Welding Technology

1. UNIT – 1
GAS AND ARC WELDING PROCESSES

2. “Welding is the fabrication process of joining
together two pieces of metal so that bonding
takes place at their original boundary surfaces”.
When two parts to be joined are melted
together, heat or pressure or both is applied
and with or without added metal for formation
of metallic bond.

4.  Edges of metals to be welded are melted by using a gas flame
 No pressure is applied during welding except pressure gas
welding
 Flame is produced at the tip of a welding torch
 Welding heat is obtained by burning a mixture of oxygen and
combustible gas
 Gases are mixed at the required proportion in a welding torch
which provides a control for welding flame
 Commonly used gases are acetylene, hydrogen, propane and
butane
 Flame only melts the metal, so, the additional metal required to
the weld is supplied by the filler rod
 Flux is used during welding to prevent oxidation and remove
impurities
 Metal having 2 mm to 50 mm thick are welded by gas welding
3 types
(a) Air-acetylene welding
(b) Oxy-acetylene welding
(c) Oxy-hydrogen welding

5.  .

6.  Most common form of gas welding is OXY-
ACETYLENE WELDING

7.  The combination of oxygen and acetylene
produces a flame temperature about 3200oC.
 The gases Oxygen (O2) and Acetylene can be
stored at high pressure in separate cylinders.
There are two types of oxy-acetylene system
employed
1. High pressure system (Oxygen at
120 bar and acetylene compressed to 1.5 bar)
2. Low pressure system (Acetylene is
produced at the place of welding using calcium
carbide and water)

8. Ca C2 + 2H2O Ca (OH)2 + C2H2 + 127.3 KJ/mol
Heat generated in this reaction is very high.

9.  Similar to Oxy-Acetylene welding. Air is used
instead of oxygen.
 Air taken from the atmosphere is compressed
in a compressor
 Compressed air is mixed with acetylene to
the required proportion in the torch.
 It is used in lead welding and many low
melting temperature metals and alloys.

10. By varying the ratio of Oxygen and acetylene
the following three types of flames can be
obtained.
1. Neutral flame
2. Carburising flame
3. Oxidising flame

12. Neutral Flame
 The neutral flame is obtained by supplying
equal quantity of oxygen and acetylene.
 Neutral flame has two zones
 Sharp bright inner cone
 Bluish outer cone
 The reaction of the inner cone for the neutral
flame is given by
C2H2 + O2 2CO + H2

13. Neutral Flame
 The inner cone develops heat to melt the
metal.
 The maximum temperature neutral flame
obtained at the inner cone which is about
3200oC.
 In this type flame all acetylene gas is
completely burned and all available heat in
the acetylene is released.
 It is most desirable flame to be used in oxy-
acetylene welding.

14. Neutral Flame
The reactions of the outer cone are given by
2CO + O2 2CO2
H2 + 1/2O2 H2O
The oxygen is supplied from the air.
The outer cone protects the molten metal from
oxidation reaction because oxygen in the
surrounding air is absorbed by gases from the
flame.

16. Carburising flame:
 A carburising flame is also called reducing
flame.
 It is obtained by more acetylene than oxygen.
 This flame has three zones
1. Sharp inner cone
2. White intermediate cone called feather cone.
3. Bluish outer cone

17. Carburising flame:
 The theoretical ratio of mixture
O2 : C2H2 = 0.85 to 0.95
 Length of the intermediate cone is an
indication of excess acetylene in the flame.
 It is suitable for bracing and flame hardening.
 The excess carbon causes the steel to
become extremely hard and brittle.

19. Oxidising flame
 It is obtained by supplying more oxygen than
acetylene.
 It is similar to neutral flame except the inner
white cone is somewhat small.
 It is giving higher temperature about 3300oC
 This flame has two zones
1. Smaller inner cone
2. Outer cone

20. Oxidising flame
The theoretical mixture of oxidizing flame is
O2 : C2H2 = 1.15 to 1.5
This flame is harmful for steels because it
oxidizes steel.
Oxidising flame is desirable for non ferrous
alloys such as copper and zinc base alloys.

21. .

22.  In arc welding process heat is developed by
an electric arc.
 An arc is a sustained electric discharge
through ionized gas column called plasma
between two electrodes.
 The arc formation is similar for both AC and
DC welding.

