Selection of right welding electrode is a quite challenging decision because we need to consider many factors while choosing an electrode for a particular job. In this article, we will discuss about the various factors responsible for choosing a welding electrode for Shielded Metal Arc welding Process (SMAW) for Carbon steels. The Shielded metal arc welding electrodes are always covered by flux and the electrode’s name starts with E followed by either a four-digit number or a five-digit number for example: E7018, E6010, E6013, E10018 etc.
Each letter and digit has a specific meaning, for example: in E7018 the letter ‘E’ stands for Electrode, ‘70’ denotes the minimum tensile strength i.e. 70 Ksi (or 70000 Psi) for this case, the second last digit i.e. ‘1’ indicates the position in which the electrode can be used. In particular, the letter ‘1’ depicts all position and the last two digits together shows the polarity and the flux composition of the electrode. To read more about the electrode symbols and their meaning please read this article: https://www.weldingandndt.com/welding-electrodes-understanding-the-electrode-symbols/
As discussed earlier, we need to consider various factors before selecting an electrode. These factors are as under;
1. Base metal or Parent Metal
2. Welding Position
3. Power Supply
4. Joint Preparation
5. Welding Quality/Weld finish
6. Welding Cost
We will discuss each factors one by one;
1. Base Metal or Parent Metal: Base Metal or Parent Metal is one of the most important factor to be considered while selecting an electrode. We need to take into consideration three following parameters;
a. Base metal mechanical properties: The mechanical properties especially the tensile strength of the electrode and of the base metal should be similar or as close as possible. In case of wide differences in tensile strength between welding electrode and the base metal, there will always be chances of cracking and other welding discontinuities. Hence to prevent cracking and other welding discontinuities we should always choose and electrode having minimum tensile strength equivalent to the parent metal being welded.
b. Base metal chemical properties: The chemical properties shall also be checked before choosing an electrode. The electrode chemical composition should match the chemical composition of the base metal, especially the carbon percentage.
c. Base metal thickness: Base metal thickness is very important while choosing an electrode. For thinner materials an electrode with soft arc and less penetrating power can give good result but for thicker material we need and electrode with digging arc for deep penetration, maximum ductility and low hydrogen for a defect free welding.
We should always keep in mind that the electrode size (diameter) should not be more than the thickness of the parent metal.
2. Welding Position: The second factor is welding position. Each electrode is meant for welding at some particular position. Hence we need to choose electrode as per the position of our job. The welding position is specified in the electrode name itself i.e the second last digit give the position for example: in E7018 electrode, the second last digit i.e. 1 give the position for which the electrode can be used. A general summary of the welding position for electrodes are given below (Table 1);
3. Power Supply: Some electrodes can be used with alternating Power (AC) supply, some can be used with direct current power (DC) supply. Depending upon the available power supply we can choose the electrode. However, a welding machine can give AC or DC or both AC as well as DC power supply too.
When using Direct Current (DC) power supply, if electrode is connected with the positive terminal of the machine then it is known as Direct current electrode positive (DCEP) or reverse polarity and, if electrode is connected with the negative terminal of the machine then it is known as Direct current electrode negative (DCEN) or straight polarity.
The last two digit of an electrode name denotes the polarity as well as flux of the electrode. Please see the table given below (Table 2) for a summary of electrodes and the polarity in which they can be used.
4. Joint Preparation: The fourth factor for to be considered while selecting an electrode is the joint preparation or groove configuration. It there is tight fit-up or unbevelled root face then an electrode with digging arc can be used, which can provide deeper penetration. Electrodes such as E6010 or E6011 can be used for this purpose. If there is sufficient root gap and bevelled root face, then we can use electrodes which can give shallow to moderate penetration such as E6013 or E7018.
5. Welding Quality/Weld finish: The required quality and the weld finishing is also an important factor to be considered before selecting an electrode. Electrodes can be chosen depending upon the final finishing requirement for example Flat weld bead, concave weld bead or convex weld bead. Quality of the weld must suit the service requirements of the job. For example, to weld a cryogenic vessel parts which is supposed to work at very extreme temperature and pressure with high impact loading or parts which are subjected to corrosive atmosphere, we need low hydrogen electrode such as E7018, which can produce very sound welding with higher ductility. So the chances of getting any weld defect during operation will be minimized. Table 3 gives a summary of features of welding electrodes, which you may find useful while selecting an electrode for a particular job.
6. Welding Cost: Finally, the cost of electrode should be considered according to your project requirement. We cannot choose costly electrode for a low budget project. Similarly, for critical jobs where quality needs to be given utmost care a low hydrogen electrode with higher ductility such as E7018 can be considered to meet the quality requirements.