SAND CASTING EQUIPMENT.

1. The role of spheroidizing agent and spheroidizing elements in the production of nodular cast iron
Although there are many types of spheroidizing agents at home and abroad, rare earth magnesium alloys are the most widely used in our country. Now we mainly discuss the role of such alloys and spheroidizing elements. The so-called spheroidizing elements refer to those elements that can promote the spheroidization of graphite and generate or increase graphite spheres.

Spheroidized elements generally have the following common properties: there are one or two valence electrons in the outermost electron layer of the element, and 8 electrons in the second inner layer. This electronic structure makes the element have a strong affinity with sulfur, oxygen and carbon, reflecting the stability of the product, and can significantly reduce the sulfur and oxygen in the water; the element has a low solubility in the molten iron, and has a significant segregation tendency during the solidification process; It has a certain affinity, but has low solubility in the graphite lattice. According to the above characteristics, Mg, Ce, Y, and Ca are effective spheroidizing elements. One is the high vapor pressure in the molten iron, which makes the molten iron stiff. The atomic weight and density of magnesium are smaller than that of molten iron, with a melting point of 650 degrees and a boiling point of 1108 degrees. At the processing temperature of molten iron, the vapor pressure generated by magnesium is very high (over 1Mpa). The heat of fusion of magnesium is 21J/g and the latent heat of vaporization is 406J/ g. Therefore, when magnesium is added to the molten iron, it will vaporize and make the molten iron churn. Second, it has a strong affinity with sulfur and oxygen. The formed MgO and MgS have a high melting point and a density far less than iron. They are easily separated from molten iron. Therefore, the content of sulfur and oxygen in molten iron after magnesium treatment is very low; third, there is a tendency to segregate in graphite during the solidification of molten iron. When the residual amount in the molten iron exceeds 0.035%, the powder can be spheroidized, but when the residual amount of magnesium exceeds 0.07%, a part of the magnesium segregates in the grain boundary, and the carbon, phosphorus, etc. in the grain boundary generate heat The reaction produces MgC2, Mg2C3, Mg3P2 and so on. When the amount of residual magnesium increases, the intergranular carbides increase.

The rare earth elements that have a significant effect on graphite spheroidization are cerium in light rare earth elements and yttrium in heavy rare earth elements. One is that the boiling point of rare earth elements is higher than that of magnesium, and when added to molten iron, it will not cause tumbling and splashing of molten iron; second, cerium and yttrium-based rare earth elements have stronger desulfurization and deoxidation capabilities than magnesium, resulting in rare earth sulfide and oxidation Compounds such as rare earths have high melting points and good stability; thirdly, rare earth elements and spheroidizing interference elements in molten iron can also form stable compounds, so the spheroidizing agent containing rare earths has stronger anti-interference ability than magnesium spheroidizing agent.

The residual amount of rare earth elements has a significant effect on graphite spheroidization. Light rare earth has been treated with eutectic molten iron. When the residual cerium content is 0.04%, graphite can be spheroidized and is very stable; when treating hypoeutectic molten iron, the amount of light rare earth added must be increased. The light rare earth treatment produces ductile iron, and the graphite roundness is worse than that of magnesium treatment ductile iron, and fragmented graphite appears; in addition, the light rare earth treatment produces ductile iron with a greater tendency to become white, so it is necessary to control its addition. The heavy rare earth yttrium itself has a high melting point, and the oxides and sulfides produced by its deoxidation and desulfurization are relatively stable at high temperatures, so its resistance to spheroidization and recession is strong. When the molten iron at 1400 degrees is kept for 1 hour, the spheroidization rate is reduced by no more than 10%, and the molten iron with sulfur content of 0.06% can be treated with yttrium-based heavy rare earth alloy to obtain complete spheroidal graphite. The residual yttrium in the molten iron is 0.10—0.15%, and the graphite spheroidization is good; below this limit, irregular graphite and vermicular graphite appear at one time as the amount of yttrium decreases; when the residual yttrium exceeds 0.15 and less than 0.30%, the white mouth tendency gradually increases , The roundness of graphite becomes worse, and YTe4 appears at higher residue levels.
Ca: The solubility of calcium in molten iron is very low, and its effect on the metallographic structure is achieved indirectly through the combination of oxygen and sulfur. Compared with magnesium, calcium has a stronger affinity for sulfur and oxygen and can effectively desulfurize and remove oxygen. When the calcium residual amount is very low, the branching tendency of graphite increases, and when the residual amount is large, the graphite size decreases and the branching tendency decreases. When the calcium residue reaches 0.2%, the white mouth tends to increase significantly.
1.1 The role of despheroidizing elements (spheroidizing interference elements)
Such elements mainly refer to elements that damage and hinder the spheroidization of graphite. According to their mechanism of action, they can be roughly divided into three categories:
1.11 Consumable despheroidizing elements, such as sulfur, oxygen, selenium, tellurium, etc., form compounds with magnesium and rare earth elements, and prevent the formation of spherical graphite by consuming nodular elements.
1.12 Interfering elements of spheroidization segregated at the boundary, including tin, antimony, arsenic, copper, boron, titanium, aluminum, etc., these elements are enriched in the grain boundary to promote the formation of deformed dendritic graphite when carbon is crystallized in the late eutectic phase If the content of these elements is high, it can also contribute to graphite distortion in the middle of eutectic, forming clusters or thick flake graphite.
1.13 Some intermediate spheroidization interference elements, such as aluminum and bismuth, lead to graphite distortion through segregation when the content is low, and the spheroidization elements can also be consumed when the content is high. In addition, despheroidizing elements also have different effects on the ductile iron matrix. Te\B strongly promotes the formation of white mouth, As\Sn\Sb\Pb\Bi stabilizes pearlite, Al\Zr promotes the formation of ferrite, and Se has no effect.

