U.S. patent application number 11/250514 was filed with the patent office on 2006-03-16 for method for treating organs subject to erosion by liquids and anti-erosion coating alloy.
This patent application is currently assigned to Nuovo Pignone Holding S.P.A.. Invention is credited to Massimo Giannozzi.
Application Number | 20060057305 11/250514 |
Document ID | / |
Family ID | 31972215 |
Filed Date | 2006-03-16 |
United States Patent
Application |
20060057305 |
Kind Code |
A1 |
Giannozzi; Massimo |
March 16, 2006 |
Method for treating organs subject to erosion by liquids and
anti-erosion coating alloy
Abstract
The present invention relates to a method for treating organs
subject to erosion by liquids, in particular vapour turbine
components, which contemplates laser plating with a cobalt-based
alloy comprising chromium from 28 to 32% by weight; tungsten from 5
to 7% by weight; silicon from 0.1 to 2% by weight; carbon from 1.2
to 1.7% by weight; nickel from 0.5 to 3% by weight; iron from 0.01
to 1% by weight; manganese from 0.01 to 1% by weight; molybdenum
from 0.2 to 1% by weight; possible impurities or other elements
from 0 to 0.5% by weight and cobalt the complement to 100%.
Inventors: |
Giannozzi; Massimo;
(Florence, IT) |
Correspondence
Address: |
NIXON & VANDERHYE P.C.
901 NORTH GLEBE ROAD, 11TH FLOOR
ARLINGTON
VA
22203
US
|
Assignee: |
Nuovo Pignone Holding
S.P.A.
Florence
IT
|
Family ID: |
31972215 |
Appl. No.: |
11/250514 |
Filed: |
October 17, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10697973 |
Oct 31, 2003 |
6984458 |
|
|
11250514 |
Oct 17, 2005 |
|
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|
Current U.S.
Class: |
427/585 ;
420/436 |
Current CPC
Class: |
C23C 30/00 20130101;
F01D 5/286 20130101; Y10S 428/926 20130101; Y10T 428/12937
20150115; Y10T 428/12861 20150115; C23C 24/10 20130101 |
Class at
Publication: |
427/585 ;
420/436 |
International
Class: |
C22C 19/07 20060101
C22C019/07 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 27, 2002 |
IT |
MI2002A 002057 |
Claims
1. A method for the treatment of organs subject to erosion by
liquids, comprising the application of a cobalt-based alloy on the
surface of said organs to form a layer of anti-erosion coating,
wherein said alloy comprises: TABLE-US-00003 chromium from 28 to
32% by weight tungsten from 5 to 7% by weight silicon from 0.1 to
2% by weight carbon from 1.2 to 1.7% by weight nickel from 0.5 to
3% by weight iron from 0.01 to 1% by weight; manganese from 0.01 to
1% by weight; molybdenum from 0.2 to 1% bby weight cobalt the
complement to balance.
2. The method according to claim 1, characterized in that said
application is effected by means of laser plating (laser
cladding).
3. The method according to claim 1, characterized in that said
organs comprise the components of a vapour turbine.
4. The method according to claim 3, characterized in that said
components are vapour turbine blades.
5. The method according to claim 2, characterized in that said
laser plating is effected with a CO.sub.2 or YAG laser.
6. The method according to claim 1, characterized in that the layer
of coating applied has a thickness ranging from 0.1 to 5 mm.
7. The method according to claim 1, characterized in that it also
comprises a preliminary heating phase of the surface of the organ
to be treated.
8. The method according to claim 1, characterized in that it
comprises a series of application passages of said alloy.
9. (canceled)
10. The cobalt-based alloy according to claim 9, characterized in
that it has the following composition: TABLE-US-00004 Cr 30 g W 6 g
Si 1 g C 1.5 g Ni 1.5 g Fe <0.3 g Mn <0.3 g Co 48 g Mo 0.75 g
Other <0.25 g (Imp.)
11. The cobalt-based alloy according to claim 9, characterized in
that it has the following composition: TABLE-US-00005 Cr 30 g W 6 g
Si 1 g C 1.5 g Ni 1.5 g Fe 0.20 g Mn 0.20 g Co Balance Mo 0.75 g
Other 0.20 g
12. The cobalt-based alloy according to claim 9, characterized in
that it has the following composition: TABLE-US-00006 Elem.
