Arc welding with coated electrodes is a manual process where the heat source consists of the electric arc. When the arc strikes between the coated electrode by means of an electrode holder clamp and the piece to be welded base material , it generates heat which causes rapid melting of both the base material and the electrode weld material. The purpose of the power source is to feed the electric arc, which is present between the base material and the electrode, through the output of a current sufficient in quantity to keep the arc struck. Electrode welding is based on the constant current principle i.
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Shielded metal arc welding SMAW , also known as manual metal arc welding MMA or MMAW , flux shielded arc welding  or informally as stick welding , is a manual arc welding process that uses a consumable electrode covered with a flux to lay the weld. An electric current , in the form of either alternating current or direct current from a welding power supply , is used to form an electric arc between the electrode and the metals to be joined.
The workpiece and the electrode melts forming a pool of molten metal weld pool that cools to form a joint. As the weld is laid, the flux coating of the electrode disintegrates, giving off vapors that serve as a shielding gas and providing a layer of slag , both of which protect the weld area from atmospheric contamination. Because of the versatility of the process and the simplicity of its equipment and operation, shielded metal arc welding is one of the world's first and most popular welding processes.
It dominates other welding processes in the maintenance and repair industry, and though flux-cored arc welding is growing in popularity, SMAW continues to be used extensively in the construction of heavy steel structures and in industrial fabrication. The process is used primarily to weld iron and steels including stainless steel but aluminium , nickel and copper alloys can also be welded with this method.
In , the consumable metal electrode was invented by Nikolay Slavyanov. Later in , C. Coffin received U. Patent , for his arc welding method that utilized a metal electrode. The process, like SMAW, deposited melted electrode metal into the weld as filler. Around , A. Strohmenger and Oscar Kjellberg released the first coated electrodes. Strohmenger used clay and lime coating to stabilize the arc, while Kjellberg dipped iron wire into mixtures of carbonates and silicates to coat the electrode.
In , the development of an extrusion process reduced the cost of coating electrodes while allowing manufacturers to produce more complex coating mixtures designed for specific applications. In the s, manufacturers introduced iron powder into the flux coating, making it possible to increase the welding speed.
In K. Madsen described an automated variation of SMAW, now known as gravity welding. It briefly gained popularity in the s after receiving publicity for its use in Japanese shipyards though today its applications are limited. Another little used variation of the process, known as firecracker welding , was developed around the same time by George Hafergut in Austria. Due to the large force of energy coupled with the small area of focus, this laser became a powerful heat source for cutting and tooling.
To strike the electric arc, the electrode is brought into contact with the workpiece by a very light touch of the electrode to the base metal. The electrode is then pulled back slightly. This initiates the arc and thus the melting of the workpiece and the consumable electrode, and causes droplets of the electrode to be passed from the electrode to the weld pool.
Striking an arc, which varies widely based upon electrode and workpiece composition, can be the hardest skill for beginners. The orientation of the electrode to workpiece is where most stumble, if the electrode is held at a perpendicular angle to the workpiece the tip will likely stick to the metal which will fuse the electrode to the workpiece which will cause it to heat up very rapidly.
The tip of the electrode needs to be at a lower angle to the workpiece, which allows the weld pool to flow out of the arc. As the electrode melts, the flux covering disintegrates, giving off shielding gases that protect the weld area from oxygen and other atmospheric gases. In addition, the flux provides molten slag which covers the filler metal as it travels from the electrode to the weld pool.
Once part of the weld pool, the slag floats to the surface and protects the weld from contamination as it solidifies. Once hardened, it must be chipped away to reveal the finished weld. As welding progresses and the electrode melts, the welder must periodically stop welding to remove the remaining electrode stub and insert a new electrode into the electrode holder.
This activity, combined with chipping away the slag, reduces the amount of time that the welder can spend laying the weld, making SMAW one of the least efficient welding processes. The actual welding technique utilized depends on the electrode, the composition of the workpiece, and the position of the joint being welded.
The choice of electrode and welding position also determine the welding speed. Flat welds require the least operator skill, and can be done with electrodes that melt quickly but solidify slowly. This permits higher welding speeds.
Sloped, vertical or upside-down welding requires more operator skill, and often necessitates the use of an electrode that solidifies quickly to prevent the molten metal from flowing out of the weld pool. However, this generally means that the electrode melts less quickly, thus increasing the time required to lay the weld. The most common quality problems associated with SMAW include weld spatter, porosity, poor fusion, shallow penetration, and cracking.
Weld spatter, while not affecting the integrity of the weld, damages its appearance and increases cleaning costs. Secondary finishing services are often required due to the aesthetic appearance caused by the occurrence of molten splatter.
Arc blow can also cause porosity in the weld, as can joint contamination, high welding speed, and a long welding arc, especially when low-hydrogen electrodes are used. Porosity, often not visible without the use of advanced nondestructive testing methods, is a serious concern because it can potentially weaken the weld.
Porosity is caused when the gaseous shield did not sufficiently shield the molten weld metal while the bead is laid or while it is cooling. What happens is that the weld bead is overexposed to and absorbs; nitrogen, oxygen, and hydrogen from the atmosphere. When a porous weld cools, the newly absorbed gaseous molecules are released and the welder is left with a porous weld bead.
