Method for machining metal workpiece

Abstract

The invention relates to a method for machining a metal workpiece. The method has the known steps of spraying a powdery auxiliary substance (2) onto the workpiece (20) by means of a spraying device (10), the auxiliary substance at least partially melting before it impacts the surface of the workpiece when exiting the spraying device. In order to be able to apply an anti-scale coating to the workpiece in a convenient and easy-to-remove manner, the method according to the invention further provides that the temperature of the workpiece is lower than the melting point of the powdery auxiliary substance (2), preferably lower than 50 DEG C, before the workpiece (20) is sprayed with the auxiliary substance (2); the powdery auxiliary substance has an anti-oxidation effect; and forming a uniform auxiliary substance coating on the workpiece by melting the auxiliary substance when the auxiliary substance is sprayed.

Description

Technical Field

[0001] The invention relates to a method for machining a metal workpiece. Background Art

[0002] In the production of steel, workpieces such as continuously cast blanks or hollow blocks are often produced in large quantities for further processing in extruders or radial forging machines before being introduced again into the hot forming process. To this end, the workpieces must be heated to forming temperature in a furnace. This can take several hours, during which a very thick oxide layer often forms on the surface of the workpiece. This oxide layer, also known as scale, represents a loss of material. Depending on the material, this material loss can amount to 1% to 3% of the weight of the workpiece.

[0003] In order to prevent the formation of such a thick oxide layer, it is generally known in the prior art to apply a powdered auxiliary substance, which is also solid, to the surface of the workpiece. The known auxiliary substances are, for example, in powder form and only exert their desired antioxidation effect when they are at least partially melted. Therefore, a known method for applying such a powdered auxiliary substance to the surface of the workpiece is to heat the workpiece at least at its surface to a temperature corresponding to the melting point of the auxiliary substance. The melting temperature of the known auxiliary substances is above 400° C. and up to 1000° C. The auxiliary substance then becomes liquid and wets the surface. The auxiliary substance then forms a protective film on the surface, which protects the surface of the workpiece from oxidation, i.e. from the formation of oxide scale. Typical applications of this method are the glazing of the hold-down bolts of extruders or the deoxidation of hollow blocks of seamless pipes, etc.

[0004] Necessarily heating the workpiece to such high temperatures is expensive and time consuming.

[0005] Another known application for applying powdered auxiliary substances to the surface of metal workpieces is thermal spraying, which also includes the well-known powder flame spraying; see Wikipedia. In this case, the powdered auxiliary substance is at least partially melted in a burner flame and then shot at a very high speed onto the surface of the workpiece to be coated. The speed varies from 70 m / sec to 800 m / sec, depending on the auxiliary substance to be applied and the method. Due to such high speeds, the particles of the auxiliary substance are deformed when they hit the surface of the workpiece and remain on its surface. This creates a very strong connection between the auxiliary substance layer and the surface of the workpiece. The application rate of the auxiliary substance to the surface of the workpiece and the resulting layer thickness are very low, usually only a few micrometers per spraying process, and the surface must be specially prepared beforehand. Flame spraying is a classic process for coating workpieces with metals or oxides.

[0006] However, this known powder flame spraying is not suitable for the application of the oxidation-resistant auxiliary substance in accordance with the method according to the invention. On the one hand, this is because in the present invention, no such strong connection is desired between the auxiliary substance layer and the workpiece. On the contrary, the auxiliary substance layer applied according to the invention should only be a "one-time application" which should be partially or completely worn away from the surface of the workpiece during the subsequent forming process or pre-processing process. On the other hand, for certain planned subsequent processing steps of the workpiece, the layer thickness that can be achieved per spraying process using powder flame spraying is too low. Summary of the invention

[0007] The object of the invention is to apply a coating which protects against oxidation scale to a workpiece in a simple and easily removable manner for the known methods for machining workpieces.

[0008] This object is achieved by a method according to claim 1 .

[0009] The method is characterized in that before the workpiece is sprayed with the auxiliary substance, the workpiece has a temperature lower than the melting point of the powdered auxiliary substance, preferably lower than 50°C; the powdered auxiliary substance has an antioxidant effect; and a uniform auxiliary substance coating is formed on the workpiece by melting the auxiliary substance when spraying the auxiliary substance.

[0010] In order to use the method according to the invention, the workpiece to be coated advantageously does not need to be preheated; room temperature is usually sufficient. In particular, the workpiece does not have to be heated to the melting point of the auxiliary substance to be applied, as is the case with other methods for applying auxiliary substances to workpieces, in particular metallic workpieces. This firstly results in considerable cost savings, since heating the workpiece to the melting temperature of the auxiliary substance generally consumes significantly more energy than correspondingly heating the auxiliary substance. The claimed method can also be used in systems which explicitly exclude the use of liquid auxiliary substances, such as application by spray gun in a converter, in order to reduce adhesion.

