SE1050616A1 - 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 analysis 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

10 15 20 25 30 However, this known method does not provide an exact measure of how they are included the structures are nucleated or grow but give only 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 in the cast iron by adding grafting agent. With the known methods it has however, it has proved difficult to optimize the amount of inoculum to be added.

The nucleation of the constituent structures in the cast iron is thus large impact on the properties of cast iron, i.a. on defect formation and strength.

For the final properties of the finished casting is special the structure of the eutectic phase important. Prior art methods provide however, not 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 determine the amount of inoculant to be added to a cast iron melt, which method solves the above problems or at least reduces them to one minimum.

SUMMARY OF THE INVENTION This object is achieved by the method of determining the amount of inoculum as a cast iron melt shall be added in a particular casting process, comprising steps: provide a first sample container and a second sample container each comprising a thermocouple connected to one analysis equipment; -fill each sample container with a quantity of molten cast iron; register a first cooling curve while solidifying the cast iron in the first the sample container and a second cooling curve during solidification of the cast iron in the second sample container; 10 15 20 25 30 characterized by that in one of the sample containers, before filling in molten cast iron, a predetermined amount of inoculum representing the determined the saturation level of the grafting process for inoculants, the amount inoculants to be added to the cast iron melt in the determined the casting process is determined from the difference between the lowest eutectic the temperature (TEiow) of the first cooling curve and the lowest the eutectic temperature (TEiow) of the second cooling curve.

The method entails a precise control of the casting process, which in turn gives giving rise to minor variations in the quality of the finished castings. This causes savings in the form of fewer scraps during casting, processing and assembly while reducing the risk of breakdowns.

According to an alternative, the predetermined amount of inoculum is as the casting process represents an average value based on saturation levels for inoculants of a plurality of cast iron melts in the particular one the casting process.

According to an alternative, the predetermined amount of inoculum is as the casting process represents a selected value from saturation levels for inoculants of a plurality of cast iron melts in the particular casting process.

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

The amount of inoculant to be added to the cast iron melt in the determined the casting process can, if necessary, be controlled against over- or under-grafting of the melt. 10 15 20 25 30 Preferably, the method is intended to determine the structure of a cast iron mountain graphite type.

DESCRIPTION OF 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 it the method according to the invention.

Figure 4 A diagram showing the difference between the eutectic the temperature TE | 0W of an inoculated or inoculated 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. Formation of these cores, i.e. nucleation, is controlled by the total energy in the melt. One systems, such as a molten metal, strive to have as low an energy content as possible. Which state of solid or liquid has the lowest energy content for a substance depends on its temperature. When a solid phase is formed in a melt, it is formed a volume where the atoms are arranged in an energy-efficient way. At the same time a surface is formed between the solid and the liquid phase. The atoms in the surface become pressed into positions they would not normally be in, which requires energy. When a melt solidifies into a solid body, there is thus a reduction 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 system's total energy content can be described with the equation: AG = AGvV + yA, where AG is the total change of energy, V is the body volume and A is the surface area of the body. 10 15 20 25 30 Nucleation occurs only if it leads to the total energy content of the system decreases, ie AG becomes negative. Because the surface energy counteracts the phase transformation, the melt will remain liquid even then the temperature exceeds the solidification temperature. This is called submerging the melt. The more the temperature is lowered below the solidification temperature, the more greater becomes the driving force to change phase. Nucleation occurs when the temperature has become so low that the decrease in volume energy is greater than that amount of energy used to create the surface. For homogeneous melts can this hypothermia amounts to several hundred degrees, but is normal significantly lower. How much subcooling is needed for nucleation to take place spoon is an indicator of how easily nucleation can occur in the melt, which also is called the nucleation potential of the melt.

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 of the vertical axis and carbon content 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 limits the different phases that the iron assumes at different temperatures and carbon content. In the figure, the liquidus line is 1 and a line marks it eutectic temperature 2 marked. These two lines are important to predict the structure of the cast product. At the liquidus temperature secreted austenite, also called y-iron. At the eutectic temperature begins also carbon to be separated from the remaining melt. The carbon is excreted in the form of graphite, which, due to its shape, has a great influence on the properties of the material.

