Main Causes of Porosity in Stainless Steel Castings
Release time:
2017-08-14
In the process of stainless steel casting, the problem of porosity often occurs, which brings a lot of trouble to the casting processing. The porosity is the bubble defect formed by the gas precipitated in the ingot during the cooling and solidification process of the metal liquid. Here's an analysis of the main causes of pores:
01
The permeability of the coating is poor or the negative pressure is insufficient, the permeability of the filling sand is poor, the gas and residue in the cavity cannot be discharged in time, and the pores are formed under the filling pressure.
02
The pouring speed is too slow to fill the pouring cup, exposing the sprue, involving air, inhaling slag, forming entrained pores and slag holes.
03
The gasification and decomposition of the foam model generates a large amount of gas and residues that cannot be discharged from the casting mold in time. The dry sand filled with the foam and coating layer is poorly dried. Under the high temperature surrounding of the liquid alloy, a large amount of hydrogen and oxygen cracking into the casting is the main reason for the formation of pores.
04
Due to the unreasonable design of the gating system, the filling speed of the metal liquid is greater than the foam gasification concession and gas discharge speed, resulting in the filling front sandwiching the gasification residue in the metal liquid and gasifying again to form the black decomposition pores of the inner wall smoke.
05
The connection between the gate cup and the sprue and the gating system is not well sealed, especially the connection between the sprue and the gate cup is not well sealed, and it is easy to form sand and pores under the action of negative pressure. This phenomenon can be calculated and explained by Bernoulli's equation.
06
The particle size of the molding sand is too fine, the dust content is high, the air permeability is poor, and the internal blockage of the negative pressure pipe causes the negative pressure distortion, so that the negative pressure value around the cavity is far lower than the indicated negative pressure, and the gasification material cannot be discharged from the coating in time to form pores or wrinkled skin.
07
The pouring temperature is low, the metal liquid at the filling front can not fully gasify the foam, and the undecomposed residual substances can not float to the riser and solidify to form pores in the casting.
08
Poor deoxidation of molten steel, unclean slag removal in the furnace, furnace and package, too short sedation time, ineffective slag blocking during pouring, and unreasonable pouring process, resulting in slag holes.
09
The opening position of the in-gate is unreasonable, and a dead corner area is formed during filling. Due to the gas pressure in the cavity, gasification residues accumulate at the dead corner to form pores. The cross-sectional area of the in-gate is too large, so that the filling speed is greater than the foam gasification retreat speed, the foam is swallowed, and the gas is decomposed and gasified inside the alloy, and the gas cannot be discharged to form pores.
10
The capacity of the gate cup is too small, and the metal liquid forms a vortex and invades the air to generate pores.
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