Cast Iron Impact Performance

- Aug 07, 2017 -

Impact properties refer to the nature and ability of materials to resist fracture under impact force. This ability is measured by the magnitude of the energy absorbed by the material at fracture. A shock test is often used to measure the impact absorbed energy. This indicator describes the work consumed by the unit cross section or the total energy absorbed by the specimen when it breaks. These include the elastic work, the plastic work and the fracture work during crack propagation. The results of a single impact test can not accurately represent the magnitude of the fracture work, and the fracture work is the index that really reflects the impact toughness of the material. Therefore, impact absorption work is not generally used in product strength design. It is only used to compare the material brittleness or ductile brittle transition temperature, or as a large energy impact, that is the failure of the material performance indicators.

Because of the presence of graphite, the cast iron can not be used when the rapid deformation of gray iron occurs under the impact of the impact, so the energy absorbed is only the elastic energy needed to overcome the fracture stress. This indicates that the impact fracture of the grey iron is essentially the same as that resulting from continued tension or bending. The impact absorption work of gray iron is almost unaffected by temperature. The ductile brittle transition temperature has practical significance for gray cast iron. In addition, the fracture sensitivity of gray iron is low, and the notched test bar is usually used in impact test. The impact absorption work of nodular cast iron is much higher than that of gray iron. In terms of impact toughness, Ferritic Ductile iron is larger than Pearlite Nodular Cast iron. In the mixed matrix, the impact toughness increases with the decrease of pearlite content.  The pearlite content has a significant influence on the ductile brittle transition temperature of ductile iron, and the transformation temperature increases with the pearlite content increasing. 35% pearlite matrix ductile cast iron transition temperature is generally above room temperature. Therefore, ductile iron under low temperature shall be made of ferritic spheroidal graphite cast iron.

The impact fracture of most engineering components is significantly different from that of a single impact test. Fracture is usually caused by small energy, multiple shocks leading to crack nucleation and expansion, which is a process of damage accumulation. The mechanism and dominant factors of a single impact fracture and multiple impact fracture are different, and the impact resistance mainly depends on the strength of the impact of the material and the comprehensive properties of the plastic. Besides the effect of strength, the plastic work that can be absorbed by the fracture of material must be considered.

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