SE534912C2 - Method for determining the amount of inoculant to be added to a cast iron melt - Google Patents

Abstract

Method for determining the amount of inoculum to be added to cast iron melt in a particular casting process, comprising the steps of: - providing a first sample container (1) and a second sample container (2) each comprising a thermocouple (3, 4) connected to an assay equipment (5) filling a respective sample container (1, 2) with a quantity of molten cast iron; register a first cooling curve during solidification of the cast iron in the first sample container (1) and a second cooling curve under solidifying cast iron in the second sample container (2); before filling molten cast iron, the sample containers are placed with a predetermined amount of inoculum representing the saturation level of inoculum of the particular casting process, the amount of inoculum to be added to the cast iron melt in the particular casting process being determined by the temperature difference between the minimum temperature and the minimum temperature. ) of the second cooling curve.

Description

However, this known method does not provide an exact measure of how the constituent structures are nucleated or grow, but only gives a rough estimate of the amounts of the primary phase and the eutectic phase, respectively.

It is also known to control the nucleation of the constituent structures of the cast iron by the addition of grafting agents. With the known methods, however, it has proved difficult to optimize the amount of inoculum to be added.

The nucleation of the constituent structures in the cast iron thus has a great impact on the properties of the cast iron, e.g. on defect formation and strength.

For the final properties of the finished casting, the structure of the eutectic phase is especially important. However, prior art methods do not provide a good measure of the structure of this phase, which can lead to strength problems and disposal.

The object of the invention is therefore to provide a reliable method for determining the amount of inoculant to be added to a cast iron melt, which method solves the above-mentioned problems or at least reduces them to a minimum.

SUMMARY OF THE INVENTION This object is achieved by the method for determining the amount of inoculum to be added to a cast iron melt in a particular casting process, comprising the steps of: - providing a first sample container and a second sample container each comprising a thermocouple connected to an analysis equipment; - fill the respective sample container with a quantity of molten cast iron; registering a first cooling curve during solidification of the cast iron in the first sample container and a second cooling curve during solidification of the cast iron in the second sample container; 534 912 characterized in that in one of the sample containers, before filling of molten cast iron, a predetermined amount of inoculant representing the saturation level of inoculum of the particular casting process is placed, the amount of inoculum to be added to the cast iron melt being determined in the difference between the lowest eutectic temperature (TEiow) of the first cooling curve and the lowest eutectic temperature (TEiow) of the second cooling curve.

The method entails a precise control of the casting process, which in turn gives rise to minor variations in the quality of the finished casting. This results in savings in the form of fewer scraps during casting, processing and assembly, while at the same time reducing the risk of accidents.

In an alternative, the predetermined amount of grafting agent representing the casting process is an average value based on saturation levels of grafting agent of a plurality of cast iron melts in the determined casting process.

According to an alternative, the predetermined amount of grafting agent representing the casting process is a selected value from saturation levels of grafting agent of a number of cast iron melts in the determined casting process.

In the event that the inoculum contains silicon, the effect of silicon on the eutectic temperature is suitably deducted from the inoculated sample based on a predetermined relationship.

The amount of inoculant to be added to the cast iron melt in the determined casting process can, if necessary, be controlled against over- or under-grafting of the melt. Preferably, the method relates to determining the structure of a cast graphite type cast iron.

DESCRIPTION OF THE FIGURES Figure 1: Part of an Fe-C diagram Figure 2: A cooling process for a cast iron melt Figure 3: A test setup for performing an experiment according to the method according to the invention.

Figure 4 A diagram showing the difference between the eutectic temperature TEiow of a grafted and ungrafted sample of a cast iron melt.

DESCRIPTION OF THE INVENTION Initially, the theoretical background on which the invention is based is described.

Upon solidification of a melt, small solid nuclei are initiated by agglomerated atoms, around these nuclei the melt will then solidify. The formation of these nuclei, i.e. nucleation, is controlled by the total energy in the melt. A system, such as a metal melt, strives to have as low an energy content as possible. Which state of solid or surface has the lowest energy content of a substance depends on its temperature. When a solid phase is formed in a melt, a volume is formed where the atoms are arranged in an energy-efficient way. At the same time, a surface is formed between the solid and the fl-surface phase. The atoms in the surface are pressed into positions they would not normally be in, which requires energy. Thus, when a melt solidifies into a solid body, there is a decrease in free volume energy, GV, and an increase in surface energy, y, provided that it occurs at a temperature below the solidification temperature, TM.

