METHOD OF CLEANING A WORKPIECE AFTER A THERMAL JOINING PROCESS WITH CATHODIC CLEANING; CLEANING DEVICE AND PROCESSING GAS

Abstract

A method of cleaning a workpiece after a welding process is provided, wherein the cleaning is conducted by removing oxide from the surface of the workpiece which is formed on the weld and the heat-affected zone of the workpiece during the previous welding process, wherein an electric arc is generated between the workpiece and a non-consumable electrode to remove the oxide on the workpiece, wherein a power source is provided to electrically communicate the workpiece and the non-consumable electrode and wherein the non-consumable electrode is anodic connected and the workpiece is cathodic connected.

Claims

1. A method for cleaning a workpiece after a thermal joining process is provided, wherein the cleaning is conducted by removing oxides from the surface of the workpiece which are formed on the workpiece during the previous thermal joining process, characterized in that an electric arc is generated between the workpiece and a non-consumable electrode to remove the oxides on the workpiece, wherein a power source is provided to electrically connect the workpiece and the non-consumable electrode and wherein the non-consumable electrode is essentially anodically polarized and the workpiece is essentially cathodically polarized.

2. A cleaning method according to claim 1, characterized in that the power source generates a direct current circuit by which the non-consumable electrode is anodically polarized and the workpiece is cathodically polarized.

3. A cleaning method according to claim 1, characterized in that the power source generates an alternating current circuit by which the workpiece is polarized anodically for a maximum of 25% of the electrical energy in total.

4. A cleaning method according to claim 1, characterized in that the thermal joining process is a welding or brazing process.

5. A cleaning method according to claim 1, characterized in that the surface of the workpiece is cleaned after the thermal joining process in such a manner that the cleaning is carried out with a predetermined time interval after the thermal joining process.

6. A cleaning method according to claim 1, characterized in that a processing gas is introduced along the non-consumable electrode towards the workpiece.

7. A cleaning method according to claim 1, characterized in that the power source provides a current from 5 A to 100 A, preferable from 10 A to 50 A.

8. A cleaning method according to claim 1, characterized in that the workpiece is made of steel alloy, preferably stainless steel alloy.

9. A cleaning method according to claim 1, characterized in that the workpiece is made of a high alloyed material, preferably a Ni-based alloy.

10. A cleaning device for removing oxides on a workpiece which are formed by a previous thermal joining process comprises: a non-consumable electrode at least one gas nozzle surrounding the non-consumable electrode to introduce a processing gas towards the workpiece a power source electrically connecting the non-consumable electrode and the workpiece characterized in that the non-consumable electrode is essentially anode and the workpiece is essentially cathode.

11. A cleaning device according to claim 10, characterized in that the non-consumable electrode is a tungsten electrode.

12. A processing gas for the method according to claim 1, characterized in that the processing gas contains argon and/or helium.

13. A processing gas according to claim 12, characterized in that the processing gas contains at least another reducing gas selected from H2, CO, NO, N20, CnHm.

14. A processing gas according to claim 12, characterized in that the processing gas contains 0.5 to 5 vol.-%, preferable 1 to 4 vol.-% nitrogen which is preferable balanced by argon.

15. A processing gas according to claim 12, characterized in that the processing gas consists of H2 and Ar in which H2 has an amount of 0.1 to 10 vol.-%, preferable 0.1 to 5 vol.-%.

16. A processing gas according to claim 12, characterized in that the processing gas contains active gas selected from C02, 02 with an amount up to 0.1 vol.-%.

17. A processing gas according to claim 12, characterized in that the processing gas consists of argon and helium in which helium has an amount of 1 to 50 vol.-%, preferable 10 to 40 vol.-%.

Description

[0031] The present invention will now be described with reference to the following non-limiting examples and the accompanying schematic figures in which:

[0032] FIG. 1: a cleaning device in accordance with the present invention to remove the oxides

[0033] FIG. 2: an oxide layer formed in the welding process

[0034] FIG. 3: a cleaning device in accordance with the present invention in a vertical position

[0035] FIG. 4: a continuous line equipped with welding device, cleaning device and a trail.

[0036] FIG. 1 illustrates schematically a cleaning device for removing oxides which are formed during the previous welding process on the weld seam and the heat-affected zone of the workpiece. In this embodiment the workpiece 6 is a T-shaped piece which is constructed by two base materials which are vertically welded, also known as filet weld. The weld 8 finds itself on the place where the two substrates are connected by welding. The weld heat-affected zone 9 extends from the solidified weld interface to the termination of the sensitizing temperature in the substrates. The oxides are formed in this area in presence of both heat and oxygen. The oxides perform as a discoloration on the weld or the heat affected zone. The different colors (e.g. yellow, red, blue, and grey, colorless) are indicating different thickness of the oxide layers. This is the result of the different exposure of the metal surface to temperature, time and oxidizing agent. All are not desirable as all thermal oxides cause changes of the metallurgical structure and properties of the workpiece which adversely affect its corrosion resistance.

[0037] The cleaning device contains a tungsten electrode 2, a gas nozzle 3 surrounding the electrode 2, a power source 7 electrically connecting the electrode 2 and the workpiece 6. The tungsten electrode 2 is anodic connected and the workpiece 6 is cathodic connected. An electric arc is struck between the electrode 2 and the workpiece 6. The end of the arc travels on the oxides to be removed. The gas nozzle 3 is arranged to introduce a processing gas 1 along the electrode 2 towards the workpiece 6. The workpiece is made of steel and preferable made of stainless steel. The oxide to be removed is mainly chromium oxide. The power source 7 generates a DC circuit and provides a current from 5 to 100 A, preferably 10 to 50 A.

