JPH05239613A - Thermal spraying method using selectively preliminarily oxidized alloy powder - Google Patents

Abstract

PURPOSE: To form a protective film decreased in bulky oxides having no protective function by preliminarily oxidizing the grains of an alloy containing Cr, Al, or Si at the time of depositing a protective film of the alloy grains on a metallic base material.

CONSTITUTION: Grains of an alloy containing Cr, Al, or Si as oxide-forming reactive element are allowed to adhere to a metallic base material composed of Ni-base super alloy, Co-base super alloy, or Fe-base alloy. At the time of this adhesion, at least part of these grains are preliminarily oxidized. As the alloy for grains, e.g. an alloy, which has a composition containing, as essential elements, Fe, Ni, Co, or a mixture thereof and also containing ≥ about 10 wt.% Cr and/or ≥ about 3 wt.% Al, can be used. Moreover, the adhesion is performed by means of plasma thermal spraying, flame thermal spraying, thermal spraying, etc. By this method, the film with corrosion resistance and erosion resistance can be allowed to adhere to the base material composed of the alloy.

COPYRIGHT: (C)1993,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is directed to the use of conventional particle coating techniques to protect metal alloy substrates exposed to aggressive conditions including corrosion, erosion, oxidation and / or wear, using conventional particle coating techniques. The present invention relates to a method for forming an oxide film. Particles of a metal alloy having a thin metal oxide film, produced under controlled conditions, suitable for use in such metal coating methods are also described.

A method of depositing a corrosion resistant, erosion resistant, oxidation resistant and / or abrasion resistant coating on a metal substrate using a metal alloy powder by a conventional particle coating method will be described. The metal alloy powder has a protective oxide film formed on the particle surface in advance, or the metal oxide is dispersed inside the alloy particles themselves. Suitable for use in high temperature powder coating methods,
The metal alloy powder having a thin adhesion protective oxide film on the surface of each particle will also be described.

[0003]

2. Description of the Related Art Chemical reaction devices, steam generators, gas turbine parts, etc. are always exposed to aggressive conditions,
In many cases protection from such conditions is necessary. As a protection technique, a method of depositing a metal oxide film on the surface of an article to be protected is often used. For example, a metal oxide layer is deposited on a substrate requiring protection by thermal decomposition of a volatile metal alloy. .. Various proposals have been made to improve the deposition process, for example US Pat. No. 4,297,15.
As described in No. 0, it has been proposed to oxidize the surface of the article to be protected and then deposit a metal or metal oxide on the part. U.S. Pat. No. 4,532,10
No. 9 proposes to form a device for treating hydrocarbons at high temperature so as not to generate carbon deposits by forming a special aluminum-containing alloy, which itself is exposed to aggressive oxidizing conditions. Alternatively, when exposed, it oxidizes to form an oxide film directly on the surface. Protective oxide A that selectively oxidizes and adheres to the surface of the alloy itself
Nickel-based superalloys forming l ₂ O ₃ are described in US Pat. No. 3,754,902.

Alternatives to aluminide coatings such as MC
Coatings based on rAlY alloys (M is Ni or Co or both) are described in US Pat. No. 3,918,139, which exemplary alloy coatings are themselves corrosion protective and Independent of the deoxidized base metal substrate on which the coating is deposited. Particles of this alloy are deposited by plasma spraying, vapor deposition, or the like.

Protective oxide coatings formed by conventional methods are not uniform, dense coatings of the desired protective metal oxide (s), and are often the conventional thermal spray coating methods. It contains bulky, undesired, non-protective oxides. Such a non-protective film is not only unnecessary but, in practice, degrades the mechanical integrity and mechanical properties of the deposit. Unwanted oxides dilute, or reduce, the concentration of desirable corrosion resistant oxides, often resulting in less than satisfactory coatings.

