PIPE FACE TO FACE WELDING METHOD FOR

MAINTENANCE OF UNIFORMITY WELDING

TEMPERATURE AND MACHINE FOR THE SAME Technical Field [1] The present invention relates to a pipe face-to-face welding machine, and more par¬ ticularly a pipe face-to-face welding machine for maintenance of uniformity welding temperature and machine for the same, which may keep a welding temperature of the entire welding area uniformly on an average and also restrain a rising time for preheating to reduce a welding time so that a specific welding area is not dug or unsmooth when a pipe connection is not welded at a regular temperature as a whole in case cylindrical pipes are welded in a face-to-face manner. Background Art [2] Generally, a plurality of pipes are welded to form a pipeline so as to construct a long fluid passage such as an oil pipeline. For forming such a long pipeline, a pipe face-to-face welding machine is used. [3] A conventional pipe face-to-face welding machine used for this purpose is schematically shown in FIG. 1. Referring to FIG. 1, the conventional pipe face-to-face welding machine includes a clamp 14 for pressing and fixing circumferences of pipes 11, 12, a welding table 18 having a welding torch 16 mounted adjacent to connection of the pipes 11, 12, a driving motor 20 for rotating the welding table 18 around the cir¬ cumference of the connection of the pipes 11, 12, a welding machine body 22 connected to the welding torch 16 to apply a welding current to the welding torch 16, and a wire supply motor 26 for supplying a welding wire 24 to the welding torch 16. [4] Such a conventional pipe face-to-face welding machine controls rotating speeds of the driving motor 20 and the wire supply motor 26 in consideration of diameter and thickness of the pipes 11, 12 so that the connection of the pipes 11, 12 may be welded uniformly. [5] However, the conventional pipe face-to-face welding machine has a preheating area at a welding start point and a down slope at a welding end point as shown in the graph of FIG. 2. However, in an actual welding area, a constant welding current is supplied to the welding torch 16 so that the connection of the pipes 11, 12 is heated higher as the welding work is progressed, which often causes the welding area is dug at a rear portion of the welding area. [6] Thus, it is difficult to form the welding surface of the entire connection uniformly, and it is a factor of high inferiority rate. [7] Meanwhile, such a temperature difference in the welding area may not be solved by just controlling a rotating speed of the driving motor 20 for driving the welding table 18 and a rotating speed of the wire supply motor 26 for supplying the welding wire 24. In addition, there is also difficulty to realize a controller for minutely adjusting a rotating speed of the motor. Disclosure of Invention Technical Problem [8] The present invention is designed to solve the above problems of the prior art, and therefore it is an object of the invention to provide a pipe face-to-face welding machine for maintenance of uniformity welding temperature and machine for the same, which may keep a welding temperature of the entire welding area uniformly on an average and also restrain a rising time for preheating to reduce a welding time so that a specific welding area is not dug or unsmooth when a pipe connection is not welded at a regular temperature as a whole in case cylindrical pipes are welded in a face-to-face manner. Technical Solution [9] In order to accomplish the above object, the present invention provides a pipe face- to-face welding method for maintenance of uniform welding temperature, which includes (a) setting a welding start temperature and a temperature at a welding end point in accordance with a welding temperature and then subdividing the welding temperature into a plurality of welding temperatures; (b) setting a magnitude of a welding current for maintaining the corresponding temperature in accordance with the subdivided welding temperatures; (c) setting a linear current slope for lowering a magnitude of the welding current according to the temperature at the welding end point to a zero potential so as to stabilize the welding work for a predetermined time after the welding end point; (d) selecting a start welding current and an end welding current having a relatively lower magnitude from the welding current set in the (b) step while an object is welded, and then downsizing a magnitude of current from the start welding current to the end welding current as the welding work is progressed; and (e) lowering the magnitude of the welding current to a zero potential by means of the slope set in the (c) step corresponding to the magnitude of the end welding current after the step (d) is completed. [10] In another aspect of the invention, there is also provided a pipe face-to-face welding machine including a clamp for pressing and fixing circumferences of pipes, a welding table having a welding torch mounted adjacent to a connection of the pipes, a driving motor for rotating the welding table around a circumference of the connection of the pipes, a welding machine body connected to the welding torch to apply a welding current to the welding torch, and a wire supply motor for supplying a welding wire to the welding torch, the machine including a welding sectional current setting unit for dividing a welding area of the pipes into a plurality of sections according to a control signal, and supplying the welding current to the welding machine body with lowering the welding current gradually for each section; and a welding current control unit for controlling the welding sectional current setting unit according to setting conditions requested by a user, the welding current control unit including a PLC (Programmable Logic Controller) for opening or closing a plurality of contact points according to a program so as to maintain a normal operation according to the welding sectional current setting unit. [11] Additionally, the PLC may include a welding material setting unit for setting material