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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 , ..
therefore been made to provide these heat-resistant alloys . ~
with oxidation resistance and corrosion resistance, and various surface treatmentmethods using, for example, chemical .i .
and physical techniques, have been employed. ~lowever, none of these methods has been really satisfactory as regards efficiency and cost.
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:: -2-.. 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 '; .
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 "
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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Table 2 illustrates the results of fly-ash errosion ., .
; resistance test, corrosion resistance test, and practical ~ . .
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 i:
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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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