Transcript Fusion Welding - Agricultural engineering

SMAW Welding
1
Definition
Shield Metal Arc Welding is an arc welding process that
uses a consumable electrode to initiate the arc and provide
the filler material.
2
Arc Welding Safety
What are the principle hazards associated with SMAW?
What can be done to manage the hazards associated with
SMAW?
3
SMAW Process
• The arc temperature over 9,000 oF melts the base metal, the
wire core and the coating on the electrode.
• The high temperature causes some of the ingredients in the
flux to form a gaseous shield.
• The electric energy is provided by a power source.
• As the weld cools slag forms on top of the weld puddle.
What’s the purpose of slag?
4
Equipment
A. Polarity switch
B. Power cord
C. Electrode holder
D. Electrode
E. Base metal
F. Ground clamp
G. Electrode lead
H. Ground lead
I. Amperage scale
J. Amperage adjustment
K. On/Off switch
L. Welder case
5
Six (6) Common SMAW Power Supplies
What welding current is available from each of these power supplies?
Type
Transformer
Transformer with rectifier
Generator
Alternator
Alternator with rectifier
Inverter
Current
Initiating The Arc
List the six (6) steps for initiating the arc.
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Brushing Method
 Hold end of electrode about 1/4 - 1/2
inch above the surface.
 Lower helmet
 Gently brush surface of the metal with
the end of the electrode.
 When arc starts, lift electrode 1/8 inch.
 If electrode sticks, twist it back and forth. If it does not break loose, release
electrode from electrode holder.
 Do not shut off the welder with the electrode stuck to the metal.
Why?
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Tapping Method
•
•
•
Set up welder
Hold the electrode at the travel
angle and 1/4 - 1/2 inch above
the metal.
Quickly lower the electrode
until it touches the metal and
then lift it 1/8 inch.
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Five (5) Factors Controlling the
Quality of SMAW Welds
1. Heat
2. Electrode
3. Electrode angle
4. Arc length
5. Speed of travel
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Five (5) Factors
1. Heat
The arc welder must produce sufficient heat (BTU’s) to melt the
electrode and the base metal to the desired depth.
How is the amount of heat at the weld controlled?
What factors determine the amount of heat required for a weld?
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Five (5) Factors
1. Heat – cont.
What are the characteristics of excessive heat when
completing a SMAW joint?
What are the characteristics of insufficient heat when
completing a SMAW weld?
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Five (5) Factors
2. Electrodes
What are the two (2) primary requirements for an
SMAW electrode?
What metals can be welding with the SMAW process?
How are SMAW electrodes classified?
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American Welding Society (AWS) Classification System
The AWS system designates:
1 tensile strength,
2 weld position
3 coating (flux)
4 current.
The primary difference in the
performance of electrodes is the
flux.
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Arc Welding Electrode Flux
Flux: A material used during arc welding, brazing or braze
welding to clean the surfaces of the joint chemically,
to prevent atmospheric oxidation and to reduce
impurities and/or float them to the surface. (British
Standard 499)
SMAW fluxes are naturally occurring minerals.
The quality, and cost, of the flux is directly related to the amount of
resources the manufacture invests purifying the minerals for the
flux.
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Seven (7) Classifications of Flux constituents
calcium, manganese, calcium
fluoride and cellulose
calcium carbonates, rutile,
silica, talc
nickel and iron powders,
sodium, feldspar, clay, talc
ferroalloys, ferrosilicon, iron
powder
manganese, iron, rutile,
alumina, silica, calcium fluoride
1. Protection from atmospheric
contamination & slag formers
2. Fluxing agents
3. Arc initiators and stabilizers
4. Deoxidizes
5. Physical properties of the flux
6. Fillers and metallic additions
7. Binders and flux strength improvers
waterglass, mica organic binders
ferrow alloys, iron and nickel
powders
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Electrode Performance Groups
Fast-freeze
Mild steel
Quick solidification of weld pool
Deep penetrating
Recommended for out of
position welds
Deep penetrating arc
Fast-fill
Highest deposition rate
Stable arc
Thick flux
Flat position and horizontal
laps only
Fill-freeze
General purpose electrodes
Characteristics of fast-freeze and
fast-fill
Low hydrogen
Welding characteristics of fillfreeze
Designed for medium carbon and
alloy steels
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Selecting Electrode Size
What factors determine the optimum diameter of electrode that
should be used?
Is it permissible to use more than one diameter of electrode to complete a
joint?
Yes
Explain!
ROT: the diameter of the electrode should not exceed the
thickness of the metal.
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Electrode Storage
 Electrodes are damaged by rough treatment, temperature
extremes and moisture.
 The should be kept in their original container until used.
 They should be stored in a heated cabinet that maintains them
at a constant temperature.
 The storage of low hydrogen electrodes is very critical.
•
•
•
Designed to reduce underbead cracking in alloy and medium
carbon steels by reducing the the amount of hydrogen in the weld
pool. What is the primary source of hydrogen in the weld pool?
The flux is hydroscopic-- What does that mean?
Moisture in the flux also causes excessive gasses to develop in
the weld pool and causes a defect in the weld called worm holes.
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Five (5) Factors
3. Electrode Angle
Two angles are important:
Travel
Work
The travel angle is the angle of the
electrode parallel to the joint.
 The correct travel angle must be used for each joint.





