FIELD OF THE INVENTION

The present invention relates to a method for producing a Yankee drying cylinder from steel and a Yankee cylinder made of steel.

BACKGROUND INFORMATION

In a paper making machine for making paper, a newly formed fibrous web which is still wet is dried on a Yankee drying cylinder. The Yankee drying cylinder is a large cylinder, typically having a width of 2-6 m, but larger cylinders wider than 6 m are common. When producing tissue paper Yankee cylinders having a width of 12 m or more may be conceivable but Yankee cylinders having a smaller width than 2 m may also be conceivable, e.g. in pilot machines. However, the diameter of a Yankee cylinder is commonly within the interval of 2 - 6,5 m.

The cylindrical shell of a Yankee cylinder commonly consists of cast iron. The interior of the shell is typically filled with hot steam which may have a temperature of up to l80°C or even more. The hot steam heats the Yankee drying cylinder such that the external surface of the Yankee cylinder reaches a temperature suitable for effective evaporation of water contained in a wet fibrous web, such as a tissue paper web, in contact with the external surface of the Yankee cylinder. The steam is normally pressurized to such an extent that the Yankee cylinder is subjected to substantial mechanical stress due to the internal pressure. The overpressure inside the Yankee cylinder during operation may be about 1 MPa (10 bar). The weight of the Yankee cylinder as well as centrifugal forces may also contribute to the mechanical stress. Hence, the thickness of the shell must have a certain minimum level so as to being able to withstand said stresses. It is known that a Yankee cylinder can also be made of welded steel. WO 2008/105005 discloses a Yankee cylinder for drying paper which is made of steel and has a cylindrical shell joined to two ends through a respective circumferential end weld seam made between opposing surfaces of each end and the cylindrical shell. A Yankee cylinder made from steel has the advantage of being lighter in weight while still being able to withstand substantial mechanical stress. In WO2012/126603 a method is disclosed for producing a Yankee cylinder from steel, said Yankee cylinder consisting of individual pre-fabricated or finished cylinder sections that are welded together.

However, paper products dried on Yankee cylinders made of welded steel has shown to have somewhat inferior quality properties as compared to the quality properties of paper dried on conventional Yankee drying cylinders made from cast iron.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome or at least minimize at least one of the drawbacks and disadvantages of the above described manufacturing methods. This can be obtained by a method of manufacturing a Yankee cylinder from steel as defined in claim 1.

Thanks to performing a final machining step of an outer cylindrical surface of the Yankee cylinder until a variation in a wall thickness in a circumferential direction of the cylindrical wall of said Yankee drying cylinder is 2,5 mm or less, the effect of a wall thickness variation introduced by welding together different steel parts may be significantly reduced. Furthermore, said surface becomes more uniform, smoother and flusher in the areas where the welding has been performed. Areas surrounding the axial and radial weld seams as well as the weld seams themselves will be very even and smooth if they are machined after a welding operation has been performed. Welding may cause certain unevenness in the welding areas and this unevenness will then remain on the surfaces of welded shell sections and/or cylinder sections if performed only after a machining step, and said unevenness may reduce the quality of the product to be dried on the Yankee cylinder. The inferior paper qualities are probably caused by an uneven heat distribution in Yankee cylinders made of steel causing an uneven heat flow from an inside to an outside of the Yankee cylinder which results in an uneven drying temperature profile of the wet fibrous web to be dried on the outside of the Yankee cylinder shell. An uneven drying temperature profile in the machine direction leads to variations in strength properties of the dried fibrous web which indeed is unwanted. It is the believe that the uneven heat distribution is caused, at least in part, by a too large variation in wall thickness in the circumferential direction of the cylindrical wall of said Yankee drying cylinder. Reducing the wall thickness variation leads to a more even heat distribution and a more even drying temperature profile and as a result thereof, improved quality properties of the dried paper product.

Another advantage of the aspects as defined in the independent claims is that it is preferred to transport the Yankee cylinder to the final destination where the Yankee cylinder is to be used, in its finished and complete state or at least to transport the complete cylindrical shell to the final destination. The reason to this is that the Yankee cylinder as well as the complete cylindrical shell are more robust and that they hence better withstand unwanted situations, such as sudden braking of the transportation vehicle or traveling on bumpy roads causing the goods to move inside the transportation vehicle, that may arise during transportation, situations which could most probably affect the conditions of the different single, separate cylinder sections but not of Yankee cylinders and complete cylindrical shells which are much heavier and hence not that easily moved. Furthermore, it is easier to securely fasten one large and heavy piece inside a transportation vehicle than to securely fasten several pieces. When transporting several pieces, the pieces could collide with themselves and cause damage to themselves.

Thanks to the present invention a Yankee cylinder may be manufactured having an even heat flow from an inside to an outside of the Yankee cylinder which result in an even drying temperature profile of the fibrous web to be dried on the outside of the Yankee cylinder shell. An even drying temperature profile in the machine direction minimizes variations in strength properties of the dried fibrous web, i.e. the fiber product, which indeed is desired. In addition, since it is more advantageous to transport an already manufactured Yankee drying cylinder due to the risk of damaging the Yankee drying cylinder during transport as compared to transporting several, separate pieces, an undamaged Yankee drying cylinder also contributes to a more even drying temperature profile of the fibrous web to be dried on the outside of the Yankee cylinder shell, and, hence, to minimized variations in strength properties of the dried fibrous web.

According to one aspect of the invention cutting, the step of heat-treatment of the cylindrical shell and / or heat- treatment of the Yankee cylinder is performed. Thanks to performing a heat treatment step, tensions introduced by welding operations performed may be released or at least reduced.

