|Title of Invention||
A COBALT-BASED ALLOY FOR THE COATING OF ORGANS SUBJECT TO EROSION BY LIQUID
|Abstract||The present invention relates to a cobalt-based alloy for the coating of organs subject to erosion by liquid comprising chromium 28-32% by weight,tungsten 6-8% by weight, silicon 0.1-2% by weight, carbon 1.2-1.7% by weight, nickel 3-6% by weight, molybdenum 1-3%, cobalt the complement to 100%. The invention also relates to an application method of the alloy on organs subject to erosion by liquid, in particular vapour turbine blades, to reduce the metal erosion rate following impact with liquids.|
|Full Text||The present invention relates to a cobalt-based alloy for
the coating of organs subject to erosion by liquid.
In particular, the present invention relates to a cobalt-
based alloy in powder form suitable for the coating of
organs subject to erosion by liquid such as the blades of
vapour turbines and the relative application method to
increase its resistance to erosion following impact with
It is known that in vapour turbines the condensation
pressure values must be as low as possible in order to
obtain the highest outlet power in simple and combined
Under these operating conditions, the low pressure rotor
blades are subjected to different chemical and physical
stress and therefore undergo erosion processes due both
to the presence of numerous water particles in the vapour
flow and also to the high peak rates of the blades.
The erosion phenomena of vapour turbine components, which
occur as a result of repeated impact with liquids under
prolonged operating conditions, have already been the
subject of studies and are documented in Wear, M. Lesser
In order to avoid the drawbacks due to these erosion phe-
nomena, attempts were made to solve the problem, from the
design point of view, by increasing the axial spacing be-
tween the stator and rotor or by extracting the humidity
between the rows of blades through holes or air gaps
situated on the blades of the stator.
These remediations did not prove to be particularly suit-
able for solving the problem, as they cause a reduction
in the performances of the turbine.
Attempts were then made to prolong the average operating
life of the turbine blades, by studying new coating mate-
rials which are capable of reducing the erosion rate of
the metals caused by impact liquid separation (F.J. Hey-
mann, ASM Handbook Vol. 18, page 221).
The improvements in this field have so far been reached
by resorting to specific treatment on the metal surface
of the blades, such as induction or local flame harden-
ing, by means of stellite plate brazing or with tool
steels, or by means of hard coatings applied by welding.
In order to evaluate the resistance to erosion, the coat-
ing materials of the known art have been subdivided, ap-
proximately, into two groups, that of carbides and that
of metallic materials among which Stellite 6, according
to what is already described in literature for example in
the publication "Erosion-resistant Coating for Low-
Pressure Steam Turbine Blades, Euromat '99".
Ionic nitriding with PVD coating using titanium nitride
and chromium or zirconium nitride were selected for the
The blades subjected to ionic nitriding treatment fol-
lowed by two subsequent PVD coatings were made up of a
layer of titanium nitride followed by a coating of zirco-
nium nitride or chromium nitride.
All the PVD coatings had a thickness of about 3-4 µm. The
coating tests showed a coating discontinuity of the mod-
els and the behaviour was considered unsatisfactory.
A SEM test revealed that the PVD coating was not substan-
tially capable of opposing impact erosion whereas the ni-
tride layer was subject to lesions as a result of micro-
fractures together with the foil nitrides present in the
Blades with metallic coatings were then tested with HVOF
(Triballoy 800) .
The performances of the Triballoy 800 alloy, as coating
material against erosion from liquids, proved to be in-
From the indications obtained in the tests effected, it
can in fact be held that these metal alloy coatings are
not even as effective, in limiting erosion phenomena, as
uncoated surfaces of the base material.
This behaviour on the part of the Triballoy 800 alloy is
verified both by the results of the adhesion tests (all
the coatings tested did not pass this test) and also
through SEM micrographic observation which revealed the
presence of numerous micro-fractures in the coating
layer. The microstructure of these coatings, in fact, has
a high oxide content and a marked porosity which make it
unsuitable for resisting erosion by liquids.
Blades with metallic coatings (Stellite 6) were then
tested with HVOF.
Although stellite alloys are known as being a material
suitable for coating, they show all their limits when ap-
plied by means of HVOF. Micrographic analysis, in fact,
demonstrates that low content particles are also envel-
oped in a film of oxide.
This fact is also confirmed by the surface morphology re-
vealed by means of SEM, which shows a detachment or un-
gluing of the material specifically along these parti-
Blades treated with coatings with HVOF and SD-Gun TM car-
bides were then tested.
The results obtained with these types of coatings are in
some cases comparable to or better than those obtained
with the hardened base material (WC-10Co-4CrSD-Gun TM and
88 WC-12CO HVOF).
The cases in which an unsatisfactory behaviour is veri-
fied can be explained by the reduced adhesion of the
coating and through the known intrinsic fragility (due to
the presence of chromium carbides).
Vice versa, the coatings of the known art which provide
better results are those made of tungsten carbides with a
cobalt or chromium-cobalt matrix, depending on the coat-
ing process used.
