Roll in the press of a paper machine and a method for the manufacture of the roll

Abstract: The invention concerns a roll used in the manufacture of paper, for example a centre roll in the press or a calender roll, with which the web is in direct contact and from which the web is detached. Onto the cylinder mantle of the roll, a coating has been formed, which is partly made of metal and which also includes a ceramic material. The outer face of the coating consists of carbide-rich areas and of matrix areas placed between said areas. The invention further concerns a method for the manufacture of such a roll.

Description: The invention concerns a roll used in the manufacture of paper, for example a centre roll in the press or a calender roll, with which the web is in direct contact and from which the web is detached, a coating having been formed onto the cylinder mantle of said roll, which said coating is partly made of metal and which also includes a ceramic material. The invention also concerns a method for the manufacture of a roll in accordance with the invention. As is well known, in the press section of a paper machine, a rock roll is used, which is made of granite. The popularity of granite is based on its surface properties, which provide a controlled detaching of the paper web from the rock face. Moreover, granite withstands the wearing effect of a doctor well. However, granite has certain drawbacks. Being a natural material, its properties vary, and internal flaws in granite and its tendency of cracking constitute a serious obstacle for its use in some applications. Moreover, a granite roll is heavy, which increases the tendency of vibration of the structures. The weight of the rock roll is also reflected in the dimensioning of the lifting equipment and of the foundations of the paper machine. In prior art, synthetic rock rolls are also known, which are, in principle, polymer-faced rolls in which rock powder, such as quartz sand, has been added to among hard rubber or polyurethane. Drawbacks of these rolls have been excessive adherence of the paper web to the roll face as well as poor mechanical strength. In the applicant's GB-A-2 169 381 a press roll is described whose surface layer is composed of a mixture of metal powder and an inorganic substance. The function of the metal is to act as a binder agent and to increase the toughness of the roll coating. The function of the inorganic substance is to provide a wear-resistant face of suitable surface energy, because the surface energy of the roll face must be within certain limits in order that the detaching of the paper web from the face of the press roll should be controlled. GB-A-2 180 624 discloses a similar technique. In a roll in accordance with the applicant's FI Pat. Appl. No. 853544 & GB-A-2 180 624 a suitable surface energy has been carried into effect even better, so that the metal component is stainless steel expressly containing chromium, the proportion of chromium in the metal being 9...35 %. A stainless steel that contains an abundance of chromium is a hydrophilic material (chromium increases the hydrophily). On the other hand, by means of the alloying of chromium, wear-resistant chromium carbides were obtained in the structure. Chromium also increases the resistance of steel to corrosion. In such an "alloy", the ceramic material is separated from the steel itself as a chromium carbide. The primary object of the present invention is to provide a roll and a method for the manufacture of same by means of which the detaching of the paper web from the roll face is controlled and the resistance of the roll to temperature and to mechanical strains is even better. In view of achieving the objective stated above, the roll in accordance with the invention is characterized by the features of claim 1. On the other hand, the method in accordance with the invention for achievement of the above objectives is characterized by the features of claim 12. While applying the method of the invention, it is possible to use roll frames manufactured by means of the prior-art casting technique, together with their ends and axle journals, which said roll frame primarily provides for the mechanical strength of the roll, whereas the surface properties of the roll and the durability of the roll face are achieved in a novel way. The roll in accordance with the invention can be manufactured by means of a number of different methods, which are described in GB-A-2 169 381 and in the FI Pat. Appl. 853544. The function of the carbide coating is in particular to act as a press face of the roll, whose specific feature is good properties of detaching of the paper web. The detaching properties are based on the properties of the carbide coating, which are microporosity, low friction, suitable roughness of the surface, and retention of these properties. In the following, the morphology, the chemical composition, and the other properties of the coating in accordance with the invention will be described in detail. The roll coating in accordance with the invention consists of carbide-rich areas, whose diameter is about 10...50 µm, as well as of matrix areas placed between them, whose diameter is about 10...50 µm. In the carbide areas, the size of the carbides is about 0.5...10 µm, and the proportion of the carbides is about 50...95 %. Carbides also occur in the matrix area, but they are of sub-microscopic size of about 0.001...0.01 µm, and their proportion varies within the range of 5...50 vol-%. Some of the carbides are still unseparated, and the carbon reinforces the matrix by means of a solution-reinforcement mechanism. The proportion of the carbide areas varies in accordance with the purpose of use. When the proportion is low, about 5...20 vol-%, the coating is softer and tougher than when the proportion is high, e.g. 70...95 vol-%, in which latter case the abrasive wear resistance increases along with hardness and brittleness. For example, in the tungsten carbide face of a G-roll, the proportion of the carbide areas is appropriately 20...30 vol-%. Chemically the roll coating is a mixture of tungsten, chromium and carbon or a mixture of tungsten carbide, tungsten, cobalt, chromium and carbon. The percentages of the constituents in the composition are in the matrix area of an order of W 70...88 wt-%, preferably 81 wt-%, Co 7...13 wt-%, preferably 10 wt-%, Cr 4...10 wt-%, preferably 