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Quality control of molten iron in induction furnace, common problems and countermeasures

(Summary description)It is often said that "composition determines organization, and organization determines performance." This statement is generally true, but it is not comprehensive. Because, in the past, chemical composition was usually only five elements and alloy elements; In addition to component factors, crystallization control is also important for tissue formation, such as the cooling rate associated with cast iron wall thickness, casting conditions, presetting of crystallization core, and orientation of graphite morphology. Putting aside the external cause of cooling speed, the smelting of cast iron should focus on four aspects: accurate chemical composition, clean molten iron, less interfering elements and controlled graphite core.

Quality control of molten iron in induction furnace, common problems and countermeasures

(Summary description)It is often said that "composition determines organization, and organization determines performance." This statement is generally true, but it is not comprehensive. Because, in the past, chemical composition was usually only five elements and alloy elements; In addition to component factors, crystallization control is also important for tissue formation, such as the cooling rate associated with cast iron wall thickness, casting conditions, presetting of crystallization core, and orientation of graphite morphology. Putting aside the external cause of cooling speed, the smelting of cast iron should focus on four aspects: accurate chemical composition, clean molten iron, less interfering elements and controlled graphite core.

Information

1. Four aspects of iron liquid quality control in induction furnace

It is often said that "composition determines organization, and organization determines performance." This statement is generally true, but it is not comprehensive. Because, in the past, chemical composition was usually only five elements and alloy elements; In addition to component factors, crystallization control is also important for tissue formation, such as the cooling rate associated with cast iron wall thickness, casting conditions, presetting of crystallization core, and orientation of graphite morphology. Putting aside the external cause of cooling speed, the smelting of cast iron should focus on four aspects: accurate chemical composition, clean molten iron, less interfering elements and controlled graphite core.

1 iron liquefaction components to accurate and stable

It is very important to make the casting stronger, improve the international competitiveness of the casting, win the right to say in casting pricing, and meet the requirements of customers with stable performance. One thing at a time to do the chemical composition, not skill, always accurate, stable production level.

Induction furnace than cupola burning loss of much less, but after all there is a loss. The burning loss should be minimized and factors for the fluctuation of the burning loss should be reduced.

Want to remind a few points in particular: (1) the net material into the furnace is the best, the burden is dirty and miscellaneous is taboo. (2) the use of higher than the power of the furnace, and supplemented by a reasonable charge feeding operation, in order to achieve rapid melting. Shortening the melting period and preventing bottom overheating are particularly important, since oxidation occurs mainly during the melting stage of iron. Excessive overheating of molten iron, iron and casting delay will cause fluctuations in the composition. (4) on the basis of making clear all kinds of charge ingredients, we should carefully calculate the ingredients. When batching, the amount of dec and Si increase caused by "self-boiling" at the later stage of melting must be taken into account.

The purification of iron liquid

The purified iron solution refers to the removal of non-metallic inclusions and gases from the iron solution.

By electromagnetic stirring and "self boiling", induction furnace has a better slag degassing effect. If the charge is relatively clean and the slag is carefully removed with a slag remover, it is not a problem to remove the non-metallic inclusions. The presence or absence of non-metallic inclusions can also be easily found in the microscopic examination. H, O, and N gases are not so direct. But where dissolve H, that is, [H], as long as the excipients used in the later smelting stage are not wet, [H] will not be a problem and [H] < 2ppm can be guaranteed. Pinholes in a casting are caused by water in the casting mold or casting ladle, not by [H] passes. There is not much oxygen in the induction furnace molten iron because [O] and [FeO] are reduced by C in the middle and later smelting stage. In the production of gray iron, sometimes even try to increase the O. So degassing is actually a divided by N problem.

With the increase of scrap stock in China, the amount of scrap in induction furnace will increase inevitably. The use of scrap steel not only reduces the cost of charge, but also improves the performance of cast iron, killing two birds with one stone. High N content of scrap steel, in addition to the use of scrap less pig iron, it is necessary to use carburizer to increase C, carburizer will bring some N. Therefore, [N] may exceed the standard (e.g., > 100ppm for thin-walled parts and b> 80ppm for thick-walled parts), resulting in fissured porosity in the casting and unfavorable stability of the performance. Therefore, do not use scrap steel and carbonizing agent with high N content. If necessary, additional measures should be taken to divide N, such as blowing Ar. Individual factories borrow the experience of casting steel, using LF furnace refining, production of pure high-end casting. If [N] is not very high and has no harm to machining performance, then adding Ti et al to make [N] generate TiN et al., for solid N treatment, is the most simple method.

