Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2001-02-26
2003-03-18
Koehler, Robert R. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C118S400000, C266S275000, C428S627000, C428S632000, C428S633000, C428S655000, C428S656000, C428S663000, C428S664000, C428S678000, C428S380000, C428S381000, C428S384000, C428S389000, C428S926000, C428S934000, C428S937000, C492S049000, C492S053000, C492S054000
Reexamination Certificate
active
06534196
ABSTRACT:
TECHNICAL FIELD
The present invention relates to refractory coatings on articles, such as bearings, bushings, couplings or rollers, used in a molten metal environment, such as in aluminizing or galvanizing processes.
BACKGROUND OF THE INVENTION
Aluminizing and galvanizing processes take place at very high temperatures. For example, a 100% aluminum aluminizing bath typically can have a temperature of 1,325° F. (720° C.). In order to run these processes effectively, in an industrial context, it is necessary to have various metal parts, such as bearings, bushings, couplings or rollers, immersed wholly or partly in the aluminizing or galvanizing bath such that the metal pieces or strip to be coated can effectively and efficiently move through the bath. Such baths tend to be very destructive of these metal parts due to rapid metallurgical alloying of the reactive molten metal with the commonly used structural alloys. This results in frequent stoppage of the process so that the parts can be replaced to allow the process to continue. The more frequently the process has to be stopped to replace such parts, the less efficient it is and the greater the running cost of the process. Therefore, designing metal parts which can remain intact in the aluminizing or galvanizing baths for a longer period of time would be highly desirable. It is that goal which the present invention addresses.
The use of refractory metals as coatings for metal parts is known in the art. For example, U.S. Pat. No. 2,839,292, Bellamy, issued Jun. 17, 1958, describes that molybdenum and tungsten coatings on metal parts are highly resistant to attack when immersed in molten aluminum. The coatings can be applied by spraying (which is preferred), plating or cladding. There is no disclosure of the use of high density refractory metal coatings. The patent utilizes an outer ceramic layer to protect the parts from oxidation.
U.S. Pat. No. 5,360,657, Wood, et al., issued Nov. 1, 1994, describes a molybdenum (3% to 9% Mo) boron alloy which is said to show excellent resistance to attack by molten zinc. The patent deals very specifically with molten zinc, discussing its low viscosity and surface tension, and does not deal with molten aluminum at all. Further, there is no discussion of tungsten coatings or of application by any means other than thermal spray. Finally, the patent (at column 5, lines 20-27) demonstrates that a pure molybdenum coating is clearly inferior to the molybdenum-boron alloy coating.
U.S. Pat. No. 5,759,142, Perdikaris, issued Jun. 2, 1998, discloses an aluminizing process guide roll, said to provide longer wear, which utilizes a multi-layer coating on the roll's surface including a first coating layer comprising MCrAlY metal in which M is Ni or Co, and a second coating layer of a refractory oxide of aluminum, zirconium, silicon or chromium. The patent teaches that the MCrAlY coating must be placed directly against the surface of the guide roll and that there not be any intermediate layers between them.
U.S. Pat. No. 5,310,476, Sekhar, et al., issued May 10, 1994, teaches the formation, utilizing a micropyretic heating process, of a coating made from a refractory material, such as molybdenum or niobium compounds. These coatings are said to be inert to damage from molten aluminum.
U.S. Pat. No. 5,370,372, Eckert, issued Dec. 6, 1994, describes a ladle used for working with molten metal (such as aluminum) which is said to be resistant to damage caused by the molten metal. The ladle may be made from niobium or molybdenum, among other materials, and has a refractory oxide coating which comprises at least one of alumina, zirconia, yttria-stabilized zirconia, magnesia, magnesium titanate, mullite, or a combination of alumina and titania.
SUMMARY OF THE INVENTION
The present invention relates to two different, but related, refractory metal coatings used to protect metal parts placed in a molten metal environment.
The first embodiment is an article used in contact with molten aluminum or molten zinc, particularly molten aluminum, which is coated with a high density coating consisting of a metal selected from the group consisting of Group Vb, VIb and VIIb metals (in pure or alloyed form). The coatings generally have a thickness of from about 0.01 to about 0.30 inch. The articles are generally in the form of a bearing, bushing, sleeve, coupling or roller. Preferred metals are the Group VIb) metals, particularly molybdenum and tungsten, most particularly in pure form. Examples of high density coating application methods which may be used in this embodiment of the present invention include plasma-transferred arc, welding overlay, and high-velocity arc spraying processes. These parts may optionally be provided with an aluminum-based or similarly effective overlay to provide oxidation protection to the part prior to immersion.
The second embodiment of the present invention is a roll for guiding strip steel through a high temperature aluminizing bath, said roll comprising a roll body having a surface in guiding contact with said steel strip within said bath and a multi-layer coating on said roll's surface for contact with said steel strip, said multi-layer coating including a first primer layer on said roll's surface, comprising a group Vb, VIb or VIIb metal, particularly tungsten or molybdenum, either as pure metal or an alloy; a second layer comprising MCrAlY metal in which M is Ni or Co, said MCrAlY metal being thermally sprayed on said primer layer; and a third layer comprising a refractory metal oxide of aluminum, zirconium, silicon or chromium, said third layer having a higher porosity than said second layer to better accommodate thermal expansion coincident with bath immersion without disruption of the third layer surface, while the less porous MCrAlY metal second layer and the first layer maintain effective metallurgical corrosion protection of said roll surface.
All percentages and ratios given herein are “by weight,” unless otherwise specified.
DETAILED DESCRIPTION OF THE INVENTION
In its first aspect, the present invention encompasses articles used in contact with molten aluminum or molten zinc which are coated with a high density coating consisting of certain specific refractory metals. The articles are any of a variety of articles which typically would be immersed either wholly or partially in a bath of molten aluminum or molten zinc during the aluminizing or galvanizing process and can include, for example, bearings, bushings, sleeves, couplings and rollers. The metals which are utilized for the coating are selected from the group consisting of Group Vb, VIb and VIIb metals, including, for example, molybdenum, tungsten, niobium, tantalum, and rhenium. The Group VIb metals are preferred, with molybdenum and tungsten being particularly preferred.
The metals may be applied in either pure or alloyed form, with pure form being preferred. The metals are applied to the article in the form of a high density coating having controlled and minimized microstructural porosity. As used herein, “high density” means a coating which has a porosity of no greater than about 15%, preferably no greater than about 5%, based on the metallographic examination technique common in metallurgical practice. Such application methods include, for example, plasma transferred arc (which is preferred), welding overlay, and high velocity plasma arc spraying in either air, protective cover (gas or water) or low atmospheric pressure chamber. These application methods provide a coating which is dense and which forms an intimate bond with the substrate. The coating could also be formed as a sleeve which is applied and bonded to the substrate. In the plasma transferred arc method, the coating material (e.g., molybdenum) is melted to form the coating, but the substrate is not extensively melted; the coating therefore is not diluted significantly upon application. Lower density application methods, such as thermal spray, plating or cladding are not suitable for use in the present invention. When applied as described herein, the art
Cincinnati Thermal Spray
Frost Brown Todd LLC
Koehler Robert R.
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