23.  The electrodes consists of baked carbon or
pure graphite which is inside a copper
blanket.
 The electrodes is not consumed as welding
progresses over time.
 However the electrodes eroded after prolong
usage.
 The average dimensions are 150 mm long,
diameter 5mm to 12.5 mm.

25.  In this welding metals are melted and joined
by heating them with an arc between
consumable electrode and work piece.
 The welding electrode consists of core metal
wire with an outer coating called flux.
 The flux assists in creating and stabilizing the
arc and provides shielding gas which
prevents the reaction of the molten metal
with atmosphere.
 The slag protects the welding from rapid
cooling.

26.  In this process, the work pieces and electrode
are melted by the arc and hence, both work
pieces become a single piece with out
applying external pressure.
 The temperature of arc is about 5000oC to
6000oC.
 The depth at which the metal is melted and
deposited is called depth of fusion.
 For better depth of fusion the electrode is
kept at 70o inclination to the vertical.

27. 1. Bare Electrodes
2. Lightly coated electrodes
3. Heavily coated electrodes.

29.  It is used where large quantity of flux is
required.
 It is supplied separately instead of using flux
coated welding rod.
 It is also called as hidden arc welding.
 The arc is completely submerged.
 The metal electrode is continuously fed from
a reel.
 The flux is supplied in front of the moving
head.

30.  The flux powder is made up of silica, metal
oxides and other compounds.
 Normally DC power is employed using 600A
to 1000A and AC power 200A.
 It is used to weld carbon steels and alloy
steels.
 Plates of 12 mm to 50mm can be welded with
one pass.

31.  Very high quality welds can be produced.
 Deep penetration can be obtained.
 Safe for eyes.
 No edge preparation needed.

32.  It is not suitable for inclined and vertical.
 Only specific metals can be welded.
 Health issues can be caused.
 Slag removal needed after welding.

33.  Heavy steel plate fabrication work.
 Pressure vessel, tanks, Nuclear reactors,
Chemical vessels.
 It is widely used in shipbuilding.

35.  In this welding the electric arc is produced
between an non-consumable tungsten
electrode and work piece.
 The inert gas from the cylinder is passed
through the nozzle of the TIG torch.
 The inert gas (Argon, Helium, Nitrogen and
CO2) surrounds the arc and protects the weld
from atmospheric effects.

36.  The tungsten has high melting point (3430oC)
and it will not melt during welding.
 The power supply through the column of
ionized gases.

37. TIG welding equipment
1. TIG torch
2. Power transformer
3. Shielding gas cylinder
4. Pressure regulator
5. Work clamp
6. Coolant system

38. Various functions of TIG torch
1. It holds the tungsten electrode.
2. It delivers welding current to the electrode.
3. It delivers the shielding gas.
4. Water cooling can be provided.

39. Advantages
1. It can be used for Aluminum, Stainless steel,
Copper alloys.
2. More suitable for thin sections.
3. It does not create much sputter and spark.
4. No flux is required.
5. The welding speed is high.

40. Limitations
1. It is generally restricted for flat and
horizontal welding.
2. It emits brighter UV rays.

41. Applications
1. Aluminum, magnesium, copper alloys can be
welded.
2. It can be used in atomic power plants,
aircraft industries.
3. Used in rocket motor fabrication.

43.  Consumable electrode wire having chemical
composition similar to the parent material is
continuously fed from a reel.
 The welding zone is surrounded by inert gas
such as Argon, Helium, CO2.
 The welding can be done manually or
automatically.

44.  Either DC generator or AC transformer is
used.
 The current ranges from 100A to 400A.
 This process is used for welding thick plates.
 It is used for welding aluminum, stainless
steel, nickel and magnesium without defects.

45. Advantages
 It is used for variety of ferrous and non
ferrous metals.
 No flux is required hence no slag to remove.
 Welding productivity is double as compared
to the shielded metal arc welding.
 High welding speed.

46. Limitations
 It cannot be used for vertical or overhead
welding.
 Process is more expensive.
 Contact tips may seize due to weld spatter.

47. Applications
 It is suitable for thin sheet sections. Hence it
is used for automobile repair works.
 It can be used to establish hard facing.

