The arc welding process is one of the most used and encompasses various techniques. One of those techniques is arc welding with covered metal electrode (SMAW), also known as arc welding with covered electrode, stick welding or manual metal arc welding.
It is a technique in which the welding heat is generated by a Electric arc between the work piece (base metal) and a consumable metal electrode (filler metal) coated with chemical materials in a suitable composition (flux). We can visualize the process in the following figure:
All the elements that participate in the SMAW welding they perform an important function. Let’s see why:
The arc: The beginning of any arc welding process is precisely the arc formation. Once this is established, the filler metal and the flux that covers it begin to be consumed. The force of the arc provides the action of digging into the base metal to achieve the desired penetration. This process continues as the weld flares and the electrode advances along the work piece.
The filler metal: when melted, it forms drops that are deposited on the workpiece, giving rise to weld puddle, which fills the weld gap and joins the parts in what is called a solder joint.
the flux: melts together with the filler metal forming a gas and a layer of human waste, which protect the arc and the weld puddle. The flux cleans the metal surface, supplies some alloying elements to the weld, protects the molten metal from oxidation, and stabilizes the arc. The slag is removed after solidification.
SMAW welding equipment
Known for its simplicity and ease of use, the equipment for performing SMAW welds is the one shown in the figure below.
This equipment consists of:
Power supply: Depending on the type of electrode and the type and position of the workpiece, the source can be from DC or alternating current. If it is direct current, and again depending on the type of electrode and the nature of the weld to be obtained, the electrode can be connected to the source in two ways:
- Connection to negative terminal: in this case we speak of a negative electrode or direct polarity (DCEN). It is used when high deposition rates and low penetration are desired.
- Connection to the positive terminal: in this case speaking of a positive electrode or reverse polarity (DCEP). It is used when deep penetration is desired.
Electrode holder: connects to the welding cable and conducts the welding current up to the electrode. The insulated handle is used to guide the electrode over the weld joint and feed electrode into the puddle as it is consumed. Electrode holders are available in various sizes and are classified according to their ability to carry current.
Electrode cable and work cable: both are an important part of the welding circuit. They must be highly flexible and have heat-resistant insulation. The connections to the electrode holder, the dough clamp and the power supply terminals must be welded or perfectly made to ensure low electrical resistance. The cross-sectional area of these cables must be of sufficient size to carry the welding current with a minimum of voltage drop. The longer the cable, the larger its diameter should be, in order to reduce resistance and voltage drop.
dough clamp: used to connect the work lead to the work piece. It can be connected directly to the workpiece, to the table or to the workpiece holder. As part of the welding circuit, the work clamp must be able to carry the welding current without risk of overheating due to electrical resistance.
Steps to make a SMAW weld
Once we have chosen the Coated electrode to be used, which will depend on the type and thickness of the workpiece, as well as the welding position and the characteristics of the welding that we want to obtain, we have to perfectly clean the work piece by means of a steel brush, eliminating the particles of dirt, grease, paint or rust. With the part clean and the connections correctly made, we follow a series of steps, such as those detailed below.
Step 1. Arc striking: the first step in making a SMAW welding, is the operation of establishing or lighting the arc, known as “ignition”. The principle of striking is based on the shock of the electrode tip with the base metal or work piece. This shock can be done in two ways, as shown in the figure:
- per hit: that is, hitting the metal and lifting the electrode.
- by scraping: sliding the electrode across the metal at a slight angle, as if lighting a match.
In both cases, the arch must form and remain stable. When stability is achieved, it is primed and welding can begin.
Step 2. To plot the Weld, we direct the electrode to the starting point of the welding, trying to make the distance between the electrode and the piece constant and approximately the diameter of the electrode. The choice between straight or oscillating cords will depend on the demands of the procedure and the type of cord:
- If the separation at the root is not very large, the first passes are generally made with straight laces.
- If the joint has excessive separation from the root, the first passes must be deposited giving, in addition to the oscillating movement, a small forward and backward swing of the electrode, in order to give time for the weld puddle to solidify, which prevents the falling molten material.
Step 3. The length of the arc must always be as constant as possible (between 2 and 4 mm in length, depending on the thickness of the electrode) bringing the electrode evenly closer, as it is consumed, towards the piece and along the joint in the direction welding.
Step 4. If we want to reinforce the weld, we must deposit several parallel beads, separated from each other by 8-10 mm, then remove the slag and deposit a new pass between the beads.
Step 5. The advance of the electrode must always be uniform, since the good appearance and quality of the weld, as well as the uniform distribution of heat, depend on this. To obtain a good weld, it is necessary that the arc is successively in contact along the welding line, since if it moves irregularly or too quickly, porous parts with little or no penetration will be obtained. Penetration also depends on the intensity of the current used: if it is low, the piece does not heat up sufficiently; if it is too high, a crater excessively large with the risk of burning or perforating the piece.
Step 6. When we finish welding or we have to replace the consumed electrode, we should never break the bow abruptly, since defects in the welding could occur. There are several ways to correctly interrupt the arc:
- Quickly shorten the arc and then move the electrode laterally out of the crater. This technique is used when the electrode already consumed is to be replaced to continue welding from the crater.
- Stop the forward motion of the electrode, allow the crater to fill, and then withdraw the electrode.
- Give the electrode an opposite inclination to the one it was wearing and move back about 10-12 mm, on the same bead, before interrupting the arc; in this way the crater is filled.
Step 7. When we replace the electrode we must always observe the following steps:
- arc interruption.
- Descaling or removal of the slag with an appropriate hammer.
- Cleaning with a steel brush to allow the correct deposition of the next weld bead.
- Electrode replacement.
- New arc striking operation… and so on.
Step 8. In fact, during welding we must take personal protection measures, such as:
- Wear suitable clothing and footwear: gloves, boots, aprons and leggings.
- Take care of the eyes and face from the radiation of the arc by using goggles and welding shields.
- Prevent electrical shocks: make sure you work on dry surfaces and that both the equipment and the insulation work properly and the connections are perfectly made.
SMAW welding applications and utilities
For reasons of greater productivity, quality and profitability, the process SMAW has been gradually replaced. However, the process capacity SMAW to achieve welds in restricted access areas means it still finds considerable use in certain situations and applications.
Heavy construction, such as in the industry naval, and “in-field” welding are largely based on the process SMAW. And while the process finds wide application for welding virtually all steels and many non-ferrous alloys, it is primarily used for joining steels, such as mild low-carbon steels, low-alloy steels, high-strength steels, hardened steels, and tempered, high-alloy steels, stainless steels and various cast irons. The SMAW process is also used to join nickel and its alloys and, to a lesser degree, copper and its alloys, although it is rarely used to weld aluminum.
- Simple, portable and low cost equipment.
- Applicable to a wide variety of metals, welding positions and electrodes.
- It has relatively high metal deposition rates.
- Suitable for outdoor applications.
- The process is discontinuous due to the limited length of the electrodes.
- As it is a manual welding, it requires great skill on the part of the welder.
- The weld may contain slag inclusions.
- The fumes make it difficult to control the process.
Tutorial on the use of a SMAW welder