[0011] The invention claims that the sprayed auxiliary substance has an anti-oxidation effect on the metal workpiece. This advantageously achieves that when the metal workpiece is heated in a furnace to the forming temperature for the subsequent hot forming process, significantly less oxide scale is formed on the metal workpiece than if the workpiece had no anti-oxidation layer according to the invention. This also means correspondingly less material loss, because the oxide scale formed on the workpiece during the heating process ultimately means material loss, because this oxide scale is usually removed before further forming. Since only a small amount of oxide scale is produced due to the application of the anti-oxidation auxiliary substance, the time required for cleaning the workpiece, or more precisely for removing the remaining oxide scale that is still formed, is also reduced.

[0012] According to an embodiment of the present invention, a gas mixture consisting of at least an auxiliary substance, ambient air and a power gas is heated at least to the melting temperature of the auxiliary substance contained therein, so that the auxiliary substance contained therein is at least partially melted. The gas mixture can be heated by means of a heat source that does not form a flame or by means of a burner that is to form a flame. In the case of using a heat source that does not form a flame, for example in the form of an electrically operated heating spiral, the gas mixture flow does not necessarily experience additional translational energy in the conveying direction during heating. This is not the case, in particular, if the heating is performed by means of a burner that is to form a flame; in the case of heating by means of a burner that is to form a flame, the gas mixture may be accelerated in the direction of the outlet of the spraying device due to the explosion of the gas mixture. In this regard, the method selected for heating the gas mixture to the melting temperature of the auxiliary substance also affects the outlet velocity of the auxiliary substance leaving the spraying device. Compared with the case where no flame is formed, the outlet velocity is significantly greater in the case of an explosion, more precisely, the formation of a flame, than in the case where no flame is formed.

[0013] The powder is conveyed to the outlet of the spraying device by means of a motive gas. Therefore, the flow rate of the motive gas must be adapted to the corresponding amount of auxiliary substance. Furthermore, when a burner is used, the amount of burner gas must be adapted to the amount of motive gas so that the gas mixture remains ignitable.

[0014] According to another embodiment, the spraying device is configured in the form of a Venturi nozzle. The use of a Venturi nozzle provides the advantage that the motive gas only needs to be introduced into the Venturi nozzle with a significantly lower initial volume flow rate to achieve a significantly larger volume flow rate of the gas mixture at the outlet of the Venturi nozzle. In this regard, the Venturi nozzle advantageously acts as an amplifier of the volume flow rate due to its design.

[0015] According to another embodiment of the invention, 10 g to 80 g of at least partially melted auxiliary substance can be applied to the surface of the workpiece per second and / or the liquid or pasty particles of the melted auxiliary substance hit the surface of the workpiece at a particle speed of 4 m / sec to 20 m / sec. The required application amount per second of the auxiliary substance on the surface or the very low particle speed compared to other application methods for applying the auxiliary substance to the metal workpiece advantageously improves the application efficiency because, firstly, fewer particles rebound from the surface of the workpiece and secondly, the particles stay longer in the spraying device for heating. In this regard, the required application amount per second and the very low particle speed make it possible to form a thicker layer of auxiliary substance on the workpiece than in the prior art.

[0016] In the method according to the invention, when spraying the auxiliary substance, the distance between the outlet of the spraying device and the surface of the workpiece is between 15 cm and 60 cm. This distance depends on whether a heat source without flame formation or a burner with flame formation is selected to heat the auxiliary substance, and in the latter case also depends on the flame length and the amount of burner gas used.

[0017] Advantageously, the auxiliary substance applied by the method according to the invention has not only the aforementioned antioxidative effect, but also the property of removing existing oxides, in particular existing scales, from the surface of the workpiece and / or a lubricating effect. Depending on which of the above-mentioned properties the auxiliary substance used in each case has, the amount of auxiliary substance applied to the workpiece surface is 15 g / m 2 Up to 400g / m 2 between.

[0018] In principle, all industrial deoxidants or lubricants containing a certain proportion of meltable substances can be used as auxiliary substances. The melting temperature is usually in the range of 500° C. to 1200° C. The infusible components of the auxiliary substance may be incorporated into the melted particles. Common base substances in auxiliary substances are phosphates, borates or silicates, which are optionally enriched with soaps and / or graphite, etc.

