Welding & its Necessity, Types, Gas Welding, Arc Welding Principle, Electrodes, Polarity and Welding Defects
Introduction:
The
welding is defined as the process of joining similar or dissimilar metals by application
of heat or without. Welding can be obtained with or without application of
pressure or filler material. The welding process is used for making permanent
joints which is obtained by homogenous minute of two materials The heat may be
developed in several ways for welding operation. A good welded joint is as
strong as the parent metal.
Necessity:
● To produce 100% efficient joint.
● To fabrication and assemble steel
components.
● To join dissimilar metals.
● It's noiseless.
● It forms a more stronger joint.
Types of welding process:
There
are two main types of welding process, which are classified based on energy employed
for heating the metal and state of metal at the place being used.
● Fusion welding
● Plastic welding
Fusion welding:
● In fusion welding the metal at the joining area
is heated to molten state.
● Pressure is not applied during welding process
hence it is also called as non-pressure welding.
● Filler material may be required during this type
of welding process. Ex. Gas welding, arc
welding.
Plastic welding:
● In plastic welding, the metal pieces are heated
to plastic state.
● They are pressed together to make a joint.
● Inter atomic bond may be established by bringing
the atoms of two surfaces into close proximity.
Differences between pressure welding and fusion
welding:
Pressure Welding
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Fusion Welding
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1.Requires heat and pressure.
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1.Requires only heat.
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2.Joint area is heated to plastic state.
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2.Joint area is heated to molten state.
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3.Requires lower temperature.
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3.Requires higher temperature.
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4.Composition is not much affected.
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4.Composition and structure are affected.
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5.No need of filler metal.
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5.Filler metal is required.
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GAS WELDING
Principle of Gas Welding:
Gas welding is a process in which the required heat to melt the surfaces
to be welded is supplied by a high temperature flame obtained by a mixture of
two gases. The gases are mixed in proper proportions in a welding blow pipe
[torch]. For controlling the welding flame, there are two regulators on the
torch by which the quantity of either gas can be controlled. Usually, the
mixture of oxygen and acetylene [C2H2] is used for welding purposes. It
produces temperature in the range of 3200 - 3300“ C. This flame is known as
Oxy-acetylene flame. The mixture of hydrogen and other combustible gases can
also be used, but the temperatures obtained are lower than the oxy-acetylene
flame. In gas welding, the two surfaces to be welded are properly prepared and
placed near each other. The metal in the joint is brought to melting
temperature by heat from the flame and then weld is completed by supplying
additional metal as the filler metal obtained by a filler rod.
Procedure of Gas Welding:
• Confirm that the two gauges on the cylinders are
registering zero pressure.
• Open the cylinder valves and check the
correctness of pressure registered on the pressure gauges.
• Open the pressure regulating screw by turning
them clockwise.
• Open acetylene gas slightly to ignite and then to
the correct quantity.
• Ignite the gas mixture using a lighter.
• Then open the oxygen slowly to give out the
required flame.
Precautions to be taken in gas welding:
• Both the cylinders should be kept away from heat
and fire.
• Cylinders should be placed in upright position.
• Do not use oil or grease to lubricate various
controls on oxygen cylinder
• Maintain acetylene at low pressure [below 2N/mm^2].
• Draw acetylene always through the pressure
reducing valve.
• In case of backfire immediately turn off the
acetylene valve first and then oxygen valve.
Types of Flames in Gas Welding:
Neutral flame:
• This type of flame is obtained by supplying equal
volumes of oxygen and acetylene.
• The neutral flame has the following two sharply
defined zones.
(i) An inner luminous cone and
(ii) An outer cone or envelope of bluish colour.
• The first one develops heat and the second
protects the molten metal from oxidation.
• The most of the oxy-acetylene welding is done
with the neutral flame.
Oxidizing flame:
• This type of flame is obtained when there is an
excess of oxygen.
