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In India nearly 90% of the welded fabrication is done by this process and even in the most advanced countries like USA, USSR, Japan, and the west European countries it accounts for nearly 60% of the metal deposited by welding machine. Though its use is slowly decreasing but it is expected to remain indispensable for repairs and short-run jobs. One of its attractive features is the lowest initial cost for a workable installation. Welding power source for SMAW welding machine or arc welding machine are available which can be plugged-in, if required, in domestic single phase electric supply, hence its popularity even with small volume fabricators.
The major welding equipment for SMAW is the power source which may be a welding transformer, a dc rectifier or dc motor-generator set. The selection of equipment depends upon the provision for initial investment and the range of the materials to be handled. The size and type of electrodes that are used and the penetration and welding speeds desired determine the current supply requirements. The welding power sources employed for SMAW welding machine are almost invariably of the constant current type as they serve the purpose best in maintaining the arc current undisturbed even when the welder’s hand is inadvertently disturbed through temporarily.
Of the three types of welding power sources each one has its own definite advantages. The dc welding power source is very versatile in welding a variety of metals in any desired thickness. It permits portable operation and uses efficiently a large variety of coated electrodes. The welding transformer has the lowest initial cost as well as low operation is quite. The dc rectifier welding power source is simple in design and it combines the advantages of a welding transformer and a dc welding set.
Welding Equipment Accessories
The welding equipment accessories for the welding power source include the connecting cables or leads, an electrode holder, cable connectors and the ground clamp. The cables that carry the current in welding circuit are quite flexible and are generally made of copper or aluminum wires. These wires are very find (0.2 mm diameter) and number between 800 to 2500 depending upon the current carrying capacity of the cable. Aluminum cables are much lighter and weigh only one-third of copper cables but their current carrying capacities are also lower being about 60% that of copper cables.
Electrode holder: Electrode holder is generally matched to the welding cable and the cable size depends upon the current required to be carried in the welding circuit. Usually electrode holders are specified depending upon the current that they can carry; the normal range being 150 to 500A. The electrode holders of the popular design have grooves cut in the jaws which facilitate the holding of electrode at different angles for easy manipulation.
The ground clamp is used to connect the other terminal of the welding circuit. It sometimes resembles the electrode holder but often it is like a C clamp is fitted tightly to the work table to avoid sparking, however most often it is rather loosely attached to facilitate easy detachment.
Bare welding wires and rods are used in short lengths of about 1 meter or in coiled form in spools. Whereas short lengths are used for processes like tig welding machine and plasma arc welding machine wherein they are not part of the welding circuit, long wires are employed for processes like mig welding machine and saw welding machine where a part of the wire conducts current. When a welding wire forms a part of the electrical circuit is called a welding electrode otherwise it is referred to as a welding rod.
Most wires used for welding structural steel usually contain 0.10% carbon and 0.35 to 0.60% manganese content. Some other types have increased amounts of carbon, manganese and silicon.
Excess silicon in welding wire results in heavy spatter, gassing in the weld pool, and non-metallic materials in the weld metal. Maximum silicon content permitted, therefore, is upto 0.95%.
The contents of harmful impurities like sulphur and phosphorous should not exceed 0.04% each. In some wires, particularly those used for welding alloy steels the maximum amount of sulphur and phosphorous allowed is each 0.03% each.
The range of wire diameter extends from 0.5 to 2.5 mm with 0.5, 0.6, 0.8, 0.9, 1.0, 1.2, 1.6, 2.0, 2.4 and 2.5 mm diameter wires being normally available. Welding machines use continuous wires in coils. Depending on the wire diameter, a coil may weigh anywhere between 5 to 500 Kg and measure 150 to 1000 mm across.
The welding wires are usually copper coated to prevent rusting and to improve current pickup from the contact tube, it also helps during drawing of wires through dies. To avoid harmful effects and peeling of copper coating it is usually kept very thin and the maximum amount of copper is specified at 0.4% by weight of the wire.
Apart from low carbon steels, welding wires are also produced from stainless steels, aluminum and its alloys, nickel alloys, magnesium alloys, titanium alloys, and copper alloys.
