Electricity: measuring and testing – Determining nonelectric properties by measuring electric... – Particle counting
Reexamination Certificate
2001-09-11
2003-08-05
Le, N. (Department: 2858)
Electricity: measuring and testing
Determining nonelectric properties by measuring electric...
Particle counting
Reexamination Certificate
active
06603296
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to an apparatus for the detection and measurement of particulates in molten metal and, more particularly, to an improved apparatus which includes a liquidus depressing material which lowers the temperature at which sampled molten metal would normally begin to solidify, thereby, allowing for more molten metal to be sampled over a longer time period for enhanced particulate measurement.
Molten metals, particularly molten aluminum and steel, are frequently contaminated to some extent by entrained non-metallic inclusions that potentially give rise to a variety of shortcomings or defects in the resulting finished metal products. More often, a certain size or size range of non-metallic inclusions, such as alumina in deep drawing stock, is known to be harmful to the performance of the drawing stock. Knowledge of the quantity of such inclusions would be beneficial in determining the fitness for service of the finished product.
An apparatus for electrical zone sensing of suspended particles in a liquid is disclosed in U.S. Pat. No. 2,656,508 issued Oct. 20, 1953 to Wallace A. Coulter. In a typical apparatus, a tube having an aperture in its wall is positioned within a larger vessel. A liquid electrolyte suspension containing the particles to be detected and measured is placed in the vessel and is induced to flow into the tube through the aperture by establishing a fluid pressure differential between the interior of the tube and the vessel. The vessel and the tube are both fabricated of an insulator, e.g. glass, and a constant electric current is placed across the aperture. The presence of a particle in the liquid flowing through the aperture causes a change in the electrical resistance detected at the aperture and the electric voltage producing the constant current varies directly with the resistance change each time a particle passes through the aperture. A detecting circuit determines the size of the particles passing through the aperture from the change in resistivity caused by each particle, this depending upon the volume of electrolyte at the aperture displaced by the particle and by the resistivity of the kind of particles. The information is amplified and processed by suitable electronic circuits.
U.S. Pat. No. 4,555,662 describes a method and apparatus for the detection and measurement in a molten metal sample of suspended particulates of greater than a predetermined size whose electrical conductivities differ from that of the suspending molten metal. The apparatus comprises an electrically insulating vessel having a small passage (typically 200 to 500 microns in diameter) extended therethrough; a pair of electrodes disposed within and outside of the vessel to establish a current path between them through the molten metal of the sample and passing through the small passage; means for passing a sample of the molten metal through the passage; and means for passing an electric current between the two electrodes through the molten metal in the current path and for detecting a voltage change resulting from the flow of particulates through the passage. The apparatus also includes means for counting the number of voltage changes during a particular measuring period as representative of the number of particulates, and for measuring the magnitude of each of the voltage changes as representative of the size of the particulates causing the changes. The device described comprises a refractory tube with the small hole at its lower end, which is dipped into the molten metal, for example in a trough along which the molten metal is flowing. One electrode is positioned within the tube and the other outside of the tube. Molten metal is caused to pass through the small hole by means of a differential pressure applied to the tube.
The principle of operation of the apparatus described in the two above-identified patents generally refers to the measurement of non-metallic particles in molten aluminum. The devices used for the particulate measurement in molten aluminum are unsuitable for use in molten steel due to the large difference in the respective processing temperatures. The described particle counters commonly evaluate molten aluminum at a temperature of around 750° C., however, the temperature of a steel measurement would be closer to about 1550° C. Simple substitution of more suitable materials for the components of the apparatus cannot be assumed. The availability of materials which are capable of withstanding such high temperatures and are stable at such temperatures for the relatively long periods of time needed to make meaningful particulate measurements are limited as well as very expensive.
U.S. Pat. No. 5,198,749 attempts to address the numerous differences in apparatus construction due to the high processing temperature of steel and its alloys and provide a measuring strategy to overcome the difficulty of relatively long measurement times at high temperatures. The device of the '749 patent comprises a single use disposable probe that is detachably connected to a support member. The detachability of the device is common to those skilled in the art of disposable sensors for the molten iron and steel industry. The probe comprises electrode and orifice configurations of the prior more continuous devices and a jet limiting insert which serves to help cool the incoming metal immediately upon immersion of the probe into the molten metal. A meltable cover closes the orifice prior to immersion of the probe and the cover is protected by a meltable shield (slag cap) enabling the probe to be passed through an overlying slag layer without entry of slag into the probe interior. Such capping is also well known to those skilled in the art of disposable molten metal sensors. The filling of the inner chamber with the molten metal may be assisted by a reduced pressure established within the tube, or may be slowed by a positive pressure to maintain the Reynolds number of the flow below 2000. The inner chamber, is divided by a narrow bore into two compartments so that when metal enters and fills one compartment it will freeze in the bore so that it cannot enter the second compartment, protecting the vacuum source, if provided, and establishing a prescribed quantity of metal entering the probe.
Although the device of the '749 patent uses the principle of disposable, short term measurements of approximately 2 minutes to overcome the problems of long term high temperature measurements, the solution of the '749 patent has introduced a new set of problems. A short term measuring device of the above-described construction does not provide suitable time for preheating of the internal components of the probe. The entering molten metal is cast against the interior probe material that is close to room temperature and is quickly cooled. The lack of suitable preheating results in premature solidification of the molten metal entering the inner chamber effectively limiting the amount of metal which may be sampled. The liquidus temperature of a molten material is the temperature at which a solid phase begins to precipitate from the cooling liquid. The difference between the molten metal processing temperature and the liquidus temperature is called the superheat. An additional problem arises when such probes are intended for immersion in a tundish of molten steel during continuous casting. The temperature of molten steel in the tundish is generally on the order of 20-40° C. above the liquidus temperature of the steel, providing a super heat 20-40° C. The liquid steel possesses a low heat content and an inability to raise the temperature of the inner chamber walls of the probe so as to maintain a non-freezing sampling condition. The mass of the sampling apparatus itself chills the liquid metal in the chamber during filling by thermal conduction to cooler portions of the probe, thus limiting the useful application of such probes to metals having a suitable super heat.
In accordance with the present invention there is provided a molten meta
Conti Richard F.
Kopansky Gregory
McCauley William
Akin Gump Strauss Hauer & Feld L.L.P.
Heraeus Electro-Nite Co.
Le N.
Teresinski John
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