METHOD OF APPLYING A WEAR-RESISTANT COATING ON A YANKEE DRYING CYLINDER, SUCH COATINGS AND YANKEE CYLINDERS WITH SUCH COATINGS

Abstract

A method of applying a long lasting wear-resistant coating on a Yankee drying cylinder is described, whereby the method includes: providing a Yankee drying cylinder having a cylindrical shell with a circular cross-section and an outer surface; and performing a thermal spray operation to form a wear-resistant coating layer on the outer surface of the Yankee drying cylinder during which thermal spray operation coating feedstock is fed to at least one spray device, heated to become plastic and/or semi-molten and/or molten and sprayed onto the outer surface of the Yankee drying cylinder to form the wear-resistant coating layer. The coating feedstock for the thermal spray operation consists of a specific set of elements, by percent weight, with the remainder being iron and impurities. Coatings and Yankee cylinders with such coatings are also disclosed.

Claims

1. A method of applying a wear-resistant coating on a Yankee drying cylinder (1), the method comprising: a step of providing a Yankee drying cylinder (1) having a cylindrical shell (2) with a circular cross-section and an outer surface (3); a step of performing a thermal spray operation to form a wear-resistant coating layer (4) on the outer surface of the Yankee drying cylinder (1) during which thermal spray operation coating feedstock (6) is fed to at least one first spray device (5), heated to become plastic and/or semi-molten and/or molten and sprayed onto the outer surface (3) of the Yankee drying cylinder (1) to form the wear-resistant coating layer (4), the coating feedstock (6) for the thermal spray operation consisting of: 0.0 to 2.1 weight percent Al 0.0 to 10.0 weight percent Ti, 0.0 to 10.2 weight percent Si, 1.7 to 10.1 weight percent B, 15.0 to 16.1 weight percent Mo, 9.5 to 11.4 weight percent V, 2.0 to 4.2 weight percent C, 0.000 to 0.050 weight percent Cr, 0.0 to 0.3 weight percent Mn, 0.0 to 0.2 weight percent Mg, 0.0 to 1.0 weight percent Ni, 0.0 to 0.5 weight percent Nb, the sum of Al+Mn+Si is equal to or greater than 5.00 weight percent and equal to or less than 10.00 weight percent, and the remainder being iron and impurities.

2. The method of claim 1 wherein the step of thermal spraying the outer surface of the Yankee drying cylinder (1) is performed until the wear-resistant coating layer has obtained a thickness of 680 μm-2000 μm.

3. The method of claim 1 wherein the coating feedstock (6) for the thermal spray operation consists of: from 1.40 to 2.02 weight percent Al, from 0.00 to 10.00 weight percent Ti, from 0.10 to 10.08 weight percent Si, from 1.80 to 10.00 weight percent B, from 15.00 to 16.10 weight percent Mo, from 10.00 to 11.36 weight percent V, from 2.10 to 2.50 weight percent C, from 0.000 to 0.020 weight percent Cr, from 0.10 to 0.30 weight percent Mn, from 0.00 to 0.10 weight percent Mg, from 0.00 to 1.00 weight percent Ni, from 0.00 to and 0.50 weight percent Nb, the remainder being iron and impurities.

4. The method according to claim 1 comprising the subsequent step of performing a grinding and/or polishing operation of the coated outer surface such that, after the grinding and/or polishing operation, the wear-resistant coating layer (4) has a thickness from 100 μm-1990 μm.

5. The method according to claim 1, wherein the step of performing thermal spray operation to form the wear-resistant coating layer (4) is preceded by the steps of: performing an initial first grinding operation on the outer surface (3) of the shell (2); grit blasting the outer surface (3) after the initial first grinding operation; coating the ground and grit blasted outer surface (3) with a bond coating layer (10) having a composition different than the composition of the coating feedstock (6), and wherein the wear-resistant coating layer (4) is subsequently applied on top of the bond coating layer (10).

6. The method according to claim 5, wherein the coating of the ground and grit blasted outer surface (3) with a bond coating layer (10) is performed by an initial thermal spraying operation during which a bond coating feedstock is fed to at least one spray device (13), heated to become plastic and/or semi-molten and/or molten and sprayed onto the ground and grit blasted outer surface to form the bond coating layer (10), wherein the bond coating feedstock constituting a Ni—Al mixture or alloy consisting of from 85 to 98 percent by weight Ni and from 15 to 2 percent by weight Al and unavoidable impurities.

