Gray cast iron is one of the most widely used cast iron. Gray cast iron is a cast iron with a large amount of flake graphite dispersed in pearlite (or ferrite) matrix. When pouring, slow cooling can promote graphitization, and gray cast iron can be obtained. Because the fracture is often gray black, it is different from white cast iron. Gray cast iron is generally classified as brittle material, but there is still one A material that absorbs the deformation work of external force and exhibits a certain toughness. Its carbon content is generally 2.8% ~ 4.0%. Because of its good pouring performance, it is widely used in castings with complex structure, and even for pouring pressure vessels (such as papermaking dryer).
Ordinary gray cast iron is mainly cast iron with carbon precipitated in the form of flake graphite. Gray cast iron is widely used. In the total output of all kinds of cast iron, gray cast iron accounts for more than 80%.
The composition range of gray cast iron is 2.5% ~ 4.0% C, 1.0% ~ 3.0% Si, 0.25% ~ 1.0% Mn, 0.05% ~ 0.50% P and 0.02% ~ 0.20% s. When the liquid iron with the above composition range is cooled and solidified slowly, graphitization will occur and flake graphite will be precipitated. The appearance of the fracture is dark gray, so it is called gray cast iron.
The structure of ordinary gray cast iron is composed of flake graphite and steel matrix. Under the optical microscope, the graphite is discontinuous flake, straight or curved. The matrix can be divided into ferrite, ferrite + pearlite and pearlite. The inoculated gray cast iron (HT250, HT300, ht350) has the microstructure of small flake graphite distributed on the fine pearlite matrix because the number of graphite nuclei increases and the graphite flakes become thinner during crystallization.
The application of gray cast iron is mainly determined by tensile strength. It is used for various castings with complex shapes. When selecting cast iron grade, the wall thickness and corresponding strength value of the casting must be considered. For example, the wall thickness of a casting is 40mm, and the tensile strength value is required to be 200MPa. At this time, HT250 should be selected instead of HT200.