Transcript SHIELDED METAL ARC WELDING (SMAW)

SHIELDED METAL ARC WELDING
(SMAW)
•Shielded metal arc welding (SMAW),
•Also known as Manual Metal Arc (MMA) welding
• Informally as stick welding
is a manual arc welding process that uses a
consumable electrode coated in flux to lay the weld.
•An electric current, in the form of either alternating
current or direct current from a welding power supply, is
used to form an electric arc between the electrode and
the metals to be joined.
• As the weld is laid, the flux coating of
the electrode disintegrates, giving off
vapors that serve as a shielding gas
and providing a layer of slag, both of
which protect the weld area from
atmospheric contamination.
• Because of the versatility of the
process and the simplicity of its
equipment and operation, shielded
metal arc welding is one of the world's
most popular welding processes.
• It dominates other welding processes in the
maintenance and repair industry, used
extensively in the construction of steel
structures and in industrial fabrication.
• The process is used primarily to weld iron
and steels (including stainless steel) but
aluminum, nickel and copper alloys can also
be welded with this method.
• Flux-Cored Arc Welding (FCAW) , a
modification to SMAW is growing in
popularity
SAFETY PRECAUTIONS
• Uses
an open electric arc, so
risk of burns – to be prevented
by protective clothing in the
form of heavy leather gloves
and long sleeve jackets.
•The brightness of the weld area
can lead arc eye, in which
ultraviolet light causes the
inflammation of the cornea and
can burn the retinas of the eyes.
•Welding helmets with dark face
plates to be worn to prevent this
exposure
• New helmet models have been produced that
feature a face plate that self-darkens upon
exposure to high amounts of UV light
• To protect bystanders, especially in
industrial environments, transparent welding
curtains often surround the welding area.
• These are made of a polyvinyl chloride
plastic film, shield nearby workers from
exposure to the UV light from the electric arc,
but should not be used to replace the filter
glass used in helmets.
ARC EYE
Arc eye, also known as arc flash or welder's flash or
corneal flash burns, is a painful condition sometimes
experienced by welders who have failed to use adequate
eye protection.
It can also occur due to light from sunbeds, light
reflected from snow (known as snow blindness), water
or sand. The intense ultraviolet light emitted by the arc
causes a superficial and painful keratitis.
Symptoms tend to occur a number of hours
after exposure and typically resolve
spontaneously within 36 hours.
It has been described as having sand poured
into the eyes.
Signs
Intense lacrimation
Blepharospasm
Photophobia
Fluorescein dye staining will reveal corneal ulcers
under blue light
Management
• Instill topical anaesthesia
• Inspect the cornea for any foreign body
• Patch the worse of the two eyes and prescribe analgesia
• Topical antibiotics in the form of eye drops or eye
ointment or both should be prescribed for prophylaxis
against infection
EQUIPMENT
Various welding electrodes and an electrode holder
SUBMERGED ARC WELDING (SAW)
CONTROL PANEL
Submerged Arc Welding (SAW)
• Is a common arc welding process.
• A continuously fed consumable solid or tubular
(metal cored) electrode used.
• The molten weld and the arc zone are protected
from atmospheric contamination by being
“submerged” under a blanket of granular fusible
flux.
• When molten, the flux becomes conductive, and
provides a current path between the electrode
and the work
• Normally operated in the automatic or
mechanized mode.
• Semi-automatic (hand-held) SAW guns with
pressurized or gravity flux feed delivery are
available.
• The process is normally limited to the 1F, 1G, or
the 2F positions (although 2G position welds
have been done with a special arrangement to
support the flux). Deposition rates approaching
45 kg/h have been reported — this compares to
~5 kg/h (max) for shielded metal arc welding.
• Currents ranging from 200 to 1500 A are
commonly used; currents of up to 5000 A have
been used (multiple arcs).
• Single or multiple (2 to 5) electrode wire
variations of the process exist
• SAW strip-cladding utilizes a flat strip
electrode (e.g. 60 mm wide x 0.5 mm
thick).
• DC or AC power can be utilized, and
combinations of DC and AC are common
on multiple electrode systems.
• Constant Voltage welding power supplies
are most commonly used, however
Constant Current systems in combination
with a voltage sensing wire-feeder are
available.
Material applications
• Carbon steels (structural and vessel
construction);
• Low alloy steels;
• Stainless Steels;
• Nickel-based alloys;
• Surfacing applications (wearfacing, buildup, and corrosion resistant overlay of
steels).
Advantages of SAW
• High deposition rates (over45 kg/h) have been
reported;
• High operating factors in mechanized
applications;
• Deep weld penetration;
• Sound welds are readily made (with good
process design and control);
• High speed welding of thin sheet steels at over
2.5 m/min is possible;
• Minimal welding fume or arc light is emitted.
Limitations of SAW
• Limited to ferrous (steel or stainless steels) and
some nickel based alloys;
• Normally limited to the 1F, 1G, and 2F positions;
• Normally limited to long straight seams or
rotated pipes or vessels;
• Requires relatively troublesome flux handling
systems;
• Flux and slag residue can present a health &
safety issue;
• Requires inter-pass and post weld slag removal.
Key SAW process variables
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Wire Feed Speed (main factor in welding current control);
Arc Voltage;
Travel Speed;
Electrical Stick-Out (ESO) or Contact Tip to Work (CTTW);
Polarity and Current Type (AC or DC).
Other factors
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Flux depth/width;
Flux and electrode classification and type;
Electrode wire diameter;
Multiple electrode configurations.
GAS TUNGSTEN ARC WELDING (GTAW)
GTAW
GTAW
• Fusion Welding Process
• Arc Between Non-Consumable Tungsten
Rod And Work
• Arc & Weld Pool Shielded By Argon/Gas
• Filler Wire Separately Added To Weld Pool
• Welding Torch & Tungsten Rod Cooled by
Flow OF Argon / Cooling Water
GTAW Equipment & Accessories
• Power Source – Inverter, Thyrister, Rectifier,
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Generator
High Frequency Unit
Water Cooling System
Welding Torch- (Ceramic Cup, Tungsten Rod, Collet,
Gas-lens)
Pedal Switch
Argon Gas Cylinder
Pressure Gauge, Regulator, Flow Meter
Earthing Cable With Clamp
Equipment & Accessories
Pressure Regulator
Flow Meter
Tungsten Rod
Argon Gas In
Cooling Water In
Solenoid
Valve
Argon Cylinder
Gas Lens
Ceramic Cup
Welding Cable & Cooling
Water In Tube
Cooling Water Out
Argon Shielding
Arc
+
HF Unit &
Water Cooling
System
High Frequency
Connection
Work
Pedal Switch
Power Source
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Equipment
GTAW torch with various
electrodes, cups, collets and gas
diffusers
GTAW torch, disassembled
Gas tungsten arc welding (GTAW),
commonly known as Tungsten Inert Gas
(TIG) welding
• Is an arc welding process that uses a
nonconsumable tungsten electrode to produce
the weld.
