of pouring, or the encounter of two (or more) metal streams from different directions when pouring liquid metals, because the semi-solid film formed by cooling the liquid metal surface remains. A diaphragm-like area defect is formed in the casting body.

8. Skin Turning

This is a layered defect formed when pouring ingot from ladle to ingot mold in steelmaking. Because of interruption and pause of pouring, the first poured liquid metal surface rapidly cools in air to form oxide film. When pouring continuously, the newly poured liquid metal breaks it into ingot body and turns it into ingot body. It is in follow-up. It is impossible to forge the ingot without forging.

9. Anisotropy

When casting or ingot is cooled and solidified, the cooling rate from the surface to the center is different, so different crystalline structures will be formed, which are characterized by the anisotropy of mechanical properties, and also lead to the anisotropy of acoustic properties, i.e. different sound velocity and sound attenuation from the center to the surface. The existence of this anisotropy will have a negative impact on the size and location of defects in ultrasonic inspection of castings.

casting valve

Improving Measures

(1) Melting equipment has poor ability to guarantee the composition of molten iron and poor stability of sand mixing equipment. The composition of molten iron is restricted by many factors, such as coke, furnace type, air flow rate, raw material condition, and resin sand is affected by temperature, resin and acid addition. For example, sand often does not undergo regeneration and cooling bed, which makes the temperature of sand very high, seriously affects the strength of sand mold, causes serious sand expansion of castings, and increases the tendency of shrinkage and porosity defects of castings.

(2) Sand hole and sand inclusion defects are directly caused by loose sand in the cavity and impact of molten iron during casting.

(3) Slag always occurs in molten iron in melting equipment. Solid and liquid slags in molten iron enter the cavity together with molten iron to form slag holes during pouring.

(4) In the production process, the nitrogen content in molten iron increases with the increase of temperature and decreases with the increase of carbon equivalent. When nitrogen and hydrogen are together, it is easy to form stomata, which is the main source of stomata.

(5) The rigidity of the bottom plate of the mold is poor, and the irregular placement before the molding results in the irregular parting surface of the sand mold. The gap between the upper and lower parting surfaces is large, resulting in the bad size and shape of the parting surface.

(6) The downward drift of sand core at the support foot of 2.2m valve body is the main cause of uneven wall thickness at the support foot.

According to the causes of defects in valve castings, we have taken improvement measures from the following aspects:

(1) Increase the carbon equivalent of molten iron appropriately and enhance the self-feeding ability of materials by graphitizing expansion.

(2) Ensure the compactness of molding sand, increase the strength of molds and promote the self-feeding ability of castings.

(3) Before closing, the loose sand in the cavity is blown clean, and the cavity is carefully examined.

(4) The valve body sand mold left behind after field pouring should be tightly covered with the gate cup and vent to prevent sand from entering.

(5) Clean the solid slag on the surface of molten iron before pouring; increase the initial pouring temperature of molten iron to reduce the tendency of secondary oxidation slag; arrange the initial pouring of the valve castings after the start of the furnace as far as possible to reduce the large amount of thin slag produced by the lining after repeated use; For 610mm (24in) F body valves, for the overlap of the runner, the joint In order to improve the slag retaining effect of the pouring system, the filter screen is installed at the inlet and outlet of the joint pouring and the multi-piece lapped fiber filter screen is improved to single-piece type.

(6) Carbon steel, common gray cast iron or nodular cast iron should be recycled as far as possible; alloying elements such as Cr and MN in molten iron should be reduced to reduce the gas content of molten iron itself; all sand cores before core setting should be brushed over the air and stored for a limited period to prevent moisture absorption of sand cores; in rainy or humid season, it is better to use a blowout lamp before core setting. The surface of the cavity and core is baked once to reduce the gas production of the sand mold, and the small valve is poured at high temperature to facilitate the self-exhaust of the molten iron and reduce the slag production.

stainless steel  Flat valve casting

(7) When pouring 1067mm (42in) F body valves, it is required to ladle the upper and lower shapes on the same bottom plate before fixing the sand mixer, and there must be no impurities on the bottom plate; it is not allowed to place them in other places to reduce the source of variation; it is prohibited to hoist the sand mould and the bottom plate together to prevent the bottom plate from deforming.

(8) Casting 2.2n, when the valve body, put a proper amount of resin sand on the core head of the supporting sand core, and fit as soon as possible.

After taking the above improvement measures, a total of 2413.78t valves were produced in the whole year. The internal waste rate was 1.15%, the external waste rate was 1.73%, and the comprehensive waste rate was 2.88%. Compared with the scrap rate before and after improvement, the internal scrap rate decreased by 2.39 percentage points, the external scrap rate decreased by 2.85 percentage points, and the comprehensive scrap rate decreased by 5.24 percentage points.