According to another aspect of the invention, at least two shell sections to form at least two cylinder sections are welded together. This means that smaller sections may be utilized and handled as compared to starting from only one single shell section.

According to a further aspect of the invention, at least two cylinder sections are welded together to form the cylindrical shell of said Yankee drying cylinder. This means that smaller sections may be utilized and handled.

According to still another aspect of the invention, a said final machining step is followed by a finishing step comprising a crowning step of said outside cylindrical surface of the Yankee cylinder to a convex shape of said surface, and preferably followed by a metallization step of the outside cylindrical surface. This gives the Yankee cylinder a robust surface having a long expected lifetime.

According to another aspect of the invention, the variation in wall thickness in the circumferential direction of the outer cylindrical surface of said Yankee drying cylinder is equal to or less than 2 mm, preferably equal to or less than 1,5 mm, and more preferred equal to or less than 0,5mm. Thanks to this aspect, a very low variation in heat transfer is achieved and an even drying temperature profile in the machine direction resulting in almost no or at least minimal variations in strength properties of the dried fibrous web.

According to yet another aspect of the invention, an inside and an outside of said at least two cylinder sections are simultaneously welded to form a complete cylindrical shell of said Yankee cylinder. Thanks to this aspect tensions introduced during the welding may be reduced. Further, a greater integrity of the welding is achieved as well as greater safety for the welding operator/-s. According to an aspect of the invention, an inner surface of said cylindrical shell is machined, preferably by lathing, so as to form circumferential grooves on a part of said inner surface of said cylindrical shell. Thanks to his aspect, heat transfer from the steam to the cylinder wall may be enhanced. According to another aspect of the invention, a run-out value is measured, comparing the measured run-out value with a target run-out value and continuing the finishing step until the target run-out value has been reached, which target run-out value is equal to or less than 1,0 mm. Thanks to this aspect, it is ensured that the cylindrical surface of the Yankee drying cylinder is even and smooth, with wall thickness variation, meaning that a very low variation in heat transfer is achieved as well as an even drying temperature profile in the machine direction resulting in almost no or at least minimal variations in strength properties of the dried fibrous web.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying figures, wherein: Fig. 1 is a side view over a schematic representation of Yankee cylinder in operation,

Fig. 2 shows a longitudinal section of a Yankee drying cylinder,

Fig. 3 shows an enlargement of a portion of a Yankee cylinder where the

cylindrical shell of the Yankee cylinder has been welded to an end wall, Fig. 4 shows a cylindrical shell of the Yankee cylinder and according to a p referred embodiment of the invention, and

Fig. 5 shows a Yankee cylinder manufactured in accordance with the

embodiment shown in Fig. 4. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following detailed description, and the examples contained therein, are provided for the purpose of describing and illustrating certain embodiments of the invention only and are not intended to limit the scope of the invention in any way.

With reference to Figure 1, a Yankee drying cylinder 1 is shown in operation. The Yankee drying cylinder 1 in Figure 1 gives an example of how such a Yankee drying cylinder may be used in practice. As can be seen in Figure 1 , a fibrous web W is carried by a fabric 50 to a nip between a roll 51 and the Yankee drying cylinder 1. The fabric 50 may be a felt which is permeable to air and water and capable of receiving water. The roll 51 may be, for example, a suction roll or a shoe press roll but it may also be some other kind of roll such as a deflection-compensated roll. In the nip between the roll 51 and the Yankee drying cylinder 1, the fibrous web W is transferred to an external surface 30 (also termed outer cylindrical surface) of the Yankee drying cylinder 1. The Yankee drying cylinder 1 has a smooth outer cylindrical surface 30. In the nip between the roll 51 and the Yankee drying cylinder 1, the web W will be transferred from the fabric 50 to the surface 30 of the Yankee drying cylinder 1. The fibrous web W has a strong tendency to follow a smooth surface rather than a rough surface and the smoothness of the outer cylindrical surface 30 of the Yankee drying cylinder 1 results in a very strong tendency of the fibrous web W to follow the outer cylindrical surface 30 of the Yankee drying cylinder. The outer cylindrical surface 30 of the Yankee drying cylinder 1 is normally made smooth by special surface treatment such as, for example, grinding while the fabric 50 normally has a surface that is rough, at least compared to that of the outer cylindrical surface 30 of the Yankee drying cylinder 1. The fibrous web W follows the surface of the Yankee drying cylinder 1 as the Yankee drying cylinder rotates in the direction of arrow R. The Yankee drying cylinder 1 is heated and normally heated from within by a heating medium such as hot steam. A source of hot steam is symbolically indicated by the reference numeral 16. As the fibrous web W is in contact with the outer cylindrical surface 30 of the Yankee drying cylinder 1, the hot surface 30 of the Yankee drying cylinder 1 will cause water in the fibrous web W to evaporate such that the fibrous web W is dried. The fibrous web W may then be removed from the surface 30 of the Yankee drying cylinder 1. For example, it may be creped away from the outer cylindrical surface 30 by means of a doctor blade 53 as schematically indicated in Figure 1. It should be understood that the Yankee drying cylinder 1 is journalled for rotation and the reference numeral 92 indicates a journal for the Yankee drying cylinder 1. The Yankee drying cylinder of the present invention is intended for drying wet fibrous webs and may be used in the way explained with reference to Figure 1 and the Yankee drying cylinder of Figure 1 may be understood as a representation of the inventive Yankee drying cylinder. The Yankee drying cylinder may be followed by other equipment such as a reel-up.

