Transcript Chapter 11 Shielded Metal Arc Welding Principles

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
• 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
Miller Electric Mfg. Co.
The Lincoln Electric Co.
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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 suited
for heavy work
Miller Electric Mfg. Co
The Lincoln Electric 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
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
Two leads are
required because
the arc is created
between the two
electrodes.
Larger than the
metal electrode
holder
Metal shield to protect welder's
hand from intense heat.
The Lincoln Electric Co.
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
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
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