MXPA00008371A

MXPA00008371A - Device and process for thermal analysis of molten metals

Info

Publication number: MXPA00008371A
Application number: MXPA/A/2000/008371A
Authority: MX
Inventors: Rudolf Valentin Sillen; Kjell Pettersson; Hakan Fransson
Original assignee: Novacast Ab
Priority date: 1998-02-26
Filing date: 2000-08-25
Publication date: 2002-07-25

Abstract

A device for thermal analysis of molten metals comprising two thermocouples is described. It comprises a mould (1) with a spherical cavity (2), the one thermocouple (3) being placed such that it extends over the central portion of the cavity (2), a cylindrical duct (5) which communicates with the cavity (2) and a cylindrical part (7) which communicates with the lower portion of the cavity (2), the other thermocouple (6) being placed in the transition between the cavity (2) and the cylindrical part (7). A process for thermal analysis of molten metals with the aid of the device is also described. The difference in temperature (11) in the temperature/time curve for the thermocouple which is centrally placed and the lower thermocouple which is peripherally placed when the solidus temperature determined by means of the centrally placed thermocouple (3) has been reached is used as a measure of the thermal conductivity.

Description

DEVICE AND PROCEDURE FOR THE THERMAL ANALYSIS OF CASTED METALS DESCRIPTIVE MEMORY The present invention relates generally to the thermal analysis of molten metals and in particular to a device and method for the thermal analysis of molten metals. Industrially used alloys almost always consist of a base metal that has formed an alloy with one or more elements. In the liquid state, the alloy additives are in many cases soluble in the base metal. The solidification normally takes place within a solidification scale which is typical of the alloy composition. After solidification, different solid phases are separated from the molten metal, releasing latent heat. Following the temperature and the duration of the solidification time, it is possible to indirectly obtain a reference for the composition of the alloy and its solidification form. The method has been standardized by the use of test bowls or crucibles made of refractory material with an integrated consumable thermocouple. The method, which is called thermal analysis, is widely used for iron and aluminum alloys. The cavity in the test bowls used industrially is square or crosswise and the test bowls are provided with a centrally placed thermocouple. The typical dimensions are 37 x 37 mm and a height of 40 mm. The bowls are made of sand for shell molding and have a wall thickness of approximately 5 mm. The cavity is completely open up where the metal is poured when a test is carried out. From a test you can get a lot of information about the molten metal and its behavior, for example, when it is cast. The crucial point is to provide a high degree of repeatability of the test. In the prior art, the repeatability can vary, inter alia, depending on the degree of filling of the test bowl and variations in heat emission by radiation and convection coming from the upper surface. One problem is that the centrally placed thermocouple records only temperature conditions in the center of the bowl where the molten metal is liquid for a fairly long time. It is desirable to be able to simultaneously monitor the temperature on the surface of the test bowl and carry out a more detailed analysis of the test piece by comparing the process in the center and the surface of the test bowl. The test bowls are already known with a thermocouple placed in the center and others on the surface. In this way, the description of the Swiss patent 626,450 details a crucible receiving a molten metal, a thermocouple being disposed in the molten metal and another inside, or in the wall of the crucible. In other known examples, cylindrical or cubic test cups have been used, the surface thermocouple being placed at a distance of 1-3 mm from the wall. One problem is that the slightest error in the placement of the peripheral thermocouple makes the result of the measurement uncertain. The object of the present invention is to solve these problems and provide a device and a method for the thermal analysis of molten metals providing a high repeatability and high resolution. Accordingly, the device and the method have the features indicated in claims 1 and 4, respectively. In the device according to the invention, the spherical cavity has a cylindrical duct that is connected in the upper part and a cylindrical part that is connected in the lower part. Since the cavity is spherical, the solidification will take place in a concentric way, which makes the impulses much clearer from the solidification to the thermocouple placed in the center than in the known cylindrical or cubic structures. By the provision of a cylindrical filling duct in which the molten metal has a shorter solidification time than in the spherical cavity, the effect of fluctuations in the emission of heat from the upper surface due to changes in emission per radiation. In addition, the variations caused by different degrees of filling will be eliminated because it is assumed that the duct is constantly filled after the casting of a test piece.

By placing the lower thermocouple in the transition between the spherical cavity and the lower cylindrical part, the position can vary in some way without altering the repeatability. The purpose of the lower cylindrical part is that the molten metal located therein solidifies relatively quickly and before the molten material in the spherical cavity. Accordingly, the thermal condition occurs in the solid phase through the lower part and during a large part of the solidification in the spherical cavity. Therefore, the lower thermocouple can indirectly record the thermal conductivity of the alloy in the semi-solid to solid phase. This is useful in particular when testing cast iron alloys in which the carbon is precipitated in the form of graphite with high thermal conductivity during solidification. Graphite can be precipitated in different ways that affect the casting capacity and physical properties of the alloy. If the graphite precipitates in the form of spheres, the alloy is called nodular iron. If the graphite is precipitated in the form of agglomerates with thin graphite flakes, the alloy is called gray cast iron by iron cast with flake graphite. The thermal conductivity of cast iron with graphite in flakes can be up to 25% higher than if the graphite were precipitated in the form of spheres. An intermediate form is the so-called dense graphite iron, which is distinguished because the graphite precipitates in a rounded and "plump" type bar. In this way, thermal conductivity can be used to analyze the shape of the graphite.

