HT250 and HT200 gray cast iron are made of lost foam casting and clay sand casting to improve the tensile strength and hardness. It can be seen that the tensile strength and hardness of lost foam casting samples are lower than those of clay sand casting.
The properties of gray cast iron are affected by its chemical composition, nucleation conditions during solidification and cooling rate, which affect its properties by changing the metallographic structure of gray cast iron. Flake graphite in gray cast iron is equivalent to microcracks in the matrix. Because graphite is soft and brittle, its sharp corner cuts the metal matrix, which will not only cause stress concentration, but also destroy the continuity of the metal matrix. Therefore, it can be considered that the distribution, shape, quantity and size of graphite basically determine the mechanical properties of gray cast iron.
In lost foam casting, due to the use of dry sand modeling, the sand consumption is large and the cooling speed is slow; During negative pressure pouring, some nucleation particles in molten iron will move to the surface of the casting, which reduces the nucleation particles during solidification. The two effects lead to the reduction of the number of eutectic clusters per unit area, the coarseness of eutectic clusters, the weakening of graphite branching ability in eutectic clusters, and finally the coarse and long flake graphite is obtained. The longer and coarser the graphite is, the more obvious the splitting effect of graphite on the metal matrix, and the lower the strength and hardness of gray cast iron.
The matrix structure also has a great influence on the properties of gray cast iron. In lost foam casting, due to the slow solidification and cooling rate, the amount of ferrite in the matrix is more than that in clay sand casting under the same chemical composition of molten iron and the same pouring conditions, while the amount of pearlite is reduced. On the other hand, due to the slow cooling rate, the pearlite formed is coarse, which shows that the strength and hardness of lost foam casting are lower than those of clay sand casting.
It is worth noting that in the test, it is found that when HT250 and HT200 are cast with clay sand, the tensile strength is more than 250Mpa and 200MPa respectively, while when lost foam casting is used, the tensile strength is lower than 250Mpa and 200MPa, and its mechanical properties can not reach Φ Performance requirements of 30mm test bar. This is mainly because the cooling rate of the casting is slow, the flake graphite is long and thick, the amount of ferrite in the matrix increases and the amount of pearlite decreases. Therefore, when adopting lost foam casting process, in order to ensure the mechanical properties of castings, corresponding measures to improve the mechanical properties should be taken according to factors such as casting structure and wall thickness, such as appropriately adjusting the control range of chemical composition, adopting alloying strengthening, improving charge quality, strengthening inoculation treatment of molten iron, etc.