Polarity in DC Arc welding


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Polarity in DC Arc welding:

Polarity in DC Arc welding Objectives : State the kinds and importance of polarity in arc welding. Describe the uses of straight and reverse polarity. Determination of polarity and the effects of using wrong polarity.

What is polarity?:

What is polarity? Polarity indicates the direction of current flow in the welding circuit.

Direct current (D.C.) always flows from::

Direct current (D.C.) always flows from: The positive (higher potential) terminal to the negative (lower potential) terminal, as per the conventional theory. Negative terminal to positive terminal as per electronic theory. In the latest machines a polarity switch is used to change the polarity.

Importance of polarity in welding: :

Importance of polarity in welding: In D.C. welding 2/3 rd of the hit is liberated from the positive end and 1/3 rd from the negative end. To have this advantage of unequal hit distribution in electrode and base metal, the polarity is an important factor for successful welding.

Kinds of polarity:

Kinds of polarity Straight polarity / electrode negative (DCEN): In straight polarity the electrode is connected to the negative and the work to the positive terminal of the power source. Reverse polarity / electrode positive(DCEP): In reverse polarity the electrode is connected to the positive and the work to the negative terminal of the power source.

Application of polarity:

Application of polarity Straight polarity is used for: Welding with bare light coated and medium coated electrodes. Welding the thicker sections in down hand position to obtain more base metal fusion and penetration. Reverse polarity is used for: Welding of non-ferrous metals. Welding of cast iron. Welding with heavy and super heavy coated electrodes. Positional welding. Sheet metal welding.

Determination of polarity:

Determination of polarity In order to get the best results it is essential to attach the electrode with the correct terminal of the welding machine. Positive/negative terminals on a DC welding machine can be identified by the following tests. Carbon electrode test. Water test.

Terminal test:

Terminal test Carbon electrode test. Strike an arc using normal range current with the help of a carbon electrode pointed at its end using D.C. The pointed end of carbon will become blunt soon if it is connected with the positive terminal, but there will be no change with the negative. Water test Put both the terminals of the welding cable (connected with D.C.) in a container of electrolyte water separately. More and quick arising bubbles will indicate NEGATIVE while slow arising bubbles will indicate POSITIVE.

Terminal test:

Terminal test Carbon electrode test Water test

Indication of wrong polarity:

Indication of wrong polarity If the electrode is used on wrong polarity it will result in: Excess spatter and poor penetration. Improper fusion of the electrode. Heavy brownish deposition on the face of the weld metal. Difficulty in manipulation of the arc. Abnormal sound of the arc. Poor weld bead appearance with surface defects and more spatter.

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