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Shielded metal arc welding

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   Shielded metal arc welding
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   Shielded metal arc welding

   Shielded metal arc welding (SMAW), also known as manual metal arc (MMA)
   welding or informally as stick welding, is a manual arc welding process
   that uses a consumable electrode coated in 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. 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 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 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.

Development

   After the discovery of the electric arc in 1800 by Humphry Davy, arc
   welding began to develop slowly, and by the end of the 19th century, an
   early welding process called carbon arc welding was developed. Nikolai
   N. Benardos and Stanislaus Olszewski were awarded patents in the 1880s
   showing a rudimentary electrode holder, and later, in 1890 C. L. Coffin
   received a U.S. patent for his arc welding method that utilized a metal
   electrode. The process, like SMAW, deposited melted electrode metal,
   serving as filler metal, into the weld.

   Around the turn of the 20th century, A. P. Strohmenger and Oscar
   Kjellberg released the first coated electrodes. Strohmenger used clay
   and lime as a coating to stabilize the arc, while Kjellberg dipped iron
   wire into mixtures of carbonates and silicates to coat the electrode.
   In 1912 Strohmenger released a heavily coated electrode, but because of
   the high cost and complex production methods, none of these early
   electrodes became popular. In 1927, however, an extrusion process was
   released that reduced the cost of coating electrodes while opening the
   door to more complex coating mixtures designed for specific
   applications. Later, in the 1950s the use of iron powder in the
   electrode covering became popular, making it possible to increase the
   welding speed.

   In 1938 K. K. Madsen described an automated variation of SMAW, now
   known as gravity welding. It briefly gained popularity in the 1960s
   after receiving publicity for its use in Japanese shipyards. However,
   today its applications are limited. Another little used variation of
   the process, known as firecracker welding, was developed around the
   same time by George Haferguy in Austria.

Operation

   SMAW weld area
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   SMAW weld area

   To strike the electric arc, the electrode is brought into contact with
   the workpiece in a short sweeping motion and then pulled away 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. As the electrode melts, the flux
   covering disintegrates, giving off vapors 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, reduce the amount of
   time that the welder can spend laying the weld, making SMAW one of the
   least efficient welding processes. In general, the operator factor, or
   the percentage of operator's time spent laying weld, is approximately
   25%.

   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.

Quality

   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. It can be caused by
   excessively high current, a long arc, or arc blow, a condition
   associated with direct current characterized by the electric arc being
   deflected away from the weld pool by magnetic forces. 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. 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.

Safety

   SMA 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, presenting a risk of burns which is
   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 the 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, transparent 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.

Application

   Shielded metal arc welding is one of 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 0.05 in (1.5 mm). 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.

Equipment

   SMAW system setup
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   SMAW system setup

   Shielded metal arc welding equipment typically consists of a constant
   current welding power supply and an electrode, with an electrode
   holder, a work clamp, and welding cables (also known as welding leads)
   connecting the two.

Power supply

   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 and the workpiece negatively charged
   increases the weld penetration. With alternating current the polarity
   changes over 100 times per second, creating an even heat distribution
   and providing a balance between electrode melting rate and penetration.
   A high output welding power supply for SMAW and GTAW
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   A high output welding power supply for SMAW and GTAW

   Typically, the equipment used for SMAW consists of a step-down
   transformer and a rectifier, for converting 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. As a result,
   instead of 220 V at 50 A, for example, the power supplied by the
   transformer is around 17–45 V at currents up to 600 A. 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.

Electrode

   Various welding electrodes and an electrode holder
   Enlarge
   Various welding electrodes and an electrode holder

   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. Electrodes can be divided into three
   groups—those designed to melt quickly are called "fast-fill"
   electrodes, those designed to solidify quickly are called "fast-freeze"
   electrodes, and intermediate electrodes go by the name "fill-freeze" or
   "fast-follow" electrodes. 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.
   However, sometimes it is desirable to use electrodes with core
   materials significantly different from the base material. For example,
   stainless steel electrodes are sometimes used to weld two pieces of
   carbon steel, and are often utilized to weld stainless steel workpieces
   with carbon steel workpieces.

   Electrode coatings can consist of a number of different compounds,
   including rutile, calcium fluoride, cellulose, and iron powder. Rutile
   electrodes, made of 25%–45% TiO[2], are characterized by ease of use
   and good appearance of the resulting weld. However, they create welds
   with high hydrogen content, encouraging embrittlement and cracking.
   Electrodes containing calcium fluoride (CaF[2]), sometimes known as
   basic or low-hydrogen electrodes, are hygroscopic and must be stored in
   dry conditions. They produce strong welds, but with a coarse and
   convex-shaped joint surface. Electrodes made of cellulose, especially
   when combined with rutile, provide deep weld penetration, but because
   of their high moisture content, special procedures must be used to
   prevent excessive risk of cracking. Finally, iron powder is a common
   coating additive, as it improves the productivity of the electrode,
   sometimes as much as doubling the yield.

   To identify different electrodes, the American Welding Society
   established a system that assigns electrodes with a four- or five-digit
   number. Covered electrodes made of mild or low alloy steel carry the
   prefix E, followed by their number. The first two or three digits of
   the number specify the tensile strength of the weld metal, in thousand
   pounds per square inch (ksi). The penultimate digit generally
   identifies the welding positions permissible with the electrode,
   typically using the values 1 (normally fast-freeze electrodes, implying
   all position welding) and 2 (normally fast-fill electrodes, implying
   horizontal welding only). The welding current and type of electrode
   covering are specified by the last two digits together. When
   applicable, a suffix is used to denote the alloying element being
   contributed by the electrode.

   Common electrodes include the E6010, a fast-freeze, all-position
   electrode with a minimum tensile strength of 60 ksi (410  MPa) which is
   operated using DCEP. Its cousin E6011 is similar except that it is used
   with alternating current. E7024 is a fast-fill electrode, used
   primarily to make flat or horizontal welds using AC, DCEN, or DCEP.
   Examples of fill-freeze electrodes are the E6012, E6013, and E7014, all
   of which provide a compromise between fast welding speeds and
   all-position welding.

Process variations

   Though SMAW is almost exclusively a manual arc welding process, one
   notable process variation exists, known as gravity welding or gravity
   arc welding. It serves as an automated version of the traditional
   shielded metal arc welding process, employing an electrode holder
   attached to an inclined bar along the length of the weld. Once started,
   the process continues until the electrode is spent, allowing the
   operator to manage multiple gravity welding systems. The electrodes
   employed (often E6027 or E7024) are coated heavily in flux, and are
   typically 28 in (0.8 m) in length and about 0.25 in (6 mm) thick. As in
   manual SMAW, a constant current welding power supply is used, with
   either negative polarity direct current or alternating current.

   Due to a rise in the use of semiautomatic welding processes such as
   flux-cored arc welding, the popularity of gravity welding has fallen as
   its economic advantage over such methods is often minimal. Other
   SMAW-related methods that are even less frequently used include
   firecracker welding, an automatic method for making butt and fillet
   welds, and massive electrode welding, a process for welding large
   components or structures that can deposit up to 60 lb (27 kg) of weld
   metal per hour.
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