Changes in the total energy content of the system can be described with the equation: AG = AGVV + yA, where AG is the total change of energy, V is the volume of the body and A is the surface area of the body. 10 15 20 25 30 534 912 Nuclear formation takes place only if it leads to a reduction in the total energy content of the system, ie that AG becomes negative. Since the surface energy counteracts the phase transformation, the melt will remain liquid even when the temperature exceeds the solidification temperature. This is called subcooling of the melt. The more the temperature is lowered below the solidification temperature, the greater the driving force to change phase. Nucleation occurs when the temperature has become so low that the decrease in volume energy is greater than the amount of energy used to create the surface. For homogeneous melts, this subcooling can amount to fl your hundred degrees, but is normally significantly lower. How much subcooling is needed for nucleation to take place is an indicator of how easily nucleation can take place in the melt, which is also called the melt's nucleation potential.

The solidification process of a cast iron can be described with an Fe-C phase diagram, see Figure 1. The Fe-C diagram indicates the temperature on the vertical axis and the carbon content in percentage by weight on the horizontal axis. Figure 1 indicates the carbon content in the area relevant for cast iron, i.e. up to 5%. The lines in the diagram limit the different phases that the iron assumes at different temperatures and carbon contents. In the figure, the liquidus line 1 and a line marking the eutectic temperature 2 are marked out. These two lines are important in predicting the structure of the cast product. At the liquidus temperature, austenite, also called y-iron, is precipitated. At the eutectic temperature, carbon also begins to separate from the remaining melt. The carbon is excreted in the form of burr, which due to its shape has a great influence on the properties of the material.

Upon cooling from fully molten state to solidified state, a melt passes through the various phases of the Fe-C diagram. Figure 2 schematically shows a solidification curve for a cast iron. In Figure 2, the plateau “a” at the temperature of 1200 ° C indicates the liquidus liquid temperature. At this point, austenite begins to form in the melt. As the temperature passes 1150 ° C, the eutectic solidification begins. This is marked with "b" which is the lowest eutectic temperature in the TElow melt. The eutectic solidification is indicated by a slight increase in temperature in the melt. When the temperature returns to a steady decrease, area "c" has all melted to solid form.

How much subcooling is needed for the eutectic discrimination to begin, ie TEiow has been shown to be a good indicator of how good the melting potential of the melt is. Slight subcooling means good nucleation potential. It is important to achieve a high nucleation potential during the eutectic solidification as this gives rise to an even graphite separation during the solidification. The even graphite separation is necessary to obtain good mechanical properties in the casting because the increase in the volume of the graphite during solidification counteracts the decrease in the volume of the austenite.

The nucleation potential can be controlled by the addition of nucleation points in the form of inoculants. By adding inoculants, the temperature at which the eutectic solidification takes place is raised. That is, less cooling of the melt is required for the eutectic solidification to begin.

It is important to optimize the content of inoculants added to the melt. If too small an amount of inoculant is added, uneven or insufficient graphite precipitation can be caused, which leads to carbide formation and in the worst case so-called white solidifying.

Added too much inoculant leads to high production costs and can also lead to disadvantages for the structure of the cast product, for example due to graphite expansion.

As mentioned above, it is thus advantageous to determine the nucleation potential of the melt at the eutectic minimum at point "b" in the cooling curve.

This is because the subcooling during the eutectic solidification is directly linked to the final structure of the cast material. 10 15 20 25 30 534 512 The inventors have found through measurements that a cast iron melt can be "saturated" with inoculants. This means that when inoculants are added, the temperature of the eutectic solidification stops increasing at a certain level even if more inoculants are added. This means that at this "saturation level" an absolute limit is obtained for the TEiow of the melt, which is the highest eutectic temperature, ie the lowest subcooling, which in practice can be achieved in the melt.

For the method according to the invention, which will be described in more detail below, this connection is important, since it thereby obtains a fixed level for how high the eutectic temperature in a cast iron melt can be in practice. The nucleation potential in an ungrafted melt can thus be compared with a stable guide value.

