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Essay on Arc Welding
Essay Contents:
- Essay on the Introduction and Principle of Arc Welding
- Essay on the Characteristic of Arc Welding
- Essay on the Methods of Arc Welding
- Essay on the Power Supply for Arc Welding
- Essay on the Electrodes in Arc Welding
- Essay on the Types of Arc Welding
- Essay on the Equipment Employed for Arc Welding
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Essay # 1. Introduction and Principle of Arc Welding:
The process of arc welding has replaced the ordinary methods of bolting and riveting to a large extent. It is used for the repair of the damaged castings, deposition of metal, welding of steel plates, construction of ships pipelines, bridges, welding of tank plates, steel frames and machine parts made of plates etc. In arc welding heat is produced from an arc struck between an electrode and metal to be welded. An arc is a spot of electric discharge.
The principle of arc welding is the production of heat due to an arc struck between the welding electrode and metal to be jointed. The heat of the arc which produces temperature around 3500°C is sufficient to melt the metal in a localised space. A filler material is also supplied to the weld as the local fusion takes place. The composition of this filler material depends upon the type of work. The heat developed by arc is also used for cutting of metal.
The voltage necessary to strike an arc is higher than that required maintaining it because some voltage is required to ionize the air gap also. The maximum safe voltage specified for welding is about 100 volts and current ranges from 30 to 500 A for manually operated welding sets and 75 to 600 A for automatic operated sets.
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The arc welding may be used for producing lap joints, butt joints, Tee joints or corner joints.
Essay # 2. Characteristic of Arc Welding:
The welding arc has a –ve resistance characteristic i.e. its resistance decreases with the increase of current. Due to this characteristic it is impossible to maintain a steady arc. Thus, if the process is allowed to accumulate, it may result in a virtual short circuit. So this is a need for the production of a stabilized arc. For the production of the same it is necessary that the welding transformer (For A.C. welding) or the D.C. welding generator must have drooping characteristics which means that the voltage of the set falls down appreciably with the production of arc.
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The arc welding process is widely used for the joining of metal parts instead of riveting or bolting; the repair of fractured casting and the filling in, by the deposition of new metal, of worn out parts.
The arc welding may be carbon arc welding or metal arc welding. Again, the metallic welding electrodes may be bare electrodes or coated electrodes. Coated electrodes have many advantages over bare electrodes.
Essay # 3. Methods of Arc Welding:
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The following are the methods of arc welding:
1. Carbon arc welding
2. Metallic arc welding
3. Atomic hydrogen arc welding.
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(1) Carbon Arc Welding:
Arc is struck between a carbon electrode and the pieces to the joined. This method is commonly used for welding copper, brass and their alloys. The electrode is made of carbon and is kept negative and job to be welded is kept positive. If the electrode is make positive with respect to the job, carbon particles, being positive will go to the weld and make it brittle.
For this type of welding only D.C. can be used. The current is around 800 amperes. The heat of the arc melts the parts of the job to be welded and filler material is provided by a separate electrode (called filler rod) to make the weld. It was one of the earliest welding processes but it rarely used now a days.
For welding non-ferrous metals there is no need of using flux but for the welding of ferrous metals flux in the form of powder or paste is used to prevent the weld from oxidation.
(2) Metallic Arc Welding:
In metallic arc welding a metal rod of the same material as being welded forms one of the electrode and also serves as filler and no filler rod is used. The electrode itself is used as a filler material. The composition of the electrode will, therefore, depend upon the materials to the welded.
This process is suitable for welding iron and steel. For d.c. supply 50-60 volts and for a.c. supply 70-100 volts are used for welding. To ensure good welding an un-interrupted supply of current and correct voltage is essential.
The secondary of the welding transformer or the supply from the d.c. welding generator depending upon whether a.c and d.c. is used, is connected across the job and the electrode. The electrode is suddenly touched to the work and separated a little. Due to this an arc is struck between the job and the electrode to melt. The two pieces to be welded fuse together and when electrode is removed, they cool giving strongly welded joint.
