Q.6 What are the different power sources used in welding? What are the advantage and limitations of each? (AKTU. 2011 - 12)
Ans. Power Sources in Arc Welding: -
To supply the electrical energy necessary for arc welding processes, a number of different power supplies can be used. The most common classification is constant current power supplies and constant voltage power supplies. In arc welding, the voltage is directly related to the length of the arc, and the current is related to the amount of heat input.
Constant current power supplies are most often used for manual welding processes such as gas tungsten arc welding and shield metal arc welding, because they maintain a relatively constant current even as the voltage varies. This is important because in manual welding, it can be difficult to hold the electrode perfectly standby, and as a result, the arc length and thus voltage tend to fluctuate.
Constant voltage power supplies hold the voltage constant and vary the current, and as a result, are more often used for automated welding processes such as gas metal arc welding, flux cored arc welding, and submerged arc-welding. In these processes, arc length is keep constant, since any fluctuation in the distance between the wire and the base material is quickly rectified by a large change in current. For example, if the wire and the base material get too close, the current will rapidly increase, which in turn causes the heat to increase and the tip of the wire to melt, returning it to its original separation distance.
Consumable electrode processes such as shielded metal arc welding and gas metal arc welding generally used direct current, but the electrode can be charged either positively or negatively. In welding, the positively charged anode will have a greater heat concentration, and as a result, changing the polarity of the electrode has an impact on weld properties. If the electrode is positively charged, it will melt more quickly, increasing weld penetration and welding speed. Alternatively, a negatively charged electrode results in more shallow welds. Nonconsumable electrode processes, such as gas tungsten arc welding, can use either type of direct current, as well as alternating current. However, with direct current, because the electrode only creates the arc and does not provide filler material, a positively charged electrode causes shallow welds, while a negatively charged electrode makes deeper welds. Alternating current rapidly moves between these two, resulting in medium-penetration welds. One disadvantage of AC, the fact that the arc must be re-ignited after every zero crossing, has been addressed with the invention of special power units that produce a square wave pattern instead of the normal sine wave, making rapid zero crossing possible and minimizing the effects of the problem.
Q.7 Write two welding defects. (AKTU 2009-10)
Ans. (1) Inclusions: -
Normally the slag, being lighter, is expected to float over the surface of the Molten Metal Pool. But, several times, specially in case of multi-pass welds, it is not fully squeezed out and a portion of it remains entrapped in the weld metal and is known as Slag Inclusion. Such inclusions may also be added due to many foreign materials like dirt, mill scale, rust, etc. present on the surface of the base metal. The Slag Inclusions appear as large patches and other foreign material inclusions are dispersed finely in the weld (See. Fig.). Such inclusions render the welded joint weak.
(2) Cracks: -
A Crack is the discontinuity of metal. This discontinuity may occur in the Base metal or Weld metal or at Fusion face Between the weld metal and base metal. If the Crack is large enough to be visible by naked eye, it is called a Macrocrack. If it is too small to be detected by the naked eye, it can be revealed through a Microscopic Examination only, then it is known as a Micro-Crack. A Crack may also appear in the Crater, in the root of the bead or on the surface of the weld. Presence of cracks in a welded joint renders it unsound and weak and may ultimately lead to the failure of the welded joint during service. For details of cracks refer to fig.
(1) Root Crack
(2) Fusion face Crack
(3) Under bead Crack
(4) Toe Crack
(5) Weld-metal Crack (Longitudinal)
(6) Heat effected zone (base metal) Transverse Cracks
(7) Weld metal Transverse Cracks.