The welding wires are available both in solid and tubular forms, the latter contains flux in it.
Specifications for Solid Wires and Rods
Several systems are in use to specify welding electrodes and rods. AWS specification is one of the well known systems of codification. It consists of a prefix letter or letter S and then a suffix which may be figure or a letter or both.
|E||a welding electrode|
|R||a welding rod|
|RB||a welding rod/brazing filler|
|ER||an electrode or a welding rod|
AC Arc Welding Machine
DC Arc Welding Machine
Striking of arc with electrode is relatively difficult maintenance of a short arc is also difficult except with iron powder electrode.
Developing an arc is easier maintenance of short arc is also easier.
No problem of arc blow in AC arc welding machine. Work piece do not get magnetized in DC.
Arc blow is a serve problem and minimised with the use of proper corrective measure. Work piece may get magnetized due to current flow in one direction.
Arc is never stable.
Arc is more stable.
No polarity change possible and hence no suitable for welding all metals. It is used for welding ferrous metals.
Polarity (DCSP or DCRP) can be changed and hence suitable for welding both ferrous and non-ferrous metal s quality efficient.
More suitable for higher current value. It is less suitable for use at low current value with small dia of electrode.
It is most suitable with lower current value is also, for example at low amperage with small diameter electrode.
Bare electrode cannot be used. Only flux coated electrode with arc stabilizing agent influx can be used.
Bare and coated electrode can be used.
Not suitable for thin sheets or sheet metal work due to difficulty in striking the arc.
It is suitable for welding of sheet metal as striking arc is easier and arc remains steady.
Distribution of heat in arc is equal at electrode and job.
Most of heat (upto 66.67%) is liberated in the positive side of arc i.e. DC RP.
Voltage drop in welding is less and hence welding is suitable for longer distance from welding plant using long welding lead.
Voltage drop is relatively higher and hence start cables are used to weld only close to the welding plant.
Welding transformer has no moving part and working is salient.
A DC generator set has several moving parts therefore operation is noisy.
AC transformer welding set is not costly, simpler in operation maintenance cost is also very low.
A dc generator set is costly, difficult to operate and very high maintenance cost.
This type of welding power source usually consists a transformer and a bank of rectifying cells. A unit rectifying cell is called a diode and it allows the electrical current to pass only in one direction and thus helps in converting alternating current to direct current.
The welding industry uses solid-state devices like semi-conductors to make rectifying cells. Earlier selenium was mainly used to make these cells but because of the demand for higher economy, reliability, and efficiency most rectifying cells are now made of silicon.
In comparison with motor-generator welding set, a rectifier welding power source has the following advantages.
- No rotating parts, easy maintenance,
- Higher efficiency,
- Smaller weight, size and cost.
General theory of DC Rectifier Design
Due to easy control of heat balance as well as the ease of arc initiation and its maintenance, dc is used particularly for bare wire welding like mig welding machine. However, ac is more easily and commonly available. The best solution, therefore, appears to be to use ac to produce the required dc and that is done with the help of a dc generator, a convertor or a dc rectifier.
Basically an electric rectifier is a device which permits flow of current on one direction only and can thus convert ac into a fluctuating dc. In the case of sinusoidal supply voltage suppression of the negative half of the curve results in intermittent pulses of energy which show no reversal in polarity. Obviously, such a source of supply would not be suitable for welding as periods of interrupted supply of energy between consecutive pulses would make it impossible to maintain a stable arc.
This difficulty can, however, be overcome if the source of supply is a three-phase current, each phase displaced by 1200 with reference to preceding or succeeding phases. When the negative half cycles of all three phases are suppressed the resulting graph for the uni-directional current shows an approach to straight line with much smaller fluctuations than that in the case of single phase. On similar basis, systems with more than three phase ac would attain better closeness to true dc transient. However, it is common to come across systems with more than three phases, thus three phase system is the one most used. But there is another way of improving the shape of the rectified current, and that is referred to as full-wave rectification.
