1. CE
The mechanical properties of gray cast iron are affected not only by the basic elements in cast iron, but also by the alloy elements and trace elements. The effect of carbon and silicon on the mechanical properties and microstructure of gray cast iron is usually expressed by carbon equivalent CE. The hypoeutectic cast iron with carbon equivalent less than 4.3% selected in this paper has high strength, The size and quantity of graphite will decrease with the decrease of carbon equivalent. The determination of carbon equivalent should also be considered in combination with the relative mass fraction of carbon and silicon. The relative mass fraction of carbon and silicon is usually measured by silicon carbon ratio (mass fraction). When the carbon equivalent is high, the increase of silicon carbon ratio increases the volume of primary austenite, the dendrite tends to be refined, and the mechanical properties are improved accordingly. The mass fraction of carbon is 3.05% ~ 3.25%, and the mass fraction of silicon is 1.85% ~ 2.05%
2. Manganese and sulfur
Manganese and sulfur in gray cast iron will affect the mechanical properties to a certain extent. Manganese and sulfur are stable carbides and elements that hinder graphitization. They exist in molten iron at the same time and will combine into manganese sulfide and iron sulfide compounds, which are distributed in the matrix in granular form. The melting point of these compounds is above 1600 ℃, which can be used as effective graphitization nuclei. In order to ensure sufficient manganese combined with all sulfur, the minimum mass fraction of manganese in gray cast iron should be w (MN) = 1.7w (s) + (0.30% ~ 0.50%). An appropriate amount of manganese can refine pearlite and improve hardness and strength, but excessive manganese will lead to internal shrinkage and porosity of thick and large section of gray cast iron.
The mass fraction of manganese is 0.5% ~ 0.70%, and the mass fraction of sulfur is 0.10 ~ 0.12%
3. Molybdenum
The main purpose of adding molybdenum to gray cast iron is that it can play a beneficial role in the process of solid phase transformation. It diffuses slowly in austenite and inhibits the diffusion of carbon. Its role in promoting pearlite formation is limited, but the role of refining pearlite is strong. In most cases, molybdenum and elements promoting pearlite formation are added at the same time and used together. The mass fraction of molybdenum in this test is 0.45% ~ 0.65%
4. Chromium
About 85% of chromium in gray cast iron is dissolved in austenite during hot metal solidification. The chromium dissolved in austenite reduces its carbon activity and can stabilize austenite, which has the following effects on solid phase transformation: first, inhibit ferrite nucleation, reduce ferrite volume and promote pearlite formation; The second is to prolong the incubation period of pearlite and delay the eutectoid transformation; The third is to improve the undercooling of eutectoid transformation, which can refine pearlite. Generally, only 0.3% ~ 0.35% chromium is added to gray iron castings. If the mass fraction of chromium exceeds 0.35%, there may be a small amount of free cementite and increase the shrinkage porosity tendency of castings. When the mass fraction of chromium exceeds 0.5%, there may be more free cementite and affect the machining and cutting performance of castings. Cooling in the sand mold can generally avoid the formation of eutectic carbide, but in the thinner section, it needs to be determined according to the specific cooling rate. The mass fraction of chromium in the test is 0.25% ~ 0.35%
5. Nickel
Nickel can be dissolved into liquid and solid iron in any proportion. It does not form carbides with carbon in gray cast iron, but exists in ferrite and austenite. It has an effect on solid phase transformation, which can promote graphitization and inhibit the formation of carbides. The addition of nickel and chromium to gray cast iron at the same time can also prevent the precipitation of ferrite and offset the tendency of white cast iron caused by chromium. Nickel has many beneficial effects on the structure of gray cast iron, which can reduce the critical rate and temperature of austenite eutectoid transformation. By promoting the formation and refinement of pearlite in the thicker casting section, the mechanical properties of gray cast iron can be improved. By adding 1% Ni, the tensile strength of gray cast iron under as cast condition can be increased by 15 MPa ~ 20 MPa The mass fraction of nickel in this test is 0.5% ~ 0.7%
6. Copper
Solid solution copper atoms accumulate near the interface between austenite and graphite, which hinders the diffusion of carbon in austenite, prolongs the incubation period of eutectoid transformation and reduces the eutectoid transformation temperature, which is conducive to the refinement of pearlite, which is similar to nickel. The main function of adding copper to gray cast iron is to promote the formation of pearlite, reduce the amount of ferrite in thick wall castings, and effectively reduce the sensitivity of casting wall thickness to structural changes. The mass fraction of copper in this test is 0.8% ~ 1.0%
7. Antimony
Antimony is also an element that promotes the formation of pearlite. The appropriate mass fraction of antimony in gray cast iron is 0.015% ~ 0.025%. In this range, the graphite is refined, the pearlite layer spacing is reduced, and the strength and hardness of gray cast iron are improved. Pure antimony has a melting point of 631 ℃ and a boiling point of 1440 ℃. It is usually added in the molten iron ladle or in the tapping tank together with the inoculant. If the tapping temperature of molten iron is higher than the boiling point of antimony, the absorption rate of antimony will be significantly reduced. The mass fraction of antimony in the test is 0.01% ~ 0.02%.