Shielded Metal Arc Welding (SMAW)

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Shielded Metal Arc Welding (SMAW)

• Sometimes called "Stick Welding"
• most popular form of electric arc welding.

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Electric arc between flux covered electrode and
base metal
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• relatively inexpensive - 1500
• mobile when used with a generator
• used for maintenance, small production welding,
  heavy pipe welding
• heat from arc melts both base metal and electrode

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Arc welding station includes

• 1. arc welding machine (power source) 2.
  electrode lead and work lead terminals3.
  electrode holder4. ventilation5. safety
  equipment

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Electrodes

• Electrode solid metal wire core with a coating
  material
• electrodes are specified by the wire diameter and
  by the type of flux covering.
• common wire dia
• 1/16, 3/32, 1/8, 3/16, 7/32, 1/4, 5/16,
  3/8"lengths of 9 to 18" with 14" most common.
• usually come in 50lb packages
• electrodes must be kept very dry - some use ovens
  
• used for welding many metals
• carbon and low alloy steels most common

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Functions served by the electrode other than
adding molten metal to the weld area

• 1. shielding gas - prevent air from contacting
  and oxidizing2. fluxing agents, impurity
  scavengers and deoxidizer to clean the weld
  pool3. produce (slag) to prevent oxidation - and
  cool slowly - prevent brittle weld

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Functions (Cont.)

• 4. establish the polarity and electrical
  characteristics of the electrode5. adds alloying
  ingredients to the weld metal area to change the
  physical properties.

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Selecting the proper electrode
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Electrode type normally determined by the welding
procedure specification and or code used.

• 1. weld groove design2. tensile strength of the
  required weld3. the base metal composition4.
  position of the weld joint5. rate at which you
  want to deposit the weld metal

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Electrode Selection (Cont.)

• 6. type of arc welding current used7.
  penetration required8. metal thickness9.
  experience of the welder10. specifications for
  the weld to be madeboth AC and DC arc welding
  are done

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American Welding Society (AWS) electrode ID system

• 4 or 5 digit numbers

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Letter E welding electrode used for arc welding
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First two or three digits tensile strength

• 60 60,000 psi
• 70 70,000 psi
• 100 100,000 psi

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Next digit (second from right) recommended
position

• 1 all positions
• 2 flat or horizontal

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Last two digits together are type of current (AC,
DC, DC-) and electrode covering
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Occasionally additional two digits (letter and
number) - indicates alloying elements
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Common Electrodes
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6011

• XX11
• organic covering - paper ect.
• unstable arc - bead looks poor
• good penetration - good quality
• good for out of position welds

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6013 and 7018

• XX13
• Rutile covering - iron powders
• stable arc - bead looks good
• less penetration
• not good for out of position
• deposits lot of metal
• cools slower

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Electricity as it arcs across the gap -
temperatures - 6500 to 7000 deg F.Welders are AC
and DC
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DC welding machines of the following types

• 1. motor driven generator2. motor driven AC
  alternator with DC rectifier3. AC transformer
  with DC rectifier

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DC electrode negative (DCEN) and electrode
positive (DCEP) fundamentals
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• DCENElectrons flow from neg terminal of machine
  to electrode. Electrons continue to flow across
  arc into the base metal and to positive terminal
  of the machine.
• electrode (-) polarity
• base metal () polarity

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DCEN - direct current electrode negative (DCEN)

• also known as Direct Current Straight Polarity
  (DCSP)
• electrode melts faster
• deposits filler metal at a faster rate
• heats base metal more slowly
• used for welding thin metals

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DCEP

• By reversing the direction of the electron flow
  or polarity in the arc welding circuit.
• This may be done by disconnecting the electrode
  and work leads and reversing their positions.
• Also known as Direct Current Reverse Polarity
  (DCRP).

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Electrons flow from neg pole of the welding
machine to the work. Electrons travel across
arc to electrode and then return to positive
terminal of machine from electrode side of arc.

