Sensor for determining the oxygen content of cast iron

(54) Title: METHOD FOR INFLUENCING CAST IRON PROPERTIES AND OXYGEN SENSOR (30) Unionist Priority: 22/01/2007 de 102007004 147.2 (73) Holder (s): Heraeus Electro-Nite International NV (72) Inventor (s): Danny Habets (74) Attorney (s): Dannemann, Siemsen, Bigler & Ipanema Moreira (57) Summary: method to influence CAST IRON PROPERTIES, AND OXYGEN SENSOR. The present invention relates to a method for influencing the properties of cast iron by adding magnesium to the melting of the metallic iron, in which the oxygen content of the melting of the cast iron is measured and the magnesium is added to the melting of the cast iron. until the oxygen content of the melting of the cast iron is approximately 0.005 to 0, ppm at a temperature of approximately 1. 420 ° C, and a sensor for measuring the oxygen content in the melting of the cast iron by means of an electrochemical measuring cell comprising a solid electrolyte tube.

(86) International Order: pct EP2008000226de 14/01/2008 (87) International Publication: W0 2008 / 089894de3i / 07/2008

ΡΙ8885850 -4

The success of magnesium treatment on cast iron can be demonstrated, for example, by means of metallographic or spectrographic analyzes of solidified white samples or also by means of thermal analyzes.

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

Sensors for determining the oxygen activity of a metal melt are known from DE 10310387B3, for example. A solid electrolyte tube is described here which has, on its external surface, a coating of a mixture of calcium zirconate and fluoride so that, for example, the concentration of sulfur, silicon or carbon can be measured in metal melts.

It is the aim of this invention to propose a method as well as a sensor to regulate the method by which the prior art is improved, and the mechanical properties of the cast iron must be influenced specifically already in the liquid phase.

The problem is solved by the characteristics of the independent claims. Advantageous configurations are indicated in the subclaims. In particular, the method according to the invention is characterized by the fact that the melting oxygen content of the cast iron is measured and that the magnesium is added to the melting of the cast iron until the oxygen content of the melting of the cast iron is approximately 0.005 at 0.2 ppm at a temperature of approximately 1,420 ° C as the reference temperature. Since the measurement of oxygen is more accurate than the measurement of magnesium possible so far (magnesium is present in fusion as free magnesium and as bound magnesium so that an accurate analysis is not possible), the determination of the mechanical properties of iron cast will be more accurate.

The person skilled in the art can detect and use a correlation between the existence of, on the one hand, a few large particles of graphite at a low oxygen content and, on the other hand, many small particles of graphite at a higher oxygen content . Thus, a correlation to mechanical properties is possible, as already described in US 5,675,097, for example in terms of tensile strength, elongation, and resistance to deformation. It has been surprisingly shown to cast iron that it has a maximum elongation when magnesium is added until the oxygen content is less than 0.1 ppm, preferably 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 to add approximately 200 to 750 ppm of magnesium to the melting of the cast iron to achieve the desired oxygen content.

The sensor according to the invention is characterized by the fact that a layer of zirconium dioxide is applied to the outer surface of the solid electrolyte tube. In particular, the zirconium dioxide in 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 approximately 4 to 6% by weight of calcium oxide. Advantageously, the sensor layer is sprayed with plasma. Preferably, it has a thickness of approximately 30 to 50 pm, in particular approximately 40 pm. The solid electrolyte tube in which the layer is provided is preferably a zirconium dioxide tube that can be stabilized with approximately 2% by weight of magnesium oxide.

An exemplary embodiment of the invention is described on the basis of a drawing. In the drawing:

Figure 1 shows the correlation between the number of graphite particles and the oxygen content (oxygen activity aO);

Figure 2 shows the correlation between the relative elongation and the oxygen content;

Figure 3 shows a cross section through the sensor head according to the invention; and Figure 4 shows a partial section through another sensor configuration.

Figure 1 shows that the number of graphite particles increases with increasing oxygen content (aO oxygen activity). Thus, the number of graphite particles can be adjusted by regulating the oxygen content by adding magnesium. Consequently, the properties of cast iron are especially influenced in the melting process. Maximum modularity is provided at an oxygen activity between approximately 0.10 and 0.12 ppm (valid for 1,420 ° C). Modularity reduces when oxygen activity drops below 0.10 ppm. This corresponds to known experiences in the casting practice that an excessive magnesium level has negative effects on modularity.

Figure 2 shows the correlation between the relative elongation of the cast iron and the oxygen content. The maximum elongation is noticeable at approximately 0.08 ppm. At lower oxygen activity, the elongation is slightly less, probably due to less modularity. If the oxygen activity exceeds the optimal value, the elongation will be constantly reduced. The graphical presentation shows that it is possible to influence the relative elongation of the cast iron by adjusting the oxygen content in the melting of the cast iron through the addition of magnesium.

Figure 3 shows a sensor according to the invention, in a metallic tube 1, the electrical lines 2 (Cu / CuNi / conductor) are arranged in a sand filling 3. Through the connection piece 4, the electrical lines are connected with a boom or other support and also with an analyzer unit. The other end of the lines 2 is connected with a thermocouple 5 and the electrochemical measuring cell 6. The electrochemical measuring cell 6 has a solid electrolyte tube (ZrO cell₂) with a steel shock shield as an outer covering. On its outer surface, the ZrÜ2 cell has a layer of zirconium dioxide that is stabilized with 5% by weight of calcium oxide. This layer is approximately 40 μιτι thick. It is not shown in detail in the drawing since solid electrolyte tubes are basically known.

Thermocouple 5 is fixed in position on a waterproofing cement of thermocouple 7. The measuring cell 6 is also fixed in position on a cement 8; its end provided inside the sensor is closed with a waterproofing cap 9 through which the electrical contacts are passed. The two sensor elements 5; 6 are connected using a plastic clamp 10. Extending through the thermally insulated part 11, the lines are passed through the interior of the metal tube 1. At the immersion end of the sensor, a sand body 12 is provided on the outside of the metal tube 1 to protect it.

Figure 4 shows a similar arrangement in which the sensor is shown contacting the conveyor tube 13. The conveyor tube 13 is formed of cardboard and wrapped on its front side facing the sand body 12 by a splash protection tube 14 which is formed from foundry sand or cement. For protection during transport and during immersion in the melt, the sensor elements 5; 6 are initially wrapped with a metal cover 15 that melts during or, respectively, after the sensor is immersed in the metal melt and exposes the sensor elements 5; 6 .