Arc welding power sources provide either AC or DC power; however, depending on the connection made, DC power can provide two different polarities, Straight Polarity and Reverse Polarity. Direct Current Straight Polarity (DCSP), also called Direct Current Electrode Negative (DCEN) occurs when electrode is connected with the negative terminal of the power source and base metals are connected with the positive terminal. When the connection is made opposite, it is termed as reverse polarity. That means in Direct Current Reverse Polarity (DCRP), also called Direct Current Electrode Positive (DCEP), base metals are connected with the negative terminal of the power source and electrode is connected with the positive terminal.
All three polarities have their respective pros and cons and a suitable polarity must be selected prior to welding in order to get desired result. Straight polarity or electrode negative tends to provide maximum penetration. However, there exist a number of factors to judge suitable polarity; required depth of penetration is just one of such factors. You may read: Comparison among DCSP, DCRP, and AC polarities in arc welding.
Which polarity gives maximum penetration in arc welding?
Well the answer is Direct Current Straight Polarity (DCSP) or Direct Current Electrode Negative (DCEN). Although penetration depends on a number of factors, DCSP tends to enhance the achievable penetration. Depending on the achievable penetration, polarities can be compared as provided below.
- DCSP—Maximum penetration.
- AC—Moderate penetration.
- DCRP—Minimum penetration.
Why DCSP offers best penetration?
With DCSP connection, electrons, liberating from the electrode (negative polarity), are accelerated towards base metal (positive polarity) due to potential difference between them and finally strike the base metal surface at a very high velocity. Upon striking, kinetic energy of the electrons is converted into thermal energy and therefore high heat is generated at the base metal surface. As a thumb rule, it is considered that about 2/3rd of total arc heat (i.e., around 66%) is generated at the vicinity of base metal; whereas, rest of the heat (around 33%) is generated near electrode. This results in quick melting of the base metal and sufficient penetration.
Lack of penetration, high reinforcement, insufficient melting, etc. are basically welding defects found in arc welding when base plates are not allowed to fuse properly. These defects can be eliminated by utilizing DCSP as majority of heat is generated near base plates.
Other parameters that influence penetration in arc welding
It is to be noted that polarity is not the only parameter that affect the achievable penetration in arc welding. Some other welding parameters that influence penetration, directly or indirectly, are enlisted below.
- Edge preparation.
- Compatibility between electrode and base metals.
- Wettability of molten metal.
- Root gap.
- Welding current and heat input.
- Arc temperature.
- Arc length.
- Pre heating or post heating.
- Presence of coating or dirt on surface.
- Coating or flux on electrode.
- Sputter level.