Cracks, Shrinkage & Loosening of Steel Sleeve
The inclusion and pore defects of centrifugal casting are much less than those of gravity casting. However, due to centrifugal force, cracks, shrinkage, pore and component segregation defects of centrifugal casting are outstanding.
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Common Defects and Causes
The inclusion and pore defects of centrifugal casting are much less than those of gravity casting.
But, due to centrifugal force, cracks, shrinkage, pore, and component segregation defects of centrifugal casting are outstanding.
Cracks in Centrifugal Casting Alloy Steel Sleeve
Type and morphology of cracks
Cracks in centrifugal casting alloy steel sleeve can be divided into longitudinal cracks and transverse cracks according to the direction of cracks.
A transverse crack is a radial crack. This defect often occurs in the supporting area of the carrier wheel, especially near the pouring end. Cracks are usually intermittent or continuous snake-shaped. Longitudinal cracks are cracks that occur. This defect often occurs in the area between the two-wheel supports and is usually in a continuous or intermittent spiral shape.
Kinetic analysis of crack generation
Under centrifugal casting conditions, a liquid cylinder is formed when molten steel is injected into the pipe mold. Due to the characteristics of the sequential solidification of molten steel and the high-temperature gradient of the outer layer of the cylinder, the outer surface solidifies into a thin solid shell.
Under the action of the centrifugal force field, the shell is subjected to centrifugal pressure as follows:
P = gamma 2 (R2-r2) / (2g) = gamma 2 (R +r) B/ (2g)
P is centrifugal pressure, Pa; gamma is the weight of alloy liquid, N/m3; _is angular velocity of rotation, _=pi.n/30, where n is casting speed, r/min; G is gravity acceleration, 9.81 m/s2; R is the outer diameter of cast pipe, m; R is the inner diameter of cast pipe, m; B is the wall thickness of cast pipe, M.
It can be seen from the formula that: (1) when the initial sequence of the solidification layer is not strong enough to withstand centrifugal pressure, the outer surface will crack to form longitudinal and transverse crack sources; (2) the thicker the cylinder wall, the larger the diameter, the greater the possibility of the cylinder cracking. In this case, the reason for the crack is that the G value is too large, and the crack propagation is due to the casting can not solidify and shrink after the initiation of the crack source under the long working environment of the centrifugal casting system.
shrinkage and loosening of centrifugal casting
shrinkage and loosening characteristics of centrifugal casting
Shrinkage holes are located on the last solidified surface of the centrifugal casting alloy steel sleeve, on the inner surface, and some also appear on the inner surface and coexist with the molten slag floating on the upper surface.
It is found in production that shrinkage holes are located on the surface of the casting end and middle section of the casting pipe; especially when the wall thickness and diameter of the casting pipe are large and the material contains elements that increase the shrinkage of the solidification point body, shrinkage holes are more likely to occur, and the depth and volume are also larger.
Causes of shrinkage and loosening
Under centrifugal casting conditions, except for the heat transfer system “liquid metal/casting”, there is convection heat transfer between the free surface of liquid metal (inner surface) and air and radiation heat transfer to the surrounding space. The heat transfer process is aggravated by strong convection “cyclones” in the mold at high speed of rotation, which causes the temperature of the free surface of the molten steel to be lower than the internal temperature and when the final solidification stage is reached, the internal surface temperature drops below the liquid line.
Due to the layer-by-layer solidification characteristics of the molten steel, there is still some incomplete solidification of the molten steel in the inner layer adjacent to the shell after the inner surface solidification. When this part of the molten steel solidifies and shrinks under the centrifugal force field, it can not get the necessary supplement. Thus, shrinkage and loosening defects occur in the final solidification part.