Quality and selection of the hottest rare earth no

2022-07-27
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Quality and selection of rare earth nodulizer (3)

2, 2 smelting process control

first, the feeding sequence should be correct. Attention shall be paid to avoid direct contact between magnesium and scrap iron. Magnesium with low melting point should first react with silicon to form mg Si phase after melting, so as to reduce the burning loss of magnesium

second, the melting composition should be uniform. In addition to using the self induction stirring of the medium frequency furnace, it is also necessary to manually stir at the right time and with the right force to homogenize the alloy composition in the smelting process. During the smelting process, the phenomena of magnesium running, shed material and furnace touching shall be prevented

thirdly, the thickness of alloy ingot should be appropriate. If the ingot thickness after liquid alloy pouring cooling is too thin, its surface area is large, and it is easy to cause more magnesium combustion and oxidation during alloy cooling. If it is too thick, it is easy to cause component segregation during solidification because of different specific gravity of alloy elements. The appropriate thickness is generally 5mm according to Mo Ke, chief analyst of 101 true lithium research

fourth, the screening particle size should be classified. The surface of solidified alloy ingots shall be cleaned of oxides and inclusions before crushing and screening. According to the size of the molten iron ladle of the user, the particle size shall be graded and packed, but there shall be no alloy powder

2, 3 chemical composition inspection

a qualified spheroidizing agent, in addition to its dense appearance and no inclusions, the more important thing is the content and uniformity of its chemical composition

in the nodulizer, in addition to the conventional analysis of RE, Mg, Si, Ca and other elements, the manufacturer and user often ignore the analysis of MgO in the alloy. This is also related to the fact that there is no unified national analytical standard for MgO. The composition conclusions of the same alloy are different in different manufacturers with different analytical methods. This requires alloy manufacturers and foundry users to reach a unified acceptance analysis standard to comply with

3. Evaluation on the quality of spheroidizing agent

what kind of spheroidizing agent is the best? This will often become a perennial topic for many foundries to provide services such as innovation and entrepreneurship, utilization and implementation, information consultation, financing, intellectual property protection, talent introduction and training for new material enterprises in the initial stage. In fact, it can be said that there is no best spheroidizing agent, only the one that is suitable for you, and the one that is suitable for you is the best. So, how to evaluate the quality of nodulizer

based on the reaction thermodynamics and kinetics of spheroidizing elements in molten iron, the factors affecting the quality of spheroidizing agent production process, and the factors affecting the use process of foundry, the standards for judging the quality of spheroidizing agent are put forward as follows:

(1) the affinity between spheroidizing elements and sulfur, oxygen and other elements in molten iron. It has strong affinity and forms compounds with it as the foreign core in the solidification process of molten iron, such as rare earth, magnesium, calcium and other nodular elements. (2) The ability of spheroidizing elements to change the morphology of graphite from flake to sphere. (3) The density and boiling point of nodulizer are lower than those of molten iron. The low density can automatically float up in the molten iron, the boiling point is lower than the treatment temperature of molten iron, and magnesium can be transformed into gas at this treatment temperature, with self stirring effect, so as to improve the spheroidization effect. (4) The content of magnesium oxide in the nodulizer is equal to about 10% of the magnesium content. (5) The spheroidizing agent is dense, without segregation, shrinkage porosity and pulverization. (6) The spheroidizing agent has uniform particle size distribution, no powder alloy, and obtuse angle and multi deformation particles. (7) The above conditions basically cover the quality requirements during the production and use of spheroidizing agent, so it can be used as a unified standard for both the supplier and the demander to determine the quality

4. Selection of spheroidizing agent

the above quality evaluation standards of spheroidizing agent can also be said to be the basis for our selection of spheroidizing agent, but they are explained according to the general principles and common phenomena. As for the selection of spheroidizing agent, it is now analyzed according to the habits of our foundry

the most common problems encountered by foundry in the use of nodulizer are:

(1) the composition of nodulizer is inaccurate and fluctuates. (2) The particle size of the nodulizing agent powdered alloy is not up to the requirements. (3) The nodulizer is not dense, floats up quickly and burns seriously. (4) The MgO content is too high, the reaction is too intense, the spheroidizing treatment is poor, and the amount of spheroidizing agent is too large. (5) After spheroidizing treatment, it decayed rapidly. (6) After spheroidization, the tendency of white mouth is large

so, what kind of spheroidizing agent can be used to avoid the above problems

of course, the specific problems should be analyzed according to the melting conditions, casting structure, structure and performance requirements of the factory. For the convenience of narration, this paper is divided into the following types

4, 1 melting conditions

cupola melting ductile iron is adopted by about 80% of enterprises in China. Because the cupola molten iron has low temperature and high sulfur and other impurities, it requires a strong desulfuration and slag removal capacity of nodularizing agent, so it is appropriate to select high-grade nodularizing agents, such as fesimg10re7, fesimg8re7, fesimg8re5; For electric furnace or duplex hot metal melting nodular iron, nodularizing agents with low rare earth and low magnesium content are commonly used, such as those containing Mg and re

4, 2 casting thickness size

for castings with different wall thickness and weight, the choice of spheroidizing agent cannot be the same because their solidification cooling conditions are different. For small thin-walled parts, the solidification is fast and the undercooling is large, which is suitable for the growth of spheroidal graphite. At the same time, it is easy to appear carbides and increase the tendency of the maximum size gap of the white mouth. When the residual magnesium exceeds 0.07%, carbides are more likely to occur. Therefore, low rare earth and low magnesium spheroidizing agents should be selected; For ductile iron parts with thick and large section (wall thickness above 100mm), due to the small solidification rate at the center and the existence of spheroidization recession, it is easy to select high-grade spheroidizing agent or increase the addition amount of spheroidizing agent (0.01 0.02% higher than the residual magnesium content of general ductile iron). However, too high residual rare earth will also cause explosive graphite and anti white mouth. Therefore, it has been studied to add a small amount of anti spheroidizing elements (such as 0.005% antimony, or bismuth and tin) to molten iron to neutralize the excessive rare earth elements. There is also the choice of yttrium based heavy rare earth nodulizer, which is more resistant to decay than cerium based nodulizer and has a small tendency to white mouth

4, 3 pearlite and ferrite castings

the main factors affecting the pearlite content in ductile iron are solidification structure characteristics, comprehensive study of the Soviet Union, carbon silicon content, alloy element types and contents after the liberation of cooling rate in eutectoid zone, etc. There are few graphite balls and large size in the solidification structure, which is not conducive to the full diffusion of carbon, and is conducive to the increase of pearlite and the decrease of ferrite; Austenite has high carbon content and the cooling rate of castings tends to increase pearlite. The pearlite structure can be stabilized by using nodulizer containing copper, antimony or nickel or adding copper, antimony, nickel, tin and other elements to molten iron. For Ferritic Ductile Iron, the content of these elements must be controlled. In addition, as rare earth elements increase the supercooling tendency of molten iron, it is appropriate to select nodularizing agent with low rare earth content (RE content should not be higher than 5%) when producing ferritic castings

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