METHOD OF AND APPARATUS FOR CONDUCTING PLASMA WELDING

Abstract: A plasma welding method has successive steps including pre-heating of members (15, 16) to be welded, tack welding of the for the members (15, 16) for tentatively fixing these members (15, 16), keyhole welding for completely welding the members (15,16) to each other, and finish padding. These steps are automatically conducted by a single plasma welding apparatus operative in accordance with a programmed sequence including data concerning the optimum conditions of operation in each step in relation to the conditions such as sizes and materials of the members (15,16) to be welded. The apparatus has a welding torch (7), a welding head on which the welding torch (7) is mounted for rotation, a controller (5) adapted for storing the optimum welding conditions for successive steps of the welding process in relation to the conditions of the members (15,16) to be welded, a power supply system (1) for transmitting welding current and welding speed signal to the welding head (6) in accord­ ance with a command provided by the controller (1), and a gas supply system (11,14a,18) for supplying a plasma gas at a rate determined in accordance with a command from the controller (5).

Description: The present invention relates to a method of and an apparatus for automatically conducting plasma welding. More particularly, the invention is concerned with plasma welding method and apparatus suitable for all-position welding of an object, such as a circumferential joint of a pipe, which requires uniform penetration bead formed by, for example, keyhole welding. Plasma welding method has been put into practical use in recent years. This welding method features a higher energy density of arc and a greater length of arc so that welding can be effected with higher heat input rate and, hence, at higher efficiency than conventional welding methods such as TIG (Tungsten-Inert-Gas) welding. Briefly, the plasma welding employs a plasma jet of a high temperature which penetrates a base metal to form a keyhole in the base metal while arc welding is executed. Plasma welding apparatus which have been used hitherto have been designed for flat welding or down hand welding. Recently, however, there has been an increasing demand for all-position welding. All-position welding by plasma welding method, however, requires a considerably high level of skill because the plasma welding apparatus was intended for a single purpose skill such as keyhole welding or melt in welding. Actual welding operation by plasma welding method is conducted by a licensed operator. The operator is required to handle different types of welders and other machines in different steps of the welding process, such as tack welding, pre-heating, keyhole welding and finish padding. More specifically, tack welding is conducted for the purpose of fixing welding objects together in advance of the welding and also for the purpose of preventing any thermal distortion which would otherwise be caused by the heat applied during the welding. The tack welding has thus been conducted typically by TIG welding. The pre-heating is necessary for preventing cracking which may occur as a result of contraction of the weld metal during solidification thereof and also for avoiding any influence of welding heat which may cause an undesirable effect on the metallurgical structure of the base metal. The pre-heating has been effected typically by induction heating or by means of flame of a burning gas. Thus, the known plasma welding method requires various preparatory operations such as tack welding and pre-heating and, in addition, no specific consideration has been directed hitherto to the application of the plasma welding method to a welding object which requires a strict control of the welding operation, e.g., all-position welding of pipe joints. In consequence, it has been necessary that a licensed operator conducts the different steps such as the tack welding and the pre-heating by employing specific devices such as a TIG welder and an induction heating apparatus or a gas burner. Alternatively, these different steps have been carried out respectively by making use of different single-purpose robots designed specifically for carrying out such steps. TIG welding which has been used conventionally provides only a small heat input rate so that the localised portions of members to be welded together are molten uniformly and slowly. For the purpose of successfully carrying out the tack welding, therefore, it is necessary to oscillate the arc between these two members or to insert an additional welding wire so as to increase the amount of the melt of the welding metal. In any case, the welding operation has to be conducted under careful visual check of a skilled person. The necessity for the preparatory works such as tack welding and pre-heating, which requires high degree of skill and high strictness of check, seriously impairs the efficiency and causes an impediment on the effort for saving labor when plasma welding is applied to all-position welding of circumferential pipe joints. US-A-4,168,406 discloses a method of and an apparatus for automatically conducting a plasma welding of pipes according to the precharacterizing part of claim 1 and claim 2, respectively, but does not disclose any automatically conducting successive plasma welding operations including tack-welding. On the other hand, US-A-4,195,216 discloses a keyhole welding by plasma wherein the supplies of a welding current, a plasma gas and a wire are controlled, but does not disclose any automatic plasma welding method wherein several welding steps such as pre-heating, tack-welding, keyhole welding and finish padding are successively conducted automatically. