gas welding

1. Welding Processes

2. Fabrication Process
• Joining of 2 metals in order to make into single component.
• Types of Joints:
1. Temporary Joints – Nuts, Bolts, etc,.
2. Semi-permanent Joints – Riveted joints etc,.
3. Permanent Joints – Welded Joints.

3. Welding Process
 Welding is a materials joining process
which produces coalescence of materials
by heating them to suitable temperatures
with or without the application of
pressure or by the application of pressure
alone, and with or without the use of filler
material.
Example:
• Butt joints, Lap joints, Cotter joints, Edge joints etc,.

4. Advantages:
• Permanent joint is produced, which becomes an integral part
of work piece.
• Joints can be stronger than the base metal if good quality filler
metal is used.
• Economical method of joining.
• It is not restricted to the factory environment.

5. Disadvantages:
• Labor cost is high as only skilled welder can produce sound
and quality weld joint.
• Creates the problem in dissembling if the sub-component is
required.
• Hazardous fumes and vapours are generated during welding.

6. • Applications of welding processes
General applications:
• Fabrication of pressure vessels, LPG cylinders, Transport
tankers (oil, water, milk, chemicals), bridges, building
structures, aircraft and space crafts, railway coaches,
electrical, electronic and defense industries, laying of pipe
lines and railway tracks and nuclear installations.

7. Specific Applications:
• Automobiles (Resistance welding),
• Rail Joints in railways (Thermit welding),
• Aerospace and nuclear reactors (TIG welding),
• Ship work (Submerged arc welding),
• Joining of metals like stainless steel, aluminum and
magnesium (MIG welding).

8. Types of Welding processes
• Plastic Welding or Pressure Welding
The piece of metal to be joined are heated to a plastic state
and forced together by external pressure.
Example:
Forge Welding, Spot Welding, Seam Welding, Projection
Welding, Butt (or) Upset, Thermit, Friction, Ultrasonic Welding
etc,.

9. • Fusion Welding or Non-Pressure Welding
The material at the joint is heated to a molten state and
allowed to solidify.
Example:
Electron Beam Welding, Laser Welding, Soldering, Brazing, Arc
Welding, Gas Welding, Resistance welding.

10. Classification of welding processes
• Gas Welding
• Oxy-acetylene
• Air-acetylene
• Oxy-hydrogen
• Arc welding
 Carbon arc
 Metal arc
 Metal inert gas
 Tungsten inert gas
 Plasma arc
 Submerged arc
 Electro-slag
• Resistance Welding
• Butt
• Spot
• Seam
• Projection
• Percussion
• Thermit Welding
• Solid State Welding
• Friction
• Ultrasonic
• Diffusion
• Explosive
• Newer Welding
• Electron-beam
• Laser
• Related Process
• Oxy-acetylene cutting
• Arc cutting
• Hard facing
• Brazing
• Soldering

11. Gas Welding
• A group of welding processes where in coalescence is
produced with a gas flame with or without use of pressure
and filler material.
• The temperature generated during the process is 33000c.

12. Gases
• Oxyacetylene welding process uses two principal gases
• Oxygen and acetylene
• Other gases can be used for cutting and heating
• Propane
• Natural gas
• Mapp® gas
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13. Various Fuel Gas Efficiencies
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Normal
Btus Usable heat Flame Oxygen velocity
Fuel Gas (ft3) (s/ft2) temp. per ft3 of fuel (ft/s)
Acetylene 1433 12,700 5,420ºF 1.04 17.7
Mapp®2 2381 5,540 5,301ºF 2.4 7.9
Propane 2309 5,500 5,190ºF 4.00 11.9
Natural gas3 918 5,600 5,000ºF 1.50 15.2
Hydrogen 275 7,500 4,600ºF 0.25 36

14. Acetylene
• Most widely used of all fuel gages
• Both welding and cutting
• Generated as result of chemical reaction that takes place when
calcium carbide comes in contact with water
• Tests show oxyacetylene flame temperatures up to approximately
6,300ºF
• Very rapid rate of preheating
• Burns with smoky flame, gives off carbon, has peculiar odor
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15. Characteristics of an Effective Welding
Fuel Gas
• High flame temperature
• High rate of flame propagation
• Adequate heat content
• Minimum chemical reaction of flame with base and filler metal
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Acetylene most closely matches all these
requirements and used for welding purposes.

16. Oxy – Acetylene Welding
Equipment
• Oxygen Cylinder,
• Acetylene Cylinder,
• Pressure Regulators,
• Pressure Gauges,
• Welding Torch (or) Blow Pipe,
• Hose & Hose Fittings,
• Nozzle (or) Tip
• Goggles,
• Gloves,
• Spark Lighter,
• Chipping Hammer,
• Wire Brush.

18. • The oxyacetylene welding process uses a combination of
oxygen and acetylene gas to provide a high temperature
flame.
• Acetylene produces high temperatures of about 32000C.
• The chemical reaction involved in burning of acetylene is
2C2H2 + 5O2 → 4CO2 + 2H2O + Heat

19. • During the welding, heat from the flame is concentrated on
the joint edges until the metal melts and starts to flow.
• When the molten metal from both sides melts it starts to fuse.
• when the metal cools down the two parts become
Permanently joined.

20. • Acetylene can be supplied in two ways:
1. High pressure system.
2. Low pressure system.
High Pressure Welding
• oxygen and acetylene are stored at high pressure from their
respective high pressure cylinders.
• Oxygen is normally stored in strong cylinder at a pressure
ranging from 13.8 to 18.2 MPa.