Since the protective film of metal oxide is formed under the conditions of volatile temperature and environmental conditions (with respect to the reaction gas), various oxidation in the coating process is also an important problem, and the concentration of the protective oxide is also important. Is reduced. These conventional methods have the feature of depositing the metal alloy directly on the surface of the substrate, in order to oxidize the alloy during the coating process and / or upon impact on the substrate during coating, in a high temperature oxidizing environment. It depends on the deposition conditions. This leads to the undesired consequences mentioned above.

[0007]

SUMMARY OF THE INVENTION In the method of depositing a protective film on a metal substrate of the present invention, the oxide-forming reactive elements Cr and Al are used.
Alternatively, particles of Si-containing alloy are deposited on the substrate to be protected, but at least some of these particles are pre-oxidized prior to deposition on the substrate. The partially oxidized metal alloy particles are deposited by a suitable particle coating method such as plasma spraying, flame spraying, thermal spraying, vacuum plasma spraying, isotropic pressing.
It is desirable that a thin adherent oxide film be distributed almost uniformly on the surface of the alloy particles before deposition. Cr, Al inside the particles
Alternatively, the Si oxide is preferably dispersed.

In the method for coating a Ni or Co-based superalloy substrate with a protective film according to the second invention, particles of a metal alloy containing at least one of oxide-forming reactive elements Cr, Al or Si. To wear. Prior to deposition, at least a portion of the particles contains 20% by weight or less of Cr, Al or S
It contains an oxide of i. The obtained adhesion protective film is rich in Cr, Al or Si oxides having corrosion resistance, erosion resistance, oxidation resistance and abrasion resistance and having a protective action, and substantially contains a bulky oxide having no protective action. Not included. According to this invention,
Ni or Si-based superalloy articles with thin, adherent, corrosion-resistant, erosion-resistant, oxidation-resistant, wear-resistant protective coatings are also provided, and also Cr or Al or Cr and Al on the surface.
Cr or A with both thin adherent protective oxide films
Fe, Ni or C containing both 1 or Cr and Al
Particles of o-based metal alloys are also provided.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention relates to a thermal pretreatment of a pre-alloyed metal powder, after which the particles are sprayed onto a metal substrate for corrosion resistance, erosion resistance,
It relates to forming a protective layer or coating on a substrate to provide oxidation or abrasion resistance. The particles are deposited by techniques such as plasma spraying and flame spraying. The invention is applicable to alloys containing reactive elements that form strong oxides, such as Cr, Al or Si.

According to the present invention, in order to obtain an excellent powder metallurgy deposited layer, the powder is selectively pre-oxidized before spraying to form a thin adherent protective oxide film on the surface of each powder particle. .. Pre-oxidation is performed under controlled time, temperature and environmental conditions to produce a minimum thickness of the preferred oxide composition on the alloy particles. Examples of grain-forming alloys suitable for this invention include Fe, Ni or Co or mixtures thereof as the basic element and contain about 10% by weight or more of Cr and / or
Alternatively, there is an alloy containing about 3% by weight or more of Al. The purpose is to prevent the formation of bulky, unprotected oxides that normally occur during the thermal spraying process. The presence of such oxides in the deposited layer reduces the integrity and mechanical properties (strength and ductility) of the deposited layer. Since various elements are involved in the oxidation reaction, various oxidations during deposition also change the composition balance of the deposited layer. If the primary function of the deposited layer is to provide oxidation, corrosion or wear resistance, this ability is diminished by uncontrolled or unavoidable oxidation during thermal spraying.

In thermal spraying or plasma spraying with conventional oxyacetylene or other combustion gases, the metal powder is heated in an oxidizing environment to extremely high temperatures of 3000 ° F. and above for a short time. Momentum and heat are transferred to the powder particles by the high gas flow velocity and high temperature obtained by plasma generation, and the particles are blown away and become in a molten state. The impingement of this material on the substrate results in adiabatic heating, near plastic deformation of the droplets, and crushing of the powder surface oxide. However, because of the fast solidification rate and the dissipation of heat to and around the substrate, the degree of in situ oxidation of the deposit is minimized if the substrate is preheated less. This minimizes post-deposition oxidation of the deposit.