With reference to Figure 2, the inventive Yankee cylinder 1 manufactured by the innovative manufacturing method comprises a cylindrical shell 2. The cylindrical shell 2 has axial ends 3, 4. A cylindrical wall 31 of the Yankee cylinder 1 is shown. An end wall 5, 6 is connected to each axial end 3, 4 by means of a circumferential end weld seam 7. The end walls 5, 6 are preferably also made of steel and may be made of the same steel material as the cylindrical shell 2. The cylindrical shell 2 and said end walls 5, 6 define an enclosed space.

In Fig. 2, it is shown that the Yankee cylinder 1 has journals 10, 11. During operation, when the wet fibrous web W is running on the external surface of the Yankee cylinder, the interior of the Yankee cylinder 1 is filled with hot steam. The hot steam can be supplied, for example, through the journals 10, 11.

Inside the cylindrical shell 2, there may be an internal tie 12 having a tubular form and which is provided with holes 13, for the passage of ducts of a condensate removal system (not shown). For an example of a condensate removal system, reference is made to WO 2012/033442 Al. Said internal tie 12 assists in joining the two end walls 5, 6 with the cylindrical shell 2. Preferably, the internal tie 12 is further provided with holes of a smaller diameter (not shown) for distribution and circulation of the hot steam. The tie may also be provided with suitable human passages for access and maintenance within the Yankee cylinder. In Fig. 2 it is shown that said tie 12 extends between the end walls 5, 6 and is joined to the end walls 5, 6 by screws. Flowever, other joining methods may be used, e.g. by welding and one example of welding method may be as described in SE 1650417-7. Other welding methods may also be conceivable.

Preferably, a longitudinal axis of the internal tie 12 coincides with a longitudinal axis A of said cylindrical shell 2.

In the Yankee cylinder the cylindrical shell 2 has an inner surface 8. Said inner surface is preferably but not necessarily provided with circumferential grooves 9 formed in the inner surface 8 of the cylindrical shell 2 and running in a radial direction of said inner surface, see Fig. 3. In the circumferential grooves 9 hot steam is condensed and heat energy is transferred to the outer surface of the Yankee cylinder 1 such that water in a fibrous web W, which web W is caused to ran over the surface of the cylindrical shell 2, is evaporated. The circumferential grooves thus serve to facilitate heat transfer such that a fibrous web W which is passed over the Yankee cylinder is dried by evaporation. However, in some embodiments it may be preferred that the inner surface is a smooth inner surface not provided with said grooves. In Fig. 3, the end wall 5 is shown and said end wall 5 fixed by an end weld seam 7 to the axial end 3 of the cylindrical shell. Said end weld seam 7 is located in a circumferential joint spacing 77 arranged to provide a destination for said weld seam 7. Furthermore, a wall thickness T of the cylindrical shell 2 is shown at the part of the cylindrical shell 2 where the wall thickness T is constant in a longitudinal direction of said shell 2 (i.e. the longitudinal axis of the shell). The cylindrical shell 2 corresponds to the cylindrical wall 31 of the Yankee cylinder. In the embodiment shown in Fig. 3 the grooves 9 are arranged in the inner surface 8 only in the part of the inner surface 8 where the wall thickness T is constant in the longitudinal direction. As can be seen in Fig. 3 the wall thickness decreases from a point 14 located somewhere between the axial end 3, 4 of the cylindrical shell 2 and the outermost groove 9a.

In Figure 4, a cylindrical shell 2 according to a preferred embodiment of the invention is shown. As seen in Figure 4 the cylindrical shell 2 comprises cylinder sections 18. In the embodiment shown there are three cylinder sections 18A, 18B, 18C. The cylindrical shell 2 could however comprise only two cylinder sections or more than three cylinder sections, e.g. four or five cylinder sections or even more than five cylinder sections depending e.g. on the predetermined final width of the Yankee cylinder to be produced. Said cylinder sections 18A, 18B, 18C may preferably be made from shell sections 17 which shell sections 17 are jointed together preferably by welding of weld seams 20 to form single cylinder sections 18A, 18B, 18C. The three single cylinder sections 18A, 18B, 18C are then jointed, preferably by welding of radial circumferential weld seams

19 along their respective circular end surfaces to form the cylindrical shell 2.

Said shell sections 17 are made of steel. The steel used could be any kind of steel, for example carbon steel or stainless steel. The steel used may be, for example, rolled steel. For example, it may be steel that has been hot rolled and/or cold rolled. The shell section 17 is suitably a sheet of metal. Said sheet has been processed, e.g. by bending, to adapt a curved shape. One or more shell sections 17 is/are fixedly attached/joined to itself/each other at the abutting edges by welding thereby forming the cylinder sections 18A, 18B, 18C. As seen in Figure 4 several shell sections 17 are welded together at their abutting edges by two kinds of weld seams; axial weld seams 20 meaning that said axial weld seams 20 are extending in an axial direction of said cylindrical shell 2 as viewed when the shell sections 17 are in a mounted position, and which weld seams 20 joint sides 22 of the shell sections 17 - henceforth these sides 22 are termed axial sides 22 due to their extension in an axial direction of the cylindrical shell 2 when being in a mounted position of the cylindrical shell 2 - and circumferential, radial weld seams 19, which are extending in a radial direction of said cylindrical shell 2 and jointing together sides 23 of the shell sections 17 - henceforth said sides 23 are termed circumferential sides 23 due to their radial extension when being in a mounted position of the cylindrical shell 2. Depending on the length of the sides 22, 23 of the shell sections 17, one or several shell sections is/are welded by the axial weld seam 20 to form a cylinder section 18A, 18B, 18C. If, for instance, the length of the circumferential sides 23 of the shell section 17, equals the circumferential length of the cylindrical shell 2, then only one shell section 17 is needed to be welded to form the cylinder section 18 A. The two adjacent axial sides 22 of the shell section 17 are then welded together in the axial direction of the cylindrical shell 2 by an axial weld seam 20. If, however, the length of circumferential sides 23 of the shell section 17, that are to be placed in the radial direction of the cylindrical shell 2, equals one third of the circumferential length of the cylindrical shell 2, then three shell sections 17 are needed to be welded together at axial sides 22 in the axial direction of the cylindrical shell 2 respectively to form the cylinder section 18B. If instead the length of circumferential sides 23 of the shell section 17, that are to be placed in the radial direction of the cylindrical shell 2, equals one half the circumferential length of the cylindrical shell 2, then two shell sections 17 are needed to be welded together at axial sides 22 in the axial direction of the cylindrical shell 2 respectively to form the cylinder section 18C. It is also conceivable to weld together adjacent axial sides 22 of two or more shell sections 17 having different lengths of the circumferential sides 23.

