Download Shield Metal Arc Welding

Welding, Soldering, Brazing
Shield Metal Arc Welding (SMAWW)
School of Renewable Energy…Maejo
University
Work shop Practice & Renewable Energy Safety
1
Arc Welding Safety
1. Recognize that arc welding produces a lot of heat.
2. Use equipment according to manufacturers
recommendations.
3. Insure fire extinguishers are available
4. Provide a first aid kit
5. Use water filled containers to receive hot metal from
cutting operations.
6. Practice good housekeeping
7. Use appropriate PPE
2
Arc Welding Safety-cont.
7. Insure all wiring is correctly installed and
maintained.
8. Remove or shield all combustible materials in work
area.
9. Do not use gloves or clothing which contain
flammable substances
10. Protect others from arc flash.
11. Protect equipment from hot sparks.
12. Use a fume collector.
13. Never work in damp or wet area.
14. Shutoff power source before making repairs or
adjustments, including changing electrode.
15. Don’t overload the welding cables or use cables with
damaged insulation.
3
Arc Welding PPE
 Helmet




Shade 10 or darker
Face protection
Always wear safety glasses underneath
Auto helmet recommended
 Clothing




Long sleeves
Button up shirt
Work shoes
Protective apron, sleeves, jackets or pants if available. (Fig 26-6)
4
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 special power
source.
• As the weld cools slag forms on top of the weld
puddle.
5
SMAW Power Supplies
 SMAW requires a constant current (CC) of either DC or
AC.
 Some power supplies will supply both DC and AC.
 Power supply capacity determines the maximum diameter
of electrode that can be used.
6
Equipment
Power Supply
Polarity Switch
Power
Cord
Electrode Holder
Power Switch
Electrode
Amperage
Adjustment
Amperage
Scale
Base Metal
(work Piece)
Ground Cable
Ground Clamp
Electrode Cable
7
Open Circuit Voltage (OCV)
 Open circuit voltage is the potential between the welding
electrode and the base metal when the machine is on, but
there is no arc.
 The higher the OCV a machine has, the easier it will be to
strike an arc.
 Only adjustable of dual control machines.
8
Arc Voltage
 Arc voltage is the potential between the electrode and
the base metal when the arc is present.
 Arc voltage is less than OCV.
 Adjustable on dual control machines.
9
Polarity
 The polarity of an object is its physical alignment of
atoms.
 The term is often used to describe the positive and
negative ends of batteries and magnets.
 The negative end has an excess of electrons
 The positive end has a deficiency of electrons.
10
Five (5) Common Power
Supplies
 Transformer
 AC only
 Rectifier
 DC only
 Transformer/rectifier
 AC or DC
 Generator
 DC and/or AC
 Inverter
 AC and DC
11
Striking The Arc






Select the best electrode
Set the welder (Fig 26-8)
Turn on welder
Warn bystanders
Lower helmet
Start arc (two methods)
 Brushing
 Tapping
12
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.
Recommended method for beginning weldors.
13
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.
More difficult method to learn
14
Arc Welding Bead Nomenclature
Electrode
Flux
Slag
Gas
shield
Electrode
metal
Penetration
Base metal
Bead
Molten
puddle
15
Running Beads
 Practice running stringer beads
 No weaving or pattern.
 Remember the electrode burns off as the weld is made.
 Speed used should result in a bead 2-3 times wider than
the diameter of the electrode.
 Cool metal between beads.
 Practice holding a long arc for a couple of seconds after
striking the arc.
 Preheats the weld
 Practice filling in the crater.
16
Five (5) Factors of Arc Welding
1. Heat
2. Electrode
3. Electrode angle
4. Arc length
5. Speed of travel
17
Five (5) Factors
1. Heat
 The arc welder must produce sufficient heat (electric arc) to melt
the electrode and the base metal to the desired depth.
 The amount of heat produced is determined by the amperage.
 Amperage is limited by the diameter of the electrode and the capacity of the
welder.
 The amount of heat needed to complete the weld is determined
by several factors:
 Thickness of the
metal
 Type of joint,
 Electrode type
 Electrode
diameter
 Weld position
 Excessive heat.
 Insufficient heat.
 Electrode easier to
start
 Excessive penetration
(burn through)
 Excessive bead width
 Excessive splatter
 Electrode overheating
 Hard to start
 Reduced
penetration
 Narrow bead
 Coarse ripples
18
Five (5) Factors
2. Electrodes
 The SMAW process uses
a consumable electrode.
 Electrode must be
compatible with base
metal.
 Electrodes are available
for different metals.







