For the role of cold iron in the anti shrinkage process of castings, the sequential solidification theory and the equilibrium solidification theory are both attached great importance
(1) . place it at the place where it is difficult to make up the shrinkage to prevent shrinkage defects;
(2) . place it at the cross position of wall thickness and the position with large change to avoid cracks at this position;
(3) . it can be used together with the riser to strengthen the feeding capacity of the riser and reduce the size of the riser;
(4) Eliminate the hot spot at some positions to reduce or even avoid the deformation of the casting
In addition, place the casting in a position with mechanical property requirements to improve the surface hardness and wear resistance of the position, such as the guide surface and assembly contact surface of some large castings. The theory of equilibrium solidification also suggests that chill can enhance the self feeding effect of the whole casting. The following will discuss the process design for the size and location selection of chill.
Classification and material of cold iron
Cold iron is generally divided into external cold iron and internal cold iron. Although the quenching effect of internal cooling iron is better than that of external cooling iron, and it can effectively prevent the component segregation in the parts with large wall thickness of castings, the technical requirements of internal cooling iron are high and casting defects are easy to occur. Therefore, the internal chill can only be used under the condition of limited effect of external chill. External chill can also be divided into bright chill and dark chill. The surface of the open cooling iron is in direct contact with the surface of the die, and its chilling effect is good, but it is easy to cause color difference and crack, white mouth layer and undercooled graphite layer at the contact position of the casting and the cold iron. However, the surface of dark cast iron is generally separated by a sand layer of 10-15mm. Although the chilling effect of dark cast iron is worse than that of bright cast iron, the surface quality of contact surface of casting is better and the processing performance is better.
Generally, the shape of external cooling iron is long square with uniform wall thickness, and part of it is arc-shaped for convenient contact with arc-shaped chilling position. The material of external cooling iron is mainly cast iron and carbon steel which are different from the casting material. In recent years, graphite chill has also been widely used. Compared with ordinary cast iron and carbon steel chill, it has the following advantages:
(1) The weight of graphite cold iron of the same volume is 1 / 4 of that of common cold iron, which is easy to shape and operate;
(2) . if the soldering point is dimple, no coating is needed;
(3) , good heat conduction and storage performance, the contact surface of casting is not easy to stick cold iron;
(4) , the coefficient of thermal expansion is less, and the tendency of casting deformation is relatively small;
(5) , the feeding performance is better than that of common cold iron, and the improvement of mechanical properties of casting surface is relatively high. Because of the above advantages, the application of graphite chill is more and more extensive with the higher requirements of the market for the surface quality of castings.
Internal cooling iron is generally required to be welded with the casting, so its material is generally the same or similar to the casting material. For iron castings, low carbon steel is often used as internal cooling iron.
Design and placement of cold iron
Because of the classification of the cold iron, the design and placement of the cold iron are also divided into two parts: the outer cold iron and the inner cold iron.
The size design of external chill is usually determined by experience method, and its design size is mainly the thickness of chill. Because the thickness of the cold iron is small, the chilling effect is poor, and it is easy to stick the cold iron; but when the thickness is large, the chilling effect is strong, and it is easy to crack the contact surface. Therefore, the thickness and size should be appropriate.
Generally, the thickness of cold iron is 0.7-1 times of the thickness of boss. However, for the general casting wall location, the cold iron placement is 0.5-0.7 times of the wall thickness.
As the function of cold iron says, the external cold iron is placed in the following positions:
(1) . the position where the riser is difficult to be fed, such as the position where shrinkage defects are easy to appear at the bottom and side;
(2) , and the larger wall thickness surface at the intersection of wall thickness and the position with larger change;
(3) , riser and casting feeding channel position;
(4) . hot spot position W prevents casting deformation;
(5) . the position where the casting has special mechanical property requirements and metallographic structure requirements. Generally, the external chill is directly in contact with the surface at the above position, except for the necessary sand consumption and chill clearance, to ensure the coverage of the above position.
The design principle for the size and quantity of internal cooling iron is to ensure that the chilling capacity of internal cooling iron is enough to control the solidification of casting, and at the same time, it is required to solder with the casting itself without affecting the strength of casting. Based on this principle, the diameter of internal cooling iron material is 12-16mm, and it is selected according to the size of hot spot. There are also requirements for the weight of internal cooling iron. For castings with higher quality requirements, the weight of internal cooling iron is 5% – 8% of the weight of parts to be quenched, while for castings with lower requirements, it is 8% – 10%.
At the same time, the following points should be paid attention to when using and placing the internal cooling iron:
(1) , for some castings with high temperature, high pressure and densification requirements, internal chill cannot be used;
(2) The internal cooling iron shall be placed to ensure that it cannot be exposed after processing;
(3) When the internal cooling iron is placed in the thick wall position, it shall be placed in the middle position as far as possible to ensure the chilling effect.