EP0546359B1 - Thermal spray coating process with cooling - Google Patents

Abstract

The invention relates to a process for coating a surface by means of a thermal spray method, in which a spray jet of hot carrier gas and molten material particles is directed at the surface in question and at the same time cooling is effected adjacent to the spray jet by means of a cooling jet which consists of substantial proportions of carbon dioxide and contains cold gas and snow particles. Hitherto, such a process was carried out based on liquid carbon dioxide, in order to ensure that a useful cooling jet with correspondingly high snow particle fractions is obtained. According to the invention, the carbon dioxide fraction in the cooling jet is obtained from gaseous carbon dioxide which is at a pressure of at least 45 bar, specifically by the carbon dioxide gas first being expanded, via a slit nozzle or another slit-like orifice, into an expansion volume which is disposed about said expansion slit and is largely enclosed with respect to the environment, and starting from this expansion volume and its outlet orifice the cooling jet is formed and is directed at the region to be cooled. <IMAGE>

Description

The invention relates to a method for coating a surface by means of a thermal spray method, for example flame or high-speed flame spraying, arc or plasma spraying or detonation spraying, a jet of hot transport gas and molten material particles being directed onto the surface in question and in the process adjacent to the spray jet is cooled with a cooling jet consisting essentially of carbon dioxide containing cold gas and snow particles.

Such a method is known from DE-PS 26 15 022. In this pure carbon dioxide (CO₂) is used as a coolant, which is supplied to the nozzle generating the cooling jet in a liquid state. When it emerges from this, a mixture of gaseous and solid CO₂, ie CO₂ snow particles is formed, which achieves a particularly high cooling performance when it hits a workpiece and is particularly advantageous in connection with the thermal application of layers. Would be used in the manner shown in DE-PS 26 15 022, gaseous instead of liquid carbonic acid (CO₂), so there would be a significant reduced cooling effect, since the resulting cooling jet would have a lower coldness and in particular a considerably lower proportion of CO₂ snow particles. This is due firstly to the fact that the amount of cold that is to be supplied to the CO₂ in liquid gaseous form for the phase transition is no longer present in the pure gas expansion, and secondly to the fact that in the nozzle shown in the aforementioned DE-PS and other common round nozzles the CO₂ sucks in surrounding air immediately after leaving the nozzle, consequently an intimate heat exchange takes place with it and as a result significantly less snow forms than with the liquid CO₂ expansion. However, this CO₂ snow contributes significantly to the known cooling effect and forms a latent cold storage, which cools the workpiece directly. The result would be a significantly reduced cooling effect on a loaded workpiece compared to the method according to DE-PS.

The same applies to the processes known from EP-PS 0 263 469, in which also a cooling jet generated from liquid CO₂, but mixed, consisting of CO₂ gas, CO₂ snow and other gases, for example helium and / or hydrogen, for Cooling of the thermally sprayed surfaces is applied.

However, the use of liquid carbon dioxide, with CO₂ being at ambient temperature and under the condensing pressure, means that a special form of CO₂ supply must be guaranteed, namely one in which the carbon dioxide from the respective storage containers is in the liquid phase is applied. This means that the carbon dioxide, which is in standard storage at 20 ° C with about 57 bar pressure in the associated storage tanks, can be discharged from them by means of a riser pipe or in another special way, ie the storage tanks must be provided with a liquid phase extraction.

Another difficulty with the use of carbon dioxide for cooling purposes in connection with thermal spraying methods is that with a carbon dioxide jet emerging from a conventional round nozzle, a turbulent, widely diversified snow-gas mixed jet is formed which does not allow targeted action on a suitably limited surface area . When such a turbulent and divergent carbon dioxide jet is used adjacent to a spray jet, the spray jet and the cooling jet can also be adversely affected.

The object of the present invention was therefore to provide a thermal spray process with CO₂ cooling which avoids or eliminates the disadvantages described and in particular also enables the use of gaseous carbon dioxide.

This object is achieved in that the carbon dioxide fraction in the cooling jet is obtained from gaseous carbon dioxide which is at least under 45 bar pressure, and in such a way that the carbon dioxide gas is initially largely closed via a narrow slot nozzle or another slot-like opening in an expansion slot arranged around this expansion slot Expansion volume is expanded into it and, based on this expansion volume and its outlet opening, the cooling jet is formed and directed onto the surface to be cooled.

