The present invention relates to a method for detecting defects in a welding joint between two metal surfaces by means of ultrasonic noise and to an apparatus in order to put into practice the method.

When welding different kind of methods are used to detect possible defects in a welding joint in order to prevent damages which can be caused because of leakages through the welding joint. A method using eddy current is required in standards for testing a weld seam in a welded tube. Testing by eddy current, however, cannot detect cracks or small holes, which diameter is smaller than 0,5 millimetre. Another manner to detect possible defects in a welding joint is to make a water leakage pressure test for each individual welded object. Even if this water pressure test is very reliable for finding all leakage points in a welded object, the water pressure test is time demanding and expensive. Therefore, it is not practical, when a huge amount of welded objects should be tested. It is also possible to test a welding joint by means of X-ray in order to detect defects. However, this kind of method is really expensive and, further, has difficulties to detect small cracks.

From the DE patent application 3338615 is known an apparatus for leakage testing of welded tubes, in which apparatus helium gas is conducted inside the welded tube and the leakage of helium is controlled by a mass spectrometer. The helium gas is conducted inside the welded pipe in the whole length of the tube. Thus a huge amount of helium gas is needed in order to keep the sufficient pressure inside the tested tube. The helium leakage measurement is also used in the method of the KR patent application 20050015594, wherein leakage in the welding part of a tank is tested by filling the tank with helium. The JP patent application 7-077477 describes a device and method for detecting leakage in a fuel tank by filling an inspecting pressurized air into the tank. When the tank has a defective hole or a defective gap, air is leaked from the tank and ultrasonic noise of about 40 kHz is generated. Then a leak detector is moved to trace the periphery of the tank to detect the noise. This kind of test arrangement is suitable for separate and closed objects, but the arrangement is not suitable for an essentially continuous testing of the objects.

The object of the present invention is to eliminate some drawbacks of the prior art and to achieve an improved method and apparatus for detecting defects in a welding joint by ultrasonic noise generated by pressurized medium directed towards the welding joint, particularly when detecting defects in a seam weld of a welded tube or pipe. The essential features of the present invention are enlisted in the attached claims.

In accordance with the method of the invention, first a gas flow from a gas source is directed towards a welding joint essentially immediately after the welding joint to be detected for possible defects is produced between the metal surfaces. The gas nozzle and the welding joint are installed to each other in a manner that at least one is movable installed and thus during the operation of the apparatus of the invention moving with respect to another and thus the detection is essentially continuous or at least periodically essentially continuous. When a defect exists in the welding joint, the gas flows through the defect in the welding joint and generates ultrasonic noise which is detected by at least one ultrasonic sensor. In the ultrasonic sensor ultrasonic noise is changed to electric signal. The ultrasonic sensor is advantageously electrically connected with a signal processing device, such as a microprocessor, which operates as a central processing unit of a computer, and further electrically connected to a marker equipment. The aim of the marker equipment on the basis of received signals is to make for further operations a visible marker in that point of the welding joint where the defect was detected. The apparatus of the present invention also contains the gas source which is mechanically connected to a gas nozzle, which directs the gas towards a surface of the welding joint. The gas nozzle, the ultrasonic sensor, the microprocessor and the marker equipment as parts for the apparatus of the invention advantageously form a compact unit and are advantageously positioned in an essentially immediate vicinity to the welding joint to be tested, which is movable with respect to the apparatus of the invention. In another embodiment of the invention the welding joint to be tested is arranged in a fixed position to the apparatus of the invention which then is movable with respect to the welding joint to be tested. Regardless of that the welding joint to be tested is movable or in a fixed position with respect to the apparatus of the invention the gas nozzle is separately positioned on the opposite side of the welding joint to be tested in relation with the other parts for the apparatus of the invention. When there is need after the detection of defects in a welding joint for a heat treatment, such as annealing, and a possible following pickling operation, the marker equipment is advantageously positioned after these process stages. However, the marker operation is carried out for the detected material before any operation stage, which will separate the material from the process line of the method, such as cutting. In the preferred embodiment the method and apparatus of the invention is utilized in the connection with the manufacture of a welded metal tube or pipe after the seam weld or the welding joint is achieved and advantageously cooled. In the manufacture of a metal welded tube or pipe the material to be welded is moving with respect to the welding apparatus and thus the parts for the apparatus of the invention form a compact unit and are positioned in a fixed position in relation to the metal welded tube or pipe. The gas nozzle is in this embodiment positioned in the interior of the metal welded tube or pipe, and the gas, which is directed through the nozzle to the seam weld, is advantageously the same gas which is used as a protecting gas during welding. The ultrasonic sensor, the microprocessor and the marker equipment are positioned outside the welded tube or pipe and thus on the opposite side of the seam weld than the gas nozzle. The gas nozzle can be shaped also so that the structure of the gas nozzle makes possible a radial flow through the nozzle in order to create pressure against the surface of a metal tube or pipe, the gas nozzle having the radial angle between 0 and 360 degrees. Thus at least part of the interior of the metal tube or pipe is pressurized by gas radially directed through the gas nozzle towards the entire internal surface of the welded tube or pipe essentially at the same point with the gas nozzle when relating to the moving direction of the gas nozzle or the welded metal tube or pipe. This arrangement is particularly advantageous when the welding joint is not in a fixed position or the welding joint is not straight, but has for instance at least one curved portion. The connection element between the gas source and the gas nozzle is installed coaxially with the longitudinal axis of the welded tube or pipe. It is then advantageous to use one ultrasonic sensor which is installed outside and around the whole tube or pipe to be detected or to use more than one ultrasonic sensor outside and around the tube or pipe to be detected.