Upon cooling from a completely molten state to a solidified state, a melt passes through the different phases of the Fe-C diagram. Figure 2 schematically shows one solidification curve for a cast iron. In Figure 2, the plateau indicates "a" at the temperature 1200 ° C melting temperature of the melt. At this point, austenite begins to formed in the melt. As the temperature passes 1150 ° C, the eutectic begins 10 15 20 25 30 solidification. This is marked with "b" which is the lowest eutectic the temperature in the melt TE | 0W. The eutectic solidification is indicated by a the melt. small temperature increase in When the temperature returns to even reduction, area "c", all melt has changed to solid form.

How much subcooling is needed for the eutectic distinction should start, ie TE | 0W has proven to be a good indicator of how good the nucleation potential of the melt. Slight subcooling means good nucleation potential. It is important to achieve a high nucleation potential in the eutectic solidification as this gives rise to an even graphite separation during solidification. The even graphite separation is necessary to obtain good mechanical properties in the casting because graphite volume increase during solidification counteracts the volume decrease of the austenite.

The nucleation potential can be controlled by adding nucleation points in in the form of inoculants. By adding inoculum the temperature is raised at which the eutectic solidification occurs. That is, less subcooling is required of the melt to begin the eutectic solidification.

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

Added too much inoculum leads to high production costs and can also cause disadvantages for the structure of the cast product, e.g. as a result of graphite expansion.

As mentioned above, it is therefore advantageous to determine the melting point nucleation potential in the eutectic minimum at point “b” of the cooling curve.

This is because the subcooling during the eutectic solidification is directly connected to the final structure of the cast goods. 10 15 20 25 30 The inventors have found through measurements that a cast iron melt can become "Saturated" with inoculants. This means that when added inoculum stops the temperature of the eutectic solidification to increase at a certain level even if more inoculants are added. This means that at this "saturation level" one is obtained absolute limit of the TE | OW of the melt which is the highest eutectic the 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, because one 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 inoculated melt can thus be compared with a stable guide value.

The following method according to the invention will be described in detail.

Step 1 In a first step, a cast iron melt is manufactured. This is done by a starting materials, such as scrap, recycled, pig iron and shavings are melted. The melt composition and temperature are checked and any adjustments are made.

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

The test equipment (see figure 3) comprises two test cups 1 and 2, e.g. 3 and 4.

The thermocouples are connected to an analysis equipment 5 on which software sand cups, each of which comprises a thermocouple for thermal analysis is run. A predetermined amount is placed in one test specimen grafting agents that represent the saturation level of the particular casting process for inoculants. 10 15 20 25 30 The level of saturation of the inoculant, ie the amount of inoculant that must Cast iron melts are added to saturate them with inoculants varies with the process conditions in the manufacture of the melts. These conditions include to (scrap) example of the starting material composition and settings of the process equipment.

In industrial processes for the manufacture of cast iron components, so-called casting processes, however, one strives to keep the conditions in process as constant as possible, i.e. keep these within certain borders. The manufacture of a certain component, such as an engine block, is sought to always start from scrap with a composition kept within established limits and / or always use the same casting equipment when manufacturing one certain component. This is done to ensure as uniform a result as possible obtained between different castings.

The amount of inoculum to be added to one sample cup to melt which is filled therein must reach the saturation level can therefore be determined in advance as a representative value for a particular casting process. The representative the value can then be used each 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 co-determined for a plurality of melts for a particular casting process, for example, a process for casting engine blocks of a certain type. The representative the value can also be selected from several different saturation levels for inoculants and determined for a plurality of melts for a particular 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 for it certain casting process is then stored, for example in a non-volatile memory in a computer, from where it can be retrieved and used each time it is determined the casting process must be performed. 10 15 20 25 30 It is also possible to determine saturation values for inoculants for a plurality different cast iron melts and then adapt a linear relationship to these saturation values.

