Patent Application
20100304036
The present invention relates to a thermal spraying technique, in which corrosion resistance or abrasion resistance is applied to the surface of a base plate by forming a corrosion-resistant thermally sprayed metallic film by thermal spraying, so as to prolong the lifetime of a structure or various kinds of industrial equipment and, more particularly, to a metallic film forming method using a high velocity oxy-fuel (hereinafter abbreviated as “HVOF”) thermal spraying gun and a thermal spraying apparatus for the method.
It is necessary to prevent (any) corrosion by subjecting a material poor in corrosion resistance to some surface treatment when such a material is used in seawater or in a coastal environment even if the material such as steel has excellent characteristics as a structural material. Means for preventing (any) corrosion include many methods for painting, plating and the like. However, the painting or plating has raised problems yet from the viewpoints of durability and lifetime. On the other hand, an attempt has been made to spray corrosion-resistant powder onto the surface of a base plate by thermal spraying at high temperature (for example, flame spraying, plasma spraying, arc spraying and the like), so as to fabricate corrosion resistant film. However, a resultant film has not been satisfactory in regard to density. Therefore, even if a technique for forming a corrosion-resistant film has been adopted and carried out, there has arisen such an ironic result under the current circumstances that the film must be subjected to post-treatment by different means, for example, impregnation of a resin in the film or partial fusion by overheating after the thermal spraying, that is, after the formation of the film.
Furthermore, there has been put into practical use a corrosion-prevention method of a sacrificial anode type in which a material electrochemically less noble than iron such as zinc or aluminum is coated and selectively eluded, to thus protect a steel base member. In this case, although pores in the film cannot raise any problem, it is construed that a corrosion-resistant lifetime can become long if a resin penetrates the film. However, this has raised a problem of an increase in dissolution rate depending upon the mechanical strength and environment of the film, thereby shortening a lifetime of a designed product.
In the meantime, a so-called HVOF thermal spraying method has been put into practical use and has become a focus of attention in recent years, in which material powder is hardly fused but is sprayed to a base member at a high velocity in a softened state, and then, the powder is instantly welded by kinetic energy, thereby forming a film. For now, this technique is most used to, for example, an abrasion-resistant (superhard) film made of WC-Co cermet. This is because tungsten carbide (WC) is readily decomposed when it is exposed to, for example, a hot plasma at high temperature; in contrast, WC is much less decomposed at a heat source temperature of about 2,500° C. at the maximum in the case of the HVOF, and further, a dense film is formed at a high velocity. From the above-described example, it has been found that the HVOF has the feature of formation of a dense barrier type film in the atmosphere, and therefore, has the possibility of formation of a dense film made of a corrosion-resistant material.
In view of such circumstances as described above, the inventors of the present application have studied to form dense films made of various kinds of corrosion-resistant alloys by the HVOF thermal spraying method. As a result, it has been found that even if an Ni-based alloy such as Hastelloy is thermal sprayed under a standard condition by a commercially available HVOF thermal spraying apparatus, a considerably dense film excellent in corrosion resistance can be formed. This invention has been patentable already (see Literature 1). Literature 1: Jpn. Pat. No. 3,069,696, “Corrosion-Resistant Thermally Sprayed Film and Method for Fabricating the Same”
However, a film having satisfactory density could not be made of stainless steel under spraying conditions in which the commercially available HVOF thermal spraying apparatus can operate. When thermal power of combustion flames is increased to enhance the film density, a base member is undesirably overheated, thereby raising a problem of marked oxidation of a film. Thus, the present invention has been accomplished to solve the above-described problems in a contradictory relationship experienced in the prior art according to existing and simple means. In other words, an object of the present invention is to provide a thermally sprayed dense metallic film of low oxidation by using HVOF thermal spraying means without overheating.
As a result of an earnest study conducted by the inventors of the present invention, it has been found that a cylindrical attachment (hereinafter referred to as “a gas shroud” or simply referred to as “a shroud”) is attached to a commercially available HVOF thermal spraying gun, and thereby oxidation of thermally sprayed particles can be suppressed by supplying a great quantity of inert gas into the cylindrical attachment. Further, when the particles are thermally sprayed to the surface of a base plate at an increased particle velocity, a thermally sprayed dense film with remarkably low oxidation can be formed without increasing the temperature of combustion flames very much. These findings have reached the present invention.
