Causes of shrinkage and shrinkage of castings
Causes of shrinkage and shrinkage of castings
1. Reasons for the structure of the castings Because the section of the casting is too thick, the shrinkage is poor and the shrinkage holes are formed. The wall thickness of the casting is not uniform, and shrinkage or shrinkage occurs at the hot section of the wall thickness. Since the diameter of the casting hole is too small, the sand core of the casting hole is heated by the high-temperature molten metal, and is in a high temperature state for a long time, which reduces the solidification speed of the metal on the surface of the casting hole, and at the same time, the sand core provides a channel for gas or atmospheric pressure, resulting in a hole wall. The shrinkage hole and the embroidered looseness are produced. The radius of the concave corner of the casting is too small, so that the heat transfer capacity of the sand at the sharp corner is reduced, the solidification speed at the concave corner is lowered, and the gas generated by the sharp corner is heated, the gas pressure is high, and the gas is evolved. It can penetrate into the unsolidified molten metal, causing the casting to produce air shrinkage holes.
2, the reasons for melting
The gas content of the liquid metal is too high, causing it to precipitate as bubbles during the cooling of the casting, preventing the adjacent liquid metal from flowing to the place for feeding, resulting in shrinkage or shrinkage.
When the carbon equivalent of the gray cast iron is too low, the precipitation of the eutectic graphite is reduced when the molten iron is solidified, the effect of the graphitization expansion is lowered, the solidification shrinkage is increased, and the fluidity of the molten iron is also lowered. It is recognized that the self-retracting ability of molten iron is reduced, so that the casting is prone to shrinkage or shrinkage.
When the phosphorus content or the sulfur content of the molten iron is high, phosphorus is an element that expands the solidification temperature range, and at the same time, a large amount of low-melting-point phosphorus eutectic is formed, and the shrinkage ability is reduced upon solidification. Sulfur is an element that hinders graphitization, and sulfur also reduces the fluidity of molten iron. At the same time, the molten iron is seriously oxidized, which also reduces the fluidity of the liquid metal, causing shrinkage or shrinkage of the casting.
When inoculation of cast iron or ductile iron is carried out with inoculant such as ferrosilicon before pouring, if it is poorly bred, it will cause a large amount of cementite to precipitate when the molten iron solidifies, thereby increasing the solidification shrinkage and causing shrinkage or shrinkage.
3. Reasons for process design
(1) Unreasonable design of the gating system When the design of the gating system conflicts with the solidification principle of the casting, it may cause shrinkage or shrinkage of the casting. The main performance is that the pouring position is not suitable, which is not conducive to sequential solidification, and the position and size of the ingate are not correct. For gray cast iron and nodular cast iron, if the inner gate is opened at the thick wall of the casting and the inner gate is thicker, the inner gate is in a liquid state for a long time after pouring. Under the action of graphitization expansion of the solidification of molten iron, the molten iron will flow back to the sprue through the ingate, thereby causing shrinkage and shrinkage of the casting.
(2) Irrational design of the riser The position, quantity, size and riser neck size of the riser fail to promote the sequential solidification of the casting, which may cause shrinkage and shrinkage of the casting. If the gas riser is not placed at the top of the dark riser, or if the cold iron is used improperly, it will cause shrinkage and shrinkage of the casting.
(3) Causes of sand and core sand The fire resistance and high temperature strength of the sand (core sand) are too low, and the amount of thermal deformation is too large. When the static pressure or the graphitization expansion force of the molten metal acts, the wall or the core wall moves. Increasing the amount of shrinkage actually required for the casting or creating a new heat knot at the expansion site results in shrinkage and shrinkage of the casting. This phenomenon is very sensitive to large and medium-sized castings. In addition, if the moisture content in the molding sand is too high, it will make
The thickness of the dry layer on the surface of the wall is reduced and the moisture in the water condensation zone is increased, and the range is expanded, so that the moving ability of the wall is increased, resulting in shrinkage and shrinkage.
(4) Pouring reasons The pouring temperature is too high, which increases the liquid shrinkage of the liquid metal; when it is too low, it reduces the feeding capacity of the riser, especially when using the bottom injection type casting system, the castings are often The lower part produces shrinkage cavities and shrinkage. When the riser is not filled or the large and medium-sized castings are not refilled with molten metal, this will reduce the feeding capacity of the riser and cause shrinkage or shrinkage of the casting.