The method and apparatus of the invention are also utilized in testing of already existing and completely welded metal tubes or pipes. In this embodiment the parts for the apparatus of the invention form a compact unit and are positioned either movable or in a fixed position in relation to the welding joint to be tested. Also in this embodiment the gas nozzle is advantageously positioned in the interior of the tube or pipe, while the other parts for the apparatus of the invention are positioned outside the tube or pipe.

In one embodiment of the invention the method and apparatus are utilized in testing plate-like metal objects essentially immediately after the plate-like metal objects are welded to each other. In this embodiment the parts for the apparatus of the invention are positioned either movable or in a fixed position in relation to the welding joint to be tested. In this embodiment the gas nozzle is positioned on the opposite side of the welding joint than the other parts for the apparatus of the invention so that the welding joint of the plate-like metal plates is positioned between the gas nozzle and the other parts of the apparatus of the invention.

The gas to be used in the method and apparatus in accordance with the invention is advantageously inert gas, such as argon or nitrogen especially when the method and apparatus is utilized in connection with welding operation. Advantageously, the gas is the same gas as the protecting gas in the welding operation. When the method and apparatus is utilized in connection with the welding joint which has been achieved essentially earlier than the detection of defects in accordance with the invention an oxidizing gas, such as pressurized air or air in the atmospheric pressure can be used.

The invention is described in more details referring to the attached drawings wherein

Fig. 1 shows a preferred embodiment of the invention schematically as a partly cut side view,

Fig. 2 shows another embodiment of the invention schematically as a side view, Fig. 3 shows schematically the embodiment of Fig. 2 seen from the direction A- A of Fig. 2.

Fig. 4 shows still another embodiment of the invention schematically as a cut side view.

Fig. 5 shows the embodiment of Fig. 1 as seen from the direction B-B.

In accordance with Fig. 1 and Fig. 5 a metal strip 1 is moving with respect to the direction shown by an arrow 2 to bending rolls 3. The rolls 3 bend the edges 15 and 16 of the strip 1 towards to each other so that the edges of the strip 1 is welded with a welding machine 4 in order to have a position fastened to each other by a welding joint 5 in a shape of a metal tube 6. For the apparatus of the invention, a gas source 7 is placed so that the gas source 7 is mechanically connected by a connecting element 14 with a gas nozzle 8 which is positioned in the interior of the welded metal tube 6. The outlet of the nozzle 8 is directed towards the welding joint 5 of the welded metal tube 6. The ultrasonic sensor 9, one part for the apparatus of the invention, is positioned outside of the metal tube 6 essentially at the same point with the gas nozzle 8 when relating to the moving direction 2 of the welded metal tube 6. At least one ultrasonic sensor 9 is electrically connected to a microprocessor 10, which microprocessor 10 is further electrically connected with a marker equipment 12. The marker equipment 12 is positioned outside the welded metal tube 6 after the ultrasonic sensor 9 when relating to the moving direction 2 of the welded metal tube 6. The ultrasonic sensor 9 and the marker equipment 12 are positioned essentially close to the welding joint 5 of the welded metal tube 6.

