Method for influencing the elongation of cast iron

0001The invention relates to a method for influencing the properties of cast iron by adding magnesium to the cast iron melt.

0002In general, the free magnesium content in a cast iron melt is regarded as a determining factor for the formation of spheroidal or vermicular graphite in the magnesium-treated cast iron. The present practice for controlling the production of ductile cast iron consists in the determination of the total magnesium content, ie the free and the bound magnesium, by means of spectrographically investigated samples. However, this method gives an incomplete picture because the content of free magnesium is not known and the measurement does not give any information on the oxygen activity. However, the oxygen activity, which is in equilibrium with the free magnesium, is a determining factor in the formation of the graphite form. The so-called ductile cast iron is a normal gray cast iron treated with a ball-forming additive so that the principal part of the graphitic carbon in the cast is so-called nodular graphite or spherical graphite. Nodular graphite in cast iron must be analyzed for shape, size and number of particles since these parameters influence the mechanical properties of the cast iron. A visual analysis is complex or subjective, even with partially automated analyzes. Measurements for this are, for example, from<patcit id="pcit0001" dnum="US5675097A"><text>US 5,675,097</text></patcit> known. In the<patcit id="pcit0002" dnum="DE19928456A1"><text>DE 199 28 456A1</text></patcit> Measurements for the determination of the spatial structure of graphite in castings are based on oxygen determination and do not show the disadvantages of visual methods. This can be reactivated more quickly and the targeted influence on the production increases the yield or reduces the rejects during casting. The quality of the cast iron is well controlled.

0003Further examples of the measurement of oxygen for controlling the composition of Mg-treated cast iron are shown in FIG <patcit id="pcit0003" dnum="JP2006063396B"><text>JP2006063396</text></patcit>, <patcit id="pcit0004" dnum="US6544359B1"><text>US6,544,359 B1</text></patcit> and <patcit id="pcit0005" dnum="WO9945156A"><text>WO 99/45156</text></patcit> known.

0004The success of the magnesium treatment in cast iron can be carried out, for example, by metallographic or spectrographic analyzes of white-solidified samples or also by thermal analyzes.

0005In general, pure magnesium or a magnesium alloy is used to promote the spherical shape of the cast iron. A portion of the added magnesium extracts oxygen and sulfur from the iron, while the remaining part is the so-called free magnesium portion, which regulates oxygen activity. The free magnesium content in the melt is the determining factor for the nodularity of the cast iron. The free magnesium content decreases in the melt over time, while the oxygen activity increases. This affects the structure and mechanical properties of the cast iron.

0006Sensors for determining the oxygen activity of a metal melt are, for example, <patcit id="pcit0006" dnum="DE10310387B3"><text>DE 103 10 387 B3</text></patcit> known. In this case, a solid electrolyte tube is disclosed which has a coating of a mixture of calcium zirconate and a fluoride on its outer surface so that, for example, the concentration of sulfur, silicon or carbon can be measured in iron melts.

0007It is an object of the present invention to provide a method for regulating the method with which the prior art is improved, the mechanical properties of the cast iron being already influenced in a targeted manner in the liquid phase.

0008The object is achieved by the features of the independent claim. Advantageous embodiments are specified in the subclaim. In particular, the process according to the invention is characterized in that the oxygen content of the cast iron melt is measured and the magnesium is fed to the cast iron melt until the oxygen content of the cast iron melt at a temperature of 1.420 ° C. as a reference temperature is 0.08 to 0.1 ppm. Since the oxygen measurement is more accurate than the magnesium measurement that has hitherto been possible (magnesium is present in the melt as free magnesium and as bound magnesium, so that accurate detection is not possible), the determination of the mechanical properties of the cast iron is more accurate. The person skilled in the art can determine and make use of a correlation between the existence of less large graphite particles with a low oxygen content on the one hand and many small graphite particles with a higher oxygen content on the other hand.

0009This correlates with the mechanical properties, as already described in <patcit id="pcit0007" dnum="US5675097A"><text>US 5,675,097</text></patcit> , Such as, for example, tensile strength, elongation, and resistance to deformation. For cast iron, it has surprisingly proved that it has a maximum elongation when the addition of magnesium takes place until the oxygen content is less than 0.1 ppm, according to the invention between 0.08 and 0.1 ppm. At a lower or higher oxygen content, the elongation of the cast iron decreases again. It is advantageous that about 200 to 750 ppm magnesium are added to the cast iron melt in order to achieve the desired oxygen content.

