1.Interaction between rare earth and sulfur
Sulfur can cause “hot brittleness” of cast iron during smelting of gray cast iron and reduce the corrosion resistance, welding performance and mechanical properties of cast iron. However, when rare earth inoculates gray cast iron, the sulfur content must not be lower than a specific value, which is determined by the process of inoculating gray cast iron. In the process of inoculating gray cast iron with rare earth, rare earth can react with sulfur in gray cast iron to form rare earth sulfide, The rare earth sulfide in gaoguandian, as an external core, promotes the nucleation of graphite, thus promoting the graphitization of gray cast iron and protecting the inoculation process. Therefore, in the process of rare earth inoculation of gray cast iron, the sulfur content of gray cast iron must be ensured to ensure the inoculation effect. The research shows that the existence of sulfur in molten iron will reduce the spheroidizing ability of rare earth and aggravate the formation of vermicular graphite layer on the surface of gray cast iron. When gray cast iron is cast, the sulfur content in molten iron is too high or too low, which will lead to the deterioration of gray cast iron graphite. Even if rare earth inoculation is used, it is difficult to produce inoculation effect. When the sulfur content of original molten iron is less than 0.035%, the inoculation effect of rare earth is not ideal, The inoculation amount is not easy to control, and it is easy to produce a large number of carbides; When the sulfur content of molten iron is more than 0.1%, the number of eutectic clusters inoculated by rare earth increases sharply, and the tendency of intergranular porosity and slag inclusion increases; Only when the sulfur content of molten iron is 0.035% – 0.1%, it is most suitable for rare earth inoculation.
2.Eliminate the harmful effects of oxygen and phosphorus
Oxygen mainly exists in the form of oxidation inclusions in gray cast iron. Gray cast iron samples with different content of oxidation inclusions are obtained by treating gray cast iron with different inoculants such as silicon strontium, silicon strontium manganese, silicon strontium rare earth and 0.11% s + silicon strontium. The relationship between oxidation inclusions and tensile strength and Young’s modulus is measured. As shown in the figure, the amount of oxidation inclusions is inversely proportional to tensile strength and Young’s modulus, That is, in a certain range, the tensile strength and Young’s modulus of gray cast iron decrease relatively with the increase of the amount of oxidation inclusions. On the contrary, with the decrease of the amount of oxidation inclusions, the tensile strength and Young’s modulus of gray cast iron increase relatively. The application of rare earth in the process of raising gray cast iron can effectively reduce the influence of oxygen on the properties of gray cast iron. This is mainly because the strong affinity between rare earth and oxygen produces stable rare earth oxides in the casting process. One part leaves the molten iron in the slag raking process, and the other part exists in the molten iron as an external core to promote graphitization, so as to refine the grains, Enhance the properties of gray cast iron.
Phosphorus will cause low temperature brittleness of iron carbon alloy and serious segregation of castings. However, increasing phosphorus content in a certain range can improve the fluidity of molten iron, increase the wear resistance and strength of gray cast iron, promote graphitization and reduce the tendency of white cast iron; Excessive phosphorus will increase the brittleness of gray cast iron. When the phosphorus content in the casting exceeds, phosphorus will exist in the grain boundary in the form of binary or ternary phosphorus eutectic. These fine dispersed phosphorus eutectic will lead to shrinkage porosity defects of the casting; In the process of rare earth inoculating gray cast iron, only when the number of rare earth atoms remaining in the molten iron is phosphorus atoms, the rare earth can react with the phosphorus element dissolved in the molten iron to form phosphorus compounds and reduce the brittleness of gray cast iron caused by phosphorus. It is found that rare earth plays a significant role in high phosphorus cast iron. Rare earth can be used as modifier to break the continuous network distribution of phosphorus eutectic and refine graphite and matrix, It is high phosphorus gray cast iron, which is raised by half to one brand.
3.Effect on other impurity elements
There are some impurity elements in gray cast iron, such as lead, aluminum and arsenic. These impurity elements are difficult to be discharged from molten iron in the casting process. Therefore, other means can only be used to eliminate the adverse effects of these impurity elements. The function of impurity elements in gray cast iron is described in detail, and the means to eliminate some adverse effects are simply put forward, among which the more effective method is to treat with rare earth.
The arsenic content in gray cast iron is usually controlled below 0.013%, because when the arsenic content exceeds this value, the graphite morphology of gray cast iron will deteriorate. D-type, E-type and B-type graphite will slowly appear in type a graphite, which greatly reduces the mechanical properties of gray cast iron. It is the increase of white tendency and section sensitivity of gray cast iron, which increases the reduction of section The shrinkage cavity and shrinkage porosity tendency of gray cast iron increase. The research shows that rare earth elements can greatly eliminate the harmful effect of arsenic on gray cast iron, improve the properties of gray cast iron, and reduce the crack tendency of gray cast iron. It is pointed out that when 0.3% – 0.5% rare earth ferrosilicon is used to inoculate Longyan pig iron (with high arsenic content), the properties of Longyan pig iron can be greatly improved, The problems of cracks or microcracks in gray castings produced by Longyan pig iron are effectively solved, and the quality of gray castings is obviously improved; Lead is an element that has a great impact on the properties of gray cast iron. Rare earth can effectively reduce the harm of lead. Inoculation with rare earth in the melting process of gray cast iron can effectively reduce the tendency of white cast iron, promote the growth of eutectic, improve the mechanical properties of gray cast iron and reduce the internal stress of gray cast iron; Aluminum has strong chemical activity and strong ability to combine with oxygen. It will react with water and oxygen in molten iron to produce pinholes and dissolved slag. The addition of rare earth elements can effectively improve the defects caused by aluminum, because rare earth elements have strong affinity with oxygen and hydrogen, which can purify molten iron and reduce water content. At the same time, rare earth is easy to react with aluminum to form metal compounds and become an external core, Improve the properties of gray cast iron.