It is to be understood that the cylinder sections 18 A, 18B, 18C may have the same or different lengths in the axial direction of the cylindrical shell 2. In Fig. 4 it is shown that the cylinder sections 18B and 18C have substantially the same axial lengths while the cylinder section 18A has an axial length which is longer than said lengths of the cylinder sections 18B and 18C.

In the above described embodiment the shell section 17 is suitably a sheet of metal having edges that are perpendicular and where the jointing is performed in a direction that is parallel with the axis of the cylindrical shell 2 to be produced. In other embodiments it may however be conceivable that the shell section 17 is suitably a sheet of metal having edges that are not perpendicular and where the jointing is performed along a sloped line in relation to the axis of the cylindrical shell 2 on the surface of the cylindrical shell 2. In Figure 5, a cylindrical shell 2 having two end walls 5, 6 at its axial ends 3, 4 is shown. The cylindrical shell 2 is made of several shell sections 17 as shown in Figure 4. The dashed lines indicate the positions of the radial and axial weld seams 19, 20. Said cylindrical shell 2 having two end walls 5, 6 at its axial ends 3, 4 now forms the Yankee cylinder 1, said Yankee cylinder 1 having an outside cylindrical surface 30. In another preferred embodiment (not shown) the cylindrical shell are made of four bent metal plates, i.e. four shell sections, that have been welded together two and two to form two cylinder sections which are welded together to form said cylindrical shell. The inventive production method of the inventive Yankee cylinder 1 is now to be described. It is to be understood that the production method is the same irrespective of the starting number of metal bands or metal plates and the number of cylinder sections to be jointed. With reference to Figure 4, a first preferred embodiment of a method of producing a cylindrical shell 2 of said Yankee cylinder 1 will be described.

Metal bands or sheets of an appropriate steel quality are punched or cut to metal sheets of appropriate sizes. Said metal sheets of appropriate sizes are then bent to a curved shape forming shell sections 17 with appropriate curvature with regard to the diameter of the cylindrical shell 2 to be produced. Appropriate thickness of the metal bands/sheets may be in the interval of 40-150 mm, preferably 60-120 mm, and more preferred 70-100 mm. In some embodiments a thickness of about 115 mm may be conceivable. Flowever, the appropriate thickness is also dependent on the diameter of the Yankee cylinder to be produced. For a Yankee cylinder having a diameter of 12 ft (3660 mm), a thickness of 70 mm may be suitable while for a 16 ft (4872 mm) Yankee cylinder the suitable thickness may be 95 mm. The next step of the method is then to joint together the formed shell sections 17.

In embodiments where the length of the circumferential sides 23 of the shell sections 17 are of the same or approximately same size as the circumferential length of the cylindrical shell 2 to be produced, the jointing/welding procedure preferably starts with welding together the axial sides 22 of each shell section 17 so as to form single cylinder sections 18. Said jointing is preferably performed by welding.

In other embodiments two curved metal bands or metal sheets may serve as starting material. Said two curved plates are mounted to each other by welding thereby forming a single cylinder section. This procedure may be repeated until an appropriate number of single cylinder sections has been reached. In a preferred embodiment four curved metal bands or metal sheets are welded two and two, forming two single cylinder sections.

It is also to be understood that only one shell section 17 may be welded to form only one single cylinder section 18, which single cylinder section 18 then forms the cylindrical shell (2) of the Yankee cylinder (1). This is the simplest embodiment of the present invention.

Irrespective of the number of single cylinder sections and of the starting number of metal plates or metal sheets, the mounting procedure of the single cylinder sections to a cylindrical shell 2 preferably starts with placing a first cylinder section 18 on a supporting structure so that one of the ends of said cylinder section 18 rests on the supporting structure and a longitudinal axis of said cylinder section 18 is orthogonal in relation to the supporting structure. A second cylinder section 18 is placed with its one end on an upper end (the end not facing the supporting structure) of said first cylinder section and the longitudinal axes of said sections coincide. The first and second cylinder sections are then welded together, preferably but not necessarily by manual welding by a welding operator. The cylinder sections are rotated horizontally so as to expose the entire circumference of the sections to the welding device and the welding operator. However, it is also possible to manufacture the cylindrical shell 2 in in the other direction, i.e. that the longitudinal axis of said cylinder section 18 is horizontal in relation to the supporting structure meaning that the cylinder sections are resting on their mantle surface during the welding operation. Said two single cylinder sections 18 are welded along their respective circumferential sides 23, where the term circumferential indicates an extension in the circumferential direction of the cylinder as viewed in a mounted position. When all cylinder sections (two or more) have been jointed at their adjacent sides by circumferential jointing in a plane orthogonal or non-orthogonal to the axis of the cylinder sections 18, a complete cylindrical shell 2 has been produced. The welding may be performed first from an outside of the cylinder sections, or first from an inside of said sections. It is also conceivable that the welding of the outside and inside of the cylinder sections are done simultaneously by using two welding devices.