0010A sensor for measuring the oxygen content in molten cast iron with an electrochemical measuring cell comprising a solid electrolyte tube is characterized in that a layer of zirconium dioxide is applied to the outwardly directed surface of the solid electrolyte tube. In particular, the zirconium dioxide of the layer can be stabilized with calcium oxide, yttrium oxide and / or magnesium oxide. It is advantageous that the layer is stabilized with up to 30% by weight of calcium oxide, up to 25% by weight of magnesium oxide and / or up to 52% by weight of yttrium oxide. In particular, it is advantageous that the layer is stabilized with about 4 to 6% by weight of calcium oxide. Advantageously, the layer of the sensor is plasma sprayed. It preferably has a thickness of about 30 to 50 μm, in particular about 40 μm. The solid electrolyte tube on which the layer is placed is preferably a zirconium dioxide tube which can be stabilized with about 2% by weight of magnesium oxide.

0011An exemplary embodiment of the invention is described below with reference to a drawing. In the drawing: FIG.<ul><li><figref idrefs="f0001">FIG</figref> The relationship between the number of graphite particles and the oxygen content (oxygen activity aO)</li><li><figref idrefs="f0002">FIG</figref> The relationship between the relative extent and the oxygen content,</li><li><figref idrefs="f0003">FIG</figref> A cross - section through a sensor head and</li><li><figref idrefs="f0004">FIG. 4</figref> 3 shows a partial section through a further embodiment of the sensor.</li></ul>

0012In <figref idrefs="f0001">FIG</figref> It can be seen that the number of graphite particles increases with increasing oxygen content (oxygen activity aO). By means of the control of the oxygen content via the addition of magnesium, the number of graphite particles can thus be adjusted. Thus, the properties of the cast iron are already specifically influenced in the melt. Maximum nodularity occurs with oxygen activity between about 0.10 and 0.12 ppm (valid for 1,420 ° C). If the oxygen activity falls below 0.10 ppm, the nodularity is reduced. This corresponds to the known experience in foundry practice that an excessively high magnesium content has a negative effect on the nodularity.

0013<figref idrefs="f0002">FIG</figref> Shows the relationship between the relative extent of the cast iron and the oxygen content. A maximum of the expansion (elongation) is recognizable at about 0.08 ppm. At lower oxygen activity the elongation is slightly smaller, presumably because of the lower nodularity. If the oxygen activity exceeds the optimum value, a steady reduction in the elongation occurs. The graph shows that it is possible to influence the relative expansion of the cast iron by adjusting the oxygen content in the cast iron melt by adding magnesium.

0014In <figref idrefs="f0003">FIG</figref> A sensor is shown. In a metal tube 1, the electrical leads 2 (Cu / CuNi / conductors) are arranged in a sand filling 3. Via the connecting piece 4, the electrical leads are connected to a lance or another holder and also to an evaluation unit. The other end of the lines 2 is connected to a thermocouple 5 and the electrochemical measuring cell 6. The electrochemical measuring cell 6 has a solid electrolyte tube (ZrO₂Cell) with a steel shock shield as an outer shell. The ZrO₂Cell has a layer of zirconium dioxide stabilized with 5% by weight of calcium oxide on its outer surface. This layer is about 40 μm thick. In the drawing, it is not shown in detail since solid electrolytic tubes are basically known.

0015The thermocouple 5 is fixed in a thermocouple sealing cement 7. The measuring cell 6 is likewise fixed in a cement 8, its end disposed inside the sensor is closed by a sealing plug 9, through which the electrical contacts are led out. The two sensor elements 5; 6 are connected to one another by means of a plastic clip 10. Through the thermally insulating part 11, the lines are guided through the interior of the metal tube 1. On the immersion of the sensor, a sand body 12 is arranged on the outer side of the metal tube 1 in order to protect it.

0016<figref idrefs="f0004">FIG. 4</figref> Shows a similar arrangement, in which the contacting of the sensor in the carrier tube 13 is shown. The carrier tube 13 is formed from cardboard and is surrounded, on its front side facing the sand body 12, by a splash guard tube 14, which is formed from foundry sand or cement. The sensor elements 5; 6 are themselves surrounded by a metal cap 15 for protection during transport and immersion into the melt, which metal chip 15 melts during or after the immersion of the sensor into the metal melt and the sensor elements 5; 6.

0017The sensor elements 5; 6 are themselves surrounded by a metal cap 15 for protection during transport and immersion into the melt, which metal chip 15 melts during or after the immersion of the sensor into the metal melt and the sensor elements 5; 6.