Abstract:
The present invention provides a novel method and a system of modules for carrying out thermal analysis of cast iron melts. The characterizing step of the method is de-termination of the position of the thermocouples used for recording cooling curves. The system comprises a thermocouple unit, a thermocouple holder, and a sampling unit. The system comprises means for ensuring that the thermocouples are correctly positioned before starting the thermal analysis. The system also comprises means for automatically transferring calibration data regarding the thermocouples.

Description:
[0001]     The present invention relates to an improved apparatus for carrying out thermal analysis of molten metals, in particular cast iron melts. The invention also relates to thermal analysis methods involving use of the improved apparatus.  
       TECHNICAL BACKGROUND  
       [0002]     Thermal analysis, is a technique for determining and predicting the microstructure in which a certain molten substance, such as a molten metal or a molten alloy, will solidify. Such a thermal analysis is basically carried out by recording a so-called cooling curve showing the temperature variations as the molten sample transitions from the liquid state to the solid state, and then comparing the curve with pre-recorded calibration data. Cooling curves have been extensively used in the experimental study and production control of cast irons.  
         [0003]     Thermal analysis is typically carried out by taking a sample of the melt to be analysed, and bringing the sample into a sample vessel. Then, cooling curves are recorded in the sample during solidification by means of temperature-responsive means, such as thermocouples or pyrometers.  
         [0004]     Methods for predicting the microstructure with which a certain cast iron melt will solidify are known in the art. WO 99/25888 and WO 92/06809 disclose two examples of such methods. When carrying out the methods of both WO 99/25888 and WO 92/06809, a sample of molten cast iron is taken and subsequently the sample is transferred to a sample vessel. When the sample solidifies in the vessel, two cooling curves are recoded. One of the curves is recorded in the centre of the sample, whereas the other is recorded close to the sample vessel wall. Examples of suitable sample vessels that can be used in these methods are disclosed in WO 96/23206 and WO 99/28726.  
         [0005]     When carrying out such prediction methods, it is essential that the underlying temperature measurements are obtained under constant conditions. A substantial amount of calibration has to be carried out. The sample amounts must be substantially identical. The sample vessels in which the cooling curves are recorded must also be substantially identical. Small differences regarding the sample vessels, sample amounts, thermocouple locations, etc. can lead to prediction errors. The industrial tolerance level regarding castings having an erroneous microstructure is very low, due to the substantial costs involved in quality control and potentially recalling of sold products. It is therefore essential to be able to carry out such thermal analysis methods for predicting the microstructure in which a certain cast iron melt will solidify under as constant conditions as possible.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention provides a novel method and a system of modules for carrying out thermal analysis of cast iron melts. The characterizing step of the method is determination of the position of the thermocouples used for recording cooling curves. The system comprises a thermocouple unit, a thermocouple holder, and a sampling unit. The system comprises means for ensuring that the thermocouples are correctly positioned before starting the thermal analysis. The system also comprises means for automatically transferring calibration data regarding the thermocouples.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0007]     As already mentioned, the present invention provides a method as well as a sampling module kit which is useful in thermal analysis of molten cast irons. By using the method and the kit as well as at least one of its components, it is possible to substantially increase the accuracy of such a thermal analysis, and to eliminate some sources of measurement variation and thus prediction error. Accordingly, the method, the kit and the components of the invention should be attractive to use in industrial processes for manufacturing cast iron products, where the tolerance level regarding erroneous castings is very low.  
         [0008]     As disclosed herein, the term “cooling curve” refers to graphs representing the temperature as a function of time, which graphs have been recorded in the manner disclosed in WO 99/25888 and WO 92/06809.  
