A PROCESS FOR CONTROLLING WELDING WHEN USING A STRIPELECTRODE

Abstract: In order to overcome the problem that nothing can be done to prevent welding defects such as undercut, or slag inclusion that can occur at the end of a wide bead by using electromagnetic correction means to correct the irregular flow of the molten pool caused by variations in the distribution of the welding current because of the impossibil­ ity or difficulty of monitoring the conditions of the molten pool; a magnetic flux detector is arranged in the vicinity of the rod electrode to detect the magnetic field generated by the welding current, and the magnetic field of the electro­ magnetic correction means, so as to regulate the direction of flow of the arc and the molten pool using the operation of the magnetic field as a synthetic magnetic flux; thereby automa­ tically controlling the operation of the electromagnetic correction means and forming a high quality, uniform welding bead without monitoring the molten pool effectively and it is thus suitable for efficient build-up welding by the rod electrode.

Description: A PROCESS FOR CONTROLLING THE WELDING USING A STRIP ELECTRODE Technical Field The present invention relates to a process for controlling the welding in which a strip electrode is used and particularly to a surfacing welding which needs no controlling operation in the course of welding and can provide welded portions having no defects such as undercut, lack of penetration or irregular bead edge. The welding carried out by using a strip electrode can provide weld beads having a broad width and a shallow penetration in one pass, so that this can be advantageously used in the surfacing welding which aims at the improvement of surface hardening or corrosion resistance. The welding using a strip electrode includes (1) arc fusing welding and (2) electro-slag fusing welding and the present invention intends to be applied to both the above described processes (1) and (2). Submerged arc welding process is carried out by forming arc between a base metal to be welded and a strip electrode and electro-slag welding process utilizes resistant heating of a molten slag bath formed just below or back an electrode and passes electric current to a slag by directly immersing a strip electrode in a molten slag bath having a depth of about 10 mm. Thus, the above described processes (1) and (2) have a fairly large difference in the welding phenomenon per se but both the processes use a large direct current, so that complicated factors, such as the action of magnetic field caused by the welding current, the pulling phenomenon of parallel currents and Pinch effect, are combined, and although in the process (1), a specific phenomenon that the arc shifts to the central portion in the width direction of the strip electrode, is included, in both the processes the inward flows toward a central portion from both edge portions of a strip electrode are caused in molten slag and metal and therefore defects, such as undercut and lack of penetration are caused at both edge portions or one edge portion of the bead and further an irregular bead edge is formed and these defects are common in both the processes (1) and (2) and the similar behaviors are found. The generation of the above described defects is more noticeable as the width of the strip electrode used for the welding is broader and this is common in both the processes (1) and (2) and recently an electrode having a width of more than 100 mm has been used and it has been strongly demanded to prevent and obviate these defects. Background Art In the arc fuse welding process, it has been proposed that the direction of the arc is oriented to the same direction as the orienting direction of the electrode or to the slightly lateral outward direction by an external magnetic field or an induced magnetic field respectively in Japanese Patent Application Publication Nos. 8,348/69 and 8,349/69. Japanese Patent Application Publication No. 21,187/79 discloses that a parallel portion to a welding line along a given length in the vicinity of surface of a metal to be welded is formed with a conductor which connects an electric power source for welding and a welding energizing chip and the arc and the molten metal are pulled toward the side edges. Recently, the applicant has made development regarding the electro-slag welding process wherein the flow direction of the molten pool is oriented to both the lateral edges of the electrode from the portion just beneath the electrode by applying an external magnetic field. However, even though the above described prior technics have the effect that the orienting direction of the arc and the flow direction of the molten pool are forcedly varied and the flow can be oriented to the lateral direction of the strip electrode, in order that these technics are commercially practised, the fundamental problems mentioned hereinafter have never been solved, so that these technics have not been practically used and are troublesome and unstable. When the arc welding processes as in the above described prior arts are applied to the actual submerged arc welding, the molten metal pool and the molten slag bath formed behind the electrode excluding the arc cavity just below the strip electrode, are completely covered with the welding flux, so that the welding status behind the just below portion of the electrode, particularly the deflection of the arc and the -flow state of the molten pool can not be observed