Elemental tin, ytterbium, etc., which promote pearlite generation, are given full attention

Higher carbon equivalents and increased silicon content tend to promote a ferrite matrix, which is beneficial for certain applications. However, excessive silicon can significantly reduce the material’s toughness. Manganese, on the other hand, encourages the formation of pearlite, carbides, and segregation, all of which can negatively impact the material’s ductility. Phosphorus tends to segregate during solidification, leading to the formation of phosphorus eutectic at grain boundaries. This is a serious issue that severely compromises the toughness of ferritic ductile iron. Sulfur has a detrimental effect on the spheroidization process, influencing the formation of graphite nodules and increasing the risk of inclusions. Therefore, it is crucial to strictly control the levels of manganese, phosphorus, and sulfur in the alloy.

The residual amounts of rare earth elements and magnesium should be carefully maintained within a lower limit range to ensure effective spheroidization. The selection of chemical composition must be tailored to the specific production conditions, with particular attention given to elements that influence spheroidization, such as tin and antimony, which can promote pearlitic structures. Consistent quality raw materials and fuels are essential for stable and high-quality casting of ductile iron pipes. The molten iron should meet specifications such as high carbon content, low silicon, and minimal levels of manganese, phosphorus, and sulfur, ensuring a stable base composition. Coke used in the process must provide sufficient heat value, mechanical strength, and low sulfur content to support the melting process effectively.

The type and quantity of spheroidizing agents play a critical role in the quality and consistency of as-cast ductile iron tubes, especially in controlling the tendency for white cast iron formation. In China, rare earth magnesium spheroidizing agents are commonly used, typically added during the spheroidizing process. The addition of rare earths helps purify the molten metal by removing sulfur, oxygen, and gases. Rare earth oxides and sulfides can be easily removed at the melting temperature of cast iron. These elements also form compounds with low-melting-point metals like arsenic, lead, zinc, and tin, which remain solid and do not dissolve into the molten iron. Additionally, rare earths counteract the negative effects of anti-graphitizing elements such as titanium and arsenic, reducing their harmful impact on graphitization and allowing for more flexibility in the composition of the original molten iron. This makes the use of rare earths a significant advantage in the production of high-quality ductile iron.