[0019] The cleaner the workpiece surface is before the auxiliary substance is applied, the better the effect of the anti-oxidation auxiliary substance applied to the workpiece. Therefore, it is particularly recommended to remove the oxides already present there, in particular the existing scale, from the workpiece surface before applying the auxiliary substance according to the method of the invention. This increases the anti-scaling effect of the anti-oxidation auxiliary substance in the reheating furnace.

[0020] Finally, it should be mentioned that the method according to the invention can also be advantageously used in cases where previously the workpiece had to be preheated before the auxiliary substance could be applied, such as in the case of extruders in which the hold-down screws are covered with a layer of enamel. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The specification is accompanied by two drawings, including:

[0022] Figure 1 A method according to the invention according to a first variant is shown; and

[0023] Figure 2 The method according to the invention is shown according to a second variant. DETAILED DESCRIPTION

[0024] The present invention is described in detail in the form of embodiments with reference to the accompanying drawings. In the two drawings, the same technical elements are represented by the same reference numerals.

[0025] Figure 1A spraying device 10 in the form of a venturi nozzle 12 with a heat source 16' is shown. The output of the venturi nozzle 12 is connected to the heat source 16' via a delivery line 14. At the heat source 16', the gas mixture is heated without forming a flame. The heat source 16' is designed, for example, in the form of an electrically operated heating coil. The heat source 16' forms the outlet of the spraying device 10 for the heated gas mixture.

[0026] The method according to the invention is carried out using the spraying device 10 as follows:

[0027] First, the auxiliary substance 2 and the ambient air 3 are sucked into the spraying device 10, where they are mixed with the motive gas 5 (for example, compressed air) to form a gas mixture 7. Driven by the kinetic energy of the motive gas, the gas mixture 7 flows as a continuous gas mixture stream along the conveying direction FR to the outlet of the spraying device 10 so as to flow out there.

[0028] When the powdered auxiliary substance 2 is sucked into the spraying device 10, it usually, but not necessarily, has the temperature of the ambient air 3 sucked in together. It is not until in the spraying device, preferably at its outlet, that the gas mixture is heated by means of a heat source 16' without forming a flame at least to the melting temperature of the auxiliary substance 2 contained in the gas mixture, so that the auxiliary substance at least partially melts in the gas mixture. After melting, the auxiliary substance 2 is contained in the gas mixture in the form of liquid or pasty particles.

[0029] By means of the spraying device 10, the gas mixture 7 containing the molten particles of the auxiliary substance 2 discharged at its outlet is sprayed onto the surface of the workpiece 20. On the surface of the workpiece 20, the auxiliary substance 2 previously at least partially melted forms a uniform coating, such as Figure 1 As shown. In the method according to the invention, before the auxiliary substance is applied, the workpiece 20 is not preheated to a specific temperature, but is usually at room temperature or ambient temperature, i.e., a temperature usually below 50° C. In particular, the workpiece does not have to be heated to a temperature higher than the melting point of the applied auxiliary substance in powder form, because the auxiliary substance is already melted in the spraying device 10 and does not melt until it is on the workpiece 20. Since the sprayed auxiliary substance has an antioxidant effect in the method according to the invention, during the subsequent heating of the workpiece, oxidation, in particular the formation of an oxide scale, on the coated surface of the workpiece is effectively reduced, if not completely prevented.

[0030] When spraying the auxiliary substance, the spraying device is usually at a distance of between 15 cm and 60 cm from the surface of the workpiece 20. If a venturi nozzle 12 is used, the power gas is introduced into the venturi nozzle 12, for example, at an initial volume flow rate of 150 l / min to 300 l / min. The venturi nozzle amplifies this volume flow rate so that the gas mixture is ejected from the venturi nozzle at a volume flow rate of 800 l / min to 4000 l / min. With this gas mixture flow, 10 g to 80 g of at least partially melted auxiliary substance are applied to the surface of the workpiece per second. In other words, the liquid or pasty particles of the melted auxiliary substance 2 hit the surface of the workpiece 20 at a particle speed of 4 m / sec to 20 m / sec. In the case of this smaller particle speed compared to other application methods, advantageously only a small amount of particles rebound from the workpiece, so that a larger layer thickness can be achieved on the workpiece using the method according to the invention.

[0031] In addition to the anti-oxidation effect, the auxiliary substance 2 can also be suitable for removing existing oxidation, in particular removing oxide scale already present on the workpiece surface. In addition, the auxiliary substance preferably also has a lubricating effect.

