Method for modifying tungsten carbide coated coater blades to perform better in overcoming paper making defects

Abstract

 The invention relates to a Tungsten Carbide coated coating doctor blade used for the coating of paper and board. The doctor blade comprises:
a substrate in the form of a strip,
a wear resistant top deposit covering the working edge of the blade intended for being in contact with the moving web,
the wear resistant top deposit comprises WC particles in a Ni-based matrix,
the Ni-based matrix comprises in weight %:
Cr 1-32
Mo 1-32

optional components, balance Ni and impurities.

Description

TECHNICAL FIELD

[0001] The invention relates to a doctor blade for applying and metering the coating to a paper or board. The doctor blade is made from a hardened steel strip that has its wear resistance enhanced by application of a thin Tungsten Carbide coating, using a Thermal Spray coating technique.

BACKGROUND OF THE INVENTION

[0002] In the paper industry, coating doctor blades are used for the manufacture of paper and board. Coating doctor blades may be made of different materials such as Carbon steel strip, tool steel strip, composites and polymers. In addition, different types of coating may be applied by Thermal Spray coating in order to reinforce the working edge of the coating doctor blade such as described in US20130174779 A1.

[0003] In many paper mill operating conditions, Tungsten Carbide Coated coating doctor blades can generally operate for long service lives with only a relatively slight progressive reduction in the quality of the coating produced. This makes them the optimum product for many paper coating applications.

[0004] However, even Tungsten Carbide Coated coating doctor blades can give rise to a paper making defect known as microlines. These micro lines are lines that appear on the paper where the localised thickness of the coating has not been effectively metered and is thicker than the coating on the bulk of the paper.

[0005] In addition, it has been found that the working edge of Tungsten Carbide coated coating doctor blades often progressively suffers from deterioration of the blade edge, which often results in gradual deterioration in key paper quality measures such as roughness expressed by the PPS-value.

DISCLOSURE OF THE INVENTION

[0006] An object of the present invention is to provide a method of modifying the standard Thermal Spray coating on such a Tungsten Carbide coated coating doctor blade to produce a coating that minimises the occurrence of a major paper defect, which often limits the application of existing Tungsten Carbide coated blades. A further object is to provide a Tungsten Carbide coated coating doctor blade that has an improved ability to maintain the quality of the paper coating for longer service time, increasing the opportunity to extend the life of the blade.

[0007] The foregoing objects, as well as additional advantages, are achieved to a significant measure by modifying the composition of the matrix of the Tungsten Carbide coating that is applied to the coating doctor blade, as set out in the claims.

[0008] The invention is defined in the claims.

DETAILED DESCRIPTION

[0009] The present inventor has surprisingly found that the defects mentioned above are linked to the exposure of the WC top coat to the coating pigments.

[0010] In some situations, such as high solids/viscosity content in the pigment, in combination with high machine speeds and efficient drying, the primarily CoCr-matrix, that is usually used as a binder for the Tungsten Carbides, can suffer from erosion-corrosion from exposure to the coating pigment, typically Calcium Carbonate. The present inventor has found that this progressive erosion-corrosion of the matrix allows the Tungsten Carbides to become detached and leave behind 'craters' that negatively affect the ability of the blade edge to meter the coating consistently, leading to the above mentioned paper making defects.

[0011] The present inventor has found that the elimination of the conditions that cause Tungsten Carbide coated coating doctor blades to produce defective paper due to microline defects or high PPS-values, is therefore related to the management of the susceptibility of the matrix of the Tungsten Carbide coating to significant erosion-corrosion, when exposed to Calcium Carbonate based coating pigments.

[0012] Consequently, the concept of modifying the matrix of the Tungsten Carbide coating is the means by which a Tungsten Carbide coated coating doctor blade can be made that does not cause defects in the same way that they are formed with existing Tungsten Carbide coated coating doctor blades in some operating conditions.

[0013] If a Tungsten Carbide coating is modified to minimise the erosion-corrosion susceptibility of the Tungsten Carbide coating, it is expected that the deterioration in these key paper quality measures will be more gradual and this may well give the opportunity to extend the working life of the blades on some paper/board products. After intense research the present inventor surprisingly found that a Ni-based binder alloyed with Cr and Mo have improved resistance against erosion-corrosion than conventional CoCr binders and therefore result in improved paper quality as well as increased life of the doctor blade. The Ni-based binder preferably comprises at least 30, 40, 50 or 55 wt. % Ni.