(3) control harmful trace elements

Harmful trace elements, which are mainly brought by pig iron and scrap steel in some places, have adverse effects by dissimilar graphite morphology, producing brittle phase and affecting matrix structure. The way to control harmful trace elements is to close the charge well to prevent harmful elements from mixing. Secondly, high purity cast iron is used to dilute the content of harmful trace elements.

Regarding the adverse effects and limits of harmful trace elements, please refer to the article "modern cast iron" (2014.2, pp. 86-88), "trace impurities > in gray cast iron and ductile iron" written by the author.

(4) Graphite core layout

It is well known that the induction furnace iron liquid supercooling tendency is large, so the white mouth tendency is large, the shrinkage tendency is large, affecting the cutting performance. The source is traced back to the long smelting time in the induction furnace, so that the graphite brought by pig iron and the return charge has little or all dissolved, while the amount of some SiO2 in the iron liquid can be used as the foreign core of graphite is insufficient, leading to the increase of the supercooling tendency of the iron liquid.

The solution is twofold. On the one hand, it is fast melting, especially the cleaning process which accounts for 70% of the melting time. The later overheating temperature should not be too high (generally 1490-1540℃ according to different circumstances), and the holding time should not be too long. The purpose is to retain more fine graphite in the charge. On the other hand, more important, is how to use the good carbon agent. As mentioned above, the induction furnace used more scrap steel is needed to increase C, should choose a good crystalline graphite carburizer. If the amount of C increase is large, you can use a part of carbon carburizer, in the loading period, but must leave some graphite carburizer in the cleaning after adding. Metallurgical SiC can increase C and Si and play the role of graphite nucleation. SiC is a highly recommended pretreatment agent internationally.

The above is the idea of graphite core layout in furnace. The various treatments outside the furnace are another matter

The author has written several articles about induction furnace smelting in the past two years, the reader can find them from "casting", "today's casting", "casting industry" and "hundred casting network" to supplement.

2. Common problems and countermeasures in induction furnace work

There are a lot of problems in induction furnace work. The following is only an introduction to some common problems.

The product element burns too much

The burning loss of Si, Mn, Cr and other easy oxidizing elements in induction furnace is mostly in 3%-5%. Overfiring and chemical composition fluctuation of cast iron will inevitably cause a series of structural and performance problems. The element burn loss is large, generally occurs in the melting time is too long, and did not pay attention to the slag protection. If the amount of scrap steel is large, more light and light material, charge belt rust, the problem is more serious. Avoid element burn too big way is: (1) charge as clean as possible, do not branch fork shape, size can not be too large, too thin. (2) put an end to wear materials, and create all the conditions for fast melting. (3) smelting early to timely slag, late under the high temperature slag cover. Give full play to the protective effect of slag. (4) if the factory has chips to use, the bottom of the furnace can be spread, melt to melt pool in batches to add some.

The iron liquid O is on the high side

The induction furnace does not have the oxidizing atmosphere of the culotte, and the Fe is protected by C due to the reaction of [O] and [FeO] in the iron solution with [C]. Dissolved oxygen in the iron solution is not much (see 2(5) for data). However, in the late melting stage, in order to promote the dissolution and absorption of C increasing agent, the electric frequency is often reduced to enhance the agitation of the melting pool. If the hump is too high and the frequency modulation is too long, the chances of contact between the iron solution and the atmosphere will increase, and the dissociated O ions will enter the iron solution. In the late melting stage, the addition of unbaked materials will also increase [O] and [H]. Recently, some people in the industry put forward: "Insulation above 1500℃, [O] will not decrease, but increase." For your reference. The way to prevent O on the high side is: (1) smelting late FM not too much. (2) Late do not use damp materials and tools. Don't overheat. Do not keep warm at high temperatures for long periods of time.

