Abstract:
The present invention provides a possibility to evaluate cooling curves recorded in near-eutectic cast iron melts. The curves are evaluated by generated in the melt in the center of the sample to the melt as a function of time. This information is then used to identify the part of the centrally recorded cooling curve that can be used as a basis for determining the amount of structure-modifying agent that must be added to produce compacted graphite cast iron, and/or spheroidal graphite cast iron, and to identify the part of said curve that is associated with formation of primary austenite.

Description:
This application is the National Phase of International Application PCT/SE99/02392 filed Dec. 16, 1999 which designated the U.S. and that International Application was published under PCT Article 21(2) in English. 
    
    
     The present invention relates to an improved method for predicting the microstructure in which a cast iron melt, having a composition with a carbon equivalent near the eutectic point of the iron-carbon phase diagram, 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 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. However, the cooling curves disclosed in this document are rather uniform and no variations are disclosed. 
     In order to accurately determine the graphite microstructure in cast iron specimens, conventional thermal analysis techniques, such as the one disclosed in WO 86/01755, require cooling curves where the first thermal arrest caused by austenite formation is distinctly separated from heat release caused by the onset of eutectic solidification. However, sometimes cooling curves are obtained without such a distinctly separated thermal arrest. This is the case when the molten cast iron solidifies as eutectic or hyper-eutectic iron. Until now, it has not been possible to use cooling curves corresponding to near eutectic cast iron for monitoring graphite growth. 
     WO93/20965 teaches that hyper-eutectic melts, where graphite nucleates before iron, do not provide a distinct plateau as the temperature crosses the liquidus line. This is correctly attributed to the fact that graphite crystallisation has a lower latent heat release than iron. 
     By placing a small amount of low-carbon iron in the melt, the low-carbon iron partially dissolves locally while the sample is still molten. As the sample cools, the relatively pure iron surrounding the remaining solid portion of the nail begins to solidify because of its lower carbon equivalent (CE). Ultimately, as the sample volume cools below the liquidus line, the remaining solid volume of nail, the surrounding low CE and the melted portion of the nail begin to solidify and “trigger” the solidification in an otherwise hyper-eutectic melt. The net result is that an austenite arrest plateau appears on the cooling curve. 
     WO93/20965 also states that the temperature difference (ΔT) between T 67  and T c max  can be used to establish a correlation with the carbon equivalent. However, WO93/20965 refers to hyper-eutectic melts, i.e. melts where the carbon equivalent is so high that the release of heat from the primary solidification does not coincide with the minima of the cooling curve (in the hatched region of FIG.  2 ( a ) of WO93/20965). Accordingly, the primary solidification and the eutectic solidification are separate. 
     None of the above cited references discuss anything about carrying out thermal analysis on cast iron melts in order to determine the carbon equivalent of melts which are near-eutectic. Moreover, WO93/20965 suggests measurements on melts having a carbon equivalent of up to 4.7%. It is disadvantageous to reach such high values because of graphite flotation and the degeneration of graphite shape. Likewise, the method of WO93/20965 is disadvantageous in that it requires an extra addition of low-carbon steel or iron to the sampling vessel, and accordingly lead to higher costs and a more laborious method. 
     SUMMARY OF THE INVENTION 
     The present invention provides a possibility to evaluate cooling curves recorded in near-eutectic cast iron melts. The curves are evaluated by determining the net amount of heat generated in the centre of the melt sample as a function of time. This information is then used to identify the part of the centrally recorded cooling curve that can be used as a basis for determining the amount of structure-modifying agent that must be added to produce compacted graphite cast iron, and/or spheroidal graphite cast iron, and to identify the part of said curve that is associated with formation of primary austenite. 
     DEFECTIONS 
     The term “cooling curve” as utilised herein refers to graphs representing temperature as a function of time, which graphs have been recorded in the manner disclosed in WO86/01755, 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. For the purposes of the present invention, all heat generation curves herein are determined for a zone located in the centre of a sample of molten cast iron. This zone is generally referred to as the “A zone”. 
     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 is then recorded during solidification in a suitable way preferably the sample vessel is designed in the manner disclosed in WO86/01755, WO92/06809, WO91/13176 (incorporated by reference) and WO96/23206 (incorporated by reference). 
     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 with molten metal. The sample vessel is preferably equipped with said sensors in the manner disclosed in WO96/23206. 
     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 and the graphite morphology of solidified cast irons have already been discussed in the above cited documents WO92/06809 and WO86/01755. 
     The term “CGI” as utilised herein refers to compacted graphite cast iron. 
     The term “SGI” as utilised herein refers to spheroidal graphite cast iron. 
    
