Relevance

To obtain durable seams on small- and medium-diameter stainless pipes made of high-alloy steel, as well as aluminum, titanium, nickel and other alloys, methods of fusion welding are used - arc welding under inert gas protection and plasma welding. Along with the most common types of pipes d x s = (8 + 102) x (1 + 3) mm, welding of pipes with especially thin walls (8 + 40) x (0,2 + 0,35) mm, which production process requires very precise adjustment of welding thermal conditions, is widespread.

Gas-electric welding

An electric arc is used between a tungsten non-consumable electrode and a pipe billet. The heat concentration of the arc is in a small area in the combustion zone, which causes fairly rapid melting of the edges. 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. Using such welding, tubes of high-alloyed steel and various alloys with very high quality and strong weld without saw-tooth bevel are produced.

Welding technology

In addition to the tungsten electrode with a diameter of 3-5 mm, a ceramic nozzle for gas supply to the welding point is necessary. The heater is attached to the welding head, which is equipped with devices for longitudinal (parallel to the weld) vertical and transverse adjustments. 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.

Arc

Tubes of the usual variety are welded with a continuous arc at about 10 kV with an amperage of 100-300 A. This arc is not much longer than the wall thickness of the product. For stainless steel especially thin-walled pipes use current of 20-30 A, but even with this current the arc is unstable, burns and edge deformation can occur. Therefore, for welding these pipes using pulsed arc. It combines a low-amperage arc (1-1.5 A), which burns constantly, as well as pulsed arc (with a length of 0.8-1 mm, the current strength of 20-30 A), which burns periodically. Separate arc power sources are connected to a single electrode. Pulsed arc melts the metal, and low-amperage arc is needed to excite the pulsed arc and welding craters.

Welding especially thin-walled pipes

A special feature is very precise edge joining. This is achieved by replacing the support rolls with a cutter with an adjustable diameter of the passage.
Protection of the welding area, as well as cooling of the weld pool, is done with argon, helium or helium-based mixtures. Helium is the most suitable - it helps to stabilize the arc and heat transfer thus improving seam quality and increasing welding speed. 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 widespread

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. For especially thin-walled pipes the welding speed is 0.01 m/s. This is due to the underwater heat capacity limit (due to possible edge deformations and burns), as well as to the time required for solidification of the molten metal.

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, the increase in current concentration with the same heat output practically 2 times speeds up welding. Argon consumption is reduced. Plasma welding, as well as argon arc welding, is performed both in continuous and pulsed modes.

Using microplasma welding

It is used to make especially thin-walled 12x18n10t pipes. Due to the higher degree of arc compression and current density concentration with minimum nozzle diameter of 1 mm and 1 mm electrode, 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 a narrow weld is obtained. During such welding, the speed of ionized gases can reach near-sonic speeds due to the high temperature and the passage of gas through narrow nozzle channels. At critical plasma outflow rates, welding turns into cutting.

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