Another defect affecting the strength of the weld is poor fusion, though it is often easily visible. It is caused by low current, contaminated joint surfaces, or the use of an improper electrode.
Shallow penetration, another detriment to weld strength, can be addressed by decreasing welding speed, increasing the current or using a smaller electrode. Any of these weld-strength-related defects can make the weld prone to cracking, but other factors are involved as well. High carbon, alloy or sulfur content in the base material can lead to cracking, especially if low-hydrogen electrodes and preheating are not employed.
Furthermore, the workpieces should not be excessively restrained, as this introduces residual stresses into the weld and can cause cracking as the weld cools and contracts. SMAW welding, like other welding methods, can be a dangerous and unhealthy practice if proper precautions are not taken.
The process uses an open electric arc, which presents a risk of burns which are prevented by personal protective equipment in the form of heavy leather gloves and long sleeve jackets. Additionally, the brightness of the weld area can lead to a condition called arc eye , in which ultraviolet light causes inflammation of the cornea and can burn the retinas of the eyes.
Welding helmets with dark face plates are worn to prevent this exposure, and in recent years, new helmet models have been produced that feature a face plate that self-darkens upon exposure to high amounts of UV light. To protect bystanders, especially in industrial environments, translucent welding curtains often surround the welding area.
These curtains, made of a polyvinyl chloride plastic film, shield nearby workers from exposure to the UV light from the electric arc, but should not be used to replace the filter glass used in helmets. In addition, the vaporizing metal and flux materials expose welders to dangerous gases and particulate matter. The smoke produced contains particles of various types of oxides.
The size of the particles in question tends to influence the toxicity of the fumes, with smaller particles presenting a greater danger.
Additionally, gases like carbon dioxide and ozone can form, which can prove dangerous if ventilation is inadequate. Some of the latest welding masks are fitted with an electric powered fan to help disperse harmful fumes. Shielded metal arc welding is one of the world's most popular welding processes, accounting for over half of all welding in some countries.
Because of its versatility and simplicity, it is particularly dominant in the maintenance and repair industry, and is heavily used in the construction of steel structures and in industrial fabrication. In recent years its use has declined as flux-cored arc welding has expanded in the construction industry and gas metal arc welding has become more popular in industrial environments.
However, because of the low equipment cost and wide applicability, the process will likely remain popular, especially among amateurs and small businesses where specialized welding processes are uneconomical and unnecessary. SMAW is often used to weld carbon steel , low and high alloy steel , stainless steel, cast iron , and ductile iron.
While less popular for nonferrous materials, it can be used on nickel and copper and their alloys and, in rare cases, on aluminium. The thickness of the material being welded is bounded on the low end primarily by the skill of the welder, but rarely does it drop below 1.
No upper bound exists: with proper joint preparation and use of multiple passes, materials of virtually unlimited thicknesses can be joined. Furthermore, depending on the electrode used and the skill of the welder, SMAW can be used in any position. Shielded metal arc welding equipment typically consists of a constant current welding power supply and an electrode, with an electrode holder, a 'ground' clamp, and welding cables also known as welding leads connecting the two.
The power supply used in SMAW has constant current output, ensuring that the current and thus the heat remains relatively constant, even if the arc distance and voltage change.
This is important because most applications of SMAW are manual, requiring that an operator hold the torch. Maintaining a suitably steady arc distance is difficult if a constant voltage power source is used instead, since it can cause dramatic heat variations and make welding more difficult. However, because the current is not maintained absolutely constant, skilled welders performing complicated welds can vary the arc length to cause minor fluctuations in the current.
The preferred polarity of the SMAW system depends primarily upon the electrode being used and the desired properties of the weld. Direct current with a negatively charged electrode DCEN causes heat to build up on the electrode, increasing the electrode melting rate and decreasing the depth of the weld.
Reversing the polarity so that the electrode is positively charged DCEP and the workpiece is negatively charged increases the weld penetration. With alternating current the polarity changes over times per second, creating an even heat distribution and providing a balance between electrode melting rate and penetration. Typically, the equipment used for SMAW consists of a step-down transformer and for direct current models a rectifier , which converts alternating current into direct current.
Because the power normally supplied to the welding machine is high-voltage alternating current, the welding transformer is used to reduce the voltage and increase the current. A number of different types of transformers can be used to produce this effect, including multiple coil and inverter machines, with each using a different method to manipulate the welding current.
The multiple coil type adjusts the current by either varying the number of turns in the coil in tap-type transformers or by varying the distance between the primary and secondary coils in movable coil or movable core transformers.
Inverters, which are smaller and thus more portable, use electronic components to change the current characteristics. Electrical generators and alternators are frequently used as portable welding power supplies, but because of lower efficiency and greater costs, they are less frequently used in industry. Maintenance also tends to be more difficult, because of the complexities of using a combustion engine as a power source. However, in one sense they are simpler: the use of a separate rectifier is unnecessary because they can provide either AC or DC.