DC rectifier, as mentioned earlier, is a device which permits current flow in one direction only, or more correctly it suppresses most of the current flow in reverse direction. Although the relation between voltage and current in the first quadrant of the graph is not linear, it can be noticed that for the third quadrant even a very large increase in voltage results in transmission of very small amount of current.
As is evident from the different transients of rectified current, there is inherent fluctuation in such a system. One method of obtaining smooth dc from a rectifier unit is the use of capacitors. If a capacitor is connected in the circuit it stores energy and supplies the same at nearly constant output voltage, although the energy received by it from the rectifier is in variable pulses.
Initially mercury arc rectifiers were developed but they are very fragile and have now been completely replaced by solid-state rectifiers.
V-I Characteristics of Welding Machine
Means for different welding plant different V-I characteristics. It shows the relationship between the arc voltage and arc current. During the welding arc length between the electrode tip and the workpiece determines the arc resistance and consequently the potential drop across the arc. In other manner, the arc length find out the arc voltage longer the arc length higher the arc voltage and it is this voltage which allow certain flow of current according of the characteristics of welding plant (unit).
There are basically three type of characteristics:
- Drooping characteristics (or constant current)
- Flat (or constant voltage)
- Rising voltage type
Our all concentration will be on only drooping type characteristics as it is used mostly in arc welding machine plant, both ac and dc type.
1. Drooping Type (Constant Current): Drooping V-I characteristics is used on constant current type welding machine. When arc is struck in arc welding machine (GMAW Welding Machine) electrode is essentially in short-circuit which would immediately required a sudden of current otherwise machine is design to prevent this. A constant current machine is design to minimise theses sudden surges.
As we know that a manual metal arc welding plant consist a drooping V-I characteristics. Drooping means that the terminal voltage of welding machine decreases as the welding current increases. In arc welding machine (MMA Welding) , arc length (gap between the workpiece and electrode) length, from a shorter arc B to longer arc A, there is a marked variation(K) in the voltage but the corresponding variation (c) in the current is very small.
Drooping V-I characteristic is applicable for both AC and DC welding machine which is used for SMAW Welding Machine, TIG Welding Machine, and Submerged Arc Welding Machine (SAW Welding Machine) and Plasma Arc Welding Machine and the MMA (Manual Metal Arc) Welding Machine, voltage at the time of welding is approximate 30-40 V.
2. Flat or constant voltage type characteristics are used with semi-automatic MIG Welding Machine and other automatic welding machines.
3. Rising voltage type characteristics are used with fully automatic welding machine.
a) Open circuit voltage usually is in the range of 70 to 80 volt.
b) System to adjust welding current is usually in the AC section of the machine before the rectifiers control of current is based on the principle of variable inductance or impedance various method for varying impedance for current control are as:
a) Moving shunt
b) Tapped reactor
c) Moving coil
d) Saturable reactor
e) Moving reactor core
In welding circuit flow of current is controlled by inductor in line between the electrode and transformer current can be vary by varying the inductance. For current control during welding a means of varying this inductance is must.
a) Tapped type reactor
b) Moving core type reactor
c) Saturable type reactor
This process is growing in popularity. It is being used for more than 20% of arc welding machine. Some FCAW still uses CO2 shielding, but the use of flux cored wire alone is increasing. In many cases, the flux-cored wire alone produces welds equal to or better than the original metal and its uses eliminates the need for the gas shield equipment and cost of the gas.
Definition and Concept
The FCAW is a process in which coalescence is produced by heating with an electric arc between a continuous tubular consumable electrode and the work. The electrode is flux cored, i.e. the flux is contained within the electrode which is hollow. In addition to flux, mineral and ferro alloys in the core can provide additional protection and composition control.
The flux cored electrode is coiled and supplied to the arc as a continuous wire as in CO2 welding. The flux inside the wire provides the necessary shielding of the weld pool. Additional shielding may (or may not) be obtained from an externally supplied gas (e.g. CO2) or gas mixture.
