Patent Description:
A machine for making tissue paper typically includes a Yankee drying cylinder which, during operation, is heated internally by hot steam. In the following, the term "Yankee" will be used for Yankee drying cylinders. A wet fibrous web that is transferred to the Yankee is dried on the outer surface, the "drying surface" of the Yankee, and subsequently creped from the drying surface (the outer surface) of the Yankee by a creping doctor. During the twentieth century, Yankees were usually made of cast iron but in recent years, Yankees have been made in welded steel. A steel Yankee weighs less than a cast iron Yankee having the same required strength and steel Yankees are thus seen as advantageous. An example of a steel Yankee is disclosed in <CIT>. As mentioned above, a creping doctor is used to crepe the ready-dried fibrous web from the surface of the Yankee. In practice, this means that the surface of the Yankee will be subjected to pressure from the doctor blade. A cast Yankee has a surface with a relatively high hardness since cast iron may typically have an HB hardness (i.e. Brinell hardness) of <NUM> - <NUM>. For this reason, the surface is not so easily worn out by contact with the doctor blade that is pressed against its surface. However, experience has showed that even Yankees made of cast iron are subjected to a considerable degree of wear. For this reason, it has long been a practice to metallize the surface of Yankees in order to obtain a harder surface. In the context of this patent application, the terms "to metallize" and "metallization" refer to a process in which the outer surface (the drying surface) of the Yankee is coated with a hard layer which may be based on an element or an alloy or a mixture of metal powder and at least one carbide or nitride or possibly other elements, or a metal matrix containing at least one carbide or nitride or possibly other elements. The coating may typically be applied to the surface of the Yankee by spraying. An example of this technology is disclosed in <CIT>. Other examples are disclosed in <CIT>; <CIT> and <CIT>. It should be noted that many different compositions for the hard layer have been proposed. For example, the hard layer may be an iron alloy containing from about <NUM> to about <NUM> weight per cent chromium, about <NUM> to about <NUM> weight per cent boron, about <NUM> to about <NUM> weight per cent silicon, and less than about <NUM> weight per cent molybdenum as suggested in the above-mentioned <CIT>. In the context of Yankees, it should be noted that the term "coating" may also be used to refer to liquid coating that is continuously sprayed onto the surface of the Yankee during operation which is fundamentally different from the hard coating that is applied as a part of the manufacturing process. When discussing application of a hard layer in order to increase hardness and resistance to wear, a technical term often used by persons skilled in the art is "metallization" and this term and "to metallize" (except when used in reference to other documents that are cited herein and that use the term in a different meaning) will be used in this patent application and all patents that may be granted based on it to refer to the application of a hard and wear-resistant layer permanently joined to the steel drying surface of the Yankee, regardless of the exact composition of that layer. Such a layer will be called "metallization layer" in the following. The term "coating" as used henceforth in this patent application will refer to the application of a temporary liquid coating during operation of a tissue paper making machine (except when used in reference to other documents that are cited herein and that use the term in a different meaning).

Steel used for making steel Yankees has a hardness which is markedly lower than that of cast iron. A typical value for the hardness of such steel may be on the order of about <NUM> HB or, possibly, in the range of <NUM> - <NUM> HB.

Since steel grades used for Yankee drying cylinders typically have a hardness (on the order of <NUM> HB) that is substantially lower than that of cast iron (<NUM> - <NUM> HB), it has been a requirement that Yankees made of steel be metallized. Different metallization layers for steel Yankees have been proposed. In a paper from <NUM>, Mr. Jörg Bauböck discussed a steel Yankee with a "metallic coating" with a surface hardness which was said to be double that of cast iron ("<NPL>) although no details were given about the exact composition. In a <NUM> paper by <NPL>), Mr. Mignani suggested that a steel Yankee dryer can be provided with a "metal coating" (i.e. metallization layer) and that the "metal coating" may be for example an alloy with a high content of Cr and Ni and have a hardness of <NUM> HRC. Although hardness measured according to HRC does not correspond directly to Brinell hardness, the value of <NUM> HRC can be said to correspond to about <NUM> HB. In the same paper, Mr. Mignani also suggests that INFINIKOTE® can be used. Infinikote® is a trade mark owned by Valmet Corporation and refers to metallization of Yankees with a thermal sprayed metal coating. The paper by Mr. Mignani gives the hardness value in this case as "<NUM> to <NUM> HRC", i.e. on the order of about <NUM> - <NUM> HB.

