The selection of right welding electrodes is a quite challenging job because we need to consider many factors while choosing the best alternative. In this article, we will discuss the various factors responsible for choosing the right welding electrode for the 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 choosing 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 these factors one by one. Please note that a selection chart for choosing the correct welding electrode based on the parent material (in PDF format) is given at the end of this article.
1. Base Metal or Parent Metal: Base Metal or Parent Metal is one of the most important factors 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 that of the base metal should be similar or as close as possible. In case of wide differences in tensile strength between the 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 an 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 chemical composition of the electrode should match the chemical composition of the base metal, especially the carbon percentage. However, practically it’s not possible to match every parameter of an electrode with the base metal. Hence we need to choose an electrode having chemical properties as close as possible with the parent metal.
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 results but for thicker material we need and electrode with digging arc for deep penetration, maximum ductility, and low hydrogen for 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 the 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 gives the position for example: in the 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 and Some electrodes work well with both AC as well as DC. Hence, depending upon the power output of the welding machine we can choose the electrode. For example, if the machine is capable of giving only AC output, then we will not be able to use the electrodes which are not meant for AC. However, if the welding machine is capable of both AC as well as DC output then we can choose from a broader range of electrodes.
When using Direct Current (DC) power supply, if the 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 the electrode is connected with the negative terminal of the machine then it is known as Direct current electrode negative (DCEN) or straight polarity. DCEP is the preferred polarity for most of the Shielded metal arc welding (SMAW) operations. However please refer to Table-2 for the correct polarity in which welding electrodes can be used.
The last two digits of the electrode name denotes the polarity as well as the 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 to be considered while selecting an electrode is the joint preparation or groove configuration. It there is a tight fit-up or unbevelled root face then an electrode with a 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 beveled root face, then we can use electrodes which can give shallow to moderate penetration such as E6013 or E7018.
Depending upon the operating characteristics, Grouping of SMAW electrodes can be done in the following ways;
- Fast-Freeze electrodes
- Fast-Fill electrodes
- Fill-Freeze electrodes
- Low Hydrogen electrodes
Fast-Freeze electrodes (EXX10/EXX11) are those which can solidify quickly and are capable of giving forceful arc which helps in deep penetration and slag formation is quite low. Whereas The Fast-Fill electrodes (EXX22/EXX24/EXX24/EXX27/EXX28) can be melted very quickly with a high deposition rate and travel speed is also very high. However, these electrodes produce heavy slag.
The third type of electrodes i.e. Fill-Freeze electrodes (EXX12/EXX13/EXX14) is a compromise between the fast-freeze electrodes and fast-fill electrodes, hence the Fill-Freeze electrodes possess characteristics in-between the Fast-fill and the Fast freeze electrodes. These electrodes are having medium penetration and medium deposition rate hence very useful for Thin/Sheet metal welding.
The fourth type of electrodes is Low Hydrogen Electrodes (EXX15/EXX16/EXX18). Some Low Hydrogen Electrodes are having characteristics similar to fast-fill and some are having similar to Fill-Freeze. But they have been grouped separately because of their Low Hydrogen properties. These electrodes produce excellent welding with high ductility and good notch toughness.
In case of a low diameter pipe, back-chipping and welding from the other side (ID side) can,t be done due to inaccessibility, hence a deeper penetration is always required. Hence we can select Fast-Freeze electrodes like E6010 for that case. To weld Sheet Metal or material with very low thickness, we can choose Fill-Freeze electrodes like E6013, because of it’s medium penetration characteristics. Hence Selection of electrodes can be done in this way too.
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. The quality of the weld must suit the service requirements of the job. For example, to weld cryogenic vessel parts that are supposed to work at very extreme temperature and pressure with high impact loading or parts that are subjected to a corrosive atmosphere, we need a 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 the features of welding electrodes, which you may find useful while selecting an electrode for a particular job.
6. Welding Cost: Finally, the cost of the electrode should be considered according to your project requirement. We cannot choose a 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.
Hence, While selecting the welding electrodes the above factors can be considered to get the required result.
To download the Welding electrode selection chart/guide according to the base metal (in pdf format), please click on the below link;