When operating the method and apparatus of the invention in connection with the embodiment of Fig. 1 , inert gas is fed from the gas source 7 to the gas nozzle 8. By means of the gas nozzle 8 nitrogen is directed to the welding joint 5 of the metal tube 6. If any defect exists in the welding joint 5, the defect causes a gas leakage from the interior of the welded metal tube 6 to the exterior of the welded metal tube 6. This gas leakage generates ultrasonic noise, which noise is detected by the ultrasonic sensor 9. In the ultrasonic sensor 9 the ultrasonic noise is changed to electric signal. This electric signal from the ultrasonic sensor 9 is electrically transferred to the microprocessor 10. Based on the electric signal from the ultrasonic sensor 9 the microprocessor 10 controls the marker equipment 12 to put a marker on that point of the welding joint 5, where the gas leakage was detected. For controlling the operation of the marker equipment 12 for finding the leakage point in the welding joint, the microprocessor 10 is electrically connected with a measuring equipment (not shown) measuring the moving speed of the welded metal tube 6.

According to the embodiment of the Figs. 2 and 3, a gas nozzle 21 as a part for the apparatus of the invention is installed beneath two metal plates 23 and 24 and is directed to the welding joint 22 between two metal plates 23 and 24. The metal plates 23 and 24 are installed movable to the gas nozzle 21 and the welding machine 29. The moving direction of the metal plates 23 and 24 is shown as an arrow 25. An ultrasonic sensor 26 is positioned essentially at the same point as the gas nozzle 21 when comparing to the moving direction 25 of the metal plates 23 and 24, but above the metal plates 23 and 24. The microprocessor 27 is installed after the ultrasonic 26 when comparing with the moving direction 25 and the marker equipment 28 after the microprocessor 27 above the metal plates 23 and 24.

During the operation of the method and the apparatus of the invention, ultrasonic noise generated by gas leakage through the welding joint 22 is detected in the ultrasonic sensor 26. The ultrasonic noise detected is also changed to electric signal in the ultrasonic sensor 26. The electric signal is further conducted to the microprocessor 27 which gives a signal for the marker equipment 28 to make a marker in order to indicate a leakage point in the welding joint 22. According to Fig. 4, the method of the invention is used for the welding joint 31 of a welded metal tube or pipe 32. Before positioning the gas nozzle 33 in the interior of the welded metal tube or pipe 32, the gas nozzle 33 is positioned in a connecting element 34, which is essentially tightly connected with the starting end of the welded metal tube or pipe 32, through which the gas nozzle 33 is moved in the interior of the welded metal tube or pipe 32. The other end of the welded metal tube or pipe 32 is also provided with a connecting element 35, which is essentially tightly connected to the end of the welded metal tube or pipe 32. The connecting elements 34 and 35 make possible to use pressurized air as a gas in accordance with the invention, which pressurized air is conducted to the gas nozzle 33 through the connecting element 37. When the gas nozzle is moving in the interior of the welded metal tube or pipe 32, the ultrasonic sensor 36 is moved with the same speed as the gas nozzle 33 in order to detect a possible defect. The ultrasonic sensor 36 sends the signal to a microprocessor and further to a marker equipment 38 in the similar manner as described in connection with the embodiments of Figs. 1 and 2. The marker equipment 38 is also installed to move at the speed as the ultrasonic sensor 36 in order to make possible the marker operation after a defect has been detected.

The embodiment of Fig. 4 can be utilized also in a manner that the gas nozzle 33 is positioned in a fixed position, and pressurized air is fed through the gas nozzle 33 in order to fill the metal tube or pipe 32 with pressurized air. Then the surrounding ultrasonic sensor 36 is moved along the metal tube or pipe 32 for detecting possible defects.