As an alternative to metallization, it has been suggested that the shell of a steel Yankee be hardened by laser and such a solution is disclosed in <CIT>. According to that document, it is possible to obtain a hardness value of up to <NUM> HB.

However, both currently known technologies for metallization and possible hardening by laser represent a further step in the manufacturing process of a Yankee which makes the manufacturing process more complicated.

With regard to metallized steel Yankees, the inventors of the present invention have also noted that streaks can occur in the paper that has been dried on such dryers.

The inventors have also seen that the metallization layer of a Yankee may crack and/or delaminate which makes it necessary to halt production and shut off the machine. When this occurs, production is seriously disturbed and the costs for this may be very high.

Therefore, it is an object of the present invention to provide a steel Yankee that is properly protected against surface wear and which can yet be manufactured in a simple way and to provide a machine having such a steel Yankee. A further object of the invention is to provide a steel Yankee that prevents or reduces the occurrence of streaks in the paper that is dried on the machine.

Patent application <CIT> discloses a steel and its manufacturing methiod in the field of metallurgical technology and relates to a low-alloy high-strength, high toughness steel plate. The steel is aimed to have improved hardness and wear resistance.

<CIT> describes a Yankee paper making machine. It does not detail the chemical composition of the Yankee cylinder, not its surface.

The invention relates to a steel Yankee drying cylinder comprising a cylindrical shell having two axial ends. An end wall is joined to each axial end by means of a circumferential weld bead. The cylindrical shell further has an outer surface and an inner surface in which inner surface circumferential grooves are formed. According to the invention, the cylindrical shell is made of the steel grade P690 and the outer surface of the shell is formed by such steel i.e. steel of the grade P690.

The invention also relates to a tissue paper making machine comprising the inventive steel Yankee drying cylinder and a creping doctor with a creping blade which is arranged to act against the outer surface of the Yankee drying cylinder.

In embodiments of the invention, the tissue paper making machine further comprises a device for applying liquid coating onto the outer surface of the Yankee drying cylinder.

The paper making machine may optionally also comprise a Yankee drying hood that is arranged to be capable of blowing hot air against the outer surface of the cylindrical shell over a part of the circumference of the shell.

In embodiments of the invention, a cleaning doctor may be arranged to act against the outer surface of the shell at a location which, in the direction of rotation of the Yankee, is located after the creping doctor but before the coating device.

The machine may additionally comprise at least one through-air drying cylinder. In such cases, the imprinting fabric will be an air permeable fabric which is arranged to carry a fibrous web over a part of the circumference of the at least one through-air drying cylinder and to carry the fibrous web from the at least one through-air drying cylinder to the transfer nip formed between the press roll and the Yankee.

With reference to <FIG>, the Yankee drying cylinder <NUM> of the present invention is a steel Yankee drying cylinder that comprises a cylindrical shell <NUM> having two axial ends <NUM>, <NUM>. It should be understood that the cylindrical shell <NUM> is a circular cylindrical shell. An end wall <NUM>, <NUM> is joined to each axial end <NUM>, <NUM> by means of a circumferential weld bead <NUM>. The shell can be joined to the end walls in, for example, the way disclosed in <CIT> or in <CIT>. The end walls may optionally be provided with thermal insulation, for example as disclosed in <CIT> or in <CIT>. As can be seen in <FIG>, the cylindrical shell <NUM> has an outer surface <NUM> and an inner surface <NUM>. With further reference to <FIG>, preferably circumferential grooves <NUM> are formed in the inner surface <NUM> of the cylindrical shell <NUM>. In the groves <NUM>, condensate water will be accumulated during operation and the Yankee drying cylinder <NUM> is preferably provided with means for evacuating condensate water from the grooves <NUM>, for example as disclosed in <CIT> or in <CIT>.

