Welding 2010 (New)

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Title:

Shielded Metal Arc Welding (SMAW), Gas Metal Arc Welding (GMAW)

Objective:

To expose and practically hand on in welding / joining processes.

Introduction:

Welding is one of joining processes method which is widely used. During welding

processes the interface of two parts to be joined are brought to a temperature above the melting point and then allowed to solidify so that a permanent joining rakes place. Because of the

permanent nature of the joint and strength being equal to or sometimes greater than that of the

parent metal makes welding one of the most extensively used fabrication method. Welding is not

only used for making structures but also repair work such as the joining of broken casting.

Shielded Metal Arc Welding (SMAW) / Stick Welding /Arc Welding

Theory SMAW

Shielded Metal Arc Welding (SMAW) is frequently referred to as stick welding or

covered electrode welding. Stick welding is among the most widely used welding processes. It is

also 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.

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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.

The flux covering the electrode melts during welding. This forms the gas and slag to

shield the arc and molten weld pool. The slag must be chipped off the weld bead after welding.

The flux also provides a method of adding scavengers, deoxidizers, and alloying elements to the

weld metal.

Apparatus

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Schematic diagram of Shielded SMAW circuit

SMAW procedure

To strike the electric arc, the electrode is brought into contact with the work piece in a

short sweeping motion and then pulled away slightly. This initiates the arc and thus the melting

of the work piece 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 protectsthe 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

work piece, 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.

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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.

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 gasesand 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.

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Gas Metal Arc Welding (GMAW) /MIG Welding

GMAW Theory

Gas Metal Arc Welding (GMAW) is frequently referred to as MIG welding. MIGwelding is commonly used high deposition rate welding process. Wire is continuously fed from a

spool. MIG welding is therefore referred to as a semiautomatic welding process.

Gas metal arc welding (GMAW), sometimes referred to by its subtypes metal inert gas

(MIG) welding or metal active gas (MAG) welding, is a semi-automatic or automatic arc

welding process in which a continuous and consumable wire electrode and a shielding gas are

fed through a welding gun. A constant voltage, direct current power source is most commonly

used with GMAW, but constant current systems, as well as alternating current, can be used.

There are four primary methods of metal transfer in GMAW, called globular, short-circuiting,

spray, and pulsed-spray, each of which has distinct properties and corresponding advantages and

limitations.

Originally developed for welding aluminum and other non-ferrous materials in the 1940s,

GMAW was soon applied to steels because it allowed for lower welding time compared to other

welding processes. The cost of inert gas limited its use in steels until several years later, when

the use of semi-inert gases such as carbon dioxide became common. Further developments

during the 1950s and 1960s gave the process more versatility and as a result, it became a highly

used industrial process. Today, GMAW is the most common industrial welding process,

preferred for its versatility, speed and the relative ease of adapting the process to robotic

automation. The automobile industry in particular uses GMAW welding almost exclusively.

Unlike welding processes that do not employ a shielding gas, such as shielded metal arc welding,

it is rarely used outdoors or in other areas of air volatility. A related process, flux cored arc

welding, often does not utilize a shielding gas, instead employing a hollow electrode wire that is

filled with flux on the inside.

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Apparatus

Schematic diagram of MIG MIG circuit

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GMAW Procedure

The basic technique for GMAW is quite simple, since the electrode is fed

automatically through the torch. By contrast, in gas tungsten arc welding, the welder must handle

a welding torch in one hand and a separate filler wire in the other, and in shielded metal arc

welding, the operator must frequently chip off slag and change welding electrodes. GMAW

requires only that the operator guide the welding gun with proper position and orientation along

the area being welded. Keeping a consistent contact tip-to-work distance (the stickout distance) is

important, because a long stickout distance can cause the electrode to overheat and will also

waste shielding gas. Stickout distance varies for different GMAW weld processes and

applications. For short-circuit transfer, the stickout is generally 1/4 inch to 1/2 inch, for spray

transfer the stickout is generally 1/2 inch. The position of the end of the contact tip to the gasnozzle is related to the stickout distance and also varies with transfer type and application. The

orientation of the gun is also important it should be held so as to bisect the angle between the

work pieces; that is, at 45 degrees for a fillet weld and 90 degrees for welding a flat surface. The

travel angle or lead angle is the angle of the torch with respect to the direction of travel, and it

should generally remain approximately vertical. However, the desirable angle changes somewhat

depending on the type of shielding gas used with pure inert gases, the bottom of the torch is out

often slightly in front of the upper section, while the opposite is true when the welding

atmosphere is carbon dioxide.

Safety

Gas metal arc welding can be dangerous if proper precautions are not taken. Since

GMAW employs an electric arc, welders wear protective clothing, including heavy leather

gloves and protective long sleeve jackets, to avoid exposure to extreme heat and flames. Inaddition, the brightness of the electric arc can cause arc eye, in which ultraviolet light causes the

inflammation of the cornea and can burn the retinas of the eyes. Helmets with dark face plates

are worn to prevent this exposure, and in recent years, new helmet models have been produced

that feature a liquid crystal-type face plate that self-darkens upon exposure to high amounts of

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UV light. Transparent welding curtains, made of a polyvinyl chloride plastic film, are often used

to shield nearby workers and bystanders from exposure to the UV light from the electric arc.

