METHOD FOR PREDICTING THE MICROSTRUCTURE OF SOLIDIFYING CAST IRON

Abstract: By studying heat transfer in a sample vessel containing a sample of molten cast iron, it is possible to carry out accurate predictions of the microstructure in which the molten cast iron sample will solidify. This method is also highly suitable for automatisation by using a computer.

Description: METHOD FOR PREDICTING THE MICROSTRUCTURE OF SOLIDIFYING CAST IRON The present invention relates to an improved method for predicting the micro- structure with which a certain cast iron melt will solidify. The invention also relates to an apparatus for carrying out the method. Background of the invention WO86/01755 (incorporated by reference) discloses a method for producing compacted graphite cast iron by using thermal analysis. A sample is taken from a bath of molten cast iron and this sample is permitted to solidify during 0.5 to 10 minutes. The temperature is recorded simultaneously by two temperature responsive means, one of which is arranged in the centre of the sample and the other in the immediate vicinity of the vessel wall. So-called cooling curves representing temperature of the iron sample as a function of time are recorded for each of the two temperature responsive means. According to this document it is then possible to determine the necessary amount of structure-modifying agents that must be added to the melt in order to obtain the desired microstructure. WO92/06809 (incorporated by reference) describes a specific method for evalu- ating the cooling curves obtained by the method of WO86/01755. According to this document, thermal analysis is carried out in a sample vessel coated with a material consuming the active form of the structure-modifying agent. This material may comprise oxides of Si, Mn, Fe, K and Na. An early plateau in the cooling curve recorded by a certain temperature-responsive means located near the vessel wall indicates that flake graphite has been formed due to interaction with the coating. It is then possible to determine whether any structure-modifying agent has to be added to the melt in order to obtain compacted graphite cast iron by using calibration data. When casting compacted graphite cast iron in a commercial large-scale foundry, it is of utmost importance that accurate and reliable predictions of the micro- structure of the castings can be carried out. Sometimes cooling curves that are difficult to interpret are accepted as CGI despite the fact that some flaky graphite has been formed. In order to improve the accuracy of the evaluation, there is thus a need for alternative methods for evaluating cooling curves which methods can compensate for deviations from the normal appearance of the cooling curves. Summary of the invention Now, it has turned out that by studying heat transfer in a sample vessel containing a sample of molten cast iron, it is possible to carry out accurate predictions of the microstructure in which the molten cast iron sample will solidify. This method is also highly suitable for automatization by using a computer. Definitions The term "cooling curve" as disclosed herein refers to graphs representing temperature as a function of time, which graphs have been recorded in the manner disclosed in WO86/01755 and WO92/06809. The term "heat generation curve" as utilised herein relates to a graph showing the heat that is generated in a certain zone of a molten cast iron as a function of time. For the purposes of the present invention, the heat generation curves herein are determined for a zone located in the centre of a molten cast iron sample (the A zone), and in the periphery of a molten cast iron sample (the B zone), respectively. Below, methods for determining heat generation curves will be further described. The term "sample vessel" as disclosed herein, refers to a small sample container which, when used for thermal analysis, is filled with a sample of molten metal. The temperature of the molten metal is then recorded during solidification in a suitable way. Preferably the sample vessel is designed in the manner disclosed in WO86/01755, WO92/06809, W091/13176 (incorporated by reference), WO96/23206 (incorporated by reference) or PCT/SE98/02122. The term "sampling device" as disclosed herein, refers to a device comprising a sample vessel equipped with at least two temperature responsive means for