7. The method according to claim 6 wherein the thickness of the bond coating layer is greater than or equal to 1.00 μm and less than or equal to 2000 μm.

8. The method according to claim 6 wherein the thickness of the bond coating layer is greater than or equal to 10 μm and less than or equal to 30 μm.

9. The method according to claim 1, wherein the method comprises applying an aqueous solution comprising dissolved mono ammonium phosphate or di ammonium phosphate over the surface of the wear-resistant coating layer (4).

10. The method according to claim 4, wherein the grinding and/or polishing operation is performed until the surface of the wear-resistant layer (4) has obtained a surface roughness Ra in the range of 0.1 μm-1.2 μm.

11. The method according to claim 1, wherein the coating feedstock (6) for the wear-resistant coating layer (4) comprises from 16.00 to 16.10 weight percent Mo, from 10.85 to 10.98 weight percent V, from 1.81 to 1.85 weight percent B, from 2.17 to 2.19 weight percent C, from 0.00 to 0.10 weight percent Ti, from 0.00 to 0.10 weight percent Mg, from 0.00 to 0.10 weight percent Ni, from 0.00 to 0.50 weight percent Nb, and from 0.000 to 0.005 weight percent Cr.

12. The method according to claim 11, wherein the coating feedstock for the wear-resistant coating layer (4) comprises 16.00 to 16.04 weight percent Mo, 10.85 to 10.98 weight percent V, 1.81 to 1.85 weight percent B, 2.17 to 2.19 weight percent C, and from 0.000 to 0.010 weight percent Cr.

13. The method according to claim 1 wherein the coating feedstock for the wear-resistant coating layer (4) comprises from 15.00 to 15.20 weight percent Mo, from 11.26 to 11.40 weight percent V, from 1.80 to 1.90 weight percent B, from 2.19 to 2.29 weight percent C, from 0.00 to 0.01 weight percent Ti, from 0.00 to 0.01 weight percent Mg, from 0.00 to 0.03 weight percent Ni, from 0.00 to 0.03 weight percent Nb, and from 0.000 to 0.010 weight percent Cr.

14. The method according to claim 13 wherein the coating feedstock for the wear-resistant coating layer (4) comprises 15.01 weight percent Mo, 11.36 weight percent V, 1.85 weight percent B, 2.24 weight percent C, from 0.00 to 0.01 weight percent Ti, from 0.00 to 0.01 weight percent Mg, from 0.00 to 0.03 weight percent Ni, from 0.00 to 0.03 weight percent Nb, and from 0.000 to 0.005 weight percent Cr.

15. The method according to claim 1 wherein the coating feedstock used to form the wear-resistant coating layer comprises: from 10.00 to 11.36 weight percent V, from 2.10 to 2.50 weight percent C, from 1.80 to 10.00 weight percent B, from 15.00 to 16.10 weight percent Mo, from 0.00 to 2.10 weight percent Al, from 0.00 to 0.30 weight percent Mn, from 0.00 to 10.20 weight percent Si, from 0.00 to 10.00 weight percent Ti, from 0.00 to 0.20 weight percent Mg, from 0.00 to 1.00 weight percent Ni, from 0.00 to 0.50 weight percent Nb, and, from 0.000 to 0.020 weight percent Cr.

16. The method according to claim 15 wherein the coating feedstock for the wear-resistant coating layer of this embodiment consists of from 15.00 to 16.00 weight percent Mo, 1.40 to 2.02 weight percent Al, 0.10 to 0.30 weight percent Mn, 0.10 to 10.10 weight percent Si, from 0.10 to 10.00 weight percent Ti, from 0.00 to 0.10 weight percent Mg, from 0.00 to 1.00 weight percent Ni, from 0.00 to 0.50 weight percent Nb, and from 0.000 to 0.020 weight percent Cr with the balance being iron and impurities.

17. The method according to claim 1, wherein, when the at least one first spray device (5) acts against a part of the outer surface (3) of the shell (2) to apply the wear resistant coating to that part of the outer surface, the at least one first spray device (5) is operated at a distance from that part of the outer surface (3) which is in the range of 50 mm-260 mm.