[0014] A traditional Tungsten Carbide Thermal Spray coating typically has the following approximate composition, when measured by SEM/EDS:

Element	Semi- Quantitative Composition of Coating measured (% by weight)
Co	11
Cr	3
C	7
W	78
Ni	-
Fe	-
Mo	-

[0015] A Tungsten Carbide Thermal Spray coating with a modified binder to minimise susceptibility to erosion-corrosion according to the invention might have the following approximate composition, when measured by SEM/EDS:

Element	Semi- Quantitative Composition of Coating (% by weight)
Co	0.5
Cr	3
C	9
W	70
Ni	12
Fe	2
Mo	3

[0016] It should be noted that the SEM/EDS analysis is semi-quantitative and involves a certain inaccuracy, in particular for the light elements. The measured analysis for carbon is thus inaccurate and given for completeness only.

[0017] The Tungsten Carbide Thermal Spray coating with a modified binder can be applied by any one of the following Thermal spray coating techniques: Plasma, HVOF or HVAF, or any combination of one or more of them.

[0018] The content of the Ni-based binder in the top deposit can be regulated according to the needs in different applications. However, the amount of binder is is generally 10-30 wt. %, preferably 12-25 wt. %. For many applications it is suitable to have a binder content of about 18 wt. % and about 82 wt.% WC in the top coat, because this results is a dense coating having a good wear resistance and a superior resistance against cavitation erosion.

[0019] The hardness of the top coat can be influenced by varying the composition and amount of the binder phase. A hardness of 1100-1400 HV 0.3 (test force 2.942 N) is suitable for most applications. The hardness of a top coat with 82 wt. % WC can be in the range of 1200-1300 HV 0.3.

EXAMPLES

Pilot Plant Trial

[0020] A trial was undertaken on a Pilot coating line. The blades compared in the trials were a reference blade with a traditional Tungsten Carbide coating that had been used for 28 hours in a full scale paper mill coating line and a blade of this invention that had been run for a similar time in the same paper mill. These two blades were run in the pilot coating line using the following conditions:

Parameter	Value
Base Paper Quality	Duplex
Base Paper Weight	256 g/m2
Coat Weight	15 g/m2
Machine speed	800 m/min
Tip Angle	22°
Pigment	80% Calcium Carbonate:20% Kaolin Clay
Pigment solids content	68%
Pigment Viscosity (CPS)	1730

[0021] The semi-quantitative composition of the two blades were as shown in the above tables. The inventive coating was formed by using Amperit® 529 powder from H.C. Starck. Very shortly after starting to run the machine with the reference blade severe microlines were apparent on the produced paper and in the judgement of the operators the paper produced was far from being of an acceptable quality. The blade of this invention was then placed in the machine in the place of the reference blade and the machine was run using identical conditions. There was an immediate improvement in the paper quality with very few fine microlines being present.

[0022] Subsequent examination of the wear surfaces of the blades revealed a greater presence of large pits in the reference blade than were present in the blade of this invention. Fig. 1 discloses the wear surface of the reference blade showing large black areas corresponding to the craters. Fig. 2 reveals a very much reduced attack on the inventive blade. This observation supports the theory that the microlines originate from large pits that are thought to be displaced carbides from erosion-corrosion of the matrix.

Paper Mill trials

[0023] A series of six blades of this invention with the same composition as in the pilot plant trials were trialled at a paper mill that routinely uses traditional Tungsten Carbide coated coating doctor blades. The trial parameters were as follows:

Parameter	Value
Base Paper Weight	229 g/m2
Coat Weight	14 g/m2
Machine speed	368 m/min
Tip Angle	5°
Pigment	80% Calcium Carbonate:20% Kaolin Clay
Pigment solids content	69%
Pigment Viscosity (CPS)	600

[0024] The conditions of the trial were not those that were likely to create a situation where severe microlines were present on the paper. Consequently, the main aim of the trial was to establish the relative lifetime of the blades of this invention with the same configuration as the traditional Tungsten Carbide coated coating doctor blades that the paper mill historically use. The trial blades of this invention were used for a similar period as the previous blades, confirming the durability of these blades. One major observation from the paper mill was that the PPS-values were more stable throughout the period of operation than is common on traditional Tungsten Carbide coated coating doctor blades.