thermal analysis, said means being intended to be immersed in the solidifying metal sample during analysis, and a means for filling the sample vessel with molten metal. The sample vessel is preferably equipped with said sensor in the manner disclosed in fig. 2 in WO96/23206 or PCT/SE98/02122. The term "structure-modifying agent" as disclosed herein relates to compounds affecting the morphology of graphite present in the molten cast iron. Suitable compounds can be chosen from the group of magnesium and rare earth metals such as cerium, or mixtures of these compounds. The relationship between the concentration of structure-modifying agents in molten cast irons have already been discussed in the above cited documents WO92/06809 and WO86/01755. The term "CGI" as disclosed herein refers to compacted graphite cast iron. The term "SGI" as disclosed herein refers to spheroidal graphite cast iron. Figures The invention will now be described with reference to the accompanying figures in which: Fig. 1 schematically outlines a diagram of a sample vessel that can be used in connection with the present invention. In such a sample vessel the heat is uni- formly transported in all directions. A sample of molten cast iron comprised in such a vessel can therefore be regarded as a freezing sphere. In this figure, the freezing sphere is divided into two zones A and B. The radii ri and r 2 relate to the average radii of zones A and B, respectively; Fig. 2 discloses cooling curves and corresponding heat generation curves for CGI. In figures 2 - 7, the following abbreviations are used: T A = centrally recorded cooling curve, T B = peripherically recorded cooling curve, P A = heat generation in the centre, and Pβ = heat generation in the periphery; Fig. 3 discloses cooling curves and corresponding heat generation curves for a low nodularity CGI. During the measurement flake graphite is formed in the sample vessel due to reaction with the wall coating; Fig. 4 discloses cooling curves and corresponding heat generation curves for cast iron comprising a high carbon equivalent content; Fig. 5 discloses cooling curves and corresponding heat generation curves for a near eutectic iron; Fig. 6 discloses cooling curves and corresponding heat generation curves for a eutectic or hypereutectic iron; and Fig. 7 discloses cooling curves and corresponding heat generation curves for grey flake iron; and Fig. 8 is a schematic presentation of an apparatus for controlling production of compacted graphite cast iron according to the present invention. Detailed description of the invention As already disclosed above, the present invention relates to predicting the micro- structure in which a certain molten cast iron sample will solidify by measuring the heat evolution in the sample. In particular, the invention relates to determining the heat transfer between two sample zones (one zone in the centre of the sample and one peripheral zone encompassing the central zone) and the sur- roundings. By studying graphs presenting the derivative of heat as a function of time, it is possible to carry out accurate predictions. The thermal balance of any uniform element can be described by the relation: stored ~ Qgenerated " • " Vin " Vout ( 1 ) where Q st0red is the amount of heat stored by the heat capacity of the material, Qg enerat ed is the amount of heat generated by the volume of material, Q in is the heat transferred into the material from its surroundings and Q out is the heat transferred from the sample volume to its surroundings. When carrying out the present invention, it is advantageous to use a sample vessel as disclosed in SE 9704411-9. In such a sample vessel, the heat transport in a sample contained in the vessel is approximately the same in all directions. From now on, the heat transport between the centre (Figure 1, zone A) and the more peripheral part (Figure 1, zone B) of the molten cast iron contained in a sample vessel is described. As the A zone is situated in the centre of the freezing sphere, no heat will be transported into the zone and Q m is therefore equal to zero. Appropriate substitutions in relation (1) above give the following equation: C p π A dT A /dt = Q g e„A + 0 - 4πke [(T A -T B )/( 1/r, - l/r 2 )] (2) where C p is the heat capacity per unit mass, m A is the mass of the A zone,, dT A /dt is the temperature change of the A zone per unit time, ke is