Due to the fact that the invention is based on gaseous and non-liquid carbon dioxide, any special equipment of the CO₂ storage container can be omitted. A slight limitation arises from the fact that the CO₂ pressure in these containers for carrying out the invention must not drop below 45 bar, since the cold for the cooling jet alone from the Expansion cooling of the CO₂ gas is obtained and on the other hand no contribution from the "latent cold" of the liquid carbon dioxide is available. For this reason, the cold yield is too low below pressure values of 45 bar, which can easily occur under unfavorable conditions - e.g. when the storage bottles are stored outdoors and at low outside temperatures. However, if the standard pressure values of approx. 57 bar, as they occur at room temperature, are available, this results in an excellent function.

Above all, the type of expansion of the carbon dioxide gas according to the invention via a slot nozzle or the like into a closed expansion volume is essential for the function and effectiveness of the invention. The slot nozzle with its elongated and, on the other hand, narrow cross-sectional opening namely generates an expansion gas jet with a surface which is substantially enlarged in comparison to an expansion gas jet originating from a round nozzle. This enlarged surface area results in an increased interaction of the expansion gas jet with its surroundings, which - according to the further essential feature of the invention - is formed by an expansion volume in which, during operation, cold carbon dioxide gas is almost exclusively already expanded. Warmer ambient air therefore has no direct access to the expanded carbon dioxide. This means that initially only a little heat can flow from the environment to the carbon dioxide and therefore there is an increased formation of carbon dioxide snow particles in the expansion volume - due to the heat deficit there. Compared to an unshielded expansion of gaseous carbon dioxide, a significantly increased proportion of snow particles is generated, which above all provides the strong cooling effect desired when cooling thermally sprayed layers cause. The gas-snow mixture created in the expansion space is then formed into a cooling jet over the further course of the expansion volume and directed onto the workpiece through the outlet opening. With a cooling jet generated in this way, there is effective cooling of the workpiece in its inflow region, and if necessary, other gases which increase the cooling effect can be added to the cooling jet.

With regard to the proposed mode of expansion, it should be pointed out that it is already known from another technical field, namely medical technology. For example, DE-PS 36 24 787 shows a cooling and freezing probe for local cooling of human or animal body areas and - in a secondary aspect - also of electronic components, which is based on the principle described, but the cooling of the respective area from close proximity takes place and no long-range cooling gas jet, but a suitably guided cooling gas flow is formed. However, the transfer, adaptation and modified application according to the present invention is not obvious.

Preferred embodiments of the present invention are now specified in subclaims 2 to 5.

It should be noted that a cooling jet following and / or preceding the spray jet primarily protects temperature-sensitive spray material or heat-sensitive workpieces from overheating. With thermal spray processes cooled in the manner described, however, an increase in performance is generally possible compared to uncooled spray processes.

Finally, the method according to the invention is advantageously carried out with an expansion nozzle, which has a connectable to a CO₂ gas source inner tube 6 with a final slot nozzle, and has an outer tube 9 enveloping the inner tube at the end of the slot nozzle, projecting significantly beyond and forming the expansion volume. With this expansion nozzle, a diameter to length ratio of 1 to 3 to 1 to 10, preferably 1 to 5, is maintained with regard to the cylindrical expansion volume formed with the outer tube.

With the expansion nozzle according to the invention, a distance from the workpiece of at least approximately 3 cm is advantageously maintained in process operation in order to obtain a favorable process function.

In principle, with the cooling jets according to the invention there is also the possibility of conveying spray particles rebounding from the spray area away from the workpiece and thus avoiding coating errors.