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide plasma welding method and apparatus which enable various preparatory steps such as tack welding and pre-heating, and various further steps of welding such as keyhole welding after the tack welding, and finish padding subsequent to the keyhole welding to be conducted easily and which enable even those who are not licensed to conduct all the steps of the plasma welding operation, thereby to improve the efficiency while saving labor significantly. To solve said object, the present invention provides, in one aspect, a method of automatically conducting a plasma welding of pipes, including the steps of: placing the pipes in abutting relation one to the other; mounting a welding torch on one of the pipes so that the welding torch is movable around the surfaces of the pipes to be welded together; and moving the welding torch around the surfaces of the pipes to be welded to weld the pipes together by a plasma flame, characterized by successively conducting welding operations by the plasma flame, including pre-heating weld portions of said pipes, tack-welding said pipes to tentatively fix them together, conducting a keyhole welding of the thus tentatively fixed pipes and conducting a finish padding of the thus welded portions of said pipes; and during the successive welding operations, controlling the supplies of a welding current and a plasma gas such that the welding current and the flow of the plasma gas supplied during the tack-welding are smaller than those in the keyhole welding. To solve said object, the present invention provides, in the other aspect, a plasma welding apparatus comprising a welding torch, a gas supply system for supplying a plasma gas to said welding torch, an electric power supply system, and means for supporting a fixed pipe to be welded, which supporting means include a welding head adapted to be mounted on said fixed pipe and having a torch rotating means supporting said welding torch for rotation around the outer periphery of said fixed pipe at a desired speed, and means on said welding head for driving said torch rotating means, said apparatus being provided with a controller storing therein time sequence of welding current and welding speed determined based on the sizes, materials and temperatures of the pipes to be welded together and on the welding conditions of successive steps of the welding process including pre-heating, tack welding, keyhole welding and finish padding, said controller also storing therein data of optimum conditions for respective welding steps, said power supply system transmitting a welding current signal and a welding speed signal to said driving means of said welding head in accordance with a command provided by said controller, characterized in that said power supply system includes a flow controller which forms a part of said gas supply system and controls the rate of the plasma gas supply to said welding torch in accordance with a command from said controller. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic illustration of the whole of an embodiment of a plasma welding apparatus in accordance with the present invention; Fig. 2 is a front elevational view of the plasma welding apparatus shown in Fig. 1, illustrating the positional relationship between the welding head, welding torch and members to be welded; Fig. 3 is a side elevational view of the portion of the plasma welding apparatus shown in Fig. 2; Fig. 4 is a diagram showing the arrangement of a gas control section of a power supply system; Fig. 5 is a sectional view of the welding torch suitable for use in the plasma welding apparatus in accordance with the present invention; Fig. 6 shows time charts for respective welding operations performed by the plasma welding apparatus of the present invention; Fig. 7A is an illustration of the manner in which a pre-heating is conducted by the plasma welding apparatus of the present invention; Fig. 7B is an illustration of the manner in which a tack welding is conducted by the plasma welding apparatus of the present invention; Fig. 7C is an illustration of the manner in which keyhole welding and finish padding are conducted by the plasma welding apparatus of the present invention; and Fig. 8 shows time charts of welding operations in respective steps of the welding work conducted by the plasma welding apparatus of the present invention to connect carbon steel pipes. DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described hereinunder with reference to the accompanying drawings. Referring first to Fig. l, an embodiment of the plasma welding apparatus in accordance with the present invention has a welding torch 7 which is rotatably mounted on a welding head 6 through a torch mount 8. The welding head 6 has a clamp l0 for suspending and fixing members l5 and l6 which are to be welded together. The plasma welding apparatus further has a controller 5 which stores a time sequence for the optimum welding operation in relation to the optimum values of factors of the welding operation, such as welding current, welding speed and so forth corresponding to the conditions of the members to be welded, such as sizes, materials and temperatures of these materials, for each of the steps of the welding operation, i.e., the pre-heating, tack welding, keyhole welding and finish padding. The plasma welding apparatus further has a power supply source l which is adapted to feed to the welding head 6 welding current and welding speed signal which are determined in accordance with a welding command issued by the controller 5. In addition, a plasma gas cylinder ll and