the effective heat transfer coefficient of the material, and (1/rι -l/r ) * ' is the mean distance for heat transport. The radii ri and r 2 are both defined in fig. 1. T A and T B are the temperatures in the A zone and B zone, respectively. From equation (2) we can isolate the heat generation term and calculate the aver- age bulk heat generation in the A zone: Q ge „ A = Cpm A dT A /dt + 4π ke [(T A -T B )/(l/r l/r 2 )] (3) In equation (3) all variables are constants except dT A /dt and (T A - T B ). Accord- ingly, equation (3) can be simplified to: Q g e nA = k, dT A /dt + k 2 (T A - T B ) (4) where ki and k are constants. Hence a heat generation curve corresponding to the A zone can be calculated from a set of cooling curves recorded in the centre and periphery of a sample of molten cast iron. Heat balance for the B zone resembles the balance of zone A, but heat is transferred both into the zone (from the A zone) and out from it (to the surroundings). Hence term Q ιn in relation (1) is not equal to zero. Appropriate substitutions in relation (1) give the following equation: C p ni B dTβ/dt = Q g e n B + 4πke [(T A -TB)/(l/r,-l/r 2 )] -[h(T B -T s )+εA B σ(T B 4 -Ts ;] (5) where, in addition to those variables defined in connection with equations (2) and (3) above, h is the convective heat loss to the surroundings, T s is the temperature of the surroundings, and εA B σ is the appropriate constant in Stefan-Boltzmann law of radiation. Regarding the last term, we can assume that the ratio between total radiative heat loss and convective heat loss is constant: C = εA B / h (6) Accordingly, we can isolate the heat generation term of zone B, Q genB : QgenB = Cpm B dTβ/dt - 4πke [(T A -T B )/(l/r l/r 2 )] + h[(T B -T s ) + Cσ(T B 4 -T s 4 )] (7) In equation (7) all variables are constant except dTβ/dt, (T A - T B ) and T B . Ac- cordingly, equation (7) can be simplified to: Qge nB = 3 dT B /dt - l t (T A - T B ) + k 5 T B + k* T B 4 - k 7 (8) By applying equations (4) and (8) on cooling curves recorded in the manner dis- closed in WO86/01755 and WO92/06809 we can determine the corresponding heat generation curves. Figs. 2A-6A disclose different kinds of cooling curves and Figs. 2B-6B corresponding heat generation curves. As already mentioned, these calculations are based on a situation where heat is uniformly transported in all directions. The skilled person can of course determine other equations corresponding to other heat transport conditions. There are larger differences between different kinds of heat generation curves compared to corresponding cooling curves. For CGI (Fig. 2), the curve repre- senting heat release in the B zone comprises two distinct peaks, one smaller and one larger, whereas the heat release for low nodularity CGI (Fig. 3) is so much larger that these two peaks hardly can be separated. This also applies to curves related to high carbon equivalent cast iron (Fig. 4). A primary austenite peak is visible the A zone curve. In heat generation curves representing near eutectic iron (Fig. 5) a small austenite peak is still visible, whereas the peak has disappeared in the curves relating to eutectic or hypereutectic iron (Fig. 6). As regards grey flaky iron (Fig. 7), the first peak of the heat generation curve representing the B zone is dominating. It is preferred to carry the prediction method by using a computer-controlled system, especially when a large number of measurements must be carried out. Such a system is outlined in fig. 8. A sample of a molten cast iron is taken and transferred to a sample vessel 22. During the measurement of a particular sample the two temperature-responsive means 10,12 send signals to a computer means 14 in order to generate cooling curves and corresponding heat generation curves, e.g. by applying equations (3) and (6). The computer has access to calibration data, e.g. previously recorded model curves corresponding to a known amount of structure-modifying agent or a known microsrructure, in a ROM unit 16 and calculates the amount of structure-modifying agent that has to be added to the melt. In a preferred embodiment this is implemented as an expert system. The amount of structure-modifying agent to be added is signalled to a means 18 for administrating structure-modifying agent to the melt 20 to be corrected, whereby the melt is supplied with an appropriate amount of such agents.

