Scheme of resistance welding is shown in the example of pipe welding (Fig. 1). The two pipes to be welded (1-1 '), and coils of electromagnets (2-2') are arranged coaxially. Pipes connected to the welding power source (one of the welded pipe is the cathode, the other - the anode). Electromagnet coils generate magnetic fluxes directed along the tube axis towards each other. As a result, the gap between the tubes, the magnetic field has a radial component. The interaction of the arc current I and the radial component of the magnetic field strength Bp created Fg, resulting in an arc motion. This force, whose direction is perpendicular to the direction of current in the arc and the direction of the magnetic field causes the arc to move along the surface at the ends, reaching several tens of meters per second. Repeated traversal arc butt weld pipe formation occurs almost simultaneously along the entire perimeter. In the observation of the rotating arc is a continuous ring of glowing plasma. Upon reaching the welding temperature at the ends is compression (settlement) of pipes, just as is done in resistance welding. When moving in a magnetic field, the arc is influenced by various forces acting on the arc column and the active spots. These forces can be divided into the driving forces and resistance forces. The driving forces are due to the interaction of arc current with the radial component of the magnetic field. Doug can be regarded approximately as the current-carrying conductor placed in a magnetic field. On it the force Fg, defined by Ampere's law Fg = B1Il, where B1 is the component of the magnetic field perpendicular to the polar current; I - current in a conductor; L - the length of the conductor. In the case of axial and radial arc of the magnetic field strength at each moment of Fg Time is tangential to the circumference of the pipe and tends to move at the ends of the arc tube. to resistance forces include aerodynamic drag force F1, the friction flow of gases entrained arc F2 on the walls of the gap, which is a narrow slit, and the force F3, opposing the movement of the active spots of the arc.
Force of aerodynamic resistance depends on the velocity of the arc and the magnetic field. The friction force depends on the gas flow velocity of the arc, as well as the size and geometry of the gap. Force opposing movement of the active spots of the arc depends on the electrode material, shape, condition and temperature of their surfaces. for rotating arc is characterized by three stages of the arc during the whole period of heating the ends of articles (Figure 2). One of the distinguishing features of each of these stages is the nature of the change speed of the arc. Stage I - acceleration and rotation of the arc with increasing speed, at the end of the period of the velocity reaches its maximum value. Applied to the driving force behind the arc than at this stage aimed to meet her resistance force Fg> Fl + F2 + F3. At the beginning of this period, F1 and F2 are small, because small velocity of the arc. The greatest strength of the resistance is the force Fs, which is high in the initial period due to unfavorable conditions for moving spots on the pipe ends. During the period the force F3 is significantly reduced (ends of the tubes are heated, fused microroughnesses, ionized gases arc gap). The main resistance forces are forces of aerodynamic drag and friction. The difference between the driving force Fg and the resistance forces (F1 + F2 + F3) determines the acceleration of the arc in the initial period. After the initiation of an arc is displaced to the inner surface of the pipe due to the fact that there is a significant gap in the magnetic field gradient. As the heated ends of the magnetic field gradient is reduced and the arc is part of the gap. Settling time, maximum speed, ie, the duration of stage I depends on the induction in the gap and the magnitude of welding current. Stage II - the steady motion of the arc with the maximum speed is the most time-consuming. Its duration is characterized by the time required for the appearance at the ends of the film of liquid metal. At this stage, the driving force is balanced by the forces of resistance. Fg = F1 + F2 + F3. During the second stage of induction in the gap is reduced, which reduces the driving force. However, the rate of displacement of the arc remains nearly constant, since a simultaneous decrease in the friction force F3. In Fig. 3 and 4 shows the variation curves of maximum speed, depending on the arc welding current and the radial induction in the gap, taken when welding pipes with a diameter of 27 mm. From these curves it is clear that at this stage, the main parameter determining the rate of displacement of the arc, is the magnitude of the radial induction in the gap. Stage III - the rotation of the arc of a variable rate, observed during heating above the melting point edge. Almost the entire surface of the ends of welded products covered with a film of liquid metal. Applied to the driving force behind the arc is less than the resistance: FgSila Fg continues to fall, as due to the increase of the gap decreases the induction of it. Force of aerodynamic resistance and friction flow entrained gases are also reduced. The power of active resistance to movement of sunspots varies unstable, its mean value increases. This is due to the formation of bridges of molten metal ejected in the form of sparks, which leads to fluctuations in the velocity of the arc. Connections made by resistance welding have the same structure as that obtained by other methods, with application of pressure, for example, butt welding fusion. However, there are a number of structural features considered by the example of steel pipe welding St20. Most of the compounds of the seam is observed bright streak decarburized. All joints in the form and the presence of light strips can be divided into three groups:
- joints without light strip;
- joints with white stripe, with blurred boundaries;
- joints with white stripe, which has sharply defined borders.
See also:
Electric Resistance Welding
Theory Fundamentals of Resistance Welding
Resistance Welding Technology
Equipment for Resistance Welding
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