Shielded
metal arc welding
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 non destructive 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.
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, 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.
Equipment:-
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.
A
high output welding power supply for Stick, GTAW, MIG, Flux-Cored, & Gouging
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 100 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.
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.
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.
In some units the
alternator is essentially the same as that used in portable generating sets
used to supply mains power, modified to produce a higher current at a lower
voltage but still at the 50 or 60 Hz grid frequency. In higher-quality
units an alternator with more poles is used and supplies current at a higher
frequency, such as 400 Hz. 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.
Electrode:-
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 flouride, 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 hygtroscopic 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 that increases the rate at which the electrode fills
the weld joint, up to twice as fast.
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. E6011 is similar except its flux coating allows it to be
used with alternating current in addition to DCEP. 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.
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.
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 1.5 mm (0.06 in). 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.
|
, Gas tungsten arc welding (GTAW), Gas metal arc welding (GMAW), Flux-cored arc welding (FCAW), Submerged arc welding (SAW), Electroslag welding (ESW), Electric resistance welding (ERW) ){kind=link}
1 Comments
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