To produce durable joints on stainless steel pipes of small and medium diameters from high-alloy steel and alloys of aluminum, titanium, Nickel, etc., use the methods of fusion welding — arc welding under protection of inert gases and plasma. Along with the most common types of pipe d x s = (8 + 102) x (1 + 3) mm, widely common is the welding of pipes with very thin walls (8 + 40) x (0,2 + 0,35) mm, and whose manufacturing process requires very precise adjustment of the thermal conditions of welding.
Used electric arc between a tungsten non-consumable electrode and the tubular workpiece. The concentration of the arc heat concentrated in a small area in the combustion zone, causing a fairly rapid melting of the edges. The bath of molten metal before the formation of the weld is protected from exposure to air by an inert gas which prevents oxidation of the metal. Seam kristallizuetsya independently, edges not compressed. With this weld pipe is produced from high alloy steel, and various alloys with a very high quality and durable seam without sawtooth grata.
In addition to the tungsten electrode diameter of 3−5 mm ceramic nozzle for gas supply to the weld. The heater is attached to the welding head, which is equipped with devices for longitudinal (parallel to weld) vertical and lateral adjustments. Welding is made by alternating currents of frequencies (180−360 Hz). But the most often used DC. Depending on the welded material it can give a different polarity, given the higher temperature of the anode arc spots. Alternating current increases the thermal power of the arc, but makes it less stable.
Pipe the usual assortment of weld arc continuous operation under the voltage of about 10 KV with a current of 100−300 A. This arc is not much longer than the wall thickness of the product. For particularly thin-walled stainless steel pipe apply a current of 20−30 a, But even in this current arc are unstable and can cause burn-through, warping the edges. Therefore, for welding of such pipes using pulsed arc. It combines Malampaya arc (1−1. 5 A), which is always on, and pulse (with a length of 0.8−1 mm, the strength of the current 20−30 A), which is lit periodically. Separate power supplies for arcs connected to the same electrode. Pulsed arc melts the metal, and malaupaina needed for the excitation pulse and brewing craters.
The welding of particularly thin-walled tubes
The peculiarity lies in the very exact information of the edges. This is accomplished by setting instead of the backup rolls split die which has an adjustable diameter through hole.
The protection of the welding zone and cooling weld bath is carried out with argon, helium or mixtures based on them. Helium is most suitable it is easy to stabilize the arc and the heat transfer improving quality seams and increased welding speed. However, the helium dissipates quickly due to the small density. Argon is denser than air, consumed more economically, more securely protects the welding area and cheaper. Argon arc welding is widely distributed
Another important task is the production of pipe having a minimum internal «grat» — the protrusion of weld above the surface of the pipe. Welding bath is suspended by surface tension forces. The thicker the pipe and the weight of such baths, the more its sagging inward. To create additional vertical forces and protection from oxidation in the cavity is injected argon. Obtained with this small «grat» allows the use of welded pipe as blanks for mills HPT.
The main disadvantage of the argon-arc method is the low welding speed of 0.01−0.03 m/s For extremely thin-walled pipes welding speed of 0.01 m/s. This is due to the limit of underwater thermal capacity (due to possible deformation of the edges and burn-through), as well as the time required for solidification of the molten metal.
To speed up the welding edges, pre-heated to 150−200°C by using high-frequency inductor. Further increase in welding speed because of the use of plasmatrons. With their help implement microplasma welding and welding closed arc compressed. Such an arc is obtained after installation of the electrodes inside the burner and the formation of a thin channel nozzle with a diameter of arc to 3 mm. Through the hole also leave the plasma-forming gases. Arc flashing between the electrode and the edges (called a direct arc). Argon insulates and compresses the arc at the nozzle exit. The current density in the arc increases, the gas is ionized and becomes plasma. In the middle part of this arc t* 1500−3000*C. Since the heat spot is reduced more than in 2 times, the increase in the concentration of current at the same heat almost 2 times faster for welding. The cost of argon is reduced. Plasma welding as well as argon arc, performed both in continuous and in pulsed mode.
The use of microplasma welding
With its help, produce extremely thin-walled tubes 12x18н10т. Because of the greater degree of compression of the arc and the concentration of current density at the minimum nozzle diameter of 1 mm and electrode 1 mm, the arc is ignited in a separate low-voltage power supply. In the formation of the plasma jet of the arc is shut off, the voltage switch on the tube, and the formation of a plasma arc of direct action. Due to the small spot heating does not occur warping of the edges, and the result is a narrow seam. During such a welding speed of ionized gases can reach transonic speeds, due to the high temperature and the passage of gas through the narrow channels of the nozzle. At the critical velocity of efflux of plasma welding goes into cutting.
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