According to the relationship between the elongation and Si content of nodular cast iron, as well as the internal control standard for the production of nodular cast iron by our company, after several discussions and studies by the project team, the chemical composition is determined according to the following principles.
1. CE value
Carbon equivalent has a great influence on the fluidity of nodular cast iron. Increasing carbon equivalent can improve the fluidity of nodular cast iron. When the carbon equivalent is 4.4% ~ 4.6%, the fluidity of molten iron is the best, which is conducive to casting forming and feeding. The CE value of nodular cast iron is generally controlled at 4.45% ~ 4.55%.
Si is an element that strongly promotes graphitization, promotes ferrite formation and reduces cementite production. According to the relationship between elongation and Si content ofafter fracture, it can be seen from Figure 2 that when the Si content exceeds 4.5%, the elongation decreases sharply, so the Si content is controlled at 3.3% ~ 4.3%.
Mn is an element that strongly stabilizes austenite, reduces the transformation temperature of austenite to pearlite, promotes the formation of pearlite, and is prone to segregation at the boundary of eutectic. Because the matrix of the material developed is an all ferrite matrix, the Mn content should be less than 0.2%.
P enters into nodular cast iron with metal charge, which does not affect spheroidization, but p is a harmful element. It dissolves in molten iron, reduces the eutectic carbon content of iron carbon alloy, and has a weak anti graphitization effect. Because nodular cast iron solidifies in the same way as porridge, P is easy to segregate and produce phosphorus eutectic. With the increase of casting wall thickness, the more serious the segregation is, the more phosphorus eutectic will increase at the hot spot. Due to the low melting point of phosphorus eutectic, it will eventually be distributed at the boundary of eutectic group, which will sharply deteriorate the mechanical properties of cast iron. The lower P should be, the better. However, due to the restriction of raw materials, the content of P ＜ 0.04% is sufficient.
Cu is also an element that strongly promotes the formation of pearlite and reduces the eutectoid temperature. When the Cu content in nodular cast iron exceeds a certain amount, a part of pearlite will be produced in the matrix. Because the matrix of the developed material is full ferrite matrix, the Cu content should be less than 0.1%.
S is harmful and indispensable in nodular cast iron, so its content should be reduced as much as possible. According to the internal control standard of the author company, in order to ensure a good spheroidization rate, the content of s should be controlled at 0.006% – 0.012%.
MG is a necessary element for the production of nodular cast iron, and its content directly affects the graphite morphology. As the developed material uses high silicon for solid solution strengthening treatment, the Si content in molten iron is high, but when the Si content is high, it is easy to lead to the appearance of broken graphite, thus affecting the graphite morphology. In order to obtain relatively round graphite balls, the Mg content should be controlled at 0.045% ~ 0.065%.