The amount of inoculum required for an individual cast iron melt shall be saturated is determined as follows. The eutectic temperature TE | 0W determined simultaneously for two samples taken from the cast iron smelter. One the sample is inoculated and in the second sample inoculants have been added. For to determine the saturation level, a sample is poured from the cast iron melt at the same time in two identical sample cups and TEjow is then determined for each sample from cooling curves during solidification of the cast iron samples. which is registered The nucleation potential of the inoculated sample is then determined from the difference between TEjow in the inoculated sample and TEjow in the inoculated tried. The procedure is repeated several times and each time it is inoculated one sample with more inoculum than in the previous test round. Through to compare the nucleation potentials of the repeated samples it can level is found where the nucleation potential stops increasing with increasing vaccine content. This level is the saturation level of the current cast iron melt for inoculants. As mentioned above, the saturation level is determined for several cast iron melts and based on these can an average saturation level for a specific casting process is determined or a representative value can selected.

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

The saturation level of inoculants is thus determined by repeated experiments there each experiment involves two measurements simultaneously in two separate identical test cups and under the same conditions. This is because at thermal analysis, in which cooling curves are recorded during solidification of cast iron in sample cups, 10 15 20 25 'IO the cooling curve is affected by several factors that are not related the nucleation potential of the melt. These factors are ex: 0 Sample cup material 0 Variations in the location and design of the thermocouple 0 Variations in Air draft due to walls, ventilation and the like 0 The amount of melt in the cup 0 Chemical composition of the melt 0 The temperature of the melt when it is poured into the cup The above-mentioned error factors affect the measurement results by, for example, whole the cooling curve is shifted upwards or sideways, whereupon the eutectic temperature ends up at the wrong level.

If errors occur during measurements performed in two test cups below identical conditions, however, both measurements will be encumbered with the same error. By the eutectic temperatures TE | 0W from each sample is subtracted from each other all of these are eliminated felfa ktorer.

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

Because the temperature when the eutectic solidification starts is based on the chemical composition of the melt increases the eutectic temperature in it grafted the sample when adding silicon. To exclude this error factor as well the effect of silicon on the eutectic temperature must be subtracted from it grafted the sample. 10 15 20 25 30 11 It is possible to develop a connection for the effect of silicon on the eutectic the temperature. This connection can be developed by studying phase diagrams and engineering adaptation. It is also possible to create the connection with one suitable calculation program, eg ThermoCalc. Using the connection can the effect of silicon on the eutectic temperature is excluded.

It also happens that the inoculum, instead of silicon, is alloyed with others substances that affect the equilibrium lines in the phase diagram, ie it affects it eutectic temperature. It is also possible to create connections for how these substances affect the eutectic temperature as well as to utilize these connection to compensate for the eutectic temperature in the inoculated tried.

Step 3 In a third step, the sample cups are then filled with cast iron melt and cooling curves registered during solidification of the samples. From the two cooling curves are determined then the lowest eutectic temperature TEjow for each sample. The lowest the eutectic temperature of the ungrafted sample is then subtracted from it lowest eutectic temperature in the inoculated sample.

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

Step 4 In a fourth step, based on the calculated difference in ° C, it is determined how much inoculum to be added to the cast iron melt to the finished casting shall obtain a desired structure. The resulting difference in ° C, i.e. the nucleation potential of cast iron smelting is a measure of how far it is The eutectic temperature of the cast iron melt is from the practically achievable 10 15 20 25 30 12 eutectic temperature. The resulting difference in ° C can be related to the final structure of the casting.

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

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

It is also possible to control the amount of inoculum to be added 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 melt an individual cast iron add exactly the amount of inoculant corresponding to the saturation level of it determined the casting process for the individual melt. This is due to over over time, the saturation levels of the individual cast iron melts have one spread around the representative value of the saturation level of the casting process. individual established borders. This means that the saturation level of inoculants for each individual For example, the spread can be caused by them the compositions of cast iron melts vary within the melt differs from the saturation level that represents the casting process.