• This flame has two zones:
(i) The small inner cone which has purplish tinge
and
(ii) The outer cone or envelope.
• It is similar to neutral flame, but the inner cone
is less luminous and shorter.
• It is used for welding brass and bronze.
Reducing or Carburizing Flame:
• This type of flame is obtained by supplying an
excess of acetylene.
• This flame has the following three zones:
(i) Sharply defined inner cone.
(ii) An intermediate cone of whitish colour.
(iii) An outer cone of bluish colour.
• The length of intermediate cone indicates the
proportion of excess acetylene in the flame.
• This flame is used where it is required to keep
oxidation to a minimum.
• It is used for welding of metal, certain alloy
steels, hard surfacing materials such as stellite.
Gas Welding Techniques:
Depends on the following factors:
• Material being welded
• Thickness of the work piece
• Shape and size of the work piece
• Properties anticipated of the joints
Types of gas welding techniques:
• Leftward or forward welding
• Rightward or backward welding
• Vertical welding
Forward or Leftward welding:
• The weld is commenced at the right hand side of
the join and blow pipe is given by steady forward movement, with slight slide
ways motion, zig-zag along the weld towards the left.
• The blow pipe is kept an angle 60°-70° to the
surface of work, so that flame on stage in front of it.
• The filler rod held an [angle 30°-40°] just ahead
of flame and progressively fed into it.
Advantages of Forward or Leftward welding:
• Vertical joints are welded by this technique.
Disadvantages of Forward or Leftward welding:
• This technique is restricted to welding of mild
steel plates[ upto 5mm thickness], cast iron and non-ferrous metals.
Backward or Rightward welding:
• Flame is directed towards the completed part of
joint and welding proceeds from left hand to right hand.
• Filler rod is fed into the flame in circular
movement.
• Horizontal and overhead weldings can be done by
this technique.
Advantages of Backward or Rightward welding:
• Rightward welding is faster by 20-25% than
leftward welding 15-25% acetylene less consumption compared to leftward
welding.
• The mechanical properties of the weld are better
due to annealing effect of flame, which is directed on the completed weld.
• The amount of distortion in the work is minimum.
Disadvantages of Backward or Rightward welding:
• Only weld thicker materials.
Vertical welding:
• Welding is commenced at the bottom and proceeds
vertically upwards.
• Generally two welders are employed for vertical
welding, one on either side of the joint.
• If one operator is employed the angle of the
torch varies with the plate thickness.
• Does not require edge preparation.
• Requires less amount of filler material.
• Advantageous for plates more than 6 mm thick.
Welding Practice and Safety:
• Edges are to be prepared and position is
maintained with fixtures.
• Acetylene gas is first released and ignited and
correct flame is adjusted.
• Torch is held at an angle of 45° and filler rod
at 30 to 40° to the work piece
• Filler rod is to be moved uniformly in the joint
in both speed and feed.
• Surface is to be cleaned by wire brushes.
Safety measures:
• Goggles, face shields, gloves and protective
clothing is to he used properly.
• Hoses should be connected properly.
• Oxygen and acetylene cylinders have different
threading to avoid mistake.
• Gas cylinders should not he dropped or
mishandled.
Advantages –
Disadvantages & Application of Gas Welding
Advantages of Gas Welding:
● Temperature of flame can be easily
controlled.
● The flame can be used for welding and
cutting.
● Cost and maintenance of gas welding
equipment is low.
● Equipment is versatile and used for
preheating, brazing and is readily converted to oxygen cutting.
● High portability and convenient.
● Oxy-Acetylene flame is more easily controlled
and can be used for all metals and alloys.
● Welding skills are relatively easy.
● Welder has control over filler metal
deposition.
Disadvantages of Gas Welding:
● Takes longer time to weld as gas flame takes
a long time to heat the metal.
● Heat affected zone and distortions are
longer.
● Refractory and reactive metals cannot be gas
welded.
● Oxygen and acetylene gases are expensive.
