A YANKEE CYLINDER AND A METHOD FOR COATING A YANKEE CYLINDER

CYLINDRE DE MACHINE YANKEE ET METHODE DE DEPOSITION D'UN REVETEMENT SUR CE CYLINDRE

English Abstract

The invention relates to a Yankee cylinder for a paper-making
machine, in which a coating which is a mixture of a
metal powder and a carbide or nitride and withstands well
the wearing effect of the doctor blade and other corrosive
and thermal stresses produced in paper making is formed on
top of the cylinder mantle. The invention also relates to
methods for coating a Yankee cylinder with such a coating
by using a detonation, plasma or supersonic method.

Claims

CLAIMS
1. A Yankee cylinder for a paper-making machine, having ends and
axle pins, as well as a cylinder mantle which is made of metal and primarily
gives the cylinder its mechanical strength, characterized in that a coating is
formed on top of the cylinder mantle and constitutes the surface layer
thereof,
the coating being obtained from a mixture containing 60 to 94 percent by
weight of a carbide or nitride of tungsten, titanium, vanadium or boron and 6
to 40 percent by weight of a powder of cobalt, nickel or iron by spraying the
mixture by a denotation, plasma or supersonic method.
2. A Yankee cylinder as claimed in claim 1 wherein the thickness of
the coating constituting the surface layer is less than 0.5 mm, preferably in
the
range of from 0.2 to 0.3 mm.
3. A method for coating a Yankee cylinder, characterized in that on
top of the cylinder mantle or on top of a metal coating first sprayed onto the
cylinder mantle and forming an intermediate layer, a coating is applied by
spraying a mixture containing 60 to 94 percent by weight of a carbide or
nitride of tungsten, titanium, vanadium or boron and 6 to 40 percent by weight
of a cobalt, nickel or iron powder by a detonation, plasma or supersonic
method the coating constituting the surface layer of the cylinder mantle and
being resistant to the wearing effect of the doctor blade and to other
corrosive
and thermal stresses produced in paper making.
4. A method as claimed in claim 3 wherein the particle size of the
coating material mixture is in the range of from 5 to 70 µm.
5. A method as claimed in claim 3 or claim 4 wherein the cylinder
mantle is ground to precisely the correct shape and dimension before the
spraying of the coating layer.
6. A method as claimed in any one of claims 3 to 5, wherein the
coating material used in the intermediate layer is a martensitic stainless
steel,
a NICrMoAl metal alloy or a Mo-based metal alloy.

7. A method as claimed in any one of claims 3 to 5 or claim 6,
wherein the coating used in the intermediate layer is produced by thermal
spraying.
8. A method as claimed in any one of claims 3 to 4, claim 6 or claim
7, wherein the coating used in the intermediate layer is first ground
precisely
to the correct shape and dimension and is then roughened either by grinding
or grain blasting before the spraying of the surface layer thereon.
9. A method as claimed in any one of claims 3 to 8 wherein the
surface layer is ground by using either a diamond stone or diamond bands and
is further superfinished, when necessary, by using a diamond-containing
liquid.

Description

1
A Yankee cylinder and a method for coating a Yankee
cylinder
The invention relates to a Yankee cylinder for use in a
paper-making machine and to coating methods used in its
manufacture and repair.
The Yankee cylinders used in the paper-making process are
in the main manufactured by casting from cast-iron. Cylin-
der mantles have also been assembled from steel sheets by
welding. The largest cylinders have diameters.of 6-7 m, and
their length may be 5-6 m. The wall thickness of a new cyl-
finder is 40-60 mm.
During paper-making, a so-called doctor blade, which de-
taches the paper from the cylinder surface and crinkles it,
scrapes against the cylinder surface.
The doctor blade wears away the cylinder surface and.espe-
cially the edge parts of the cylinder. For this reason the
cylinder must occasionally be ground to give the cylinder
surface the correct shape and a suitable surface quality.
When a cylinder has been ground many times, its wall thick-
ness will reach the limit set by the pressure vessel autho-
rities. The thinner the wall thickness, the more the oper-
ating pressure of the cylinder must be reduced. Reducing
the operating pressure will mean a slowing down of the pro-
duction rate. At this stage the Yankee cylinders are usual-
ly repaired by coating.
Another reason for the coating is that the mantle surface
may have pores which produce holes in the paper. If there
are few holes, they can be plugged, but small cavities in

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large numbers will necessitate the coating of the mantle.
Yankee cylinders have been coated for nearly 20 years. The
coating materials used have included:
- martensitic stainless steel (AISI 420), arc extruded
- martensitic stainless steel (AISI 431), plasma extruded
- CrNiMoAl alloy, plasma extruded
- Mo-based alloys, plasma extruded
Arc-extruded coatings have in certain cases been a success,
but in the manufacture of new paper grades and as the
speeds of paper-making machines increase, they do not meet
the requirements.
Plasma-extruded coatings are considerably more resistant to
corrosion, and also their resistance to wear is better than
that of arc-extruded coatings. However, both CrNiMoAl al-
loys and Mo-based alloys have the drawback that they wear
away too rapidly when very thin papers are being made. The
reason is the wearing effect of the doctor blade and the
fact that flint particles from the paper stock adhere to
the doctor blade during disturbances, and they "lathe" the
coating.
The coatings currently known axe 0.8-2.0 mm thick, so that
they can be ground often enough before re-coating. A thick
coating decreases the thermal conductivity of the cylinder
wall, slowing down production and increasing the energy
costs.
Furthermore, the present-day coatings have to be ground at
approximately one-year intervals, some even at 4-6 month
intervals, as the quality of the surface deteriorates. An
intermediate grinding will cause a stoppage of 5-8 days,

