Metal working – Method of mechanical manufacture – Roller making
Reexamination Certificate
1999-11-23
2001-03-13
Cuda Rosenbaum, I (Department: 3726)
Metal working
Method of mechanical manufacture
Roller making
C029S895210, C492S058000, C427S453000, C427S456000
Reexamination Certificate
active
06199281
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of preparing a hearth roll with a coating. More particularly, the present invention relates to a method of thermally spraying electroless-plated ceramic powder and metal powder simultaneously onto a hearth roll.
2. Description of the Related Arts
In the process of continuous annealing and continuous galvanization, a rolled steel strip is conveyed by means of a hearth roll. Conventionally, a hearth roll is installed in a high temperature zone (800-1200° C.), thus, in addition to having thermal oxidation resistance, the surface of a hearth roll has to retain hardness under high temperature, thereby maintaining the necessary surface roughness to produce the necessary friction to convey and avoid the slippage of a steel strip. In addition, there is also a need for a hearth roll to have excellent chemical inertness in order to prevent ferric oxide or other metal oxides of the surface of a steel strip from adhering to the surface of a hearth roll to form the buildup and cause defects on the surface of a steel strip.
An earlier technique to prevent the formation of the buildup on the surface of a hearth roll was to plate hard chromium on the surface of a hearth roll to form a hard layer. However, such hard chromium layer has the disadvantage of decreasing in hardness when the temperature is higher than 450° C. Thus, recently a new technique has been developed, in which heat-resistant alloy powder with high hardness or ceramic powder are melted, thermally sprayed, and thus adhered to the surface of a hearth roll. The alloy coating has the advantages of easy-fabrication and an excellent adhesion; however, it is restricted by insufficient hardness under high temperature. In contrast, the ceramic coating has the advantages of hardness under high temperature, heat-resistance and wear-resistance; however, due to the poor adhesion, it has the disadvantage of easy-collapse under high temperature, and thus the surface roughness cannot be maintained.
Therefore, the cement coating technique, with the advantages of both alloy and ceramic coatings, has been widely used in the treatment of a hearth roll recently. The technique used is to mix ceramic powder and metal powder first; afterward, the mixed powder was sprayed simultaneously onto the surface of a hearth roll so that this coating has the advantages of both excellent adhesion from the alloy coating and hardness under high temperature from the ceramic coating.
The technology of the cement coating is described in U.S Pat. No. 4,470,802. Niobium powder (metal powder) and inorganic material (ceramic powder) are mixed mechanically, and then the mixture is sprayed onto the surface of a hearth roll.
However, this process has the following disadvantages. During the period of storage and transportation, the metal powder tends to settle due to the different densities between metal powder and ceramic powder. Thus, when the powder for spraying is obtained by mixing the two powders mechanically, the ratio between metal powder and ceramic powder will be changed. Thus, the different densities between metal powder and ceramic powder can cause the loss of the ceramic powder during the spraying, making the hardness of the coating insufficient. Moreover, because of the different surface tensions between metal powder and ceramic powder, the melted metal powder has insufficient wettability to the ceramic powder, so that the coating will have higher porosity and lower buildup-resistance.
SUMMARY OF THE INVENTION
In view of the above problems, it is the object of the present invention to provide a hearth roll with a coating of high hardness, low porosity and high buildup-resistance.
Another object of the present invention is to provide a hearth roll with a coating, in which the ceramic content can be controlled precisely.
In order to accomplish the aforementioned objects and advantages a method of preparing a hearth roll with a coating according to the present invention, comprises the steps of: (a) plating a metal on a ceramic powder by using electroless plating; (b) mixing the metal-plated ceramic powder and a metal powder to form a cermet powder; and (c) thermally spraying the cermet powder onto a hearth roll.
Another method of preparing a hearth roll with a coating according to the present invention comprises the steps of: (a) plating a metal on a ceramic powder by using electroless plating; (b) mixing the metal-plated ceramic powder, a metal powder, and a binder to form a slurry, and subjecting the slurry to granulating and sintering to obtain a sintered powder; and (c) thermally spraying the sintered powder onto a hearth roll.
Other objects, features, and advantages of the present invention will become apparent from the following detailed description.
DETAILED DESCRIPTION OF THE INVENTION
The method of preparing a hearth roll with a coating according a first embodiment of the present invention is to thermally spray a ceramic powder plated with a metal using electroless plating and a metal powder onto the hearth roll.
The ceramic powder used herein can be oxides, carbides, nitrides, or mixtures thereof. The embodied examples include alumina (Al
2
O
3
), yttria (Y
2
O
3
), zirconia (ZrO
2
) tungsten carbide (WC), and chromium carbide (Cr
2
C
3
) etc. The major reason why the ceramic powder is plated with metal is to increase the density of the ceramic powder, and thus the metal type used herein is not to be restricted. However, cobalt and nickel are preferred. The electroless-plating solution can be adjusted according to the metal which is plated on the ceramic powder; for example, a solution containing cobalt when the cobalt is the metal we want to plate on.
According to the present embodiment, the powder used for thermally spraying is the cermet powder obtained by mixing the ceramic powder plated with metal and the metal powder. The resulting cermet powder can be sprayed onto a hearth roll directly.
In a second embodiment of the present invention, in order to increase the adhesion of the electroless metal-plated ceramic powder to the metal powder, the cermet powder (electroless metal-plated ceramic powder and metal powder) is mixed with a binder to form a slurry, followed by granulating and sintering to obtain a sintered powder. Then, the sintered powder is thermally sprayed onto a hearth roll. The process of granulating is to dry the above-described slurry at 40~80° C., and then shatter for granulating. The condition of sintering is at 900~1100° C. under vacuum. By the process of mixing the binder, granulating ,and sintering, there is provided a powerful bond strength between the ceramic and metal powders, thus makes preventing ceramic powder from separating from the metal powder during the spraying process.
The metal powder used to mix with the metal-plated ceramic powder is selected from the group consisting of cobalt powder, cobalt alloy powder, nickel powder, nickel alloy powder, iron powder, iron alloy powder, niobium powder, niobium alloy powder, and mixtures thereof. The cobalt alloy powder and nickel alloy powder are preferred; for example, cobalt-chromium alloy powder or nickel-chromium alloy powder.
The thermally spraying of the sintered powder or the cermet powder onto a hearth roll, according to the present invention, can be conducted by flame spraying, plasma spraying or high velocity oxygen fuel spraying. During the process of spraying, the metal powder can be melted sufficiently, and the ceramic powder can be still maintained as solid and dispersed in the metal substrate evenly, so that the object of strengthening the coating can be accomplished.
The main feature of the present invention is to replace the conventional ceramic powder with ceramic powder plated with metal by using electroless plating. Due to the metal plating, the ceramic powder of the present invention has a higher density than the conventional ceramic powder. Thus, during the period of storage and transportation, the powders for spraying (cermet powder or sintered powder) of the p
Chang Wei-Jieh
Hsu I-Chung
Lee Yuh-Wen
Lih Wei-Cheng
Shiau Wei-Deam
Cuda Rosenbaum I
Industrial Technology Research Institute
Knobbe Martens Olson & Bear LLP
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