It is however conceivable that instead of jointing together each shell section 17 at its axial sides 22 as a first step, the jointing process could start with jointing together the shell sections 17 with each other at their respective circumferential sides 23. The next step would then preferably be to joint together the respective axial sides of each shell section 17. It may be facilitated if the shell sections 17 are jointed together such that all of their respective axial sides 23 are in line with each other in the axial direction of the cylindrical shell 2 and thereby form a predestination line along the length of the cylindrical shell 2 in its axial direction intended for the weld seam to be placed within, when the jointing is performed by welding. The welding may then be performed as a single line weld seam from one axial end 3, 4 of the cylindrical shell 2 to the other axial end 3, 4 without having to interrupt the welding, moving the welding equipment radially to the next place on the surface of the cylindrical shell 2, and continue the welding of the axial sides 23 of the next shell section 17. Finally, when all the axial sides of the shell sections 17 have been welded a cylindrical shell 2 has been produced.

As mentioned above, when welding together the cylinder sections 18A, 18B, 18C, the welding operation may be carried out from the outside or the inside or from both sides of the Yankee cylinder resulting in an outer weld seam or an inner weld seam or both. When welding from both sides, one of the welding beads is preferably a back bead, i.e. a secondary welding bead, which guarantees a greater integrity of the welding and greater safety.

The welding method used can be arc welding, for example manual metal arc welding (MMA), submerged arc welding (SAW), shielded metal arc welding (SMAW), or gas metal arc welding (GMAW). It is further understood that the jointing weld may have different kinds of shapes, e.g. U-shape, V-shape, K-shape, X-shape etc. depending on what is the most advantageous shape of the welding for the specific Yankee cylinder to be produced. Preferably the welding is performed by gas metal arc welding (GMAW) and/or submerged arc welding (SAW).

As already described the welding starts with jointing together said single pieces of shell sections 17. This step is preferably performed manually by a welding operator using gas metal arc welding. After the welding of shell sections to cylinder sections the welding of cylinder sections preferably continues by submerged arc welding preferably by a mechanical and manual method operated by a welding operator. After having welded the cylinder sections manually, an automated welding process is preferably performed for the complete welding.

Due to the shrinkage of solidification of the weld metal caused by the heat supply, rest tensions will be in and around the weld metal after the welding has finished. Rest tensions may affect ductility, endurance limit and corrosion resistance. One way to reduce the rest tensions is to heat-treat the weld after the welding has been performed. The heat- treatment step may be performed e.g. locally or in an furnace. In the latter case the complete cylindrical shell may be heat-treated in the furnace so as to release or at least reduce the rest tensions within the weld and in the areas surrounding the weld metal.

Irrespective of performing the heat-treatment step locally or in an, the goods to be heated is heated to a temperature preferably in the interval of 400-800 °C, preferably 580-680 °C, and most preferred about 605-640 °C. For example, the goods can be heat- treated at a temperature of 620 °C. The heating should preferably not be too quick and a suitable heating rate may be to increase the temperature of the goods by 20-45 °C per hour. More preferred, the temperature is raised at a rate of 25-35 °C per hour and a suitable rate of heating may be, for example 30 °C per hour. When the goods have reached its final temperature, it may be kept at that temperature for about 10 minutes to 3 hours and then be allowed to slowly cool to approximately room temperature. In one embodiment said cooling may be performed in a controlled manner by lowering the temperature of the goods with 30 °C per hour until the goods has reached a predetermined temperature and thereafter the goods may cool in still air down to room temperature.

Next step following the steps of jointing (welding), heat-treatment (including cooling) is preferably a first machining step of the inner and/or outer surface of the cylindrical shell 2. In the context of this text, the term“machining” should be understood as an operation in which material is removed. For instance, it can consist of or comprise a turning operation. Flowever, the inventors have surprisingly found that one reason to uneven heat distribution and heat transfer from an inside of the Yankee cylinder to an outside may be a problem with a so called run-out, i.e. a too high value of run-out, resulting in different wall thicknesses in the radial and/or the axial direction of the Yankee cylinder. Run-out may be present in any rotating system and, depending on the system, different forms of run-out may either combine so as to increase total run-out, or reduce total run out. Run-out has two main forms, namely radial run-out and axial run-out.

In the manufacture of Yankee cylinders, mounting together two or more already finished cylinder sections to form a cylindrical shell may result in problems with run out for the formed cylinder, e.g. the respective longitudinal axes of the cylinder sections forming the cylinder may have the same direction but are displaced in relation to each other. At any point along the cylinder it is not possible to determine whether run-out is axial or radial; only by measuring run-out along the longitudinal axis of the cylinder they can be differentiated. If the longitudinal axes of the cylinder sections forming the cylindrical shell is not exactly in line with or correspond with a predetermined longitudinal main axis when machining (e.g. by lathing) is performed on the inside of the cylinder surface, the machining will be performed about a secondary axis resulting in a larger hole than an expected nominal diameter of the cylinder shell due to the lathe being rotated eccentrically (off-axis instead of in-line). This is radial run-out and said radial run-out will be measured about the same all along the longitudinal axis. The inside hole of the cylinder will have a deviation from perfect cylindricity in its axial extension. Axial run-out is caused by the axes of the cylinder sections forming the cylindrical shell being at an angle to the predetermined longitudinal axis of the shell. Axial run-out will vary according to how far from the longitudinal axis in the radial direction it is measured.