• The weld area is protected from atmospheric
contamination by a shielding gas (usually an
inert gas such as argon), and a filler metal is
normally used, though some welds, known as
autogenous welds, do not require it.
• A constant current welding power supply
produces energy which is conducted across the
arc through a column of highly ionized gas and
metal vapors known as a plasma.
• Most commonly used to weld thin sections
of stainless steel and light metals such as
aluminum, magnesium, and copper alloys.
• The process grants the operator greater
control over the weld than competing
procedures such as shielded metal arc
welding and gas metal arc welding, allowing
for stronger, higher quality welds.
• GTAW is comparatively more complex and
difficult to master, and furthermore, it is
significantly slower than most other welding
techniques.
• A related process, plasma arc welding, uses
a slightly different welding torch to create a
more focused welding arc and as a result is
often automated.
GTAW system setup
Applications
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Aerospace industry is one of the primary users of gas
tungsten arc welding, the process is used in a number of other
areas.
Many industries use GTAW for welding thin workpieces,
especially nonferrous metals.
It is used extensively in the manufacture of space vehicles, and
is also frequently employed to weld small-diameter, thin-wall
tubing.
Is often used to make root or first pass welds for piping of
various sizes.
In maintenance and repair work, the process is commonly used
to repair tools and dies, especially components made of
aluminum and magnesium.
Because the welds it produces are highly resistant to corrosion
and cracking over long time periods, GTAW is the welding
procedure of choice for critical welding operations like sealing
spent nuclear fuel canisters before burial.
GTAW ranks the highest in terms of the
quality of weld produced.
Operation must be with free from oil,
moisture, dirt and other impurities, as
these cause weld porosity and
consequently a decrease in weld
strength and quality.
To remove oil & grease, alcohol or
similar commercial solvents used, while
a stainless steel wire brush or chemical
process remove oxides from the
surfaces of metals like aluminum.
Rust on steels removed by first grit
blasting the surface and then using a
wire brush to remove imbedded grit.
These steps important when DCEN
used, because this provides no cleaning
during the welding process, unlike
DCEPor AC.
To maintain a clean weld pool during welding, the shielding gas flow should be
sufficient and consistent so that the gas covers the weld and blocks impurities in
the atmosphere. GTA welding in windy or drafty environments increases the
amount of shielding gas necessary to protect the weld, increasing the cost and
making the process unpopular outdoors.
Quality
• Because of GTAW's relative difficulty and the
importance of proper technique, skilled
operators are employed for important
applications.
• Low heat input, caused by low welding
current or high welding speed, can limit
penetration and cause the weld bead to lift
away from the surface being welded.
• If there is too much heat input, the weld
bead grows in width while the likelihood of
excessive penetration and spatter increase.
• If the welder holds the welding torch too far
from the workpiece, shielding gas is wasted
and the appearance of the weld worsens.
• If the amount of current used exceeds the
capability of the electrode, tungsten
inclusions in the weld may result. Known as
tungsten spitting, it can be identified with
radiography and prevented by changing the
type of electrode or increasing the electrode
diameter.
• If the electrode is not well protected by the
gas shield or the operator accidentally allows
it to contact the molten metal, it can become
dirty or contaminated. This often causes the
welding arc to become unstable, requiring
that electrode be ground with a diamond
abrasive to remove the impurity.
• GTAW welding torches designed for either automatic
or manual operation and are equipped with cooling
systems using air or water. The automatic and
manual torches are similar in construction, but the
manual torch has a handle while the automatic torch
normally comes with a mounting rack.
• The angle between the centerline of the handle and
the centerline of the tungsten electrode, known as
the head angle, can be varied on some manual
torches according to the preference of the operator.
• Air cooling systems are most often used for lowcurrent operations (up to about 200 A), while water
cooling is required for high-current welding (up to
about 600 A).
• The torches are connected with cables to the power
supply and with hoses to the shielding gas source
and where used, the water supply.
• The internal metal parts of a
torch are made of hard alloys
of copper or brass in order to
transmit current and heat
effectively.
• The tungsten electrode must
be held firmly in the center of
the torch with an
appropriately sized collet,
and ports around the
electrode provide a constant
flow of shielding gas.
• The body of the torch is
made of heat-resistant,
insulating plastics covering
the metal components,
providing insulation from
heat and electricity to protect
the welder.
GTAW TORCH
Torch Handle
Cap with collet For
Holding Tungsten
Cooling Water Outlet
Argon Gas Inlet
Cooling Water Inlet Tube with cable
Ceramic Cup
Argon Shielding Gas
Tungsten Rod
Base Metal
Earthing Cable
Arc
• The size of the welding torch nozzle depends
on the size of the desired welding arc, and
the inside diameter of the nozzle is normally
at least three times the diameter of the
electrode.
• The nozzle must be heat resistant and thus is
normally made of alumina or a ceramic
material, but fused quartz, a glass-like
substance, offers greater visibility.
• Devices can be inserted into the nozzle for
special applications, such as gas lenses or
valves to control shielding gas flow and
switches to control welding current.
Power supply
• GTAW uses a constant
current power source,
meaning that the current (and
thus the heat) remains
relatively constant, even if
the arc distance and voltage
change.
• This is important because
most applications of GTAW
are manual or semiautomatic,
requiring that an operator
hold the torch.
• Maintaining a suitably steady
arc distance is difficult if a
constant voltage power
source is used instead, since
it can cause dramatic heat
variations and make welding
more difficult.
• The preferred polarity of the GTAW system depends largely on
the type of metal being welded.
• DCEN is often employed when welding steels, nickel, titanium,
and other metals. It can also be used in automatic GTA welding
of aluminum or magnesium when helium is used as a shielding
gas. The negatively charged electrode generates heat by
emitting electrons which travel across the arc, causing thermal
ionization of the shielding gas and increasing the temperature
of the base material. The ionized shielding gas flows toward the
electrode, not the base material, and this can allow oxides to
build on the surface of the weld.
• DCEP is less common, and is used primarily for shallow welds
since less heat is generated in the base material. Instead of
flowing from the electrode to the base material, as in DCEN,
electrons go the other direction, causing the electrode to reach
very high temperatures. To help it maintain its shape and
prevent softening, a larger electrode is often used. As the
electrons flow toward the electrode, ionized shielding gas flows
back toward the base material, cleaning the weld by removing
oxides and other impurities and thereby improving its quality
and appearance.
• AC commonly used when welding aluminum and
magnesium manually or semi-automatically, combines
the two direct currents by making the electrode and
base material alternate between positive and negative
charge. This causes the electron flow to switch
directions constantly, preventing the tungsten electrode
from overheating while maintaining the heat in the base
material. This makes the ionized shielding gas
constantly switch its direction of flow, causing
impurities to be removed during a portion of the cycle.