during operation and after the molten slag bath and the molten metal pool are completely solidified, the flux thereon is removed and further the solidified slag is removed, then firstly the finished state of the solidified metal bead can be confirmed, so that the status of the molten pool just below and back the electrode, which is most important, can not be known during welding and there is no way other than that said status is presumed from the bead form after lapse of several time. In the welding using a strip electrode through the electro-slag fusing, the molten pool just below and behind the electrode is exposed, so that the flow status of the molten pool can be observed by naked eyes and therefore the welding can be carried out while confirming the flow status and controlling the intensity of the external magnetic field. However, even in this case, the external magnetic field should be always manually controlled with respect to the behavior of the molten pool which varies every moment, so that the operator is exposed to the strong radiant heat from the exposed molten pool for a long time from the welding start to the finishing and there is problem in this point and the flow status of the molten pool is controlled by perception through naked eyes, so that it is difficult to obtain the satisfactory uniform bead. Disclosure of Invention The present invention aims to solve the above described problems of the prior arts and to provide a process for automatically controlling the operation of electromagnetic correcting means which is applicable to both the submerged arc welding process and the electroslag welding process when a welding is carried out under the action of magnetic field due to the welding current between a strip electrode and a base metal to be welded while energizing the electromagnetic correcting means, which generates an external magnetic field opposing to the above described action of magnetic field and this object can be advantageously attained by providing a detector which detects a combined magnetic flux of magnetic field due to the above described welding current and the above described external magnetic field due to the electromagnetic correcting means which modifies the magnetic field due to the above described welding current and the molten metal flow, in the vicinity of a strip electrode and controlling the action of the above described electromagnetic correcting means corresponding to the detected output. The inventors have made study with respect to these prior technics in order to solve the problems of the prior arts and fully clarified the causes of the problems. When the states of arc and molten pool are controlled by the above described external magnetic field or induced magnetic field in the prior submerged arc welding process, the most important deflection of the arc and flow state of the molten pool can not be observed by naked eyes during the welding operation but the actual state is as follows. The welding condition in the case of a usually used strip electrode having a thickness of about 0.4 mm and a width of about 100 mm is mentioned as follows as one embodiment. Welding current (DC, Base metal: : 1500 Amp. Welding voltage : 30 Volt. Welding rate : 150 mm/min. In this case, the state where the form of the weld bead can be confirmed, that is the state where the slag is completely solidified and can be removed from the bead, is obtained at more than 150 mm, usually more than 200 mm, that is more than 1 minute in time after the arc has passed through. Thus, the states of the arc and the molten pool are confirmed only indirectly by the solidified bead form fairly later in view of position and time. Even though the melting behavior can be observed by naked eyes in every points in the course of welding, the health of the operators is damaged by the severe radiant heat stimulation and the manual control relying upon the perception of naked eye observation is apt to become uncertain and the problem can not be solved unless relying upon the automatic control. According to the present invention, in the surfacing welding process using a strip electrode wherein a magnetic field opposing to the action of magnetic field due to the welding current which is applied between the strip electrode and a base metal to be welded, is generated and an electromagnetic correcting means which modifies the flow of the molten metal in molten pool, is energized, a device for detecting the magnetic flux, such as hall element, that is an element causing hall effect, is arranged near the strip electrode to detect the degree of the current distribution in a magnetic flux density and by the magnitude of the detected signal, an output of an electromagnetic coil, an apparatus for applying parallel current or other electrically operating apparatuses, which changes the orienting direction of an arc and a flow of molten pool, is varied, whereby the stable welding is automatically conducted by means of the strip electrode. The present invention is a process for automatically controlling a welding by means of a strip electrode as mentioned above irrelative to whether the behavior of the arc and molten pool can be observed by naked eyes or not, so that the present invention can effect a uniform and stable welding in both the submerged arc welding process and the electro-slag welding process. Brief Explanation of Drawings Figs. l(a) and (b) are diagrammatic views for illustrating the welding behavior in a surfacing welding