(3) Liquid iron C is lower than expected

When the iron liquid temperature exceeds the equilibrium temperature, the reaction SiO2+2C=Si +2CO moves to the right, causing the iron liquid to decrease C and increase Si. So you can't forget to fill C with the ingredients. To master the factory's C reduction, C quantity as the number of complement. Also remind a point, gray cast iron late adjustment composition, to take the order of Mn and Then C after Si.

If the casting is machined, it is found that there is a slit porosity

The fissured porosity is the characteristic of N porosity. When [N] exceeds the limit (see 1 (2)), it is easy to occur, and the occurrence rate is higher because there are more non-metallic inclusions in the iron solution. "Disease comes from the mouth", so the amount of arc furnace waste steel should be limited. The amount of arc furnace waste steel [N] is high, while converter waste steel is not. It is also necessary to prevent the mixing of waste alloy steel with high N content, such as high manganese steel, heat resistant high chromium ferritic steel and chromium-manganese nitrogen steel, and austenitic steel. Of course, Mn, Cr, N and Ni brought by these alloy steels are also taboo for ferrite nodular iron. The N content of different C intensifiers varies greatly. The N content of calcined petroleum coke is much higher than that of artificial graphite intensifiers. Comparing the N content of two products, the former is 500ppm and the latter is only 20-25ppm. SiC contains less N than synthetic graphite and is safe to use. If it is found that the amount of N in the liquid iron is high, it should be decided immediately and treated with solid N by Ti (Fe), Al, Zr(Fe), etc.

It must be noted that, of course, fissured pores should be avoided, but the lower N is not the better. For gray cast iron, N can shorten the graphite length and tend to make the graphite end smooth. The dissolution of N in solid solution can promote the refinement of pearlite and increase the number of pearlite. N also has an inoculating effect, which promotes graphitization. Therefore, soluble N has utilization value for gray cast iron. In the technical requirements of GE special alloy gray cast iron, it is stipulated that N quantity should be 60-120ppm and Ti should be limited below 0.025%. When talking about grey cast iron in the automobile industry, Aiken company believes that the ideal content of [N] is 95-160ppm and points out that Ti, Al and Zr should not be used to fix N.

5) grey cast iron breeding effect is not good

In some factories, gray iron inoculating effect is not good, even if more inoculant is of no avail. This is related to the low O and S content in the induction furnace molten iron. Different data, the data is different, but very close: [O] < 10ppm, < 15ppm, around 20ppm; [S] < 0.06%, < 0.05%, in the range of 0.02%-0.04%. The basic solution is to use THE FeS system to increase The S to 0.07%-0.10%. A few plants also add O agents later, such as sponge iron or sintered iron or chip, to bring [O] above 30ppm. Oxysulfide inoculants are also used in some factories. These methods produce nothing more than oxysulfide, which acts as the graphite core. It must be noted that in low S iron solution, the graphite formed is relatively easy to disappear. This is probably also low S gray cast iron iron liquid "not on" a reason.

About MnS, for a long time, often said that MnS is the core of graphite. Under the microscope, however, there is no shortage of pale gray-blue MnS particles. Therefore, whether MnS is the graphite core and under what conditions can it become the core needs to be demonstrated. Moreover, there is usually 0.6%-1.2% Mn in gray cast iron, increasing S to 0.07%-0.10%, which is far from enough to meet the balance requirement of Mn= 1.73s. Obviously, the purpose of S is not to balance Mn, nor to form simple compound MnS to play a core role. Only when S and O are combined with more active elements, such as Ba, Ce, Zr, Al, etc. to form thioxo compounds, can they play the foreign core role of graphite.

6. Nodular cast iron nodulation level fluctuates greatly

The spheroidization grade of nodular cast iron in a certain factory showed great fluctuation. After analysis of the examination, the original iron liquid S is 0.01991, 0.02872, 0.02399, 0.02660, 0.03338, 0.03885, 0.01559, content of ups and downs, spheroidizing natural instability. Only by strengthening the management and stabilizing the content of raw iron solution S, can the spheroidization grade be stable.