of an object to be welded; a section division setting unit for setting the number of divided sections, which are work environments according to a diameter of pipe to be welded; a memory storing data according to welding environments; a sensing unit for sensing change of state according to the welding work by means of a plurality of sensors; and a controller for accessing system operation data from the memory to form a data table for a start welding temperature and a welding temperature gradually lowered in the case that the welding material setting unit sets the material of pipe to be welded and the section division setting unit sets divided sections according to the welding work, the control unit then opening or closing the contact points accordingly and receiving a sensing signal according to the operation of the welding system through the sensing unit to conduct start operation and correction of an error generated during operation according to the welding system. [12] Also additionally, the welding sectional current setting unit may include a welding current supply unit composed of the N number of welding current generators for generating current set by an enable operation signal and a down slope current generator for linearly lowering a current from a predetermined current to a zero potential for a predetermined time; and a welding current setting unit for selectively operating any of the N number of welding current generators of the welding current supply unit according to a control signal input from the PLC, and adjusting a linear current down slope of the down slope current generator of the welding current supply unit. Brief Description of the Drawings [13] FlG. 1 is a schematic view showing a conventional pipe face-to-face welding machine; [14] FlG. 2 is a graph showing a welding current in a welding area according to the prior art; [15] FlG. 3 is a schematic view showing a pipe face-to-face welding machine for maintaining a uniform welding temperature according to the present invention; [16] FlG. 4 is a detailed diagram showing a welding current control unit, which is an essential component of the present invention; [17] FIG. 5 is a partial view showing a controller ladder diagram of FIG. 4 in case the welding area is classified into 6 sections in the present invention; and [18] FlG. 6 is a graph showing a welding current of each section of the welding area according to the ladder diagram of FlG. 5. Best Mode for Carrying Out the Invention [19] The present invention will be described in detail on the basis of the drawings. [20] FlG. 3 is a schematic view showing a pipe face-to-face welding machine for maintaining a uniform welding temperature according to the present invention, and FIG. 4 is a detailed view showing a welding current control unit, which is an essential component of the present invention. [21] Referring to FlGs. 3 and 4, the pipe face-to-face welding machine for maintaining a uniform welding temperature according to the present invention includes a clamp 14 for pressing and fixing circumferences of pipes 11, 12, a welding table 18 having a welding torch 16 mounted adjacent to connection of the pipes 11, 12, a driving motor 20 for rotating the welding table 18 around the circumference of the connection of the pipes 11, 12, a welding machine body 22 connected to the welding torch 16 to apply a welding current to the welding torch 16, and a wire supply motor 26 for supplying a welding wire 24 to the welding torch 16, as in the conventional one. In addition, the pipe face-to-face welding machine of the present invention further includes a welding current control unit, designated by reference numeral 100 so as to gradually a current for operation of the welding machine body 22 at each welding time or each welding area so that a welding temperature of the pipes 11, 12 to be welded may be maintained uniformly as a whole. [22] The welding current control unit 100 includes a welding sectional current setting unit 120 for dividing a welding area of the pipes 11, 12 into a plurality of sections according to a control signal and supplying a welding current to each section with lowering the welding current gradually, and a PLC (Programmable Logic Controller) 110 for controlling the welding sectional current setting unit 120 according to welding conditions requested by a user and opening or closing a plurality of contact points according to a program so as to maintain a normal operation accordingly. [23] In the configuration of the welding current control unit 100 generally classified into two parts, the welding sectional current setting unit 120 may be included in the welding machine body 22. [24] FlG. 4 shows the welding sectional current setting unit 120 and the PLC 110 of the welding current control unit 100. Referring to FlG. 4, the PLC 110 includes a welding material setting unit 111 for setting material of an object to be welded, a section division setting unit 112 for setting the number of divided sections as work en¬ vironments according to a diameter of pipe to be welded, a memory 113 for storing data related to welding work environments, a sensing unit 114 for sensing change of state (e.g., a welding temperature, an operation state of the motor, a welding standby state, etc.) for the welding work through various sensors, and a controller 115 for accessing system operation data from the memory 113 to form a data table for a start welding temperature and a welding temperature gradually lowered in the case that the welding material setting unit 111 sets the material (e.g., nonferrous metal, metal, etc.) of pipe to be welded and the section division setting unit 112 sets divided sections according to the welding work, the control unit then opening or closing the contact points accordingly and receiving a sensing signal according to the operation of the welding system through the sensing unit 114 to conduct start operation and correction of an error generated during operation according to the welding system. [25] In addition, the welding sectional current