lect 3 shielded metal arc welding

8/8/2019 Lect 3 Shielded Metal Arc Welding

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Lecture 3Arc Welding:

Shielded Metal Arc Welding

Welding and Joining TechnologyEDPT 701

Dr. Mohamed Harraz5 Oct 2010 1


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Arc Welding

Arc welding is a process that uses an electric arc to join the metals being welded. A

distinct advantage of arc welding over gas welding is the concentration of heat. In gaswelding the flame spreads over a large area, sometimes causing heat distortion. Theconcentration of heat, characteristic of arc welding, is an advantage because less heatspread reduces buckling and warping. This heat concentration also increases the depthof penetration and speeds up the welding operation; therefore, you will find that arcwelding is often more practical and economical than gas welding.

All arc-welding processes have three things in common: a heat source, filler metal, andshielding. The source of heat in arc welding is produced by the arcing of an electricalcurrent between two contacts.

There are several types of arc-welding processes with which you should becomefamiliar. They are:

Shielded metal arc welding (SMAW) Flux-cored arc welding (FCAW)

Gas Shielded Arc Welding (GSAW)

Submerged Arc Welding (SAW)

Shielded Metal Arc Welding Dr. Mohamed Harraz5 Oct 2010 2


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Shielded Metal Arc Welding (SMAW)

Shielded metal arc welding is performed by striking an arc

between a coated-metal electrode and the base metal.Once the arc has been established, the molten metal fromthe tip of the electrode flows together with the moltenmetal from the edges of the base metal to forma sound

joint. This process is known asfusion. The coating from the

electrode forms a covering over the weld deposit, shieldingit from contamination; therefore the process is calledshielded metal arc welding. The main advantages ofshielded metal arc welding are that high-quality welds aremade rapidly at a low cost.

Shielded Metal Arc Welding is sometimes referred to asManual Metal Arc (MMA) welding or Stick Welding.



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Shielded Metal Arc Welding (SMAW)

Equipment required to perform the SMAW welding process includes a

constant current power source that supplies the power to the consumablerod electrode.


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SMAW Characteristics and Features


Shielded Metal Arc Welding:

Uses a electrode rod that is quickly consumed, Uses equipment that is simple, inexpensive, and highly portable,

Uses an electrode that provides and regulates its own flux,

Provides all position flexibility,

Is less sensitive to wind or drafts,

Yields a weld with a variable quality and appearance based on operator

skill.

During the SMAW welding process the arc is established, the flux coating

on the rod disintegrates and then forms a gas that shields the weld from

the atmosphere. The slag that is produced by the flux coating prevents the

weld metal from oxidizing. 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.


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SMAW Process


Electrode Wire

Base Metal


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Welding Electrode


Flux-coated electrodes are available in many core wire

diameters and lengths. Matching the electrode properties to

the base materials as a general rule for choosing the type of

electrode. Available electrodes types include aluminum

bronze, bronze, mild steel, nickel, and stainless steel.

Electrodes are made in lengths varying from 200mm to450mm and range in diameters from 1.5mm to 8mm.

The core wire is selected to match the chemical composition

of the metal being welded as closely as possible.

Materials commonly welded using the SMAW process includemild steel, cast iron, and stainless steel. The flux covering on

the electrode melts during welding. This forms the gas and

slag to shield the arc and molten weld pool.


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Types of Joints for SMAW


The SMAW welding process typically is capable of producing four types of

welded joints. They are:

- Butt joint,

- Lap joint,

- T-joint ,

- Fillet Joint.


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Welding Current

In arc welding, the heat required is obtained from electrical arc

energy. Electric arc is generated by touching the tip of the electrode

against the wp then withdrawing it quickly to a distance sufficient

to maintain the arc.

Arc is produced between the tip of the electrode and the work

piece to be welded by the use of an arc of an AC or DC powersupply

Welding Current ranges from 50Amp to 300Amp . Current is

generally 40Amp per millimeter of electrode diameter. power

required is less than 10 kW AC or DC. For most small applications

DC is preferred due to arc stability and shallow penetration.

AC is used for welding thicker sections using larger diameter

electrodes

Voltages generally ranges from 20 to 50 volts.



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Electrode CoatingA variety of electrode coatings have been developed. For

example cellulose and titania coatings contain SiO2, TiO2, smallamounts of FeO, MgO and Na2O and volatile matter.

Under decomposition the volatile matter may releasehydrogen which can dissolve in weld metal and causeembrittlement or cracking in the joint.

Low hydrogen electrodes are available which provideshielding without hydrogen emission.

Since many electrode coatings may absorb hydrogen, coatedelectrodes are baked just before use.