• Some power supplies enable operators to use an unbalanced
alternating current wave by modifying the exact percentage of time
that the current spends in each state of polarity, giving them more
control over the amount of heat and cleaning action supplied by
the power source.
• In addition, operators must be wary of rectification, in
which the arc fails to reignite as it passes from straight
polarity (negative electrode) to reverse polarity (positive
electrode).
• To remedy the problem, a square wave power supply
can be used, as can high frequency voltage to
encourage ignition.
Tungsten Rod
Tungsten Rod
• Non Consumable Electrode.
• Maintains Stable Arc
• Tip to be Ground to a cone Shape of 60º to 30º
angle
• Thoriated Tungsten for General Application,
Zerconiated Tungsten for Aluminium Welding
• Sizes :- 2, 2.4 & 3 mm Ø
Ground to
50º ankle
•The electrode used in GTAW is
made of tungsten or a tungsten alloy,
because tungsten has the highest
melting temperature among metals,
at 3422 °C.
• The electrode is not consumed
during welding, though some erosion
(called burn-off) can occur.
•Electrodes can have either a clean
finish or a ground finish—clean finish
electrodes have been chemically
cleaned, while ground finish
electrodes have been ground to a
uniform size and have a polished
surface, making them optimal for
heat conduction.
•The diameter of the electrode can
vary between 0.5 mm and 6.4 mm,
and their length can range from 75 to
610 mm .
ISO
Class
ISO Color
AWS Class
AWS
Color
Alloy [18]
WP
Green
EWP
Green
None
WC20
Gray
EWCe-2
Orange
~2% CeO2
WL10
Black
EWLa-1
Black
~1% LaO2
WL15
Gold
EWLa-1.5
Gold
~1.5% LaO2
WL20
Sky-blue
EWLa-2
Blue
~2% LaO2
WT10
Yellow
EWTh-1
Yellow
~1% ThO2
WT20
Red
EWTh-2
Red
~2% ThO2
WT30
Violet
~3% ThO2
WT40
Orange
~4% ThO2
WY20
Blue
~2% Y2O3
WZ3
Brown
WZ8
White
EWZr-1
Brown
~0.3% ZrO2
~0.8% ZrO2
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A number of tungsten alloys have been standardized by the International
Organization for Standardization and the American Welding Society in ISO
6848 and AWS A5.12, respectively, for use in GTAW electrodes- refer table
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Pure tungsten electrodes (classified as WP or EWP) are general purpose
and low cost electrodes. Cerium oxide (or ceria) as an alloying element
improves arc stability and ease of starting while decreasing burn-off. Using
an alloy of lanthanum oxide (or lanthana) has a similar effect. Thorium oxide
(or thoria) alloy electrodes were designed for DC applications and can
withstand somewhat higher temperatures while providing many of the
benefits of other alloys.
However, it is somewhat radioactive, and as a replacement, electrodes with
larger concentrations of lanthanum oxide can be used. Electrodes
containing zirconium oxide (or zirconia) increase the current capacity while
improving arc stability and starting and increasing electrode life.
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Electrode manufacturers may create alternative tungsten alloys with
specified metal additions, and these are designated with the classification
EWG under the AWS system.
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Filler metals are also used in nearly all applications of GTAW, the major
exception being the welding of thin materials. Filler metals are available with
different diameters and are made of a variety of materials. In most cases,
the filler metal in the form of a rod is added to the weld pool manually, but
some applications call for an automatically fed filler metal, which is fed from
rolls.
shielding gases
• Necessary in GTAW to protect the welding area from atmospheric
gases such as nitrogen and oxygen, which can cause fusion
defects, porosity, and weld metal embrittlement if they come in
contact with the electrode, the arc, or the welding metal. The gas
also transfers heat from the tungsten electrode to the metal, and it
helps start and maintain a stable arc.
• The selection of a shielding gas depends on several factors,
including the type of material being welded, joint design, and desired
final weld appearance.
• Argon is the most commonly used shielding gas for GTAW,
since it helps prevent defects due to a varying arc length. When
used with alternating current, the use of argon results in high
weld quality and good appearance.
• Another common shielding gas, helium, is most often used to
increase the weld penetration in a joint, to increase the welding
speed, and to weld conductive metals like copper and
aluminum.
• A significant disadvantage is the difficulty of striking an arc
with helium gas, and the decreased weld quality associated
with a varying arc length.
Shielding Gas
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Inert Gas - Argon , Helium
Common Shielding Gas – Argon
When Helium Is Used – Called Heli – Arc Welding
When Argon Is Used – Called Argon Arc Welding
Inert Gas Prevents Contamination Of Molten Metal
It Prevents Oxidation Of Tungsten Rod
It Ionizes Air Gap and Stabilizes Arc
It Cools Welding Torch & Tungsten Rod
Shielding Gas
• Argon - Purity 99.95%
• Impure Argon Results In Porosities
• Purity Verified by Fusing BQ CS plate
• Leakage of Argon in Torch Results in
Porosity.
• Check Leakage by Closing the Ceramic Cup
With Thump
Argon Gas Cylinder
• Light Blue In Colour
• Full Cylinder Pressure: 1800 psi ( 130 Kgs / Cm2 )
• Volume Of Argon In Full Cylinder: 7.3 M3
• Commercial Argon (99.99%) Cost: Rs 70/- Per M3
• High Purity Argon (99.999) Cost: Rs 87/- Per M3
Back Purging
Purging Gas Commercial Argon or• Applicable to Single
Nitrogen
Sided full penetration
• Prevents oxidation of
Filler Wire
Welding Torch
root pass from opposite
side of weld
• Essential for high alloy
steels, nonferrous
Purging
Purging Gas In
Gas Out
metals and alloys
Root Pass
Purging
• Desirable For All
chamber
Material
• Argon-helium mixtures are also frequently utilized in
GTAW, since they can increase control of the heat input
while maintaining the benefits of using argon. Normally,
the mixtures are made with primarily helium (often about
75% or higher) and a balance of argon. These mixtures
increase the speed and quality of the AC welding of
aluminum, and also make it easier to strike an arc.
• Argon-hydrogen, is used in the mechanized welding of
light gauge stainless steel, but because hydrogen can
cause porosity, its uses are limited.
• Nitrogen can sometimes be added to argon to help
stabilize the austenite in austentitic stainless steels and
increase penetration when welding copper. Due to
porosity problems in ferritic steels and limited benefits,
however, it is not a popular shielding gas additive.
Materials
• Most commonly used to weld stainless steel
and nonferrous materials, such as aluminum
and magnesium, but it can be applied to
nearly all metals, with notable exceptions
being lead and zinc.
• Its applications involving carbon steels are
limited not because of process restrictions,
but because of the existence of more
economical steel welding techniques, such
as gas metal arc welding and shielded metal
arc welding.