[0025] Subsequent examination of the wear surfaces on the two types of blade used in this trial revealed a similar observation to the ones shown previously, with a greater degree of pitting present on the traditional Tungsten Carbide coated coating doctor blades than was present on the blades of this invention. This supports the observation of the consistency of the PPS-values as is known that PPS-values are related to the roughness of the blade edge.

Claims

1. A doctor blade for the manufacture of paper or board comprising:

a substrate in the form of a strip,

a wear resistant top deposit covering the working edge of the blade intended for being in contact with the moving web,

the wear resistant top deposit comprises WC particles in a Ni-based matrix,

the Ni-based matrix comprises in weight %:

Cr	1-32
Mo optionally	1-32
Fe	≤ 20
Co	≤ 20
Si	≤ 3
Mn	≤ 5
W	≤ 10
V	≤ 2
Ti	≤ 4
Zr	≤ 4
Nb	≤ 6
C	≤ 0.8
B	≤ 0.6
Al	≤ 1
Cu	≤ 5
Ni	balance apart from impurities.

2. A doctor blade according to claim 1, wherein the amount of Ni-based binder in the top deposit is 10-30 wt. %, preferably 12-25 wt. %, most preferably 15-20 wt. %.

3. A doctor blade according to claim 1 or 2, wherein the composition of the Ni-based binder fulfils at least one of the following requirements:

Cr	10-25
Mo	5-25
Fe	1-15
Co	≤ 5
Si	≤ 1
Mn	≤ 1
W	≤ 5
V	≤ 1
Ti	≤ 1
Zr	≤ 4
Nb	≤ 1
Al	≤ 0.5
Cu	≤ 1
C	≤ 0.3
B	≤ 0.01
Ni	≥ 30

4. A doctor blade according to any of the preceding claims, wherein the composition of the Ni-based binder fulfils at least one of the following requirements:

Cr	20-25
Mo	10-20
Fe	1-5
Co	≤ 5
Si	≤ 0.1
Mn	≤ 0.5
W	≤ 4
V	≤ 0.4
Ti	≤ 0.5
Zr	≤ 0.5
Nb	≤ 0.5
Al	≤ 0.1
Cu	≤ 0.3
C	≤ 0.1
B	≤ 0.001
Ni	≥ 40

5. A doctor blade according to any of the preceding claims, wherein the composition of the Ni-based binder fulfils at least one of the following requirements:

Cr	20-25
Mo	10-20
Fe	2-4
Co	≤ 4
V	≤ 0.35
Ni	≥ 50

6. A doctor blade according to any of the preceding claims, wherein the composition of the Ni-based binder fulfils the following requirements:

Cr	21-24
Mo	12-14
Fe	2-4
Ni	≥ 55

7. A doctor blade according to any of the preceding claims, wherein the top coat has a hardness of 1100-1400 HV 0.3.

8. A doctor blade according to any of the preceding claims, wherein the top coat has a hardness of 1200-1300 HV 0.3.

9. A doctor blade according to any of the preceding claims, wherein the wear resistant top deposit comprising WC particles in a Ni-based matrix has the following composition in wt.%:

C	4.5-6.2
Ni	8.0-14.5
Mo	1.0-4.5
Cr	1.0-4.5
Fe	≤ 2.5
Co	≤ 1.8
V	≤ 2
Ti	≤ 4
Zr	≤ 4
Nb	≤ 6
B	≤ 0.6
Al	≤ 1
Cu	≤ 5
W and impurities balance.

10. A doctor blade according to any of the preceding claims, wherein the wear resistant top deposit comprising WC particles in a Ni-based matrix has the following composition in wt.%:

C	5.0-5.6
Ni	10.0-12.5
Mo	2.0-3.5
Cr	2.0-3.5
Fe	0.8-1.5
Co	0.3-0.6
Si	≤ 0.1
Mn	≤ 0.5
V	≤ 0.4
Ti	≤ 0.5
Zr	≤ 0.5
Nb	≤ 0.5
Al	≤ 0.1
Cu	≤ 0.3
B	≤ 0.001
W and impurities balance.

11. A doctor blade according to any of the preceding claims, wherein the wear resistant top deposit comprising WC particles in a Ni-based matrix has the following composition in wt.%:

C	5.0-5.6
Ni	10.0-12.5
Mo	2.0-3.5
Cr	2.0-3.5
Fe	0.8-1.5
Co	0.3-0.6
W and impurities balance.

Drawing