METHODE DE TRAITEMENT SUPERFICIEL D'UN ALLIAGE RESISTANT AUX TEMPERATURES ELEVEES

SURFACE TREATMENT METHOD OF HEAT-RESISTANT ALLOY

Abrégé anglais

ABSTRACT OF THE DISCLOSURE
A method of surface treating an article made of
heat-resistant alloy comprises spraying onto the surface
of the article, a first layer of a coating of a heat
resistant material comprising for example a metal such
as Ni or Cr or a Ni-Cr alloy or a compound thereof.
liquid coating containing a corrosion resistant material
is then applied onto the first layer. The article is then
heat treated to effect penetration by diffusion of one
coating into the other.

Revendications

The embodiments of the invention in which an exclusive
property or privilege is claimed are defined as follows:
1. A method of treating a surface of a metal alloy
article, which comprises:
coating said surface with a member selected from
the group consisting of Ni, Cr, a Ni-Cr alloy, a Ni compound
and a Cr compound to form a first layer on said surface,
coating said first layer with a liquid containing
(1) a member selected from the group consisting of Al, an
alloy thereof and a compound thereof, and (2) a member
selected from the group consisting of Si, an alloy thereof
and a compound thereof, to form a second layer on said
first layer, and
heat-treating said coated article to diffuse one of
said layers into the other of said layers.
2. A method according to Claim 1, wherein said liquid
comprises a slurry.
3. A method according to Claim l,wherein the liquid
comprises a slurry formed by dispersing A?and SiO2 in a
liquid carrier.
4. A method according to Claim 3, wherein said A?
and SiO2 have a particle size of about 0.1µ to 1µ.
5. A method according to Claim 1, wherein the heat
treatment includes the step of holding the coated article at
about 1080°C.
6. A method to Claim 5, wherein said step in the heat
treatment is preceded by a heating step to evaporate the liquid,
followed by heat treatment at about 330°C.
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Description

