As an advanced joining technique Plasma Arc welding process has gained importance in the pressure vessel industry. The deep penetration characteristic of plasma arc process enables it to join the materials in a single operation. As an improved productivity technique, this process was introduced for welding stainless steel vessels for a number of applications: one such application being the fabrication of stainless steel vessels to handle cryogenic liquids (Liquid O2 , N2, etc).
Salient Features of Plasma Arc Welding Process
Plasma is an ionized gaseous discharge. Upon de-ionization, it results in very high temperatures. This high-energy arc can be used to fuse materials. The plasma arc characteristics are very much different from conventional TIG arc. In GTAW, the arc is open and soft in nature; it takes a conical shape and penetration is shallow, whereas in PAW, the arc is constricted in water-cooled copper nozzle; here it is stable and stiff, unlike in GTAW process. Tungsten electrode is set back in PAW torch for the purpose of arc constriction in the copper nozzle. The arc is established between tungsten electrode and metallic work piece (transferred arc).
Characteristics of Plasma Arc
1) Plasma arc is not highly sensitive to arc length variation like TIG arc.
2) Arc lengths are longer than in TIG and vary from 5-10 mm.
3) Shielding gases should be either neutral or slightly reducing in nature.
4) Plasma gas flow rates are generally low and vary between 5-8 LPM. Since it causes ineffective shielding to the weld, sheath gas or annular gas is required for protection of weldment, as well as to thermally shield the plasma arc to maintain directional arc stream.
Bi-cathode Plasma Arc Welding Working Principle
Forming a keyhole at the joint through which plasma escapes carries out welding. Molten metal moves from front to the rear portion of the keyhole by surface tension force thus bridging the hole. Since this is an autogenous process, a filler pass may be required on plasma weld to compensate the reduction in weld volume due to solidification from molten state and oxidation losses as well as to provide reinforcement to the weld. TIG cosmetic pass with filler is recommended for bright finish and smooth weld profile.
Both PAW and TIG processes can be run simultaneously and electrodes are connected to negative terminals and hence aptly coined is "bi-cathode welding". The leading PAW torch fuses the base metal and trailing TIG torch provides cosmetic pass to the plasma weld.
Special Features Recommendable for Better Performance of the Joint
i) TIG arc oscillation
ii) Arc voltage regulation (AVC-within 0.1V)
iii) PAW and GTAW in Tandem Mode Applications: CS, LAS, SS, Ti, Zr, and Ta. etc.
Bi-cathode welding can be employed for a wide variety of materials with the following limitations:
CS/LAS Steel should be fully killed. In semi-killed or un-killed steels, the presence of gases creates defects in the compactness of weld resulting from the incomplete degassing.
SS stabilized varieties and molybdenum bearing steels are less easy. By virtue of their high fluidity may result in collapse of weld metal.
Recommended Welding Practices:
For joining plates of thickness above 8mm, single "V" bevel is adopted maintaining a nose of 5-8 mm and included bevel angle of 45 degree.
Gas recommended for Plasma Arc Welding
For stainless steels generally upto 5-10% hydrogen is added to argon for shielding purpose to achieve narrow weld beads and good bead appearance.
Argon + 5% hydrogen is used for shielding purposes for both PAW and GTAW processes. Argon backing is provided for the joint while plasma arc welding to avoid any root defects. Joint tracking system will be of great help in order to facilitate seam alignment during welding. The weidment toughness can be of the very high order and to the extent of meeting the liquid nitrogen temperatures.
Defects Noticed During Plasma Arc Welding
The cause and effect relation to be investigated and corrected accordingly during the welding procedure establishment and this exercise does factor the aspect of cost reduction further.
The expected savings are maximum when PAW and GTAW processes are operated in Tandem mode.
1) Stainless steel plates of different thickness can be welded successfully using bi-cathode welding.
2) Higher conductivity can be obtained by using plasma arc welding which enables to employ square butt joint faster welding speeds.
3) The charpy V notch impact test results of plasma arc welds are satisfactory down to -196 degree Celsius.
4) Stainless steel plates up to 8mm can be welded in a single operation by fusing the joint by PAW and depositing cosmetic pass by TIG
5) The amount of distortion in plasma are welds are far less when compared to the conventional welding process. This is due to minimum number of passes to complete the joint and symmetry of weld with respect to neutral axis of the joint.