    
     FIGURES 
     The invention is described with reference to the accompanying figures, in which: 
     FIGS. 1A-4A disclose cooling curves. A continuos line relates to the temperature in the centre of the melt and a dashed line relates to the temperature close to the wall of the sample vessel. 
     FIGS. 1B-4B show A zone heat generation curves corresponding to the cooling curves in FIGS. 1A-4A. FIG. 1 relates to normal hypo-eutectic cast iron and FIGS. 2-3 show curves for near-eutectic cast irons comprising increased values of the carbon equivalent. FIG. 4 discloses curves relating to hypo-eutectic cast iron. 
     FIG. 5 is a schematic presentation of an apparatus for controlling production of compacted graphite or spheroidal graphite cast iron according to the present invention. 
     FIG. 6 is a schematic representation of a sample vessel in which the heat is approximately uniformly transported in all directions. During measurements the vessel is filled with molten cast iron. This molten cast iron can be considered as a freezing sphere. The A zone relates to a sphere in the centre of the molten sample and the B zone relates to the molten iron surrounding the A zone. The radii r 1  and r 2  relate to the mean radii of the A zone and the B zone, respectively. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As already mentioned above, the present invention relates to an improved method for interpreting cooling curves in neareutectic cast iron melts. One of the important aspects of a cooling curve is the maximum slope of the recalescence peak of the centrally recorded cooling curve. This point is referred to as a in FIGS. 1A,  2 A and  3 A. In cooling curves corresponding to near-eutectic cast iron melts, the inflexion point (referred to in the figures as “t p ”) is located much closer to α and it can be very difficult to determine the slope correctly. Examples of such curves can be found in FIGS. 2A and 3A. However, it has now turned out that the determination of the maximum slope at a can be considerably simplified if a heat generation curve corresponding to the central zone (the A zone) of the molten cast iron is calculated. Such a heat generation curve renders it possible to determine the location of t p  in the centrally recorded cooling curve and to check whether it affects the slope of the recalescence peak or not. 
     The thermal balance of any uniform element can be described by the relation: 
       Q   stored   =Q   generated   +Q   in   −Q   out   (1). 
     where Q stored  is the amount of heat stored by the heat capacity of the material, Q generated  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 out of the material 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 (FIG. 6, zone A) and the more peripheral (FIG. 6, zone B) part of a molten cast iron contained in a sample vessel is described. As the A zone is situated in the centre of the molten cast iron sample, and as that zone can be considered to be a freezing sphere, no heat is transported into the zone and Q in  is therefore equal to zero. Appropriate substitutions in relation (1) above give the following equation: 
     
       
           C   p   m   A   dT   A   /dt=Q   genA +0−4 πke [( T   A   −T   B )/(1 /r   1 −1 /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, Q genA  is the heat generated in zone A, ke is the effective heat transfer coefficient of the material, and (1/r 1 −1/r 2 ) −1  is the mean distance for heat transport. The radii r 1  and r 2  are both defined in FIG. 6. 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 average bulk heat generation in the A zone: 
     
       
           Q   genA   =C   p   m   A   dTA/dt +4 πke [( T   A   −T   B )/(1 /r   1 −1 /r   2 )]  (3) 
       