• GTAW can be performed in a variety of otherthan-flat positions, depending on the skill of
the welder and the materials being welded.
A TIG weld showing an
accentuated AC etched zone
Closeup view of an
aluminium TIG weld AC etch zone
• Aluminum and magnesium are most often welded using
alternating current, but the use of direct current is also
possible, depending on the properties desired. Before
welding, the work area should be cleaned and may be
preheated to 175-200 °C for aluminum or to a maximum
of 150 °C for thick magnesium workpieces to improve
penetration and increase travel speed.
• AC current can provide a self-cleaning effect, removing
the thin, refractory aluminium oxide (sapphire) layer that
forms on aluminium metal within minutes of exposure to
air. This oxide layer must be removed for welding to
occur. When alternating current is used, pure tungsten
electrodes or zirconiated tungsten electrodes are
preferred over thoriated electrodes, as the latter are
more likely to "spit" electrode particles across the
welding arc into the weld.
• Blunt electrode tips are preferred, and pure argon
shielding gas should be employed for thin workpieces.
Introducing helium allows for greater penetration in
thicker workpieces, but can make arc starting difficult.
• Direct current of either polarity, positive or negative,
can be used to weld aluminum and magnesium as
well.
• DCEN allows for high penetration, and is most
commonly used on joints with butting surfaces, such
as square groove joints. Short arc length (generally
less than 2 mm or 0.07 in) gives the best results,
making the process better suited for automatic
operation than manual operation. Shielding gases
with high helium contents are most commonly used
with DCEN, and thoriated electrodes are suitable.
• DCEP is used primarily for shallow welds, especially
those with a joint thickness of less than 1.6 mm.
While still important, cleaning is less essential for
DCEP than DCEN, since the electron flow from the
workpiece to the electrode helps maintain a clean
weld. A large, thoriated tungsten electrode is
commonly used, along with a pure argon shielding
gas.
Steels
• For GTA welding of carbon and stainless steels, the
selection of a filler material is important to prevent
excessive porosity. Oxides on the filler material and
workpieces must be removed before welding to prevent
contamination, and immediately prior to welding, alcohol
or acetone should be used to clean the surface.
• Preheating is generally not necessary for mild steels less
than one inch thick, but low alloy steels may require
preheating to slow the cooling process and prevent the
formation of martensite in the heat-affected zone.
• Tool steels should also be preheated to prevent cracking
in the heat-affected zone. Austenitic stainless steels do
not require preheating, but martensitic and ferritic
chromium stainless steels do. A DCEN power source is
normally used, and thoriated electrodes, tapered to a
sharp point, are recommended. Pure argon is used for
thin workpieces, but helium can be introduced as
thickness increases.
Dissimilar metals
• Welding dissimilar metals often introduces new difficulties to
GTA welding, because most materials do not easily fuse to form
a strong bond. Welds of dissimilar materials have numerous
applications in manufacturing, repair work, and the prevention
of corrosion and oxidation. In some joints, a compatible filler
metal is chosen to help form the bond, and this filler metal can
be the same as one of the base materials (eg:, using a stainless
steel filler metal stainless steel and carbon steel as base
materials), or a different metal (such as the use of a nickel filler
metal for joining steel and cast iron). Very different materials
may be coated or "buttered" with a material compatible with a
particular filler metal, and then welded. In addition, GTAW can
be used in cladding or overlaying dissimilar materials.
• When welding dissimilar metals, the joint must have an
accurate fit, with proper gap dimensions and bevel angles. Care
should be taken to avoid melting excessive base material.
Pulsed current is particularly useful for these applications, as it
helps limit the heat input. The filler metal should be added
quickly, and a large weld pool should be avoided to prevent
dilution of the base materials.
Process variations
Pulsed-current
• In the pulsed-current mode, the welding current rapidly
alternates between two levels.
• The higher current state is known as the pulse current,
while the lower current level is called the background
current.
• During the period of pulse current, the weld area is
heated and fusion occurs. Upon dropping to the
background current, the weld area is allowed to cool and
solidify.
• Pulsed-current GTAW has a number of advantages,
including lower heat input and consequently a reduction
in distortion and warpage in thin workpieces. In addition,
it allows for greater control of the weld pool, and can
increase weld penetration, welding speed, and quality. A
similar method, manual programmed GTAW, allows the
operator to program a specific rate and magnitude of
current variations, making it useful for specialized
applications.
Dabber
• The Dabber variation is used to precisely place
weld metal on thin edges. The automatic
process replicates the motions of manual
welding by feeding a cold filler wire into the weld
area and dabbing (or oscillating) it into the
welding arc. It can be used in conjunction with
pulsed current, and is used to weld a variety of
alloys, including titanium, nickel, and tool steels.
Common applications include rebuilding seals in
jet engines and building up saw blades, milling
cutters, drill bits, and mower blades
Heat-affected zone
The cross-section of a welded butt joint, with the
darkest gray representing the weld or fusion zone,
the medium gray the heat affected zone, and
the lightest gray the base material.
• The heat-affected zone (HAZ) is the area of base
material, either a metal or a thermoplastic, which has
had its microstructure and properties altered by welding.
The heat from the welding process and subsequent recooling causes this change in the area surrounding the
weld. The extent and magnitude of property change
depends primarily on the base material, the weld filler
metal, and the amount and concentration of heat input
by the welding process.
• The thermal diffusivity of the base material plays a large
role – if the diffusivity is high, the material cooling rate is
high and the HAZ is relatively small. Alternatively, a low
diffusivity leads to slower cooling and a larger HAZ. The
amount of heat inputted by the welding process plays an
important role as well, as processes like oxyfuel welding
use high heat input and increase the size of the HAZ.
Processes like laser beam welding give a highly
concentrated, limited amount of heat, resulting in a small
HAZ. Arc welding falls between these two extremes, with
the individual processes varying somewhat in heat input
• To calculate the heat input for arc welding
procedures, the formula used is:
where Q = heat input (kJ/mm), V = voltage (V), I =
current (A), and S = welding speed (mm/min). The
efficiency is dependent on the welding process used,
with shielded metal arc welding having a value of
0.75, gas metal arc welding and submerged arc
welding, 0.9, and gas tungsten arc welding, 0.8.
Types Of GTAW Power Source
• Inverter- DC
• Thyrister – DC
• Motor Generator – DC
• Rectifier – DC
• Transformer – AC (For Aluminium Welding Only)
Power Source
• Provides Electric Energy – Arc – Heat
• Drooping Characteristic
• OCV – Appx. 90V,
• Current Range 40 A to 300 A ( Capacity Of M/s)
• Arc Voltage 18V to 26V
Characteristic Of GTAW
Power Source
Drooping – Constant Current
V
V1
Vertical
Curve
V2
A
A1 A2
High Frequency Unit
• Provides High Voltage Electric Energy With Very
high Frequency – 10000 Cycles / Sec.