[0032] The amount of the auxiliary substance applied to the surface of the workpiece 20 is substantially 20 g / m 2 and 400g / m 2 This unit dosage of the auxiliary substance also indicates the thickness of the layer of the auxiliary substance applied on the workpiece surface. The amount of auxiliary substance applied ultimately depends on the effect and properties of the auxiliary substance 2. If the auxiliary substance only has an antioxidant effect, the amount of auxiliary substance applied is 15 g / m 2 Up to 60g / m 2 If the auxiliary substance 2 has the property of removing the oxide already existing on the workpiece 20 in addition to the anti-oxidation effect, it is preferred to use 60 g / m 2 Up to 120g / m 2 If the auxiliary substance 2 has the aforementioned lubricating effect in addition to its anti-oxidation effect and the property of removing the oxide scale already present on the workpiece, then the method according to the invention recommends spraying the auxiliary substance at 120 g / m 2 Up to 400g / m 2 amount is applied to the surface of the workpiece.

[0033] In order to apply the stated amount of auxiliary substance to the surface of the workpiece 20 , a volume flow of the aforementioned gas mixture of 800 l / min to 4000 l / min is required at the outlet of the Venturi nozzle, or more precisely of the spraying device 10 .

[0034] The initial volume flow of the motive gas required for this purpose must be adapted to the respective desired amount of the auxiliary substance to be applied. The amount of auxiliary substance introduced into the spraying device 10, more precisely the volume flow, and the desired resulting layer thickness on the workpiece surface depend on the type of auxiliary substance and the goals to be achieved by coating the workpiece. The motive gas 5 is preferably introduced into the spraying device in the form of a continuous volume flow in order to achieve a continuous volume flow of the gas mixture at the outlet of the spraying device and so as to be able to apply the auxiliary substance uniformly to the workpiece.

[0035] The at least oxidation-resistant auxiliary substances used according to the invention are, for example, phosphates, borates or silicates or mixtures thereof. Optionally, soap and / or graphite can also be contained in the auxiliary substances. The melting temperature of the auxiliary substances according to the invention is in the temperature range between 500° C. and 1200° C. Therefore, the gas mixture must be heated to a temperature in this temperature range by means of a heat source in order to at least partially melt the auxiliary substance 2.

[0036] The method according to the invention can in principle be carried out on its own, i.e. without any temporally close pre-processing or any temporally close post-processing of the workpiece. Preferably, however, the method is integrated into an overall processing of the workpiece, in which temporally close pre-processing of the workpiece and / or temporally close post-processing of the workpiece can be carried out. Temporally close post-processing of the coated workpiece can specifically include heating the coated workpiece in a reheating furnace to a predetermined forming temperature and subsequently forming it. In particular, during the described heating of the workpiece, the applied auxiliary substance layer exerts its antioxidant effect, i.e. reduces the formation of oxide scale during heating.

[0037] Figure 2 A second variant of the method according to the invention and of the spraying device 10 according to the invention is shown. Figure 2 Spraying device and Figure 1 The first variant shown differs in that the gas mixture is heated at the outlet of the spraying device 10 by means of a burner 16. In order for the burner 16 to be able to function, it requires a fuel, in particular a fuel gas 6. The fuel gas 6 is sucked into the spraying device 10 together with the auxiliary substance 2 and the ambient air and is therefore a component of the gas mixture 7 in the spraying device. Alternatively or additionally, the fuel gas 6 can also be supplied directly to the burner at the location of the burner, i.e. at the outlet of the spraying device 10, as in Figure 2 In both cases, the fuel gas may be, for example, propane, natural gas or hydrogen, which is ignited in the burner 16 to form a flame 8. The auxiliary substance 2 contained in the gas mixture 7 is then melted in the flame 8 and sprayed onto the surface of the workpiece 20.

[0038] In the case of an auxiliary substance 2 for heating a gas mixture Figure 1 The first variant shown and Figure 2 In the second variant shown, the auxiliary substance 2 cools very quickly on the surface of the workpiece 20 , so that at least the application of the auxiliary substance as described does not significantly heat up the workpiece.

[0039] List of reference numerals:

[0040] 2 Auxiliary substances

[0041] 3 Ambient air

[0042] 5 Power gas, such as compressed air

[0043] 6 Fuel gas

[0044] 7 Gas mixture

[0045] 10Spraying device

[0046] 12 Venturi nozzle

[0047] 14Transmission pipeline

[0048] 16 Burner as heat source

[0049] 16' heat source that does not form a flame

[0050] 20 workpieces

[0051] FR conveying direction.