using a strip electrode in submerged arc welding process; Figs. 2(a) and (b) are the same diagrammatic illustrating views in electro-slag welding process; Fig. 3 is a diagrammatic view for illustrating a surfacing welding manner; Fig. 4 is a diagrammatic view for illustrating a manner of electromagnetic correction in submerged arc welding process; Fig. 5 is cross-sectional views of essential parts showing the manner where defects of weld beads are formed in the process shown in Fig. 4; Fig. 6 is a diagrammatic view for illustrating an embodiment of applying the present invention to the electro-slag welding; Figs. 7(a) and (b) are views for illustrating the controlling manner of electromagnetic correcting means; ; Fig. 8 is a view for illustrating test positions of weld beads; Figs. 9(a) and (b) are views for showing a control circuit used in an example of the present invention; and Figs. 10, (a)-(f) are views for showing examples of arrangement of the detector. Best Mode of Carrying Out the Invention The construction of the present invention will be explained hereinafter in detail with respect to a practical mode of carrying out the present invention but firstly, explanation will be made with respect to the foundation of the welding using a strip electrode with reference to the drawings. Fig. 1 shows a welding behavior in the surfaceing welding using a strip electrode in the submerged arc welding process and Fig. 2 shows that in the electroslag welding process. In these figures, e is a strip electrode, a is an arc cavity, b is a molten slag bath, f and f' are a welding flux, p is a molten metal pool, s is a solidified slag, w is a solidified metal and m is a base metal to be welded. In this case, the flow of the molten slag bath b and the molten metal pool p owing to the magnetic action under complicated distribution of the welding current prevents the formation of a stable surfacing weld bead and the problems in the prior arts concerning the application of various electromagnetic correcting means are already mentioned but it has been found that the problems in application of the electromagnetic correction as mentioned hereinafter are important in the practical welding working. As shown in Fig. 3, grounds for supplying a welding power source to a base metal were provided at the central position of the width of the base metal in front and rear with respect to the advance direction of welding on the base metal having a width of 1000 mm, a length of 1000 mm and a thickness of 100 mm, a strip electrode e was perpendicularly directed to an upper surface of the base metal to be welded and the polarity of the base metal and the electrode was made to be negative and positive respectively and the surfaceing welding was effected under the following condition. Size of strip electrode : Thickness : 0.4 mm, Width : 100 mm Component of electrode : SUS 309 Flux : KBS-150 (Firing type) Welding current : 1500 Amp Welding voltage : 30 Volt. Welding rate : 15 cm/min. In this case, among the above described prior arts for correcting the orienting direction of arc, a process wherein electromagnetic coils C and C' are arranged arround the strip electrode as shown in Fig. 4, was adopted. The welding was effected in one pass along each of three rows of line Q connecting between both the ends of grounds t-t and lines #' and Q" positioning at left and right side of said line Q as shown in Fig. 3, Concerning an original pass #, the exciting current of said current controllers (A) and (B) was previously adjusted to the condition that a good bead form not causing undercut at both edge portions of the bead when the welding starts and a bead was welded on the surface of the base metal to be tested along a length of about 900 mm under the adjusted condition without effecting the adjustment thereafter. The bead form of each of the obtained passes Q, i' and Q" is shown in Fig. 5 and in the welded pass Q at the central portion in the width of the steel sheet, the bead form when starting the welding was substantially held until the welding was finished and one substantially stable bead was obtained, but in the beads in the welded passes Q' and Q" along the left and right lateral edge portions of the steel sheet, when the advance of the welding exceeded the middle position which is half of the distance between the grounds t-t, the undercut u was formed on one side and the slag inclusion r was partially formed and the good state when starting the welding was not maintained. This experiment results show the following fact. The behavior of the arc and the molten pool is varied every moment according to the relative relation to the position of the ground t and the thermal and electric behavior at the central portion and the edge portion of the steel sheet, so that it is impossible to deal with the above described variations only by controlling the exciting current only when starting the welding. Therefore, it is necessary to control the condition (in this case, exciting current) based on the variation of the above described behavior in every point but in the case of the submerged arc welding, the behavior at every moment in the course of welding can not be obserbed by naked eyes based on the already mentioned reason, so that it has been impossible to deal with the above described variation of behavior by the prior arts. This point is similar in the electroslag welding wherein even if the observation through naked eyes is feasible, the continuation is