What is Shrinkage Cavity?
Shrinkage Cavity refers to void defects caused by shrinkage. It happened when steel ingots or other castings solidify on the top of the casting during casting.
If the shrinkage cavities are not cleaned, wrinkles or holes are formed in the center of the section after the ingot is formed. There are often serious loose and oxygen-like accumulations in the vicinity. This will seriously affect the quality of the material and cause excessive deformation or fracture of the engineering components.
To prevent the casting from producing defects such as shrinkage Cavity. First of all, we should understand that the dynamic fluid shrinkage of the molten metal in the cavity after the molten metal flows into the cavity during casting.
Cast iron shrinkage three stages.
I. Liquid (body) shrinkage. During the pouring, the high-temperature molten iron flows into the cavity. It will be filled with the mold, and then the liquid starts to cool to solidify. The shrinkage at this stage is called liquid shrinkage. 6%。 In the liquid state, the temperature is reduced by 100 ° C, the volume shrinkage is 1.6%. For example, a cast iron of a certain composition has a solidification temperature of 1,250 ° C and a pouring temperature of 1,400 ° C. It will be lowered by 150 ° C when the solidification starts. And the volume shrinkage will be 2.4%. So, the higher the temperature of the molten iron, the greater the liquid shrinkage of the molten iron.
II. Solidification shrinkage. Also known as crystal shrinkage. The white iron begins to solidify until the solidification is completed. The contraction at this stage is called solidification contraction. During the solidification process, the molten iron not only changes from a liquid state to a solid state, but also precipitates graphite (graphitization) in the molten iron. The precipitation of graphite will cause the cast iron to expand in volume. Each of the gray cast iron precipitated 1% of graphite, and the volume expansion amount was 2%. Thus, the shrinkage of molten iron during solidification is determined by both the shrinkage of the metal matrix and the expansion of graphite precipitation.
According to the influence of chemical composition on the graphitization, it can be known that the higher the carbon and silicon content in the cast iron. The more the graphite precipitates and the smaller the solidification shrinkage. So, gray cast iron with high carbon and silicon content not only does not shrink during solidification, but also may cause volume expansion. The lower the carbon and silicon content in the molten iron, the greater the solidification shrinkage. Thus, the shrinkage of high grade cast iron is greater than that of low grade cast iron.
III. Solid state shrinkage. After the solidification of the molten iron, it becomes solid and the cast iron is cooled to normal temperature. The contraction at this stage is called solid state shrinkage. In the solid state shrinkage process, the castings are reduced from all sides. So solid shrinkage is one of the factors that determine the shrinkage of the casting.Butr, it has no effect on the formation of defects such as shrinkage, shrinkage and shrinkage of the casting.
It can be seen from the above that the shrinkage of each stage has different effects on the quality of the casting. The liquid shrinkage and solidification shrinkage determine the size distribution of the shrinkage, shrinkage and shrinkage of the casting. The solid shrinkage determines the size, internal stress, deformation and cracking of the casting.
Gray iron castings produce shrinkage holes, which can be divided into open shrinkage and Invisible shrinkage
1 Open shrinkage. After the casting, the casting is solidified and cooled, and the riser is peeled off to remove the riser, and the hole is exposed at the raised part of the casting or the flat riser portion. The naked eye is known as the shrinking hole.
2 Invisible shrinkage . The hole of the shrink hole is a hole hidden in the hot section of the casting. The naked eye is not noticeable. It must be mechanically machined or when the scrap iron is broken.
(l) Open shrinkage characteristics
In order to prevent the shrinkage or exhaust of the castings during the molding, it is common to provide a cylindrical riser, a bottle type riser and a crimping riser in the plane of the casting or the raised portion of the casting. Due to insufficient iron in the riser, when the riser is knocked out, the surface of the casting is exposed. That is, shrinkage holes. The edges of the holes are flat or have rounded corners and sharp corners. The inner wall of the hole is dark gray or shiny with oxidized color. The holes are irregular in shape and resemble a funnel. Sometimes there are small holes in the hole, which are more than 20mm deep.
(2) Invisible shrinkage characteristics
The Invisible shrinkage , also known as the inner shrinkage hole, is hidden in the hot section of the wall thickness of the casting. A single hole appears and is invisible to the naked eye. It can only be found during machining or after machining. The shape of the hole is similar to an irregular shape such as a pear shape, an elliptical shape, a circular shape, and a long flat shape. The inner wall of the hole is rough and has a dark gray or lustrous oxidized color.
1.The correct casting process and modeling method are the basis for ensuring the quality of the casting. If the casting process or the modeling method is wrong, the next few processes cannot be saved, resulting in the loss of the former work and the loss.
For this reason, in the preparation of the casting process and the modeling method, it is necessary to follow the rules of each process of casting. For example, pouring metallurgy by pouring workers. Before pouring molten iron, the nozzle should first be aligned with the gate cup. After the iron is poured into the pouring cup, the molten iron is gradually poured. The ladle moves anywhere and anytime to adjust the position of the gate cup. Slow down when pouring the mold. Wait until the molten iron rises smoothly from the riser to stop the bag and stop pouring, and the iron sinks in the riser stops. The sinking is the return of molten iron in the mold. When the molten iron is stopped, the molten iron is immediately replenished. The molten iron rises slowly from the riser and fills the riser. Sometimes it is filled in the riser to increase the temperature and static pressure of the molten iron in the riser. The riser has sufficient filling capacity to avoid defects such as shrinkage and shrinkage of the casting. The above is the law of casting. Also called the pouring operation procedure. Careful implementation of the pouring operation procedures can ensure the quality of the casting.
Do not ignore the quality of castings, violate the casting law, and cause defects. Such as shrinkage, shrinkage, shrinkage and porosity. For this reason, it is necessary to operate according to the casting rule. When the casting process or the modeling method is prepared. Or when the casting plane needs to be processed, the casting plane must be placed below and the larger the casting plane is placed below.
2. Pouring the structure, size, height and location of the gating system (riser)
The gating system (riser) is one of the components of the casting process and is the main passage in the human cavity during the pouring of molten iron. The correctness of the structure, size, height and location of the gating system (riser) directly affects the quality of the casting. In the preparation of the casting process or the selection of the modeling method, if the importance of the structure, size, height and location of the casting system (riser) is neglected, the casting system (riser) can be arbitrarily excavated or fixed. After the pouring, the molten metal will not obtain sufficient replenishment and static pressure of the molten metal during the solidification and shrinkage process. The castings are defective in shrinkage, shrinkage, and shrinkage, and are largely scrapped.
The production of 2-+90 diesel engine block castings, material HT200, cast castings, accidentally collapsed (collapse) on the protruding steps of the cylinder castings, a large number of scraps can not be solved in batches, resulting in tight production in the foundry Machine-free machine tools without cylinder castings are discontinued.