• electrode () polaritybase metal (-) polarity

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Results in deeper penetration
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The decision to use DCEN or DCEP often depends on
such variables as

• 1. depth of penetration2. rate at which filler
  metal is deposited3. position of the joint4.
  thickness of the base metal5. type of base metal
• Normally use DC

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Alternating Current AC arc welding fundamentals
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Two basic types of machines

• 1. motor driven alternator2. transformer type

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Current reverses its direction of flow 120 times
per second - (requires 1/60th of a second to
complete a cycle or hertz - Hz) called 60 Hz
(cycle) current
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Most AC welders have transformers which step down
the voltage and increase the current (amperes)
for welding purposes.
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Voltage and current are zero 120 times per
second. Each time the current crosses the zero
point, the welding arc stops. To reestablish the
arc, the voltage must increase enough to enable
the current to jump the arc gap and maintain the
arc.
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It is important that the voltage lead the current
as each passes through zero to help make the arc
stable.
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Another method to stabilize arc is to increase
ionization of the material in the arc.

• IONIZATION is the phenomenon in which a particle
  obtains an electrical charge. These ionized or
  charged particles in the space between the
  electrode and the work make it easy for the arc
  to jump the gap. electrodes intended for AC
  welding have ionizing agents in the electrode
  covering.

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Selection of either AC or DC arc welding machine
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ADV of DC

• 1. the ability to choose DCEP or DCEN

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DISADV of DC

• 1. the welder is generally more expensive than an
  AC machine of the same specs
• DC arc blow
• AC arc is quite stable after starting DC arc
  may wonder from the weld line. this wandering is
  caused by the forces of the magnetic field around
  the DC electrode called ARC BLOW - the magnetic
  lines of flux ten to travel more easily in metal
  than air. concentration of the magnetic flux at
  the ends of the metal forces the arc toward the
  center of the base metal.

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ADV of AC

• 1. welds made with AC have moderate
  penetration2. large dia electrodes can be used
  with high AC currents to obtain greater filler
  metal deposition rates and faster welding
  speeds3. AC arc welding machines are generally
  less expensive

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DISADV of AC

• 1. not all SMAW electrodes can be used with AC

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Choice on which to buy is made on basis of

• 1. type of welds to be made2. economics of the
  welding machine purchase3. personal preferences

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Can buy combination AC and DC welders - more
expensive.
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Starting / Stopping / adjusting the machine

• never start or stop the machine under
  loadusually a switch to turn off and on - may
  need to start the engine of an engine driven
  machine

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SMAW utilizes a constant current welding machine
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On a constant current machine, current (amps)
is set on the machine . voltage on the constant
current machine is not set. It varies as the
welding circuit resistance changes to maintain
constant or relatively constant current.
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Amperage (current) controls vary in appearance,
location, and operation on various machines -
hand crankstap type control with holes to plug
into for course and fine adjustmentshandwheel
for course and fine adjustments
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Striking an arc
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Electrode must first touch the base metal then be
withdrawn to the correct arc distance
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2 methods

• 1. glancing or scratching motion2. straight down
  and up motion

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If the arc breaks continually, regardless of how
careful the operator may be, it is probably due
to too low a current adjustment on the machine.
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If the electrode spatters excessively, and if it
becomes overheated while welding, the current
setting is too high
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Running a bead

• once the arc is struck and the arc stabilizes,
  the ARC CRATER will begin to form.

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To run a good bead, the following must be
controlled manually by the welder
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1. arc gap distance - 3/16 to 1/4

• arc length too short, shorting out may occurarc
  length too long, spattering and metal drops seen
  on the base metal

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2. speed of forward motion

• a. stringer bead - no motion is made except
  forwardb. weaving bead - electrode is moved
  uniformly back and forth across weld line

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3. electrode angle or position

• tipped forward 15 degrees in the direction of
  travel
• it is kept in line with the weld line for a
  stringer bead
• Slag Drag

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4. bead width

• a. stringer bead 2 to 3 times dia of
  electrodeb. weaving bead weld bead no wider
  than six times the electrode diameter

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Weld height is normally about 1/4 of the bead
width
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Restarting a weld bead end of existing bead
must be cleaned. Restart the arc about 3/8" ahead
of the forward edge of the crater then move back
rapidly until the new rater just touches the rear
edge of the previous crater
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Finishing a bead without leaving a crater

• 1. Run-off tab piece of metal tack welded tot
  the end of the base metal being welded. (also
  use for starting a weld)2. reverse electrode
  motion by moving the electrode to the trailing
  edge of the crater.

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Cleaning the arc bead must remove slag prior to
restarting or welding over a bead or painting.
slag inclusions are pieces of slag trapped or
included in the weld use chipping hammer and a
wire brush (shot peening)
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To make a good weld must consider the following
variables

• 1. current (amperage) 2. type and size of
  electrode3. arc and it's manipulation4. type of
  base metal5. preparation of the base metal