Metallurgical apparatus – Means for introducing fluent into vessel – e.g. – tuyere – Having means preventing damage to introducing means – e.g.,...
Reexamination Certificate
2001-03-19
2003-01-07
Kastler, Scott (Department: 1742)
Metallurgical apparatus
Means for introducing fluent into vessel, e.g., tuyere
Having means preventing damage to introducing means, e.g.,...
C266S280000
Reexamination Certificate
active
06503442
ABSTRACT:
TECHNICAL FIELD
This invention relates to coatings for high temperature-corrosive applications. In particular, it relates to coatings useful for extending the service life under severe conditions, such as those associated with metallurgical vessels' lances, nozzles and tuyeres.
BACKGROUND OF THE INVENTION
Tuyeres, often mounted on a bustle pipe inject air, oxygen and fuel into blast furnaces and smelters, such as Pierce-Smith converters. Similar to tuyeres, gas injection nozzles inject oxygen and fuel into electric arc furnaces' bath of molten steel. In addition, lance nozzles inject oxygen and fuel into basic oxygen furnaces used to manufacture steel. These lances, nozzles and tuyeres are usually water-cooled and made of high conductivity copper or copper-base alloys that have minimal resistance to molten slag or metal attack. In addition to these, metallurgical vessels' lances and nozzles typically experience both hot particle erosion and molten slag or metal attack.
An additional problem is the presence of corrosive gases. These corrosive gases include acids and non-acidic reactive metal vapors. The corrosive gases, such as chlorine and sulfur dioxide often originate from fuels or the oxidation of metal sulfides in the feed stock or melt. Similar to acidic gases, reactive vapors such as, cadmium, lead, zinc, etc. typically originate from their inclusion in scrap steel feed to blast and electric arc furnaces. These gases aggressively attack metal injection devices. For example, sulfur dioxide readily reacts with copper and forms sulfides such as, copper sulfide (CuS).
Nakahira, in U.S. Pat. No. 3,977,660, discloses a blast furnace tuyere coating. This coating consists of a cermet deposited on either a nickel-base or cobalt-base self-fluxing alloy and an alumina or zironia ceramic layer covering the cermet. The major disadvantage of this coating is that the self-fluxing powder requires a two-step process to obtain an adequate bond to the tuyere. The process first spray coats the self-fluxing powder to the tuyere. Then it heats the powder (and tuyere) to bond the self-fluxing alloy to the tuyere. This heating process often imparts significant distortion upon the tuyere.
Watanbe et al., in U.S. Pat. No. 4,189,130, disclose a three-layer coated copper tuyere for blast furnaces. This coating contains a metal bond layer, a cermet layer containing ceramic in a metal matrix and a ceramic top coat. As far as known, this coating did not receive widespread commercial application due to spalling of the multi-layer coating.
Yet another problem with coated tuyeres and nozzle tips is cracking after a period of service under extreme cyclic heating and cooling. This cracking can propagate toward the inner wall, causing eventual water leakage.
Schaffer et al., in U.S. Pat. No. 4,898,368, disclose a bi-layer coated tuyere deposited by flame spraying, plasma spraying, plasma deposition, detonation gun or hypersonic deposition. Most advantageously, this process used a non-transferred arc plasma deposition process. Unfortunately, Schaffer et al.'s design provided inadequate protection to justify its relatively high cost--ceramic coatings add significant cost to tuyeres, nozzles and lances—especially in comparison to tuyeres, nozzles and lances fabricated out of low cost copper-base alloys. The inadequate increase in tuyere life most likely originated from the coating's insufficient resistance to sulfidation.