In the inventive method, next step may therefore be to measure run-out of the cylindrical shell 2. This is a first measure of run-out. Typically, run-out is measured using a dial indicator pressed against the outer surface of the cylindrical shell.

Typically, dial indicators measure variation in tolerance during the inspection process of a machined part, measure the deflection of a beam or ring under laboratory conditions, as well as many other situations where a small measurement needs to be registered or indicated. Here, the dial indicator measures radial run-out at several points along a revolution of the cylindrical shell by measuring variation in tolerance of the outer surface of the Yankee cylinder. However, it may not be necessary to measure run-out at this stage but it is often preferred. Machining allowance is most often sufficient and if there is any run-out it will be taken care of later in the manufacturing process, as will be described below.

As an alternative, instead of measuring run-out, a cylindricity control could be done defining how much a cylindrical surface or a portion of a cylindrical surface may deviate from a perfect cylinder surface. Perfect cylindricity is when all points of a surface of a revolution are at the same distance from a common longitudinal axis.

Cylindricity is a form control. The cylindricity control defines how much a cylindrical surface on a real part of a cylinder may vary from an ideal cylinder that is perfectly round, perfectly straight and has no taper. The cylindricity tolerance zone is the volume between two coaxial cylinders, e.g. an inside surface of a Yankee cylinder compared to an outside surface of said Yankee cylinder. The radial distance between the two cylinders (inside surface/outside surface) is the value of the cylindricity control tolerance. The surface being controlled must lie within the volume defined by the tolerance zone. The cylindricity control is used to limit‘out of roundness’,‘taper’ and to ensure straightness of a shaft. If a shaft has too much cylindrical error, it could cause bushing or bearing failure. The cylindricity control may be inspected as follows: 1.The part is placed on a turntable.

2. A measuring instrument contacts the part at the circular element to be measured.

3. As the part is rotated, the measuring instrument must up or down the part and the profile of the cylinder is captured.

4. The profile data may be plotted on a polar graph or analyzed using a computer algorithm to determine if this particular circular element is within specifications.

The first machining step, machining of said outer and inner surface of the cylindrical shell 2, is performed until the wall thickness T has reached a predetermined target level. A starting level of wall thickness T before performing the machining step may typically be 40-150 mm, while the target level after the machining step may be 50-90 mm. It is understood that scaling off material of the outer surface by reducing the wall thickness T by means of machining results in a very smooth and linear surface having no inclinations. It is further to be understood that also the weld seams and the adjacent areas surrounding the weld seams are machined in the machining step meaning that the weld seams will no longer protrude but be in line with the cylindrical surface and be part of the smooth surface. If the internal surface of the cylinder section is to be provided with internal grooves said grooves are formed during the first machining step. There is no need to avoid the weld seams 19, 20 while forming the grooves. Some grooves may be placed such that they either cross or follow a weld seam. The machining step preferably further comprises machining of the axial ends 3, 4 of the cylindrical shell 2 so as to form a circumferential joint preparation, a joint spacing (not shown) for the weld seam 7 to be placed within when welding together the end walls 5, 6 with the cylindrical shell 2. The form of the circumferential joint spacing is chosen depending on the desired weld shape, e.g. U-shape, V-shape, K-shape, X-shape etc. The inner surface is now finished. There is preferably no remaining radial displacement of the inside surface at this stage. Said first machining step is preferably followed by measuring run-out. This measure of run-out, which may be either a first or a second measure of run-out depending on if run out was measured already after the first cooling step, is to check whether the run-out value is within acceptable limits by measuring variation in tolerance of the outer surface of the Yankee cylinder at several points along each measured revolution of the cylindrical shell. Preferably, several points along several revolutions at different positions of the outer cylindrical surface 30 are measured. For every revolution, the measured value of run-out may preferably be less than 0,5 mm and more preferred less than 0,2 mm.

The next step following the first machining step of the cylindrical shell 2 and measurement of run-out, is a mounting step for mounting the end walls 5, 6 to the cylindrical shell 2. The mounting is preferably performed by welding a weld seam 7 in the formed circumferential joint spacing of the cylinder shell 2 and the welding method is preferably but not necessarily SAW-welding. Said weld seam 7 may for example be made in accordance with the teachings in EP 2126203 or WO 2014/077761 Al. The material of the weld seams 7, 19, 20 is preferably of the substantially same material as the starting material, i.e. the metal band or the metal sheets. The material of the weld seams 7, 19, 20 may differ somewhat with regard to its alloying material. Preferably the material of said weld seams 7, 19, 20 may have 0,1-2%, preferably 0,5-1 %, more of the alloying material than said starting material. This is due to the fact that some of the alloying material will be burnt off in the welding arc during welding. The finished welding bead will after the burning off have a lowered content of alloying material that is very near or almost equal to the content of alloying material of the starting material. If not using a somewhat higher content of alloying material in the weld seam, the final content of alloying material in the weld seam would probably be too low and it is preferred that the alloying material contents of the weld seam and the shell

sections/cylinder sections are as near as possible.