The following, the method according to the invention will be described in detail. âšgl In a first step, a cast iron melt is manufactured. This is done by melting a starting material, such as scrap, return, pig iron and shavings. The melt composition and temperature are checked and any adjustments are made.

Step 2 In a second step, a test equipment for thermal analysis is provided.

The test equipment (see figure 3) comprises two test cups 1 and 2, eg a thermocouple 3 and 4.

The thermocouples are connected to an analysis equipment 5 on which software sand cups, each comprising for thermal analysis are run. In one sample a predetermined amount of inoculant is placed which represents the saturation level of inoculum of the particular casting process. The level of saturation of the inoculant, i.e. the amount of inoculant which must be added to cast iron melts in order for them to become saturated with inoculants, varies with the process conditions in the manufacture of the melts. These conditions include, for example, the composition of the starting material and the settings of the process equipment (scrap).

In industrial processes for the manufacture of cast iron components, so-called casting processes, however, strive to keep the conditions in the process as constant as possible, ie keep these within certain limits. When manufacturing a certain component, eg engine blocks, the aim is to always start from scrap with a composition that is kept within established limits and / or always use the same casting equipment when manufacturing a certain component. This is done in order to obtain as uniform a result as possible between different castings.

The amount of inoculant to be added in one sample cup in order for the melt filled therein to reach the saturation level can therefore be determined in advance as a representative value for a particular casting process. The representative value can then be used every time a new cast iron melt is to be manufactured in the casting process.

This representative value can, for example, be an average value of saturation levels for inoculants determined for a number of melts for a specific casting process, for example a process for casting engine blocks of a certain type. The representative value can also be selected from your different saturation levels for inoculants and determined for a number of melts for a specific casting process. For example, the representative value can be a median value, or a maximum value or a minimum. The representative value of the saturation level of the particular casting process is then stored, for example, in a non-minne memory memory in a computer, from which it can be retrieved and used each time the particular casting process is to be performed. 10 15 20 25 30 534 912 It is also possible to determine saturation values for inoculants for a number of different cast iron melts and then to adapt a linear relationship to these saturation values.

The amount of inoculum required for an individual cast iron melt to be saturated is determined as follows. The eutectic temperature TEiow is determined simultaneously for two samples taken from the cast iron melt. One sample is inoculated and in the other sample inoculants have been added. For the determination of the saturation level, a sample from the cast iron melt is poured simultaneously into two identical sample cups and TEiow is then determined for each sample from cooling curves.

The nucleation potential in the difference between TEjow in the inoculated sample and TElow in the inoculated sample. The procedure is repeated several times and each time one sample is inoculated with more inoculum than in the previous test round. By comparing the nucleation potentials of the repeated samples, the level can be found where the nucleation potential stops increasing with increasing inoculum content. This level is the saturation level of the current cast iron melt for inoculants. As mentioned above, the saturation level of a plurality of cast iron melts is determined and based on these, an average saturation level of a particular casting process can be determined or a representative value can be selected.

Figure 4, which will be discussed in more detail later, shows how the eutectic temperature TEiaw stops increasing when more than 0.20% by weight of inoculant is added to a certain cast iron melt.

The saturation level for inoculants is thus determined by repeated experiments where each experiment includes two measurements simultaneously in two separate identical test cups and under the same conditions. This is because in thermal analysis, where cooling curves are recorded during solidification of cast iron in sample cups, the cooling curve is affected by several factors that are not related to the nucleation potential in the melt. These factors are, for example: ~ Sample cup material - Variations in the location and design of the thermocouple v Variations in draft due to walls, ventilation and the like O The amount of melt in the cup 0 The chemical composition of the melt v The temperature of the melt when poured into the cup The above mentioned factors affect the measurement results by, for example, shifting the entire cooling curve upwards or sideways, whereupon the eutectic temperature ends up at the wrong level.

If errors occur during measurements performed in two test cups under identical conditions, however, both measurements will be subject to the same error. By subtracting the eutectic temperatures TElow from each sample from each other, all these error factors are eliminated.

An additional factor that affects the result is the silicon content of the inoculum. It is common for the vaccine to contain silicon, for example in the form of FeSi. Silicon changes the chemical composition of the cast iron melt in the grafted specimen.

