The chemical composition of grey cast iron is composed of two parts: one is the basic composition, i.e. the five elements of C, Si, Mn, P, s; the other is to add a small amount of alloy elements, such as Cu, Cr, Mo, Ni, Sb, Sn, etc., as required [1-3]. C and Si contents play a decisive role in the five elements. Due to the different graphitization action of the two elements, CE is usually expressed as: CE ≈ w (c) + 1 / 3W (SI).
(1) CE must ensure that the requirements of tensile strength or hardness in cast iron brand are met.
(2) The level of CE is the main decisive factor to the strength of gray cast iron. The higher the grade of gray cast iron is, the higher the strength is, the lower 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 rate has different strength. With the increase of wall thickness, the strength of castings decreases, and with the decrease of wall thickness, the strength of castings increases. Therefore, the CE of thin wall castings should be increased and that of thick wall castings should be reduced.
(4) With the decrease of CE, the strength of gray cast iron is increased, but the casting and processing properties are deteriorated. Therefore, under the same strength, the gray cast iron with high CE is superior.
(1) Mn is the element that hinders the graphitization and the stabilization of pearlite. Generally, the pearlite matrix structure is required for gray cast iron. Therefore, the mass fraction of Mn is high. The content of thin-walled w (MN) is 0.5% – 0.8%, and that of thick-walled w (MN) is 0.9% – 1.4%.
(2) Mn has a large affinity with s and forms compounds of Mn s or (Fe, Mn) s with S. therefore, Mn can partially counteract the anti graphitization effect of S, and the generated MNS is dispersed in the molten iron in a granular form, with a melting point above 1600 ℃, which can be used as heterogeneous crystal nucleus, which is conducive to the precipitation of graphite. Therefore, Mn also indirectly promotes graphitization.
(3) After neutralization of Mn and s in grey 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 iron, hinders graphitization and increases the tendency of white cast iron. When the content of Mn in grey cast iron is low and S is high, in addition to forming MNS with Mn, s can also form the sulfur eutectic of Fe FES or the ternary sulfur eutectic of fe-fe3c-fes distributed on the interface, which reduces the mechanical properties of grey cast iron and is easy to produce hot cracking. Therefore, it is generally considered that s is a harmful element in grey cast iron, requiring w (s) < 0.12%.
(2) S has double effects on graphitization. S itself hinders graphitization, but with MNS, it can form crystal core and promote graphitization. Therefore, when smelting in induction furnace, it is necessary to increase s so that the w (s) content is not less than 0.06%; when smelting in cupola, the w (s) content in molten iron is generally 0.06% – 0.10%.
(1) P in gray cast iron often exists in the form of binary phosphorus eutectic (Fe + fe3p) or ternary phosphorus eutectic (Fe + fe3p + Fe3C). The P eutectic with low melting point distributes at the interface, which reduces the mechanical properties, especially the plasticity and toughness of the gray cast iron, and easily produces cracks. Therefore, the w (P) content of the gray cast iron is generally less than 0.15%.
(2) The w (P) content can be increased to 0.3% ～ 1.5% for gray cast iron with wear resistance requirements, and to 0.06% for gray cast iron with compactness requirements.
5. Alloy elements
In production practice, a small amount of alloy elements are often added in front of the furnace, which are combined with inoculation technology to improve the mechanical properties and service properties of gray cast iron. This kind of low alloying measure can make the same molten iron produce castings of different grades, and also 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 that of microalloys is generally less than 1%.
The addition of alloying elements has an important effect on the structure of gray cast iron: (1) promote the formation of pearlite; (2) refine pearlite; (3) strengthen ferrite; (4) refine graphite; (5) significantly improve the mechanical properties of castings.