Gas-electric welding
Gas-electric welding
An electric arc is initiated between a tungsten electrode and the pipe workpiece. The heat concentration of the arc is in a small area in the combustion zone, which causes the edges to melt quite quickly. Bath with molten metal before the formation of the weld is protected from exposure to air by a layer of inert gas, which prevents oxidation of the metal. The weld crystallizes on its own, the edges are not squeezed. The weld is of high quality and strong, without sawtooth burr.
Welding technology
In addition to the tungsten electrode with a diameter of 3−5 mm you need a ceramic nozzle for gas supply to the welding point. The heater is attached to the welding head, which is equipped with devices for longitudinal (parallel to the weld) vertical and transverse adjustment. Welding is carried out with alternating current frequencies (180−360 Hz). But most often direct current is used. Depending on the material to be welded, it may be given a different polarity, given the higher temperature of the anode spots of the arc. Alternating current increases the thermal capacity of the arc, but makes it less stable.
Welding of extremely thin-walled pipes
A special feature is the very precise edge joining. This is achieved by replacing the support rolls with a cut die that has an adjustable bore diameter. The weld pool is protected and cooled with argon, helium or helium-based mixtures. Helium is the most suitable — it helps to stabilize the arc and heat transfer and thus improves the quality of welds. However, helium dissipates quickly due to its low density. Argon is denser than air, it is more economical, protects the welding area more reliably and is cheaper. Argon arc welding is widely used.
Grat
Another important task is to produce pipes that have a minimum internal «grat» — the protrusion of the weld over the surface of the pipes. The welding tub is suspended due to the forces of surface tension. The greater the thickness of the pipes and the weight of such a tub, the greater its sagging inward. To create additional vertical forces and to protect against oxidation, argon is injected into the pipe cavity. The small «grit» obtained with this allows the welded tubes to be used as billets for HPT mills.
Disadvantages
The main disadvantage of the argon-arc method is the low welding speed of 0,01−0,03 m/s. Welding speed of especially thin-walled pipes is 0,01 m/s. This is due to the underwater heat capacity limit (due to possible edge deformations and burn-throughs), as well as to the time required for the molten metal to solidify.
Optimization
To accelerate welding, the edges are preheated to 150−200 ° C with a high-frequency inductor. A further increase in welding speed is possible through the use of plasmatrons. They are used for microplasma welding and closed compressed arc welding. Such arc is obtained after installing electrodes inside the torch and forming a thin nozzle channel with the diameter of arc burning up to 3 mm. Plasma-forming gases also escape through this hole. The arc flashes between the electrode and the edges (so-called direct arc). Argon, insulates and compresses the arc at the exit of the nozzle. The current density in the arc increases, the gas is ionized and transformed into plasma. In the middle part of such an arc, the t* is 1500−3000*C. Since the heating spot decreases more than 2 times, increasing the current concentration with the same heat output practically 2 times speeds up welding.
Using microplasma welding
It is used to produce a particularly thin-walled pipes 12x18n10t. Because of the higher degree of arc compression and current density concentration with a minimum nozzle diameter of 1 mm and the electrode 1 mm, the arc is ignited by a separate low-current power source. When the plasma jet is formed, this arc is switched off, the voltage is switched to the tube, and direct plasma arc is formed. Due to the small heating spot, there is no edge distortion and the weld is very narrow.
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