Thermal measuring and testing – Temperature measurement – Of molten metal
Reexamination Certificate
2002-12-02
2004-07-27
Gutierrez, Diego (Department: 2859)
Thermal measuring and testing
Temperature measurement
Of molten metal
C374S157000
Reexamination Certificate
active
06767130
ABSTRACT:
BACKGROUND
The present invention relates to a sampling device for thermal analysis of solidifying metal, especially for thermal analysis in the production of castings.
Thermal analysis is a technique for monitoring variations in temperature change of molten substances during solidification to be able to determine the microstructure and hence properties of the substances in solid form. This is accomplished by taking a sample from the melt, transferring it into a sampling vessel and recording and evaluating a time-dependent temperature change in the sample during solidification, by means of temperature responsive sensor means, such as thermocouples or other devices known in the art.
When using thermal analysis for recording the solidification of molten metals, such as compacted graphite iron (CGI), it is important that the analysis is done under the same geometric and thermal conditions, as will occur in the castings. The contribution of the sampling vessel is to control the cooling during solidification of the sample of the molten metal so that the geometric and thermal conditions, in the sampling vessel are similar to those obtained in the castings. The ability to accurately measure the solidification behaviour of the molten metal allows a foundry to control its process and guarantee high quality in production.
Sampling vessels for thermal analysis are known in a large number of designs. They can be made of graphite, for use in aluminum melts, or made of a ceramic material, when intended for use with molten cast iron. However, they cannot be made of steel due to dissolution and/or thermal imbalance.
A drawback for many vessels is that they are made of materials, which are difficult to machine. Another drawback is that since they are immersed in the bulk of the metal, when taking a sample, the risk of thermal shock cracking is a problem, especially when they are made of ceramic materials, whereby they easily crack.
WO-A1-96/23206 (incorporated by reference) describes a sampling vessel to be immersed in a bath of molten metal to be analyzed. A sampling vessel is disclosed, a double-walled steel vessel with a space between the walls, having low radial thermal conductivity. The space can be filled with an insulating gas, such as air. The inner wall of the vessel is thin and thus provides a low heat capacity, so as it will easily obey a steady-state thermal condition in a short time. Furthermore, heat lost from the outer surface of the inner wall, is not let out into the ambient atmosphere, because of an outer wall, acting as a radiation shield, surrounding the inner wall and the insulating space between the walls.
The previously mentioned sampling vessel in WO-A1-96/23206 is very well suited for thermal analysis for use in CGI production on account of its special properties. However, it is expensive to manufacture, which is a disadvantage from the point of view that it can only be used once. The thermocouples are at proper locations, i.e., one near the inner wall and one at a position, which attempts to simulate the centre of a hypothetical sphere of molten metal with uniform heat-loss per unit area. In fact the heat-loss from the bottom part is much lower, as compared to the top. One reason for this non-uniform behaviour is that the open top part emits much more heat per unit area than the rounded bottom part. Another reason is that the contact between the two surfaces at the upper joint allows heat to go around the insulting air space. This is a considerable disadvantage, since proper results are not always obtained.
It is of great importance to cool at a similar rate as the castings which are to be controlled. Equilibrium cooling would take too long to be of any practical value for this process control situation, for instance CGI production, since results would not be available before the casting process was completed, nor would it form a similar material micro-structure.
Furthermore it is essential that the sampling device is not expensive, since it can only be used once. Since especially accurate measuring elements, such as thermocouples are expensive, it is preferred to reuse them several times. A major drawback with many known sampling devices is that the rather expensive thermocouples are only used once.
Yet another drawback is that it is difficult to produce large series of sampling vessels, at low cost wherein all vessels show similar properties regarding geometric and thermal conditions etc.
DISCLOSURE OF THE INVENTION
The object of the invention is to overcome these considerable disadvantages by using an improved sampling device, with a sampling container having controlled hear-loss per unit area, which simulates a sphere of molten metal, since a sphere is the most uniform, and therefore most reliable and accurate shape for thermal analysis. This sampling device having controlled heat-loss per unit area simulates a spherical solidification of the molten metal inside the sampling container, but is not spherical in shape, because of manufacturing limitations for instance. The sampling device according to the invention comprises a double-walled container, provided with a radiation shield at the top and controlled space between the walls, which has much more controlled heat-loss, does not fail at high temperatures, is not expensive and has unproved positioning of the temperature responsive sensor means, such as thermocouples, which can easily be removed and reused.
Another important problem that is solved by the invention is the shifting of a thermal centre of the simulated sphere of molten metal, which shifts downwards once the exposed top surface of the sample inside the container solidifies.
All these requirements are achieved by providing the container having the features disclosed in appended claim
1
. Up till now no such sampling device has been available.
The sampling device is intended for single use, is cheap, gives reproducible geometric and thermal conditions.
It has been found that heat issuing from the exterior surface of the inner wall of the container must not immediately be let out into the ambient atmosphere, as this would make it very difficult to accomplish a controlled, slow and reproducible heat removal rate. Thus, the purpose of the outer wall is to define, together with the inner wall, a space between the walls that controls where hear is lost from the bottom and sides of the solidifying metal.
Thus, the space between the inner and the outer wall is an important parameter in regulating the heat loss due to radiation and thermal conduction. When this space is evacuated, or filled with a transparent material, such as air, radiation will be an important heat transfer mechanism. As temperature of the solidifying metal in the sampling device increases, radiation will be of increasing importance, since it's effect increases with the fourth power of absolute temperature.
By selecting and fully or partly filling the space with a suitable medium, and/or by altering the thickness of the space, it is possible to adapt the geometry of the heat removal rate of the sampling device to the values required by thermal analysis. The medium may bc any known and suitable medium, such as, sand, vermiculite, mica, magnesia, chlorite, various ceramics or combinations thereof, but is preferably a gas, such as air, because of cost. In one preferred embodiment, a distance (d1) between the walls in the flattened bottom part of the container is only 5-50%, preferably about 20% of a distance (d2) between the side-walls of the container, thereby increasing the heat loss due to conduction from the bottom. Because of the reduced space at the bottom of the container, heat-loss due to conduction is increased at the bottom, balancing heat-loss from the open top of the container thereby simulating heat-loss as occured by spherical solidification of a sphere of molten metal.
Another parameter of greatest importance is the shape of the container. To be able to position temperature responsive sensor means for thermal analysis, enclosed in a protective tube, at a certain distance fro
Nyström Henrik
Popelar Patrik
Guadalupe Yaritza
Gutierrez Diego
Nixon & Vanderhye P.C.
Sinter-Cast AB
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