4 wt-%, C 4...8 wt-%, preferably 4...5 wt-%. In the carbide-rich area the percentages of the constituents in the composition are of an order of WC 70...95 wt-%, preferably about 85 (e.g. 86 %), Co 7...13 wt-%, preferably 10 %, Cr 4...8 wt-%, preferably about 5 % (e.g. 4 %). The composition of the tungsten carbides consists of different carbides, whose percentages are of an order of WC 30...100 %, preferably 50 %, WC 1-x 0...50 %, preferably 40 %, W₂C 0...30 %, preferably 10 %. In the matrix area the composition corresponds to the average composition, but part of the carbon is bound in sub-microscopic carbides and part of it is dissolved in the metal matrix. The roughness of the surface is regulated by means of the porosity of the face, size of the carbides, and by means of the degree of finish-grinding. The porosity is affected by means of the coating parameters. With increased porosity the surface roughness is also increased. The size of the carbides is mainly determined by the size of the carbides in the powder of which the coating is formed. By means of the coating parameters, the carbide size can be reduced only little. With an increasing carbide size, the surface roughness is also increased. The surface roughness is affected by means of the finish-grinding by grinding-off the peaks of carbides present in the surface, and thereby the surface roughness is also reduced at the same time, and at the same time any macroscopic unevennesses are ground off. The roughness of the surface that is aimed at depends on the purpose of use; when increased friction and detaching properties are desired, a higher surface roughness is chosen, e.g. R a 1...2 µm, and when it is desirable to emphasize low friction and low abrading quality, the surface roughness is, e.g., R a 0.2...1 µm. The hardness of the coating is higher than 1000 HV 300, and its internal strength is higher than 100 N/mm². The coating of the roll in accordance with the invention is made of, a powder, whose properties will be dealt with in detail in the following. The powder consists of a carbide-rich powder and of a matrix powder. The particle size in the powder is 5...50 µm, and the distribution depends on the method and is, for example: < 45 µm 99 %, < 20 µm 20 %, and < 15 µm 7 %. The shorter the time of subjecting to heat or the lower the coating temperature, the smaller is the particle size. The powder must, however, be sufficiently large-sized in order that it could be fed in the coating device. Moreover, the surface area of the particles has influence; the larger the area, the more efficient is the heating. The size of the carbides present in the powder particles varies in accordance with the purpose of use, being 1...10 µm, depending on the requirement of roughness. A carbide size of 1 µm corresponds to a surface roughness of about 0.5 µm R a , and a carbide size of 3 µm corresponds to a surface roughness of about 1.5 µm R a . The matrix powder has been made of a melt by means of rapid solidification (rapid solidification technique) so that the carbon remains in the structure as sub-microscopic carbides and/or as dissolved in the metal matrix. The shape of the carbide particles in the coating and in the powder is round. The chemical composition of the powder corresponds to the composition of the coating. The matrix may be crystalline, amorphous, or a mixture of the structures of a crystalline and an amorphous substance. The powder (matrix powder) is manufactured, e.g., by means of the technique of milling-sintering and crushing. The coating is carried out by means of methods known in prior art, e.g. band welding, by thermal spraying, casting, by winding a sintered and/or rolled band around the cylinder, or the roll coating is produced by using a flexible mat, which is fixed to the roll frame by melting by using induction, laser, or any other suitable heating. These methods have been described in detail in the applicant's GB-A-2 169 381 and in the FI Pat. Appl. 853544. When the coating is produced by spraying, it is carried out by spraying of two powder components, which are a) carbide and binder metal and b) matrix metal. In respect of its properties of moistening and surface tension, the coating is close to the properties of granite, which has been used traditionally in the roll concerned. The long-term operating quality of the carbide coating is ensured by the high wear resistance produced by the high hardness (1100 HV) and by the good resistance to corrosion based on the alloying. Both of these factors retain the microporosity and surface roughness, which are essential in view of the detaching of the web. The original level of microporosity can be regulated by means of the parameters of the coating process. The carbide coating will also withstand the alterations predictable in future paper machine conditions, which are corrosion strain and higher temperatures, filler contents, and increasing linear loads. The coating will also endure situations of thermal shock. Moreover, the invention will be illustrated by means of the following examples and the figure. On the basis of trial runs carried out in the applicant's test paper machine, with the carbide coating small detaching angles and low detaching forces were ascertained, compared with other test faces. The figure illustrates the web detaching angles as a function of the difference in speed between the web and the roll at a paper machine running speed of 18 m/s. Examples In the test machine, trial runs were carried out with the following rolls: 1. A Old rock roll 2. B Metal roll 3. C Conventional polymer roll 4. D Roll in accordance with the invention at a paper machine running speed of 18 m/s. In the figure, the results are recorded as web detaching angles as a function of the difference in speed between the web and the roll. From the figure it is seen that the carbide coating D has the best detaching properties, because with a certain tensile force the web detaching point is early (small detaching angle). In connection with the trial runs, the relative detaching work was also measured. The results can be seen in Table 1. The detaching work with the carbide coating D is only about 40 % of the detaching work of the reference roll A. In the following, the patent claims will be given, whereat the details of the invention may show variation within the scope of the inventive idea defined by said claims.