         [0009]     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 in a suitable way. The walls of the sample vessel may be coated with a material which reduces the amount of structure-modifying agent in the melt in the immediate vicinity of the wall. Examples of such sample vessels are given in WO 99/28726 and WO 96/23206  
         [0010]     The term “structure-modifying agent” as disclosed herein, relates to compounds either promoting spheroidization or precipitation of the graphite present in the cast iron. Suitable compounds can be chosen from the group of inoculating substances well-known in the art, and shape-modifying agents, such as magnesium, cerium and other rare earth metals. The relationship between the concentration of structure-modifying agents in molten cast irons and the graphite morphology of solidified cast irons have already been discussed in WO 92/06809 and WO 86/01755. 
     
    
       [0011]     The present invention will be described with reference to the enclosed figures, in which:  
         [0012]      FIG. 1  shows a thermocouple unit;  
         [0013]      FIG. 2  discloses a thermocouple holder, which is adapted for containing a thermocouple unit. Details of the thermocouple unit are shown with broken lines;  
         [0014]      FIG. 3  shows a sampling unit adapted to be used together with, and to contain, a thermocouple holder and a thermocouple unit. Details of a thermocouple holder and a thermocouple unit are shown with broken lines. 
     
    
       [0015]     In a first aspect, the present invention relates to a method for predicting the microstructure in which a certain cast iron melt will solidify. The method is based on a known procedure, where a sample of a certain cast iron melt is obtained in a sample vessel. Then, cooling curves are recorded in the centre of the sample and in the vicinity of the sample vessel wall using two thermocouples. Finally, the cooling curves are evaluated using pre-recorded calibration data in order to predict the microstructure.  
         [0016]     Thermal analysis methods involving recording cooling curves require pre-determined and constant conditions. An essential feature of such methods is use of pre-determined calibration data. The results of determinations made under slightly different conditions compared to the conditions during the calibration cannot be trusted. A common reason for erroneous results is that at least one of the thermocouples has been in a different position compared to the position during the calibration measurements. The present method therefore comprises a step where the positions of the thermocouples are determined before recording the cooling curves. If the position of one of the thermocouples differs from the calibration position with more than a predetermined value, a fault signal is activated and the sampling procedure cannot be initiated until the fault has been rectified.  
         [0017]     The thermal analysis methods of WO 99/25888 and WO 92/06809 are all carried out within the temperature range 1100-1300° C. and with a tolerance of +/−1° C. When measuring temperatures within the above disclosed range and precision, the exact location of the temperature-responsive means is extremely important. An erroneous localisation of the temperature-responsive means of 1 mm in a typical sample vessel such as those vessels disclosed in WO 99/28726 and WO 96/23206, corresponds to an erroneous temperature measurement of 1.5° C.  
         [0018]     The most important temperature measurements are all carried out within a subrange or “window” of +/−20° C. Small differences (˜1.5° C.) regarding temperature measurements within this window may lead to very different predictions regarding the microstructure of the produced casting. Moreover, as the desired subrange or window comprises as much as +/−20° C. it is not possible for computer-based systems to detect erroneously located temperature responsive means by just monitoring the measured temperature.  
         [0019]     An erroneous reading can therefore effect the production of castings with production stop, or worse, faulty products, because the process control system receives faulty data. In case any products are produced out of specification without any indication by the process control system, it may lead to quality problems.  
         [0020]     It is therefore very important to be able to detect the exact location of the temperature-responsive means.  
         [0021]     There are several reasons why a temperature-responsive means could be erroneously located. There could be small particles in its way. Alternatively, the temperature-responsive means could be bent and thus not be able to slide into the protective tube of the sample vessels normally used in these thermal analysis methods. There is no visual way to detect whether said means is in the correct position after mounting the sample vessel. Finally, the sample vessel could have been damaged during transport or mounted in a wrong manner, which also results in an erroneous location. There are several ways of determining the position of the thermocouples. The positions can for example be determined mechanically, optically or magetically. In the methods of WO 99/25888 and WO 92/06809, the thermocouples are moved from a resting position above the sample vessel to a measuring position in the cast iron melt. If position indication means are fixed to the thermocouples, or alternatively, to a protective tube completely surrounding the thermocouples it is possible determine the exact location of the thermocouple in the sample in relation the calibration position.  
