The Selection Principles for the Five Major Elements in Ductile Iron

The chemical composition of ductile iron primarily includes five common elements: carbon, sulfur, manganese, silicon, and phosphorus. For certain castings with specific requirements for microstructure and performance, it also contains small amounts of alloying elements. Unlike ordinary gray cast iron, to ensure the spheroidal shape of graphite, ductile iron must additionally include trace amounts of residual nodulizing elements.

1、 The selection principles for carbon and carbon equivalent: Carbon is the fundamental element in ductile iron, and a high carbon content promotes graphitization. Once the graphite has formed spherical shapes, its impact on mechanical properties is significantly reduced. Very Low, so the carbon in ductile iron Existing stock Generally higher, at 3.5% to 3.9% Between.

2、 The selection principle for silicon: Silicon is a strong graphitizing element. In ductile iron, silicon not only increases the amount of ferrite and effectively reduces the tendency toward white cast iron, but also enhances the roundness of graphite spheres. ， Refine the eutectic clusters. However, silicon enhances the ductility and ductile-to-brittle transition temperature of cast iron while reducing its impact toughness—thus, silicon Existing stock It should not be too high, especially when the cast iron contains manganese and phosphorus. Existing stock When levels are higher, stricter control of silicon is even more essential. Existing stock . The silicon in ductile cast iron Existing stock Generally, it is 1.4–3.0% ， After the carbon equivalent is selected, the principle of high-carbon, low-silicon inoculation reinforcement is generally adopted.

3、 The selection principle for manganese: Due to the sulfur in ductile iron Existing stock It's already very low, so there's no need for excessive manganese to neutralize sulfur. In ductile iron, manganese primarily serves to enhance the stability of pearlite and promote (Fe ， Mn)3C The formation of these carbides, which segregate at grain boundaries, significantly influences the toughness of ductile iron. Moreover, manganese can enhance both the toughness and the ductile-to-brittle transition temperature of ferritic ductile iron. ， Manganese Existing stock Each additional 0.1% , the ductile-to-brittle transition temperature will increase 10 to 12 °, So, manganese in ductile cast iron Existing stock The lower, the better. Even in pearlitic ductile iron, manganese Existing stock It also should not exceed 0.4 ~ 0.6% However, this excludes medium-manganese ductile iron and bainitic ductile iron, which are specifically designed to enhance wear resistance.

4、 The selection principle for phosphorus: Phosphorus is a harmful element. ， It has very low solubility in ductile iron. When its content is less than 0.05% At that time, it dissolves into the matrix with almost no impact on the mechanical properties. When Existing stock Greater than 0.05% At this point, phosphorus tends to segregate at the boundaries of eutectic colonies, forming binary, ternary, or complex phosphorus eutectics that reduce the toughness of cast iron. To improve the ductile-to-brittle transition temperature of phosphorus-containing cast iron and enhance its toughness, phosphorus Existing stock Each additional 0.01% , the ductile-to-brittle transition temperature increases 4 ~ 4.5 Degree Therefore, phosphorus in ductile iron Existing stock The lower, the better. Generally, it should be below 0.08% For important castings, phosphorus Existing stock Should be lower than 0.05% 。

5、 The selection principle for sulfur: Sulfur is an anti-spheroidizing element that exhibits a strong affinity for spheroidizing elements such as magnesium and rare earths. The presence of sulfur can deplete significant amounts of spheroidizing elements in the molten iron, leading to the formation of magnesium and rare earth sulfides. This, in turn, results in casting defects like slag inclusion and porosity. Sulfur in ductile iron Existing stock Generally required to be less than 0.06% 。

Currently, the spheroidizing elements used in industry are mainly magnesium and rare earths. ， Meanwhile, magnesium and rare earth elements can neutralize the effects of anti-spheroidizing elements like sulfur, enabling graphite to grow in a spherical shape. The residual amounts of magnesium and rare earths should be adjusted based on the levels of anti-spheroidizing elements such as sulfur present in the molten iron. Existing stock To determine ， Under the condition of qualified spheroidization, the residual amounts of magnesium and rare earth elements should be kept as low as possible. Moreover, excessively high levels of residual magnesium and rare earths can increase the tendency for molten iron to form open pores, while their segregation at grain boundaries may also adversely affect the mechanical properties of ductile iron.