Analysis and Improvement of Common Defects in Valve Castings

1. Air hole

 

This is a small cavity formed by the gas that cannot be escaped during metal solidification and remains inside the metal. Its inner wall is smooth, contains gas, and has high reflectivity for ultrasonic waves, but because it is basically spherical or ellipsoidal, It is a point defect that affects its reflection amplitude.

 

2. Shrinkage and looseness

 

When the casting or ingot cools and solidifies, the volume shrinks, and in the final solidification part, void-like defects are formed due to lack of complement of the liquid metal. Large and concentrated voids are referred to as shrinkage pores. Small and scattered voids are referred to as loose pores. They are generally located in the final solidified part of the center of the ingot or casting. The inner wall is rough, surrounded by many impurities and tiny pores. Due to the law of thermal expansion and contraction, the shrinkage cavity is inevitably present. However, the shape, size and position of the shrinkage hole are different depending on the processing method of the processing. When it extends to the casting or ingot body, it becomes a defect. When the billet is forged, it will become residual shrinkage (remaining shrinkage, residual shrinkage) if it is not cut clean and brought into the forging.

 

3. slag inclusion

 

The slag in the smelting process or the refractories on the furnace body is peeled off into the liquid metal, and is entrained into the casting or the steel ingot body during pouring, thus forming a slag inclusion defect. Slag inclusions do not usually exist singly, they tend to be dense or disperse at different depths. They are similar to volumetric defects but often have a certain degree of linearity.

 

4. Inclusion

 

The reaction products in the smelting process (such as oxides, sulfides, etc.) - non-metallic inclusions, or the addition of certain components in the metal components are not completely melted and remain to form metal inclusions, such as high density, high melting point component - tungsten Mo, etc.

 

5. Segregation

 

Segregation in castings or ingots mainly refers to the segregation of components formed during the smelting process or the melting process of metals due to uneven composition distribution. The mechanical properties of the areas where segregation is present are different from the mechanical properties of the entire metal matrix, and the differences exceed the allowable standards. The scope becomes a defect.

 

6. Cast crack

 

Cracks in castings are mainly caused by shrinkage stress when the metal is cooled and solidified, which exceeds the ultimate strength of the material. It is related to the shape design of the casting and the casting process. It is also sensitive to cracking caused by high levels of some impurities in the metal material. Sex related (for example, hot brittleness when the sulfur content is high, cold brittleness when the phosphorus content is high, etc.). Axial core intergranular cracking also occurs in steel ingots. If it cannot be forged during subsequent forging, it will remain in the forgings as an internal crack in the forgings.

 

7. Cold isolation

 

This is a peculiar delamination defect in the casting, which is mainly related to the casting process design of the casting. It is when the liquid metal is poured, due to splashing, wave breaking, pouring interruption, or two or more strands coming from different directions. ) Metal flow meets and other reasons, because the semi-solid film formed by the cooling of the liquid metal surface remains in the casting body and forms a diaphragm-like area-type defect.

 

8. Peeling

 

This is when the steel ingot is poured from the ladle to the ingot mold during steelmaking. Because of casting interruptions, pauses, etc., the surface of the liquid metal poured first is quickly cooled in the air to form an oxide film, and the newly poured liquid metal will be A delamination (area type) defect formed by breaking into the steel ingot is not eliminated by the subsequent forging of the steel ingot.

 

9. Anisotropy

 

When the casting or ingot cools and solidifies, the cooling rate from the surface to the center is different. Therefore, different crystalline structures will be formed. The anisotropy of the mechanical properties will also lead to the anisotropy of the acoustic properties, that is, from the center to the The surface has different sound speed and sound attenuation. The presence of this anisotropy can have an adverse effect on the size and position of flaws during the ultrasonic testing of castings.

 

Third, improvement measures

 

(1) The melting equipment has a poor ability to guarantee the composition of molten iron and the stability of the sand mixing equipment is not good. The composition of molten iron is constrained by various factors such as coke, furnace type, air volume, and raw material conditions; resin sand is affected by factors such as temperature, resin, and acid addition amount. If the sand often does not pass through the regeneration and cooling beds, the temperature of the sand is very high, which seriously affects the strength of the sand mold, resulting in serious sand swell in the casting and increasing the tendency of the casting to produce shrinkage and shrinkage defects.

 

(2) The blasting of molten iron in the cavity and the impact of the molten iron during the pouring process directly result in the defects of sand holes and sand inclusions.

 

(3) There will always be slag formation in the molten iron in the melting equipment. During casting, the solid and liquid slag in the molten iron enter the cavity together with the molten iron to form the slag hole.

 

(4) In the production process, the nitrogen content in the molten iron increases with the increase of the temperature and decreases with the increase of the carbon equivalent. When nitrogen and hydrogen are together, pores are easily formed. This is the main cause of the pores. source.

 

(5) The rigidity of the mold base plate is poor, and the shape is uneven and deformed before the molding. As a result, the sand type parting surface is uneven, and the gap between the upper and lower parting surface is large at the time of forming, resulting in a poor dimension and shape at the parting surface.

 

(6) The drift of the sand core of the 2.2m valve body foot during the pouring process is the main reason for the uneven wall thickness at the foot.

 

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

 

(1) Properly increase the carbon equivalent of the molten iron and use the graphitized expansion to enhance the self-feeding ability of the material.

 

(2) To ensure the compaction of the molding sand, improve the strength of the sand mold, and promote the self-feeding ability of the casting.

 

(3) Blow out the loose sand in the cavity before closing and carefully inspect the inside of the cavity.

 

(4) Unfilled valve body sand remains after pouring on site, and the gating cup and outlet hole should be tightly covered to prevent loose sand from entering.

 

(5) Clean the solid slag on the surface of the molten iron before pouring; increase the initial pouring temperature of the molten iron and reduce the tendency of the molten iron to produce secondary oxidized slag; valve castings should be arranged as early as possible after the furnace is opened to reduce the lining of the lining. After use, a large amount of lean slag is produced. For the 610mm (24in) F-body valve, for the place where the runner overlaps, combined with other parameters during pouring, the filter is placed at the inlet and outlet, and multi-piece lap-type fibers are filtered. The net was improved to monolithic to improve the slag blocking effect of the gating system.

 

(6) Raw materials should be made of carbon steel, common gray cast iron or ductile iron return materials as much as possible; reduce the content of alloy elements such as Cr, Mn, etc. in the molten iron to reduce the iron content of the molten iron; all the sand cores before the lower core should be overhead brush coatings. And the deadline for storage, to prevent sand core moisture absorption; in the rainy season or high humidity season, the next core is best to use torches to bake the cavity, sand core surface again to reduce the sand type gas output; using high temperature pouring small valve, In order to facilitate the exhaust of molten iron and reduce the production of cast slag.

 

(7) When casting 1067mm (42in) F-body valves, it is required that the same bottom plate before the fixed mixer be raked up and down, and there should be no debris on the bottom plate; it is not allowed to put in other places to reduce Source of variation; it is forbidden to lift the sand mold together with the mold base plate to prevent the base plate from being deformed.

 

(8) When casting 2.2n, when the valve body is placed, place the appropriate amount of resin sand on the core of the support foot core and close the mold as soon as possible.

 

According to several suggestions in the improvement measures, the sand compactness, air permeability, and gas generation volume are all crucial conditions. Therefore, professional sand detection equipment must be equipped for testing to ensure the quality of castings. XQY-II intelligent sand strength tester, ZTY smart air permeability tester, high intelligence computer energy-generating tester, SEL-A computer-controlled clay suction blue tester are all necessary for sand testing equipment.