Since the temperature when the eutectic solidification starts is based on the chemical composition of the melt, the eutectic temperature in the inoculated sample increases upon the addition of silicon. To exclude this error factor as well, the effect of silicon on the eutectic temperature must be subtracted from the inoculated sample. 10 15 20 25 30 534 912 11 It is possible to develop a relationship for the effect of silicon on the eutectic temperature. This connection can be developed through the study of phase diagrams and linear adaptation. It is also possible to develop the relationship with a suitable calculation program, eg ThermoCalc. With the help of the connection, the effect of silicon on the eutectic temperature can be calculated.

It also happens that the inoculant, instead of silicon, is alloyed with other substances that affect the equilibrium lines in the phase diagram, ie it affects the eutectic temperature. It is also possible to develop relationships for how these substances affect the eutectic temperature and to use these relationships to compensate for the eutectic temperature in the inoculated sample.

S293 In a third step, the sample cups are then filled with cast iron melt and cooling curves are recorded during solidification of the samples. From the two cooling curves, the lowest eutectic temperature TEjow is then determined for each sample. The lowest eutectic temperature in the inoculated sample is then subtracted from the lowest eutectic temperature in the inoculated sample.

In case the inoculum contains silicon, according to the invention, the effect of silicon on the eutectic temperature in the inoculated sample is eliminated before it is subtracted from the eutectic temperature in the inoculated sample.

Step 4 In a fourth step, it is determined, on the basis of the calculated difference in ° C, how much inoculant is to be added to the cast iron melt in order for the finished casting to obtain a desired structure. The resulting difference in ° C, i.e. the nucleation potential of the cast iron melt, is a measure of how far the eutectic temperature in the cast iron melt is from the practically achievable eutectic temperature. The resulting difference in ° C can be related to the final structure of the casting.

The relationship between the temperature difference, the amount of inoculum to be added and the final structure of the cast material can be determined empirically. This is done by manufacturing several cast iron melts over a longer period of time, the amount of inoculant to be added is determined by the method according to the invention and the structure of the cast goods is checked.

For each cast iron melt produced, the relationship between the number of degrees of temperature difference and the amount of inoculant to be added can be adjusted until there is sufficient good agreement with the final structure of the casting.

It is also possible to control the amount of inoculant to be added to the cast iron melt against over-grafting or under-grafting if this is desired for a particular product.

It should be noted that it is not possible to add to an individual cast iron melt exactly the amount of inoculant corresponding to the saturation level of the particular casting process of the individual melt. This is because over time the saturation levels of the individual cast iron melts have a spread around the representative value of the saturation level of the casting process.

For example, individuals can set boundaries. This means that the saturation level of inoculants for each individual spread is caused by the fact that the compositions of the cast iron melts vary within the melts differ from the saturation level which represents the casting process.

If inoculants were added to the individual melts in exactly the amount indicated by the saturation level representing the casting process, this would lead to the melts being over-grafted and under-grafted. As mentioned earlier, over- resp. inoculation various disadvantages m a p costs and material properties. Step 5 In a fifth step, the desired amount of inoculant is added to the melt, after which the cast iron melt is cast in a suitable mold.

It may, of course, be the case that a single cast iron melt is to be manufactured for which there is no predetermined value for the saturation level of the melt for inoculants. In this case, the saturation level is determined by repeated experiments with two sample cups in the manner described above. The determined amount of inoculum is then added to the cast iron melt which is then cast.

DESCRIPTION OF EMBODIMENTS The invention will be described in the following with a concrete example.

A 10 ton cast iron smelter of the gray iron type with scale graphite was prepared in a medium frequency furnace from the manufacturer ABB. In a first step, the amount of inoculant needed for the melt to be saturated in inoculum was determined. For this, four test rounds, 1 - 4, were prepared. In each test round, the sampling equipment, see Figure 3, consisted of two sample containers 1 and 2 in the form of sand cups in which a thermocouple 3, 4 was each arranged. The sand cups and thermocouples had the following specifications: ~ Cup material: 65 gram shell-shaped sand cups made of silica sand.

° The sand was coated with 3.5% phenolic resin, iron oxide and stearin-based lubricant.

~ Thermocouple: NiCr- NiAl alloy, insulated with glass tube of high-purity quartz glass.

