Metal working – Method of mechanical manufacture – Roller making
Utility Patent
1999-09-07
2001-01-02
Cuda, Irene (Department: 3726)
Metal working
Method of mechanical manufacture
Roller making
C029S895300, C427S454000, C427S456000
Utility Patent
active
06167623
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a press roll for a paper or board machine, in particular a center roll for such a machine, a method for manufacturing a press roll for a paper or board machine, and a composition of a coating for a roll for use in a paper or board machine which is subject to difficult corrosion conditions.
BACKGROUND OF THE INVENTION
In a press section of early paper and board machines, a roll was used which was commonly made of granite. In spite of the excellent surface properties of granite rolls, rolls having a ceramic coating have been gradually substituted for granite rolls. In prior art rolls with metal, metal-ceramic and ceramic coatings, however, corrosion of various degrees has occurred in particular in more severe conditions, which corrosion causes corroding of the roll, delamination of the coating, and wear of the roll so that it becomes unusable.
These types of rolls are coated by means of thermal spraying, in which case the coating unavoidably remains porous, and the roll face susceptible to corrosion. In view of corrosion, the process conditions in paper industry have become more and more severe because, for reasons of protecting the environment, process water is recirculated to an ever greater extent, and the rolls also must operate under ever more corroding conditions. For this reason, it is necessary to renew the coatings on rolls and/or to replace the rolls quite frequently which, of course, causes high expenses.
Attempts have been made to improve the resistance of rolls to corrosion by means of a variety of different methods.
In Finnish Patent Application No. 853544, a method for coating a roll is described in which a metallic core of a roll has been coated fully or partly with a metal, such as stainless steel, and in which the proportion of chromium is from about 9% to about 35%. It has been believed that the presence of chromium in the steel expressly increases the resistance of steel to corrosion.
In Finnish Patent No. 89,950, the metal core of a roll has been coated with molybdenum- and nickel-based metals, such as nickel-chromium alloys, nickel-chromium aluminum alloys, etc., in order to improve the resistance of the roll to corrosion. This intermediate layer is applied onto the roll by means of flame spraying or plasma spraying. The thickness of the intermediate layer is from about 100 &mgr;m to about 500 &mgr;m. Between this porous metal layer and the core of the roll, it is further possible, by means of flame spraying or plasma spraying, to apply a tight metal film that inhibits corrosion and that has a thickness of about from about 100 &mgr;m to about 500 &mgr;m. Metals suitable for this are nickel, aluminum alloy, copper, stainless steel, etc., and the particles that are used for the spray coating material are very small.
In Finnish Patent No. 86,566, a roll coating is described including a corrosion-protection layer on the roll core and which has a thickness of at least about 0.5 mm. The tightness of the corrosion-protection layer is higher than about 96% and, if necessary, the corrosion-protection layer may have been sealed by means of laser, induction, plasma, flame, or electron-jet melting, or it may also have been produced by means of thermal spraying. The corrosion-protection layer consists of stainless steel whose chromium content is from about 10% to about 29%, and it is placed between the core mantle of the roll and the ceramic outer layer.
In Finnish Patent No. 82,094, a coating for a Yankee cylinder in a paper machine is described, whose resistance to corrosion permits manufacture of special papers in acid solutions, in which the pH can be in the range from about 3 to about 5. The coating has been made of a mixture of a metal powder and a carbide or nitride.
In Finnish Patent No. 84,506, a press roll for a paper machine is described in which the roll core has been coated with an intermediate layer which consists of a composite compound made of a ceramic and a metal, so that the ratio of the components in the composite compound is different in different parts of the intermediate layer in the direction of the radius of the roll. The intermediate layer is then coated with a ceramic material over the intermediate layer.
In European Patent Application No. 0 657 237, coating of a roll with tungsten carbide and chromium carbide by means of thermal spraying is described. The function of this coating is also, among other things, to improve the resistance to wear of the carbide coatings on rolls.
In European Patent Application No. 0 481 321, a press roll is described in which an intermediate layer made of a molybdenum-based or nickel-based alloy has been applied onto the metal core, for example, by means of plasma spraying. A ceramic layer is applied onto this layer by means of the plasma spraying method, which ceramic layer consists of metal oxides or mixtures of same. The roll is then coated with an organic polymer to fill the pores in the porous ceramic coating.
In the prior art methods and in the coatings that are used currently on the rolls in a paper machine, particularly the center roll of a press section of a paper machine, a problem is the softness of the polymer coating or the porosity of the ceramic coating. As a result, the medium that produces corrosion of the roll frame, such as moisture and other corroding materials, can penetrate through the small pores through the coating to the roll frame. In this connection, corrosion arises, which deteriorates, among other things, the adhesion of a ceramic coating to the roll and promotes damage to the metal core of the roll. In particularly difficult corrosive environments, such as rolls that operate under highly acid or alkaline conditions, very rapid corroding has been noticed in spite of a metallic corrosion-protection layer that has been applied underneath a ceramic coating by means of plasma spraying or high-velocity flame spraying (HVOF) and whose thickness is from about 200 &mgr;m to about 300 &mgr;m. In such a case, the coating layer of the roll often falls apart partially, i.e., delamination takes place, and the roll is unusable.
OBJECTS AND SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide new and improved press rolls with a ceramic coating which can withstand difficult corrosion conditions.
It is another object of the present invention to provide new and improved methods for manufacturing press rolls with a ceramic coating which can withstand difficult corrosion conditions.
It is yet another object of the present invention to provide new and improved coating compositions for press rolls which can withstand difficult corrosion conditions.
It is still another object of the present invention to provide novel press rolls with a ceramic coating, novel methods for manufacturing a press roll, and novel compositions of a coating for a press roll which avoid the drawbacks of the prior art discussed above.
In order to attain these objects and others, the press roll in accordance with the invention includes a frame part, an adhesion/corrosion-protection layer having a thickness from about 150 &mgr;m to about 300 &mgr;m arranged on the frame part, a tight-ceramic layer having a thickness from about 50 &mgr;m to about 150 &mgr;m arranged on the adhesion/corrosion protection layer and a porous ceramic layer arranged on the tight-ceramic layer. The adhesion/corrosion protection layer contains a nickel-chromium alloy and the tight-ceramic layer includes from about 50% to 100% by weight of Al
2
O
3
and optionally comprises up to about 50% by weight of TiO
2
. In one preferred embodiment, the thickness of the adhesion/corrosion-protection layer is from about 180 &mgr;m to about 220 &mgr;m and the thickness of the tight-ceramic layer is from about 80 &mgr;m to about 120 &mgr;m. The tight-ceramic layer may consists of from about 55% to about 65% of A
2
O
3
and from about 35% to about 45% of TiO
2
. The tight-ceramic layer may further comprise an amount of Al
2
TiO
5
. The adhesion/corrosion-protection layer is applied ont
Lehtonen Pentti
Niemi Kari
Wahlroos Juha
Cuda Irene
Steinberg & Raskin, P.C.
Valmet Corporation
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