How should these defects of ductile iron castings be solved?

(1) Control the residual aluminum content

The hazardous residual aluminum content of wet ductile iron castings is 0.03%-0.05%. At this time, subcutaneous porosity will occur. When it is less than 0.03%, it generally will not occur. Under the premise of not affecting the metallographic structure, adding more than 0.2% of aluminum before casting can eliminate subcutaneous pores. However, the aluminium in cast iron mainly comes from inoculants. For wet-cast ductile iron castings, when too much ferrosilicon inoculant is added to the molten iron, it is one of the reasons for the formation of subcutaneous pores in the castings.

(2) Control the amount of titanium

Both residual aluminum and residual titanium are present in cast iron. Excessive residual titanium can cause severe subcutaneous porosity in cast iron. When the residual aluminum content in ductile iron castings is less than 0.03%, subcutaneous porosity generally does not occur. However, if the residual titanium content exceeds 0.01% at this time, subcutaneous porosity will occur. Under the combined action of titanium and aluminum, the reduction of water vapor at the interface is intensified, resulting in a higher hydrogen content in the molten iron at the interface and making it easier to form subcutaneous pores. Residual titanium mainly comes from the pig iron ingots of the smelting furnace. Attention should be paid to the titanium content of the pig iron ingots. Those with high and low titanium content should be used in combination to control the titanium content

(3) Reduce sulfur content

From the perspective of preventing subcutaneous porosity in wet ductile iron castings, sulfur is a harmful element. When the sulfur content exceeds 0.094%, subcutaneous pores are prone to occur. The higher the sulfur content, the more severe the defect situation. In addition, the H2S produced may make the defect even more severe. The oxide and sulfide slag produced after spheroidizing treatment should be cleaned up thoroughly. Otherwise, the generation of H2S gas at the interface will also form subcutaneous pores, and the graphite spheroidization around such subcutaneous pores is poor.

(4) Add rare earth elements

Adding rare earth elements can deoxidize and desulfurize, increase the surface tension of the liquid cast iron, and effectively prevent the formation of subcutaneous pores. Ductile iron generally uses rare earth magnesium silicon iron alloy as the spheroidizing agent. Before casting, 0.002% (20ppm) of tellurium (Te) is added to the molten iron. A very small amount can achieve a significant effect.

(5) Control the pouring temperature

To prevent the formation of subcutaneous pores in ductile iron castings, the correct pouring temperature should be selected to avoid falling within the dangerous pouring temperature range (dangerous pouring temperature: 1310℃-1250℃).

(6) Control the original hydrogen content of molten iron. Practical experience shows that when the hydrogen content of molten iron drops to 4-5ppm, subcutaneous pores are prone to occur. Generally, the original hydrogen content of molten iron should be controlled below 2-2.5 PPM. Within 1.5-3.5 hours after the furnace is opened, the hydrogen content is relatively high and it is not suitable for pouring.