• Initiates low energy Arc / Spark & Ionize Air Gap.
• Electrically charges Air Gap For welding Current to
Jump Across the Tungsten Tip & BM to Form An
Arc.
• HF Gets Cut Off, Once Welding Arc Struck.
Water Cooling System
• Provides Cooling Water To Welding Torch.
• Cools Tungsten Rod, Torch handle & Welding
Cable.
• Cooling Water Returns through Flexible Tube Which
Carries welding cable within.
Pedal Switch
When Pedal Pressed
• Solenoid valve opens, Argon gas flows
• High Frequency current jumps from
tungsten rod generating sparks
• Welding current flows generating an
Switches system
arc across tungsten rod and work.
on And off in sequence
• High frequency gets cut off from the
system & welding continues.
When Pedal Released
1 Current gets cut off, Arc extinguishes
2 Gas flow remains for few more
seconds before it stops.
Argon Gas Cylinder- Pressure Regulator +
Flow Meter
Cylinder Valve
Pressure gauges
Flow Meter
Flow Regulator
• Cylinder Stores Argon At
High Pressure
• Regulator Regulates
Cylinder Pressure to
Working Pressure
Pressure Regulator
Connection To Torch
Argon Cylinder
• Flow Meter Controls Flow
Rate
Tools For GTAW
• Head Screen
• Hand gloves
• Chipping Hammer
• Wire Brush
• Spanner Set
Filler Wire
• Added Separately to the weld pool.
• Compatible to base metal
• Used in cut length for manual welding.
• Used from layer wound spool for automatic
welding.
• Sizes :- 0.8, 1, 1.2, 1.6, 2, 2.4 & 3 mm
ASME Classification Of Filler Wire
SS Filler Wire:
SFA-5.9, ER 308, 308L, 316, 316L, 347, 309
LAS Filler Wire:
SFA 5.28, ER 70S A1, ER 80S B2, ER90S D2,
ER 80S Ni2
CS Filler Wire:
SFA- 5.18 , ER 70S2
C = 0.07%, Mn = 0.9% – 1.4%, Si = 0.4 – 0.7%, P = 0.025%, S = 0.035%
Dos & Don'ts In GTAW
Dos
• Always Connect
Electrode – Ve
• Keep Always Flow
Meter Vertical
• Check & Confirm
Argon Purity
• Clean Groove & Filler
wire With Acetone
• Grind Tungsten Tip to
Point
Don’ts
• Don’t Strike Arc With
Electrode + Ve
• Don’t strike Arc Without
Argon Flow
• Don’t Strike Arc By
touching Tungsten Rod
• Don’t Touch Weld Pool
With Tungsten Rod
• Don’t Lift and break Arc
Dos & Don'ts In GTAW
Dos
• Break The Arc Only By
Pedal Switch
• Lift The Torch only After
5 Sec Of Arc Break.
• Ensure Pre Purging &
Post Purging of 5Sec
• Ensure Argon Flow &
Water Circulation To
Torch
Don’ts
• When Arc is Stopped Don’t
Lift Torch immediately.
• Don’t Weld With Blend
Tungsten Rod
• Don’t Weld With Argon
Leaking Torch
• Don’t Weld Without Water
Circulation
Dos & Don'ts In GTAW
Dos
• Provide Back Purging For
Single Sided Full
Penetration Welds
• Use N2 or Argon as Back
Purging Gas For CS &
LAS
• Use Argon As Back
Purging Gas For SS &
Non Ferrous Alloys
Don’ts
• Don’t Weld Single Sided
Full Penetration Welds
Without Back Purging
• Don’t Use N2 As Back
Purging Gas For Non
Ferrous Alloys
• Don’t Empty Ag Cylinders
Fully.
Defects In GTAW
1. Cracks
2. Lack Of Fusion
3. Porosity
4. Undercut
5.Lack Of Penetration
6. Excess Penetration
7.Overlap
8. Suck Back
9. Under Flush
10. Burn Through
11. Tungsten Inclusion 11.Stray Arcing
Crack
1)
2)
3)
4)
Cause
Wrong Consumable
Wrong Procedure
Improper Preheat
Inadequate Thickness
In Root Pass
crack
1)
2)
3)
4)
Remedy
Use Right Filler Wire
Qualify Procedure
Preheat Uniformly
Add More Filler Wire
in root Pass
Lack Of Fusion
Cause
Remedy
1) Inadequate Current
1) Use Right Current
2) Wrong Torch angle
2) Train /Qualify welder
3) Improper bead placement 3) Train/Qualify Welder
Lack Of Fusion
Porosity
Cause
1) Impure Argon Gas
2) Argon Leak Within Torch
3) Defective Filler Wire
4) Wet surface of BM
5) Rusted / Pitted Filler wire
6) Improper Flow Of Argon
Porosity
Remedy
1) Replace Argon Cylinder
2) Replace Leaking Torch
3) Replace Filler Wire
4) Clean & Warm BM
5) Clean Filler Wire
6) Provide Gas lens
. .
Undercut
Cause
1) Excess Current
2) Excess Voltage
3) Improper Torch angle
Under cut
Remedy
1) Reduce the Current
2) Reduce Arc length
3) Train & Qualify the Welder
Lack Of Penetration*
Cause
1) Excess Root Face
2) Inadequate Root opening
3) Over size Filler Wire
4) Wrong Direction of Arc
5) Improper bead placement
6) Improper weaving technique
* Applicable to SSFPW
LOP
Remedy
1) Reduce Root Face
2) Increase Root Opening
3) Reduce Filler Wire size
4) Train / Qualify Welder
5) Train / Qualify Welder
6) Train & Qualify Welder
Excess Penetration*
Cause
1)Excess root opening
2) Excess Current
3) Inadequate root face
4) Excess Weaving
5) Wrong Direction Of Arc
* Applicable to SSFPW
Excess Penetration
1)
2)
3)
4)
5)
Remedy
Reduce root gap
Reduce Current
Increase Root face
Train Welder
Train Welder
Overlap
Cause
1) Wrong Direction Of Arc
2) Inadequate Current
3) Excess Filler Wire
Overlap
Remedy
1) Train & Qualify Welder
2) Increase Current
3) Reduce Filler Metal
Suck Back*
Cause
1) Excess weaving in root
2) Excess Current
3) Inadequate root face
4) Wrong Electrode angle
Remedy
1) Reduce weaving
2) Reduce Current
3) Increase root face
4) Train / Qualify Welder
* Applicable to SSFPW in 4G, 3G & 2G
Suck Back
Under flush
Cause
Remedy
1) Weld some more beads
1) Inadequate weld beads in
final layer
in final layer
2) Inadequate understanding on 2) Train / Qualify welder
weld reinforcement
3) Wrong selection of filler wire 3) Train / Qualify Welder
size
Under flush
Burn through*
Cause
1) Excess Current
2) Excess Root opening
3) Inadequate Root face
4) Improper weaving
Remedy
1) Reduce the Current
2) Reduce root opening
3) Increase root face
4) Train / Qualify Welder
*Applicable to root pass
Burn trough
Tungsten Inclusion
Cause
1) Ineffective HF
2) Improper Starting of Arc
3) Tungsten Tip Comes in
Contact With Weld
Tungsten Inclusion
Remedy
1) Rectify HF Unit
2) Never Touch Weld
With Tungsten Rod
3) Train / Qualify welder
Stray Arcing
Cause
Remedy
1) HF Not In Operation
1) Rectify HF Unit
2) Inadequate Skill of Welder 2) Train the Welder
Arc Strikes
Gas Metal Arc Welding
What Is GMAW ?
