In order to reduce the occurrence of defects such as porosity, shrinkage porosity and heterogeneous liquid metal hedge, the general principle of dual liquid compound pouring is: pour carbon steel liquid with high melting point first, and then pour high chromium cast iron with low melting point.
Firstly, the pouring sequence of the two metal liquids was studied. It was found that it was difficult to obtain good composite castings in the process of pouring molten iron first. It was shown that the joint zone of the two materials was prone to slag inclusion, porosity and other casting defects, and the two metal liquids were easy to mix. There are two main reasons: first, the temperature of molten steel after pouring is much higher than the melting point of cast iron, which is easy to cause large-area mixing; Second, oxides of chromium, manganese, silicon and other elements are easy to form on the surface of cast iron. These oxides are relatively stable and difficult to be reduced in the composite process. Under the scouring of molten steel, defects such as slag inclusion and porosity are easy to form. In addition, the crystallization temperature range of iron is wider than that of steel, the melting point of steel is higher than that of iron, and the superheat of molten steel is generally smaller than that of iron, so the liquid flow time is shorter. The higher the temperature of molten steel, the faster the heat dissipation in the mold and the faster the dendrite precipitation. After pouring, the solidification layer of steel is continuously thickened with the decrease of temperature, and its speed to reach the center is faster than that of iron. Therefore, many scholars believe that during the horizontal pouring process of wear-resistant castings with double liquid and no diaphragm, it is appropriate to pour molten steel first and then molten iron.
For traditional casting, the above analysis should be correct, but for lost foam casting, cast iron should be poured first and then molten steel. First of all, in the whole lost foam casting process, the replacement of space is carried out around three parts: foam, reducing atmosphere and liquid metal. This reducing atmosphere can effectively inhibit the oxides of chromium, manganese, silicon and other elements formed on the surface of cast iron, and reduce the inclusion and porosity of oxides due to the scouring of liquid steel; Secondly, in the process of lost foam casting, because the forming speed of vaporized foam is lower than that of traditional casting, the high chromium cast iron poured first can burn most of the foam in advance, and the carbon steel poured later can fill the mold cavity at a faster speed, which is very beneficial to the liquid steel with short liquid flow time; Finally, according to the basic principle of casting process design, the parts with high strength and quality requirements should be placed at the bottom of the mold, and the structural defects and impurities will float on the riser at the top of the workpiece through bottom-up solidification. Therefore, the high chromium cast iron material layer as the working layer of the bimetal liner should be poured first.
Based on the above analysis, molten iron should be poured first and then molten steel should be poured in the horizontal pouring process of double liquid bimetal without diaphragm based on the lost foam process. It should be noted that in order to ensure the quality of interface bonding, the temperature gradient of steel and iron should be increased. The surface of high chromium cast iron should be cooled rapidly to form a crystalline layer near the interface, so as to avoid the impact mixing phenomenon.