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Main Causes of Porosity in Stainless Steel Castings

May. 12, 2020

One, In order to avoid the defects of shrinkage and porosity in casting, the methods of risers, chillers, and subsidies are mostly adopted in the casting process to complete the condensation sequence.

In order to avoid the defects of shrinkage cavity, porosity and crack in stainless steel cast fittings, the wall thickness should be even, sharp Angle and right-angle structure should be avoided, sawdust should be added to molding sand for casting, coke should be added to mold core, and hollow core and oil sand core should be selected to improve the conceding and air permeability of sand mold or mold core.

Second, Due to the poor fluidity of liquid steel, the wall thickness of Chinese stainless steel pipe fittings shall not be less than 8mm in order to avoid cold insulation and lack of casting. Choose dry or hot casting; Proper improvement of pouring temperature, generally 1520° ~ 1600 ° c, due to the high pouring temperature, the excess heat of steel, liquid for a long time, liquidity can be improved. But pouring temperature is too high, can cause grain coarse, hot crack, stoma, and sticky sand and other shortcomings.

Stainless Steel Cast Fittings

Stainless Steel Cast Fittings

Therefore, generally small, thin wall and disorderly shape of fine casting parts, its pouring temperature is about the melting point of steel temperature +150℃; The structure of the casting system is short and the cross-section is larger than that of cast iron. The pouring temperature of large, thick wall castings is about 100℃ higher than their melting point.

From the box, trolley resistance furnace plate, before the use of a high-temperature tempering, and as far as possible to control the temperature above 950 degrees, the purpose of this is to eliminate the internal stress in the casting process. And the furnace plate in the use process, must ensure that the workpiece is placed evenly, can not work in a local location, otherwise in the heating process will lead to the furnace plate in the heat dissipation process is not uniform, easy to cause deformation and cracking of the furnace plate, reduce the life of the furnace floor.

In the process of stainless steel casting, the problem of air holes often occurs, which brings a lot of trouble to the casting process. Air holes are the defects formed when the metal liquid is cooled and solidified, and the precipitated gas remains in the ingot. Below Stainless Steel Elbow 90°supplier to analyze the main cause of blowholes:

01: poor permeability of paint or insufficient negative pressure, poor permeability of sand filling, unable to timely discharge the gas and residues in the mold cavity, forming pores under the filling pressure.

02: the casting speed is too slow to fill the spout cup, expose the spout, draw in the air, suck in slag, and form blowhole and slag hole.

03: a large number of gases and residues generated by the gasification and decomposition of the foam model could not be discharged out of the casting mold in time. The poor dry condition of the foam and the coating layer filled with dry sand and a large amount of hydrogen and oxygen from pyrolysis invading the casting under the high temperature of the liquid alloy were the main reasons for the formation of pores.

04: due to the unreasonable design of the pouring system, the filling speed of the metal liquid is higher than that of the foam gasification concession and the gas discharge speed, which causes the gasification residue to be regasified in the metal liquid in the filling front to form the inner wall smoke black decomposition pores.

05: the connection between the sprue cup and the sprue and the casting system is not well sealed, especially the connection between the sprue cup and the sprue cup is not well sealed. Under the effect of negative pressure, it is easy to form sand and pores. This phenomenon can be calculated and explained by the Bernoulli equation.

06: the grain size of molding sand is too fine, the dust content is high, the permeability is poor, the negative pressure distortion is caused by the blockage inside the negative pressure pipeline so that the negative pressure value around the mold cavity is far lower than the indicated negative pressure, and the gasifier can not discharge the coating in time and form pores or wrinkled skin.

07: the casting temperature is low, the metal liquid in front of mold filling can't fully gasify the foam, and the undecomposed residual material can't float to the riser and solidify in the casting to form pores.

08: the steel deoxidization is bad, slag removal is not clean in the hearth, furnace, and bag, the sedation time is too short, slag retaining is not strong in the pouring process, and slag hole is caused by the unreasonable pouring process.

09: internal sprue opening position is unreasonable, the filling form when a corner area, it is due to the air in the cavity pressure, gas hole making gasification residue accumulation in a corner, the runner cross-sectional area is too large, the filling speed is greater than the bubble gasification concession, to devour the bubble, internal decomposition of the alloy gasification, and gas cannot discharge gas hole.

10: the capacity of the pouring cup is too small, and the metal liquid forms eddy current, which intrudes into the air and generates pores.