a shield gas cylinder l2 are provided for the purpose of storing plasma gas and shield gas which are to be supplied through the power supply source l to the welding torch 7 at rates which are determined in accordance with a command given by the controller 5. The plasma gas and the shield gas are supplied from the respective cylinders ll and l2 to the power supply source through gas hoses l4a and l4b. The controller 5 is connected to a commercial electric supply 2 through a cable l3 and the power supply system l through a communication cable l9 so that outputs of the controller 5, including both the welding current and the welding speed signal, are delivered to the welding head 6 through a power cable hose l8 via the power supply system l. Meanwhile, the plasma gas and the shield gas which have been supplied to the power supply source l also are delivered to the welding head 6 also through the power cable hose l8. Figs. 2 and 3 show the relationships between the welding head 6, the welding torch 7 and the members l5 and l6 to be welded. As will be seen from these Figures, the welding head 6 is fixed to the members l5 and l6 by means of the clamp l0 provided thereon. The welding head 6 is constituted by a case 6a and a rotary gear 6b. The arrangement is such that the rotary gear 6b is driven by a drive motor 24 through a pinion 25 so that the welding torch 7 is moved around the member l6 such that it traces a circumferential line X determined by the bevel. Fig. 4 shows the detail of a gas control section incorporated in the power supply source l. It will be seen that the gas hoses l4a and l4b which connect the plasma gas cylinder ll and the shield gas cylinder l2 to the power supply source l are coupled to a plasma gas inlet 29 and a shield gas inlet 30, respectively, through couplings which are adapted to open the inlets 29 and 30 only when the hoses are coupled to these inlets. The plasma gas circuit leading from the plasma gas inlet 29 to a plasma gas outlet 3l includes a pressure switch 34, a flow meter 36 and a solenoid valve 38. Similarly, the shield gas circuit leading from the shield gas inlet 30 to a shield gas outlet 33 has a pressure switch 34, a flow meter 36 and a solenoid valve 38. A back shield gas circuit, which branches from an intermediate portion of the shield gas circuit and leading to a back shield gas outlet 32, also has a flow meter 36 and a solenoid valve 38. These outlets 3l, 32 and 33 of the respective circuits are connected to the respective lines assembled in the power cable hose l8. A flow controller 37 is disposed at an intermediate portion of the plasma gas circuit. The plasma gas circuit and the shield gas circuit are connectable with each other through a communication valve 35. Thus, the welding system can operate with only one gas cylinder provided that the shield gas and the plasma gas have an identical gas composition. Gas supply rate control commands issued from the controller 5 are delivered to the flow controller 37 and the respective solenoid valves 38 so that the gases are supplied at the controlled flow rates through the outlets of the respective gas circuits. Fig. 5 shows the detail of the welding torch 7 used in this embodiment of the plasma welding apparatus. The welding torch 7 has a torch body 2l which surrounds an electrode rod 20 leaving a predetermined annular gap therebetween. The electrode rod 20 is supported by the torch body 2l through the intermediary of an electrically insulating supporting means, not shown, and is spaced from the members l5 and l6 to be welded by a predetermined stand-off distance G. The torch also has a shield cover 22 which surrounds the torch body 2l leaving a predetermined annular gap therebetween. The annular gap between the electrode rod 20 and the torch body 2l constitutes a passage for the plasma gas 3, while the annular gap between the torch body 2l and the shield cover gas 22 constitutes a passage for the shield gas 4. Normally, the electrode rod 20 constitutes a cathode, while the torch body 21 constitutes an anode so that a pilot arc voltage is developed between these anode and cathode. The members 15 and 16 to be welded also constitute anodes. The voltage applied between the electrode rod 20 serving as the cathode and the members 15 and 16 serving as anodes is the plasma arc voltage. The welding current which directly contributes to the welding is supplied in the form of a plasma arc. Namely, the welding current is supplied in the form of an arc column 23 which is jetted through an open end of the torch body 21 together with the plasma gas 3, thereby welding the members 15 and 16 to connect them together. Meanwhile, the shield gas 4 is supplied from the space inside the shield gas cover 22 in such a manner as to envelope the plasma gas 3. The welding torch 7 having the described construction is adapted to be mounted on the welding head 6 through the torch mount 8 such that it can be directed optimumly for each welding operation in the welding process. The welding head 6 carrying the welding torch 7 has the clamp 10 which clamps either one of the members 15 and 16 to be welded. The welding head 6 also carries a wire feeding unit adapted to store and feed a wire 26 which is used in the finish padding. Fig. 6 shows a time chart of a process for conducting a plasma welding including successive steps of pre-heating A, tack welding B, keyhole welding C and finish padding D, with the time represented by the axis of abscissa. The process is started by commencing the supply of the plasma gas 3 and the shield gas 4 at a moment Sl. After the lapse of a predetermined time E which is necessary for the flow of the gases to be stabilized, welding current 9 