Figur 1

einen autogenen Flammspritzvorgang in schematischer Darstellung;

Figur 2

eine erfindungsgemäße CO₂-Expansionsdüse in Seitenansicht im Schnitt;

Figur 3

eine erfindungsgemäße CO₂-Expansionsdüse in Vorderansicht;

Zunächst sei von einer standardgemäßen CO2-Versorgung ausgegangen, d.h. z.B. von einem unter Umgebungstemperatur stehenden Mittel- oder Hochdrucktank für CO2. Dieser enthält im Normalfall im Gleichgewicht sowohl flüssiges als auch gasförmiges CO2 mit einem Druck von ca. 57 bar. Das unter diesem Druck stehende Kohlendioxidgas wird nun gemäß der Erfindung über eine sehr enge, schlitzartige, in ihrer Längsausdehnung im Bereich einiger Millimeter liegende Düse in ein abgegrenztes Expansionsvolumen hinein entspannt. Auf diese Weise entsteht ausgangsseitig des z.B. durch ein Röhrchen gebildeten Expansionsvolumens ein relativ eng begrenzter und wenig turbulenter Strahl aus kaltem CO2-Gas und Schnee, der sich insbesondere für die Kühlung bei thermischen Spritzvorgängen ausgezeichnet eignet.The method according to the invention and a corresponding expansion nozzle are explained in more detail by way of example below with reference to the drawings. It shows:

Figure 1

an autogenous flame spraying process in a schematic representation;

Figure 2

a CO₂ expansion nozzle according to the invention in side view in section;

Figure 3

a CO₂ expansion nozzle according to the invention in front view;

First of all, a standard CO2 supply is assumed, that is, for example, a medium or high-pressure tank for CO2 that is at ambient temperature. This normally contains both liquid and in equilibrium also gaseous CO2 with a pressure of approx. 57 bar. According to the invention, the carbon dioxide gas under this pressure is now expanded into a limited expansion volume via a very narrow, slit-like nozzle with a longitudinal extension of a few millimeters. In this way, on the output side of the expansion volume formed, for example, by a tube, a relatively narrow and less turbulent jet of cold CO2 gas and snow is created, which is particularly suitable for cooling during thermal spraying processes.

Through further tests, the applicant has determined that an even higher and more advantageous cooling effect of the cooling jet formed as described can be obtained by starting from a carbon dioxide gas with a pressure of more than 65 bar, preferably 70 to 80 bar. However, special precautions have to be taken, since - as described above - CO₂ is only available at around 57 bar in standard storage tanks. According to the invention it is proposed to solve this problem that said higher pressures are produced by heating the gas storage device together with its contents and thus by generating a higher vapor pressure of the liquid CO₂ or that the pressure increase is generated by a pump connected downstream of the storage device. A storage container is particularly advantageously heated, for example, by arranging an electrical heating conductor in it. Containers equipped with heating conductors are also available, since such heating devices are provided in any case when large amounts of CO2 gas are provided in the associated storage containers. This circumstance therefore accommodates the "high-pressure variant" of the invention, and a suitably equipped storage tank with, for example, pressure-sensitive heating control can easily supply the pressures above 65 bar. In any case, however, a particularly effective cooling jet is formed with this method variant, the effect of which lies in the relatively high proportion of snow in the jet.

FIG. 1 now shows a thermal spraying process, for example a flame spraying or high-speed flame spraying process operated with fuel gas and transport gas. Shown is a spray nozzle 1, as well as an expansion nozzle 2 and a workpiece 3. To apply the surface layer, the workpiece shown, namely a shaft 3, is rotated according to arrow 4 and the spray jet of the spray nozzle 1 is directed approximately perpendicularly onto its surface. For example, a wear-resistant layer containing tungsten carbide can be applied, the flame spray nozzle 1 and the coolant nozzle 2 being coupled, aligned in parallel, and advanced according to arrow 5 along a parallel to the workpiece surface. The expansion nozzle follows the spray nozzle at a constant distance of approx. 5 to 15 cm (distance with respect to the two nozzle axes). It can be seen that the newly applied layer areas, after leaving the spray area, come under the influence of the coolant jet 2 'emanating from the expansion nozzle 2 and thus this surface area is cooled with a freshly applied coating and also the spread of the heat away from the spray zone into the already coated area Workpiece areas is prevented. The coolant used here is, in particular, pure carbon dioxide or - if a particularly high cooling capacity is required - mixtures of carbon dioxide together with helium and / or hydrogen according to EP-PS 0 263 469, the admixing gases preferably only directly in the area of impact of the coolant jet 2 ' be mixed on the workpiece.