Quantity Cr 28% W 5.1% Si 0.1% C 1.2% Ni 0.5% Fe 0.01% Mn 0.01% Mo
0.2% Co Balance Other 0.01% (Imp.)
13. The cobalt-based alloy according to claim 9, characterized in
that it has the following composition: TABLE-US-00007 Elem.
Quantity Cr 31.5% W 6.5% Si 1.8% C 1.6% Ni 2.8% Fe 0.9% Mn 0.8% Mo
0.9% Co Balance Other 0.005% (Imp.)
14. The cobalt-based alloy according to claim 9, characterized in
that it has the following composition: TABLE-US-00008 Elem.
Quantity Cr 30% W 6% Si 1% C 1.5% Ni 1.8% Fe 0.5% Mn 0.3% Mo 0.3%
Co Balance Other 0.05% (Imp.)
15-18. (canceled)
Description
[0001] The present invention relates to a method for treating
organs subject to erosion by liquids and an anti-erosion coating
alloy.
[0002] In particular, the present invention relates to a method for
the coating of organs subject to erosion by liquids, such as vapour
turbine components, by means of the laser plating of a cobalt-based
alloy.
[0003] It is known that the organs of equipment which undergo
repeated impact with liquids during functioning, are subject to a
slow but continuous erosion destined to jeopardize their
functionality and performances after a certain period of
operation.
[0004] This phenomenon is particularly evident and significant, for
example, in vapour turbines whose components are subject to marked
wear when specific precautionary measures are not adopted.
[0005] Specifically 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 cycles.
[0006] 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.
[0007] 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 1995, 28-34.
[0008] In order to avoid the drawbacks due to these erosion
phenomena, attempts were made to solve the problem, from the design
point of view, by increasing the axial spacing between 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.
[0009] These remedies did not prove to be particularly suitable for
solving the problem, as they cause a reduction in the performances
of the turbine.
[0010] Attempts were then made to prolong the average operating
life of the turbine blades, by studying new coating materials which
are capable of reducing the erosion rate of the metals caused by
impact liquid separation (F. J. Heymann, ASM Handbook Vol. 18, page
221).
[0011] 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 hardening, by means of stellite
plate brazing or with tool steels, or by means of hard coatings
applied by welding.
[0012] In order to evaluate the resistance to erosion, the coating
materials of the known art have been subdivided, approximately,
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".
[0013] Ionic nitriding with PVD coating using titanium nitride and
chromium or zirconium nitride were selected for the surface
treatment.
[0014] The blades subjected to ionic nitriding treatment followed
by two subsequent PVD coatings were made up of a layer of titanium
nitride followed by a coating of zirconium nitride or chromium
nitride.
[0015] All the PVD coatings had a thickness of about 3-4 .mu.m. The
coating tests showed a coating discontinuity of the models and the
behaviour was considered unsatisfactory.
[0016] A SEM test revealed that the PVD coating was not
substantially capable of opposing impact erosion whereas the
nitride layer was subject to lesions as a result of micro-fractures
together with the foil nitrides present in the structure.
[0017] Blades with metallic coatings were then tested with HVOF
(Triballoy 800).
[0018] The performances of the Triballoy 800 alloy, as coating
material against erosion from liquids, proved to be inadequate.
[0019] 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.
[0020] 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.
[0021] Blades with metallic coatings (Stellite 6) were then tested
with HVOF.
[0022] Although stellite alloys are known as being a material
suitable for coating, they show all their limits when applied by
means of HVOF. Micrographic analysis, in fact, demonstrates that
low content particles are also enveloped in a film of oxide.
[0023] This fact is also confirmed by the surface morphology
revealed by means of SEM, which shows a detachment or ungluing of
the material specifically along these particles.
[0024] Blades treated with coatings with HVOF and SD-Gun .TM.
carbides were then tested.
[0025] 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-1OCo-4CrSD-Gun and 88 WC-12Co HVOF).
[0026] The cases in which an unsatisfactory behaviour is verified
can be explained by the reduced adhesion of the coating and through
the known intrinsic fragility (due to the presence of chromium
carbides).
[0027] 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 coating process used.
[0028] 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 resistance properties are considered
unsatisfactory.
[0029] This different behaviour can be explained by considering the
surface morphology.
