The production of nodular cast iron includes the selection of raw materials and the control of chemical composition, spheroidization, inoculation and pouring. Study and determine the relationship between metallographic structure, chemical composition and mechanical properties. Through reasonable melting process, the castings can obtain appropriate ferrite matrix structure, ensure good graphite morphology, spheroidization rate grade, reduce grain boundary inclusions, and obtain low-temperature impact toughness ductile iron castings.
The selection of raw materials and the control of chemical composition are very important for the production of low temperature ductile iron. In addition to strictly controlling the basic elements such as carbon, silicon, manganese, phosphorus and sulfur, special attention should be paid to the limit of the content of trace anti-spheroidizing elements and interfering elements in raw materials. In addition to being harmful to the shape of graphite, these interfering elements are also strong pearlite and carbide promoting elements, which segregate in the final solidification area and form intergranular carbides and inclusions. The existence of these carbides and inclusions will reduce the mechanical properties of nodular cast iron, especially the impact toughness and fatigue properties.
In the low temperature ferrite matrix nodular cast iron, the control of silicon is particularly critical. Silicon solid solution strengthens the ferrite, effectively improves the tensile strength, but at the same time significantly reduces the low-temperature impact performance. The addition of excessive silicon has a significant effect on the ductile-brittle transition temperature. Carbon is the direct source of graphite spheres, and the selection of carbon content is mainly considered from the perspective of carbon equivalent. Manganese has a serious positive segregation tendency, which promotes the formation of free cementite, thus reducing the low-temperature impact toughness. Its content should be controlled at a lower level as far as possible. Phosphorus can significantly increase the ductile-brittle transition temperature in low temperature ductile iron. Too much phosphorus can easily form hard and brittle phosphorus eutectic at the grain boundary. Therefore, reducing phosphorus content is one of the keys to produce low temperature ductile iron. Sulfur is easy to form sulfide with magnesium and rare earth in nodulizing agent, resulting in slag inclusion defects in ductile iron castings, which seriously reduces the mechanical properties and surface quality of ductile iron castings. Therefore, the chemical composition control of – 20 ℃ low temperature ductile iron is shown in the table.
| Element | C | Si | Mn | P | S | Mg |
| Content | 3.6~3.9 | 1.8~2.1 | ≤0.2 | ≤0.03 | ≤0.015 | 0.03~0.06 |