21. • Acetylene is highly explosive, if stored at a pressure more than
200 kPa, it becomes very unstable and likely to explode.
• Hence acetylene must be carefully stored in a strong cylinder
filled with 80 to 85% of porous material [Calcium Silicate] and
then filled with acetone.

22. • Acetone can absorb up to 420 times of its volume of acetylene
at a pressure of 1.75 MPa.
• This helps in storing acetylene at a much higher pressure than
permitted when it is in the free form.

23. Low Pressure Welding
• In low pressure welding, oxygen is supplied from high pressure
cylinder but acetylene is generated by the action of water on
calcium carbide and supplied at low pressure.
• CaC2 + 2H2O →C2H2 + Ca (OH)2

24. • Acetylene Generator

25. • It consists of a cylinder partially filled with water.
• The calcium carbide is stored in a hopper at the top of the
generator.
• When the calcium carbide reacts with water acetylene is
formed.
• The produced acetylene is supplied to the blow pipe at a low
pressure from a gas holder incorporated in the generator.

26. • Acetylene is cleaned by passing it through a purifier.
• A pressure regulating valve controls the flow of calcium
carbide into water, depending on the requirement of the
pressure of acetylene in the generator.
• To prevent the possibility of explosion (or) air blowing back
into the generator, a back pressure valve is arranged between
the blow pipe and the gas holder.

27. Safety Precautions in Gas Welding
• Always handle the gas cylinders with care.
• The adjusting screw on the regulator must be fully released
before opening a cylinder valve.
• Never use matchsticks for lighting a torch.
• Never lubricate the regulator valve with oil or grease, it may
cause explosion.
• Always use goggles while working.

28. • Acetylene cylinders should be stored in up right position.
• Do not open acetylene cylinders near sparks or fire.
• Never remove torch tips with pliers.
• The cylinder should be leak proof.
• Always use protective caps over the valves.
• Keep in mind the location of the fire extinguishers.

29. Types of Flames
• The type of flame produced depends upon the
ratio of oxygen to acetylene in the gas mixture
which leaves the torch tip.
• There are three distinct types of oxy-acetylene
flames, usually termed:
• Neutral
• Carburizing (or “excess acetylene”)
• Oxidizing (or “excess oxygen”)

30. Neutral Flame:
• Approximately equal amounts of O2 & C2H2 are burned (1.01
to 1.14).
• No chemical effect on the metal being welded.
• It will not oxidize the weld metal;
• it will not cause an increase in the carbon content of the weld
metal.
• Recognized by two sharp zones.
1. Inner white cone,
2. Outer blue flame.

31. • Two thirds of the heat is generated in the inner core.
• The products of the primary combustion, CO and H2, react
with O2 from the surrounding air and form CO2 and H2O.
• One third of the heat is generated in the outer flame.
• The area where this secondary combustion takes place is
called the outer envelope.
• Flame temperatures can reach upto 3,2000C.

33. Carburizing Flame
• It is formed when C2H2 is supplied more than O2.
• The approximate ratio of O2 to C2H2 is 0.82 to 0.95.
• Recognized by three zones.
1. Inner white cone,
2. Outer blue flame,
3. Intermediate flame feather.
• Length of intermediate flame indicates presence of excess
acetylene.

34. • Unburnt carbon present in it goes into the weld pool and the
metal appears to boil.
• Used for high carbon steels, cemented carbides etc,.
• Flame temperature can reach upto 32000C.

35. Oxidizing Flame:
• It is formed when O2 is supplied more than C2H2.
• The approximate ratio of O2 to C2H2 is 1.15 to 1.17.
• Recognized by two sharp zones.
1. Inner white cone with sharp edge,
2. Outer blue flame.
• It will oxidize or ”burn” some of the metal being welded.
• Used for non-ferrous alloys.
• Highest temperatures upto 34000C can be achieved.

37. OXY-ACETYLENE WELDING
TECHNIQUES
On the basis of torch movement there are two types of
techniques.
1. Leftward / Forward Technique.
2. Rightward / Backward Technique.

38. LEFTWARD / FORWARD
TECHNIQUE
• Torch moved from right to left.
• Blow pipe – right hand.
• Welding rod – left hand.
• Blow pipe angle of 600 to 700.
• Welding rod angle of 300 to 400.
• Preheats the metal ahead of the molten pool.

40. RIGHTWARD / BACKWARD
TECHNIQUE
• Torch moved from left to right.
• Blow pipe – right hand.
• Welding rod – left hand.
• Blow pipe angle of 400 to 500.
• Welding rod angle of 300 to 400.
• Cooling time is high.
• Welding speed increases by 20-25%.
• Fuel consumption decreases by 15-25%.
• Used for thicker materials.

42. Advantages of OAW:
• Equipment is cheap as compared to other welding process.
• It can be used for welding of all types of metals.
• Maintenance of equipment is very less.
• It is a portable process.
• It can be used for cutting metals of small thickness.
• It is specially used for sheet metal work.

43. Disadvantages
• It takes long time for heating the job as compared to the arc
welding.
• The heat affected area is more.
• This is prone to corrosion and brittleness.
• Gasses are expensive and difficult to store.

44. Applications
• Oxy-acetylene welding is particularly used for sheet metal
work.
• All the metals can be welded with proper filler metals.
• Same equipment may be used for cutting purposes.