The author has learned that a township enterprise, originally used cupola to produce low chromium grinding ball, then switched to induction furnace, low chromium grinding ball dry, and then connected to the nodular cast iron task. But the stove uses, neither does not divide the furnace body, also does not know how to wash the stove. Spheroidizing reaction, light smoke, liquid level also channeling "fire", but the cast has carbide, graphite ball is rare, the matrix structure beyond recognition. After the reorganization, strict charge management, gray cast iron and ductile cast iron return charge no longer mixed, separate furnace melting, the problem is solved.

Graphite number is less, the size of the ball diameter is uneven

The presence of nodular graphite indicates no problem with Mg residues. The fault is due to poor or declining inoculation. If the casting does not have white mouth, the inoculation gap is not large. In terms of smelting, artificial graphite C increasing agent should be used to distribute graphite core. Industry insiders recommend SiC better than artificial graphite. It is pointed out that SiC is light and easy to float up, and the surface is obstructed by SiO2 film, which affects the dissolution. Therefore, SiC in the feeding period to add. In the case of silk-feeding, inoculation must be delayed after spheroidization to avoid accelerated decay of inoculation.

It is often asked how many graphite balls are appropriate. In the standard of ductile iron, the number of graphite balls is not specified. According to graphite size 6, the total number of balls > 150 /m㎡. ADI's process control stipulates that the number of graphite balls shall not be less than 100 /m㎡.

When it comes to graphite enhancer, it is easy to intuitively distinguish it from carbon enhancer. But in recent years, it is found that the intuition is graphite C increasing agent, but the use effect is not good. It seems that the future formulation of the C additive standard needs careful consideration.

Therefore, the cutting performance of the casting is poor

Machining performance is a complex proposition, which involves the object to be machined, the tool, the machining parameters and the operator's cognition of machining performance. As far as cast iron is concerned, the machinability is not good, there may be many reasons: (1) there is free cementite, (2) there is hard point (such as phosphates, titanium compounds...) There is FeSi which has not been melted down, there is a large amount of residual pearlite in ferrite nodular cast iron, there is an anti-white mouth caused by RE partial polymerization at the hot spot, etc. Readers can use basic professional knowledge, the right medicine, medicine to stop the disease.

Mag (mag) spectrum analysis, the residual Mg content is above 0.06%, but the casting is not white

Generally, spheroidization can be conducted for medium wall thickness parts with residual Mg content between 0.035% and 0.045%. The residual Mg was found to be more than 0.06% by spectral analysis, and there was no blank in metallographic examination, indicating that there was something wrong with the spectral analysis in this plant. Spectral analysis must always be calibrated by standard samples and matched by chemical analysis. The preparation and polishing of spectral analysis samples should be done in accordance with the rules carefully. Mg, as well as P, C and S are sensitive to manipulation and should not be careless.

We will have low impact toughness of ductile iron

Qtd800-10r in ferrite nodular cast iron and isothermally quenched nodular cast iron (if required by the demandor) provides for the minimum impact energy of V-notch specimens. If the impact energy does not meet the requirements, it must be scrapped. Determining their chemical composition is critical. Taking QT350-22L and QT400-18L in ferrite nodular cast iron as an example, low P and low Mn are required, and Si quantity is correspondingly limited. These professional questions are omitted here. From the melting point of view, should: choose carbon scrap, and use a certain amount of casting with high purity pig iron. (2) According to the tips in "1" section, complete the four aspects of iron liquid quality control work. (3) Do well in front of the furnace. This will nip it in the bud.

(11) Go wrong for unknown reasons

Sometimes, nothing seems to have changed in production, but the castings go wrong. How to find the reason? It is suggested to use advanced detection methods, such as trace elements detected by spectrometer, total oxygen and total nitrogen measured by oxygen and nitrogen meter, phase composition analyzed by energy spectrum and structure analyzed by X-ray diffraction. If this factory cannot do, should entrust concerned unit to do. Then ask a specialist to make a diagnosis.

(12) Long melting time and high power consumption

The melting time is long and the power consumption is high, which is usually caused by the inborn deficiency of induction furnace. In the 21st century, intermediate frequency induction furnace develops very fast. With the development of silicon control technology and power capacitor technology, the series energy saving medium frequency furnace is a new force. The concept of "thyristor series operation of one drag two" has been deeply rooted in people's hearts. Ductile iron can consume less than 570kwh per ton. While the traditional parallel intermediate frequency furnace, ton power consumption in 800kwh above.

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