is supplied to the welding torch 7. The welding torch 7 supplied with the welding current is rotated by means of an actuator (not shown) incorporated in the welding head 6, at a welding speed l7 commanded by the controller 5. In Fig. 6, the plus or positive portion of the welding speed l7 represents forward rotation of the welding torch, while minus or negative portion of the same represents backward rotation of the welding torch 7. It will be seen that the welding torch 7 rotates in the forward direction and in the backward direction alternatingly so as to prevent the power cable hose l8 from being wound around the members l5 and l6 to be welded. The pre-heating A can be conducted by rotating the welding torch 7 forwardly and backwardly at the same speed. However, in each of other steps, i.e., the tack welding B, keyhole welding C and the finish padding D, the welding torch 7 is reversed after the completion of the welding which is conducted during forward rotation of the welding torch 7, as will be seen from Fig. 6, so that the power cable hose l8 is prevented from being wound around the welded members. The backward rotation of the welding torch 7 after the completion of the finish padding D is necessary in order to set the welding apparatus in the initial position in which the welding head 6 is suspended from the welded member l5 or l6 through the clamp l0. Figs. 7A to 7C show the details of the successive operations of the welding process. More specifically, Fig. 7A shows the manner in which the pre-heating A is conducted. In this step, the supply of the welding current is set at a level suitable for the pre-heating of the surfaces of the members l5 and l6 to be welded. The plasma gas 3 is supplied for the purpose of providing a path for the welding current, while the shield gas 4 is used for the purpose of preventing oxidation of the surfaces of the members l5 and l6 to be welded. Fig. 7B shows the manner in which the tack welding B is conducted. The factors of the welding operation, i.e., the level of the welding current 9, welding length which is the product of the welding time T and the welding speed l7 and flow rates of the plasma gas 3 and the shield gas 4, are so controlled that tack weld beads 27 corresponding to the sizes of the members l5 and l6 to be welded are obtained. The number of the points where the tack welding is conducted is determined by the number of peaks of the welding current 9. Fig. 7C shows the manners in which the keyhole welding C and the pad welding D are conducted. This Figure shows a cross-section of the bead which is peculiar to the plasma welding. Namely, both members l5 and l6 to be welded are molten by the welding current 9 and the plasma gas 3 penetrates into the bevel so as to form a keyhole bead 24 thereby completing the keyhole welding C. The finish padding D is conducted for the purpose of forming a convex surface bead 25. This is accomplished by conducting the welding while feeding the wire 20 as shown in Fig. 5. It will be understood that the beads 27 formed in the tack welding B merge in the keyhole bead 24 formed in the keyhole welding C. The plasma welding apparatus of the present invention is used in a manner which will be described hereunder. Optimum time sequences for the welding current 9, the welding speed l7 and the flow rates of the plasma gas 3 and the shield gas 4 are determined beforehand as shown in Fig. 6 through experiments in relation to various factors such as the sizes and materials of the members l5 and l6 to be welded, welding direction, and so forth. The thus determined time sequences are stored in the controller 5. Then, the commercial electric power supply 2 is connected to the power supply source l to which also connected are the gas cylinders ll and l2. Then, the clamp l0 of the welding head is operated to clamp one of the members l5 and l6 to be welded such that the welding head 6 is suspended from the one member. A command is then given for starting the plasma welding apparatus. In response to the starting command, the plasma welding apparatus commences the welding process and conduct the successive steps, i.e., the pre-heating A, the tack welding B, the keyhole welding C and the finish padding D, in the manners explained before in connection with Figs. 7A to 7C. With the plasma welding apparatus described hereinbefore, the operator is required only to prepare and set the members l5 and l6 to be welded, to give the starting command and to support one of the members l5 and l6 during the tack welding. As stated before, the welding work for welding objects by the conventional all-position plasma welding method has required a licensed person to conduct the respective steps of pre-heating, tack welding, keyhole welding and finish padding by employing different types of welders and equipment. In contrast, according to the invention, such a welding work can be conducted by a single plasma welding apparatus which can be controlled even by an operator who is not so skilled. Thus, the present invention contributes to an improvement in the efficiency of the work and appreciably saves the labor while reducing costs. A description will be made hereinunder with specific reference to Fig. 8 as to a practical example of the welding conditions for welding carbon steel pipes made of 50A(2B)SCh40 and placed horizontally, in accordance with the plasma welding method of the present invention. In this case, the plasma welding process included only three steps, i.e., tack welding B, keyhole welding C and finish padding D. Namely, the pre-heating A was omitted in view of the facts that the members l5 and l6 are made of a carbon steel and that the welding is conducted at normal