[0038] After the weld is cooled down below 200 C., preferred to a usual room temperature, the cleaning device is applied to this workpiece 6 to remove the chromium oxides. The arc length generated amounts about 3 mm. The cleaning device moves relative to the weld 8 with a speed of 0.1 m/min to 6 m/min to remove the oxides formed on the weld and in the vicinity of the weld 8.

[0039] The processing gas fed by the gas nozzle 3 embraces the arc and the cleaning area. The composition of the process gas affects electrical arc discharge, plasma behavior, cathodic spot movement and plasma chemical reactions on the metal and metal oxides surface. The processing gas has a flowrate of about 5 to 20l/min, preferable 10 to 15l/min. The composition of the processing gas could be [0040] Inert gas selected from argon, helium, [0041] 1 to 50 vol.-% helium balanced by argon, preferable 10 to 40 vol.-% [0042] 0.1 to 10 vol.-% hydrogen balanced by argon, preferable 0.1 to 5 vol.-%, very preferred 0.1 to 2 vol.-% [0043] argon containing a reducing gas selected from CO, NO, N2O, CnHm [0044] argon containing active gas selected from O2 and CO2 having an amount from 100 ppm to 500 ppm. [0045] 0.01 vol.-% to 5 vol.-% nitrogen balanced by argon [0046] 0.1 to 5 vol.-% hydrogen and 0.5 vol.-% to 5 vol.-% nitrogen balanced by argon

[0047] By running the separate cleaning device on the workpiece after its welding process, the parameters of the cleaning device can be separately adjusted depending on the characteristics of the oxides formed. These parameters include the processing gas composition, flowrate, arc length, current, travel speed and so on. The cleaning device is therefore able to remove the oxide and treat the workpiece in a flexible and efficient way.

[0048] FIG. 2 shows schematic a cross section through chromium oxide layers 5 which are formed on the weld 8 and in the vicinity of the weld 8 on the workpiece 6 in the welding process. The oxide layer 5 has an uneven thickness due to the welding process, different temperatures and exposure times to oxidizing agents. The nearer to the weld 8, the thicker is the oxide layer 5 and the further from the weld 8, the thinner is it. The thickest area of the oxide layer 5 lies on the weld 8 which can have a thickness of about 175 to 275 nm. The thickness reduces gradually from the weld 8 outwards. The thinnest area of the oxide layer 5 has a thickness of about 5 nm. During the cleaning process the cathodic foot point of the arc can align itself automatically with the edge of the oxide layer to the steel surface of the workpiece 6 due to its highest density of electric field energy.

[0049] Due to the uneven thickness of the chromium oxide layer 5 it is especially beneficial to remove it with the cathodic cleaning method as well as the cleaning device in accordance with the present invention without using any chemicals, mechanical removal or contaminated tools. The oxides can be removed faster, safer and more efficient without costly preparation and post-treatment, without generating dust during the cleaning process in comparison with the prior art.

[0050] FIG. 3 shows schematically the cleaning device in accordance with the present invention. The cleaning device is employed to treat a workpiece 6 which was welded from two base materials adjacent to each other, also known as but weld. Similar as the embodiment in FIG. 1, a power source 7 electrically connects the tungsten electrode 2 to the workpiece 6 to generate an electrical arc 4 therebetween, wherein the electrode 2 is anode and the workpiece 6 is cathode. The power source 6 provides a circuit having a current of 10 to 50 A. A gas nozzle 3 is arranged surrounding the electrode 2 to introduce a processing gas 1 towards the workpiece 6 to enclose the arc 4, to control the cleaning and protect the area to be treated from surrounding atmosphere.

[0051] FIG. 4 shows an application of the cathodic cleaning device 14 in an automatically continuous line. In this embodiment, the workpiece 6 moves in direction from the welding unit 13 towards a treatment unit 15 to be continuously treated in the production line. The weld interface goes first under the welding unit 13 to be welded and the weld 8 generated goes then under the cleaning device 14 to remove the oxides which is formed during the welding process in the welding unit 13. The welding unit has a power source 12 which electrically connects an electrode 10 and the workpiece 6. The electrode is preferably cathodic connected and the workpiece 6 is anodic connected which built a high heat concentration on the workpiece to facilitate its melting and achieve a deep penetration. The welding unit 13 has straight polarity which is opposite to the polarity of the cleaning device 14. The cleaning device can be set with a predetermined time interval after the welding unit 13 to insure that the oxides can be removed efficiently.

[0052] The weld 8 should be preferably cooled below 200 C. before being cleaned by the cleaning device 14 to avoid generating extra oxides during the cleaning process. There could be a cooling unit arranged between the welding unit 13 and the cleaning device 14 to cool down the weld 8. After the oxides have been removed by the cleaning device 14 the weld goes further under a post-treatment unit 15 which introduces primarily inert or reducing gas onto the surface to protect it from contacting air and secondary oxidation, but also to further cool down the surface. Alternatively, this treatment unit 15 could be also arranged between the cleaning device 14 and the welding unit 13. By arranging the cooling unit, cleaning device 14 and welding unit 13 in a continuous production line it facilitates the automation of obtaining a good welding quality in an easy and high efficient way.

[0053] The cleaning device 14 has preferably a control unit which analyses input data from the welding unit 13 and evaluate it and then deliver the evaluated output data to the cleaning device 14 to be able to adjust the parameters as like the processing gas composition, flowrate, current, electrode speed and so forth to adapt to the oxide layer formed on the workpiece 6 in the previous welding process.