18. The method according to claim 1, wherein, when the at least one first spray device (5) acts against a part of the outer surface of the shell to apply the wear-resistant coating to that part of the outer surface, the plastic and/or semiplastic and/or molten feedstock is sprayed onto the outer surface of the shell (2) at an angle of 30°-90° with respect to that part of the outer surface.

19. The method according to claim 5, wherein the coating the ground and grit blasted outer surface is initiated within at most 90 minutes after the grit blasting has been completed.

20. The method according to claim 6, wherein the initial thermal spraying operation is carried out at such a rate that the entire outer surface has been covered within at most 3 hours after the grit blasting has been completed.

21. The method according to claim 6, wherein at least one, two, three or more spray devices (5) are used simultaneously during at least one of the thermal spray operations.

22. The method according to claim 6, wherein at least one or more group of spray devices (5) are used simultaneously during at least one of the thermal spray operations, each group comprising three or more spray devices (5).

23. The method according to claim 4, wherein the grinding and/or polishing operation that follows the forming of the wear-resistant coating layer (4) is initiated within at most 15 minutes after the thermal spraying operation to form the wear-resistant layer has been completed.

24. The method according to claim 1, wherein the coating feedstock (6) for the at least one first spray device comes in the shape of two wires, each wire having a diameter equal to or greater than 1.1 mm and equal to or less than 3.8 mm.

25. A method according to claim 1, wherein the at least one spray device (5) is an arc spray gun or a high velocity oxygen fuel (HVOF) device or a high velocity air fuel (HVAF) device or a plasma spray gun or a water stabilized plasma spray gun.

26. A method according to claim 24, wherein, during the thermal spraying operation, each wire is fed to the at least one spray device (5) at a rate of 40 mm/second −90 mm/second, while the at least one spray device (5) operates at a voltage in the range of 28-40 Volts and an amperage in the range of 100 Amps-350 Amps.

27. A Yankee cylinder with a coating formed by the method of claim 1.

28. A Yankee cylinder according to claim 27 wherein the coating has a diamond pyramid hardness (DPH) determined by ASTM E384-10, “Standard Test Method for Knoop and Vickers Hardness of Materials”, which is equal to or greater than 650 and equal to or less than 1004.

29. A Yankee cylinder according to claim 28 wherein the coating has a porosity equal to or less than 5% and preferably greater than or equal to 1% as determined by ASTM Standard E2109-01 (Reapproved 2014) “Standard Test Methods for Determining Area Percentage Porosity in Thermal Sprayed Coatings”.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0131] FIG. 1 is a schematic representation of a Yankee drying cylinder during operation to produce paper.

[0132] FIG. 2 is a schematic representation in perspective of a Yankee drying cylinder.

[0133] FIG. 3 is a schematic representation of a step in the process of applying a coating layer to the surface of a Yankee drying cylinder.

[0134] FIG. 4 is a schematic representation of a further step in the process of applying a coating layer to the surface of a Yankee drying cylinder.

[0135] FIG. 5 is a schematic representation of how coating material is thermally sprayed onto the surface of the shell of the Yankee drying cylinder.

[0136] FIG. 6 is a schematic representation of how several spray devices may be used simultaneously to apply a coating layer to the outer surface of the Yankee drying cylinder.

[0137] FIG. 7 is a schematic representation of how a spray device may be oriented relative to the shell of the Yankee drying cylinder during application of a coating layer.

[0138] FIG. 8 is a schematic cross-sectional representation of a Yankee drying cylinder that has been coated in accordance with one embodiment of the present invention.

[0139] FIG. 9 is a schematic representation of a possible initial spraying step that precedes the step in which thermal spraying is used to form a wear-resistant coating layer.

[0140] FIG. 10 is a schematic cross-sectional representation of a Yankee drying cylinder that has been coated with a coating in accordance with the invention.

[0141] FIG. 11 is a schematic representation of a polishing step.

[0142] FIG. 12 is a schematic representation of a possible step at the end of the process of providing a wear-resistant coating layer.