● Shielding provided by flame is not effected.
● Heavy sections can't be welded.
● More safety problems are associated with the
handling and storage of gases.
Applications of Gas Welding:
● Oxy-acetylene welding is a versatile process
and can be used for welding all commercial metals and alloys.
● Due to low temperature of gas flame, the
process is employed for welding thin sections.
● The process is mostly used in; Sheet metal
fabrication workshop, Aircraft industries, Garages, Maintenance shops etc.
ARC
WELDING
Principle of
arc welding:
In arc welding process, the heat
is developed by arc is produced between an electrode and the work. Arc welding
is the process of joining two metal pieces by melting their edges by electric
arcs. In arc welding, the electrical energy is converted into heat energy. The
electrode and work piece are brought nearer with a small air gap of 3 mm approximately.
The current is passed through the work piece and the electrode which produces
an electric arc. The work piece is melted by the electric arc. The electrode is
also melted and hence both the pieces become a single piece without applying
any external pressure. The temperature of arc is about 3500°C to 6000°C. The
electrode supplies additional filler metal into the joints and is deposited
along the joint. A transformer or generator is used for supplying the current. The
depth to which the metal is melted and deposited is called depth of fusion. Electrodes
used in arc welding are generally coated with a flux. The flux is used to
prevent the reaction of the molten metal with atmospheric air. It also removes
the impurities from the molten metal and forms a slag. This slag gets deposited
over the weld metal. This slag protects the weld seam from rapid cooling.
Metal arc welding
Most commonly used in arc welding.
Manually operated, hence it is also called as Manual Metal Arc Welding [MMAW]. Makes
use of consumable electrodes. Electrode metal will be same as the parent metal.
Setup for the
metal arc welding
● Arc is
produced with a low voltage from 20-80V and with high current 80- 500 amps.
● Circuit
consists of A.C or D.C power source.
● Power is
conveyed to electrode through electrode holder.
● Holder is
connected with leads to source.
Sequence of
steps involved in arc welding
1.Preparation
of edges.
2.Holding the
work piece in a fixture.
3.Striking
the arc.
4.Welding the
joint.
1.Preparation
of edges:
● Edges are
to be cleaned with wire brush.
2.Holding the
work piece in a fixture:
● Leads are
to be connected properly to the power source.
● Parent
metals are held in a fixture.
3.Striking
the arc:
● Tip of the
electrode is scratched on the parent metal.
● After
scratching the tip of electrode for a moment a gap of 2 to 4 mm is to be
maintained.
4.Welding the
joint:
● The welder
should move the electrode along the length of the joint maintaining the
arc and avoid
distortion during welding.
● Start
forming beads along the joint at the required speed of welding.
● At the end
of the first run, slag coating present on the bead is chipped off and then
the area is cleaned
with a wire brush.
● After
welding, the joint is to be heat treated to maintain the grain size.
Application,
Advantages & Disadvantages of Arc Welding
Application
of arc welding:
● Fabrication
of pressure vessels.
● Ships.
● Structural
steel work.
● Joints in
pipe works.
●
Construction and repair of machine parts
● Repair of
broken parts.
Advantages of
arc welding:
● Faster and
lower in cost than gas welding.
● Can be
welded in any position.
● Wide range
of metals can be welded: Ferrous, Non-Ferrous & Alloys.
Disadvantages
of arc welding:
● It is not
well suited for the welding of thin sections [below 3mm thick] or for other work
requiring care in the application of heat to the work.
● As this
process can be used in its manual mode only, it is slowly getting replaced by other
welding processes for heavy fabrication where large quantity of metal need to
be deposited.
D.C polarity
● Polarity
indicates the direction of current flow in the DC welding machine.
● In D.C
welding 2/3 heat is liberated from positive end and 1/3 heat is liberated from
negative end.
● Hence
polarity plays an important role in successful welding.