The chief applications of metallic arc welding is for the welding of iron and steel for which electrodes of low carbon contents are generally suitable.
Electrodes for Metallic Arc Welding:
The chief application of metallic arc welding is for the welding of iron and steel, for which an electrode of low carbon content (not more than 0.2 percent) is generally suitable. Certain high carbon steels, however, require electrodes containing upto 1 percent carbon.
Bare or covered electrodes may be used. Covered electrodes have a layer of a compound containing salts of silicon, calcium, magnesium or other basic substance, which forms a slag on the metal and prevents oxidation and also chilling of the metal. Modern practice is to use covered electrodes.
(3) Atomic Hydrogen—Arc Welding:
In this arc welding process an arc is maintained between two non-consumable tungsten electrodes with a stream of hydrogen gas, under a pressure of about 0.5 kg/cm2 is passed through the arc and around the electrodes. Filler rod and pressure may or may not be applied depending upon the requirement.
The process combines gas welding with electric arc welding. In this method electrode holder consists of two electrodes arranged in inclined position and the hydrogen nozzle is in between the tips of these electrodes to eject the hydrogen. The electric arc between the electrodes breaks-down the molecular hydrogen into atomic hydrogen.
This atomic hydrogen, when touches the relatively cold metal, recombines into molecular hydrogen, thus liberating considerable heat, which melts the metal to be welded. By this process smooth uniform, strong and ductile welds can be produced.
Arc currents upto 150 A can be used and the power supply arrangements are similar to for ordinary arc welding. In this type of welding transformer voltages are higher, being about 300 V on open-circuit for striking arc and between 80 and 100 V while operating.
The equipments required are:
(i) Welding torch with tungsten electrode.
(ii) A-C power source.
(iii) Hydrogen gas supply through cylinder.
Operation:
Power is switched on and supply of hydrogen gas is released. The arc is struck by touching the electrodes momentarily and then separating them by a distance of 1.5 mm the value of current can be adjusted according to job thickness. The atomic hydrogen welding arc is held over the job to be welded till a molten pool forms. Depending upon the job thickness, the filler metal may or may not be added.
Advantages of Atomic Hydrogen Welding:
(i) Smooth, uniform, strong welds can be obtained.
(ii) The job is separate from the electric circuit.
(iii) Cooling of the electrodes is done by hydrogen gas.
(iv) High heat consideration is available.
Disadvantages of Atomic Hydrogen Welding:
(i) Less welding speed.
(ii) More operating cost.
Applications:
(i) This method is used for welding stainless steel, non-ferrous metal etc.
(ii) It is used in die, tools repairs, welding of thin sheets etc.
Essay # 4. Power Supply for Arc Welding:
For electric arc welding, both d.c. and a.c. are used. The electric properties of the arc determine the requirements that should be met by source of power supply for arc welding. Higher voltage is required than to maintain it under normal conditions to initiate an arc. For this reason the open circuit voltage of the power supply source must be higher than voltage remain during arc.
With d.c. open-circuit voltage must be at least 35 V, while with a.c. it should not be low than 55 V. An open arc will be sustained at 15 to 25 V. A power supply source for welding should be able to limit the surge of current, when the arc is initiated to a small percentage above the desired value. The power supply source should be able to restore the voltage quickly after a globule of molten metal has broken away from the electrode.
To sum up, the power supply for manual arc welding should meet the following requirements:
1. The open-circuit voltage must be sufficient for an arc to strike without difficulty but safe for operator i.e. below 80 V.
2. The short circuit current must be within safe limits for transformer windings.
3. The wattage of the source of power supply should vary rapidly with change in arc length.
4. The voltage of the source of power supply should vary rapidly with changes in arc length.
Essay # 5. Electrodes in Arc Welding:
An electrode is a piece of wire or rod with or without flux covering, which carries current for welding. At one end it is gripped in a holder and arc is set up at the other end.