[0012] Classical oxidation theory and bulk materials
Experimental results show that Cr and / or Al
Oxidation of the Fe, Ni and Co-based alloys contained in three stages
Occur. Based on linear reaction rate at the beginning of oxidation
Metal oxides, ie FeO, NiO or CoO
Is made. Of these elements in the pre-alloyed powder
Because the concentration is usually as high as 55-75% by weight.
It The reactive elements Cr and / or Al also react immediately
First, spinel oxides such as MCr₂O_FourOr M
Al₂O_Four(M is Fe, Ni or Co)
Participate in the second stage oxidation. For example, 300 series
Austenitic Stainless Steel, 304S
S (18Cr-8Ni) and 310SS (25Cr-
20Ni), or 400 series ferritic steel,
That is, commercially available alloys such as 446SS (25Cr)
Protective oxide film on the oxide-metal interface during the third stage oxidation
Cr ₂O₃Containing enough Cr to finally form
ing. Steps II and III are actually reversed depending on the system.
Sometimes it becomes. Once this film is formed, oxidation
The response speed is parabolic (△ W / M = k_pt^-2+ C),
The oxide film thickness growth is Cr₂O₃Through Cr⁺³of
Determined by diffusion rate.

In fact, little is known about the phenomenon of powder particle oxidation in high velocity gas streams. Since a high temperature is generated in the arc plasma and the oxyacetylene spraying system, most of the powder particles are in a molten state.
In general, the diffusion rate is two or three orders of magnitude higher in the liquid state, so there is no doubt that the oxidation process follows the reaction-defined linear dynamics, and solid oxides where diffusion-defined parabolic dynamics prevail. Formation of a surface film is not possible with current systems. Of course, an uncertain portion of the particles will not melt during the flight, these particles will oxidize at a rather low reaction rate. Some of the finer (less than 1 micron) particles volatilize and do not participate in the coating and oxidation process.

The plasma spray stream is generated with an inert gas such as Ar or He or a stable gas such as H _2, but the stream itself is not necessarily non-reactive. The gas ejects the plasma gun at a high speed and with a rapid pressure drop, so that the surrounding atmosphere, or air, is drawn into the main gas stream. Measurement by researchers [A. Hasui, S .; Ki
tahara, T .; Fukushima, Tran. N
at. Res. Inst. Metals (Japan)
1965, 7 (5), 21], the arc gas contains as much air as 90% at a distance of 10 cm from the plasma gun nozzle. This contamination can lead to oxidation, decarburization or nitrogen absorption of the powder coating alloy contained in the stream. These oxides and other reaction products formed during thermal spraying are complex and are rarely deposited as continuous films. Therefore, the oxidation reactions of the particles in flight are variable and uncontrolled.

In order to minimize various oxidations of the powder particles in flight, the present invention employs selective oxidation as a pretreatment to produce a thin, controlled oxide on the particle surface, such as Cr ₂ O ₃ or It produces Al ₂ O ₃ . The effects of time, temperature and environment are minimized to minimize the effect on composition, microstructure and particle morphology to prevent the formation of base metal oxides and to generate a uniform adhesion protective surface oxide within a reasonable time. Select the conditions. An important variable in this method is the choice of environment, which is a reducing atmosphere to FeO, NiO or CoO, ie lower than the dissociation pressure of these oxides,
An atmosphere above the dissociation pressure of the oxides of the group must be generated, so that selective oxidation pretreatment can be easily realized. Kubaschews
ki) and N. Bredzs based on data from Evans (1967).
The graph of (1969) shows metal-metal oxide stability as a function of temperature and dew point in H ₂ O / H ₂ atmosphere (and vacuum). The points to the right of a given equilibrium curve indicate that the metal is stable, ie no oxide is formed. For example, 446SS (25Cr-balance Fe), for example 2000 ° F and -20 ° F-8.
Pretreatment with oxidation at a dew point of 0 ° F. forms Cr ₂ O ₃ and does not produce FeO or Fe ₃ O ₄ .