Should inoculants be added to the individual melts in exactly the same amount as the saturation level representing the casting process indicates this would lead until the melts are over-grafted and under-grafted. As mentioned previously entails over- resp. inoculation various disadvantages m a p costs and material properties. 10 15 20 25 30 13 Step 5 In a fifth step, the desired amount of inoculant is added to the melt and then the cast iron melt is cast in a suitable mold.

It may, of course, be the case that a single cast iron smelter is to be manufactured for for which there is no predetermined value for the saturation level of the melt inoculants. In this case, the saturation level is determined by repeated experiments with two test cups in the manner described above. The established the amount of inoculant 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 rock graphite was prepared in one medium frequency furnace from the manufacturer ABB.

In a first step, the amount of inoculum needed to determine was determined the melt would be saturated with inoculum. For this, four were prepared test rounds, named 1 - 4. In each test round passed the sampling equipment see Figure 3, of two sample containers 1 and 2 in the form of sand cups in which each a thermocouple 3, 4 is arranged.

The sand cups and thermocouples had the following specifications: ~ Cup material: 65 gram shell-shaped sand cups made of silica sand. v The sand was coated with 3.5% phenolic resin, iron oxide and stearin-based lubricant. 0 Thermocouple: NiCr- NiAl alloy, insulated with glass tube of high-purity quartz glass. 0 The thermocouples were placed as shown in Figure 3. The cups had below the measurements an average volume of 4.86 cm3 (based on average weight of 350 g and density of 7.22 g / cms). 10 15 20 25 30 14 Each thermocouple was connected to a separate channel of an analysis equipment including a computer on whose processor the evaluation software ATAS from the manufacturer Novacast was run.

The average weight of the melt contained in the sample cups was calculated and in test cup 2 in test rounds 2 - 4 placed inoculants (6) of the type Superseed, which is an alloy of FeSi with a content of strontium. The amount inoculants were calculated with respect to the amount of melt as the test cups held and were fixed in weight percent cast iron. The appointed the amounts of inoculants are shown in Table 1. In sample round 1 was not placed inoculants in one 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 evaluation program.

The lowest eutectic temperature, TE | 0W was then determined for each cooling curve using the evaluation program. This is called TE | 0W “measured.” Because the inoculum contained silicon, it was then subtracted the effect of silicon on the eutectic temperature from each of the three grafted the samples. The now corrected samples are called TE | 0W "corrected".

Table 1 shows the eutectic temperatures TE | 0W "measured" for samples 1 and 2 in each sample round as well as the eutectic temperatures of the inoculated the samples after compensation for silicon (TE | oW "corrected").

The eutectic temperature for each inoculated sample TE | 0W "measured" were then subtracted from the eutectic temperature of each inoculum sample. The result is called ATE | 0W "measured". After they inoculated the samples eutectic temperature corrected for the silicon content they were subtracted from the eutectic temperature of the inoculated samples. This result called ATE | 0W "corrected". The values are shown in Table 1. 10 15 Test Round Weight% TE | OW TE | OW grafted TE | OW ym pad ATE | 0W ATE | OW inoculum oym pad "measured" "corrected" "measured" "corrected" 1 01238.24 1138.04 1138.04 -0.2 -0.2 2 0.1 1135.975 1139.725 1138.975 3.75 3 3 0.2 1132.425 1139.075 1137.575 6.65 5.15 4 0.4 1132.075 1139.95 1136.95 7.875 4.875 Table 1: Results from determination of saturation amount of inoculant Figure 5 shows a diagram where ATE | 0W has "measured" and ATEN / "corrected" drawn for each test round.

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

Claims (6)

10 15 20 25 30 16 PATENT REQUIREMENTS 1. 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 thermocouple (3, 4) connected to an analysis equipment ( 5); -fill each 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 difference between of the first cooling curve and the lowest eutectic temperature (TEbW) 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 cast 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 saturation levels of grafting agent 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 (TElow) 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 inoculum to be added to the cast iron melt in the determined casting process is controlled against over- or under-grafting of the melt The method according to any one of claims 1 -5, wherein the cast iron is of the rock graphite type.