Beads = 15o from vertical or 75o from the work.
Butt joint = 15o from vertical or 75o from the work.
Lap joint = 45o.
What is the effect of incorrect travel angle?
o
T joint = 45 .
Corner = 15o from vertical or 75o from the work.
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Five (5) Factors
Electrode Angle-cont.
 The work angle is the angle of the
electrode perpendicular to the
joint.
 The appropriate angle must be used for
each joint.





Beads = 90o
Butt joint = 90o
Lap joint = 45o
T joint = 45o
Corner = 90o
 Can you think of a situation where
the travel angle should be modified?
 When completing a joint with metal of
different thickness.
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Five (5) Factors
4. Arc Length
The arc length is the distance from the metal part of the electrode
to the weld puddle.
The best arc length is not a fixed distance, but should be
approximately equal to the diameter of the electrode.
What are the characteristics of a weld
completed with excessive arc length?
What are the characteristics of a weld
completed with insufficient arc length?
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Five (5) Factors
5. Speed of Travel
The speed of travel is measured in inches per minute.
What factors determine the best speed of travel?
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Five (5) Factors
5. Speed-cont.
The ideal speed can be calculated
using the volume of the joint and the
deposition rate of the electrode.
Step one: determine the area of the
weld. (Assuming 1/16 inch
penetration.)
1
0.25 in x 0.25 in
Area = bh =
= 0.0625 in 2
2
2
 Step Two: knowing the deposition rate of the electrode,
determine the welding speed. (Deposition rate = 2.5 in3/min.)

in
2.5 in 3
1
=
x
= 40
min
min
0.0625 in 2
in
min
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Five (5) Factors
5. Speed-cont.
The correct welding speed is indicated by the shape of the ripples.
Too slow = excessive width,
excessive penetration
Too fast = narrower width,
elongated ripple pattern,
shallow penetration.
Recommended = width 2-3
times diameter of electrode,
uniform ripple pattern, full
penetration.
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SMAW Joints
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Square Groove
A butt joint can be completed with a groove welded on metal up to
1/8 inch thick with a single pass on one side, with no root opening.
Electrode manipulation should only be used to prevent burning
through.
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Square Groove Thicker Metal
A groove weld on metal up to 1/4 inch thick can be welded with a
single pass on one side but, if possible, it should be completed
with a single pass on both sides.
Metal this thick requires a root opening to achieve adequate
penetration.
Electrode manipulation will reduce penetration.
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Single V Groove Weld
Butt joints on metal greater than 1/4 inch thick require joint
preparation.
Note that the groove does not extend all the way. A short
distance, called the root face, is left undisturbed.
Several different combinations of passes can be used to complete
this joint.
What determines that amount of joint preparation that must be done
before welding?
Note: this is the principle use of pattern beads.
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T-Joints
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Information



In a T-joint the two welding surfaces are at an angle close to 90
degrees from each other.
The welding side and number of passes uses depends on the
thickness of the metal, the welding access and capacity of the
power supply.
Common joints include.
 Plane T
 T with joint gap
 Single preparation
 Double preparation
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Plane T-Joint
The plane T joint is very useful for thin metal.
Can be completed at angles other than 90 degrees.
Can be completed with metal of different thickness.
The work angle must be changed to direct more
heat to the thicker piece.
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T-joint--Thicker Metal
• When the metal thickness exceeds 1/8 inch the
recommendation is to gap the joint.
 Improves penetration
 May not be necessary if larger diameter electrode is used and sufficient
amperage is available.
•
The need for a joint gap varies with the type of electrode, but should
not exceed 1/8 inch.
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T-joint Single Single Bevel
As with other joints,
thicker metal must
have joint preparation
to achieve full
penetration with
smaller diameter
electrodes.
 Several different preparations can be used. A popular one is the
bevel.
 A bevel can be completed by grinding or cutting.
 The bevel joint can be completed with electrode manipulation or no
electrode manipulation.
 When when electrode manipulation is used to fill the joint, the first
pass should be a straight bead with no manipulation.
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T-joint Double Bevel
•
•
•
The double bevel T-joint is recommended for metal 1/2 inch
thick and thicker.
The root passes should be with not manipulation, but the filler
passes can be completed with either straight beads or patterns
beads.
Alternating sides reduces distortion.
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Weld Defects
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Common SMAW Defects
Under Cutting
Porosity
Hot Cracks
Slag Inclusions
 Hot cracks
 Caused by excessive contraction
of the metal as it cools.
 Excessive bead size
 May also be found at the root of
the weld.
 Slag inclusions
 Long arc
 Incomplete removal of slag on
multipass welds.
Undercutting
improper welding
parameters; particularly
the travel speed and arc
voltage.
Porosity
Atmospheric contamination
or excess gas in the weld
pool.
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SMAW Weld Defects-cont.
Incomplete fusion
Microcracks
Toe cracks
Underbead cracks
 Toe Cracks
 Excessive heat and rapid cooling.
 Underbead cracks
 Excessive hydrogen in weld pool
 Microcracks
 Caused by stresses as weld cools.
 Incomplete fusion
 Incorrect welding parameters or welding techniques.
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Questions
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