49. Types of Plasma Arc Welding
1. Non-transferred type
2. Transferred type

50. .

51.  Plasma is high temperature ionized gas.
 The power is directly connected with the
tungsten electrode and copper nozzle.
 Gas is fed into the nozzle and the arc is
generated between tungsten electrode and
nozzle.
 The gas get ionized as it pass through the arc.
 Due to the restricted shaped of the nozzle orifice
ionization is greatly increased.
 The temperature of about 10000oC can be
obtained.

53.  The restricting orifice is in an inner water-
cooled nozzle within which tungsten
electrode is centrally placed.
 Both work is connected to the anode and the
tungsten electrode is connected with the
cathode.
 Relatively low plasma gas flow is necessary to
prevent turbulence and disturbance of the
weld pool.

54.  It is difficult to initiate the first arc between
work piece and electrode.
 A pilot arc is struck between nozzle and
electrode. Thus the pilot arc touches the
work piece, main current starts flowing
between electrode and work piece. It ignites
the transferred arc.
 The temperature of a constricted plasma arc
may be in the order of 8,000oC to 25,000oC

55.  The flow of cooler gas through these orifices
squeezes the circular pattern of the jet in to
oval form.
 Since the tungsten electrode is well inside the
nozzle (about 3 mm) tungsten contamination
by touch down is avoided .

56. The following metals can be welded using PAW
1. Stainless steel
2. Titanium alloys
3. Carbon and low alloy steels
4. Copper alloys
5. Aluminum alloys

57. Advantages
1. Penetration is uniform. Deeper penetration
and narrower weld is possible.
2. Arc stability is good.
3. High accuracy weld can be obtained.
4. The production rate is high.

58. Limitations
1. Huge noise occur during welding.
2. Chances of electric hazards may occur.
3. It is limited to high thickness applications.
4. Frequent orifice replacement is necessary.
5. UV radiations is harmful.
6. The torch is delicate and complex than TIG
torch.

59. Applications
1. It is used in aerospace applications.
2. It is to weld high melting point metals.
3. It is used for welding titanium plates.
4. It is used for welding nickel alloys.
5. It is used for tube mill applications.

61. .

62.  Electroslag Welding is a highly productive,
single pass welding process for thick
materials in a vertical or closed to vertical
position.
 Heat is generated by an electric current
passing between consumable electrode and
work piece through the molten slag covering
the welding surface.
 In this process the coalescence is formed by
molten slag and molten metal pool.

63. Working
 Prior to welding the gap between two work
pieces is filled with a welding flux powder.
 The welding is initiated by an arc between
starting plate and electrode.
 Heat generated by the arc melts the flux
powder and it forms molten slag.
 Additional flux is added until the molten slag
reaches the tip of the electrode.

64. Working
 After molten slag reaches the tip of the electrode
the arc is extinguished.
 Heat is continuously produced by the electrical
resistance of the molten slag.
 The slag reaches the temperature of about
1930oC.
 Because the arc is extinguished electroslag
welding is not strictly arc welding process.
 Single of multiple solid as well as flux coated
electrodes may be used.

65. Working
 To contain the molten puddle, water cooled copper
shoes or dams are placed on the sides of the vertical
cavity.
 As the weld joint solidifies the dams move vertically
so as to always remain in contact with the molten
puddle.
 The electric current passes from the electrode to the
work piece through the slag pool.
 This welding is capable of welding plates with
thickness ranging from 50mm to more than 900 mm.
 The current required is about 600A at 40V to 50V.
 The travel speed is in the range from 12mm/min to
36 mm/min.

66. Advantages
1. Heavy thickness metals con continuously be
welded.
2. Stress formation is low.
3. Penetration of join is easier.
4. High deposition rate of up to 20 Kg/hr.
5. Distortion is low.

67. Limitations
1. It is difficult to weld cylindrical objects.
2. Hot cracking may occur.
3. Grain size becomes larger.
4. Toughness of the weld is low.
5. Only vertical position is possible.
6. The cost of equipment is high.

68. Applications
1. It is used to join low carbon steel plates.
2. It is used to weld thick sections.
3. It is used in large structural steel sections
such as heavy machinery, bridges, ships, and
nuclear reactor vessels.
4. Forgings and castings can be welded.