CHEMISTRY OF OXY-ACETYLENE WELDING FLAME

-The maximum temperature of oxy-acetylene flame is 3100 - 3500 degree Celsius and the center of the heat concentration is just at the extreme tip of the white cone. Combustion of gas mixture is recognized as taking place in two main stages.

Stage 1 : Oxygen and acetylene in equal proportion by volume burn in the inner white cone.The oxygen combines with carbon of the acetylene and form carbon monoxide ,while the hydrogen is liberated.

2 C2H2 + 2O2  ---->  4CO + 2H2 

 Stage 2: Upon passing into the outer envelope  of the flame two more reactions takes place as combustion is completed. The carbon monoxide use the oxygen supplied from the air surrounding the flame and as a result of burning forms carbon-dioxide. The hydrogen also burns with the oxygen (from the atmosphere) and forms water-vapor (H2O).

4CO + 2H2 + 3O2  --> 4CO2 + 2H2O

Combining the above two equations 

2C2H2 + 5O2 ---> 4CO2 +2H2O

It can be seen that the above two-fifth of the oxygen necessary for the complete combustion of acetylene is got from the cylinder whereas the rest comes from the surrounding  atmosphere.

  

WELDING FLAMES

Neutral Flame:
-A neutral flame is produced when approximately equal volume of oxygen and acetylene are mixed in the welding torch and burnt at the torch tip.
-The temperature of the neutral is of the order of about 3260 degree Celsius.
-The flame has a nicely defined inner core which is light blue in color. It is surrounded by an outer flame envelop produced by the combination of oxygen in the air and superheated carbon monoxide and hydrogen gases from the inner cone.This envelop is usually a much darker blue than the inner cone.
-A neutral flame is named so because it effects no chemical change in the molten metal and therefore will not oxidize or carburize the metal.
-The neutral flame is commonly used for the welding at:
1) mild steel
2) stainless steel
3) Cast steel
4) Copper
5) Aluminum
Oxidizing Flame:
-If after the neutral flame has been established the Oxygen supply is increased to produce Oxidizing flame.
-Oxidizing flame can be observed by the small white cone which is shorter,more bluer and more pointed than the neutral flame.
-An oxidizing flame burns making a roaring sound.and is much hotter than the neutral flame due to excess of oxygen supply causing the temperature to rise up to 6800 degree  Celsius
-A slightly Oxidizing flame.is helping welding most metals like  (1) Copper-base metal  (2) Zinc-base metals
(3) A few types of ferrous metals such as manganese steel and cast iron.
-The Oxidizing atmosphere in these cases creates a base metal oxide that protects the base metal.
Reducing Flame:
If the volume of Oxygen supplied in the metal flame is reduced the reducing flame will be a carburising flame
rich in acatylene.
-A reducing flame can be observed by acetylene feather which exists between the inner cone and the outer envelope.The outer flame envelope is longer than neutral flame an is usually much brighter in color
-A reducing flame does not completely consumes the available Carbon, therefore the burning temperature is
lower and the left over carbon is forced into the molten metal. With iron and steel it produces very hard  and brittle substance known as iron carbide.This chemical change makes the metal unsuitable for many applications in which the weld may need to be bent or stretched. Metal that tend to absorb Carbon should not be welded with reducing flame.
-A reducing flame has an approximate temperature of 5500 degree Fahrenheit.A reducing flame may be distinguished from a carburizing flame in the fact that a carburizing  flame contains more acetylene than a reducing flame.
-A carburizing flame is used in the welding of lead  and for surface hardening process.
-A reducing flame on the other hand does not carburize the metal rather it ensures the absence of oxidizing condition. It is used for welding of low alloy steel and for those metals ( e.g non-ferrous) that do not tend to absorb carbon. This flame is very well used for welding high carbon steels.      
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TYPES OF WELDING FLAMES

To Ignite a Flame
Open the acetylene control valve of the welding torch and after the system has been flushed to clean the torch nozzle the gas is ignited. At this stage enough oxygen is drawn in from the atmosphere to burn acetylene partially.
The acetylene control valve is then adjusted until the flame ceases to smoke. The oxygen control valve of the welding torch is then opened in order to adjust the proportion in which acetylene and oxygen are required to mix and burn.
Types of Flame:

                                                                     fig-1

                                                                         fig-2

1-Neutral flame (Acetylene Oxygen in equal proportions)
2-Oxidising flame (Excess of Oxygen)
3-Reducing flame (Excess of Acetylene)

In oxy-acetylene welding flame is the most important tool.All the welding equipment simply serves to maintain and control the flame.The correct type of flame is essential for the production of satisfactory welds.
The flame must be of proper size,shape and condition in order to operate with maximum efficiency. 

GAS WELDING PROCESS AND EQUIPMENTS

Gas welding is a fusion welding processes which joins metals using the heat oh combustion of an oxygen/air and fuel gas (i e. acetylene,hydrogen,propane or butane ) mixture. The intense heat (flame) thus produced melts and fuses together the edges of the parts to be welded, generally wit the addition of filler metal.