DETAILED DESCRIPTION OF THE INVENTION

[0143] With reference to FIG. 1, a Yankee drying cylinder 1 is shown in operation during manufacture of a paper web such as a tissue paper web. In FIG. 1, a wet fibrous web W which comes from the forming section (not shown in FIG. 1) of a paper making machine is carried by a fabric such as a water-receiving felt 15 to a transfer nip formed between a transfer roll (or press roll) 16 and a Yankee drying cylinder 1. The transfer roll (or press roll) 16 may be lightly loaded against the Yankee drying cylinder 1, just sufficiently to form a transfer nip. Alternatively, the roll 16 may be loaded with a considerable force against the Yankee drying cylinder 1 and form a dewatering nip with the Yankee drying cylinder 1 such that water is pressed out of the fibrous web W. Water can also be removed by use of an internal vacuum box combined with a transfer roll (or press roll) with a perforated shell. In the nip, the fibrous web W is transferred from the fabric 15 to the outer surface 3 of the Yankee drying cylinder that is rotating in the direction of arrow R about its axis of rotation A which is also the longitudinal axis of symmetry of the Yankee drying cylinder 1. The Yankee drying cylinder 1 is heated to a high temperature, normally by means of hot steam that is supplied to the interior of the Yankee drying cylinder. The fibrous web W is ready-dried on the outer surface 3 of the Yankee drying cylinder 1 and creped off from the Yankee drying cylinder by a doctor 14. While not shown in the figures, it should be understood that the Yankee drying cylinder 1 normally has internal grooves from which condense water is evacuated during operation. The Yankee drying cylinder may a cast iron Yankee or a Yankee drying cylinder made of welded steel.

[0144] FIG. 2 shows an example of a Yankee drying cylinder in perspective. With reference to FIG. 2, the Yankee drying cylinder 1 comprises a circular cylindrical shell 2 which is designed to be capable of receiving hot steam. The cylindrical shell with a circular cross-section has an outer surface 3. The internal grooves of the Yankee drying cylinder 1 (if present) are located on an internal surface of the shell 2. In FIG. 2, the Yankee drying cylinder also has end walls 17 of which only one can be seen in FIG. 2. The end walls 17 may be connected to the shell 2 by means of, for example, welding, screws or bolts.

[0145] A first embodiment of the present invention will now be explained with reference to FIG. 3; FIG. 5; FIG. 6; FIG. 7; FIG. 8; and FIG. 11.

[0146] With reference to FIG. 5 and FIG. 8, a spray device 5, a thermal spray device 5 which may be, for example, an arc spray gun or HVOF or HVAF or plasma spray gun or water stabilized plasma spray gun, combustion powder spray device or wire spray device or combinations thereof or the like thereof of, are used to apply a wear-resistant coating layer 4 (shown in FIG. 8) to the outer surface 3 of a circular cylindrical shell 2 which is part of a Yankee drying cylinder 1. A coating feedstock 6 is fed to the spray device 5, heated to become plastic and/or semi-molten and/or molten and sprayed onto the outer surface 3 of the shell 2. As indicated in FIG. 5, the coating feedstock 6 may comprise two separate wires 6a, 6b. The wires 6a, 6b are electrically charged—one being positive and the other being negative. The wires 6a, 6b are molten when they come into close proximity or are in contact with each other and, as schematically indicated in FIG. 5, air (or gas) is used to blow the molten feedstock onto the outer surface 3 in a spray 18 that lands as small droplets on the outer surface These droplets form a wear-resistant coating layer 4 on the circular cylindrical shell 2.

[0147] It has previously been common to use a feedstock that comprises relatively high amounts of chromium in order to form a coating layer that is resistant to wear, oxidation, corrosion, and chemical attack. While the use of chromium does indeed result in a high resistance to wear, chromium represents a health hazard to the workers, especially during the process for applying the wear resistant coating on the Yankee. For this reason, it is desirable to minimize the use of chromium and preferably eliminate chromium altogether from the process. Therefore, the inventors of the present invention have seen that there is a need to provide a coating method that eliminates or at least minimizes the use of chromium but still succeeds in achieving a coating layer that has a high degree of resistance to wear.

[0148] The inventors have found that this can be achieved if the coating feedstock 6 that is fed to at least one spray device 5 and that is used for the thermal spray operation is formed by a material that consists of: [0149] 0.00 to 2.10 weight percent Al [0150] 0.00 to 10.00 weight percent Ti, [0151] 0.00 to 10.20 weight percent Si, [0152] 1.70 to 10.10 weight percent B, [0153] 15.00 to 16.10 weight percent Mo, [0154] 9.50 to 11.40 weight percent V, [0155] 2.00 to 4.20 weight percent C, [0156] 0.000 to 0.020 weight percent Cr, [0157] 0.00 to 0.30 weight percent Mn, [0158] 0.00 to 0.20 weight percent Mg, [0159] 0.00 to 1.00 weight percent Ni, [0160] 0.00 to 0.50 weight percent Nb,

[0161] the remainder being iron and impurities.