3
resulting in extensive production losses. Repairing the
cylinder.by re-coating will for its part cause a stoppage
of 12-25 days.
The most extensive production losses, in addition to the
above, occur when a paper-making machine cannot be used for
making those paper grades for which the best prices.can be
obtained.
The primary object of the present invention is to provide a
Yankee cylinder with such a mantle surface layer that the
doctor blades wear it away very little, with an improved
thermal conductivity of the mantle, and with longer inter-
vals between its mantle maintenance sequences.
It is also an object of the invention to provide coating
methods to be used in the manufacture and repairs of a
Yankee cylinder; by using the methods a cylinder mantle
with the above-mentioned properties is obtained.
The present invention thus relates to a Yankee cylinder for
use in a paper-making machine, the cylinder having ends and
axle pins, as well as a cylinder mantle which is made of
metal and primarily gives the cylinder its mechanical
strength, the Yankee cylinder being characterized in that
on top of the cylinder mant~.e there is formed a coating w
which constitutes the surface layer and which is a mixture
of metal powder and carbide or nitride and well withstands
the wearing effect of the doctor blade as well as other
corrosive and thermal stresses occurring in paper making.
The surface layer of a Yankee cylinder according to the
invention is primarily made of a mixture of a carbide or
nitride of tungsten, titanium, vanadium or boron and a
powder of cobalt, nickel or iron. The surface layer prefer-

4
ably contains the above-mentioned carbide or nitride 60-94
weight percent and the above-mentioned metal 6-40 weight
percent. The thickness of the surface layer is in the main
less than 0.5 mm and preferably within 0.2-0.3 mm.
According to the invention, a Yankee cylinder can be coated
,so that, on top of the cylinder mantle directly or on top
of a metallic coating which has first been extruded on the
cylinder mantle and constitutes an intermediate layer,
there is extruded by a detonation, plasma or supersonic
method a coating which forms the surface~layer and is a
mixture of a metal powder and a carbide or nitride and well
withstands the wearing effect of the doctor blade and other
corrosive and thermal stresses occurring in paper making.
The particle size_of the coating material mixture forming
the surface layer is preferably 5-70 uxn.
The mantle coating is made directly on the surface of a
mantle made of._cast-iron or steel by extruding the coating
by a detonation, plasma or supersonic method, the. surface
having first been ground precisely to the correct dimension
and shape.
According to another embodiment of the method according to
the invention, the mantle surface of a Yankee cylinder is
first coated with a martensitic stainless steel or a
NiCrMoAl alloy or a Mo-based alloy by thermal extrusion,
arc extrusion or plasma extrusion, and is then ground pre-
cisely to the correct shape and dimension. ThEreafter the
surface is pre-treated by grinding or grain blasting to
roughen it, and then coated by extruding, by a detonation,
plasma or supersonic method, a coating which contains a
carbide or nitride and a metal powder.

5
After coating according to either the first or the second
embodiment of the invention, the mantle surface is ground.
Since the wear-resistant components of the coating are car-
bides or. nitrides of tungsten, titanium, vanadium or boron,
the surface has to be ground with a diamond. Diamond grind-
ing can be commenced as stone grinding, but the final
grinding must be carried out using a diamond band in order
to eliminate vibration. Furthermore, the surface quality
can be superfinished using a diamond-containing liquid.
The advantages of the Yankee cylinder and coating method
according to the invention over the prior art. are the fol-
lowing:
1.0 Better resistance to wear
The carbides and nitrides present in the coating are very
hard (2400-4500 HV), and they have been-chosen so that ad-
hesion between the doctor blade made of annealed steel and
the carbides and nitrides is very small.
Thus the wearing away of the surface of the Yankee cylinder
mantle is slight even in harsh operating conditions.
Furthermore, during disturbances in operation the hard -
flint particles brought to the doctor blade in the paper
stock cannot wear the coating since the hardness of the
flint particles (approx. 1500 HV) is considerably less than
that of the coating.
The hardness of prior-art coatings is only 350-700 HV, so
that flint particles will easily "lathe" grooves into the
coating.

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2.0 Possibility of making new paper grades .
When new, very thin paper grades are being made, the doctor
blade often comes into direct contact with the mantle sur-
face, since usually the so-called coating layer between the
mantle surface and the doctor blade may become removed. If
the mantle surface cannot withstand such wear, it becomes
'scratched and causes problems in paper-making. With the
new, wear-resistant coating, this problem does not appear.
The corrosion-resistance of the new Coating is also very
good; this enables special-purpose papers to be made in
acid solutions in which the pH may be 3-5.
3.0 Energy costs will decrease
The thickness of the new coating is only 0.2-0.3 mm. The
thickness of previously used coatings is 0.8-2.0 mm.
A thinner coating conducts heat better and thus reduces the
energy required in the drying of paper.
4.0 The costs of stoppages and maintenance will decrease
The coatings currently in use have to be ground on average
at one-year intervals. When thin paper grades are manu-
factured, the interval between grindings may be 4-6 months.
With the new coating, an average grinding interval of two
years Can be achieved. Since for a large paper-making
machine one grinding will cause a stoppage of approx. 5-8
days, resulting in production losses of 5-8 million FIM,
the savings due to the longer maintenance sequence will be
significant.

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5.0 Time required for maintenance will shorten
The coatings currently in use are maintained by grinding
the old coating off either in part or totally down to the
basic material of the mantle. Thereafter the coating is
renewed from the basic material up, and is ground.
By the new technology developed, the coating can be made
over the old coating, once the old coating has first been
pre-ground clean. The de~reloped technology 'thus shortens by
up to several days the time required for the maintenance.