Principle of Operation
As explained above, FCAW utilizes the heat of an arc between continuously fed consumable flux cored electrode and the work. The heat of the arc melts the surface of the base metal and the end of the electrode. The metal melted off the electrode is transferred through the arc to the workpiece where it becomes the deposited weld metal. Shielding is obtained from the disintegration of ingredients contained within the flux cored electrode. Additional shielding may be obtained from an envelope of gas supplied through a nozzle to the arc area. Ingredients within the electrode produce gas for shielding and also provide deoxidizers, ionizers, purifying agents and in some cases alloying elements (for composition control). These ingredients from a glasslike slag, which is lighter in weight than the deposited weld metal and which floats on the surface of the weld as a protective cover. The flux cored electrode is fed into the arc automatically from a coil. The arc is maintained automatically and arc travel can be manual or by machine.
Use of the welding in today’s technology is extensive. Welding is used in every branch of science and technology. Such as computer industries, electronic industries, mechanical industries, petrochemical industries, etc.
History of Arc Welding
Arc welding machine 1st described by Davy in England in late 1809, but beginning of arc welding machine could become possible only the improvement in generator/dynamometer. Welding of metals by carbon arc was 1st suggested by a French person in 1881. In 1889, Zerner proposed an idea for twin carbon arc welding. In 1907 in America Strohmenger obtained another patent on flux coated electrode and 1st good welding joint was produced, he used asbestors coating with sodium silicate binder. Now a lot of changes and development have occurred for constituents of flux coating and core wire composition.
In 1928 shielding of welding (CO2) possible by Alexender in USA. 1st TIG (Tungsten Inert Gas) welding machine and MIG (Metal Inert Gas) welding machine develop consequently in 1946 and 1948 respectively.
Joining Process: On the basics of composition of the joint, joining process are three types:
- Homogeneous Process: In the joining process filler rod (electrode) is used and two same metals are joining no pressure is applying during the welding process.
- Heterogeneous Process: In this joining process filler metal is soluble in both the parent metal which themselves are soluble in each other.
- Autogeneous Process: In this type of welding process no filler rod is used but pressure is extensively used. Resistance welding, forge welding, friction welding, diffusion welding are the examples of autogeneous welding.
Metals that can be welded
- Ferrous metals such as low carbon steel, high carbon steel, stainless steel, cast iron, etc. (ferrous metals are those metals which are richer in carbon means having iron as a main constituents).
- Non-ferrous metals are those materials which are richer other than carbon such as Al,Cu, brass, nickel magnesium, zinc and its alloys.
Advantages of Welding
- Welding joints are stronger as base metal.
- Machining are possible on welding joints such as (grinding, shapering etc.).
- Welding joints efficiency up to 100%.
- Appearance of welding joint is very good.
- Alteration and amendment is possible in existing structure use by welding.
- A large numbers of similar and dissimilar metals can be welded easily.
- Weld joints are light in weight.
- Welding is a permanent joint.
Limitations of Welding
- Skill person is needed for welding purpose.
- Ultra-violet rays and infrared rays generate during the welding process which harm full for operator’s skin as well as eyes.
- Distortion due to high temperature is possible in workpiece.
- Thermal stresses developed in workpiece heat treatment process are required to relieve the thermal stresses.
- Weld heat produces metallurgical changes in workpiece.
- Edge preparation is required before welding which is time and labor consuming process.
- Special jigs and fixtures are required for welding purpose.
Comparison of Welding with Other Joining Processes
- Welding joints are lighter in weight as compared to riveting and casting (process).
- Welding is more economical and much faster process as compared to casting and riveting.
- Welding joint have more tensile strength and rigid as compared to riveting and casting.
- Less cost involvement in welding as compared to riveting and casting.
- Welding produces 100% efficient joint which is not possible by any other joining process.
- Jig-fixture not so much required as compared in reverting and casting.
- Welding is a permanent joint while riveting and bolting are not permanent joint (temporary joint).
Applications of Welding
- Automobile industries
- Building construction
- Railroad industries
- Pressure vessels and tank (cylinders)
- Aircraft industries
- Storage tank
- Pipeline industries
- Petrochemical industries
- Earth moving equipment and machinery
- Shipbuilding industries
On the complex shapes, the surface curvatures may not allow good contact or coupling, the angles of surfaces may prevent back wall echoes with 00 probes and some forgings, simple or complex may be anisotropic in grain structure (different grain size in different directions).