[0162] More preferably the feedstock of this invention consists of: [0163] from 1.40 to 2.02 weight percent Al, [0164] from 0.00 to 10.00 weight percent Ti, [0165] from 0.10 to 10.08 weight percent Si, [0166] from 1.80 to 10.00 weight percent B, [0167] from 15.00 to 16.10 weight percent Mo, [0168] from 10.00 to 11.36 weight percent V, [0169] from 2.10 to 2.50 weight percent C, [0170] from 0.000 to 0.020 weight percent Cr, [0171] from 0.10 to 0.30 weight percent Mn, [0172] from 0.00 to 0.10 weight percent Mg, [0173] from 0.00 to 1.00 weight percent Ni, [0174] from 0.00 to and 0.50 weight percent Nb,

[0175] the remainder being iron and impurities.

[0176] Preferably the sum of Al+Mn+Si is equal to or greater than 5.00 and equal to or less than 10.00 weight percent.

[0177] In an exemplified embodiment of the present disclosure the coating feedstock used to form the wear-resistant coating layer consists of from 15.00 to 16.10 weight percent Mo, from 10.85 to 10.98 weight percent V, from 1.81 to 1.85 weight percent B, from 2.17 to 2.19 weight percent C, from 0.00 to 0.10 weight percent Ti, from 0.00 to 0.10 weight percent Mg, from 0.00 to 0.10 weight percent Ni, from 0.00 to 0.50 weight percent Nb, from 0.000 to 0.020 weight percent Cr with the balance being iron and impurities.

[0178] In another exemplified embodiment of the invention, the coating feedstock for the wear-resistant coating layer 4 consists of from 16.00 to 16.04 weight percent Mo, from 10.85 to 10.98 weight percent V, from 1.81 to 1.85 weight percent B, from 2.17 to 2.19 weight percent C, from 0.000 to 0.010 weight percent Cr and the balance being iron and impurities.

[0179] In another exemplified embodiment of the invention the coating feedstock used to form the wear-resistant coating layer consists of from 15.00 to 15.20 weight percent Mo, from 11.26 to 11.40 weight percent V, from 1.80 to 1.90 weight percent B, from 2.19 to 2.29 weight percent C, from 0.00 to 0.01 weight percent Ti, from 0.00 to 0.01 weight percent Mg, from 0.00 to 0.03 weight percent Ni, from 0.00 to 0.03 weight percent Nb, from 0.000 to 0.010 weight percent Cr with the balance being iron and impurities.

[0180] Even more preferred, the coating feedstock for the wear-resistant coating layer of this exemplified embodiment consists of 15.01 weight percent Mo, 11.36 weight percent V, 1.85 weight percent B, 2.24 weight percent C, from 0.00 to 0.01 weight percent Ti, from 0.00 to 0.01 weight percent Mg, from 0.00 to 0.03 weight percent Ni, from 0.00 to 0.03 weight percent Nb, from 0.000 to 0.005 weight percent Cr with the balance being iron and impurities.

[0181] In yet another exemplified embodiment of the invention the coating feedstock [Oct 21 ranges] used to form the wear-resistant coating layer consists of from 15.00 to 16.10 weight percent Mo, from 0.00 to 2.10 weight percent Al, from 0.00 to 0.30 weight percent Mn, from 0.00 to 10.20 weight percent Si, from 0.00 to 10.00 weight percent Ti, from 0.00 to 0.20 weight percent Mg, from 0.00 to 1.00 weight percent Ni, from 0.00 to 0.50 weight percent Nb, and from 0.000 to 0.020 weight percent Cr with the balance being iron and impurities.

[0182] Even more preferred, the coating feedstock for the wear-resistant coating layer of this exemplified embodiment consists of from 15.00 to 16.00 weight percent Mo, 1.40 to 2.02 weight percent Al, 0.10 to 0.30 weight percent Mn, 0.10 to 10.10 weight percent Si, from 0.10 to 10.00 weight percent Ti, from 0.00 to 0.10 weight percent Mg, from 0.00 to 1.00 weight percent Ni, from 0.00 to 0.50 weight percent Nb, and from 0.000 to 0.020 weight percent Cr with the balance being iron and impurities.