That is to say, according to the present invention, a gas shielding means, which has been already used in the field of plasma spraying, is basically used in addition to an HVOF thermal spraying gun, and then, the gas shielding means and the HVOF thermal spraying gun are connected to each other, thereby producing a technical effect which could not be achieved in the prior art. That is, producing an effect of formation of a thermally sprayed dense metallic film having a small oxygen content without overheating a base plate, with an attendant advantage of a remarkably profound significance. In other words, an object of the present invention is to provide an HVOF thermal spraying method equipped with excellent features based on a series of findings and a thermal spraying apparatus therefor.
Specifically, a first solving means according to the present invention provides a metallic film forming method by using an HVOF thermal spraying gun, in which a gas shroud having a cylindrical portion formed into a shape in conformity with that of a barrel cylindrical portion of the thermal spraying gun is disposed in the barrel cylindrical portion. Inert gas is supplied into a space defined inside of the shroud in such a manner as to suppress oxidation and energize a particle velocity, and metallic particles are accelerated to collide with a base plate without overheating the base plate, so as to form a thermally sprayed dense film having a low oxygen content at a relatively low temperature. In addition, means for supplying the inert gas into the space defined inside of the gas shroud is constituted of a slit formed in a circumferential manner, and can energize the velocity with respect to the metallic particles to be thermally sprayed by the thermal spraying gun and prevent mixture of the atmosphere.
Here, the gas shroud to be used has been already used in thermal spraying at high temperature, for example, in plasma spraying. However, the gas shroud has been used merely for controlling the atmosphere and preventing oxidation of thermally sprayed metal (see Jpn. Pat. Appln. KOKAI Publication No. 224,662/1996), unlike the present invention in which the gas shroud is used as means for increasing a particle velocity. There is no literature which suggests simultaneous achievement of density of a metallic film and a low oxidation by increased velocity achieved by a gas shroud.
A technical report entitled “A Gas Shroud Nozzle for HVOF Spray Deposition” by Pershin V. and three others (on pp. 1305 to 1308 in Proceedings of the 15th International Thermal Spraying Conference held at Nice, France from Mar. 25 to 29, 1998) discloses a test result of the comparative investigation of particle velocities in the case where gas is made to flow in a cylindrical gas shroud and where no gas is made to flow therein after the gas shroud of a water cooling structure having an inside port for introducing nitrogen is disposed in an HVOF thermal spraying gun in such a manner as to surround combustion flames, and in the case where no gas shroud is disposed. As a result, it has been reported that the velocity of the thermally sprayed metallic particle is markedly decreased in the thermal spraying gun having the gas shroud disposed therein in comparison with the thermal spraying gun without any gas shroud. That is to say, the above-described technical report discloses nowhere the suggestion of the achievement of an increase in particle velocity by disposing the gas shroud in the thermal spraying gun, but merely discloses the utterly contrary result. Moreover, U.S. Pat. Nos. 4,869,936, 5,019,429 and 5,151,308 by Moskowitz and Donald disclose a technique in which a gas shroud is disposed in an HVOF thermal spraying apparatus using hydrogen and oxygen as a heat source, so as to form a film excellent in corrosion resistance. This shroud is adapted to shield thermally sprayed particles from the atmosphere by the use of a swirl flow formed by injecting inert gas toward the inner surface of the shroud from numerous nozzles formed inside thereof, but is not intended (i.e., does not produce any effect) to accelerate the particles.
Furthermore, in order to supply the inert gas into the space defined inside of the gas shroud, a second solving means according to the present invention provides a metallic film forming method characterized in that an inclination with respect to the spraying direction of the metallic particles to be thermally sprayed is provided at the slit formed in the circumferential manner, and thus, the inert gas is supplied into the space defined inside of the gas shroud along the inclination. A third solving means provides a metallic film forming method characterized in that the inclination is within an angle of 70° with respect to a line perpendicular to the center axis of the shroud cylindrical portion.