[0030] When the layer of surface coating starts losing its
conformation following the loss of material, the liquid/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
crater.
[0031] In the case of base materials, the low resistance effected
by the surface makes the removal of the material almost completely
uniform along the whole area involved in the test.
[0032] 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 carbides).
[0033] Vice versa, the coatings of the art which provide improved
results are those consisting of tungsten carbides with a cobalt,
chromium-cobalt matrix, depending on the use of the coating
process.
[0034] In general, the performances of the coatings with HVOF
improve with an increase in the content of tungsten carbide. The
micrographic morphology of the 88WC-12Co coating is, in fact, more
homogeneous with respect to that of 83WC-17Co. On the other hand,
the difference in performance of the same material (WC10Co-4Cr),
applied by means of SD-Gun.TM. or HVOF is quite marked. The results
of the former are encouraging, whereas those of the latter are
unsatisfactory.
[0035] This confirms that at present the spraying process has a
significant importance in obtaining certain performances of the
coating.
[0036] 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.
[0037] 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 requisite for resistance to erosion is the good quality
of the microstructure of the coating.
[0038] At the moment, the necessity is consequently felt for having
new types of coating or treatment of organs subject to erosion such
as gas turbine components which are capable of effectively reducing
the metallic erosion rate due to separation by impact with
liquids.
[0039] One of the general objectives of the present invention
therefore consists in providing an alloy for the coating of organs
subject to erosion, such as vapour turbine components, which is
highly resistant to metallic erosion phenomena as a result of
impact with liquids.
[0040] A further objective of the invention consists in providing a
method for the treatment of the surfaces of metallic organs subject
to erosion by liquid, in particular vapour turbine blades, which
effectively increases the adhesion resistance of the coating
applied.
[0041] 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.
[0042] It has now been surprisingly found that it is possible to
obtain a coating for organs subject to erosion, by applying on the
metallic surfaces of said organs a cobalt-based alloy, having a
composition which is rich in tungsten and incorporating selected
quantities of other elements.
[0043] The alloy of the invention is of the stellite or Haynes
alloy type, referring to a material which belongs to the group of
hard alloys based on cobalt, chromium and tungsten, particularly
resistant to corrosion and wear.
[0044] In accordance with a first aspect, the applicant has now
identified, within the range of cobalt-based alloys, a composition
which is particularly suitable for the coating of organs subject to
erosion by liquids, such as for example, vapour turbine components,
comprising:
[0045] chromium from 28 to 32% by weight
[0046] tungsten from 5 to 7% by weight
[0047] silicon from 0.1 to 2% by weight
[0048] carbon from 1.2 to 1.7% by weight
[0049] nickel from 0.5 to 3% by weight
[0050] iron from 0.01 to 1% by weight;
[0051] manganese from 0.01 to 1% by weight;
[0052] molybdenum from 0.2 to 1% by weight
[0053] cobalt the complement to balance.
[0054] The alloy of the invention, conveniently in powder form, can
also comprise other optional elements in a quantity ranging from 0
to 0.5% by weight.
[0055] The alloy of the invention has a balanced composition of
constitutive elements which enhances the properties of anti-erosion
by liquid when it is applied to organs subject to erosion according
to the method of the invention.
[0056] It has been verified that the method and alloy compositions
of the invention allow the production of a layer of coating on
organs subject to erosion by liquids, which is highly resistant to
mechanical stress when functioning, caused by impact with liquid
particles.
[0057] 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 erosion from impact with
liquids by an order of magnitude (for example 2,000,000 of impacts
against 180,000 with traditional hardening materials) with respect
to the resistance values of other materials used in the known
art.
[0058] It has also been observed that the application of the
composition of the invention to the surfaces of vapour turbine
components, such as blades, causes an unexpectedly higher
resistance to erosion with respect to the use of stellite alloys of
the known type.
[0059] The alloy according to the invention advantageously has a
selected carbon content to form carbides with a suitable
stoichiometry, a chromium and tungsten content selected for
obtaining an improved reinforcement for a solid solution and for
optimizing the precipitation values of carbides having a suitable
stoichiometry. The alloy of the invention advantageously has a
selected nickel content to provide a suitable ductility and allow
an effective application in the method of the invention.
[0060] A selected nickel content which is particularly suitable for
optimizing the behaviour of the alloy in laser plating ranges from
0.6 to 2.8% and preferably from 0.9 to 2.5% by weight.