setting unit 120 includes a welding current supply unit 112 composed of the N number of welding current generators for generating current set by an enable operation signal and a down slope current generator for linearly lowering a current from a predetermined current to a zero potential for a predetermined time, and a welding current setting unit 121 for selectively operating any of the N number of welding current generators of the welding current supply unit 122 according to a control signal of the controller 115, and adjusting a linear current down slope of the down slope current generator of the welding current supply unit 122. [26] At this time, there are provided at least 6 welding current generators, one of which may be operated at each time zone. These welding current generators may be operated subsequently, and this order may be jumped over. [27] That is to say, assuming that a welding current generated in a first welding current generator is set to have a greatest magnitude and a Nth welding current generator is set to have a smallest magnitude and a welding area is divided into 6 sections, an enable signal may be supplied to the first to sixth welding current generators subsequently, but they may also be operated as second, third, fourth, seventh, eighth and tenth welding current generators according to material of the object and the working environments. [28] The above control manner according to generation of a welding current is to make only one welding current generator be operated at a certain time zone. However, it is also possible that several welding current generators are set to generate different start welding currents and the other welding current generators are set to generate a reverse current of a certain magnitude to attenuate a currently supplied current by a system designer, using the same spirit as the present invention. [29] That is to say, the later type has an advantage that it may minimize a relay impact caused by on/off operation of the contact points, in comparison to the former type. [30] Now, operation of the pipe face-to-face welding machine for maintaining a uniform welding temperature according to the present invention, configured as mentioned above, will be described with reference to FlGs. 5 and 6. [31] FlG. 5 is a partial view showing a controller ladder diagram of FlG. 4 in case the welding area is classified into 6 sections in the present invention, and FlG. 6 is a graph showing a welding current of each section of the welding area according to the ladder diagram of FIG. 5. [32] In addition, it is assumed that there are 6 welding current generators, and the current control type is set so that, when the first welding current generator supplies a start welding current, the second to sixth welding current generators generate a reverse current to attenuate the supplied current. [33] The PLC 110 subsequently operates the welding current setting unit 120 at each section at predetermined time intervals while the welding table 18 makes one turn by the driving motor 20. [34] Referring to the ladder diagram of FlG. 5 and the graph of FlG. 6, if a welding selection relay contact point MOOOl and a first section contact point P0008 are excited, the PLC 110 operates a first section welding current setting unit. [35] At this time, as shown in the graph of FIG. 5, the driving motor 20 and the wire supply motor 26 are delayed for a predetermined time and then initiate their operation so that an object to be welded may be preheated at a welding start area. [36] After that, if a second section contact point P0009 is excited, a second section welding current setting unit is operated to lower a welding current as shown in the graph of FlG. 5. [37] The welding table 18 passes through each section while a sixth section welding current setting unit is operated, and the welding current is lowered step by step. [38] If the welding table 18 reaches a welding end point to excite a welding end contact point M0016 or a down slope contact point M0017, a down slope is operated. [39] As mentioned above, since a welding current is lowered step by step via each section of the welding area, the connection of the pipes 11, 12 is not overheated even at the welding end point, and the welding work is conducted at a regular temperature as a whole in the welding area. [40] Meanwhile, though it is described in the embodiment that the welding area is divided into 6 sections, it may be divided more than or less than 6 sections in con¬ sideration of parameters such as diameter or thickness of the pipes 11, 12. [41] The pipe face-to-face welding machine of the present invention as mentioned above divides a welding area into a plurality of sections and then controls the welding current to be lowered step by step as the welding table 18 passes through each section, so the entire welding area may be welded under the uniform temperature condition. [42] Thus, since the connection of the pipes 11, 12 is welded using the same welding current, it is possible to solve the problem that the welding area is dug due to overheat of the pipes 11, 12. In addition, since the welding area is formed smooth as a whole, it is possible to remarkably decrease a product inferiority rate caused by deteriorated welding. [43] The present invention has been depicted and described based on the specific em¬ bodiments, but it should be understood to those skilled in the art that various modi¬ fications and changes might be possible within the scope not departing from the spirit and range defined in the appended claims. Industrial Applicability [44] According to the present invention as described above, since a welding current is lowered step by step as the welding table passes through a plurality of welding sections, the entire connection of pipes may be welded under the uniform temperature condition as a whole, so the welding area is formed smooth without being dug and it is possible to provide a pipe face-to-face welding machine that remarkably reduces a product inferiority rate caused by deteriorated welding.