     
     In equation (3) all variables are constants except dT A /dt and (T A −T B ). Accordingly, equation (3) can be simplified to: 
     
       
           Q   genA   =k   1   dT   A   /dt+k   2 ( T   A   −T   B )  (4) 
       
     
     where k 1  and k 2  are constants. Hence, a heat release curve can be calculated from a set of cooling curves recorded in the centre and the periphery of a sample of molten cast iron. 
     As already mentioned, these calculations are based on a situation where heat is uniformly transported in all directions. The skilled person can of course also calculate other equations corresponding to other heat transport conditions. 
     As mentioned earlier, the carbon equivalent of a certain cast iron melt affects the appearance of its corresponding cooling curves and heat release curves. Such cooling curves and heat generation curves can, depending on the carbon equivalent, be classified into one of four model types: 
     Type  1 , which is disclosed in FIG. 1, relates to hypo-eutectic cast iron. The cooling curve recorded in the centre of the sample is characterised by a first inflexion point associated with formation of austenite, followed by a local minimum. Interpretation of such curves does not present any particular difficulties and can be carried out in accordance with the principles outlined in WO86/01755 and WO92/06809. It should be noted that there is a peak (t p ) on the zone A heat generation curve that corresponds to the inflexion point (and accordingly with the onset of austenite formation) on the centrally recorded cooling curve. 
     Type  2 , which is disclosed in FIG. 2, relates to near-eutectic cast iron and is characterised by a very flat local minimum on the cooling curve recorded in the centre of the sample. An inflexion point cannot be seen. However, it is much easier to find a peak corresponding to the austenite formation point t p  on the heat generation curve. 
     In this case, it turns out that the austenite formation point t p  is located in connection to the local minimum resulting in a very flat minimum. From FIGS. 2A and 2B it can be seen that t p  does not affect the slope at α. 
     Type  3 , which is disclosed in FIG. 3, also relates to near-eutectic cast iron, and is characterised by a detectable inflexion point after the local minimum in the cooling curve recorded in the centre of the sample. After determining the heat generation curve it is easy to establish the position of the austenite formation point t p  to a location immediately after the local minimum. However, in this situation the recalescence, and accordingly α is affected by the austenite formation. In order to be able to disregard the effects of the “moved austenite formation point” and to find a correct α located at t α (where t α &lt;t p ), the part of the centrally recorded cooling curve immediately adjacent to t p , and having time values t such that t−t p  are larger than a predetermined threshold value tv, are searched for a maximum value α 1  of the first time derivative. This value α 1  is then used as a correct α in the method. A suitable threshold value tv can easily be determined by the skilled person, but typical examples of such values are 1-5 seconds. 
     Type  4 , which is disclosed in FIG. 4A, relates to hyper-eutectic cast iron. This type is characterised in that no evident inflexion points or wide minima can be seen. The heat generation curve of the A zone does not comprise any detectable peaks. The present invention is not applicable in this situation. 
     It is preferred to carry out the prediction method by using a computer-controlled system, especially when a large number of measurements must be carried out. In this case, the same kind of sampling device  22  that has been described above is used. Such a computer-controlled system is outlined in FIG.  6 . During the measurement of a particular sample the two temperature-responsive means  10 ,  12  send signals to a computer  14  comprising a ROM unit  16  and a RAM unit  15  in order to generate the cooling curves. The computer device  14  is coupled to a memory means  19  in which a program routine is stored. When, in the following, it is described that the computer operates, it is to be understood that the computer is directed by the program in memory  19 . The memory  19  may be a ROM circuit or a hard disc. The program may be supplied to the memory  19  from a non-volatile memory such as a CD-ROM or a diskette, e.g. via a databus (not shown). The computer has access to calibration data in a ROM unit  16  and calculates the amount of structure-modifing agents that must be added to the melt. This amount 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.