difficult. The present invention will be explaned with respect to the status wherein the electro-slag welding is effected by using a strip electrode with reference to Fig. 6. The reason why the electro-slag welding process is adopted herein is based on that the molten pool can be observed and the effect for controlling the electromagnetic correcting means can be directly and easily confirmed. When the welding is effected by arranging two electromagnetic coils C and C' in the front of the strip electrode e in the advance direction of welding so that these coils position at the substantially lateral edge portions of the electrode, one hall element h, h' is arranged at each of both the lateral edge portions of the strip electrode e. The hall elements h and h' are connected to a control device through lead lines, the hall elements h and h' detect the magnetic flux density by utilizing the hall effect of the elements and the voltage output corresponding to the magnetic flux density is obtained and in general, hall IC is advantageously used. The control mechanism is explained with reference to Fig. 7. The electromagnetic coils C and C' are arranged so that these coils position in the front of the electrode at both the edges and the flow of the molten pool is controlled by the external magnetic field caused by the electromagnetic coils C and C' as in the prior art. The subject matter of the present invention lies in that the intensity of the magnetic field generated from the electromagnetic coils C and C' is automatically varied depending upon the variation of the other external conditions so that the original good state is always maintained, whereby the given flow state of the molten pool is always maintained and a stable bead form is obtained. Heretofore, the intensity of the exciting current passing through the electromagnetic coils C and C' has been manually controlled by an operator while observing the variation of the flow state of the molten pool b, but according to the present invention, the variation of the external state is detected by both the hall elements and the intensity of the exciting current applying to the electromagnetic coils C and C' are automatically controlled by the detected signal. In this example, hall elements h and h' are arranged at both the lateral edges of the electrode. When a welding current flows through a strip electrode e, a magnetic field is formed arround the electrode and the intensity of the magnetic field is varied by the current distribution in the strip electrode e and the distance from the grounds of the base metal but the behavior of the magnetic field at both the lateral edges of the electrode gives the greatest influence upon the flow of the molten pool P and the present invention detects this behavior. For making the explanation easy, mention is made with respect to only one side of hall element. In Fig. 7, (a), the intensity of the magnetic field at the lateral edge of the strip electrode is detected by a hall element h and the detected signal is supplied to a next comparison circuit in voltage. The comparison circuit compares the input detected voltage with the set value of a setting circuit. The output of the comparison circuit is supplied to a voltage-current converter circuit which generates a current output having a value corresponding to the magnitude of the compared value of the comparison circuit. The output current of the converter is supplied to the coil c thereby the intensity of the exciting current is controlled. Explanation will be made in more detail. The coil c is arranged at the right side edge in front of the strip electrode e as mentioned above, so that when the exciting current flows to the coil c, the distribution of line by magnetic force at the lateral edge portion of the strip electrode e varies, whereby the behavior of the molten pool flow is improved, so that a value (voltage) detected by the hall element h when the flow behavior becomes best, has been preferably previously set in the setting circuit. This set value varies depending upon the position where the hall element is arranged and depending upon the thickness and width of the strip electrode e. The position of the hall element h is easily fixed by a proper means, for example by a conduct jaw and the thickness and width of the strip electrode e is constant during welding, so that there is practically no problem. In the process used in this example, the above described comparison is effected and until the difference becomes 0, the exciting current supplied to the electromagnetic coil C from the voltage-current converter circuit is increased and reversely when the detected value is smaller than the set value, the operation is effected so as to decrease the exciting current and a design is made so that the comparison value always maintains 0. The above described control is effected in the quite same manner in the electromagnetic coil C' and hall element h' in the opposite side and the variation of the status at the left and right lateral edge portions is detected in this manner, whereby the external magnetic field is controlled so that the moderate value is always obtained and the good bead can be formed. The hall element is advantageous based on the following reason. (1) The detection of the magnetic field is feasible by a usual coil but the weight and the size become large and the appratus should become a large size. In this point, the hall element can be a small size and is very advantageous. In