The smaller amount of time the high-frequency waveform spends near zero makes it much easier to strike and maintain a stable arc than with the cheaper grid-frequency sets or grid-frequency mains-powered units. The choice of electrode for SMAW depends on a number of factors, including the weld material, welding position and the desired weld properties.
The electrode is coated in a metal mixture called flux, which gives off gases as it decomposes to prevent weld contamination, introduces deoxidizers to purify the weld, causes weld-protecting slag to form, improves the arc stability, and provides alloying elements to improve the weld quality.
Fast-fill electrodes are designed to melt quickly so that the welding speed can be maximized, while fast-freeze electrodes supply filler metal that solidifies quickly, making welding in a variety of positions possible by preventing the weld pool from shifting significantly before solidifying.
The composition of the electrode core is generally similar and sometimes identical to that of the base material. But even though a number of feasible options exist, a slight difference in alloy composition can strongly impact the properties of the resulting weld. This is especially true of alloy steels such as HSLA steels. Likewise, electrodes of compositions similar to those of the base materials are often used for welding nonferrous materials like aluminium and copper.
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Shielded metal arc welding
Shielded metal arc welding SMAW , also known as manual metal arc welding MMA or MMAW , flux shielded arc welding  or informally as stick welding , is a manual arc welding process that uses a consumable electrode covered with a flux to lay the weld. An electric current , in the form of either alternating current or direct current from a welding power supply , is used to form an electric arc between the electrode and the metals to be joined. The workpiece and the electrode melts forming a pool of molten metal weld pool that cools to form a joint. As the weld is laid, the flux coating of the electrode disintegrates, giving off vapors that serve as a shielding gas and providing a layer of slag , both of which protect the weld area from atmospheric contamination. Because of the versatility of the process and the simplicity of its equipment and operation, shielded metal arc welding is one of the world's first and most popular welding processes.
A History of Welding
In the resistance welding process, the parts to be joined are pressed together and heated by an electric current until pools of molten material form at individual points between them. This operation requires high currents and large pressing forces. During the welding of thick sheets, the forces between the electrode and the parts to be joined may reach as much as MPa. This represents a considerable challenge for the welding electrodes that have to withstand these loads. Welding electrodes made from our refractory metals and their alloys have a relatively high level of electrical conductivity while also exhibiting outstanding stability at high temperatures. They therefore have a considerably longer service life than conventional materials such as copper and copper alloys.SEE VIDEO BY TOPIC: Tips for stick welding sheet metal
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A Guide to Welding Electrodes on Ships – Part 2
This Site Paypal Link to Site. Welding Timeline Years Fouch and F. Picard develops oxyacetylene torch in France.
He continues to be the chairperson of the Steering Committee for this series of conferences. In he was appointed as editor-in-chief of Elsevier Journal of Material Processing Technology and continued in this role until In , Professor Hashmi was awarded the higher doctorate degree of DSc, by the University of Manchester, the highest academic degree any university in the UK can award. Professor Hashmi has supervised or co-supervised PhD and 55 MEng research students to successful completion. He has published in excess of papers and 12 books so far. Comprehensive Materials Processing. Newnes , 7 apr. Comprehensive Materials Processing provides students and professionals with a one-stop resource consolidating and enhancing the literature of the materials processing and manufacturing universe.
A History of Welding
In this article we shall discuss the nomenclature and classification of the electrodes based on popular ISO and AWS standards. The standardization of welding electrodes is essential as they are as important as the parent metals and alloys in manufacturing and repair. A correctly chosen electrode, which is matched perfectly to the parent metal, assures the effectiveness and strength of the welding. The welding electrodes are classified on the basis of the electrode metal, flux coating, current used, position of welding, performance characteristics, chemistry and the mechanical properties of the weld metal etc. AWS stands for American Welding Society and this classification is widely used in the merchant marine.
Dyeing and dairy equipment, Chemical vessels, Brewery and food equipment. Also suitable for welding of steels with high carbon content often used in the cement industry for anchor welding. Furnace linings, furnace parts, burners, heat treatment pots and baskets. Used for cladding purpose on carbon steel. Also used on L requirement. Ador Group. Team ADFL.
Tungsten and molybdenum electrodes for resistance welding.
Morgantown, WV Elemental manganese, nickel, chromium, iron emissions per unit length of weld and labor plus consumables costs were similarly measured. Flux-cored arc welding and shielded metal arc SMAW processes were also studied. The objective was to identify the best welding processes for reducing workplace exposures, and estimate costs for all processes.
The standards insure that you choose the right material to meet the welding code. While manufacturers produce their own proprietary brands and models, the products themselves can be cross-referenced to applicable industry classifications.
Shielded metal arc welding
Published on December 17, December 1, SMAW or stick electrodes are consumable, meaning they become part of the weld, while TIG electrodes are non-consumable as they do not melt and become part of the weld, requiring the use of a welding rod. Electrode selection is critical to ease of cleanup, weld strength, bead quality and for minimizing any spatter. Electrodes need to be stored in a moisture-free environment and carefully removed from any package follow the directions to avoid damage.
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