• A Fusion Welding Process – Semi Automatic
• Arc Between Consumable Electrode &Work
• Arc Generated by Electric Energy From a Rectifier
/ Thyrester / Inverter
• Filler Metal As Electrode Continuously fed From
Layer Wound Spool.
• Filler Wire Driven to Arc By Wire Feeder through
Welding Torch
• Arc & Molten Pool Shielded by Inert Gas through
Torch / Nozzle
Gas Metal Arc Welding
• MIG – Shielding Gas Ar / Ar + O2 / Ar + Co2
• MAG – Shielding Gas Co2
• FCAW – Shielding Gas Co2 With Flux cored
Wire
Note:- Addition of 1 – 5% of O2 or 5 – 10% of Co2 in Ar.
increases wetting action of molten metal
Power Source For MIG / MAG
•
•
•
•
Inverter- DC
Thyrister – DC
Motor Generator – DC
Rectifier – DC
Characteristic Of GMAW Power
Source
Constant V / Linear Characteristic
V
Appx. Horizontal
Curve
V1
V2
A1
A2
A
Current & Polarity
DC- Electrode +Ve
Stable Arc
Smooth Metal Transfer
Relatively Low Spatter
Good Weld Bead Characteristics
–
DC- Electrode Ve, Seldom
Used
AC- Commercially Not In use
Accessories Of GMAW
•
•
•
•
•
•
Power Source
Wire Feed Unit
Shielding Gas Cylinder, Pressure gauges/
Regulator, Flow meter (Heater For Co2 )
Welding Torch
Water Cooling System (For Water cooled Torch)
Earthing Cable With Clamp
Tools For GMAW
•
•
•
•
•
•
•
•
•
Head Screen With DIN 13 / 14 Dark Glass
Hand Wire Brush / Grinder With Wire Wheel
Cutting Pliers
Hand Gloves
Chipping Hammer / Chisel & hammer
Spanner Set
Cylinder Key
Anti-spatter Spray
Earthing Cable With Clamp
GMAW Torch
On / Off Switch
Shielding Gas
Torch Handle
Spring Conduit
Gas Cup
Arc
Nozzle Tip
Filler Wire - Electrode
Job
Equipment & Accessories
Pressure Regulator
Flow Meter
Shielding Gas
Switch
Heater
(Only For
Co2)
Solenoid
Valve
Shielding Gas
Cylinder
Copper Cup
Electrode /
Wire
Arc
–
Welding Torch
Wire Inside Spring Lining
Contact Tip
Argon / Co2
Shielding
Work
Torch With Cable Max. 3Mtr
Wire Feeder
Wire
Spool
Power Source
With Inductance
+
–
Types Of Wire Feeding In
GMAW
• Push Type
– Wire fed in to The torch by Pushing through Flexible
Conduit From A Remote Spool
• Pull Type
– Feed Rollers Mounted on The Torch Handle Pulls the
Wire From A Remote spool
• Self Contained
– Wire Feeder & The Spool On the Torch
Function Of Shielding Gas In
GMAW
• Prevents Air contamination of weld Pool
• Prevents Contamination During Metal
Transfer
• Increases fluidity of molten metal
• Minimizes the spatter generation
• Helps in even & uniform bead finish
Shielding Gases For GMAW
• MIG:
•
•
•
•
Argon Or Helium
For SS, CS, LAS & Non-ferrous Mt & Al
MIG: Ar + 1 to 2 % O2, Wire With Add. Mn & Si
For SS, CS, LAS & Non-ferrous Mt & Al
MIG: Ar + 5 to 20 % Co2 Wire With Add. Mn & Si
For SS, CS, LAS & Non-ferrous Mt & Al
MAG: Co2 With Solid Wire
For CS & LAS
FCAW: Co2 With Flux Cored Wire
For CS, LAS & SS Overlay
ASME Classification For CS
GMAW Wire
• SFA 5.18 : - CS Solid Wire
ER 70 S – 2, ER 70 S – 3
ER 70 S – 6, ER 70 S – 7
• SFA 5.20 :- CS Flux Cored Wire
E 71 T-1,
E 71 T-2 ( Co2 Gas )
E 71 T-1M, E 71 T-2M ( Ar + Co2 Mix)
GMAW CS Wire
• Generally Copper Coated
– Prevents Oxidation / rusting in Storage
– Promotes Electric Conductivity in Arcing
• Available In Solid & Flux Cored
– Size in mm 0.8, 1, 1.2, 1.6, 2, 2.4, 3
• Manganese & Silicon ( Mn 1 – 2 %, Si Max 1%)
– Act As Deoxidizing Agents
– Eliminate Porosity
– Increase Wetting Of Molten Pool
Metal Transfer In MIG
• Short-Circuiting / Dip Transfer
• Globular Transfer
• Spray Transfer
Metal Transfer In MIG
Up to 120A
CS Solid Wire 1.2 mm Φ
120 to 250A
14 – 22V
Dip/Short Circuiting
Co2 or Ar
16 – 24 V
Globular
Co2 or Ar
Above230A
24 – 35 V
Spray
Only Ar / Ar+O2
Short-Circuiting / Dip Transfer
• Wire In Contact With Molten Pool 20 to 200 times per
Second
• Operates in Low Amps & Volts – Less Deposition
• Best Suitable for Out of Position Welding
• Suitable for Welding Thin Sheets
• Relatively Large opening of Root Can be Welded
• Less Distortion
• Best Suitable for Tacking in Set up
• Prone to Get Lack of Fusion in Between Beads
Globular Transfer
• Metal transferred in droplets of Size grater than
wire diameter
• Operates in Moderate Amps & Volts – Better
Deposition
• Common in Co2 Flux Cored and Solid Wire
• Suitable for General purpose Welding
Spray Transfer
•
•
•
•
•
Metal transferred in multiples of small droplets
100 to 1000 Droplets per Second
Metal Spray Axially Directed
Electrode Tip Remains pointed
Applicable Only With Inert Gas Shielding
–
Not With Co2
• Operates in Higher Amps & Volts – Higher
Deposition Rate
• Not Suitable for Welding in Out of Position.
