Causes analysis prevention of shrinkage cavity sag defects of cast iron

Shrinkage

1. Features:

There are many small dispersed shrinkage holes in the casting, the surface is rough, water seepage occurs during hydrostatic test.

2. Reason analysis:

(1) Unreasonable process design.Influence of casting structure, shape wall thickness.Too many isolated hot spots, too large size change, thick section can get enough shrinkage.

(2) Unreasonable design of pouring system, chiller riser, poor feeding effect of riser.

(3) Unreasonable pouring temperature, too high too low temperature will affect the feeding effect of the riser.

(4) Low compactness poor mold stiffness.Graphitized expansion causes cavity enlargement shrinkage due to insufficient filling during casting shrinkage.

(5) Low content of carbon silicon, high content of phosphorus;Large setting interval.Silicomolybdenum high nickel nodular cast iron is very sensitive to carbon silicon content iron oxide liquid, it is easy to appear micro loose when iron liquid is seriously oxidized carbon silicon content is low.Even in the thin wall is also prone to shrinkage.

(6) Insufficient inoculation poor graphitization effect.

(7) Excessive amount of residual magnesium rare earth.The higher molybdenum content also increases the microshrinkage porosity.

(8) The casting speed is too fast.

(9) Burden corrosion, iron oxide more.

(10) The iron solution is stored in the electric furnace for too long at high temperature, causing serious oxidation.

(11) In cupola smelting, the bottom coke is too bottom, the air volume is too large, the elements are burned badly, the molten iron is seriously oxidized.

(12) The contact hot spot formed at the riser bore produces shrinkage.

(13) Pressure box iron is enough, after pouring due to the expansion of the box caused by shrinkage.

3. Prevention methods:

(1) Appropriately increase the content of carbon silicon in molten iron.It should be fully inoculated to improve the self-feeding shrinkage ability of the casting.

(2) On the premise of ensuring spheroidization, the amount of residual magnesium should be appropriately reduced to ensure the amount of residual rare earth.Rare earth residues are even lower when sulfur content is very low, especially when high purity pig iron is used.

(3) To minimize sulfur phosphorus content.

(4) Improve the compactness of the cast to facilitate the self-replenishment of molten iron during graphitization expansion.

(5) Reasonably arrange the casting riser chiller, appropriately raise the pouring temperature, strengthen the feeding capacity of riser.

(6) Improve the structure of casting reduce the inhomogeneity of casting wall thickness.

(7) Appropriate slow pouring after the casting, in order to fully fill shrinkage.

(8) When shrinkage occurs at the contact hot spot, the riser with wide, thin short caliber shall be adopted, the slope of the connection between the riser the riser shall be increased as far as possible, the riser shall be placed on the side of the hot spot, the method of avoiding the uniform solidification of the hot spot shall be adopted.

(9) The fluidity of high nickel nodular cast iron is worse, thermal insulation riser hot riser should be used when filling shrinking riser.

(10) The longer casting is inclined to be poured, the riser is located at the highest point to enhance the filling effect.

(11) To prevent iron liquid oxygenation, especially serious iron liquid oxygenation of si-Mo nodular cast iron. When carbon equivalent is low, it is easy to produce shrinkage microshrinkage porosity at the thin-wall included Angle.

(12) The seriously corroded charge should be shot blasting before use.

(13) For isolated hot nodes that cannot be filled by riser, the following methods can be adopted: put the surface with hot nodes in the lower box;If the surface with hot joints cannot be placed in the lower box due to the structural problems of the casting, cooling bars can be placed above the hot joints;To increase carbon equivalent make full use of the self-reinforcing effect of graphitization expansion;Place cold iron at the hot spot chill sand at the local location during manual modeling;A shaped quench sand core is made above the isolated hot section to accelerate the cooling speed of the part.At the top of the isolated hot spot, a flexible sand core is made, a duck beak riser is made on the sand core to strengthen the reinforcement of the isolated hot spot.Change the casting structure appropriately increase the feeding channel;Placing a riser on the side of the isolated hot spot, with the riser neck close to avoiding the hot spot, can only play the effect of shrinkage but also facilitate casting cleaning.

(14) There should be enough pressing iron for the box, the box clamp should be firm, the sealing mud should be sealed well.

(15) Silicon carbide pretreatment can also reduce the shrinkage tendency.

(16) Can be used to add charge silicon carbide re-melting.

The shrinkage cavity

1. Features:

A concentrated hole with irregular shape rough surface is produced at the hot spot of casting.Shrinkage cavity is often accompanied by porosity

Sign, commonly known as shrinkage cavity.

2. Reason analysis:

(1) Feed shrinkage deficiency at riser.

(2) Improper setting of chillers.

(3) Improper position of internal gate.

(4) Insufficient compactness of sand mold.

(5) Low carbon silicon content, high phosphorus content, large solidification interval.

(6) Excessive casting temperature increases the liquid shrinkage value.

(7) The casting temperature is too low, which affects the feeding effect of the riser.

3. Prevention methods:

(1) Correct selection of iron liquefaction components to increase carbon equivalent.

(2) Appropriately increase the amount of inoculation.

(3) For large pieces, iron can be added at the riser.

(4) Control the appropriate pouring temperature.

(5) Increase the size of the riser appropriately use hot riser as far as possible.

(6) Place the cold iron on the thick wall use it in conjunction with the riser to create reasonable solidification conditions.

(7) Correctly the casting position casting system, resulting in balanced solidification sequential solidification.

(8) Improve the compactness of the cast to facilitate the expansion of molten iron graphitization to obtain self-shrinkage.

Sag

1. Features:

Atmospheric pressure depresses the plastic surface of the casting.

2. Reason analysis:

(1) The casting is locally thick the molten iron supplement is enough.

(2) The riser is too close to the filling shrinkage, resulting in local sand overheating slow liquid iron cooling, which is easy to shrink concave in the side of the casting near the riser.

3. Prevention methods:

(1) Appropriate introduction of hot iron solution in the depression.Cold iron can be placed in the depression during manual modeling.

(2) Reasonable setting of venturing caliber.

(3) Refer to the prevention methods of shrinkage cavity porosity for the rest.