Prevention of subcutaneous blowholes in ductile iron castings by four processes

The differential case is a mature product. The cast casting material is QT600-3. It is molded with damp film sand and has a small amount of coated sand hot core. The comprehensive rejection rate remains below 2%. However, in two consecutive shifts of products, up to 70% of the waste parts caused by air holes suddenly appeared. After dissection, it is found that the air holes are mainly distributed at the edge of the casting, with a diameter of 1~3mm, spherical, and an anatomical depth of about 10mm, Solve the problem.

1. Defect analysis and solutions

According to the appearance of stomata, we can judge it as subcutaneous stomata and divide it into two types: one is the internal shiny subcutaneous stomata, and the other is the internal blackened slag stomata. As we know, the pores formed by the reaction between molten metal and mold (including molding sand, sand core, coating, cold iron), molten metal and slag, or some elements and compounds in molten metal are called reaction pores. These pores are generally located under the casting skin, called pin holes or subcutaneous pores. During the molding of ductile iron with damp film sand, the gases in the pores are H2S, H2, magnesium vapor, etc, This is the reason for the formation of the first kind of stomata; The second kind of slag porosity is caused by the oxidation of molten metal. Based on the above analysis, we have made a comprehensive analysis of smelting and pouring, core making, sand treatment, molding and other processes, and formulated corresponding solutions to the problems found.

2. Smelting and pouring process

(1) Reduce the amount of nodulizer, reduce the nodulizer from 1.7% to 1.5% of molten iron, and strictly control the Mg content, which must be kept at wMg<0.05%.

(2) The pouring ladle and inoculation ladle shall be strictly dried, and the spheroidizing agent and inoculant shall be properly dried to ensure their dryness to the greatest extent.

(3) The silicon barium inoculant is used to replace the existing silicon strontium inoculant, and the addition amount remains unchanged.

(4) The pouring temperature was raised from 1360~1370 ℃ to 1380~1390 ℃ by raising the pouring temperature.

(5) Speed up tapping and ladle inversion, and minimize the possibility of oxidation of molten iron.

3. Core making process

(1) The sand core of this product has only one hot core with a weight of about 4kg. We mainly measure the gas generation of the hot core and control it below 16mL/g. In addition, the gas generation of the coating is required to be controlled below 30mL/g to reduce the gas generation of the sand core.

(2) Drill the vent hole at the core head of the sand core, and control the depth of the vent hole to be about 1/2 of the length of the core head. At the same time, pay attention to check whether there is paint accumulation or air vent blockage in the sand core after dip coating.

4. Sand treatment process

(1) In order to reduce the chemical reaction on the interface between the molten metal and the mold, we have appropriately increased the coal powder content in the molding sand to ensure that the cavity is filled with reducing gas. The basis for determining whether the reducing gas atmosphere is established is that the loss on ignition of the molding sand should be greater than the water content of the molding sand.

(2) On the premise that the effective bentonite content of molding sand is about 7.5%, the compaction rate of molding sand shall be appropriately reduced to reduce the moisture content of molding sand, reduce the air content of the casting surface and reduce the probability of subcutaneous porosity. In addition, the AFS of used sand is calculated to be 48~50, so 1%~2% scrub sand with particle size of 0.224~0.106mm (70/140 mesh) is added for each grinding to improve the particle size composition of molding sand.

5. Molding process

(1) On the premise of ensuring that the hardness of the horizontal surface of the sand mold is>90 and the elevation is>85 (detected by the B-type hardness tester), the compaction specific pressure of the upper box is reduced to further improve the exhaust efficiency.

(2) The Baume degree of the paint shall be strictly controlled at 37~39, and the spraying operation method shall be standardized. The paint shall be sprayed in place without piling up, so as to ensure the thickness of the paint layer.

6. Implementation effect

After the above measures have been taken, the defects of subcutaneous blowholes in the castings have been completely eliminated, and the product quality has been guaranteed. Through this process improvement, we found that if there is a little deviation in each link of production, it will lead to serious quality problems. This requires us to strictly control, operate carefully, and constantly improve the details in production to produce high-quality products.