Claims: Claims: 1. A process for producing a CGI casting or optionally a SGI casting, requiring a sampling device, at least two means for recording cooling curves, and a means for administering structure-modifying agent(s) to a molten cast iron from which said casting is to be produced, said method comprising the steps of: a) providing a sample of the molten cast iron from which said casting is to be produced, and adding said sample to a sample vessel; b) simultaneously recording at least two cooling curves, at distances τ } and r 2 , etc., from the centre of the sample vessel; c) determining at least two heat generation curves by using the information obtained in b); d) comparing the heat generation curves obtained in c) with previously determined heat generation curves representing known concentrations of structure- modifying agents which previously determined heat generation curves have been obtained under same conditions as in step a), b) and c); e) determining the amount of structure-modifying agent to be added to the melt based upon the comparison in step d); f) causing said means for administrating structure-modifying agent to adminis- trate a correct amount to the melt; and g) carrying out the casting operation in a per se known manner. 2. A process according to claim 1, characterized in that said sample vessel is of a type wherein heat is substantially uniformly transported in all directions when it is filled with molten cast iron. 3. A process according to claim 1 or claim 2, characterized in that compacted graphite cast iron is produced. 4. A process for determining the amount of structure-modifying agent that has to be added to a certain cast iron melt in order to obtain CGI or optionally SGI, requiring a sampling device, and at least two means for recording cooling curves, said method comprising the steps of: a) providing a sample of the molten cast iron from which said casting is to be produced, and adding said sample to a sample vessel; b) simultaneously recording at least two cooling curves, at distances and r 2 , etc., from the centre of the sample vessel; c) determining at least two heat generation curves by using the information obtained in b); d) comparing the heat generation curves obtained in c) with previously determined heat generation curves representing known concentrations of structure- modifying agents which previously determined heat generation curves have been obtained under same conditions as in step a), b) and c); e) determining the amount of structure-modifying agent to be added to the melt based upon the comparison in step d); 5. An apparatus for establishing, in real time, an amount of a structure-modifying agent to be added to produce a CGI casting or optionally a SGI casting, the apparatus comprising: a first temperature sensor (10), arranged at a distance τ x from the centre of a sample vessel but inside said sample vessel, for recording a first cooling curve; a second temperature sensor (12), at a distance r 2 (r 2 being larger than τ ) from the centre of said sample vessel but inside said sample vessel, for recording a second cooling curve; a computer device (14) for determining an amount value (Na) of a structure-modifying agent to be added to the melt; a memory means (16) which is provided with pre-recorded calibration data; the computer being set up to determine heat generation curves for one central zone and one peripheral zone in said sample vessel by applying i) the thermal balance formula: Vstored ~ ~ Vgenerated ' Viπ Vout where Q st0 red is the amount of heat stored by the heat ca- pacity of the material, Q gen erated is the amount of heat generated by the volume of material, Q ιn is the heat transferred into the material from its surroundings and Q out is the heat transferred to the surroundings; and ii) the first and second cooling curves recorded by said first and second temperature sensors (10, 12); the computer being set up to determine an amount value (Na) of a structure-modifying agent to be added to the melt by comparing said heat generation curves with pre-recorded calibration data. A computer program product for use in an apparatus for establishing, in real time, an amount of a structure-modifying agent to be added to a cast iron melt (20) during the process of producing a compacted graphite iron casting; wherein the apparatus has a first temperature sensor (10), arranged at a distance η from the centre of a sample vessel but inside said sample vessel, for recording a first cooling curve; a second temperature sensor (12), at a distance r 2 (r 2 being larger than ri) from the centre of said sample vessel but inside said sample vessel, for recording a second cooling curve; a computer device (14) for determining an amount value (Na) of a structure-modifying agent to be added to the melt; a memory means (16) which is provided with pre-recorded calibration data; the computer program product comprising: a recording medium and a computer-readable code means for directing the computer device to determine heat generation curves for one central zone and one peripheral zone in said sample vessel by applying i) the thermal balance formula: stored — Vgenerated " * Vtn ~ " ~ Vout where Qstored is the amount of heat stored by the heat capacity of the material, Qge n e r ated is the amount of heat generated by the volume of material, Q ιn is the heat transferred into the material from its surroundings and Qout is the heat transferred to the surroundings; and ii) the first and second cooling curves recorded by said first and second temperature sensors (10, 12); and a recording medium and computer-readable code means for determining an amount value (Va) of a structure-modifying agent to be added to the melt by comparing said heat generation curves with prerecorded calibration data.

Inventors: ANDERSSON CONNY, POPELAR PATRIK, ANDERSSON, CONNY, POPELAR, PATRIK
Applicants: ANDERSSON CONNY, POPELAR PATRIK, SINTERCAST AB, ANDERSSON, CONNY, POPELAR, PATRIK
Application year: 1999
Main classification: C22C33
IPC codes: B22D1, B22D2, C21C1, C22C33, G01N25, G01N33