         [0022]     As disclosed herein, the term “position indication means” is intended to mean anything detectable that can be joined to a specific part of the thermocouple. The position of the thermocouple can be mechanically detected if the position indication means physically contacts a detection sensor. The location can be optically detected if the position indication means affect a radiation beam between a radiation source and a radiation detector. Likewise, the position can be magnetically detected if the position indication means affects or induces a magnetic field in the vicinity of the thermocouple. The position of the position indication means is preferably detected in a non-mechanical way, i.e. optical detection and magnetic detection are preferred.  
         [0023]     In case mechanic detection is used, there is a risk that wearing out of the detection equipment might hamper the results. In a second aspect, the present invention relates to a sampling module kit suitable for carrying out the method of the first aspect. This sampling module kit comprises three parts operating together, namely a thermocouple unit, a thermocouple holder and a sampling unit.  
         [0024]     The thermocouple unit  100  is shown in  FIG. 1  and comprises 
        a) a first thermocouple  102 ;     b) a second thermocouple  104 ;     c) a central part  106  joined to the first and second thermocouples  102 ,  104 . The central part  106  also involves means  108 ,  110  for connecting the first and second thermocouples  102 ,  104  to a calculation means; and     d) information transfer means  112  for transferring data relating to the two thermocouples  102 ,  104 .        
 
         [0029]     The thermocouple unit  100  is adapted for recording cooling curves in the manners disclosed in WO 99/25888 and WO 92/06809. The first thermocouple  102  is intended to record cooling curves in the centre of a sample of molten cast iron, whereas the second thermocouple  104  is intended to record cooling curves in a sample of molten cast iron adjacent to the wall of the sample vessel that is used during the analysis. The arrangement of the thermocouples on the central part  106  of the thermocouple unit is therefore adapted to a particular sample that is to be used during the thermal analysis. However, it is easy for the skilled person to design a thermocouple unit in such a way that one thermocouple can be centrally arranged while the other is located near the vessel wall for each given sample vessel. The thermocouples might for instance be welded together in such a way that the second thermocouple  104  ends at a longer distance from the central part  106  than the first thermocouple  102 . The central part  106  also has means  108 ,  110  for connecting the thermocouples  102 ,  104  to a calculation/computer means, for subsequent presentation and/or evaluation of the results, for instance using the technology disclosed in WO 99/25888 and WO 92/06809.  
         [0030]     The first thermocouple  102  is adapted for recording cooling curves in the centre of a molten cast iron sample contained in a sample vessel, and the second thermocouple  104  is adapted for recording cooling curves in the cast iron sample adjacent to the sample vessel wall. Accordingly, the arrangement of the thermocouples  102 ,  104  on the central unit  106  is dependent on the design of the particular sample vessel that is used. It is easy for the skilled person to adapt the thermocouple arrangement of the thermocouple unit to a given sample vessel design.  
         [0031]     The thermocouple unit  100  comprises an information transfer means  112 , which preferably is located on the central part. The information transfer means  112  can be a magnetic memory means a printed bar code, or a radio frequency memory tag. The information transfer means contains calibration data relating to the thermocouples  102 ,  104 . Preferably, it also contains serial numbers etc rendering it possible to identify the individual thermocouples of the thermocouple unit, and to identify the calibration factors of these thermocouples to allow automatic correction in the software.  
         [0032]     During measurements, it is advantageous to protect the thermocouples against the hot cast iron melt. If the thermocouples are protected, it is possible to reuse them several times. Typically, the thermocouples are inserted into one or two protective tubings. Such protective tubings can either constitute an integral part of the sample vessel, or be put on as a separate fitting when the thermocouple unit is mounted in a thermocouple holder. Such protective tubings are not shown in the figures of the present application.  
