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Gas Welding
The welding process in which gases are used in combination to obtain a hot flame for welding operation is known as Gas Welding.
The commonly used gases for gas welding are acetylene, natural gas, and hydrogen in combination with oxygen. Oxyhydrogen welding was the first gas welding process to be commercially used. The maximum temperature developed by this process is about 2000° C. The most used combination these days is the oxyacetylene gas welding. It has a flame temperature of about 3500°C.
Principle of Gas Welding
Almost all gas-flame weldings are fusion welding, the coalescence of metals is produced by a gas flame. Extreme heat is concentrated on the edges or on the edge and surface of the pieces of metal being joined until molten metal flows together. The type of joint design determines whether a filler metal should be used to complete the weldment, Filler metal is added by inserting it into the molten puddle (or pool) of metals. The puddle then solidifies making the weld bead. proper technique and care are used, good quality welds can be obtained by the gas-flame welding. Control of the temperature of the work is easily done.
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However, exposure of the heated and molten to various gases in the flame and the atmosphere, without effective shielding, makes it difficult to prevent contamination.
Applications of Gas Welding
Gas flame welding now has virtually been replaced by other welding processes, except
repair work and a few special applications. The processes are specially adapted to welding of the sheet to flame hardening, and to the application of many hard-facing materials.
ADVANTAGES AND LIMITATIONS OF GAS WELDING
Advantages of Gas Welding
Some of the major advantages of gas welding processes are:
- Inexpensive Equipment: The equipment used for gas welding is comparatively inexpensive.
- Little Maintenance: The equipment requires little maintenance because of being a simple one.
- Portability: It is portable and can be used with equal facility out in the field and in the factory.
- Welding all metals: With proper technique, practically all metals may be welded.
- Other uses: The equipment for gas welding can be used for cutting as well as welding. It can also be used for the application of many hard surfacing materials such as satellite and colmonoy.
- Welding Sheet Metal: The process is specially adapted to the welding of sheet metal.
Limitations of Gas Welding
Some of the major limitations/disadvantages of gas welding processes are :
- Longer Time: It takes a considerably longer time for the metal to heat up than in arc welding. There is almost no instantaneous pool.
- Thermal Effects Aggravated: Harmful thermal effects are aggravated by prolonged heat and there is, in most cases larger heat affected area. This often results in increased grain growth, more distortion, and in some cases, loss of corrosion resistance.
- Careful Storing and Handling: The storing and handling of gases are to be done carefully as they involve safety problems.
- Less Effective Shielding of Weld: Flux applications and shielding provided by the oxy-acetylene flame are not so positive as those supplied by inert gas in TIG, MIG, or carbon dioxide shielded arc welding.
- Expensive Gases: Oxygen and acetylene gases are rather expensive.
Types of Gas Welding
The various types of gas welding in common use these days may be enumerated as follows:
- Oxy-acetylene welding
- Oxy-hydrogen Welding
- Air-acetylene Welding
- Pressure Gas-flame Welding.
when unspecified, the term 'gas welding' is usually understood to mean oxy-acetylene welding. This type of gas welding is the most commonly used one because it can give high temperature and its gases are easily available commercially.
Oxy-Acetylene Welding
The type of gas welding which utilizes the heat generated by an oxy-acetylene flame for produce weld is known as oxy acetylene welding.
There are two systems of oxyacetylene welding:
- Low-pressure oxyacetylene welding.
- High-pressure oxyacetylene welding.
Low-Pressure Oxyacetylene Welding
This oxyacetylene welding method utilizes low-pressure acetylene produced locally in the shop at low pressure in an acetylene generator. The oxygen used is derived from a high-pressure cylinder as usual. The rest of the equipment used is almost the same as for high-pressure welding with the difference of blowpipes which should be of injector type.
In the acetylene generator, acetylene is generated by the action of water on calcium carbide. The lumps of calcium carbide when dropped in the water inside the generator, the gas bubbles up through the water and collects in the upper portion of the generator under low pressure, slightly higher than atmospheric pressure. The remainder of the calcium carbide is converted into slaked lime. The reaction that takes place in an acetylene generator is as follows:
CaC2 + 2H2O = Ca(OH)2+ C2H2
Calcium Carbide + Water = Slaked Lime + Acetylene Gas
The calcium carbide used for making this gas is a hard, gray, stone-like material formed by smelting calcium with coal in an electric furnace. The various devices are incorporated in the generator to ensure safe operation.
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The welding technique, in both low and high-pressure methods, is the same. An oxyacetylene weld is obtained by heating with a flame from the combustion of acetylene in the presence of oxygen. The weld may be produced with or without the use of a filler metal rod depending on the type of joint and thickness of the base metal. In most cases, the joint is brought to a state of fusion by heating with the flame issuing from the oxyacetylene torch, and no pressure is applied to effect the joint.
Today low-pressure acetylene generator is rarely installed for production work. It is useful for small repair shops.
High-Pressure Oxy-acetylene Welding
In this method of oxyacetylene welding both oxygen and acetylene are derived for use from high-pressure cylinders.
For commercial use, acetylene and oxygen gases are supplied in compressed form in steel cylinders. The oxygen cylinder is generally painted "black', while that containing acetylene is painted 'maroon' for the distinction between the two gas cylinders. The fittings of the acetylene cylinder have left-handed screw threads for further distinction from the oxygen cylinder, which have right-handed screw-threads.
Before use, each cylinder must be fitted with an adjustable pressure regulator which incorporates a pressure regulating screw, safety valve, pressure gauge,etc. and with a flexible rubber-canvas hose for conveying the gases to the single common metal unit which incorporates, a nozzle, and is known as a 'blow Pipe or torch. This blowpipe possesses two further valves by means of which gases can be mixed or regulated as per requirement.