The present invention has economic and commercial importance in the following applications: The main field of interest is conventional thermal spraying with pre-alloyed metal powders containing reactive elements such as the aforementioned Cr, Al or Si. The deposited layer can be used to provide oxidation resistance, corrosion resistance, erosion resistance or abrasion resistance. A recent development in the field of plasma spraying is decompression (10-100 Tor) in large chambers.
r) concentrates on vacuum plasma spraying underneath, which minimizes powder oxidation and enhances the structure and properties of the deposited layer. Al-containing MC applied to superalloys in the gas turbine industry
While the rAlY coating composition is particularly effective, this approach comes with a significant investment and cost penalty.

The selective preoxidation treatment of MCrAlY material according to the present invention provides a significant cost advantage over conventional plasma spray processing. Therefore, commercial utilization of MCrAlY materials would be advantageous and expanded. For example, large articles, which are difficult or impossible to put under vacuum in their entirety, can be coated with MCrAlY material and are resistant to oxidation.
Becomes corrosion resistant.

The selective preoxidation treatment of the powder alloy composition containing Cr, Al or Si can be controlled to produce a very fine internal dispersion of the oxide rather than a thin surface oxide layer. Individual sub-micron aluminum oxide (Al ₂ O ₃ ) particles are known to significantly enhance mechanical properties and microstructural stability in many alloy systems. Therefore, surface coatings or monolithic structures using this material have inherent hardness or strength advantages. Using the available VPS equipment and processing conditions, the pretreated powder could be vacuum plasma sprayed onto a substrate or sacrificial mandrel. Or HI
By subjecting P (high temperature isotropic pressing) to thermomechanical processing, the pretreated powder could be molded (compressed) into a desired shape.

Powder treatments that produce controlled oxides as internal oxide particles or surface layers are also useful as erosion or abrasion resistant coatings, especially in solid particle erosion environments. The type, amount (% by volume) and form of oxides will vary depending on the particular application environment. Among the many applications of parts coated by the method of the present invention are steam turbine surfaces exposed to boiler scale carryover at start-up, and coal-fired pressurized fluidized bed combustors (PFBC = pressurized, fluidized bed combu
gas turbine gas flow path components used in the effluent.

Claims (10)

[Claims] 1. A method of depositing a protective film on a metal substrate, wherein particles of an alloy containing the oxide-forming reactive elements Cr, Al or Si are deposited on the substrate to be protected. A method comprising pre-oxidizing at least a part of the particles. 2. The partially oxidized metal alloy particles are deposited by a particle coating method selected from the group consisting of plasma spraying, flame spraying, thermal spraying, vacuum plasma spraying or isotropic pressing. The method according to Item 1. 3. The method according to claim 1, wherein a thin adherent oxide film is substantially uniformly distributed on the surface of the alloy particles before deposition. 4. The method according to claim 1, wherein Cr, Al or Si oxide is dispersed inside the particles. 5. The method of claim 1, wherein the particles contain about 10 wt% or more Cr. 6. The method of claim 1, wherein the particles contain about 3 wt% or more Al. 7. The method of claim 1, wherein the metal substrate is a Ni-based superalloy. 8. The method of claim 1, wherein the metal substrate is a Co-based superalloy. 9. The method of claim 1, wherein the metallic substrate is a Fe-based alloy. 10. A method of depositing a protective film on a Ni or Co-based superalloy substrate, comprising particles of a metal alloy containing at least one of oxide-forming reactive element Cr, Al or Si. A metal alloy particle containing 20% by weight or less of an oxide of Cr, Al or Si on the surface of at least a part of the particle to the substrate to obtain corrosion resistance,
Corrosion resistance, oxidation resistance, wear resistance, and protective C
A method of forming a protective film, which comprises forming an adhesive protective film which is rich in r, Al or Si oxide and substantially does not contain a bulky oxide having no protective action.