As far as known, no commercial tuyere coatings have been adapted for production with either detonation gun or Super D-Gun™ devices. A detonation gun method and apparatus are described in U.S. Pat. No. 2,714,563 and a Super D-Gun™ method and apparatus are described in U.S. Pat. No. 4,902,539. A detonation gun substantially comprises a normally cylindrical, water-cooled barrel with an inside diameter of about 25.4 mm, about 1 to 2 m in length, fitted near one end with supply valves. The gun is supplied with a gaseous mixture of at least one oxidizing gas (e.g., oxygen) and at least one fuel gas (e.g. acetylene) as well as a powdered coating material, normally less than 100 micrometers in diameter. Nitrogen may be added to the gas mixture to reduce the temperature of the detonation. The gas mixture is ignited, usually with a spark, to produce a detonation wave. As the wave travels down the barrel, it heats the powder particles and accelerates the powder particles to a velocity greater than 750 m/s for a detonation gun and 1000 m/s for a Super D-Gun device.
SUMMARY OF THE INVENTION
The coated device contains a coating for use with corrosive environments at high temperatures. The device has a bond coat consisting essentially of, by weight percent, 0 to 5 carbon, 20 to 40 chromium, 0 to 5 nickel, 0 to 5 iron, 2 to 25 total molybdenum plus tungsten, 0 to 3 silicon 0 to 3 boron and balance cobalt and essential impurities to provide sulfidation resistance at high temperatures. A zirconia-base ceramic coating covers the bond coat for heat resistance. Optionally, a boride or carbide coating covers the zirconia for additional resistance to erosion.
The method forms a coated device first coating the device with a cobalt-base bond coat. Then a thermal spray device melts at least a zirconia-base ceramic powder's outer layer to form a partially molten zirconia powder. After melting the powder, the thermal spray device accelerates the partially molten zirconia-base ceramic powder to a velocity of a least 750 m/s to coat the bond coat with a series of interlocking zirconia-base ceramic agglomerations. The layer of zirconia-base ceramic agglomerations increases the coated device's heat resistance.
DESCRIPTION OF PREFERRED EMBODIMENT
The coating consists of a zirconia-base ceramic layer over an undercoat or bond layer of cobalt-base-sulfidation-resistant alloy. Optionally, a third layer of boride or carbide coating may be applied over the ceramic for additional erosion resistance. Advantageously, the device coated is an injection device for a metallurgical vessel such as a lance, nozzle or tuyere. This coating is useful for devices constructed of various metals such as cobalt-base alloys, copper, copper-base alloys, nickel-base alloys and stainless steels. Most advantageously, this coating is applied to copper or copper-base alloys.
The undercoat is a cobalt-base alloy resistant to sulfidation at high temperatures. The cobalt-base alloys of the invention advantageously contain, by weight percent, about 20 to 40 percent chromium—unless specifically expressed otherwise, all compositions provided in this specification are expressed in weight percent. The chromium provides oxidation resistance and some additional resistance to oxidation for the cobalt matrix.
A total addition of about 3 to 20 molybdenum and tungsten greatly enhances the alloy's sulfidation resistance. This is particularly important for protecting copper and copper-base alloy devices used in connection with molten metal. At the high temperatures generated with smelting and processing molten iron and steel, copper injection devices quickly react with sulfur dioxide to form detrimental CuS. The change in density associated with the sulfidation often causes ceramic coatings to spall off. In addition, ceramic coatings generally tend to have porosity and cracks that permeate the ceramic coating. These defects in the coating provide sites subject to severe crevice corrosion. For these reasons, it is essential that the coating contain at least 2 percent tungsten or molybdenum to increase the alloy's sulfidation resistance. Most advantageously, the alloy contains at least 3 percent tungsten.
In addition, it is important to limit iron and nickel to less than 5 percent, because each of these elements reduces sulfidation resistance. Maintaining these elements at levels as low as commercially practical improves the sulfidation resistance of the alloy.
Optionally, the alloy contains up to 5 percent carbon to strengthen the alloy. Carbon levels above five percent tend to decrease the corrosion resistance of the alloy.
Biederman Blake T.
Kastler Scott
Praxair S.T. Technology, Inc.
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