The thermocouples were placed as shown in Figure 3. During the measurements, the cups had an average volume of 4.86 cma (based on an average weight of 350 g and a density of 7.22 g / cm3). 10 15 20 25 30 534 512 14 Each thermocouple was connected to a separate channel of an analysis equipment comprising a computer on whose processor the evaluation software ATAS from the manufacturer Novacast was run.

The average weight of the melt contained in the test cups was calculated and in test cup 2 in test run 2 - 4 placed inoculants (6) of the type Superseed, which is an alloy of FeSi containing strontium. The amount of inoculum was calculated with respect to the amount of melt held by the sample cups and determined in weight percent cast iron. The added amounts of inoculants are shown in Table 1. In test round 1, inoculants were not placed in any of the specimens.

The sample cups were then filled with the same amount of cast iron melt and cooling curves were recorded during solidification of the samples and analyzed in the evaluation program.

The lowest eutectic temperature, TEiow, was then determined for each cooling curve using the evaluation program. This is called TE | ow "measured". Because the inoculum contained silicon, the effect of silicon on the eutectic temperature was then subtracted from each of the three inoculated samples.

Table 1 shows the eutectic temperatures TE "measured" for samples 1 and 2 in each test run and the eutectic temperatures of the inoculated samples after silicon compensation (TE | ° W "corrected").

The eutectic temperature for each inoculated sample TEjovf measured "was then subtracted from the eutectic temperature for each inoculated sample. The result is called ATEiOv / 'measurement fï After the eutectic temperature of the inoculated samples was corrected for the silicon content, they were subtracted from the eutectic temperature of the inoculated samples. This result is called ATE | ° w "corrected". The values are shown in Table 1. 10 534 912 1 5 Sample cycle Weight% TEww TEW, grafted TEW grafted ATEN ”1 0 1138.24 1138.04 1138.04 -0.2 -0.2 2 0.1 1135.975 1139.725 1138.975 3.75 3 3 0.2 1132425 1139.075 1137.575 6, 65 5.15 4 0.4 1132075 1139.95 113Ö .95 7.875 4.875 Table 1: Results from determination of saturation amount of inoculum Figure 5 shows a diagram where ATE | ° w "measured" and ATElow "corrected" have been plotted for each test round .

The diagram shows that when the samples have been corrected for the effect of silicon on the eutectic temperature, the difference between the eutectic temperatures of an inoculated sample and an inoculated sample stops increasing when the inoculum content exceeds 0.2% by weight. This content thus constitutes the saturation level of the cast iron melt for inoculants. The diagram in Figure 5 also shows that when the samples have not been compensated for the effect of silicon, the difference between the eutectic temperatures continues to increase with increasing inoculant content. Therefore, a saturation level of the inoculum can not be determined.

Claims (1)

A method for determining the amount of inoculum to be added to a cast iron melt in a particular casting process, comprising the steps of: - providing a first sample container (1) and a second sample container (2) each comprising a thermocouples (3, 4) connected to an analysis equipment (5); - fill the respective sample container (1, 2) with a quantity of molten cast iron; registering a first cooling curve during solidification of the cast iron in the first sample container (1) and a second cooling curve during solidification of the cast iron in the second sample container (2); characterized in that in one of the sample containers, before filling of molten cast iron, a predetermined amount of inoculant representing the saturation level of inoculum of the particular casting process is placed, the amount of inoculum to be added to the cast iron melt in the determined casting process being determined by the temperature of the first cooling curve and the lowest eutectic temperature (TEiow) of the second cooling curve. . The method of claim 1, wherein the predetermined amount of grafting agent representing the casting process is an average value based on grafting saturation levels of a plurality of casting iron melts in the determined casting process. The method of claim 1, wherein the predetermined amount of grafting agent representing the casting process is a selected value from grafting saturation levels of a plurality of cast iron melts in the determined casting process. . The method according to any one of claims 1 to 3, wherein the inoculum contains silicon and that the effect of silicon on the eutectic temperature (TEiow) 534 912 17 is deducted based on a predetermined relationship between silicon and eutectic temperature. The method according to any one of claims 1 - 4, wherein the amount of inoculant to be added to the cast iron melt in the determined casting process is controlled against over- or under-grafting of the melt.