The matrix of qt400-18 is mainly ferrite. The chemical composition of qt400-18 is not specified in GB 1348-2013 ductile iron standard, so the chemical composition of qt400-18 material used in this project needs to be specified according to the needs of microstructure and properties. The chemical composition range of qt400-18 is analyzed and designed in this project.
For nodular cast iron, increasing the carbon content moderately will increase the precipitation of graphite, reduce the probability of cementite and shrinkage porosity; but if the carbon content is too high, the graphite will float, if the carbon content is too low, the precipitation of graphite will decrease, and the probability of shrinkage porosity will increase. Generally, w (c)% – is between 3.4% and 3.8%.
When the silicon content exceeds 2.74%, the tendency of producing fragmental graphite increases, and the fracture morphology changes from primary fracture to brittle fracture. In order to ensure the low-temperature properties of ductile iron, w (SI)%, 1.8% – 2.2%, is controlled.
Manganese has two functions in nodular cast iron: (1) it can stabilize pearlite and austenite; (2) it is easy to segregate and accumulate at grain boundary, which can promote the formation of carbides, increase the tendency of white cast iron and deteriorate graphite morphology. If carbides are distributed in the grain boundary network, the properties of ductile iron will be seriously deteriorated. For Ferritic Ductile Iron, w (MN)% ≤ 0.15%.
Phosphorus is easy to accumulate in the grain boundary and form phosphorus eutectic. One volume of phosphorus forms about 20 volumes of phosphorus eutectic. In addition, phosphorus can also change the fracture mode of castings and promote the formation of cold cracks. The phosphorus content increases from 0.08% to 0.1% ～ 0.12%, and the impact property at – 40 ° C decreases by more than half. In addition, it is not easy to dephosphorize in production, and the harmful effect of phosphorus can not be completely eliminated by heat treatment and inoculation. Therefore, to reduce the source of phosphorus from the source, w (P)% ≤ 0.003% is required.
Sulfur is an anti spheroidizing element and is a harmful impurity, so the less the sulfur content, the better. But the sulfur content in molten iron is too low, which is not conducive to nucleation. Therefore, a certain amount of sulfur is used in molten iron to increase the number of cores and reduce the tendency of shrinkage. The w (s)% < 0.015% of molten iron is required.
Antimony can promote the amount of pearlite, and is mostly used in the production of Pearlite Nodular Iron castings. Adding a small amount of antimony when using heavy rare earth nodularizing agent to treat molten iron can improve the roundness of graphite balls, increase the number of graphite balls, inhibit the formation of fragmented graphite, improve the microstructure uniformity, and has a significant effect on large section nodular iron castings. The abnormal graphite can be eliminated and the graphite can be refined when the bismuth content is in a certain range It can promote the formation of ferrite and is mostly used in Ferritic Ductile Iron Castings; titanium can promote the formation of abnormal graphite and increase the sensitivity of cross-section; lead can strongly interfere with spheroidizing elements and cause graphite distortion, but it can improve the graphite distortion of large cross-section ductile iron castings when it is used together with rare earth; tin can increase pearlite under appropriate conditions, similar to antimony However, adding trace tellurium to large section ductile iron can avoid the occurrence of graphite distortion.
Magnesium is a spheroidizing element, and the magnesium added to molten iron contains two parts: one part is spheroidizing and remains in molten iron, and its content is expressed by Mg residual (residual magnesium); the other part forms magnesium oxide slag without spheroidizing.
Rare earth elements are also spheroidizing elements, which can remove sulfur and oxygen, purify molten iron and eliminate interference elements. However, excessive addition will increase the tendency of white mouth. Rare earth is divided into light rare earth represented by cerium and heavy rare earth represented by yttrium. Yttrium based heavy rare earths have strong anti-aging and anti graphite deformation properties.