Claims

1. A method for machining a metal workpiece (20), the method comprising the following steps: spraying a powdered auxiliary substance (2) onto the workpiece (20) by means of a spraying device (10), wherein the auxiliary substance (2) at least partially melts when leaving the spraying device (10) before the auxiliary substance hits the surface of the workpiece (20); It is characterized in that Before spraying the workpiece (20) with the auxiliary substance (2), the workpiece has a temperature lower than the melting point of the auxiliary substance in powder form, preferably lower than 50° C.; The powdered auxiliary substance (2) has an antioxidant effect; and By melting the auxiliary substance during spraying of the auxiliary substance (2), a uniform auxiliary substance coating is formed on the workpiece.

2. The method according to claim 1, It is characterized in that At least the auxiliary substance (2) and the ambient air (3) are mixed with a motive gas (5), such as compressed air, in the spraying device (10) to form a gas mixture (7); and The gas mixture flows as a continuous gas mixture flow driven by the motive gas (5) along a conveying direction (FR) to the outlet of the spraying device (10).

3. The method according to claim 2, It is characterized in that The gas mixture (7) in the spraying device (10) is heated to at least the melting temperature of the auxiliary substance (2) contained in the gas mixture, so that the auxiliary substance (2) contained in the gas mixture is at least partially melted; heating the gas mixture flow by means of a non-flaming heat source (16'), for example in the form of an electrically operated heating coil, and / or by means of a burner (16) to form a flame; and The non-flame forming heat source (16') and / or the burner (16) are preferably arranged at the outlet of the spraying device (10), respectively.

4. The method according to any one of claims 2 to 3, It is characterized in that The spraying device (10) has a Venturi nozzle (12); and The motive gas (5) is introduced into the venturi nozzle at an initial volume flow rate of 150-300 ltr / min to produce a gas mixture volume flow rate of 800 ltr / min to 4000 ltr / min at the outlet of the venturi nozzle.

5. The method according to claim 3 or 4, It is characterized in that When using a spraying device (10) having a burner (16) at the outlet of the spraying device, the burner gas (6) is mixed with the auxiliary substance (2) together with the motive gas before the gas mixture flow reaches the burner (16) of the venturi nozzle (12), and / or injecting the burner gas (6) directly into the burner (16) at the outlet of the spraying device (10), The injection is preferably carried out with a burner gas volume flow of 4 kg / h to 50 kg / h in each case, in particular when propane gas or natural gas is used as burner gas.

6. The method according to any one of claims 2 to 5, It is characterized in that 10-80 grams of at least partially melted auxiliary substance (2) are applied to the surface of the workpiece (20) by means of the gas mixture flow per second; and / or Liquid or pasty particles of the melted auxiliary substance (2) impact the surface of the workpiece (20) at a particle speed of 4 m / s to 20 m / s.

7. The method according to any one of the preceding claims, It is characterized in that When spraying the auxiliary substance (2), the distance from the outlet of the spraying device (10) to the surface of the workpiece (20) is between 15 cm and 60 cm.

8. The method according to any one of the preceding claims, It is characterized in that In addition to its antioxidative effect, the auxiliary substance (2) also has the property of removing existing oxides, in particular existing scale, from the surface of the workpiece (20) and preferably also has a lubricating effect.

9. The method according to any one of the preceding claims, It is characterized in that If the auxiliary substance (2) has only an anti-oxidation effect, the amount of the auxiliary substance (2) applied to the surface of the workpiece (20) is 15 gr / m 2 With 400gr / m 2 Between, preferably 15-60gr / m 2 or more preferably, if the auxiliary substance (2) has the property of removing oxides already present on the workpiece (20) in addition to its antioxidant effect, the amount is between 60 gr / m 2 With 120gr / m 2 or more preferably, if the auxiliary substance (2) is configured to resist oxidation, remove oxides already present on the workpiece (20) and additionally have a lubricating effect, the amount is between 120 gr / m 2 With 400gr / m 2 between.

10. The method according to any one of the preceding claims, It is characterized in that The auxiliary substances (2) are, for example, phosphates, borates, silicates or mixtures thereof, optionally with additions of soap and / or graphite.

11. The method according to any one of claims 2 to 10, It is characterized in that The melting temperature of the auxiliary substance (2) is in the temperature range between 500°C and 1200°C.

12. The method according to any one of the preceding claims, It is characterized in that Before the auxiliary substance (2) is applied, any oxide present on the surface of the workpiece (20), in particular any oxide scale present, is removed from the surface.

13. The method according to any one of the preceding claims, It is characterized in that Optionally: heating the workpiece (20) uniformly coated with the auxiliary substance to a predetermined reforming temperature; and The workpiece coated with the auxiliary substance (2) is reshaped.

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