Shielded metal arc welding

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SMAW welding in the field

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 aluminum, nickel and copper alloys can also be welded with this
method.[1]


Development
After the discovery of the electric arc in 1800 by Humphry Davy there was little development in
electrical welding until Nikolai N. Benardos and Stanislaus Olszewski developed carbon arc
welding, obtaining patents in the 1880s showing a rudimentary electrode holder. Later in 1890 C.
L. Coffin received U.S. Patent 428,459 for his arc welding method that utilized a metal electrode.
The process, like SMAW, deposited melted electrode metal into the weld as filler.[2]

Around 1900 A. P. 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.[3] In 1912 Strohmenger released a
heavily coated electrode but high cost and complex production methods prevented these early
electrodes from gaining popularity. In 1927 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 1950s manufacturers introduced iron powder
into the flux coating, making it possible to increase the welding speed.[4]

In 1938 K. K. Madsen described an automated variation of SMAW, now known as gravity

Operation

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, with a movement like lighting a match. 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%.[6]

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.[7]

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 and contracts.[8]

Safety

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, 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.[9]

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.[10]

Application and Materials

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.[11]

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 aluminum. 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.[12]

Equipment
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 ground 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.[13]

A high output welding power supply for SMAW and GTAW

Engine driven welder mounted in Field Service Truck

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 is 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.[14]

Typically, the equipment used for SMAW consists of a step-down transformer and for direct
current models 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.[15]

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.[16] However, the engine driven units
are most practical in field work where the welding often must be done out of doors and in
locations where transformer type welders are not usable because there is no power source
available to be transformed.

Electrode

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.[17] 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.[18]
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 aluminum and copper.[19]
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.[20]

Electrode coatings can consist of a number of different compounds, including rutile, calcium
fluoride, cellulose, and iron powder. Rutile electrodes, coated with 25%–45% TiO2, 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 (CaF2), 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 coated with 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.[21]

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.[22]

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.[23]

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.[5]

References
 Cary, Howard B. and Scott C. Helzer (2005). Modern Welding Technology. Upper Saddle
River, New Jersey: Pearson Education. ISBN 0-13-113029-3.
 Jeffus, Larry (1999). Welding: Principles and Applications. Albany: Thomson Delmar.
ISBN 0-8273-8240-5 .
 Lincoln Electric (1994). The Procedure Handbook of Arc Welding. Cleveland: Lincoln
Electric. ISBN 99949-25-82-2 .
 Weman, Klas (2003). Welding processes handbook. New York: CRC Press LLC. ISBN
0-8493-1773-8 .

External links
 SMAW guidelines (.pdf)
 MMA welding from The Welding Institute

Retrieved from "http://en.wikipedia.org/wiki/Shielded_metal_arc_welding"

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