Types of
Polarity:
● Straight
polarity [Electrode -ve, Work piece +ve]
● Reverse
polarity [Electrode +ve, Work piece -ve]
Straight polarity:
Straight polarity used for welding thicker sections with
light 81 medium coated electrodes.
Reverse polarity used for welding
non-ferrous metals and cast iron with heavy coated electrodes. Also used for
sheet metal welding.
Electrodes:
In arc
welding an electrode is used to conduct current through a work piece to fuse
two pieces together. The electrodes are available in two principle forms
● Rods
[sticks]
● Wire
Welding rods
are generally 225-450 mm length and 9.5 mm diameter.
Types of electrodes:
1. Consumable
electrodes:
● It provides
the filler material also.
● During
welding, they melt and supply the filler material.
● The arc
length and the resistance of the arc path vary, as a result of change in arc
length and the presence or absence of metal particles in the path.
● The
electrodes being moved towards or away from the work to maintain the arc.
Types of
consumable electrodes
● Bare
electrode.
● Fluxed or
lightly coated electrode
Bare
electrode:
● Used to
weld wrought iron and mild steel.
● They must
be used only with straight polarity.
● It is in
the form of sticks or rods [hand arc weld] and continuous wire [automatic
welding].
Lightly
coated electrode:
● It have
coating of several layers [containing 1-5% of electrode weight].
● It is used
for increases the stability.
● It is also
called as ionizing coatings.
● This type
of coating contains chalk and water glass.
● These
electrodes are used in welding only in critical structure.
● The welds
produced poor mechanical properties, due to lack of the protection of molten
metal.
2.Non
consumable electrode:
● Non
consumable electrodes are usually made of carbon, graphite or tungsten, which
do not get consumed during welding operation.
● These are
used in carbon-arc, inert-gas arc, atomic hydrogen arc welding.
Welding Defect:
A defect is a discontinuity or
discontinuities which effect a part or a product unable to meet minimum
applicable accepted specifications. A defective weld is one containing one or
more defects.
The following are the most common defects [faults] in welded
joints;
Solid
Inclusions:
Solid inclusions are any non metallic
solid material entrapped in the weld metal. Solid Inclusions caused by,
● Dirty base
metal
● Excessive
moisture in electrode
● Dirty
filler wire.
Lack of
penetration:
Lack of penetration means that the
weld depth is not up to the desired level. Root faces have not reached to
melting point in a groove joint. Lack of penetration caused by,
● Improper
joints. [For example, it is simpler to obtain full penetration in U joint as
compared to I butt joint].
● Too large
root face.
● Root gap
too small.
● Too small
bevel angle.
● Less arc
current.
● Faster arc
travel speed.
Weld crack:
Discontjnuity in the weld zone. Welding
cracks are caused by
● Poor
ductility of base metal
● Fast arc
travel speed.
● Rigidity
of the joint.
Poor Welding
Bead Appearance:
A weld bead appears to be poor if it
is not deposited straight. The bead width varies from place to place.
Undercut:
Groove formed along the edges of
welding bead. Caused due to excessive current, excessive speed and wrong
electrode position.
Porosity:
Small voids throughout the weld
material. Porosity caused by:
● Entrapment
of refractory oxides.
● Improper
joint design.
● Poor
electrode manipulative technique.
Spatter:
Globules of metal expelled from an
electrode and deposited on the surface of the parent metal. Caused by damped
welding conditions or incorrect heat input.
Distortion:
Distortion is the change in shape and
difference between the positions of the two plates before welding and after
welding. While welding a job, base metal under the arc melts. Base metal ahead
gets preheated and the base metal portion already welded starts cooling. There
is a good amount of temperature difference at various points along the joint and
thus at any instant certain areas of base metal expand and others including weld
bead contract.
Next Topics on Welding
TIG Welding
MIG Welding
Shielded Metal Arc Welding
Submerged Arc Welding
Resistance Welding & its Types (Spot Welding, Seam Welding, Projection Welding)
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