Welders are also often exposed to dangerous gases and particulate matter. GMAW

produces smoke containing particles of various types of oxides, and the size of the particles in

question tends to influence the toxicity of the fumes, with smaller particles presenting a greater

danger. Additionally, carbon dioxide and ozone gases can prove dangerous if ventilation is

inadequate. Furthermore, because the use of compressed gases in GMAW pose an explosion and

fire risk, some common precautions include limiting the amount of oxygen in the air and keeping

combustible materials away from the workplace. While porosity usually results from

atmospheric contamination, too much shielding gas has a similar effect; if the flow rate is too

high it may create a vortex that draws in the surrounding air, thereby contaminating the weld pool as it cools. The gas output should be felt (as a cool breeze) on a dry hand but not enough to

create any noticeable pressure, this equates to between 20±25 psi (mild and stainless steel).

Above 26 volts the gas debit should be augmented slightly since the weld pool takes longer to

cool. As a factor that is often ignored, many flow meters are never adjusted and typically run

between 35±45 psi. A healthy reduction of gas will not affect the quality of the weld, will save

money on shielding gas and reduce the rate at which the tank must be replaced.

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R esult:

1. Shielded Metal Arc Welding (SMAW)

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2. Gas Metal Arc Welding (GMAW)

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Discussion:

SAFETY ISSUES

To prevent any risk, welders wear personal protective equipment in the form of heavy

leather gloves and protective long sleeve jackets to avoid exposure to extreme heat and flames.

Additionally, the brightness of the weld area leads to a condition called arc eye in which

ultraviolet light causes inflammation of the cornea and can burn the retinas of the eyes. Goggles

and 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. Welders are also often exposed to dangerous gases and particulate

matter.

DISTOR TION AND CR ACKING

Welding methods that involve the melting of metal at the site of the joint necessarily are

prone to shrinkage as the heated metal cools. Shrinkage, in turn, can introduce residual stresses

and both longitudinal and rotational distortion. Distortion can pose a major problem, since the

final product is not the desired shape. To alleviate rotational distortion, the work pieces can be

offset, so that the welding results in a correctly shaped piece.

The cracking occurs in the heat-affected zone of the base material. To reduce the amount

of distortion and residual stresses, the amount of heat input should be limited, and the welding

sequence used should not be from one end directly to the other, but rather in segments. The other

type of cracking, hot cracking or solidification cracking, can occur with all metals, and happens

in the fusion zone of a weld. To diminish the probability of this type of cracking, excess material

restraint should be avoided, and a proper filler material should be utilized.

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WELD ABILITY

The quality of a weld is also dependent on the combination of materials used for the base

material and the filler material. Not all metals are suitable for welding, and not all filler metals

work well with acceptable base materials.

HEAT-AFFECTED ZONE (HAZ)

The blue area results from oxidation at a corresponding temperature of 600 °F (316 °C).

This is an accurate way to identify temperature, but does not represent the HAZ width. The HAZ

is the narrow area that immediately surrounds the welded base metal.

The effects of welding on the material surrounding the weld can be detrimental

depending on the materials used and the heat input of the welding process used, the HAZ can be

of varying size and strength.

QUALITY

Most often, the major metric used for judging the quality of a weld is its strength and the

strength of the material around it. Many distinct factors influence this, including the welding

method, the amount and concentration of energy input, the base material, the filler material, the

flux material, the design of the joint, and the interactions between all these factors. To test the

quality of a weld, either destructive or nondestructive testing methods are commonly used to

verify that welds are defect-free, have acceptable levels of residual stresses and distortion, and

have acceptable heat-affected zone (HAZ) properties. Welding codes and specifications exist to

guide welders in proper welding technique and in how to judge the quality of welds.

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Conclusion:

Shielded Metal Gas Art Welding (SMAW) is performed with the heat of an electric arc

that is maintained between the end of a coated metal electrode and the work piece. The heat

generated melts a portion of the electrode tip, its coating, and the base metal in the immediate arc

area. The molten metal consists of mixture of the base metal, the electrode metal, and substances

from the coating on the electrode; this mixture forms the weld when it solidifies. The electrode

coating deoxidizes the weld area and provides a shielding gas to protect it from oxygen in the

environment. The SMAW process has the advantages of being relatively simple versatile and requiring a

smaller variety of electrode. The most common quality problems associated with SMAW include weld

spatter, porosity, poor fusion, shallow penetration, and cracking. Weld spatter, while not affecting theintegrity of the weld, damages its appearance and increases cleaning costs.

Gas Metal Arc Welding (GMAW) shields the weld zone with an external gas such as argon,

helium, carbon dioxide, or gas mixtures. Deoxidizers present in the electrode can completely prevent

oxidation in the weld puddle, making multiple weld layers possible at the joint. Benefit of Gas Metal Arc

Welding is the technique is easy then no need for slag cleaning and it is the extremely high productivity

that GMAW possible, high rate of filler metal deposition, narrow weld bead, minimum distortion of work piece, very easy to learn and lastly clean, precise welds. However the main disadvantages is expensive

and complex equipment and setting up equipment can be time consuming and tricky. Besides that,

GMAW also used on all thicknesses of steels, aluminums, nickel, stainless steel etc. in addition it also

suitable both for steel and unalloyed, low alloy and high alloy based materials.

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R eferences:

y P.N Rao, Manufacturing Technology foundry, forming, welding, Mc Graw Hill,1997

y http://www.welding-technology-machines.info

y http://www.welding.com

y http://www.weldingengineer.com

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