Factors affecting the shrinkage and shrinkage of machine tool bed castings

In the production of machine tool castings, the principle of high temperature baking and low temperature pouring should be followed during injection. Because increasing the room temperature of the molten metal is conducive to the complete melting of impurities and the floating oil of the slag, it is convenient to remove the slag and remove the gas, and reduce the debris and pore defects of the machine castings. Using a lower pouring temperature helps to reduce the gas solubility of the molten metal, the amount of liquid shrinkage, and the bending of the hollow surface by the high-temperature molten metal, thereby preventing defects such as pores, point shooting, and shrinkage holes. Therefore, under the premise that the mold is filled, the lowest possible pouring temperature is used.

The process of injecting molten metal from the main gun into the mold is called mainstream. Improper pouring operation can lead to mechanical casting defects such as water shortage, cold shock, pores, shrinkage holes, and debris, as well as personal injury.

In order to ensure the quality of machine tool castings, improve productivity and safe production, the following operating instructions must be strictly followed:

(1) Before using coating, pouring tools, inoculants for pre-furnace treatment, spheroidizing agents, etc., they must be fully dried before use.

(2) The pouring personnel should wear work clothes and protective glasses as needed, and the workshop should be unobstructed. The molten metal in the bag should not be too full. When transporting and brushing, people should be prevented from overflowing.

(3) Correctly select the injection speed. That is, align the gate to reduce the impact of molten metal on the sand, and slowly inject at the beginning to help gas discharge. Then pour quickly to prevent cold partitions. Pour slowly before it is almost full. In other words, follow the principle of slow, fast, slow.

(4) For machine castings with large liquid shrinkage and solidification shrinkage (such as medium and large castings), they should be replenished on the gate or riser in time after pouring.

(5) When refueling, ignite the gas generated in the mold in time to avoid the formation of pores due to air holding in the mold. Due to incomplete combustion of the machine body, it will damage human health and pollute the air.

2. Factors affecting the shrinkage and shrinkage of machine tool bed castings

(1) Phosphorus content of machine tool bed casting components: high phosphorus content in molten iron expands the solidification range, and low melting point artificial melting cannot be popularized during the final solidification, which tends to weaken the casting shell, thereby increasing shrinkage holes and shrinkage. Generally, factories control the phosphorus content below 0.08%.

(2) Carbon equivalent of machine bed castings: it can increase the carbon content, increase graphitization expansion, and reduce shrinkage holes and shrinkage. In addition, increasing the carbon equivalent will also increase the fluidity of ductile iron, which helps shrinkage. The empirical formula for producing high-quality castings is C% 1/7Si% more than 39%. However, when increasing the carbon equivalent, other defects such as graphite floating should not be allowed to appear in the casting.

Quenching of machine tool castings

The most commonly used cooling media for quenching are brine, water and oil. Workpieces immersed in brine are easy to obtain high hardness and smooth surfaces, and it is not easy to generate soft spots that are not hard, but sometimes the workpiece will be severely deformed or cracked. Using oil as quenching medium is only suitable for quenching certain alloy steels or small carbon steel workpieces with relatively high supercooled austenite stability.