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Functions of Wire CoatingThe wire is surrounded by a coating containing chemical components to:

1. Vaporize to provide a protective atmosphere ( shield around the arcand molten pool)

2. Provide ionizing elements to help stabilizing the arc, reduce weldmetal spatter and increase efficiency of deposition

3. Act as a flux to deoxidize and remove impurities from molten metal

4. Provide protective slag coating to accumulate impurities, preventoxidation and slow down the cooling of the weld metal

5. Add alloying elements

6. Add additional filler metal

7. Affect arc penetration ( depth of melting in wp)8. Influence the shape of weld bead

9. Iron powder and alloying elements can be incorporated to the coatto adjust the chemistry of the weld



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Process capabilities

SMAW has the advantages of being relatively simple

and versatile and requiring a smaller variety ofelectrodes

Is used in general construction, ship building, on pipelines, and maintenance work

Is useful for work in remote areas where portablefuel powered generator can be used for powersupply

Is suitable for work piece thicknesses of 3 to 19 mm.For thicker sections multiple-pass technique is used.

Suitable for all welding positions.



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Designation of Welding Electrode Coated electrodes are classified by the tensile strength of the deposited

metal, the welding position , preferred type of current and polarity, typeof coating

A 4 or 5 digit system of designation has been adopted by AWS( American

Welding Society)



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Fluxed-cored arc welding (FCAW) Is similar to SMAW except that the electrode is tubular and

the powdered flux is inside (cored) Can be made continuous and less bulky as no binders are

required to hold the flux in place- more flexible- can beprovided in long coiled lengths as Continuous electrode is fedautomatically through a welding gun.

Higher weld deposition rate compared to GMAW has led to itsuse in welding sections of all thicknesses

Cored electrodes produce more stable arc, improve weldcontour, and produce better mechanical properties of weld

Electrodes are usually 0.5mm to 4mm power required about

20 kW Smaller diameter electrodes allow welding of thinner

materials and to weld parts in different positions.

The flux has the same functions of the coating of theelectrodes in the SMAW.



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Submerged Arc Welding


Submerged arc welding (SAW) is a commonarc welding process. It requires a continuously

fed consumable solid or tubular (flux cored)electrode. The molten weld and the arc zoneare protected from atmospheric contaminationby being submerged under a blanket ofgranular fusible flux consisting of lime, silica,manganese oxide, calcium fluoride, and othercompounds. When molten, the flux becomes

conductive, and provides a current pathbetween the electrode and the work. This thicklayer of flux completely covers the molten metalthus preventing spatter and sparks as well assuppressing the intense ultraviolet radiation andfumes that are a part of the shielded metal arcwelding (SMAW) process.

SAW is normally operated in the automatic ormechanized mode, however, semi-automatic(hand-held) SAW guns with pressurized orgravity flux feed delivery are available. Theprocess is normally limited to the flat orhorizontal-fillet welding positions
http://en.wikipedia.org/wiki/Arc_weldinghttp://en.wikipedia.org/wiki/Flux_(metallurgy)http://en.wikipedia.org/wiki/Lime_(mineral)http://en.wikipedia.org/wiki/Silicon_dioxidehttp://en.wikipedia.org/wiki/Calcium_fluoridehttp://en.wikipedia.org/wiki/Shielded_metal_arc_weldinghttp://en.wikipedia.org/wiki/Shielded_metal_arc_weldinghttp://en.wikipedia.org/wiki/Shielded_metal_arc_weldinghttp://en.wikipedia.org/wiki/Shielded_metal_arc_weldinghttp://en.wikipedia.org/wiki/Shielded_metal_arc_weldinghttp://en.wikipedia.org/wiki/Shielded_metal_arc_weldinghttp://en.wikipedia.org/wiki/Shielded_metal_arc_weldinghttp://en.wikipedia.org/wiki/Shielded_metal_arc_weldinghttp://en.wikipedia.org/wiki/Shielded_metal_arc_weldinghttp://en.wikipedia.org/wiki/Calcium_fluoridehttp://en.wikipedia.org/wiki/Calcium_fluoridehttp://en.wikipedia.org/wiki/Calcium_fluoridehttp://en.wikipedia.org/wiki/Silicon_dioxidehttp://en.wikipedia.org/wiki/Lime_(mineral)http://en.wikipedia.org/wiki/Flux_(metallurgy)http://en.wikipedia.org/wiki/Arc_weldinghttp://en.wikipedia.org/wiki/Arc_weldinghttp://en.wikipedia.org/wiki/Arc_welding


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Metal Transfer Techniques


Multipath welding


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Welding Techniques



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Welding Techniques



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Welding Techniques



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Welding Techniques

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lect 3 shielded metal arc welding

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