The key to improve the mechanical properties of gray cast iron is to improve the shape, quantity, size and distribution of graphite flakes in gray cast iron. The fewer, finer and more uniform graphite flakes, the higher the mechanical properties of cast iron. Inoculation treatment is used to improve the properties of cast iron in production. Inoculation treatment is to add a small amount of inoculant (such as ferrosilicon) to the molten iron for inoculation treatment before pouring, so as to produce a large number of artificial nuclei in the solidification process of molten iron, so as to promote the nucleation and crystallization of graphite, so as to obtain a small amount of fine and uniform graphite flake structure distributed on the fine pearlite matrix. The inoculated cast iron is called inoculated cast iron or modified cast iron. Its strength, plasticity and toughness are higher than ordinary gray cast iron. Therefore, it is often used as more important parts such as cylinder, crankshaft and camshaft.
The matrix structure of cast iron is an important factor to determine its mechanical properties. Cast iron can strengthen the matrix by alloying and heat treatment to further improve the mechanical properties of cast iron, which is particularly important in nodular cast iron. However, heat treatment can not change the morphology and distribution of graphite in gray cast iron, so the effect of heat treatment to improve the properties of gray cast iron is not great, usually only annealing or surface quenching.
(1) Stress relief annealing. During the cooling process of the casting, due to the different shrinkage and microstructure transformation speed of each part, different degrees of internal stress are generated in the casting, which may lead to warpage and crack of the casting. In order to ensure dimensional stability and prevent deformation and cracking, stress relief annealing is often carried out for some castings with complex shapes, such as machine bed and cylinder. Its specification is generally: heating temperature is 500 ~ 550 ℃, after holding for a certain time, the furnace is cooled to 150 ~ 220 ℃ and discharged for air cooling.
(2) High temperature annealing. When the casting is cooled, because the surface and thin section are easy to form white structure due to the fast cooling speed, the hardness is high, and it is difficult to cut. In order to decompose the free cementite, reduce the hardness and improve the machinability, it is necessary to heat the casting to 850 ~ 950 ℃, after holding for 2 ~ 5h, cool it to 600 ℃ with the furnace, and then discharge it for air cooling. The final structure is ferrite or ferrite + pearlite based gray cast iron.
(3) Surface quenching. The working surface of some large castings needs high hardness and wear resistance, such as the surface of machine tool guide rail and the inner wall of internal combustion engine cylinder liner. After machining, the cast iron surface can be quenched by rapid heating.