FIG. 2 shows one of the possible expansion nozzles for carrying out the method according to the invention in section. This is composed of an inner tube 6 with a closing slot nozzle 7, and an outer tube 9 enveloping the end of the inner tube and forming the expansion volume 8, which is open at its end facing away from the expansion nozzle 7.

3 shows a front view of the expansion nozzle shown in FIG. 2, likewise in a sectional view along the section line S in FIG Expansion channel 8 relaxed into it. In the relaxation process arise in particular due to a negative pressure formation behind the slot nozzle 7 CO₂ cold gas and snow particles, and there is thus a mixture of cold gas and snow in the expansion volume 8, which leaves the expansion nozzle through the outlet opening 10 of the outer tube 9 and directed onto the workpiece becomes.

Such an expansion nozzle is to be dimensioned according to the desired throughput. A coolant nozzle of the type shown, which is suitable for common flame spraying processes, has e.g. an inner diameter D (see Figure 2) with respect to the outer tube 9 of 3 mm and thus - according to the length dimension to be maintained - a protruding length L of e.g. 15 mm. Another size that is important in relation to the invention is the opening width of the slot nozzle of the inner tube. As a rule, this is advantageously between 0.1 and 0.4 mm. This opening width is based on the selection of the basic size of the expansion nozzle, i.e. after selection of the diameter for the inner or outer tube, in the narrower sense determining for the flow of CO₂ gas.

With the expansion nozzle shown and the generation of a carbon dioxide cooling jet from gaseous carbon dioxide shown, an advantageous possibility is provided for cooling in thermal spraying methods, the essential elements being found in the way of expansion of the CO₂ and the associated expansion nozzle.

Claims (12)

1. Process for coating a surface by means of a thermal spraying method, in which a spraying jet of hot transporting gas and melted material particles is directed on to the surface concerned and cooling is effected in the vicinity of the spraying jet with a cooling jet consisting essentially of carbon dioxide and containing cold gas and snow particles, characterised in that the proportion of carbon dioxide in the cooling jet is obtained from gaseous carbon dioxide at a pressure of at least 45 bar in that the carbon dioxide gas is first expanded through a slotted nozzle or other slotted opening into an expansion volume disposed around this expansion slot and largely sealed from the environment, and the cooling jet is formed from this expansion volume and its outlet opening and directed on to the area to be cooled.
2. Process according to claim 1, characterised in that the proportion of carbon dioxide in the cooling jet is formed of gaseous carbon dioxide at a pressure of at least 65 bar, preferably 70 to 80 bar.
3. Process according to claim 2, characterised in that the raised pressure level of the carbon dioxide is produced by supplying heat to the associated storage container, producing temperatures of 25 to 35°C, preferably 30 to 32°C.
4. Process according to claim 3, characterised in that the supply of heat to the storage container is effected with an electric heating conductor housed in the storage container.
5. Process according to claim 2, characterised in that the raised pressure level is produced by means of or through the use of a compressor connected after the storage container.
6. Process according to one of claims 1 to 5, characterised in that the expansion volume with its outlet opening forming the cooling jet is guided at a distance of 2 to 15 cm, preferably 3 to 8 cm, from the workpiece.
7. Process according to one of claims 1 to 6, characterised in that the cooling with the cooling jet is effected in that the cooling jet is guided following the spraying jet.
8. Process according to one of claims 1 to 7, characterised in that a cooling jet is guided preceding the spraying jet.
9. Process according to one of claims 1 to 8, characterised in that more than one cooling jet is provided.
10. Process according to one of claims 1 to 6, characterised in that the spraying jet is surrounded on all sides by a plurality of cooling jets or by one enveloping stream of cooling gas.
11. Expansion nozzle for implementing the process according to one or more of claims 1 to 10, characterised by an inner pipe 6 provided with a terminating slotted nozzle 7 and connectable to a carbon dioxide source, and an outer pipe 9 surrounding the inner pipe at the slotted nozzle end and projecting beyond it and forming the expansion volume 8 and which at its end remote from the slotted nozzle 7 exhibits an outlet opening 10.
12. Expansion nozzle according to claim 10, characterised in that a diameter to length ratio of 1 to 3 to 1 to 10, preferably approximately 1 to 5, is maintained as regards the cylindrical expansion volume 8 formed with the outer pipe.

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