Causes And Preventive Measures for Slag Holes And Pores in Castings

Slag holes and shrinkage holes are very common defects in precision castings. The causes of these defects are multifaceted, mainly related to the smelting of molten steel, the cleanliness of recycled wax and the tree assembly process.

1.1 Molten steel smelting: 

Although it only takes 20-30 minutes to melt molten steel in a fast medium frequency furnace, the most critical quality control is in the few minutes of deoxidation and slag removal.

Deoxidizers are generally ferrosilicon, ferromanganese (or electrolytic manganese), and silicon-calcium-manganese alloy. The deoxidation ability of silicon is more than 10 times stronger than that of manganese, but the deoxidation ability of aluminum is 30 times stronger than that of silicon. The deoxidation products of silicon and aluminum are not easy to float out of the molten steel, but the deoxidation products of manganese are easy to float out of the molten steel. Therefore, electrolytic manganese must be added first and then ferrosilicon (or can be added at the same time). This will make it easy for the deoxidation products to float out of the molten steel. Before the final deoxidation, the slag should be white. If it is not white, deoxidation should be added. Japanese precision casting expert Yamaya Hiroki believes that the effective deoxidation time when using silicon-manganese deoxidation is 1-2 minutes. After the final deoxidation, it is necessary to turn off the power and let it stand for 1-3 minutes to allow as many deoxidation products as possible to float out of the molten steel.

If the power is not turned off during the precision casting smelting process, the molten steel will keep rolling like boiling water, which is called electromagnetic stirring. The surface of the molten steel is mainly composed of an oxide film composed of ferrous oxide (FeO) (so a covering agent must be added all the time). As the molten steel rolls, these oxygen pressure substances are drawn into the molten steel and react with the carbon in the molten steel at high temperature (FeO+C→Fe+C0), which will produce a large amount of gas. After pouring, as the temperature of the molten steel decreases, the gas precipitates from the molten steel. When the generated gas cannot be discharged, reactive pores are formed, which makes the casting surface form honeycomb pores, which are very similar to the invasive pores produced by wet shell pouring.

When deoxidation is poor, a large amount of ferrous oxide (FeO) in the molten steel will precipitate at the grain boundary during the solidification process of the casting, which will reduce the mechanical properties of the steel and make it more prone to thermal cracking.

In addition, there are some conventional requirements: the return material should be clean, oil-free, rust-free, and sand-free, ensure that the shell mold is burned through to improve the permeability, reduce the pouring speed, and tilt the mold shell for pouring to facilitate gas discharge.

1.2 The wax treatment process is a key link that is easy to overlook: 

American precision casting plants generally do not process the recycled wax after dewaxing, but let a special wax treatment factory process it. We basically process it ourselves. If the process is not strictly implemented or the management is not in place, a large amount of impurities will be mixed into the recycled wax after processing. Some impurities will not be burned and will form slag holes, and some hydrocarbons will form pores when in contact with high-temperature molten steel. Therefore, when checking the workshop every day, you should check whether the filter under the dewaxing frame is clean. If there are many impurities on the filter, you should pay attention to controlling the impurity content in the wax. Then look at the color of the pouring cup after dewaxing. Generally, white or light yellow indicates that it is basically normal. If dark yellow or even gray-black appears, it means that the wax material is already very dirty.

1.3. Reasonable tree assembly process is the key to eliminating shrinkage defects. 

The quality of the tree assembly process reflects the level of a factory's casting process. A good tree assembly process has no problem with high-temperature and low-temperature pouring, and the wax should be removed as cleanly as possible. If necessary, add wax strips to minimize the impact of impurities in the wax.

1.4. The shrinkage problem, especially the shrinkage problem of small parts, is closely related to the pouring temperature. 

For example, when I was making a pliers head casting in 2000, since the pliers head had to be matched at right angles, there were shrinkage holes in most corners after pouring. According to the textbooks and experience at the time, it was believed that the pouring temperature was too high. We lowered the pouring temperature. After careful comparison, we found that the shrinkage holes were more serious after the cooling pouring. We were very surprised. How could it be different from what was written in the university textbooks? Later, after analysis and more than half a year of experiments, we found that there were no shrinkage holes after the pouring temperature was increased.