With further reference to <FIG>, the inventive Yankee drying cylinder <NUM> is used in a tissue paper making machine <NUM>. When the Yankee drying cylinder is used in the machine <NUM>, a creping doctor <NUM> is used. The creping doctor <NUM> has a creping blade <NUM> that is arranged to act against the outer surface <NUM> of the shell <NUM> of the Yankee drying cylinder <NUM>. In <FIG>, the Yankee drying cylinder is represented as rotating about its axis of rotation A in the direction of arrow R, i.e. "clockwise" in <FIG>. As can be understood from <FIG>, the shell has a circular cylindrical shape. The Yankee drying cylinder is so arranged in the tissue paper making machine that it is rotatable about the axis of rotation A.

As can be seen in <FIG>, a fibrous web W is formed in a forming section <NUM> between a forming fabric <NUM> and a fabric <NUM> which may be a water-absorbing felt. The fibrous web W is formed from stock injected from a head box <NUM>. The newly formed fibrous web W is carried on the felt <NUM> to a nip N between a press roll <NUM> and the Yankee drying cylinder <NUM>. The press roll <NUM> that is used in the nip against the Yankee may conceivably be, for example, a shoe roll with a design as disclosed in <CIT>; <CIT> or <CIT> but other rolls than a shoe roll may also be considered. The wet fibrous web W may be subjected to a degree of dewatering in the nip N which then serves as a dewatering press nip. The nip N could also be essentially just a transfer nip in which no substantial dewatering occurs. The nip N may optionally be preceded by a suction turning roll <NUM>. The fibrous web W is transferred to the outer surface <NUM> of the cylindrical shell <NUM> of the Yankee drying cylinder <NUM> and the fibrous web W is then dried by heat that comes from hot steam that is supplied into the Yankee drying cylinder <NUM>. The dried fibrous web W is then creped off from the Yankee drying cylinder by the doctor <NUM> and sent to a reel-up <NUM>.

During operation, the tissue paper making machine may be running at a speed which is in the range of <NUM>/min - <NUM>/min. However, higher speeds may also be considered and machine speeds in the range of <NUM>/min - <NUM>/min may also be used or possibly even speeds up to <NUM>/min. In all embodiments of the invention, the Yankee may have a diameter in the range of, for example, <NUM> - <NUM>. For example, the diameter may be <NUM>; <NUM>; <NUM> or <NUM>. The width of the Yankee <NUM> may be, for example, in the range of <NUM> - <NUM>. For example, the width could be <NUM> or <NUM>. However, the Yankee could also have other dimensions than the ones given above.

With reference to <FIG>, the doctor <NUM> preferably includes a blade holder <NUM> that holds the creping lade <NUM>. During operation, the creping blade <NUM> will act against the outer surface <NUM> of the shell <NUM> to crepe a dried fibrous web from the outer surface <NUM>.

With reference to <FIG>, the tissue paper making machine may be provided with a Yankee drying hood <NUM> that is arranged to be capable of blowing hot air against the outer surface <NUM> of the cylindrical shell <NUM> over a part of the circumference of the cylindrical shell <NUM>. The Yankee drying hood may be, for example, such a Yankee drying hood as is disclosed in <CIT> but other Yankee drying hoods may also be considered and the inventive machine may also operate without a Yankee drying hood. A further doctor <NUM> is indicated in <FIG>. This further doctor is optional and may be a cleaning doctor for scraping off fiber residue from the Yankee drying cylinder.

The tissue paper making machine preferably also comprises a device <NUM> for applying liquid coating onto the outer surface <NUM> of the Yankee drying cylinder <NUM>. The liquid coating typically comprises polyvinyl alcohol and other chemical agents. Coating that is applied may comprise, for example, <NUM> - <NUM> % by weight polyvinyl alcohol (PVOH), an adhesive that constitutes <NUM> - <NUM> % by weight of the coating, a modifier/release agent that constitutes <NUM> - <NUM> % by weight of the coating and preferably also phosphate. Phosphate used for such coating may be, for example, mono-ammonium phosphate, Diammonium phosphate, trisodium phosphate or tetra phosphate. Instead of the above-mentioned phosphates (or in combination with one or several of them), phosphoric acid may be used in the coating. A supply system for supplying liquid coating is symbolically indicated by the reference numeral <NUM>.