The parts to be welded must be "prepared” by thoroughly cleaning from scale, rust, and dirt. The edges of the parts to be welded must be properly shaped depending upon their thickness.
"Fluxes” are used for welding some metals, but are unnecessary for mild steel and wrought iron.
Owing to the injurious effect on the naked eyes from the intense white flame produced by oxygen and acetylene gases, dark-colored goggles must be worn. A leather apron should be worn to protect clothing against sparks etc.
Lighting the Torch
To light the torch, the acetylene control valve fitted to it should be turned on first. When acetylene gas issues from the nozzle it should be ignited with a spark lighter. It may be noticed that acetylene gas burns with a yellow-colored smoke-flame, in which event the valve should then be gently opened until flame just ceases to smoke. At this stage, oxygen control-valve should be opened and the supply of oxygen be increased gradually until a sharply defined "center cone” type flame is obtained.
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The oxygen control valve should now be closed down slightly until a faint “haze" appears around the center cone. The flame thus obtained is known as “neutral” flame. A neutral flame is efficient for normal welding of steel, cast iron, etc. An "oxidizing" flame is then obtainable by closing down the acetylene control valve on the torch body until the requisite flame is obtained. This type of flame is advantageous and necessary for welding brass. In case a "carburizing" flame is needed, then after obtaining a neutral flame, the acetylene control valve on the torch body is opened further till 'feather' like flame is obtained at the end of the central white cone.
Methods or Techniques of Welding
Two methods of v elding known as “leftward" and "rightward” welding are in common use.
Leftward welding
Leftward welding is employed for flanged-edge welds, unbevelled steel plates up to 3 mm thickness. and for beveled plates up to 5 mm thickness. Usually, leftward welding is adopted for cast iron and nonferrous metals.
Rightward welding
Rightward welding is recommended only for steel plates that exceed 5 mm thickness Plate edges from 5 mm to 8 mm need not to be beveled, but plates exceeding 8 mm in thickness should be beveled to 30° to provide an included angle of 60° for welding Vee.
The weld may be produced with or without the use of a filler metal depending on the type of joint and thickness of the base metal. Use a filler rod that has the same composition as the base metal. Use a red with a diameter equal to the thickness of the base metal. Move the torch just fast enough along the joint to keep the puddle active and flowing forward. When welding with a filler rod, preferably move the torch in a semicircular motion. Do not hold the rod too high above the pool so that the molten metal falls drop by drop onto the puddle.
Applications of Oxy-Acetylene Welding
Practically all metals can be welded with proper technique. The same equipment can be used for cutting as well as welding. The oxy-acetylene welding equipment is particularly used for welding sheet metal, flame hardening, and the application of many hard-facing materials.
Advantages and Limitations: The advantages and limitations described under gas welding equally apply to oxy-acetylene gas welding.
Types of Gas Torch Flames
There are three types of gas torch flames :
- Neutral flame
- Reducing or Carburizing flame
- Oxidizing flame.
Neutral Flame
It has two clearly defined zones:
- Inner zone-bluish luminous cone.
- Outer zone-long outer flame envelope.
It is also known as a balanced flame because it is produced when equal volumes of oxygen and acetylene die drawn from the cylinders. However. 1½ parts of oxygen are taken from the atmosphere.
This flame is most often used, as it allows the welding pool to remain quiet and clear, flowing along easily. A neutral flame is efficient for normal welding of steel, cast iron, etc.
Reducing Flame (Carburizing Flame)
It is produced by mixing less than 1 part of oxygen to 1 part of acetylene. There are three distinct zones in the flame :
- Innermost zone- a bluish-white inner cone
- Middle zone - a white intermediate cone
- Outer zone - an outer envelope with reddish feathers.
This flame can be used to advantage for hard-facing apparatus and for welding non-ferrous alloys such as nickel or monel metal. When this flame is used to weld steel, the metal boils and is not clear. This type of flame is sometimes used in silver-brazing operations to give a low-temperature soaking heat to the parts being joined by silver brazing.
Oxidizing Flame
It is produced when slightly more than 1 volume of oxygen is mixed with 1 volume of acetylene. This flame has two zones :
- Inner zone - a short pointed, bluish cone.
- Outer zone -- a short outer flame envelope.
This flame is also distinguishable by its hisșing sound.
This type of flame is advantageous and necessary for welding brass. When applied to steel, an oxidizing flame causes the molten metal to foam and give-off sparks.
Gas Welding Techniques
There are two distinct methods or techniques in the common use of employing a gas welding torch to make a weld.
- Leftward Welding or Forehand Welding.
- Rightward Welding or Backhand Welding.
Leftward Welding (Forehand Welding)
In leftward welding, the torch tip is held at approximately 160 to 70 degrees to the plates being welded. This method allows preheating of the plate edges immediately ahead of the molten pool, and this method is more commonly used. By moving the tip and welding rod in opposite directions perpendicular to the weld, the heat can be carefully balanced so as to melt the end of the welding rod and sidewalls of the plates in a uniformly distributed molten puddle.
Leftward welding is employed for flange-edge welds, unbevelled steel plates up to 3 mm thickness, and for beveled steel plates up to 5 mm. Usually, leftward methods are adopted for cast iron and nonferrous metals.
Rightward Welding (Backhand Welding)
Right-ward welding is done with the torch pointing back at the molten puddle at approximately 40 to 50 degrees to the plates being Welded.
Rightward welding is recommended mainly for heavy sections (which exceed 5 mm thickness) as it permits a narrower V at the joint. In general, it requires less puddling of the molten metal, and less welding rod is used.
Plate edges from 5 mm to 8 mm thickness need not be beveled, but plates exceeding 8 mm in thickness should be beveled to 30° to provide an included angle of 60° for the weld Vee.
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