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Shielded Metal Arc Welding(SMAW)

• Arc welding– Heat for welding generated by electric arc

established between flux-covered consumable metal rod (electrode) and work

• Called stick electrode welding• Combustion and decomposition of electrode

creases gaseous shield (Gases given off)– Protects electrode tip, weld puddle, arc, and highly

heated work from atmospheric contamination• Additional shielding provided by covering of molten

slag (flux)

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SMAW

American Welding Society

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SMAW Advantages

• Equipment less complex, more portable and less costly

• Can be done indoors or outdoors, in any location and any position

• Electrodes available to match properties and strength of most base metals– Not used for welding softer metals

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SMAW Operating Principles

• Sets up electric circuit– Includes welding machine, work, electric cables,

electrode holder and electrodes, and a work clamp

• Heat of electric arc brings work to be welded and consumable electrode to molten state– Heat intense: as high at 9,000ºF at center

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SMAW Operating Principle

American Welding Society

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Welding Power Sources

• Each type of power source has fundamental electrical differences that best suit particular processes

• Welding machine– Must meet changing arc load and environmental

conditions instantly– Must deliver exact amount of electric current

precisely at right time to welding arc

• Available in wide variety of types and sizes

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Four Types of Power Source

• Engine-driven generators– Powered by gas or diesel combustion engine– Can be found with a.c. or d.c. electric motor

• No longer being manufactured and rarely found

• Inverters– Increases frequency of incoming primary power– Constant current, constant voltage, or both– Produce a.c. or d.c. welding current

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Four Types of Power Source

• A.C. transformers– Used to step down a.c. line power voltage to a.c.

welding voltage

• Transformer-rectifiers– Use basic electrical transformer to step down a.c.

line power voltage to a.c. welding voltage– Welding voltage then passed through rectifier to

convert a.c. output to d.c. welding current– May be either d.c. or a.c.-d.c. machines

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Bridge Rectifier

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Output Slope

• Two basic types– Constant current

– Constant voltage

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Current Controls

• Amperage– Quantity of current (flow)– Determines amount of heat produced at weld

• Voltage– Measure of force of current (push)– Determines ability to strike an arc and maintain its

consistency

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Constant Current Welding Machines

• Used for shielded metal arc welding and gas tungsten arc welding– Available in both d.c. and a.c. welding current

– Current remains fairly constant regardless of changes in arc length

– Total Wattage stays the same– Voltage drops as amps increase (dropping arc voltage

(DAV) machine)

• Enables welder to control welding current in specific range by changing length of arc

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Open Circuit and Arc Voltage

• Open circuit voltage runs between 50-100 volts (no welding being done, volts high, no amps)– Drops to arc voltage when arc struck

• Arc voltages (Voltage generated between electrode and work during welding, voltage lower, amps higher)

– Range: 36 volts (long arc) to 18 volts (short arc)

– Determined by arc length held by welder and type of electrode used

• Arc lengthened, arc voltage increases and current decreases

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Polarity

• Electrode negative and electrode positive used in d.c. welding

• DCEN (d.c. electrode negative)– Electrode connected to negative terminal of power source

and work connected to positive terminal (current flows from neg to pos) flow from electrode to work = more electrode consumption.

• DCEP (d.c. electrode positive)– Electrode connected to positive terminal of power source

and work connected to negative terminal

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D.C. Transformer-Rectifier Welding Machines

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The Lincoln Electric Co.

Miller Electric Mfg. Co.

• Have many designs and purposes• Flexibility one reason for wide acceptance

– Deliver either DCEN or DCEP

• May be used for:– Stick electrode welding– Gas tungsten arc welding– Submerged arc welding– Multi-operator systems– Stud welding

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Transformer-Rectifier Machines

• Have two basic parts– Transformer for producing and regulating

alternating current that enters machine– rectifier that converts a.c. to d.c.

• Third important part is ventilating fan– Keeps rectifier from overheating

• Design improves arc stability and makes it easy to hold short arc which is soft and steady

• No major rotating parts so consume little power

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A.C. Transformer Welding Machines

• Most popular a.c. welding machine

• Function of transformer– Step down high voltage of input current to high

amperage, low voltage current required for welding

• Especially suitedfor heavy work

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The Lincoln Electric Co.Miller Electric Mfg. Co

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Advantages of a.c. Power Sources

• Reduces tendency to arc blow• Can use larger electrodes

– Resulting in faster speeds on heavy materials

• Lower cost• Decreased power consumption• High overall electrical efficiency• Noiseless operation• Reduced maintenance

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D.C. and A.C.-D.C. Inverter Welding Machines

• Portable, lightweight, and versatile

• May be either constant current, constant voltage or both

• Can perform several different processes

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Miller electric Mfg. Co.The Lincoln Electric Co.

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Duty Cycle

• Percentage of any given 10-minute period that machine can operate at rated current without overheating or breaking down– Rating of 100% means machine can be used at

rated amperage on continuous basis• Required by continuous, automatic machine welding

– Rating of 60% means machine can be used at its capacity 6 out of every 10 minutes without damage

• Satisfactory for heavy SMAW and GTAW

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Twin Carbon Electrode Holder

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The Lincoln Electric Co.

Two leads arerequired becausethe arc is createdbetween the two

electrodes.

Metal shield to protect welder's hand from intense heat.

Larger than the metal electrode

holder

Holder is water cooled

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Atomic-Hydrogen Arc Welding

• Process in which electric arc surrounded by atmosphere of hydrogen– Gas shields molten metal from oxidation and

contamination from the air– Transfers heat from electrode to work– Arc formed between two electrodes

• Temperature produced by arc: 7,500ºF• Current supplied by a.c. welding transformer• Hydrogen supplied in cylinders

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Atomic-Hydrogen Electrode Holder

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General Electric Co.

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Atomic-Hydrogen Arc Welding

• Metal of same analysis as being welded can be deposited

• Welds may be heat treated– Unusually smooth, ductile, nonporous and free

from impurities– Surface free from scale

• May weld hard-to-weld metals• Advantages: increased production, low

operating cost, and low maintenance cost