ON-LINE PRODUCTION CONTROL OF CAST IRONS BY MEASURING THE SURFACE TENSION OF THE BASE TREATED IRON

Abstract: A method for producing cast irons having a certain desired morphology of graphite crystals and containing graphite shape modifiers and/or inoculants, according to which: a) a sample quantity of the molten iron is taken from the batch of molten iron; b) the surface tension of the sample is determined; c) the surface tension of the sample is compared with the surface tensions of batches of molten iron that are known to solidify as different forms of cast irons; and finally d) if the surface tension of the sample deviates from the surface tension that is known to solidify as the desired form of cast iron by more than a predetermined value, correcting the concentrations of graphite shape modifiers and/or inoculants in a known way.

Description: On-line production control of cast irons by measuring the surface tension of the base treated iron. The present invention relates to a new method for predicting the nucleation and growth of graphite crystals during the solidification of cast irons. WO-A1-86/01755 relates to a such a method. A sample from a bath of molten iron is permitted to solidify during 0,5 to 10 minutes. The temperature is recorded simultaneously by two temperature responsive means, one of which is placed in the centre of the sample and the other in the immediate vicinity of the vessel wall. The number and shape of the graphite particles which constitute the graphite phase is assessed in relation to known reference values by aid of recorded values of supercooling at the vessel wall, the recalescence at the vessel wall, the difference between the temperature at the vessel wall and at the centre of the vessel and the maximum slope of the temperature time curve during the period of con¬ stant eutectic growth temperature at the centre. The method can be used for calculating the amounts of inoculants and graphite structure modifiers that have to be added or removed in order to obtain a cast iron comprising a desired form of cast iron, such as flaky graphite iron, compacted graphite iron (CGI) and spheroidal graphite iron (SGI) . The method according to O-A1-86/01755 produces excellent results, but it also has some drawbacks. The amount of time required for the determination, up to 10 minutes may be rather high, and the measurement per se is also rather com¬ plicated. Hence, there is a need for a quick, simple and reliable method for predicting the nucleation and growth of graphite crystals during the solidification of cast irons. Summary of the invention The aforesaid problem in connection with producing cast irons having a certain desired morphology of graphite crystals and containing graphite shape modifiers and/or inoculants, is solved by a method in accordance with the invention, accor¬ ding to which: a) a sample quantity of the molten iron is taken from the batch of molten iron; b) the surface tension of the sample is determined; and finally c) the surface tension of the sample is compared with the surface tensions of batches of molten iron that are known to solidify as different forms of cast iron. Short description of the figures The invention will now be described with reference to the accompanying figures according to which: Figure 1 is a graphic representation of the surface tension of liquid iron as a function of the concentration of the highly surface active elements, oxygen, sulfur and selenium, and standard components of cast irons which are less surface active such as nitrogen, phosphorus and carbon; Figure 2 illustrates the atomic structure of graphite and defines the A-axis and C-axis directions of growth of graphi- te crystals; and Figure 3 is a schematic illustration of the measuring equip¬ ment. Detailed description of the invention The method according to the present invention is based on an accurate measurement of the surface tension of the liquid iron following base treatment with inoculants and graphite shape modifiers such as magnesium. As is shown in figure 1, it is well-known (See P. Kzakevitch and G. Urbain: J. of the Iron and Steel Institute (JISI) , Vol. 186, 1957, pp. 167- 170.) that the surface tension of liquid iron changes with varying contents of surface active elements such as sulfur and oxygen. As the concentration of these elements increases, the surface tension of liquid iron decreases thus allowing the formation of elongated graphite crystals with a high surface area-to-volume ratio such as is found in conventional grey cast irons. With lower concentrations of sulfur and oxygen, the surface tension of the liquid iron increases thus favouring the formation of spheroidal graphite crystals which have a minimized surface area-to-volume ratio. Compacted graphite crystals, which are arguably a three-dimensionally growth-distored form of flake graphite morphology, are known to grow preferentially at intermediate values of liquid iron surface tension. While several theories regarding the growth mechanism and ultimate shape of graphite have been proposed, the common denominator in all theories is that they are invariably linked to the so-called dissolved or active sulfur and oxygen contents of the molten iron, which in-turn determines the surface tension of the molten iron. Therefore, by monitoring the surface tension of the molten iron, it will be possible to predict the ultimate shape of the graphite crystals imme¬ diately after base treatment so that corrective amounts of magnesium and inoculant