particular, if hall IC is used, said hall may be made of a more small size and it is possible to simply arrange said hall in the vicinity of the electrode. (2) The accuracy of detection of the magnetic flux density is high and therefore the detecting ability to the status variation is excellent. When the welding is carried out in this manner, the following result was obtained. One bead was welded on each line of the right side, center and left side in the substantially same test pieces as shown in Fig. 5. The welding condition in this case is as follows. Size of strip electrode : Thickness : 0.4mm Width : 150 mm Component of electrode : SUS 309 Flux : KFS-150 (fusing type) Welding current : 2500 Amp. Welding voltage : 26 Volt. Welding rate : 15 cm/min. Electrode : Base metal: Electromagnetic coil (1) Direction of magnetic field Electromagnetic coil C : Downward magnetic field Electromagnetic coil C' : Upward magnetic field (2) Turn number of electromagnetic coil In both left and right : 800 turn, iron core. The control circuit in this case is shown in Fig. 9. The form of the weld beads obtained by means of three strip electrodes under the above described condition was investigated at each position shown in Fig. 8 with respect to five points from the weld-started portion to the weld-finished portion and every portion showed the good uniform bead form. The behavior of flow of the molten pool during welding was constant and stable. During this welding, it has been confirmed that the exciting current of the electromagnetic coils C and C' automatically varies in the center being 2.5 Amp within a range of +1.5 Amp and the welding is completely automatically controlled. As mentioned above, it can be seen that the present invention using the hall elements is very effective for increasing the practicality of the welding process using a strip electrode. In the above described example, explanation was made with respect to the case where the electro-slag welding process was carried out by arranging the electromagnetic coils C and C' in front of the strip electrode e but when the above described control against the electromagnetic correction was applied in the case where the strip electrode e was put between the electromagnetic coils C and C' as shown in Fig. 4, the good result was obtained. Furthermore, in an arc fusing system, the similar result can be obtained and also in this case, the hall elements are arranged at both the lateral edge portions of the electrode and the control circuit between the coil and the hall element is the same as shown in Fig. 9. In the above described examples, the hall elements h and h' were arranged at the lateral edge portions of the strip electrode but the arranging position is not limited to the lateral edge portions and as shown in Fig. 10, even if these halls are arranged at the front and rear of the welding direction (arrow mark) of the strip electrode e or at the position which slightly shifts to the center from the edge portion or at the central portion, when the behavior of the magnetic field can be preferably detected, the object of the present invention can be attained. Thus, the same effect can be expected by arranging the hall elements at the side of the electromagnetic coil. However, it is important that two or more hall elements are used, because in the detection using one hall element, the behavior of only one side can be found and even effect can not be obtained in the strip electrode. The controllability can be naturally more improved by increasing the positions to be tested by using three or more hall elements. In addition, as shown in Fig. 7(b), the setting circuit may be commonly used. Industrial Applicability As explained above, the present invention can stably form a broad weld bead having no undercut nor slag inclusion by using a strip electrode and can be applied to a broad use, such as surfacing welding having high reliability in submerged arc welding or electro-slag welding.

Claims: CLAIM 1. A process for controlling welding in which a trip electrode is used, said welding being carried out by passing electric current between a broad strip electrode and a base metal to be welded while moving said electrode along a surface of said base metal toward a direction of welding line which coincides with width direction of the electrode and energizing an electromagnetic correcting means which generates a magnetic field opposing to action of magnetic field due to a welding current flowed in the electrode and modifies a flow of molten metal in welding molten pool, characterized in that a device for detecting a combined magnetic flux caused by the above described both magnetic fields is arranged near the molten pool to control action of the above described electromagnetic correcting means corresponding to the detected output. 2. The process as claimed in claim 1, wherein the device for detecting the combined magnetic flux is a hall element. 3. The process as claimed in claim 2, wherein the hall element is arranged in a plurality of numbers including ones arranged at both sides of the strip electrode. 4. The process as claimed in claim 1, 2 or 3, wherein the passing of the welding current is a submerged arc process. 5. The process as claimed in claim 1, 2 or 3, wherein the passing of the welding current is an electroslag process.

Inventors: TATEISHI, JUNJI, YAMASHITA, ITARU
Applicants: KAWASAKI STEEL CORPORATION
Application year: 1982
Main classification: B23K9
IPC codes: B23K9, B23K25