Sensor for determining the oxygen content of cast iron

(54) Title: METHOD FOR INFLUENCING CAST IRON PROPERTIES (51) Int.CI .: G01N 27/411; C21C 1/04 (30) Unionist Priority: 22/01/2007 DE 10 2007 004 147.2 (73) Holder (s): HERAEUS ELECTRO-NITE INTERNATIONAL NV

(72) Inventor (s): DANNY HABETS (85) National Phase Start Date: 05/26/2009

1/6

Invention Patent Descriptive Report for METHOD FOR INFLUENCING CAST IRON PROPERTIES.

[001] The present invention relates to a method for influencing the properties of cast iron by adding magnesium to the melting of cast iron. The invention also relates to a sensor for measuring the oxygen content in cast iron fusions by means of an electrochemical measuring cell comprising a solid electrolyte tube.

[002] Generally, the content of free magnesium in a melting of cast iron is considered to be a determining factor for the formation of spheroidal or vermicular graphite in cast iron treated with magnesium. Current practice for regulating the production of ductile iron is to determine the total magnesium content, that is, free and bound magnesium, with the help of spectrographically analyzed samples. However, this method provides an incomplete picture since the free magnesium content is not known and the measurement does not provide any information about oxygen activity. However, the oxygen activity that is in equilibrium with the free magnesium is a determining factor in the formation of the graphite shape. The so-called ductile cast iron is a normal gray cast that is treated with a lump forming additive so that the main part of the graphitic carbon in the cast iron is called nodular graphite or spherical graphite. Nodular graphite in cast iron can be analyzed in terms of shape, size and number of particles, since these parameters influence the mechanical properties of cast iron. A visual analysis is complex or subjective, even in partially automated analyzes. Measurements in this regard are known, for example, from US 5,675,097. DE 19928456A1 describes measurements for determining the spatial structure of graphite in iron

Petition 870180046237, of 05/30/2018, p. 5/16

2/6 castings that are based on oxygen determination and do not have the disadvantages of visual methods. Thus, it can be reacted faster, and the specific influence of production increases yield or, respectively, reduces losses during melting. The quality of the cast iron can be well controlled.

[003] The success of magnesium treatment in 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.

[004] 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.

[005] 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.

[006] The objective of this invention is 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.

[007] The problem is solved by the characteristics of the independent claims. Advantageous configurations are indicated in subPetition 870180046237, of 05/30/2018, p. 6/16

3/6 claims. 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. [008] 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 lower content high oxygen. 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.

[009] 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 mag oxide. Petition 870180046237, of 05/30/2018, p. 7/16

4/6 nesium 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 mm, in particular approximately 40 mm. 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.

[0010] An exemplary configuration of the invention is described based on a drawing. In the drawing:

[0011] Figure 1 shows the correlation between the number of graphite particles and the oxygen content (oxygen activity aO);

[0012] Figure 2 shows the correlation between the relative elongation and the oxygen content;

[0013] Figure 3 shows a cross section through the sensor head according to the invention; and [0014] Figure 4 shows a partial section through another sensor configuration.

[0015] 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.

[0016] Figure 2 shows the correlation between the relative elongation

Petition 870180046237, of 05/30/2018, p. 8/16

5/6 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.

[0017] 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 (cell ZrO2) with a steel shock shield as an outer coating. On its outer surface, the ZrO2 cell has a layer of zirconium dioxide that is stabilized with 5% by weight of calcium oxide. This layer is approximately 40 mm thick. It is not shown in detail in the drawing since solid electrolyte tubes are basically known. [0018] Thermocouple 5 is fixed in position in a waterproofing cement for thermocouple 7. The measuring cell 6 is also fixed in position in 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 side of

Petition 870180046237, of 05/30/2018, p. 9/16

6/6 out of metal tube 1 to protect it.

[0019] 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 .

Petition 870180046237, of 05/30/2018, p. 10/16

1/1