SAND CASTING EQUIPMENT.

1. The main factors affecting the mechanical properties of gray cast iron:
Process and metallurgical factors: mainly include cooling speed, superheat treatment of molten iron, inoculation treatment, charge characteristics, etc.
1.1 About the influence of cooling rate
Cast iron is a material that is very sensitive to the cooling rate. The thick-walled and thin-walled parts of the same casting may have different internal and external microstructures, commonly known as microstructure inhomogeneity. Because the graphitization process is largely dependent on the cooling rate. There are many factors that affect the cooling rate of castings: wall thickness and weight of castings, types of casting materials, pouring risers and weights, etc. Since the wall thickness, weight and structure of the casting depend on the working conditions and cannot be changed arbitrarily, their influence on the structure should be taken into account when choosing the chemical composition.
1.2 About the influence of molten iron inoculation treatment
The inoculation treatment is to add an inoculant to the molten iron before the molten iron enters the casting cavity to change the metallurgical state of the molten iron, thereby improving the microstructure and properties of the cast iron. For gray cast iron, the inoculation treatment is to obtain the structure of A-type graphite, pearlite matrix, fine eutectic group, and to reduce the tendency of white mouth at the thin wall or corner of the casting and the sensitivity to the wall thickness of the casting; For malleable cast iron, it is to shorten the short annealing cycle, increase the allowable wall thickness of the casting and improve the structure of the structure; for ductile iron, it is to reduce the tendency of casting white mouth, increase the spheroidization rate and improve the roundness of graphite.
1.3 About the influence of overheating of molten iron.
Increasing the superheating temperature of molten iron can:
1.31 Increase the combined carbon content and reduce the graphitic carbon content accordingly;
1.32 Refine the graphite and make the formation of dendritic graphite;
1.33 Eliminate the "heritability" of cast iron;
1.34 Improve the uniformity of the structure on the section of the casting;
1.35 is conducive to the feeding of castings. Similarly, molten iron insulation also has a similar effect of molten iron overheating.
1.4 About the influence of charge characteristics
In actual production, it is often found that when the metal charge is changed (such as using pig iron from different origins or changing the ratio of charge, etc.) and the chemical composition does not seem to change, cast iron has different structures and properties, which shows that the nature of the raw material directly affects the use of it. The properties of the cast iron that come out are called "hereditary". For this reason, increasing the temperature of the molten iron and using a variety of iron ingredients can eliminate this "hereditary" and improve the structure and properties of the cast iron.
To sum up, the technological factors and metallurgical factors of cast iron have a great influence on the mechanical properties of cast iron, therefore, the control of these influencing factors should not be ignored.

2. Grey cast iron cannot be heat treated to meet the grade requirements
Generally speaking, heat treatment can greatly improve the structure and properties of cast alloys, but under the condition of gray cast iron, heat treatment can play a relatively small role. In gray cast iron, graphite has a great influence on the properties of cast iron, and any heat treatment method cannot change the shape and distribution of graphite, so heat treatment cannot effectively improve the properties of gray cast iron to meet the grade requirements.
However, there are many ways to improve the mechanical properties of gray cast iron, such as reasonable selection of chemical components, changing the composition of the charge, superheating the molten iron, inoculation treatment, trace or low alloying, etc., all of which can achieve good results.

3. Precautions for the production of high-grade gray cast iron (inoculated cast iron)
When producing high-grade gray cast iron (generally above HT200), in order to obtain high mechanical properties, the quantity of graphite and the length of graphite must be reduced as much as possible. The traditional method is to reduce the carbon and silicon content of the molten iron and increase the condensation rate of the molten iron, but when the amplitude is slightly larger, D-type supercooled graphite and white mouth will appear, which will reduce the mechanical properties of gray cast iron.
Adding an appropriate amount of ferrosilicon-based ferroalloy particles to the molten iron before the furnace or before pouring is called inoculation treatment. The inoculation process provides a large number of nucleation particles in the molten iron through which graphite can nucleate. Effective inoculation will promote the precipitation of graphite, thereby eliminating whitening, refining flake graphite and transforming supercooled graphite into non-directional uniform graphite (A-type graphite), which can not only greatly improve the comprehensive mechanical properties, but also improve the overall mechanical properties. Improve the uniformity of the as-cast structure and reduce the difference in mechanical properties of the casting due to uneven wall thickness and different cooling rates between the corners and the core. cast iron) essential technology.
In order to make the inoculation effective, it is necessary to meet the requirements of the inoculation on the original iron solution, that is, the original iron solution should have lower carbon and silicon content, or the original iron solution should have a lower carbon equivalent, the lower the carbon equivalent, the better the inoculation effect. Well, the higher the strength of gray iron castings; on the contrary, the higher the carbon equivalent, the poorer the inoculation effect. Since silicon can be adjusted by adding an inoculant, when considering the carbon equivalent of the original molten iron, the carbon is always maintained at about 2.8%~3.2%, and the silicon is maintained slightly below the critical value that can significantly promote graphitization, and then The inoculant is added to make the silicon content exceed the critical value to obtain the effect of inoculation treatment. In addition, the wall thickness and cooling rate of the casting also affect the structure of the inoculated cast iron, which should also be considered when choosing the chemical composition. Generally, the carbon and silicon content of thick parts is taken as the lower limit, and the thin part is taken as the upper limit.
The role of manganese in high-grade gray cast iron (inoculated cast iron), in addition to the effect of neutralizing sulfur, has a special requirement, that is, by means of it, gray cast iron can obtain pearlite structure, so high-grade gray cast iron (inoculated cast iron) has a special requirement. ) is generally higher in manganese content. Sulfur can weaken the graphitization of inoculants, so sulfur is often limited to less than 0.12%. In recent years, some people think that in order to obtain a good inoculation effect, the sulfur content of the molten iron should not be too low. Therefore, when the sulfur content in molten iron gradually decreases in the future, how to determine the sulfur content in inoculated cast iron seems to be a problem worth noting.
The phosphorus content generally starts from the requirements of mechanical properties, and is often limited to below 0.15%, but some machine tool gray iron castings (often made of inoculated cast iron) need to be wear-resistant, and the phosphorus content can be increased to about 0.3% to 0.5% or higher.

4. Four prohibitions when inoculating gray cast iron with inoculants
The inoculants for gray cast iron can be classified by function, main elements, shapes, etc. When using inoculants for inoculant treatment of gray cast iron, attention should be paid: the inoculants must be dried before use; it is forbidden to use pure silicon or pure ferrosilicon for inoculation. The inoculant should not be added hastily, and the method should be paid attention to; the particle size of the inoculant.

5. Possible defects of gray iron castings
In the production of gray iron castings, common casting defects are: pores, unqualified composition and performance, thermal cracking and cold cracking, shrinkage cavity and shrinkage porosity, slag holes and iron beans, cold insulation and insufficient pouring, blisters and sand inclusions, Succulent and misalignment, deformation, etc. Usually, the cause of these defects is not only the problem of molding cores, but also the problems of many production processes such as smelting and pouring, sand distribution quality, and falling sand cleaning.

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