may be added to the melt before it is cast into moulds and thus minimize the proportion of unaccep- table or out-of-specification castings. One such theory recognises that the atomic lattice of graphi¬ te is an hexagonal close-packed structure (Figure 2) with the possibility to grow in either the A-axis prism face direction (as in grey flake iron) or in the C-axis basal plane direc¬ tion (as in spheroidal graphite iron) . Physical measurements of liquid iron surface tension, which are disclosed in Table I (See H. Geilenberg: Recent Research on Cast Iron, H.D. Merchant (Ed), Gordon and Breach, 1968, pp. 195-210.), clear- ly shows that, when the sulfur and oxygen content of the iron is high (as in grey cast iron) , the surface tension at the interface between the A-axis and the molten iron is lower than that between the C-axis and the liquid iron. Therefore, growth is favoured in the A-direction thus producing elonga¬ ted graphite flakes. Conversely, in the absence of dissolved oxygen and sulfur, while the graphite/iron surface tension is higher than when oxygen and sulfur are present as shown in Figure 1, the surface tension at the interface between the A- axis and the molten iron is higher than that between the C- axis and the molten iron. Therefore, growth is favoured in the relatively low energy C-direction thus producing spheroi¬ dal graphite crystals. Another common theory of graphite growth mechanisms suggests that sulfur and oxygen tend to adsorb onto the A-axis prism faces thus increasing the atomic density and roughness of the A-face which improves the rate of carbon atom deposition of the A-face. Ultimately, the preferential addition of carbon atoms to the A-face results in the formation of flake graphi¬ te. By the same theory, when magnesium or cerium or lanthanum or other known graphite shape modifying elements are added to the liquid iron, they react with dissolved oxygen and sulfur thus "scavenging" or cleaning the A-faces of the graphite lattice. Under these circumstances the C-face or basal plane of the graphite lattice has a higher atomic density and carbon atoms preferentially deposit on it resulting in C-axis growth and the ultimate formation of spheroidal graphite crystals. Regardless of the exact growth mechanism, for which conside¬ rable debate has been conducted, it is clear that the presen¬ ce of sulfur and oxygen are of paramount importance. The novel contribution of the present invention is to accurately measure the surface tension of the molten iron, after the addition of magnesium and inoculant (which consumes dissolved oxygen and sulfur atoms through chemical reaction) , to deter¬ mine if the surface tension is within an acceptable range for the stable growth of compacted graphite iron, or equally, for the successful production of high quality spheroidal graphite iron. The results of the surface tension analysis will allow for the inoculant and/or magnesium contents of the base treated iron to be corrected prior to casting to ensure an optimal solidification behaviour of the melt. It must also be noted that the present invention may also be useful for the production of conventional grey cast irons. As described pre¬ viously, a certain amount of oxygen and/or sulfur must be present at the A-face to promote the growth of flakes. An accurate measurement of surface tension will allow the skil¬ led foundryman to determine the need for additional inocula¬ tion, or alternatively, oxygen sources. This is particularly true when it is realized that selenium and telurium are even more surface active in molten iron than are oxygen and sul- fur. Given the inevitable deterioration of ferrous metal charge materials in the future, it may become essential to measure the accumulation of these elements in the iron and take corrective actions in real-time, by monitoring surface tension prior to casting. A very important advantage of the present invention is that the results are available very quickly. Unlike known thermal analysis control methods, such as the one disclosed in WO-A1- 86/01755, it is not necessary for the iron sample to solidi- fy. The analysis can be fully conducted with the first bubble formation, and verification can be achieved through the surface tension analysis of subsequent bubbles. The entire analysis from taking a sample of the molten iron until is¬ suing a prediction of graphite morfology can require less than one minute. Upon completion of the analysis, corrective actions are prescribed to either increase or decrease the degree of inoculation and magnesium modification, the optimi¬ sed iron can then be cast with conventional foundry tech¬ niques. The invention will now be described with respect to the following examples of preferred embodiments. However, these examples are only given for the purpose of illustration, and are not intended to limit the scope of the invention. Description of preferred embodiments The surface tension analysis of the present invention can be performed by using an equipment as illustrated in figure 3. The analysis equipment basically comprises a sample cup (1) . A sample cup (1) made of steel is preferred to conventional high temperature ceramic crucibles in order to effectively transmit vibrations originating from within the melt. The liquid iron sample (2) , which is filled into the cup can be of any size greater than approximately 100 g, but is prefe¬ rably in the range of 200 - lOOOg. A noble gas (9) ' , such as helium, neon, argon, krypton or xenon is then transported