Ductile casting still maintains nice casting properties, shock absorption, antifriction, low notch sensitivity, machinability and so on. However, compared with gray cast iron, there are some defects such as high shrinkage, poor fluidity and large tendency of white mouth, so the requirements for raw materials, smelting and casting process are also higher.
Ductile iron is widely used in machinery manufacturing, shipbuilding, metallurgy, chemical and other industries to produce important parts with complex forces and high performance requirements, such as replacing cast steel and forged steel, making diesel crankshafts, connecting rods and gears.
In ductile iron casting, since the graphite is spherical and has the smallest surface area, the splitting effect on the substrate is greatly reduced compared with the flake graphite, and the stress concentration phenomenon is also greatly reduced, thereby improving the bearing capacity of the substrate.
Therefore, the mechanical properties of ductile iron are much higher than that of gray cast iron, the yield strength ratio reached 0.7 to 0.8, especially the strength is close to that of steel. Plasticity and toughness, although much better than gray iron, are still worse than steel. In addition, the mechanical properties can be further improved by changing the matrix structure of nodular cast iron and the roundness, size and distribution of nodular graphite.
Heat treatment of ductile casting:
The heat treatment of ductile cast iron is similar to that of steel, but due to the high content of carbon and silicon and the presence of graphite, the heat treatment temperature is slightly higher, the holding time is longer, and the heating speed and cooling speed are slower.
The annealing of ductile iron is to obtain ferrite matrix and improve ductility and toughness.
The normalizing of nodular cast iron is to increase the number of pearlite in the matrix structure, refine the structure, and improve the strength and wear resistance of nodular cast iron. After normalizing, stress relief tempering is often used.
The purpose of quenching and tempering treatment of ductile iron is to obtain tempered sorbite matrix and obtain high comprehensive mechanical properties. For the process, quenching heating temperature of 860 ~ 900 ℃, oil cooling, 550 ~ 600 ℃ tempering.
4. Isothermal quenching
The purpose of isothermal quenching of ductile iron is to obtain lower bainite matrix with high hardness, high strength and good toughness. For its process, quenching temperature is 850 ~ 900 ℃, immediately after the insulation in the 250 ~ 350 ℃ in the salt bath isothermal 1 ~ 1.5 h, then take out for air cooling.
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