Moreover, a fourth solving means provides a metallic film forming method characterized in that the inert gas supplying means constituted of the slits formed in the circumferential manner are arranged at a plurality of portions in a lengthwise direction of the gas shroud. A fifth solving means provides a metallic film forming method characterized in that the slits are arranged at two or more portions including a thermal spraying gun barrel outlet and a gas shroud outlet.
Sixth to tenth solving means relate to a thermal spraying apparatus corresponding to the above-described solving means according to the metallic film forming means, respectively. Specifically, a sixth solving means according to the present invention provides a thermal spraying apparatus including an HVOF thermal spraying gun and means for supplying inert gas into a space defined inside of a cylindrical gas shroud in such a manner as to suppress oxidation and energize a particle velocity of metallic particles thermally sprayed from the thermal spraying gun, in which the gas shroud having a shape in conformity with that of the barrel cylindrical portion of the HVOF thermal spraying gun is detachably attached to the barrel cylindrical portion. Additionally, the means for supplying the inert gas into the space defined inside of the gas shroud is constituted of a slit formed in a circumferential manner, and can energize the velocity of the metallic particles thermal sprayed by the thermal spraying gun and prevent mixture of the atmosphere.
Additionally, a seventh solving means provides a thermal spraying apparatus characterized in that an inclination with respect to the spraying direction of the metallic particles thermally sprayed is provided at the slit formed in the circumferential manner, and thus, the inert gas is supplied into the space defined inside of the gas shroud along the inclination. An eighth solving means provides a thermal spraying apparatus characterized in that the inclination is within an angle of 70° with respect to a line perpendicular to the center axis of the shroud cylindrical portion.
In addition, a ninth solving means provides a thermal spraying apparatus characterized in that the slits are arranged at a plurality of portions in a lengthwise direction of the gas shroud, and a tenth solving means provides a thermal spraying apparatus characterized in that the slits are arranged at two or more portions including a thermal spraying gun barrel outlet and a gas shroud outlet.
As described above, according to the present invention, the cylindrical gas shroud is attached to the barrel of the HVOF thermal spraying apparatus, and the thermally sprayed metallic particles are controlled in such a manner as to form the thermally sprayed dense film of a small oxygen content with the inert gas supplied into the gas shroud without overheating the base plate. This unique configuration can produce the special function and effect that the thermally sprayed dense metallic film of a small oxygen content can be formed. With this unique configuration, there has never known yet that the particle velocity of the thermally sprayed metallic particle is accelerated, and further, the thermally sprayed dense metallic film of a small oxygen content is formed without overheating the base plate. In addition, the unique configuration and the special function and effect according to the present invention cannot be expected from the test results disclosed in the prior art literature.
With the above-described configuration, the present invention has succeeded in achieving the thermally sprayed dense metallic film of a small oxygen content is formed with reproducibility. Therefore, the present invention is a very basic and important invention which widely influences on various kinds of industrial fields in addition to its technical significance, thereby socially and economically producing a prominent effect with a remarkably high value.
A description will be given below of a high velocity oxy-fuel (HVOF) thermal spraying gun and a cylindrical shroud for use in a preferred embodiment according to the present invention. First of all,
Referring to
Here, it is very important that the means for supplying the inert gas are the slits formed in the circumferential manner. The inert gas to be supplied from these circumferential slits forms a certain kind of dual-layered acceleration flow in such a manner as to cover the surroundings of the metallic particles thermally sprayed, energizes the velocity of the metallic particles, and functions to effectively suppress oxidation caused by the mixture of the atmosphere.
Although the slit in this case is preferably formed over the entire circumference, slits may be intermittently arranged in a circumferential manner, or a plurality of slits may be arranged. In order to form the above-described dual-layered acceleration flow, it is desirable that the interval between the slits and the length (i.e., the size) of the slits should be substantially the same in the arrangement of the slits.
The function and effect of the slits arranged in the circumferential manner become prominent owing to an inclined surface formed at the slit in a direction in which the metallic particles travel, as illustrated in
In the case where the inclined surface is formed at the slit for supplying the inert gas into the space defined inside of the shroud, it is preferable that the inclination angle should be set within 70° with respect to a line perpendicular to the center axis of the shroud cylindrical portion.
