1.Mechanical property
1) Tensile strength
As the most common stress state, tensile stress is closely related to other static load mechanical properties of materials. At present, the grades specified in the standards of nodular cast iron in various countries are mostly based on the tensile strength index. The tensile strength and proportional limit of pearlitic nodular cast iron are higher than that of ferritic nodular cast iron, and there will be no obvious yield point under the action of tensile stress. Before fracture, the sample deforms evenly along the whole length without necking and low elongation at break; The yield limit of ferritic nodular cast iron appears earlier, there is obvious necking before fracture, some plastic deformation is retained after fracture, and the yield strength and tensile strength are lower than those of pearlitic nodular cast iron. The tensile strength and elongation of nodular cast iron with different matrix structures. Generally speaking, the material elongation decreases with the increase of tensile strength.
2) Hardness
The hardness of nodular cast iron depends on the matrix hardness. Alloy elements such as silicon, nickel, molybdenum and copper can also improve the hardness of ferrite and pearlite based nodular cast iron because they strengthen ferrite and pearlite. In addition, heat treatment is also very effective in improving the hardness of nodular cast iron. Generally speaking, the temperature has little effect on the hardness of nodular cast iron before 420 ℃, but the eutectoid structure begins to spheroidize after the temperature exceeds 540 ℃, and pearlite decomposes when the temperature reaches about 650 ℃, so the hardness of the casting decreases with the increase of temperature. Hardness of nodular cast iron with various matrix structures.
3) Impact toughness
Impact toughness not only reflects the energy absorbed by the material when it breaks under impact load, but also reflects the ability of the material to resist crack initiation, propagation and fracture under the condition of rapid deformation. Matrix structure type, chemical composition, spheroidization rate, heat treatment process and sample notch are the factors affecting the impact toughness of nodular cast iron. Under the same conditions, the impact toughness of ferrite nodular cast iron is higher than that of pearlite based nodular cast iron. The impact toughness of notchless ductile iron samples with different substrates at room temperature.
4) Fatigue limit
Generally, the maximum stress value that can withstand the two-part fracture of countless cycles of stress cycle under the action of alternating stress is used to identify the fatigue fracture resistance of materials, which is called fatigue limit. The fatigue fracture process of nodular cast iron is the same as that of other metal materials, including crack nucleation and propagation. In nodular cast iron, the stress is often concentrated in the front end of the cavity caused by the different deformation between the spherical graphite and its surrounding matrix. In addition, irregular graphite, inclusions and shrinkage defects are also possible locations for crack initiation. The crack propagation is related to the matrix type of nodular cast iron. The propagation rate is small in the early stage, and the metastable crack propagation changes to unstable propagation in the later stage. The crack will expand rapidly in a short time, resulting in material fracture [1]. Bending fatigue strength of nodular cast iron with various substrates.
2. Physical properties
1) Density
The volume fraction of graphite and matrix type determine the density of nodular cast iron. The graphite density is 2.25 g / cm3, while the matrix density of nodular cast iron is generally about 7.6 g / cm3. Therefore, the density of nodular cast iron decreases with the increase of graphite volume fraction. Increasing the elements that promote graphitization can reduce the density, while increasing the elements that hinder graphitization can increase the density.
2) Thermal conductivity
Thermal conductivity refers to the heat transfer capacity of a material, that is, the heat flowing through a unit area when maintaining a unit temperature gradient in a unit time. The thermal conductivity of cast iron depends on composition, microstructure, temperature and graphite morphology. The thermal conductivity of graphite is higher than that of the base structure, and the thermal conductivity of graphite along the base surface is higher than that along the edge surface. Therefore, the thermal conductivity of nodular cast iron is lower than that of gray cast iron, but higher than that of steel. The thermal conductivity of nodular cast iron at 100 ℃ is 30 ~ 38 w / (m ·℃), which decreases with the increase of temperature.