FUNDAMENTALS OF WELDING
Weld Definition
Materials joining process used in
making a localized coalescence of metal either by heating the materials to
suitable temperature with or without application of pressure & with or
without the use of filler material.
SHIELD METAL ARC WELDING (SMAW)
Advantages Maximum flexibility Can weld many metals
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Can weld in any position
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Can be used on most of the thickness.
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Widely used in fieldwork
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Min. investment
Limitations
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Break-in welding
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Cleaning between passes
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Less deposition efficiency (65%)
SELECTION OF CONSUMABLE
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Base metal
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Position of welding
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Thickness & shape of base metal
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Joint design & fit-up
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Weld metal deposition efficiency
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Specification & service conditions
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Size of electrodes
SELECTION OF THE ELECTRODE
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Type of electrode
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Welding position
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Type of joint and edge preparation
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Material thickness
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Experience and skill of the welder
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Base metal (Phy. & mech. properties)
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Job specifications
FUNCTION OF ELECTRODE COVERING
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Shielding of the weld pool
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Elimination of weld metal porosity
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Elimination of weld metal cracking
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Ensure specific composition of deposited weld metal
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Desirable weld deposit contour
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Desirable weld surface finish
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minimize spatter
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Provide a stable welding arc
FUNCTION OF ELECTRODE COVERING
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Provide penetration control
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Provide a high rate of metal deposition
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Reduce electrode overheating
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Provide a strong tough durable coating
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Specific mechanical properties of the deposited weld
metal
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Ease of manipulation to control slag
CARE & STORAGE OF ELECTRODES
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Mechanical handling
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Absorption of moisture
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Deterioration due to aging
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Rusting of electrodes
WILD CRISES OF AN ELECTRODE
1. Do not leave me alone
Welding electrodes come in the packet
& its place is in the packet
2. Do not manhandle me
Electrodes are flux-coated which
has a specific purpose. Improper handling will cause cracks on the coating which
will make electrodes useless.
3. Give me due respect
Electrodes cost high. Select a suitable electrode for a particular type of welding.
4. My health deteriorates
fast in cupboards
The general practice of throwing
away half-used packets are disastrous & not expected of a conscious welder.
After a day's use keep back left out electrodes in the drying oven & not in the
packets.
EFFECT OF ALLOYING ELEMENTS
Carbon
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Increases hardness, tensile strength.
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Machinability is affected.
Nickel
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Increases toughness & resistance to impact.
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Impact strength at low temperatures increases.
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Corrosion resistance increases.
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Increases hardness, strength without scarifying
ductility.
EFFECT OF ALLOYING ELEMENTS
Silicon
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Strength & hardness increase.
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Improves oxidation resistance.
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Strengthens low alloy steel.
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Acts as a deoxidizer.
Manganese
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Improves strength & hardness.
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Counteracts brittleness from Sulphur.
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Lower ductility & weldability.
EFFECT OF ALLOYING ELEMENTS
Chromium
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Increases hardenability.
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Provides strength, wear & oxidation resistance at
elevated temperature.
Titanium
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Reduces martens tic hardness and hardenability in
medium chromium steel.
EFFECT OF ALLOYING ELEMENTS
Sulfur
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Increases machinability.
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Produces hot shortness.
Molybdenum
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Promotes the hardenability of steel.
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Makes steel fine-grained.
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Raises tensile & creep strength at a high temp.
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Enhances corrosion resistance.
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Eliminates temper brittleness.
EFFECT OF ALLOYING ELEMENTS
Vanadium
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Improves fatigue resistance.
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Promotes fine grain in steel.
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Increases hardenability.
Tungsten
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Increases hardness.
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Resist heat.
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Promotes fine grain.
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Improves strength at elevated temperatures.
GAS TUNGSTEN ARC WELDING (GTAW)
Advantages
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High-quality welds in all metals & alloy
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Very little cleaning
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No spatter
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Can be used in any position & on small Dia. Tubes
etc.
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No slag entrapment
Limitations
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Very low productivity
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Higher initial cost
SUBMERGED ARC WELDING (SAW)
Advantages
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High quality of weld metal
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High deposition rate & speed
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Smooth uniform finished weld with no spatter
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Little or no smoke & No arc flash
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High utilization of wire (100%)
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Manipulative skill not involved
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Can be easily Automated
Limitations
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Welding position
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The material type which can be welded
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High initial cost
GAS METAL ARC WELDING (GMAW)
Advantages
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Higher deposition compare to SMAW
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Higher operation factor
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Higher utilization of filler metal (92%)
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Elimination of slag & flux removal
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Can be automated
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Lesser skill than SMAW
Limitations
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Inability to reach remote areas
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with gun
• The high cost of equipment compared to SMAW
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Unsuitable for fieldwork.
FLUX CORED ARC WELDING (FCAW)
Advantages
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High-quality weld metal deposits.
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Excellent weld appearance
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Easily mechanized
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Higher operator factor
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Relatively high electrode utilization (82%)
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Relatively high travel speeds
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visible arc-easy to use High deposition rate
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Reduced distortion over SMAW
Limitations
• The process restricted to limited materials
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Produces slag which must be removed
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Electrode more expensive compared to bare wire
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Equipment more expensive than SMAW
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Cannot be used in a windy atmosphere
PREHEAT
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Preheating is the application of heat to a base metal
just before welding or cutting.
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Temperature differential occurs between the heat source
and cool base metal. This causes differential thermal expansion and
contraction, high stresses, hardened areas.
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By reducing temp. the difference in the problem of cracking,
hardness, stresses, distortion can be minimized.
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Preheating will reduce temp. differential
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Preheating temp. depends on the type of material, thickness,
joint design, welding process
POST HEAT
• Interpass temp. is the temp.
before the next pass is started.
• Minimum temp. will be
same as preheating temp.
• Maximum temp. is the
temp which shall not exceed while welding
• Post heating is the temp. to
which job shall be heated immediately after welding and maintained for allowing
the hydrogen gas to escape.
• Gas torches, gas burners,
resistance heaters, low-frequency induction heating, temporary furnace PREHEAT
AND INTERPASS TEMP.IN °C
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