A plurality of such slits may be arranged in a lengthwise direction of the gas shroud. The number of slit arrangement portions or arrangement positions may be determined in addition to the length of the gas shroud in consideration of the injection rate of the thermally sprayed metallic particles, the flow rate and quantity of the inert gas and the thickness and characteristics of the metallic film.
In arranging the slits at the plurality portions as described above, although the above-described inclined surfaces may be adopted at all of the slits or not, the formation of at least one inclination surface is effective. In this case, it is more preferable that the inclined surfaces should be formed at two or more portions, that is, at the outlet of a thermal spraying gun barrel and the outlet of a gas shroud in consideration of the formation of the above-described inclined surface at the slit formed at the outlet of thermal spraying gun barrel.
The inert gas for use is exemplified by noble gas such as argon or nitrogen. It is effective that the inner diameter of the shroud is gradually enlarged from the thermal spraying barrel outlet toward the shroud outlet, as in the embodiment illustrated in
Although the configurations of the thermal spraying gun and the shroud for use according to the present invention have been schematically described above, they need not be limited to the above-described configurations. As long as a required target is not missed, it is understood that variations and additions should be allowed.
Furthermore, the metal or base plate for thermal spraying may be selected from various kinds according to the present invention, and particularly, it is understood that the base plate may be selected from various kinds such as a flat plate, a curved plate, a bulk member and an odd-form molded product.
Hereinafter, the present invention will be described in more detail by way of preferred embodiments. Of course, the present invention is never limited to the preferred embodiments, described below.
In the present preferred embodiment, stainless steel (SUS316L) powder was thermally sprayed by using a high velocity oxy-fuel thermal spraying apparatus in which a combustion flame of kerosene and oxygen are used as a heat source. The lengths of the barrel were two kinds, that is, 10 cm and 20 cm; nitrogen was used as the inert gas; and thus, the porosity and the oxygen content in the resultant film in each of the barrels were measured by varying the combustion condition (i.e., the mixture ratio of the fuel to oxygen) and the flow rate of gaseous nitrogen inside of the gas shroud.
Here, the gas shroud had the configuration illustrated in
The flow rate of the gaseous nitrogen as the inert gas (2) on the side of the outlet of the gas shroud was constantly 0.45 m3/min.
The supplied quantities and combustion pressures of the fuel and oxygen in oxidizing flame, neutral age flame and reducing flame and other thermal spraying conditions in the experiments are shown in Table 1 below.
Table 2, below, shows below values obtained in an experiment in which the length of the barrel and the flow rate of the inert gas (1) for the gas shroud influence on the average velocity and fusion rate of the thermally sprayed particles. These values are results under the condition where the mixture ratio of the fuel to oxygen achieves complete combustion. The particle velocity was measured by a non-contact optical measuring method; and the fusion rate was measured by separating a fused portion from a not-fused portion by capturing sprayed particles with an agar gel placed at the position of the base plate (this measurement was published and introduced in detail in Journal of the Japan Institute of Metals, 65 (2001) 317-22).
The result first reveals that the particle velocity in the case of the barrel having a length of 20 cm is higher by about 100 m/s than that in the case of the barrel having a length of 10 cm. When the shroud is disposed in the barrel, and then, the gaseous nitrogen is made to flow at 2.5 m3/min, the velocity can be further increased by about 20 m/s in both of the barrels. Incidentally, a flow rate of 1.5 m3/min is insufficient in the case of the barrel having a length of 20 cm, and therefore, the velocity is decreased. This results in the findings that the flow rate should be desirably 2.0 m3/min or higher in the case of the barrel having a length of 20 cm.
Moreover, the particle fusion rate is decreased according to the shroud gas since the introduced gaseous nitrogen has a cooling effect at room temperature.
Thus, as for the barrel having a length of 20 cm, a study was made on only the neutral flame and the reducing flame (indicated by triangles).
Here, numerals “15” and “25” in “Re 15”, “Ne 15” and “No, Re 25” in
As is clear from
Next, a description will be given below of results that gas shroud thermal spraying is applied to Hastelloy C alloy as one kind of nickel-based alloys in another preferred embodiment in which another material is used. It was found that even if the Hastelloy alloy was thermally sprayed under the standard condition by the commercially available HVOF thermal spraying apparatus, a considerably dense film excellent in corrosion resistance could be formed, and therefore, this invention was patentable (Jpn. Pat. No. 3,069,696, “Corrosion-Resistant Thermally Sprayed Film and Method for Fabricating the Same”), as described above.