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This inven-tion relates to a me-thod oE surface
trea-ting a member of heat-resis-tant alloy for use in turbines,
blowers, boilers or the like to render it resistant to hiyh
temperature oxidation as well as to high temperature corrosion.
In industrial gas turbines using petroleum or na-tural
gas as the fuel, gas -temperature at the turbine inlet tends to
become higher as -the turbine efficiency is improved. On
the other hand, as the available fuel supply has changed for
the worse in recent years, -the fuels used for the turbines
have been diversified and the content of corrosive impurities
in the fuels such as sulphur (S), sodium (Na), vanadium (V),
and so forth has tended to increase. As a result~ so-called
"hot parts" such as the blades and burners of turbines, that
are exposed to these high temperature gases, are subjected
to extremely severe high temperature oxidation as well as high
temperature corrosion.
~ hese hot parts have conventionally been made
primarily oE heat-resistant alloys. In particular turbine
blades consist of Ni- and Co-based alloys called "ultra-alloys".
However, since high temperature strength is generally a top
priority requirement for these ultra-alloys, they have the
lrawback that their corrosion resistance and oxidation
resistance are not satisEactory. Various attempts have
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therefore been made to provide these heat-resistant alloys
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with oxidation resistance and corrosion resistance, and
various surface treatmentmethods using, for example, chemical
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and physical techniques, have been employed. ~lowever, none
of these methods has been really satisfactory as regards
efficiency and cost.
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.. 1 The present inven-tion is directed to providing
a me-thod which overcomes the deficiencies of -the previous
methods. Acordingly, in order -to provide an article of
heat-resi.stan-t al].oy with high temperature oxida-tion resistance
and high temperature corrosion resis-tance, the presen-t
.. invention provides a surface treatmen-t method which i.s
charac-terized by the steps of coati.ng, by spraying onto
the surface of the ar-ticle, a heat-resistant material of
metals such as Ni and Cr or Ni-Cr alloys or their compounds
- 10 as a first layer, then applying, as a second layer~ a liquid
. coating containing metals such as A~ , Si, Cr, Ta and the
like or their alloys or compounds as the corrosion-resistant
material by means of spraying-coating, brush-coa-ting or the
like, and heat-treating the coated surface.
The surface trea-tment method of the present
invention provides the characterizing features as illustrated
in Table l in comparison with the conventional methods.
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1 The present inven-tion will now be descxibed in
more de~ail by reEerence to an example.
A substrate of ~dimet 520 (by weight l9~ Cr, 12~ Co,
6% Mo, 3% Ti, 2~ A~, l~ Fe, Ni-Bal), widely used as an ultra-
alloy for the hot parts of a cJas turbine, was treated in the
following sequence:
~l) After the surface oE the substrate had been cleaned
with an alkaline emulsion cleaning agent, steam cleaning
was carried out using a Fluron type solvent. The surface
was further blasted using an AQ2O3 blast.
(2) a Ni-Cr (50/50 by weight) alloy was applied as a
coating to form a first layer having a thickness oE about
50~ by plasma spraying.
(3) The surface of the sprayed-on first layer was
blastedusing AJ~23 to remove any oxide film formed on its
; outermost surface.
(4) The surface of the sprayed-on first layer was
coated by spraying it with a coating slurry formed by dis-
persing AJ~and SiO2, each having a particle size of about
0.1 to 1~, in an organic carrier ~alcohol, solvent naph-tha,
etc) to form a second layer.
; (5) After these treatments, the substrate was placed
in an electric furnace and was held at 80C. (~ 5C) for
20 minutes to evaporate and remove the liquid. After being
further held at 330C (~ 5C) for 15 minutes, the substrate
was withdrawn from the furn~ce.
(6) The substrate was held at l,080C for 4 hours
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inside a hydrogen furnace, was cooled in the furnace and
was then withdrawn.
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1 Above mentioned s-tep (4) could be carried out
using a mix-ture oE fine A~ particles with Ai~ 2O3 powder
in a mi~ing ratio by welght of 80/20 or 50/50 or a mixture
; oE A wi-th SiO2 in a mixing ratio by weiyht of 80/20 or
50/50. Also step (6) could be carried ou-t us:in~ a vacuum
furnace in place oE -the hydrogen furnace.
Although in this example Udimet 520 has been
- treated by the method of the invention by way of example~
similar excellent results can also be obtained when treating
the surfaces of other substrates such as Ni-based alloy,
Co-based alloy and stainless steel.
The costed surface of -the substrate provided by
the above described method had an extremely smooth and flat
; sur~ace, and ~RJand Si from the second layer s~lffic:iently
penetrated by diffusion into the first layer, thereby
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r~ ~ completely eliminating the fine pores of the first layer.
Hence, the composite coating was rendered wholly homogeneous.
~:- In other words, since the melting point o~ ~ is 660C, A~,
was fused due to the heat-treatment and penetrated into the
fine pores, thus presumably rendering the surface smooth
; and flat. Further, it was confirmed that a part of A~J
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~i and Si reached and was also diffused also into the substrate~
Table 2 illustrates the results of fly-ash errosion
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; resistance test, corrosion resistance test, and practical
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application test using gas turbine blades, each test being
applied to a member treated by a method in accordance with
the present invention and a member treated by a conventional
method. The composite coating produced by the method in
accordance with the present invention had a better performance
in comparison with that produced by the conventional me-thod
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1 in the fly-ash errosion resistance test and the corrosion
resistance test. In the practical application test using
gas turbi.ne blades/ too, the coated blade produced using
- the method of the presen-t invention exhibited the tendency
that the deposition amount of the fuel ash became smaller.
In a therrnal impact -test comprising holding the testpiece
at l,100C for 15 minu-tes, then charying it into water at
. 20C, and repeating these procedures five times, the composite
coating produced by the method of the present invention
.~ 10 did not suffer peeling or cracking and had extremely good
adhesion.
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