OXY-ACETYLENE WELDING

When acetylene is mixed with oxygen wit appropriate proportion in the welding torch and ignited, the flame resulting at the tip of the torch is sufficiently hot to melt and join the parent metal.
The oxy-acetylene flame reaches a temperature of about 3200 degree Celsius and thus can melt all commercial metals during welding,actually flow together to form a complete bond. A filler metal rod is generally added to the molten metal pool to build up the seam slightly for greater strength.
Oxy-Acetylene welding does not require the components to be forced together under pressure until the weld forms and solidifies. 

PRACTICAL APPLICATIONS OF WELDING

Welding has been employed in industry as a tool for
(a) Regular fabrication of automobile cars, air-crafts, refrigerators etc.
(b)Repair and maintenance work, e.g. joining broken parts, rebuilding worn out components etc.
A few important applications of welding are listed below.

1. Aircraft Construction:
 (a)  Welded engine mounts
(b) Turbine frame for jet engine
(c)  Rocket motor fuel and oxidizer tanks.
(d0 Ducts, fittings and cowling components etc.

2 Automobile Construction:
 (a) Arc welded car wheels
(b) Steel rear axle housing.
(c) Frame side rails.
(d) Automobile frame, brackets etc.

3. Bridges:
 (a) Pier construction
 (b) Section lengths
 (c )  Shop and field assembly of lengthe etc.

4. Buildings:
 (a) Column base plates
 (b) Trusses
 (c) Erection of structures, etc.

5. Pressure vessels and tanks
 (a) Clad and lined steel plates
 (b) Shell construction
 (c) Joining of nozzle to the shell etc.

6. Storage tanks:
 (a) Oil, Gas and Water storage tanks.

 (7) Rail Road Equipment:
 Locomotive
(a) Under frame
(b) Air Receiver
(c) Engine
(d) Front and rear hoods etc 

8. Piping and Pipelines:
 (a) Rolled plate piping
 (b) Open pipe joints
 (c) Oil, Gas and Gasoline pipelines, etc

9. Ships:
 (a) Shell frames
 (b) Deck beams and Bulkhead stifeners.
 (c) Girders to shells
 (d) Bulkhead webs to plating etc.

10. Trucks and Trailers.
11. Machine tool frames, cutting tools and des
12. Earth moving machinery and cranes.

In addition to arc welding finds following application in repair and maintenance work.
 12. Repair of broken and damaged components and machinery such as tools, punches, dies, gears, shears,
       press and machine tool frames.
 13. hard facing and rebuilding of worn out or undersized parts rejected under inspection.
 14. Fabrication of jigs, fixtures, clamps and other work holding devices. 

COMMON WELDED BASE METALS

Metals can be classified as:
1) Ferrous
2) Non-Ferrous

Ferrous materials are the most important metals/alloys in the metallurgical and mechanical industries because of their very extensive use.
Ferrous materials finding day to day applications are:
1-Wrought Iron
2-Cast Iron
3-Carbon steels(Low,Medium and High Carbon Steels)
4-Cast Steels
5-Alloy Steels
6-Stainless Steels
Non-Ferrous materials are those that are not iron based.
Like ferrous materials non-ferrous materials also find extensive industrial applications.
Non-ferrous materials finding day to day welding applications are:
7-Alluminium and its alloys.
8-Copper and its alloys
9-Magnesium and its alloys.
10-Nickel and its alloys
11-Zinc and its alloys.

CLASSIFICATION OF WELDING PROCCESS

-There are about 35 different welding processes and several soldering methods in use by industry today.
-There are various ways of classifying the welding and allied processes and may be classified on the basis of:
1) Source of heat,flame,arc etc.
2)Types of interaction i.e liquid/liquid (fusion welding) or solid/solid (solid state welding).
-In general various welding and allied processes are classified as follows.
A) Gas Welding                                  C)Resistance Welding
-Air acetylene welding                         -Spot Welding
-Oxy -acetylene welding                      -Seam Welding
-Oxy - Hydrogen welding                    -Projection Welding
-Pressure Gas welding                         -Resistance Butt Welding
B) Arc welding                                    -Flash Butt Welding
-Carbon arc welding                            -High Frequency Resistance Welding
-Shielded metal Arc Welding                D)Solid State Welding
-Flux Cored Arc Welding                     -Cold Welding
-Subnerged Arc Welding                      -Explosive Welding
-TIG Welding                                       -Friction Welding
-MIG Welding                                      -Roll Welding
-Plasma Arc Welding                            -Diffusion Welding
Electro slag Welding                              -Forge Welding
Electric Gas welding                              -Hot Pressure Welding
-Stud Arc Welding                                -Ultrasonic Welding

E)Thermo-Chemical Welding Processes
-Thermal Welding
-Atomic Hydrogen Welding

F)Radient Energy Welding Processes
-Electro Beam Welding
-Laser Beam Welding
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Welding-A Fabrication Process

Welding is a method for joining different metals.The large number of materials that are welded are metals and their alloys, although the term welding refers to the joining of other materials such as thermoplastics.
Welding joins different metals alloys with the help of a number of processes in which heat is supplied either electrically or by a gas torch.
In order to join two or more pieces of metals together the most essential requirement is heat.Pressure may also be employed, but this is not essential in many processess.
The use of welding in today's technology is extensive. It had a phenomenal rise since 1930; this growth has been faster than the general industrial growth. Many common everyday use items e.g. automobiles,aircraft,ships,electronics equipments,machinery,household appliances etc, depends upon welding for their commercial construction.