[0183] Preferably, the outer surface of the shell 2 should also be sprayed until the wear-resistant coating layer 4 has obtained a thickness equal to or greater than 680 μm and equal to or less than 2000 μm, more preferably equal to or greater than 800 μm and equal to or less than 1000 μm. Thereafter, a grinding operation is performed on the wear-resistant coating layer. A grinding operation is symbolically indicated in FIG. 3 in which a grinding tool 7 acts on the now coated outer surface 3. The grinding operation is followed by polishing of the coated outer surface such that, after the grinding operation and the polishing operation, the wear-resistant coating layer 4 has a thickness equal to or greater than 100 μm and equal to or less than 1990 μm, preferably equal to or greater than 650 μm and equal to or less than 850 μm. The polishing operation is indicated schematically/symbolically in FIG. 11 in which a polishing device 8 acts on the coated surface 3. The polishing operation should preferably (but not necessarily) be carried out until the surface of the wear-resistant layer 4 has obtained a surface roughness Ra equal to or greater than 0.1 μm and equal to or less than 1.2 μm, more preferably equal to or greater than 0.2 μm and equal to or less than 0.8 μm.

[0184] The wear-resistant coating layer 4 indicated in FIG. 8 is not to scale.

[0185] When the coating feedstock 6 comes in the shape of wires 6a, 6b, each wire may advantageously have a diameter preferably equal to or greater than 1.1 mm and equal to or less than 3.8 mm.

[0186] When wires 6a, 6b are used as feedstock to form the wear-resistant coating layer 4, the thermal spraying operation is preferably (but not necessarily) carried out such that each wire is fed to the at least one spray device 5 at a rate preferably equal to or greater than 40 mm/second and equal to or less than 90 mm/second. The at least one spray device 5 preferably (but not necessarily) operates at a voltage preferably equal to or greater than 28 and equal to or less than 40 Volts and an amperage preferably equal to or greater than 100 Amps and equal to or less than 350 Amps.

[0187] With reference to FIG. 6, several thermal spray devices 5 may be used and several spray devices may be arranged in groups. In the arrangement of FIG. 6, six spray devices 5a-5f are used simultaneously and three spray devices 5a, 5b and 5c are operating together in a group 19 while three spray devices 5d, 5e and 5f are arranged in a group 20. It should be understood that each group may comprise of one, two, three or more than three spray devices and more than one group may be used simultaneously with other groups of spray devices. For example, two, three, four or five groups of spray devices could be used. Conceivably, two spray devices 5 could also be arranged in a pair and operate together. Conceivably, several pairs of spray devices could also be used simultaneously. For example, one, two, three or four pairs could be used or more than four pairs. While the method may be carried out with only one spray device 5, the simultaneous use of several spray devices makes it possible to form the coating layer in a shorter time. The use of spray devices operating close to each other in pairs or groups can result in a more even coating result.

[0188] With reference to FIG. 7, it may be advantageous to arrange the at least one spray device 5 (or several spray devices 5) such that, when the at least one spray device 5 acts against a part of the outer surface of the shell to apply coating to that part of the outer surface, the molten feedstock is sprayed onto the outer surface of the shell 2 at an angle R with respect to that part of the outer surface. The angle R is preferably in the range of 30°-90°, preferably in the range of 45°-90° and even more preferred in the range of 75°-90°.

[0189] With further reference to FIG. 7, the at least one spray device 5 is operated at a distance L from that part of the outer surface 3 against which it acts at a given moment which is in the range of 50 mm-260 mm and preferably at a distance in the range of 60 mm-225 mm.

[0190] An advantageous embodiment of the inventive method will now be explained with reference to FIG. 3; FIG. 4; FIG. 9: and FIG. 10. In some cases, it may be suitable to apply an optional bond coating layer to the outer surface 3 of the circular cylindrical shell 2 before the wear-resistant coating 4 is applied. The bond layer is intended to improve adherence of the wear-resistant coating layer to the circular cylindrical shell 2.

