The chemical composition of nodular graphite casting not only directly affects the mechanical properties of nodular graphite casting, but also affects the process properties such as fluidity and shrinkage. For ferritic nodular iron castings with high requirements, the principle of “high carbon, low silicon, low manganese, low sulfur, low phosphorus, low rare earth and magnesium” shall be adopted to determine the composition.
1. C content
For nodular cast iron, the high carbon content is conducive to the precipitation of graphite, the roundness of graphite ball is increased and the ball diameter is small; Properly increasing the carbon equivalent is also conducive to reduce the shrinkage cavity and improve the density of nodular graphite castings. However, considering that too high carbon content may lead to graphite floating, the mass fraction of carbon is set as 3.60% ~ 3.70%
2. Si content
Si is a graphitized element. With the increase of silicon content, the tensile strength, yield strength and hardness of nodular cast iron increase, while the elongation and impact toughness decrease. In the production of ferritic nodular cast iron, the control of silicon content is particularly important. High silicon content will cause fragment graphite in large section nodular iron castings. Based on the comprehensive consideration of metallographic structure, strength and prevention of graphite distortion, the appropriate silicon content is determined, and the mass fraction of silicon is controlled at 2.20% ~ 2.40%
3. Carbon equivalent
The main measure to prevent graphite floating is to control the carbon equivalent. Combined with the experience of making heavy section ductile iron parts, the carbon equivalent is controlled at 4.40% ~ 4.60%
4. Mn content
Mn is an anti graphitization element, which promotes the formation of pearlite and cementite. The effect of manganese is similar to that of silicon, which can improve the tensile strength, yield strength and hardness of nodular cast iron, but the adverse effect on elongation is more serious than that of silicon. The increase of manganese increases the amount of pearlite, and it is dissolved in ferrite and strengthens ferrite. In addition, manganese is prone to micro segregation. For as cast high toughness ductile iron, manganese is an element with more disadvantages than advantages, and the manganese content should be controlled at a lower level as far as possible. The mass fraction of manganese was controlled below 0.3%.
5. P content
P is easy to segregate in nodular cast iron and forms phosphorus eutectic hard brittle phase at grain boundary, which has a very bad effect on the mechanical properties of nodular cast iron, especially plasticity and impact toughness. Therefore, when producing ferritic nodular cast iron, we always hope that the lower the phosphorus, the better. However, due to the limitation of phosphorus content in pig iron, it is unrealistic to control too low phosphorus. Using high purity pig iron, the mass fraction of phosphorus can be controlled below 0.03%.
6. S content
S is the most important anti spheroidizing element in nodular cast iron. It is an impurity harmful element. Of course, the lower the better. It should also be noted that a lower sulfur content is not only conducive to the spheroidization reaction and maintaining the spheroidization effect, but also beneficial to reduce the “sulfur recovery phenomenon” and slag inclusion. This is worthy of attention for nodular castings requiring ultrasonic flaw detection and magnetic particle flaw detection.
7. RE content
Re is an anti graphitization element and easy to segregate. Excessive residual rare earth may lead to the formation of special-shaped graphite, which is worthy of attention in thick and large section nodular cast iron. High purity pig iron with low trace elements must be used for nodular cast iron with high production requirements, so it is not necessary to require a high amount of residual rare earth. Therefore, the mass fraction of residual rare earth should be strictly controlled below 0.02%.
8. Mg content
MG is the main spheroidizing element. Sufficient graphite balls are required for the production of thick and large section ductile iron castings. With the increase of wall thickness and solidification time of ductile iron castings, the residual magnesium in molten iron will be consumed gradually. The number of graphite balls increases with the increase of residual magnesium, but when the residual magnesium exceeds a certain value, the number of graphite balls does not increase but decreases. In addition, higher residual magnesium content will also affect the roundness of graphite and increase the shrinkage tendency and slag inclusion. Therefore, on the premise of ensuring graphite spheroidization, it is more appropriate to control the middle and lower limit of residual magnesium, and control the residual magnesium at 0.030% ~ 0.050%
As mentioned above, the control range of chemical composition is shown in Table 1.