Claims: A roll used in the manufacture of paper, for example a centre roll in the press or a calender roll, with which the web is in direct contact and from which the web is detached, a coating having been formed onto the cylinder mantle of said roll, which said coating is partly made of metal and which also includes a ceramic material, characterized in that the outer face of the coating consists of carbide-rich areas and of matrix areas placed between said areas, both of which areas consist of metal with a proportion of carbides, the amount and size of which carbides are different in the areas mentioned. Roll as claimed in claim 1, characterized in that the diameter of the carbide-rich areas is 10...50 µm and the diameter of the matrix areas is 10...50 µm. Roll as claimed in claim 1 or 2, characterized in that in the carbide areas the size of the carbides is 0.5...10 µm and the proportion of carbides in said areas is 50...95 %. Roll as claimed in claim 1 or 2, characterized in that in the matrix areas the size of the carbides is 0.001...0.01 µm and the proportion of carbides in said areas is within the range of 5...50 %. Roll as claimed in claim 5, characterized in that in the coating some of the carbides are unseparated and the carbon reinforces the matrix by means of a mechanism of solution reinforcement. Roll as claimed in any of the claims 1 to 5, characterized in that the coating is chemically a mixture of tungsten carbides, tungsten, cobalt, chromium, and carbon or a mixture of tungsten, cobalt, chromium, and carbon. Roll as claimed in any of the claims 1 to 6, characterized in that the proportions of the constituents in the coating area in the matrix area of an order of W 70...88 wt-%, preferably 80 %, Co 7...13 wt-%, preferably 10 %, Cr 4...10 wt-%, preferably 4...5 %, and C 4...8 wt-%, preferably 5 %. Roll as claimed in any of the claims 1 to 7, characterized in that in the carbide-rich area the composition is substantially WC 70...95 %, preferably 85 %, Co 7...13 wt-%, preferably 10 %, Cr 4...10 wt-%, preferably 4...5 %. Roll as claimed in claim 8, characterized in that the proportions of the different carbides in the tungsten carbides are WC 30...100 %, preferably 50 %, WC 1-x 0...50 %, preferably 40 %, W₂C 0...30 %, preferably 10 %. Roll as claimed in claim 8, characterized in that in the matrix area part of the carbon is bound in sub-microscopic carbides and part of it is dissolved in the metal matrix. Roll as claimed in any of the claims 1 to 10, characterized in that the hardness of the coating is > 1000 HV 300 and the internal strength > 100 N/mm². Method for manufacturing a roll for directly contacting a web and from which the web is detached, the coating on said roll being partly of metal and also including a ceramic material, characterized by preparing a carbide-rich powder and a matrix powder including carbides with different compositions with respect to their metal content and size and amount of carbides and applying a coating of said powders onto a mantle of a roll such that an outer face of said coating is formed of carbide-rich areas composed of metal and ceramic and of metallic matrix areas of metal and ceramic formed between said carbide-rich areas , the composition of the matrix areas differing from the composition of the carbide-rich areas. Method as claimed in claim 12, characterized in that the particle size of the powder is 5...50 µm. Method as claimed in claim 13, characterized in that the distribution of the particle size depends on the method and is, e.g., < 45 µm 99 %, < 20 µm 20 %, and < 15 µm 7 %. Method as claimed in any of the claims 12 to 14, characterized in that the matrix powder is manufactured out of a melt by means of rapid solidification so that the carbon is in the structure as sub-microscopic carbides and/or as dissolved in the metal matrix. Method as claimed in any of the claims 12 to 15, characterized in that the shape of the carbide particles is round. Method as claimed in any of the claims 12 to 16, characterized in that the matrix powder is manufactured by means of the milling-sintering and crushing technique. Method as claimed in any of the claims 12 to 17, characterized in that the coating is manufactured by means of band welding, thermal spraying, casting, or by winding a sintered and/or rolled band around the cylinder. Method as claimed in any of the claims 12 to 17, characterized in that the roll coating is produced by using a flexible mat that is fixed to the roll frame by melting by using induction, laser, or any other suitable heating. Method as claimed in any of the claims 12 to 19, characterized in that the roughness of the surface is reglulated by means of coating porosity and/or carbide size and/or finish-grinding. Method as claimed in any of the claims 12 to 20, characterized in that the porosity of the roll coating is affected by means of the coating parameters. Method as claimed in any of the claims 12 to 21, characterized in that the surface roughness is chosen in accordance with the purpose of use by means of porosity, carbide size, and the degree of finish-grinding.

Inventors: LEINO JORMA, MIIHKINEN VEIJO, SALO JUKKA, TELAMA ARI, LEINO, JORMA, MIIHKINEN, VEIJO, SALO, JUKKA, TELAMA, ARI
Applicants: VALMET PAPER MACHINERY INC, VALMET PAPER MACHINERY INC.
Application year: 1989
Main classification: D21F3
IPC codes: D21G1, D21F3, C23C4