According to the present invention, an indirect measurement of the thermal conductivity can be obtained by measuring the difference in temperature between the thermocouple placed in the center and the thermocouple placed peripherally in the spherical cavity at the transition between the spherical cavity and the cylindrical part. According to a preferred embodiment of the invention, the difference in temperature is recorded when the solids temperature of the alloy has been reached in the thermocouple placed in the center. The invention will now be described in more detail with reference to the accompanying drawing, in which Figure 1 is a front view of a preferred embodiment of the device according to the invention, Figure 2 is a section along the line 11- 11 of Figure 1 and Figure 3 illustrates the temperature curves of the peripheral thermocouple and the central thermocouple. Referring again to Figures 1 and 2, there is shown a device comprising a mold 1 consisting of two parts of refractory material. The mold parts are suitably held together during the casting of the test piece by means of a holder (not shown). In addition, the device includes a spherical cavity 2 in which a thermocouple 3 is centrally located. In this manner, this thermocouple extends over the central portion of the cavity. A pouring bowl 4 is disposed for pouring molten metal and this pouring bowl passes in a cylindrical duct 5, which in turn communicates with the spherical cavity 2. A cylindrical part 7 is connected to the lower portion of the cavity and a second thermocouple 6 is disposed at the transition between the cavity 2 and the cylindrical part 7. According to this preferred embodiment illustrated, the cold junctions 8 of the thermocouples are positioned along the longitudinal axis of the test bowl, the thermocouple one, as mentioned above, extending over the central portion of the spherical cavity 2, and the other thermocouple 6 crossing the longitudinal axis of the test bowl at a short distance above the interface between the cavity 2 and the cylindrical part 7. This distance it is generally on the 0-2 mm scale. The following dimensions may be mentioned as non-limiting examples. The external dimensions of the mold are a height of 110 mm and a width of 60 mm, the thickness of each part of the mold measuring 40 mm. The pouring bowl 4 has an upper diameter of 40 mm and a height of 20 mm. Connector duct 5 has a diameter of 20 mm and a height of 25 mm. The spherical cavity has a diameter of 40 mm and the lower cylindrical part has a diameter of 16 mm and a height of 15 mm. The thermocouples 3 and 6 are made in the style of the prior art of "Chromel-Alumel", and are enclosed in a high purity quartz tube. The thermocouples are connected to an A / D converter in a known manner by means of a compensation circuit. When a molten metal is analyzed, the device is filled with molten metal by means of a casting bucket. The casting temperature for cast iron alloys should be on the 1240-1350 ° scale. The temperature is preferably recorded once per second. After about 250 seconds the molten metal is solid. Preferably, the time / temperature data are analyzed by means of a computer program. Typical cooling curves are illustrated in Figure 3. Curve 9 shows changes in temperature of the peripheral thermocouple and curve 10 those of the central thermocouple. The point of time of the solids temperature of the thermocouple 3 which is placed in the center is for this purpose defined as the minimum point of the first derivative of the time / temperature curve. The reason why this time point has been selected is that it is not until after the solidification that the differences in thermal conductivity become clear. In order to predict the casting properties, etc., it is important to obtain a measure of the thermal conductivity at the highest possible temperature. At this point of time, the difference 11 in temperature between the thermocouples is calculated. The difference in temperature for cast gray cast iron is usually around 90 ° C, and for nodular iron alloys having a lower thermal conductivity of around 120 ° C. The difference in temperature is sufficient not only to classify the type of cast iron but also to provide information about, for example, the nodularity of nodular iron alloys and the fraction of vermicular graphite in dense graphite alloys.

Claims

NOVELTY OF THE INVENTION CLAIMS

1. - A device for the thermal analysis of molten metals comprising two thermocouples, characterized in that it comprises a mold (1) with a spherical cavity (2), the thermocouple one (3) being positioned in such a way that it extends over the central portion of the cavity (2), a cylindrical duct (5) communicating with the cavity (2) and a cylindrical part (7) communicating with the lower portion of the cavity (2), the other thermocouple (6) being placed in the transition between the cavity (2) and the cylindrical part (7).

2. A device according to claim 1, further characterized in that the diameter of the duct (5) is 30-50% of the diameter of the spherical cavity (2), and because its length is at least 50% of the diameter of the spherical cavity.

3. A device according to claim 1, further characterized in that the cylindrical part (7) has a diameter 30-40% of the diameter of the spherical cavity, and because its length is more than 50% of its diameter.

4. A method for the thermal analysis of molten metals with the aid of the device according to claim 1, characterized by the use, as a measure of the thermal conductivity, of the difference in temperature (11) in the temperature curve / time for the thermocouple that is placed centrally and the lower thermocouple that is placed peripherally, when the solids temperature determined by means of the centrally placed thermocouple (3) has been reached.