through a valve (8) and a submerged refractory lance (7) and subsequently bubbled through the liquid iron sample (2) . A thermocouple (10) is also submerged in the liquid iron sample (2) . The thermocouple (10) may be an integrated part of the refractory lance (7) , but can also be independent of the gas lance. Finally, a piezoelectric transducer (4) is attached to the wall of the steel cup (1) to monitor the vibration be- haviour of the gas bubbles which form at the tip of the injection lance (7) . The transducer (4) is inexpensive and can either be re-usable or disposable. The actual contact time for the transducer (4) to the steel cup (1) is less than fifteen seconds which allows the transducer (4) to stay well below its Currie point (for example 190 - 350°C for lead circonate titanate, 400°C for lead metaniobate; or 1210°C for lithium niobate) . The base (3) of the sample cup is flat in order to ensure good physical contact between the sample cup (1) and the transducer (4) . A retraction device (5) makes it possible adjust the verticle position of the transducer (4) with respect to the sample cup (1) . Operationally, once the sample cup (1) has been filled with molten iron (2) , a clamping device (6) is applied around around the top perimeter (11) of the cup to ensure contact between the cup and the transducer and also to eliminate external noise vibrations. The gas flow is then initiated and the transducer (4) is raised to make direct contact with the flattened base (3) of the sample cup(l) . The gas flow is adjusted in a manner which allows the bubble (12) to fully form at the tip of the lance while no other bubbles are present in the molten iron to produce extraneous noise. The growth rate of the bubble and its vibration amplitude and frequency is a function of the gas pressure and the distance H between surface of the molten iron and the lance tip (amount of ferrostatic head) and most importantly, the surfa¬ ce tension of the liquid iron. By controlling the pressure and accepting that variations in ferrostatic head are suffi¬ ciently small to be neglected, the vibration behaviour of the single bubble (12) at the tip of the lance (7) can be quanti¬ fied by the transducer. Then, with a knowledge of the gas pressure, the distance d from the tip of the lance (7) to the flattened base (3) of the sample cup (1) , and the temperature of the liquid iron as obtained from the thermocouple (10) , one can readily deduce the surface tension by evaluating the vibration frequency spectrum and comparing with known refe¬ rence samples and thus predicting the ultimate shape of graphite particles in the final casting. Table I Interfacial tension between liquid iron and graphite as a function of sulfur and oxygen contents, and the growth plane of the graphite lattice. Crystal Surface tension (mJ/m^) Plane with O & S no O & S Prism (A) 845 1720 Basal (C) 1270 1460

Claims: Claims : 1. A method for predicting the nucleation and growth of graphite crystals during the solidification of cast irons, according to which: a) a sample quantity of the molten iron is taken from the batch of molten iron; b) the surface tension of the sample is determined; and finally c) the surface tension of the sample is compared with the surface tensions of batches of molten iron that are known to solidify as different forms of cast iron. 2. A method according to claim 1, characterized in that the surface tension of the molten iron sample is determined by adding said sample to a sample cup (1) , submerging a device (7) for introducing a noble gas in the molten iron, positio¬ ning a piezoelectric transducer (4) at the bottom of the sample cup (1) , administrating the noble gas at a constant rate, measuring the temperature, and monitoring the vibra¬ tions in the molten iron by registrating the signals from the transducer (4) . 3. A method according to claim 2, characterized in that the sample cup is made of steel. 4. A method according to claim 2, characterized in that the contact for the transducer (4) to the sample cup is less than one minute, preferably less than 15 seconds. 5. A method according to claim 2, characterized in that the gas flow from the gas introduction device (7) is adjusted in a manner which allows the bubble (12) to fully form at the tip of the device (7) while no other bubbles are present in the molten iron. 6. A method for producing molten iron containing graphite shape modifiers and/or inoculants in order to provide iron solidifying as compacted graphite iron, spheroidal graphite iron, grey cast iron or any other form of cast iron, compri¬ sing: a) preparing molten iron; b) removing a sample quantity of said molten iron; c) measuring the surface tension of said molten iron by using a method according anyone of claims 1 - 5; d) comparing the surface tension of the sample with the surface tension of liquid iron previously known to solidify as the desired form of cast iron; e) if the surface tension of the sample deviates from the surface tension of the melt that is known to solidify as the desired form of cast iron by more than a predetermined value, upwardly or downwardly correcting the concentrations of said graphite shape modifiers and/or inoculants in a known way; and f) releasing the laddie (batch) of analysed and corrected molten iron for casten into conventional moulds.

Inventors: DAWSON STEVE, DAWSON, STEVE
Applicants: DAWSON STEVE, SINTERCAST AB, DAWSON, STEVE
Application year: 1996
Main classification: C22C33
IPC codes: C21C1, C22C33, G01N13, G01N33