The corrosion resistance in this case was judged from the result that the film was soaked in artificial seawater in a laboratory, and then, no corrosion could not be generated by evaluating the outside appearance, a potential and a corrosion resistance value even after a lapse of three months.
However, it was found thereafter that the corrosion resistance was insufficient in severe environment such as actual ocean in which waves wash, and therefore, a more severe corrosion resistance evaluation test was conducted. Specifically, the Hastelloy alloy was thermally sprayed onto carbon steel, and then, an iron ion eluded when it was soaked in a 0.5 M HCl solution was quantified by a very sensitive analyzing method such as an ICP (Inductively Coupled Plasma) emission spectroscopy. At this time, since the base plate was sealed with a resin, the iron ions detected was mainly iron eluded from the base plate through fine pores in the thermally sprayed film (even if it could not be detected by the Mercury Porosimeter). This is a more severe evaluation of the density of the film.
As is clear from
Summing up the test results described in the above first and second preferred embodiments, the film containing 0.3% or less of oxygen and having a porosity of 0 could be formed by using the neutral or reducing combustion flame and the barrel having a length of 20 cm and adding the gas shroud downstream to allow nitrogen to flow at 2.5 m3/min in the thermal spraying of the stainless steel SUS316L.
Furthermore, it was confirmed that the similar result could be produced even when the thermal spraying distance was varied from 50 mm to 160 mm (i.e., the distance from the outlet of the shroud tip to the base plate), or when the angle of the inclination surface was varied from 0° to 70°. Moreover, it was confirmed that the favorable effect could be produced in the same manner also in the case where the thermally sprayed metallic particles were sprayed from the outlet of the shroud tip slantwise at an angle of up to 45° to the perpendicular to the base plate.
Additionally, in the case of HVOF thermal spraying with the Hastelloy C alloy, although the film having a porosity of 0 could be already obtained under the normal thermal spraying condition as evaluated by the conventional Mercury Porosimeter, the corrosion resistance was insufficient in the severe corrosion environment (for example, in the actual ocean or in the 0.5 M HCl solution). However, no elusion of the iron ion into an acidic solution was observed by additionally disposing the gas shroud and thermal spraying the Hastelloy C alloy, and further, the film having the high corrosion resistance was formed.
The principal factor results from the effects that the higher velocity of the thermally sprayed particles, the maintenance of the inert atmosphere, and the suppression of the overheating of the base plate can be achieved at the same time according to the present invention. The present invention is applicable to other materials, and further, the principle based on the useful constituent requirement influences on other thermal spraying methods, or is applicable to other thermal spraying methods as it is.
As described above, according to the present invention, the gas shroud is disposed in the HVOF thermal spraying gun and a great quantity of inert gas is supplied into the space defined inside of the shroud in such a manner as to suppress the oxidation and energize the particle velocity, and the metallic particles are made to collide with the base plate without overheating the base plate, so as to form the thermally sprayed dense film having the low oxygen content at the relatively low temperature, so that the function and effect, which could not be predicted from the test report disclosed in various kinds of literature reported up to now, can be produced: that is, the velocity of the thermally sprayed metallic particles can be increased, and the thermally sprayed dense metallic film having the low oxygen concentration can be formed without over heating the base plate. Therefore, the present invention is an epoch-making technique which can break a barrier experienced by the prior art. In addition to the remarkable enhancement of the corrosion resistance of the steel structural member and various kinds of equipment, the present invention is expected to be widely utilized in various fields including coating on the welds or ends of the various types of clad members, as well as repairing of damaged portion. Furthermore, the present invention is expected to provide an effective mean to prolong the lifetime of various steel structure of great industrial and economic importance.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2002-296709 | Oct 2002 | JP | national |
This application is a continuation of U.S. application Ser. No. 10/515,816, which was the National Stage of International Application No. PCT/JP2003/012983, filed Oct. 9, 2003.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10515816 | Feb 2005 | US |
| Child | 12796981 | US |