Coatings which have a good resistance to erosion are
characterized by a detachment of the material on a small
portion of the sample whereas this phenomenon is extended
to a much larger surface of the materials whose resis-
tance properties are considered unsatisfactory.
This different behaviour can be explained by considering
the surface morphology.
When the layer of surface coating starts losing its con-
formation following the loss of material, the liq-
uid/solid interaction is particularly complex. In this
situation, the impulse or impact pressures which trigger
the erosion phenomenon, are greatly influenced by the
point in which there is initial contact with the drops
which fall on a crest (slope) , developing lower local
pressures with respect to the drops which fall into a
In the case of base materials, the low resistance ef-
fected by the surface makes the removal of the material
almost completely uniform along the whole area involved
in the test.
The unsatisfactory behaviour of most of the coatings of
the known art can be explained by the reduced adhesion of
the coating on the metallic substrate and the well known
intrinsic fragility (due to the presence of chromium car-
Vice versa, the coatings of the art which provide im-
proved results are those consisting of tungsten carbides
with a cobalt, chromium-cobalt matrix, depending on the
use of the coating process.
In general, the performances of the coatings with HVOF
improve with an increase in the content of tungsten car-
bide. The micrographic morphology of the 88WC-12Co coat-
ing is, in fact, more homogeneous with respect to that of
83WC-17Co. On the other hand, the difference in perform-
ance of the same material (WC10Co-4Cr) , applied by means
of SD-GunTM or HVOF is quite marked. The results of the
former are encouraging, whereas those of the latter are
This confirms that at present the spraying process has a
significant importance in obtaining certain performances
of the coating.
The thermal treatment of the known art for increasing the
hardness, however, has as yet shown a reduced increase in
resistance to erosion due to an excessive fragility.
It has been verified that in the case of coatings by
means of thermal spraying, an important parameter for
evaluating the resistance to erosion by liquids is the
adhesion resistance. A low value immediately suggests
that the coating is not appropriate. An additional requi-
site for resistance to erosion is the good quality of the
microstructure of the coating.
At the moment, the necessity is consequently felt for
having new types of coating or treatment for gas turbine
components which are capable of effectively reducing the
metallic erosion rate due to separation by impact with
One of the general objectives of the present invention
therefore consists in providing an alloy for the coating
of vapour turbine components which is highly resistant to
metallic erosion phenomena as a result of impact with
A further objective of the invention consists in provid-
ing a method for the treatment of the surfaces of metal-
lie organs subject to erosion, in particular vapour tur-
bine blades, which effectively increases the adhesion re-
sistance of the coating applied.
The last but not least important objective consists in
providing an alloy and a method for the coating of vapour
turbine blades which is simple to produce and does not
involve high production costs.
It has now been surprisingly found that it is possible to
obtain a coating for vapour turbine components subject to
erosion, by applying on the metallic surfaces of said
components a cobalt-based alloy, having a composition
which is particularly rich in tungsten and suitably se-
The alloy according to the invention is of the stellite
or Haynes alloy type, referring to a material which be-
longs to the group of non-ferrous hard alloys based on
cobalt, chromium and tungsten, resistant to corrosion and
In particular, the applicant has now identified an alloy
composition which is particularly suitable for the coat-
ing of vapour turbine components, comprising:
nickel 3-6% by weight
molybdenum 1-3% by weight
cobalt the complement to 100%.
According to an embodiment, the alloy of the invention
can also comprise iron 0-1% by weight, manganese 0-1% by
weight and other elements in a quantity ranging from 0 to
0.5% by weight.
The alloy of the invention has an optimized selected
chemical composition for enhancing the characteristics of
anti-erosion by liquid when subjected to erosion phenom-
ena such as, for example vapour turbine components.
It has been verified that the alloy compositions of the
invention advantageously supplied in powder form, allow
the production of a layer of coating on vapour turbine
components which is highly resistant to mechanical stress
caused by impact with liquid particles.
In particular, from specific tests it has been observed
that the use of the alloy of the invention allows the
production of coatings having a higher resistance to ero-
sion from impact with liquids by an order of magnitude
(for example 2,100,000 of impacts against 180,000 with
traditional hardening materials) with respect to the re-
sistance values of other materials used in the known art.
It has also been observed that the application of the al-
loy of the invention to the surface of turbine blades,
causes an unexpectedly higher resistance to erosion with
respect to the use of stellite compositions of the known
The alloy of the invention has a selected quantity of
various elements which determines the resistance proper-
ties to corrosion by liquid when applied to the surfaces
of turbine components.
In particular, the carbon content in the composition is
optimized and balanced to form carbides having a suitable
stoichiometry, the chromium content and high tungsten and
molybdenum contents are selected to obtain an improved
reinforcement for a solid solution and to obtain maximum
precipitation values of carbides having a suitable
stoichiometry. Furthermore, the alloy composition of the
invention has a high nickel content, preferably from 3.5
to 5.5%, so as to increase the ductility of the alloy and
resistance of the coating layer.