When searching the defects in forgings you should have, as a minimum, the following information, which is usually written on a techniques or instruction sheet.
- The test component identification and area to test.
- Actions to be taken when defects are found.
- The purpose of the test (defects sought and acceptance criteria).
- Equipment required.
- What method and level of test sensitivity to use.
- The method of scanning.
The instruction sheet would also contain sections giving details of any relevant safety procedures such as the cleaning of the test area afterwards. It would also have the company name, a unique technical reference number, the originator’s name and signature and an authorising signature.
The test may involve testing the whole, of a component, or just parts, this must be specified.
Actions to be taken
When defects are found it may be required that the defects are reported, e.g. on a diagram as a written description, or the component, or material, may be accepted or rejected according to the defects found. If defects are to be reported then the defect information that needs reporting would be contained in this section, i.e. Defect type, size, lateral and longitudinal position in relation to datums, etc.
Purpose of the test
This sections tells us the accept/reject criteria for particular defects, i.e. what size and type of defects to report or which defects render the component rejectable.
The section should give information on; the type of flow detector, type, size, and the frequency of probes, type of couplant, calibration blocks and reference block to use.
Method of setting and level of sensitivity need to be quoted for each scan, e.g. Set the bwe from the DGS block to 80% fsh and note the gain setting. Still on the DGS block, maximise the signal from the flat bottom hole at target depth (test material thickness) and set that to 80% fsh and note the difference in dBs between the new gain setting and the previous one. Set the bwe from the test material to 80% fsh and add the difference noted in the first two gain settings to the present gain and scan at this level.
The method of scanning of the material is either a written, step by step, instruction or technique sheet, or involves following the step laid out in the relevant national standard. And example written step by step could be:
- Prepare the material surface by removing any loose scale, rust, dirt or other debris and visually inspect for surface defects or damage.
- Calibrate the screen on the flaw detector, using a 00 probe and A2 calibration block, for a range of 0 to 200 mm.
- Set the sensitivity (as quoted in the relevant section above) and apply couplant to the test area.
- Scan the designated test area, with a probe overlap between scans of at least 20% of the probe’s diameter and at a maximum probe movement rate of 150 mm/sec.
- When defects meeting the criteria in the “purpose of the test” section are found, record the relevant defect data as in the “Action to be taken” section.
- Prepare a neat concise report giving details of the component identification, test area, equipment used, sensitivity method and settings and a drawing with the defect detailsas recorded in section five above.
Post test Procedure
This would involve cleaning any remaining couplant and dirt from the test area and covering the surface with protective coatings according to client’s requirements.
Welding joins two metal pieces by the use of heat and with or without pressure apply.
Common Welding Processes are:
- Arc Welding Machine
- MIG Welding Machine
- TIG Welding Machine
- Argon Welding Machine
- Welding Rectifier
- Spot Welding Machine
- Plasma Cutting Machine
Health threats by Gases & Fumes
Mainly gases and fumes come from different way:
- Base material and filler material
- Coating and paints
- Shielding gases and their chemical reactions
- Process and different types of consumable used
- Contaminants in the atmosphere
- Wrong welding techniques
Contact of welding gases and fumes may have Short time impact and Long time impact.
Short time Hazards
- Irritate the eyes, nose, chest and respiratory tract.
- Cause coughing, wheezing, shortness of breath, bronchitis, pulmonary edema, and pneumonitis .
- Cause nausea, loss of appetite, vomiting, cramps, and slow digestion.
Long Time Hazards
? Welders may chance a variety of chronic lung problems, including: Bronchitis, asthma, pneumonia, emphysema, pneumoconiosis, decreased lung capacity, silicosis, and siderosis.
Other Health Hazards:
1. Heat exposure
- Skin burns and skin cancer
- May also increased metal stress & blood pressure, and further cause to heart problems, exhaustion, nervousness, and bad temper.
3. Eye damage– More than 5% eye injuries in industries and construction are associated with welding and cutting
- damage to retina
- damage to cornea, resulting in cataracts
- permanent eye damage