The chemical composition of gray cast iron consists of two parts: one is the basic composition, that is, five elements: C, Si, Mn, P and S; Second, a small amount of alloy elements such as Cu, Cr, Mo, Ni, Sb, Sn, etc. are added as required. Among the five elements, the content of C and Si plays a decisive role. Due to their different graphitization, they are usually represented by CE: CE ≈ w (c) + 1 / 3W (SI).
(1) CE must ensure that the requirements of tensile strength or hardness in cast iron grade are met.
(2) CE is the main decisive factor affecting the strength of gray cast iron. The higher the grade of gray cast iron, the higher the strength, and the lower the CE should be selected. Reducing CE to improve the strength of gray cast iron is the basic principle to improve the strength of gray cast iron.
(3) Under the same CE, different cooling rates have different intensities. With the increase of casting wall thickness, the casting strength decreases; With the decrease of wall thickness, the strength of castings increases. Therefore, the CE of thin-wall castings should be appropriately increased and that of thick-wall castings should be appropriately reduced.
(4) The decrease of CE improves the strength of gray cast iron, but deteriorates the casting and processing properties. Therefore, under the same strength, gray cast iron with high CE is better.
(1) Mn is an element that hinders graphitization and stabilizes pearlite. Generally, gray cast iron requires pearlite matrix structure, so the mass fraction of Mn is high. The amount of W (MN) in thin wall parts is 0.5% ~ 0.8%, and the amount of W (MN) in thick wall parts is 0.9% ~ 1.4%.
(2) Mn has great affinity with s and forms MNS or (Fe, Mn) s compounds with s, so Mn can partially counteract the anti graphitization effect of S, and the generated MNS is dispersed in molten iron in granular form, with a melting point above 1600 ℃, which can be used as a heterogeneous crystal core, which is conducive to the precipitation of graphite. Therefore, Mn can also indirectly promote graphitization.
(3) After neutralization of Mn and s in gray cast iron, excess Mn can stabilize pearlite. The amount of W (MN) necessary for neutralization of S is: when w (s) < 0.2%, w (MN) = w (s) × 1.73%+0.3%； When w (s) ≥ 0.2%, w (MN) = w (s) × 3.3%。
(1) S reduces the fluidity of gray cast iron, hinders graphitization and increases the tendency of white cast iron. When Mn in gray cast iron is low and S is high, s will not only form MNS with Mn, but also form Fe FES sulfur eutectic or fe-fe3c-fes ternary sulfur eutectic, which is distributed on the interface, which reduces the mechanical properties of gray cast iron and is prone to thermal cracking. Therefore, it is generally considered that s is a harmful element in gray cast iron, and w (s) ≤ 0.12%.
(2) S has a dual effect on graphitization. S itself hinders graphitization, but MNS formed with Mn can form a crystalline core and promote graphitization. Therefore, when smelting in induction furnace, s-increasing treatment must be carried out to make the amount of W (s) not less than 0.06%; During cupola smelting, the amount of W (s) in molten iron is generally 0.06% ~ 0.10%.
(1) P of gray cast iron often exists in the form of binary phosphorus eutectic (Fe + fe3p) or ternary phosphorus eutectic (Fe + fe3p + Fe3C). Phosphorus eutectic with low melting point is distributed at the interface, which reduces the mechanical properties, especially plasticity and toughness, and is easy to produce cracks. Therefore, the w (P) content of gray cast iron is generally less than 0.15%.
(2) For gray cast iron with wear resistance requirements, the amount of W (P) can be increased to 0.3% ~ 1.5%; For gray cast iron with compactness requirements, the amount of W (P) shall be less than 0.06%.
5. Alloy elements
In production practice, a small amount of alloying elements are often added in front of the furnace and matched with inoculation technology to improve the mechanical properties and service properties of gray cast iron. This low alloying measure can make the same molten iron produce castings of different grades and meet the requirements of castings of the same grade and different wall thickness. The mass fraction of alloying elements is generally less than 3%, and the mass fraction of trace alloys is generally less than 1%.
The addition of alloying elements has an important effect on the microstructure of gray cast iron: (1) promote the formation of pearlite; (2) Fine pearlite; (3) Strengthened ferrite; (4) Refined graphite; (5) The mechanical properties of castings are significantly improved.