[0191] When a bond layer is applied before the wear-resistant coating layer 4 is formed, the procedure may suitably be as follows. An initial grinding operation (as symbolically indicated in FIG. 3) is performed on the outer surface 3 of the shell 2. At this stage, the outer surface 3 may be simply a surface formed by cast iron or by steel. After the initial grinding operation has been performed, the outer surface 3 is grit blasted by a grit blasting device 9 as shown in FIG. 4. The ground and grit blasted surface 3 is then coated in an initial thermal spraying operation in substantially the same way as previously described with reference to the forming of the wear-resistant coating layer except that a different coating feedstock is used. The initial thermal spraying operation will form a bond coating layer 10 for the subsequently applied wear resistant coating layer. After the bond coating layer 10 has been formed, this bond coating layer will temporarily form the (new) outer surface 3 of the shell 2. With reference to FIG. 9, coating feedstock in the shape of two wires 11a, 11b is fed to a thermal spray device 5. The coating feedstock 11a, 11b is heated to become plastic and/or semi-molten and/or molten and is sent in a spray 22 onto the surface of the shell 2. As with the coating for the wear-resistant layer, air or gas as indicated by the arrow in FIG. 9 may be used to force the molten feedstock towards the surface 3 of the shell 2.

[0192] The coating feedstock 11a, 11b that is used in the initial thermal spraying operation surface to form the bond coating layer 10, consists of from 85 to 98 percent by weight Ni and from 2 to 15 percent by weight Al and unavoidable impurities.

[0193] When the bond coating layer 10 has been formed, the wear-resistant layer 4 will subsequently be applied/formed on top of the bond coating layer 10.

[0194] With reference to FIG. 10, it can be seen that the circular cylindrical shell now has two coating layers, a bond layer 10 and a top layer 4 which is the wear-resistant layer. The inventors have found that the bond layer 10, when formed with the feedstock described above, significantly improves adherence of the wear-resistant layer.

[0195] To prevent undesired oxidation of the grit-blasted surfaces, the initial thermal spraying operation to form the bond coating layer is suitably initiated less than or equal to 90 minutes after the grit blasting has been completed, preferably less than or equal to 45 minutes and even more preferred less than or equal to 5 minutes after the grit blasting has been completed. However, other time intervals may also be considered.

[0196] The initial thermal spraying operation is preferably carried out at such a rate that the entire outer surface has been covered less than or equal to 3 hours after the grit blasting has been completed to prevent oxidation of the exposed outer surface.

[0197] The grinding operation that follows the forming of the wear-resistant coating layer 4 is preferably initiated less than or equal to 15 minutes after the thermal spraying operation has been completed, in particular less than or equal to 10 minutes.

[0198] Both when the inventive method comprises the forming of a bond coating layer and when the wear-resistant coating layer is applied directly to the shell 2 without a bond coating layer, the method can suitably comprise an optional step of applying a sealant such as mono ammonium phosphate or di ammonium phosphate dissolved in water to the wear-resistant coating layer 4 evenly over the surface of the wear-resistant coating layer 4. With reference to FIG. 12, a device 12 for the application of mono ammonium phosphate or di ammonium phosphate sends a spray or jet of mono ammonium phosphate or di ammonium phosphate dissolved in water onto the shell 2 (and thereby also onto the wear-resistant coating layer 4 which now forms the outer surface 3 of the circular cylindrical shell 2).

[0199] MAP and DAP create a uniform bonding surface on the Yankee dryer to help protect the finished surface and to assist it to accept the blended chemical coating added during the paper and tissue making processes. Much like priming a metal surface prior to painting, MAP was originally used in the metal plating industry to prep the metal surfaces to accept the plating metals. MAP and DAP do a similar job for Yankee coating—it assists the blended chemical coating applied during the paper making process to more readily adhere to the Yankee surface and to adhere in a uniform and even manner.

[0200] Thanks to the method described herein, a wear-resistant coating can be formed with minimal or no use of chromium. Preferably the amount of chromium is equal or less than 0.020% by weight, more preferably up to or less than 0.010%, even more preferably equal to or less than 0.005% and most preferably zero. By using the present method, a wear-resistant coating layer can be formed which has a hardness in the range of 650-1004 DPH. The wear-resistant layer formed by the present method will be porous to some extent and the porosity is preferably equal to or less than 5% and preferably is greater than or equal to 1% % as determined by ASTM Standard E2109-01 (Reapproved 2014) “Standard Test Methods for Determining Area Percentage Porosity in Thermal Sprayed Coatings”.