• Suitable for Welding Deep Grooves
Pulsed Spray Welding
• Power Source Provides Two different
Current Levels“Background” and “Peak”at
regular interval
• “Background” & “Peak” are above and
below the Average Current
• Best Suitable for Full Penetration Open
Root Pass Welding
• Good Control on Bead Shape and Finish
Synergic Pulse GMAW
• Parameters of Pulsed Current (Frequency,
Amplitude, Duration, Background Current)
Related to Wire feed Rate
• One Droplet detaches with each pulse
• An Electronic Control unit synchronizes wire feed
Rate with Pulse Parameters
• Best Suitable for Most Critical Full Penetration
Open Root Pass Welding
• Good Control on Open Root penetration, Bead
Shape and Finish
GMAW Process Variables
•
•
•
•
•
•
•
•
Current
Voltage
Travel Speed
Stick Out / Electrode Extension
Electrode Inclination
Electrode Size
Shielding Gas & Flow Rate
Welding Position
Parameter For 1.2 ф FC Wire
•
•
•
•
•
•
Current – 200 to 240 A
Voltage – 22-24
Travel Speed 150 to 250 mm / min
Stick Out / Electrode Extension – 15 to 20 mm
Electrode Inclination – Back Hand Technique
Shielding Gas – Co2, 12 L/Min
Parameter For 1.2 ф Solid Wire
•
•
•
•
•
•
Current – 180 to 220 A
Voltage – 20-22
Travel Speed 150 to 200 mm / min
Stick Out / Electrode Extension – 10 to 20 mm
Electrode Inclination – Back Hand Technique
Shielding Gas – Co2 – 12 L/Min
Results In Change Of Parameters
• Increase In Current
– More deposition, More Penetration, More BM Fusion
• Increase In Voltage
– More Weld Bead Width, Less Penetration, Less
Reinforcement, Excess Spatter
• Increase In Travel Speed
– Decrease in Penetration, Decrease in Bead Width,
• Decrease In Gas Flow rate
– Results In porosity
• Long Stick Out / Electrode Extension
– Excess Weld Deposit With Less Arc intensity, Poor Bead
Finish, Shallow Penetration
Common Defects In GMAW
1. Porosity
3. Lack Of Fusion
5. Over Lap
7. Crack
9. Burn Through
11. Unstable Arc
2. Spatters
4. Under Cut
6. Slag
8. Lack Of Penetration
10. Convex Bead
12. Wire Stubbing
Porosity
Cause
Remedy
1) Less Mn & Si In Wire
2) Rusted / Unclean BM / Groove
3) Rusted wire
4) Inadequate Shielding Gas
1) Use High Mn & Si Wire
2) Clean & warm the BM
3) Replace the Wire
4) Check & Correct Flow Rate
Porosity
. .
Spatters
Cause
Remedy
1) Low Voltage
2) Inadequate Inductance
3) Rusted BM surface
4) Rusted Core wire
5) Quality Of Gas
1) Increase Voltage
2) Increase Inductance
3) Clean BM surface
4) Replace By Rust Free wire
5) Change Over To Ar + Co2
Spatters
•
••
Lack Of Fusion
Cause
Remedy
1) Inadequate Current
1) Use Right Current
2) Inadequate Voltage
3) Wrong Polarity
4) Slow Travel Speed
5) Excessive Oxide On Joint
2) Use Right Voltage
3) Connect Ele. + Ve
4) Increase Travel speed
5) Clean Weld Joint
Lack Of Fusion
Undercut
Cause
1) Excess Voltage
2) Excess Current
3) Improper Torch angle
4) Excess Travel Speed
Under cut
Remedy
1) Reduce Voltage
2) Reduce Current
3) Train & Qualify the Welder
4) Reduce Travel Speed
Overlap
Cause
Remedy
1) Too Long Stick Out
1) Reduce Stick Out
2) Inadequate Voltage
2) Increase the Voltage
Overlap
Slag
Cause
1) Inadequate Cleaning
2) Inadequate Current
3) Wrong Torch angle
4) Improper bead placement
Slag
Remedy
1) Clean each bead
2) Use Right Current
3) Train / Qualify welder
4) Train / Qualify Welder
Crack
Cause
Remedy
1) Incorrect Wire Chemistry 1) Use Right Wire
2) Increase wire Feed
2) Too Small Weld Bead
3) Preheat Uniformly
3) Improper Preheat
4) Post heating or ISR
4) Excessive Restrain
crack
Lack Of Penetration*
Cause
1) Too Narrow Groove Angle
2) Inadequate Root opening
3) Too Low Welding current
4) Wrong Torch angle
5) Puddle Roll In Front Of Arc
6) Long Stick Out
* Applicable to SSFPW
LOP
Remedy
1) Widen The Groove
2) Increase Root Opening
3) Increase Current
4) Train / Qualify Welder
5) Correct Torch Angle
6) Reduce Stick Out
Burn through*
Cause
1) Excess Current
2) Excess Root opening
3) Inadequate Root face
4) Too Low Travel Speed
5) Quality Of Gas
Burn trough
Remedy
1) Reduce the Current
2) Reduce root opening
3) Increase root face
4) Increase Speed
5) Use Ar + Co2
*Applicable to root pass
Convex Bead Finish
Cause
1) Low Current
2) Low Voltage
3) Low Travel Speed
4) Low Inductance
5) Too Narrow Groove
Uneven bead finish
Remedy
1) Increase Current
2) Increase Voltage
3) Increase Travel Speed
4) Increase Inductance
5) Increase Groove Width
Unstable arc
Cause
1) Improper Wire Feed
2) Improper Gas Flow
3) Twisted Torch Conduit
Remedy
1) Check Wire Feeder
2) Check Flow Meter
3) Straighten Torch Cab
Wire Stubbing
Cause
1) Too Low Voltage
2) Too High Inductance
3) Excess Slope
4) Too Long Stick Out
Remedy
1) Increase Voltage
2) Reduce Inductance
3) Adjust Slope
4) Reduce Stick Out
Important Terminology used in
Critical Welding
•
•
•
•
•
Preheating
Post Heating or Dehydrogenation
Intermediate Stress leaving
Inter pass Temperature
Post Weld Heat Treatment
What Is Preheating?
• Heating the base metal along the weld joint to a
predetermined minimum temperature immediately
before starting the weld.
• Heating by Oxy fuel flame or electric resistant
coil
• Heating from opposite side of welding wherever
possible
• Temperature to be verified by thermo chalks prior
to starting the weld
Why Preheating?
• Preheating eliminates possible cracking of weld and HAZ
• Applicable to
Hardenable low alloy steels of all thickness
Carbon steels of thickness above 25 mm.