Types of Welding Electrodes:
The electrodes are used to provide heat in arc welding. Electrodes can be classified on the following basis:
(a) Consumable or non-consumable
(b) Bare or coated electrodes
(a) Consumable Electrodes or Non-Consumable:
When the arc is produced by the consumable electrodes, the weld metal and the tip of the electrode also gets melted under the arc. The molten metal from the electrode and the base gets mixed under the arc and provides the necessary joint.
So in this process, the electrode is continuously consumed. Thus the electrode is serving the purpose of filler rod as well as the heat input to the joint. The material used for the consumable electrodes may be made of various metals depending upon their purpose and metals to be welded.
Non-Consumable Electrodes:
These electrodes are not consumed during welding process. Separate filler rods if required can be used to fill the gap along the joints. Material used for non-consumable electrodes are made of carbon, graphite or tungsten. Non-consumable electrodes are used in carbon arc welding or tungsten inert gas welding (TIG).
(b) Bare Electrodes and Flux Coated:
These are ordinary metal electrodes, without a coating of flux. These are used for welding wrought iron (W.I.) and mild steel (M.S.), when the strength needed is small.
Flux Coated Electrodes:
These are metal electrodes with a coating of flux. The coating may be of compounds of calcium, silicon and magnesium. With these electrodes, external flux is not needed. This is why these are very much in use nowadays.
Advantages of using Flux Coated Electrodes:
The coated electrodes used in arc welding have the following advantages:
(i) It helps in maintain or stabilising the arc by the flux.
(ii) It helps in shielding the molten material from oxidisation.
(iii) It helps in preventing the weld from sudden cooling.
(iv) A coating of slag is formed that removes certain impurities along with as it floats on the surface.
The coating may be used to add another material to the weld. To obtain good welds, coated electrodes are preferred.
Essay # 6. Types of Arc Welding:
i. Tungsten Inert Gas (TIG):
In this arc welding process electric arc is struck between non-consumable tungsten electrode and work piece. A shielding is provided by argon, helium or carbon dioxide to avoid atmospheric contamination of the molten weld pool. Filler rod may or may not be used. Filler rod may or may not be used.
It is in all position welding technique. A.c. as well as d.c. supply can be used. Electrodes used in TIG welding are non-consumable tungsten electrodes which vary in size from 0.15 to 9.5 mm. The electrodes are mounted in a special electrode holder. This holder is also designed to furnish a flow of inert gas around the electrode and also around the arc.
Welding operation is done by striking an arc between the work piece and tungsten electrode in an atmosphere of inert gas like argon or helium. Inert gases prevent oxidation. The arc is struck either by touching the electrode with a scrap metal tungsten piece or using a high frequency unit.
After striking the arc, it is allowed to impinge on the job and a molten weld pool is created. Filler metal, if necessary should be added by dipping the filler rod in the weld pool. When doing so, the tungsten electrode should be taken a little away from weld pool. Fig. 4.16 shows the main parts of the tungsten inert gas welding.
Advantages:
This type of welding has the following advantages:
(i) An inert gas may be argon or helium prevents oxidation and flux is, therefore, not needed.
(ii) Heat can be concentrated and a distortion free weld can be obtained.
(iii) Clear visibility of the arc, so better control.
(iv) It can weld in all position.
(v) Heat affected zone is very less
(vi) High quality welding of thin materials can be done.
Disadvantages:
1. Costly equipments are required.
2. Non-consumable electrode is used, therefore separate filler rod is required, and so output of the work is decreased. Moreover the process is however slow in operation.
Applications:
1. This welding method is used for welding stainless steel, aluminium magnesium and their alloys.
2. Welding of sheet metals.
3. Used in aircraft and instrument industries.
ii. Metal Inert Gas (MIG) Welding:
In the metal Inert gas welding arc is produced between a continuously fed consumable wire and workpiece. The arc generated is shielded by inert gas like argon, helium, carbon dioxide. About 15 mm of electrode is projected from torch before striking the arc. However, during welding process, torch remains about 10 mm away from the workpiece. The arc length is kept between 2 to 4 mm. The electrode wire is fed from a coil with a constant speed.