welding temperature. The tack welding B was conducted with the flow rate of the plasma gas 3 of about l.5 ℓ/min, with the welding current 9 of about 70A and with the welding speed l7 of about 80 mm/min. The tack welding B was effected on three points which are located on positions of half past 2, half past 6 and half past l0 on a clock along the circumference of the pipes. The welding time T at each welding point was l5 seconds. The keyhole welding C was conducted with the flow rate of the plasma gas 3 of about 2.5 ℓ/min, with the welding current 9 of about l20A and with the welding speed l7 of about l00 mm/min. The value of the welding current 9 mentioned above is the peak level of the welding current employed in this step. More specifically, the keyhole welding C was conducted by using welding current which has a base level of about 20A and the peak level of about l20A. The time ratio between the peak level and the base level was 4 : 6 and the welding current was supplied at a frequency of about l Hz. The finish padding D was conducted with the flow rate of the plasma gas 3 of about 2 ℓ/min, with the welding current 9 of about l00A and with the welding speed l7 of amount l00 mm/min. The above-mentioned value of the welding current shows the peak level of the welding current. The base level of the welding current was about l5A and the time ratio between the peak level and the base level was 4 : 7. It was confirmed that a finish bead having a good appearance can be obtained by conducting the finish padding D under the above-specified conditions and with the welding current supplied at a frequency of about l Hz. As will be understood from the foregoing description, according to the present invention, the operator can conduct tack welding of objects with plasma welding method simply by controlling the factors such as the welding current, plasma gas flow rate and so forth. This in turn makes it possible to combine the tack welding step with other steps of welding process such as keyhole welding and finish padding which can be conducted by a plasma welding apparatus. It is thus possible to formulate an arc welding process which is composed of a series of steps including pre-heating, tack welding, keyhole welding and finish padding all of which can be conducted by the same plasma welding apparatus. It is also to be understood that the plasma welding process including these steps can be conducted automatically in accordance with a control sequence which is beforehand stored in a controller in relation to various factors or parameters such as the sizes and materials of the members to be welded. In consequence, the present invention enables even an operator who is not so highly skilled to conduct the successive steps of the plasma welding process without any fluctuation in the quality of the weld products, thus contributing greatly to the saving of labor as well as to improvements in the efficiency and economy.

Claims: A method of automatically conducting a plasma welding of pipes (15, 16), including the steps of: placing the pipes (15, 16) in abutting relation one to the other; mounting a welding torch (7) on one of the pipes (15, 16) so that the welding torch (7) is movable around the surfaces of the pipes (15, 16) to be welded together; and moving the welding torch (7) around the surfaces of the pipes (15, 16) to be welded to weld the pipes (15, 16) together by a plasma flame, characterized by successively conducting welding operations by the plasma flame, including pre-heating weld portions of said pipes (15, 16), tack-welding said pipes (15,16) to tentatively fix them together, conducting a keyhole welding of the thus tentatively fixed pipes (15, 16) and conducting a finish padding of the thus welded portions of said pipes (15, 16); and during the successive welding operations, controlling the supplies of a welding current and a plasma gas (3) such that the welding current and the flow of the plasma gas (3) supplied during the tack-welding are smaller than those in the keyhole welding. A plasma welding apparatus comprising a welding torch, a gas supply system for supplying a plasma gas to said welding torch, an electric power supply system, and means for supporting a fixed pipe to be welded, which supporting means include a welding head (6) adapted to be mounted on said fixed pipe (15 or 16) and having a torch rotating means (8) supporting said welding torch (7) for rotation around the outer periphery of said fixed pipe at a desired speed, and means (24, 25) on said welding head (6) for driving said torch rotating means (8), said apparatus being provided with a controller (5) storing therein time sequence of welding current and welding speed determined based on the sizes, materials and temperatures of the pipes to be welded together and on the welding conditions for successive steps of the welding process including pre-heating, tack welding, keyhole welding and finish padding, said controller also storing therein data of optimum conditions for respective welding steps, said power supply system transmitting a welding current signal and a welding speed signal to said driving means (24, 25) of said welding head (6) in accordance with a command provided by said controller (5), characterized in that said power supply system (1) includes a flow controller (37) which forms a part of said gas supply system and controls the rate of the plasma gas supply to said welding torch (7) in accordance with a command from said controller (5).

Inventors: KURITA, SHINICHI, SAKUMA, YASUZI, WATAHIKI, SEISHI, YAMADE, TAKASHI, YAMAGUCHI, MASAYOSHI
Applicants: HITACHI, LTD.
Application year: 1987
Main classification: B23K28
IPC codes: B23K9, B23K10