[0061] It has been observed that by maintaining the quantities of
carbon, chromium, tungsten, nickel and molybdenum within the ranges
indicated above, the alloys of the invention have a resistance to
erosion by liquids that is higher than the norm.
[0062] According to another aspect of the invention, a method is
provided for the treatment of an organ subject to erosion by
liquids, 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 a coating layer
resistant to erosion by liquid.
[0063] According to a preferred embodiment, the method of the
invention comprises the application of said cobalt-based alloy by
means of laser plating (laser cladding) on organs subject to
erosion, such as for example, vapour turbine components.
[0064] The method of the invention is particularly suitable for
reducing the erosion by liquids of vapour turbine components such
as blades, rotor, stator and plates.
[0065] The laser plating according to the present invention can
typically comprise one or more passages on the surfaces of the
metallic organs subject to erosion by liquid, so as to form one or
more anti-erosion coating layers.
[0066] The method of the invention conveniently comprises the
application, on the metallic surface to be treated, of an
anti-erosion layer having a thickness ranging from 0.1 to 5 mm,
preferably from 0.8 to 3 mm.
[0067] According to an embodiment of the invention, the metallic
material to be subjected to the treatment of the invention can be
previously heated and the alloy of the invention is subsequently
applied, conveniently by the use of laser technology.
[0068] The laser plating is typically carried out using a CO.sub.2
or Nd-Yag laser apparatus.
[0069] According to an embodiment, the method of the present
invention combines the laser application technology (laser
cladding) with the use of alloys having the formulations described
above, thus allowing structures to be obtained, with increased
anti-erosion performances due to the high solidification rate and
low thermal supply.
[0070] It has been verified that the combined use of the alloys of
the invention with laser plating gives rise to a) a matrix based on
a solid solution over-saturated with the alloy elements, b) an
extremely fine grain, c) a precipitation of fine carbines
homogeneously dispersed in the matrix, d) an extremely reduced
modified thermal area, e) an extremely limited bath dilution.
[0071] 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 products of the known
art are evident from the enclosed drawing in which:
[0072] The FIGURE illustrates a graph relating to comparative
liquid erosion tests on 4 metal samples.
[0073] 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 drops.
[0074] The graph summarizes the results of erosion by liquid drops
sprayed through a 0.13 mm nozzle on four test samples made of
martensite stainless steel, the same material but with martempering
treatment (MT), integral stellite and stainless steel coated with a
layer produced by laser plating of the alloy of the invention,
according to Example 1.
[0075] 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.
[0076] Once the coating material, according to the present
invention, has been applied to metallic surfaces of vapour turbine
components, it has a high adhesion resistance.
[0077] The high resistance properties of the coating produced with
the method of the invention are also justified by its
microstructural morphology.
[0078] It has in fact been observed that the structure of the
coating produced with the laser technique is extremely fine 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 activity.
[0079] Furthermore, the coating material 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 materials of the known art.
[0080] The application of the laser technology consequently makes
it possible to produce coatings with a high resistance to erosion
by separation due to impact with liquids, 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
avoiding any possible negative effect on the tensile strength.
[0081] 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 according to the
enclosed claims.
EXAMPLE 1
[0082] A composition was used, in powder form for the coating of
mechanical vapour turbine components having the following
formulation: TABLE-US-00001 Cr 30 g W 6 g Si 1 g C 1.5 g Ni 1.5 g
Fe <0.3 g Mn <0.3 g Co 48 g Mo 0.75 g Other <0.25 g
[0083] The powder was applied to stainless steel turbine blades by
means of YAG laser plating (laser cladding) forming an anti-erosion
layer having a thickness equal to about 1 mm.
EXAMPLE 2
[0084] The following Table indicates various formulations of
compositions in powder form according to the present invention.
TABLE-US-00002 Element Comp. 1 Comp. 2 Comp. 3 Cr 28% 31.5% 30% W
5.1% 6.5% 6% Si 0.1% 1.8% 1% C 1.2% 1.6% 1.5% Ni 0.5% 2.8% 1.8% Fe
0.01% 0.9% 0.5% Mn 0.01% 0.8% 0.3% Mo 0.2% 0.9% 0.3% Co Balance
Balance Balance Other 0.01% 0.005% 0.05%
* * * * *