Poor Nodularization:
Poor nodularization refers to the failure of the nodularization treatment to achieve the expected nodularization result. The metallographic structure of poor nodularization consists of concentrated thick flake graphite and a small amount of spheroidal and nodular graphite; sometimes, there is also seaweed-like graphite. As the degree of poor nodularization worsens, the number and area of the concentrated thick flake graphite gradually increase; poor nodularization will cause the mechanical properties of ductile iron castings to fail to meet the requirements of the corresponding grade.
The causes of poor nodularization and their prevention measures are as follows:
1. High Sulfur Content in the Original Iron Molten Iron
Sulfur is a major anti-nodularization element. High sulfur content will seriously affect nodularization. Generally, the mass fraction of sulfur in the original iron molten iron should be less than or equal to 0.06%.
To ensure nodularization, when the sulfur content of the original iron molten iron is high, the amount of nodularizing agent added must be increased accordingly. The higher the sulfur content, the more nodularizing agent is consumed.
2. Low Residual Spheroidizing Elements
For good graphite spheroidization, ductile iron must contain a certain amount of residual magnesium and rare earth elements. Under current production conditions in my country, the mass fraction of residual magnesium must not be less than 0.03%, and the mass fraction of residual rare earth elements must not be less than 0.02%.
3. Iron Oxidation
Rust and contaminants in raw materials, as well as oxidation of molten iron during melting and overheating, increase the FeO content in the molten iron. This consumes more magnesium during spheroidization, resulting in excessively low residual magnesium content.
4. Furnace Charge Contains Anti-Spheroidizing Elements
When anti-spheroidizing elements exceed the allowable range, they will affect the spheroidization effect. Attention should be paid to the possibility of titanium in scrap steel, and the inclusion of electroplating materials, aluminum shavings, and lead-based furnace charges in the furnace charge.
5. Poor Inoculation Effect
Poor inoculation effect or inoculation decline will result in a small number of graphite spheroids, making them non-spherical.
6. High Moisture and Sulfur Content in Molding Sand
Due to interfacial reactions, magnesium in the molten iron reacts with oxygen and sulfur on the mold surface, resulting in insufficient residual magnesium on the casting surface and the formation of a thin layer of flake graphite. The solution is to increase the residual magnesium content and reduce the moisture content of the molding sand; the sulfur content of the molding sand should be less than 0.1% by mass, or a coating that can achieve a reducing atmosphere can be used. In resin sand molds using sulfur-containing curing agents, coatings containing MgO and CaO can be used.
Spheroidization Fading
Spheroidization fading is characterized by good spheroidization before the furnace but poor spheroidization on the casting; or, in the same ladle, castings poured earlier show good spheroidization, while those poured later show poor spheroidization.
The cause of spheroidization fading is the decrease in magnesium and rare earth elements as the molten iron remains in the furnace. Iron and rare earth elements have a greater affinity for oxygen than for sulfur, so MeS and Ce2S3 inclusions floating on the molten iron surface react with oxygen in the air. The generated sulfur re-enters the molten iron, reacting with magnesium and rare earth elements.
Thus, as the molten iron remains in the slag longer, sulfur continuously reacts with magnesium and rare earth elements, continuously generating MgS and Ce2S3, which are then continuously oxidized by oxygen in the air, creating a cycle. As a result, the magnesium and rare earth elements in the molten iron are consumed, and sulfur re-enters the molten iron from the slag, resulting in the "sulfur re-entry phenomenon."
Rare earth elements cerium and yttrium have higher boiling points than magnesium, and they do not vaporize and escape at normal molten iron temperatures. Furthermore, the sulfides and oxides of rare earth elements cerium and yttrium have high melting points and high densities, resulting in slow buoyancy. Therefore, the decay rate of rare earth elements cerium and yttrium is smaller than that of magnesium. At 1350~1400°C, the decay rate of magnesium mass fraction is (0.001~0.004)%/min, the decay rate of light rare earth cerium mass fraction is (0.0006~0.002)%/min, and the decay rate of heavy rare earth yttrium mass fraction is 0.0008%/min. The decay rate of various spheroidizing elements is closely related to the sulfur content in the molten iron; the higher the sulfur content, the faster the decay rate.
Measures to reduce spheroidization decay are listed below:
(1) Shorten the molten iron's dwell time
The process from spheroidization treatment to casting should be completed within 15 minutes.
(2) Reduce the sulfur content of the original molten iron
A high sulfur content in the original molten iron requires more spheroidizing elements. Furthermore, a high sulfur content in the original molten iron also increases the sulfide content in the slag, exacerbating the "sulfur reversion phenomenon" and accelerating spheroidization decay.
(3) Strengthen Covering and Slag Removal
After spheroidizing treatment, covering with a slag-collecting agent (e.g., perlite) and repeatedly removing slag can reduce the "sulfur reversion phenomenon."
(4) Appropriately Increase the Dosage of Spheroidizing Agent
Increasing the dosage of spheroidizing agent according to the sulfur content in the molten iron is effective, but not optimal. The fundamental solution is to minimize the sulfur content in the molten iron. Furthermore, excessive addition of spheroidizing agent not only increases costs but also leads to a deterioration in the degree of graphite spheroidization.
Post time: Oct-31-2025