The end walls 5, 6 have preferably in an earlier stage been prepared with welding grooves for the filling by SAW-welding and their inner surfaces have preferably already been machined before the mounting. Each end wall 5, 6 is jointed to the cylindrical shell 2 by a circumferential weld between opposing surfaces 5, 6 of each end wall and the axial ends 3, 4 of the cylindrical shell 2 respectively. After the mounting/welding of the end walls 5, 6 on the cylindrical shell, the thereby produced Yankee cylinder 1 is heat-treated in a second heat-treatment step so as to release or at least reduce rest tensions within the circumferential end weld seams 7 and in the areas surrounding the weld metal. The heat-treatment may be performed at a temperature preferably in the interval of 400-800 °C, preferably 580-680 °C, and most preferred about 605-640 °C. For example, the goods can be heat-treated at a temperature of 620 °C. The heating should preferably not be too quick and a suitable heating rate may be to increase the temperature of the goods by 20-45 °C per hour. More preferred, the temperature is raised at a rate of 25-35 °C per hour and a suitable rate of heating may be, for example 30 °C per hour. When the goods have reached its final

temperature, it may be kept at that temperature for about 10 minutes to 3 hours and then be allowed to slowly cool in a second cooling step down to approximately room temperature. In an embodiment a controlled cooling may be performed by cooling the goods at a rate of 30 °C per hour down to 260 °C and from 260 °C be carried out in still air down to room temperature.

The next step following the heat- treatment (including cooling) step of the Yankee cylinder is a second, and preferably final, machining step of the outside cylindrical surface of the Yankee cylinder and of the end walls 5, 6. Preferably, a grinding machine may be used. In this step, the exact position of the openings 24, 25 in the end walls 5, 6 are to be determined and machined. The Yankee cylinder 1 is preferably placed on one of its end walls 5, 6 so that the end walls are substantially horizontal and parallel with the ground and the longitudinal axis A of the cylindrical shell is substantially orthogonal to the same ground. In order to determine what position is the preferred position for the openings 24, 25 in the end walls 5, 6, measurement of the variation in wall thickness of the cylindrical shell is performed. This measurement may be performed either from the outside of the cylindrical shell or from the inside. Measuring the wall thickness on the outside of the shell is performed at specific, pre-determined outside coordinates and from this measurement at said specific outside coordinates the corresponding inside coordinates can be calculated. From this calculation a three-dimensional form - a cylindricity form control - of the inner surface of the cylindrical surface may be determined.

A dial indicator may be used when measuring the wall thickness variation from the inside. The measurement is preferably performed radially in a two-dimensional plane. Several radial measurements may be performed at different positions along the longitudinal axis of the cylindrical shell. By measuring the wall thickness variation at different positions a variation in wall thickness may be determined as well as a three- dimensional form - a cylindricity form control - of the inner surface of the cylindrical surface may be determined.

Irrespective of measuring wall thickness and wall thickness variation from the outside or the inside of the cylindrical shell, the results from the wall thickness measurement are used to exactly determine where in the turning machine the Yankee cylinder should be positioned so as to get a desired centre of rotation of the Yankee cylinder when rotated during machining in the final machining step. Also, the understanding of the inside three-dimensional form may be used as a factor when determining the centre of rotation. The turning machine may beneficially be a vertical turning machine. Preferably, the - preferably final - machining is firstly performed on the upper one of the end walls 5, 6. After having machined the upper end wall the machining is continued on the outer surface 16 of the cylindrical shell 2. The final machining of the outer surface of cylindrical shell 2 is performed until the thickness of the wall of the cylindrical shell has reached a final and uniform target thickness and until the circumferential end weld seams no longer protrude but are smooth and in line with the cylindrical shell surface. The variation in wall thickness of the manufactured Yankee cylinder is preferably less than 1,5 mm, more preferred less than 1,0 mm and even more preferred 0,5 mm or less in the radial direction of said Yankee cylinder. The outer diameter of the cylindrical shell is substantially the same but the inner diameter may vary, thus resulting in a variation in wall thickness. Finally, the opening 24, 25 is machined to its final size/diameter. The Yankee cylinder is then turned so that the other one of the end walls 5, 6 is resting on the ground and the end wall that was resting on the ground when machining the outside of the cylindrical shell can now be machined and its opening 24, 25 machined to its final size/diameter.

Radial run-out is preferably measured again, after the machining of the end walls 5, 6 and openings 24, 25, of the Yankee cylinder. Said radial run-out may preferably be less than 0,5 mm and more preferred less than 0,2 mm, and is measured from one opening 24, 25 and along the whole longitudinal distance of the cylindrical shell to the other opening 24, 25 as the displacement of one of the openings 24, 25 to the other opening 24, 25, i.e. meaning that the centres of the openings are not aligned with each other but displaced in a radial direction.

Any additional inside equipment may be mounted, e.g. a system for condensate removal etc.

The Yankee cylinder is now completed and is pressure tested so as to test that the cylinder fulfills the requirements of pressure vessels regulated by national standards of the nation where said Yankee cylinder is to be used. This is performed by testing the Yankee cylinder hydrostatically by filling said cylinder with water and applying a required testing pressure above operating pressure.

A finishing step of the outside cylindrical surface of the Yankee cylinder may now be performed. Said finishing step may preferably comprise the steps of turning of a crowning curve, balancing of the Yankee cylinder, metallization and grinding.

The turning of a crowning curve of the outside cylindrical surface gives said surface a slightly convex shape - crowning curve - in its axial extension. As a result of the crowning the middle part of the outside surface bulges outwardly one or a few millimeters as compared to the end areas of the outside surface. It is preferred to balance the Yankee cylinder since e.g. the weight of the added additional equipment may cause lack of balance of the cylinder. Counterweights may be added so as to counteract on the unbalance and cause balance. In the metallization step, the outside cylindrical surface may be prepared with a smooth metallic coating on top of the outside surface. Thanks to this metallic coating the convex shape will be fixed and have a long-term duration. Another advantage is that the wear and corrosion resistance of the outside cylindrical surface is indeed prolonged. The coating may be applied by metal spraying and the coating may have a thickness of about 0,2 mm - 1,5 mm, and more preferred 0,5 mm - 0,9 mm. For many Yankee cylinders, a thickness of the metallic coating of 0,7 mm may be suitable.