         [0033]     A thermocouple holder  200  according to the present invention is shown in  FIG. 2 . It comprises a cylindrical bushing  202  adapted to be fixed to the thermocouples  102 ,  104  of the thermocouple unit  100 . The cylindrical bushing  202  also comprises position indication means  206 . The position indication means  206  shown in  FIG. 2  is a recess enabling free passage of a light beam (optical detection) when the thermocouples  102 ,  104  of the thermocouple unit  100  are correctly positioned in the sample vessel. Alternatively, the position indication means can be a permanent magnet (magnetic detection) or a rod (mechanical detection).  
         [0034]     As already mentioned the cylindrical bushing  202  is adapted to be fixed to the thermocouples  102 ,  104  of the thermocouple unit  100  (or optionally to protective tubes surrounding the thermocouples  102 ,  104 ) by using suitable means  204 , such as screws. Naturally, it is essential that the thermocouples  102 ,  104  are fixed to the bushing  202  in a position corresponding to the position during the calibrations.  
         [0035]     The thermocouple holder  200  also comprises a head part  208  intended to house the central part  106  of the thermocouple holder. The head part has a means  210 , such as an opening, for giving access to the information transfer means  112  of the thermocouple unit. Finally, the head part is also equipped with a fastening means  214  for attaching the thermocouple holder to the sampling unit.  
         [0036]     The cylindrical bushing  202  and the head part  208  are axially flexibly joined by a suitable means  212 , such as a spring.  
         [0037]     A sampling unit  300  is shown in  FIG. 3 . It comprises a housing  302  adapted for containing a thermocouple holder  200  equipped with a thermocouple unit. The unit further involves a means  304  for attaching a sample vessel  306 . This means  304  is specifically adapted for the sample vessel type used in a particular assay. Examples of suitable sample vessels are given in WO 99/28726 and WO 96/23206. The means  304  is located on an elongated part  322  intended to enclose the cylindrical bushing  202  of the thermocouple holder  200 .  
         [0038]     The sampling unit  300  has a means  308  for attaching the head part  208  of the thermocouple holder  200  inside the housing. This means  308  is adapted for being used together with the corresponding fastening means  214  on the head part  208 . The fastening mechanism is designed in such a way that it is easy to quickly change the thermocouple holder. It is easy for the skilled person to develop suitable fastening mechanisms. Furthermore, the upper part  318  of the housing is pivotally mounted using one or more hinges  320 , in order facilitate exchanging the thermocouple holder  200  inside the hosing  302 .  
         [0039]     The sampling unit  300  comprises means  314  for reading the information in the information transfer means  112  of the thermocouple unit  100  and to send this identity and/or calibration factor information to a calculating/computer means. The reading means  314  can be a bar code reader a magnetic transducer, or a means for detecting signals from a radio frequency memory tag etc.  
         [0040]     The sampling unit  300  further comprises means  310  for moving the cylindrical bushing  202  of the thermocouple holder  200 , and thereby the thermocouples  102 ,  104  of the thermocouple unit  100 , between a measuring position and a resting position. It is easy for the skilled person to design suitable means. The means can be controlled by a manual control means  316 , or alternatively it can be controlled automatically by the computer means.  
         [0041]     The elongated part  322  of the housing  302  also comprises means  312  for detecting whether the position indication means  206  of the cylindrical bushing  202  is in a position corresponding to the measurement position or not. In case the position indication means  206  of the bushing  202  is a recess, the detecting means can be a light source operating together with a light detector. In case the position indication means  206  is correctly positioned, there is a recess in the bushing  202  between the light source and the light detector, and the detector sends a positive signal. In case the position indication means is in another position, less or no light reaches the light detector and no positive signal is sent. The start of the thermal analysis is prevented, or in case it has already begun, it is interrupted.  
         [0042]     Alternatively, detection means  312  can be for instance a magnet or a coil when the position is magnetically detected, or for example a switch mechanism when the position is mechanically detected.