During welding process, the torch is kept at an angle of 70° from horizontal and is moved along the joint in a steady manner. Metal transfer in MIG welding may be of globular, spray type. A.C is not preferred because of unequal burning in positive and negative half cycles.
Generally d.c arc welding machines are used for MIG welding with electrode positive. Moreover power source with flat characteristics is preferred over another having drooping characteristics because for same change in arc length, there is bigger change in arc current.
Advantages:
1. Continuous welding at greater speeds and in all positions is possible.
2. The process can be easily put into operation.
3. All types i.e. thick or thin work pieces can be welded effectively.
4. As no flux is used so more visibility and neatness of weld is obtained.
Essay # 7. Equipment Employed for Arc Welding:
Both A.C. and D.C. are employed for electric welding. In case of A.C. arc, there is a tendency for it to cool when current is zero in each cycle and in order to avoid this drawback electrodes are covered. The voltage required to strike the arc is between 50 to 60 volts in case of A.C. The D.C. is preferred from the point of view of safety against shock to the operator. For maintaining the arc, a voltage of 20 V to 30 V is required.
The resistance of the arc decreases as the arc current increases. However, if the voltage of the welding set has a dropping characteristic this effect can be nullified. In other words the supply voltage must be such that it falls rapidly with the increase in current if a steady arc is to be obtained.
i. A.C. Welding Equipment:
Step down transformer is used to give low voltage supply. The stable arc or current control is obtained by connecting a reactor in series with the secondary of the transformer. As soon as the arc current increases, the voltage drop in the reactor also increases thus reducing the arc current.
The voltage on open circuit is about 100 volts. The resistance reduces the power factor and is costly in first cost. The use of resistance can also be made for current control but efficiency is reduced. Refer to Fig. 4.14 is a welding transformer. The P.F. of transformer is very poor, which is nearly 0.35 lagging due to the use of reactance.
ii. D.C. Welding Equipment:
It consists of mainly a motor-generator set, the motor is a required cage induction motor and generator is differential compound to give dropping characteristic. In differential compound generator the terminal voltage falls automatically with the increase in load current. Open circuit voltage of welding transformer is from 80 V to 100 volts and power factor is poor about 0.35 lagging.
The most commonly used equipments for arc welding are:
1. A.C. or D.C power supply and welding set source
2. Electrode
3. Electrode Holder
4. Cable, Cable connectors
5. Chipping Hammer
6. Cable lug
7. Wire brush
8. Earthing clamp
9. Helmet
10. Safety goggles/screen
11. Hand gloves
12. Aprons
Specifications of A.C Arc Welding Machine:
Sample Specifications:
(i) Current = 200 Amps
(ii) Voltage on load = 40 volts
(iii) Duty cycle = 60%
(iv) Minimum current while welding = 40 A
(v) Maximum current at rated load voltage = 250 A
(vi) Load voltage = 20 V
(vii) Duty cycle at maximum current and rated load voltage = 35
Sample Rating for a D.C. Motor Generator Welding Set:
(i) Current = 300 A
(ii) Load voltage = 40 V
(iii) Duty cycle = 60%
(iv) Minimum welding Amps = 60
(v) Maximum current at rated voltage = 375 A
(vi) Duty cycle at maximum current and rated voltage = 35%
Preparation of Work:
There are several factors responsible while preparing work for welding in order to obtain good results.
1. Provision for expansion and contraction may be made to get good result.
2. Cleaning of surface is done with the help of wire brushing. If impurities present on the weld place, it tend to make the joint weaker.
3. Material to be added for weld should as small as practicable.
Safety Measures:
1. If possible, alternating current (a.c.) may be used instead of d.c.
2. The length of arc and current should be properly maintained.
3. The ground cable may be wrapped around work piece such that the current flowing in it sets up a magnetic field in a direction, which will counteract the arc below.