1.2 The configuration of spheroidizing elements and the types of spheroidizing agents
Magnesium, rare earth and calcium are currently recognized as having the ability to promote the spheroidization of graphite, but how to prepare and use them in accordance with the actual industrial production, not only to ensure the spheroidizing ability of the spheroidizing agent, but also to be easy to prepare in production, and the raw materials are economical , Easy to use, has become the principle of formulating and using spheroidizing agent.
1.21 Configuration of spheroidizing elements
Composition principles and characteristics: It must have a strong spheroidizing ability, and obviously magnesium should be the main component. The boiling point of magnesium is low. Adding molten iron can make the molten iron violently churn, and the reaction will be even up and down; the proportion of The molten iron is not easy to float, which can reduce the oxidation and burning of magnesium. Ability to digest and neutralize de-spheroidizing elements, rare earth elements have strong desulfurization and degassing, purifying molten iron and eliminating de-spheroidizing elements. Moreover, my country is rich in rare earth resources and the cost of obtaining raw materials is low.
The spheroidizing agent reacts smoothly and is easy to operate. Although calcium cannot be used alone as an autumnal agent, it can be used to form a composite spheroidizing agent with magnesium and rare earth to reduce the content of MgO in the spheroidizing agent, stabilize the spheroidizing process, and reduce the cause of rare earth Tendency to become more white. Therefore, the principle and characteristics of formulating the composition of spheroidizing agent are to use the advantages of various spheroidizing elements to improve the spheroidizing effect, and to meet the needs of different production conditions and different structure castings by adjusting the content of spheroidizing elements.
1.22 Types of spheroidizing agent
In accordance with the principle of spheroidizing and de-spheroidizing element configuration, a variety of spheroidizing agents have been developed at home and abroad, generally the following types:
Pure magnesium: This is a commonly used spheroidizing agent abroad, and it is rarely used in China. The advantages and disadvantages of making ductile iron by adding magnesium under pressure are equally obvious. Copper-magnesium, nickel-magnesium: This alloy was used in my country in the early days, but the cost is high, and the accumulation of copper and nickel in the recharge material is difficult to control, resulting in a decrease in toughness. Silicon-magnesium-iron alloy: Generally, the minimum magnesium content is 3.5-4.5% and the maximum is 10-15%. Commonly used alloys are magnesium 5-10%, silicon 42-47%, and the rest are pastes. The lower the magnesium content, the more stable the spheroidization reaction, and the higher the recovery rate of magnesium (4% magnesium can increase the recovery rate of magnesium by 10% compared to 9% alloys), but the low-magnesium spheroidizing agent is the addition of silicon to the molten iron. Big. The spheroidizing agent is used to treat molten iron with a low content of sulfur and de-spheroidizing elements, and to cast castings with medium section thickness. At present, the mass production of ductile iron in China will cause certain contradictions with the production conditions and raw material procurement of my country''s foundry enterprises. Rare earth magnesium alloys: including rare earth silicon magnesium, rare earth calcium magnesium, rare earth copper magnesium and other alloys. It is a series of rare earth magnesium alloy spheroidizing agents developed by Chinese engineers and technicians based on my country’s actual conditions in the early 1960s. The advantages and disadvantages of these spheroidizing elements, especially rare earth magnesium-calcium alloys, are currently the main spheroidizing agents with a large and wide range of applications in China, thus blazing a trail of nodular cast iron manufacturing technology suitable for my country''s national conditions.