During operation, the creping doctor <NUM> may act against the outer surface <NUM> of the cylindrical shell <NUM> with a linear load which may, in some cases, be as high as <NUM> kN/m. This means that the cylindrical shell <NUM> is subjected to wear. For Yankee drying cylinders made of steel, this can be a very serious problem and it has been the standard procedure to give them a protective layer, a so called "metallization layer" of a hard material that is resistant to coating. However, cracks may occur in such hard layers that can quickly lead to delamination of the entire layer such that operation of the machine must be interrupted. Moreover, the doctor blade can be damaged. When a crack catches the doctor blade, the doctor blade can be deformed, and this may be a source of streaks in the paper. Therefore, elimination of the metallization layer is actually desirable from a process point of view.

The inventor of the present invention has now found that it is possible to entirely avoid the use of a hard metallization layer by using the steel grade P690 or the like for making the cylindrical shell <NUM> such that the outer surface of the shell <NUM> is formed by steel of the grade P690 or the like. In a first embodiment of the invention the cylindrical shell is formed of P690 grade steel or the like. In a second embodiment of the invention the cylindrical shell is formed of inner cylindrical shell made of steel and is covered with a continuous outer layer of P690 grade steel or the like.

In each embodiment, the surface against which the doctor blade <NUM> acts will thus be formed by steel of the steel grade P690. This is possible without laser hardening as suggested in <CIT>. The steel grade P690 has been found to be so resistant to wear that it does not need any metallization layer. Moreover, it has such properties with regard to welding, strength and heat transfer that are required for a Yankee drying cylinder.

Steel grade P690 or the like has the following composition in weight percent:.

Preferably for products made of P690 or the like which have a maximum thickness which is equal to or less than <NUM> thick the upper yield strength (ReH) measured by ISO <NUM>-<NUM>:<NUM> of the P690 is equal to or greater than <NUM> MPa and the corresponding tensile strength measured by ISO <NUM>-<NUM>:<NUM> is preferably from <NUM> to <NUM> MPa.

Preferably for products made of P690 which are greater than <NUM> thick and equal to or less than <NUM> thick the yield strength measured by ISO <NUM>-<NUM>:<NUM> of the P690 is equal to or greater than <NUM> MPa and the corresponding tensile strength measured by ISO <NUM>-<NUM>:<NUM> is preferably from <NUM> to <NUM> MPa.

Preferably for products made of P690 which are greater than <NUM> thick the yield strength measured by ISO <NUM>-<NUM>:<NUM> of the P690 is equal to or greater than <NUM> MPa and the corresponding tensile strength measured by ISO <NUM>-<NUM>:<NUM> is preferably from <NUM> to <NUM> MPa.

Claim 1:
A steel Yankee drying cylinder (<NUM>) comprising a cylindrical shell (<NUM>) having two axial ends (<NUM>, <NUM>), an end wall (<NUM>, <NUM>) being joined to each axial end (<NUM>, <NUM>) by means of a circumferential weld bead (<NUM>), the cylindrical shell (<NUM>) further having an outer surface (<NUM>) and an inner surface (<NUM>), characterized in that the outer surface (<NUM>) of the cylindrical shell (<NUM>) is formed by steel of the grade P690 as defined in European Standard EN <NUM>-<NUM>:<NUM> or a similar steel which has the following composition by weight:
C equal to or greater than <NUM> % and equal to or less than <NUM> %
Si equal to or greater than <NUM> % and equal to or less than <NUM> %
Mn equal to or greater than <NUM> % and equal to or less than <NUM> %
Cr equal to or greater than <NUM> % and equal to or less than <NUM> %
Mo equal to or greater than <NUM> % and equal to or less than <NUM> %
V equal to or greater than <NUM> % and equal to or less than <NUM> %
Nb equal to or greater than <NUM> % and equal to or less than <NUM> %
B equal to or greater than <NUM> % and equal to or less than <NUM> %
P equal to or less than <NUM> %
S equal to or less than <NUM> %
N equal to or less than <NUM>%
Cu equal to or less than <NUM> %
Ni equal to or less than <NUM> %
Ti equal to or less than <NUM> %
Zr equal to or less than <NUM> %
Al equal to or less than <NUM>%
Remainder iron and impurities.