Restrained welds of all thickness
• Preheating temperature vary from 75°C to 200°C
depending on hardenability of material, thickness & joint
restrain
How does Preheating Eliminate Crack?
• Preheating promotes slow cooling of weld and
HAZ
• Slow cooling softens or prevents hardening of
weld and HAZ
• Soft material not prone to crack even in
restrained condition
What Is Post Heating?
• Raising the pre heating temperature of the weld joint to a
predetermined temperature range (250° C to 350° C) for
a minimum period of time (3 Hrs) before the weld cools
down to room temperature.
• Post heating performed when welding is completed or
terminated any time in between.
• Heating by Oxy fuel flame or electric resistant coil
• Heating from opposite side of welding wherever possible
• Temperature verified by thermo chalks during the period
Why Post Heating?
• Post heating eliminates possible delayed cracking
of weld and HAZ
• Applicable to
Thicker hardenable low alloy steels
Restrained hardenable welds of all thickness
• Post heating temperature and duration depends on
hardenability of material, thickness & joint
restrain
How does Post Heating Eliminate
Crack?
• SMAW introduces hydrogen in weld metal
• Entrapped hydrogen in weld metal induces
delayed cracks unless removed before cooling to
room temperature
• Retaining the weld at a higher temperature for a
longer duration allows the hydrogen to come out
of weld
What Is Intermediate Stress Relieving?
• Heat treating a subassembly in a furnace to a
predetermined cycle immediately on completion of
critical restrained weld joint / joints without
allowing the welds to go down the pre heat
temperature. Rate of heating, Soaking temperature,
Soaking time and rate of cooling depends on
material quality and thickness
• Applicable to
Highly restrained air hardenable material
Why Intermediate Stress Relieving?
• Restrained welds in air hardenable steel highly
prone to crack on cooling to room temperature.
• Cracks due to entrapped hydrogen and built in stress
• Intermediate stress relieving relieves built in stresses
and entrapped hydrogen making the joint free from
crack prone
What Is Inter- Pass Temperature?
• The temperature of a previously layed weld bead
immediately before depositing the next bead over
it
• Temperature to be verified by thermo chalk prior
to starting next bead
• Applicable to
Stainless Steel
Carbon Steel & LAS with minimum impact
Why Inter Pass Temperature?
• Control on inter pass temperature avoids over
heating, there by
Refines the weld metal with fine grains
Improves the notch toughness properties
Minimize the loss of alloying elements in
welds
Reduces the distortion
What Is Post Weld Heat Treatment?
• Heat treating an assembly on completion of all
applicable welding, in an enclosed furnace with
controlled heating/cooling rate and soaking at a
specific temperature for a specific time.
• Rate of heating, Soaking temperature, Soaking time
and rate of cooling depends on material quality and
thickness
• Applicable to
All type of CS & LAS
Why Post Weld Heat Treatment?
• Welded joints retain internal stresses within the
structure
• HAZ of welds remains invariably hardened
• Post Weld Heat Treatment relieves internal stresses
and softens HAZ. This reduces the cracking
tendency of the equipment in service
Weldability
• The weldability of a material refers to its
ability to be welded. Many metals and
thermoplastics can be welded, but some
are easier to weld than others. It greatly
influences weld quality and is an important
factor in choosing which welding process
to use.
•
•
•
Steels
The weldability of steels is inversely proportional to a property known as the hardenability of the
steel, which measures the ease of forming martensite during heat treatment. The hardenability of
steel depends on its chemical composition, with greater quantities of carbon and other alloying
elements resulting in a higher hardenability and thus a lower weldability. In order to be able to
judge alloys made up of many distinct materials, a measure known as the equivalent carbon
content is used to compare the relative weldabilities of different alloys by comparing their
properties to a plain carbon steel. The effect on weldability of elements like chromium and
vanadium, while not as great as carbon, is more significant than that of copper and nickel, for
example. As the equivalent carbon content rises, the weldability of the alloy decreases. The
disadvantage to using plain carbon and low-alloy steels is their lower strength—there is a trade-off
between material strength and weldability. High strength, low-alloy steels were developed
especially for welding applications during the 1970s, and these generally easy to weld materials
have good strength, making them ideal for many welding applications.
Stainless steels, because of their high chromium content, tend to behave differently with respect
to weldability than other steels. Austenitic grades of stainless steels tend to be the most weldable,
but they are especially susceptible to distortion due to their high coefficient of thermal expansion.
Some alloys of this type are prone to cracking and reduced corrosion resistance as well. Hot
cracking is possible if the amount of ferrite in the weld is not controlled—to alleviate the problem,
an electrode is used that deposits a weld metal containing a small amount of ferrite. Other types of
stainless steels, such as ferritic and martensitic stainless steels, are not as easily welded, and
must often be preheated and welded with special electrodes.
•
•
•
•
•
•
•
•
•
•
•
Aluminum
The weldability of aluminum alloys varies significantly, depending on the chemical composition of the alloy used. Aluminum alloys are
susceptible to hot cracking, and to combat the problem, welders increase the welding speed to lower the heat input. Preheating reduces
the temperature gradient across the weld zone and thus helps reduce hot cracking, but it can reduce the mechanical properties of the
base material and should not be used when the base material is restrained. The design of the joint can be changed as well, and a more
compatible filler alloy can be selected to decrease the likelihood of hot cracking. Aluminum alloys should also be cleaned prior to welding,
with the goal of removing all oxides, oils, and loose particles from the surface to be welded. This is especially important because of an
aluminum weld's susceptibility to porosity due to hydrogen and dross due to oxygen.
[edit]
References
Lincoln Electric (1994). The Procedure Handbook of Arc Welding. Cleveland: Lincoln Electric. ISBN 9994925822.
Residual stress
From Wikipedia, the free encyclopedia
Jump to: navigation, search
Residual stresses are stresses that remain after the original cause of the stresses has been removed. Residual stresses occur for a
variety of reasons, including inelastic deformations and heat treatment. Heat from welding may cause localized expansion, which is taken
up during welding by either the molten metal or the placement of parts being welded. When the finished weldment cools, some areas cool
and contract more than others, leaving residual stresses. Castings may also have large residual stresses due to uneven cooling.
While un-controlled residual stresses are undesirable, many designs rely on them. For example, toughened glass and pre-stressed
concrete depend on them to prevent brittle failure. Similarly, a gradient in martensite formation leaves residual stress in some swords with
particularly hard edges (notably the katana), which can prevent the opening of edge cracks. In certain types of gun barrels made with two
tubes forced together, the inner tube is compressed while the outer tube stretches, preventing cracks from opening in the rifling when the
gun is fired. Parts are often heated or dunked in liquid nitrogen to aid assembly.
Press fits are the most common intentional use of residual stress. Automotive wheel studs, for example are pressed into holes on the
wheel hub. The holes are smaller than the studs, requiring force to drive the studs into place. The residual stresses fasten the parts
together. Nails are another example.