2.1 Raw material preparation
Rare earth ferrosilicon alloy: This is the only source of rare earth elements in the spheroidizing agent, which requires no moisture, no powdering, uniform composition, and no inclusions. The rare earth ferrosilicon containing 23-30% rare earth is the most commonly used. There are materials to introduce the use of high-silicon rare earth ferrosilicon (commonly known as rare earth ferrosilicon produced by one-step method, containing 55% silicon) and commonly used low-silicon rare earth ferrosilicon (commonly known as rare earth ferrosilicon produced by two-step method, containing 36-44% silicon) To produce spheroidizing agent, under laboratory conditions, the structure and performance of the produced ductile iron are basically equivalent, which can meet the production requirements. It''s just that the former has a slightly higher tensile strength and the latter has more ferrite, so pay attention to it when using it. Of course, this requires further verification of industrialized mass production.
Magnesium metal: Magnesium mainly exists in the Mg-Si alloy phase state in the alloy, which is beneficial to reduce the oxidation burning loss of magnesium. Magnesium in alloys can also be divided into effective magnesium and ineffective magnesium. Ineffective magnesium mainly refers to magnesium oxide. Therefore, the raw material metal magnesium must be of high purity (first grade magnesium, containing more than 99.7% of magnesium), with few impurities and no oxidation. Ferrosilicon: requires low aluminum, compact structure, no pulverization, no inclusions, and ferrosilicon of grades below the national standard 75 is refractory and has many impurities, so it is not suitable for use. Calcium silicon and barium silicon: It is mainly to determine and control the precise content of Ca and Ba in the alloy. Less addition will increase the content of ineffective magnesium oxide, resulting in violent burning, increased white mouth tendency, and decline when the nodulizer is used. Also fast. Scrap steel: Carbon steel is generally used, silicon steel is also possible, other alloy steels are prohibited, because the alloying elements may be de-spheroidizing elements, and production is difficult to control. In addition, scrap steel is required to be oil-free, rust-free and pollution-free, especially rust is easily reduced by magnesium to magnesium oxide.

2.2 Smelting process control
2.21 The order of feeding must be correct. Be careful not to let magnesium and scrap iron and steel come into direct contact. It is necessary to let the low melting point magnesium react with silicon to form the Mg-Si phase first to reduce the burning loss of magnesium.
2.22 The molten composition should be uniform. In addition to using the induction stirring of the intermediate frequency furnace, it is also necessary to manually stir at the right time and force to make the alloy composition uniform during the smelting process. During the smelting process, it is necessary to prevent the occurrence of "magnesium running", "shed material" and "bumping furnace".
2.23 The thickness of the alloy ingot should be appropriate. If the thickness of the ingot after casting and cooling of the alloy liquid is too thin, its surface area will be large, and it will easily cause more magnesium combustion and oxidation during the alloy cooling process. If it is too thick, the specific gravity of the alloy elements is different, which will easily cause component segregation during the solidification process. The appropriate thickness is generally 10-15MM.
2.24 Screening particle size should be classified. The solidified alloy ingot should be cleaned of oxides on the surface and pick out the inclusions before crushing and screening. and according to the size of the user''s ladle, the particle size is graded and packaged, but there should be no alloy powder.
2.3 Chemical composition inspection
For a qualified spheroidizing agent, in addition to its compact appearance and no inclusions, the more important thing is the content and uniformity of its chemical composition. In the spheroidizing agent, in addition to the conventional analysis of Re, Mg, Si, Ca and other elements, manufacturers and users often ignore the analysis of MgO in the alloy. This is also related to the lack of a unified national analysis standard for MgO. Different manufacturers use different analysis methods for the same alloy, resulting in different composition conclusions. This requires alloy manufacturers and foundry users to reach a unified acceptance analysis standard to comply with.

Xinyuanzhu Group only analyzes several major issues in the currently widely used processing technology containing one, rare earth, and punching. The role of spheroidizing elements The rare earth ferrosilicon-magnesium spheroidizing agent contains spheroidizing elements magnesium, rare earth, calcium, a certain amount of iron, silicon, and a small amount of manganese, aluminum, and titanium. The spheroidizing elements are Mg, Re, and Ca according to their spheroidizing ability. They all have strong desulfurization and degassing capabilities. The affinity to oxygen is Ca, Re, Ma in descending order, and the affinity to sulfur is Re, Ca, Mg in descending order. Obviously, magnesium plays the main role of spheroidization, plays the role of auxiliary spheroidization, and also plays the role of desulfurization, degassing, and purification of molten iron. The spheroidizing elements added to the molten iron have the following functions; there is a certain residual amount in the molten iron to make graphite into balls; combine with sulfide to form sulfides, which desulfurize the molten iron; react with oxygen in the air to produce oxidative burning loss .

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