Carbon steels
Low alloy steels
Corrosion resisting steels
Cast irons
Aluminum and alloys
Copper and alloys
Nickel and alloys
 Another useful group of
electrodes is
hardsurfacing.
 NEMA color coding
 System of of colors on the
end or dots on the bare
wire indicating the class of
electrode.
 Not very common today.
 AWS numerical coding
 Most popular method.
19
American Welding Society (AWS)
Classification System
 The AWS system
distinguishes the tensile
strength, weld position and,
coating and current.
 Manufactures may and do
use there own numbering
system and produce
electrodes that do not fit in
the AWS system.
20
Welding Currents
 Not all electrodes are designed to work with all currents.
 Common SMAW currents.
 Alternating Current (AC)
 Direct Current straight polarity (DCSP) or (DCEN)
 Direct Current Reverse polarity (DCR P) or (DCEP)
21
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)
 Seven (7) Classifications of Flux constituents
1.
2.
3.
4.
5.
6.
7.
Protection from atmospheric contamination
Fluxing agents
Arc initiators and stabilizers
Deoxidizes
Physical properties of the flux
Fillers and metallic additions
Binders and flux strength improvers
22
Electrode Grouping
 Electrodes are also grouped
according to there performance
characteristics.
 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 fastfill
 Low hydrogen
• Welding
characteristics of
fill-freeze
• Designed for medium
carbon and alloy
steels
23
Selecting Electrode Size
 The optimum electrode
diameter is determined
by the thickness of the
base metal, the welding
position and the capacity
of the welding power
supply.
 A smaller diameter is
usually recommended for
out of position welding.
 When completing root
passes in V-joints, a
smaller diameter maybe
used and then a larger
diameter is used for the
filler passes.
 A diameter of 3/32 or 1/8
inch can be used on metals
up to 1/4 inches thick
without joint preparation.
 ROT: the diameter of the
electrode should not
exceed the thickness of the
metal.
24
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.
 The flux is hydroscopic--attracts moisture (H2O).
 Moisture in the flux also causes excessive gasses to develop in the
weld pool and causes a defect in the weld caused worm holes.
25
Five (5) Factors
3. Electrode Angle
 The electrode angle influences the
placement of the heat.
 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.
T joint = 45o.
Corner = 15o from vertical or 75o from the work.
26
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
 The work angle may need to be
modified for some situations.
 For example, a butt joint with two
different thickness of metal.
27
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.
 Arc length can be adjusted slightly to
change the welding process.
 Excessive length
 Excessive spatter
 Reduced penetration
 Poor quality weld
 Insufficient length
 Electrode sticks
 Narrow weld
 Poor quality weld
28
Five (5) Factors
5. Speed of Travel
 The speed of travel (inches per minute) is an important
factor when arc welding.
 The best speed of travel (welding speed) is determined by
several factors:





The size of the joint,
The type of electrode
The size of the electrode
The amperage setting on the machine
Deposition rate of the electrode (cubic inches per minute)
 The deposition rate of an electrode will change with the
welding amperage.
29
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 =
2
bh =
2
= 0.0625 in
 Step Two: knowing the deposition rate of the electrode,
determine
the welding speed. (Deposition rate = 2.5

in3/min.)
in
2.5 in
=
min
min

3
x
1
0.0625 in
2
= 40
in
min
30
2
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.
31
SMAW Joints
32
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.
33
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.
34
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.
 The amount of joint preparation is dependent on the
diameter of the electrode and the amperage capacity of
the power supply.
 Several different combinations of passes can be used to
complete this joint.
Note: this is the principle use of pattern beads.
35
T-Joints
36
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
37
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.
38
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.
39
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.
40
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.
41
Weld Defects
42
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.
43
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.
44
Questions
45