It may be preferred that an end-finishing machine, which may be a grinding machine, works on the outside cylindrical surface such that it gives the outside cylindrical surface a very smooth and even final finish.

A run-out value of the outside cylindrical surface 30 of the manufactured/finished Yankee cylinder may be measured during or after the finishing step. Also a run-out value of the inside cylindrical surface of the Yankee cylinder may be measured. A longitudinal line is determined between the rotational centres of the journals 10, 11, since in operation the Yankee cylinder will rotate about the longitudinal line between the rotational centres of the journals. Ideally, the longitudinal line corresponds to the axis of the Yankee cylinder. Also for measuring the run-out of the finished Yankee cylinder a dial indicator is preferably used. The dial indicator is pressed against the outside surface of the Yankee cylinder. The Yankee cylinder is rotated and during the rotation of the Yankee cylinder the dial indicator measures radial run-out in terms of variation of tolerance at predetermined points of the outer cylindrical surface 30 of one revolution of the Yankee cylinder. The measure is performed of the outer surface of a revolution of the Yankee cylinder at certain distances from each end wall 5, 6 of the Yankee cylinder. It is conceivable that the run-out is measured along whole revolutions of the outer surface of the Yankee cylinder at e.g. distances of 200 mm, 500 mm and 700 mm from each end wall 5, 6, but other distances are of course possible. The run-out may preferably be measured at least at four points, preferably at eight points and more preferred at ten or more along each revolution, said points preferably being regularly distributed along each revolution.

The inventive method may further comprise the step of comparing the measured run-out value of the outside cylindrical surface 30 of the finished Yankee cylinder with a target run-out value and continuing the finishing step until the target run-out value has been reached. Said target run-out value should preferably be equal to or less than 1,0 mm, more preferred less than 0,5 mm and even more preferred 0,1 mm or less. Most preferred the target run-out is less than 0,05 mm. This run-out measurement is the final measurement of run-out. Instead of measuring run-out, a variation in wall thickness may be measured. Also for measuring a variation in wall thickness of the finished Yankee cylinder an ultrasonic equipment is preferably used. An ultrasonic probe of said ultrasonic equipment is pressed against the outside surface of the Yankee cylinder. The Yankee cylinder is rotated and during the rotation of the Yankee cylinder the ultrasonic probe measures wall thickness variation at predetermined points of the outer cylindrical surface 30 of one revolution of the Yankee cylinder. The measure is performed at the outer surface of a revolution of the Yankee cylinder at certain distances from each end wall 5, 6 of the Yankee cylinder. It is conceivable that the wall thickness variation is measured along whole revolutions of the outer surface of the Yankee cylinder at e.g. distances of 200 mm, 500 mm and 700 mm from each end wall 5, 6, but other distances are of course possible. The wall thickness variation may preferably be measured at least at four points, preferably at eight points and more preferred at ten or more along each revolution, said points preferably being regularly distributed along each revolution. The inventive method may then further comprise the step of comparing the measured wall thickness variation of the cylindrical wall 31 of the finished Yankee cylinder with a target wall thickness variation value. The final machining step of the outer cylindrical surface (30) of the Yankee cylinder (1) may preferably be performed until the variation in the wall thickness in the circumferential direction of the cylindrical wall (31) of said Yankee drying cylinder (1) is 2,5 mm or less. The Yankee cylinder may preferably be heated to common operating temperature before the measure of variation in wall thickness.

In summary, the innovative method of producing a Yankee cylinder made of steel may preferably comprise the following manufacturing steps and preferably, but not necessarily, in the following order:

(1) a welding step for welding together shell sections 17 to cylinder sections 18 and welding together said cylinder sections 18 to a cylindrical shell 2,

(2) a first heat-treatment step of the cylinder sections 18,

(3) preferably but not necessarily followed by measuring of run-out of the cylindrical shell, or by controlling its cylindricity,

(4) a first machining step of the inside surface and/or outside surface of the cylindrical shell 2,

(5) preferably followed by measurement of run-out of the cylindrical shell 2 or by controlling its cylindricity,

(6) a mounting step for mounting the end walls 5, 6 on the cylindrical shell 2 to form the Yankee cylinder 1,

(7) a second heat- treatment step for heating of the Yankee cylinder 1,

(8) a final machining step of said inside surface and/or outside surface of said complete cylindrical shell (2) and of said end walls (5, 6) and of openings (24, 25) in said end walls (5, 6), and preferably measuring radial run-out of the Yankee cylinder 1,

(9) a step for final finishing of the outer cylindrical surface 30 of the Yankee cylinder 1, comprising measurement of radial run-out of said outer cylindrical surface 30 and / or measurement of variation in wall thickness in the circumferential direction of the cylindrical wall (31) of said Yankee drying cylinder (1). The paper produced in the paper machine comprising the inventive Yankee cylinder is preferably tissue paper, but it is understood that other paper qualities may also be produced, e.g. printing paper, paper and cardboard. As will be understood by those skilled in the present field of art, numerous changes and modifications may be made to the above described and other embodiments of the present invention, without departing from the scope of the present invention as defined in the appending claims. The measurement of run-out may be performed either before the steps of heat- treatment or after said steps without departing from the scope of the invention.

The end walls could be mounted to the cylindrical shell by using bolts and nuts instead of by welding.

Other types of indicators than dial indicators may be used for measuring run-out without departing from the scope of the invention, e.g. mechanical indicators with cantilevered pointers and electronic indicators with digital displays.