Gray cast iron is a type of cast iron. Carbon is present in cast iron in the form of flake graphite. The fracture is grey. It has good casting and cutting performance and good wear resistance. Used to manufacture racks, boxes, etc. The graphite of gray cast iron is flake, the effective bearing area is relatively small, and the graphite tip is prone to stress concentration, so the strength, plasticity and toughness of gray cast iron are lower than other cast irons. But it has excellent vibration damping, low notch sensitivity and high wear resistance.
Gray cast iron has a higher carbon content (2.7%~4.0%), which can be regarded as the matrix of carbon steel plus flake graphite. According to the different matrix structure, gray cast iron is divided into three categories: ferrite matrix gray cast iron; pearlite-ferrite matrix gray cast iron; pearlite matrix gray cast iron.
Ferritic gray cast iron is a ferrite matrix with many and coarse graphite flakes distributed on it, its strength and hardness are poor, and it is rarely used;
Pearlitic gray cast iron is distributed on the pearlite matrix with uniform and fine graphite flakes. Its strength and hardness are relatively high, and it is often used to manufacture important parts such as bed and machine body;
Pearlite-ferritic gray cast iron is a mixture of pearlite and ferrite, with relatively coarse graphite flakes distributed. Although the strength and hardness of this cast iron are lower than the former, it can still meet the general requirements of the machine body. Castability, shock absorption are good, and easy to smelt, is the most widely used gray cast iron.
The difference in the microstructure of gray cast iron is essentially the difference in the form of carbon in the cast iron. The carbon in gray cast iron is composed of combined carbon (Fe3C) and graphitic carbon. When the combined carbon is 0.8%, it belongs to pearlitic gray cast iron; when the combined carbon is less than 0.8%, it belongs to pearlitic-ferritic gray cast iron; when all the carbon exists in the state of graphite, it is ferritic gray cast iron.
The mechanical properties of gray cast iron are related to the structure of the matrix and the morphology of graphite. The flake graphite in gray cast iron seriously splits the matrix, and it is easy to cause stress concentration at the sharp corners of graphite, so that the tensile strength, plasticity and toughness of gray cast iron are much lower than those of steel, but the compressive strength is comparable to steel, and it is also commonly used cast iron. Cast iron with the worst mechanical properties. At the same time, the matrix structure also has a certain influence on the mechanical properties of gray cast iron. The graphite flakes of ferritic matrix gray cast iron are coarse and have the lowest strength and hardness, so they are rarely used; The strength and hardness are mainly used to manufacture more important castings; the graphite flakes of ferritic-pearlitic gray cast iron are slightly thicker than those of pearlitic gray cast iron, and the performance is not as good as that of pearlitic gray cast iron. Therefore, grey cast iron with pearlite matrix is more commonly used in industry.