According to another aspect of the invention, a method is
provided for the treatment of organs subject to erosion,
in particular vapour turbine components, comprising the
application of a cobalt-based alloy previously described
to the surface of said organ or turbine component, to
form an anti-erosion coating layer.
According to a preferred embodiment, the application of
the coating to organs subject to erosion, such as for ex-
ample blades, rotor, stator, vapour turbine plates, is
effected by means of laser plating, advantageously using
a laser apparatus with CO2 or an Nd-Yag laser.
An alloy of the invention is in fact suitable for use in
laser plating which comprises one or more passages on the
surfaces of metallic organs subject to erosion so as to
form one or more layers of coating.
According to an embodiment, the metallic material to be
subjected to the anti-erosion treatment of the invention
can be previously heated and then plated by means of la-
The differences in the behaviour of a turbine component
treated according to the method of the invention and
metal components either non-plated or plated with prod-
ucts of the known art are evident from the enclosed draw-
ing in which:
The figure illustrates a graph relating to comparative
liquid erosion tests on 4 metal samples.
In particular, the enclosed figure illustrates a graph
which indicates in abscissa the number of impacts and in
ordinate the volume loss following impact with liquid
The graph summarizes the results of erosion by liquid
drops sprayed through a 0.13 mm nozzle on four test sam-
ples made of martensite stainless steel, the same mate-
rial but with martempering treatment (MT), integral stel-
lite and stainless steel coated with a layer produced by
laser plating of the alloy of the invention, according to
The graph indicates the increased resistance to erosion
by liquid drops of the sample treated according to the
invention with respect to the samples of the known art.
Once the coating material, according to the present in-
vention, has been applied to metallic surfaces of vapour
turbine components, it has a high adhesion resistance.
The high resistance properties of the coating produced
with the method of the invention are also justified by
its microstructural morphology.
It has in fact been observed that the structure of the
coating produced with the laser technique is extremely
thin and the removal of the material, which essentially
takes place by means of cracking along the carbide bonds,
is reduced even after prolonged periods of turbine activ-
Furthermore, the coating alloy applied according to the
method of the invention only tends to become detached,
following prolonged and repeated stress, on a reduced
portion of the sample whereas this phenomenon involves a
much wider surface area when the coating is made with ma-
terials of the known art.
The application of the laser technology consequently
makes it possible to produce coatings with a high resis-
tance to erosion by separation due to impact with liq-
uids, reducing alteration of the base material to the
minimum. The use of the laser technology also allows
stress reducing treatment to be effected at temperatures
slightly lower than the recovery temperature, thus avoid-
ing any possible negative effects on the tensile
The following examples are provided for the sole purpose
of illustrating the present invention and should in no
way be considered as limiting the protection scope ac-
cording to the enclosed claims.
A composition was used, in powder form for the coating of
mechanical vapour turbine components having the following
The powder was applied to stainless steel turbine blades
by means of YAG laser plating (laser cladding) forming a
layer having a thickness equal to about 1.2 mm.
The following Table indicates various formulations of
compositions in powder form according to the present in-
1. A cobalt-based alloy for the coating of organs subject to erosion by
chromium 28 by weight
tungsten 6.1% by weight
silicon 0.2% by weight
carbon 1.3% by weight
nickel 3.2% by weight
molybdenum 1.1.% by weight
iron 0.01% by weight
manganese 0.01% by weight
cobalt the complement
to 100% by weight
2. A cobalt-based alloy for the coating of organs subject to erosion by
3. A cobalt-based alloy for the coating of organs subject to erosion by
4. A cobalt-based alloy for the coating of organs subject to erosion by
Fe Mn Co 53.4%
5. An organ or end-product subject to erosion by liquids, wherein it
comprises a surface coating to prevent erosion from liquids comprising an
alloy coating according o claim 1.
6. The organ or end-product as claimed in claim 5, wherein it is a
component of a vapour turbine.
7. The organ or end-product as claimed in claim 6, wherein said
component is a blade of a gas turbine.
8. The organ as claimed in claim 5, wherein said surface coating has a
thickness ranging from 0.1 to 5 mm.
" A COBALT-BASED ALLOY FOR THE COATING OF
ORGANS SUBJECT TO EROSION BY LIQUID "
The present invention relates to a cobalt-based alloy for the coating of organs
subject to erosion by liquid comprising chromium 28-32% by weight,tungsten
6-8% by weight, silicon 0.1-2% by weight, carbon 1.2-1.7% by weight, nickel
3-6% by weight, molybdenum 1-3%, cobalt the complement to 100%. The
invention also relates to an application method of the alloy on organs subject
to erosion by liquid, in particular vapour turbine blades, to reduce the metal
erosion rate following impact with liquids.
|Indian Patent Application Number||498/KOL/2003|
|PG Journal Number||30/2012|
|Date of Filing||25-Sep-2003|
|Name of Patentee||NUOVO PIGNONE HOLDING SPA|
|Applicant Address||VIA FELICE MATTEUCCI, 2, FLORENCE, ITALY 1-50127 ITALY|
|PCT International Classification Number||C22C 24/08|
|PCT International Application Number||N/A|
|PCT International Filing date|