Arc Welding Concept and Process
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Arc welding is a process of the fusion welding processes which contains:-
1 - Arc - welding processes (non consumable electrodes)
Gas tungsten - arc welding
Plasma - arc welding
Atomic - hydrogen welding
2 - Arc - welding processes (consumable electrodes)
Shielded metal arc welding
Submerged - arc welding
Gas metal arc welding
Flux cored arc welding
Electro gas welding
Electro slag welding
3 - Electron - beam welding
4 - Laser - beam welding
The main concept of fusion welding is the partial melting and fusion of the joint between two members and a filler metals may be used.
In the arc welding process, the heat required is obtained from electrical energy. An arc is produced between the tip of the electrode and work piece to be welded, by using an AC or a DC power supply.
Shield metal arc welding (SMAW) is one of the oldest, simplest, and most versatile joining processes. About 50% of all industrial and maintenance welding currently is performed by this process.
The electric arc is generated by touching the tip of a coated electrode against the work piece and withdrawing it quickly to a distance sufficient to maintain the arc. The heat generated melts a portion of the electrode tip, its coating and the base metal in the immediate arc area and this mixture forms the weld when it solidifies.
Classification Of Arc Welding
The arc welding classified into many types used in the industrial field each one have advantages and defects different from the other type, So that the user select the required type according to the requirements and the economy aspects. In the below figure we show for the classification of arc welding which appear in the industrial (3.1)
SMAW: shield metal arc welding
SAW: submerged arc welding
TIG: tungsten inert gas arc welding
MIG: metal inert gas welding
PAW: plasma arc welding
ESW: electro slag welding
The above chart list for the types of the arc welding which used in the industrial fields.
Advantages of arc welding
There are some characteristic for the arc welding rather than the another methods to joining metals which it represent the advantages of the arc welding listed as below
- Strong and tight joining
- Cost effectiveness ( arc welding have low capital and running costs)
- Simplicity of welded structure design
Figure 3.2: arc welding usage in the industry
Principals And Definition
Shield metal arc welding (SMAW) consider as process in which coalescence of metal produced by the heat which generated from the electric arc that it maintained between the tip of the consumable electrode and the base metal in the welding region. The next figures show the circuit used in the shield metal arc welding
The shield arc welding power source provides constant current (cc) it may be alternative current (Ac) or direct current (DC) depending on the electrode being used, but the best welding usually used the direct current (DC) as power supply for the circuit.
The amount of power required to welding process calculated from the relation
(P= IV) Where I represent the current & V represent the voltages
In the selection process for the power supply we notice that the voltage equal to constant value so that the current is the main parameters in the selection process which discussed in the next section
The amount of current needed to melt the work piece and the electrode tip depends on several factors as
- Type and position of the joint
- Work piece type and its thickness
- Electrode type and its diameter
In facts most of welder select the current depending on the diameter of the used electrode which it measured from the inner section without the coated diameter as showing in the below figure:
A basic guide of arc welding electrodes by Bruce Bauerlein
The electrode used in (SMAW) process consist from metal rod coated in metal mixture called flux which gives off gases as it decomposes to prevent weld contamination , introduces deoxidizer to purify the weld , causes weld- protection slag and provides alloying elements to improve the weld quality. there are three types from the electrode used in (SMAW) process which discussed below.
Fast fill electrode: this type from electrode designed to melt quickly so that the welding speed be maximized. This type used when we weld in the normal case in vertical welding so we can weld with high speed so we use this electrode type.
Fast freeze electrode: this type from electrode designed to solidify quickly making welding in a variety of positions possible by preventing the weld pool from shifting.
Intermediates electrodes : the commonly used type and it intermediate in two case melting and in freezing but if we weld with high speed with using this type we get poor quality so that this type is not suitable in case of high speed.
The American welding society established system that used to assign electrodes by using four or five digital number. Covered electrodes made of mild steel or low alloy steel carry the symbol of (E) followed by their number. the first two . The first two or three digits of the number specify the tensile strength of the weld metal, in thousand pounds per square inch . The penultimate digit generally identifies the welding positions permissible with the electrode, typically using the values 1 (normally fast-freeze electrodes, implying all position welding) and 2 (normally fast-fill electrodes, implying horizontal welding only). The welding current and type of electrode covering are specified by the last two digits together.
Example for electrode serial number
E indicates electrode
70 indicates 70,000 psi tensile strength
1 indicates use for welding in all positions
8 indicates low hydrogen
2. Cables: there are two cables work cable which connected the base metal and the electrode cable which connected to the electrode one of them represent positive and the other represent the negative side this according to the polarity of the welding process
Welding Trouble Shooting
This section discuss to the trouble shooting may faced it during the welding speed as
1-Porosity: small cavities or holes resulting from gas pockets in weld metal.
The porosity in welding it may be caused by the dirty on work piece and the corrective action to face it by Remove all grease, oil, moisture, rust, paint, coatings, slag, and dirt from work surface before welding.
Excessive Spatter: scattering of molten metal particles that cool to solid form near weld bead as shown in the below figure.
The main causes to excessive spatter is the current too high for electrode and it avoid by decrease current or select larger electrode.
3- Distortion: contraction of weld metal during welding that forces base metal to move.
Distortion in the welding appear as result to excessive heat input. And there is many ways to solve this problem as
- Use restraint (clamp) to hold base metal in position
- Make tack welds along joint before starting welding operation.
- Select lower amperage for electrode.
- Increase travel speed
- Lack of Penetration: shallow fusion between weld metal and base metal.
The lack of penetration appears as result for many causes as showing below:
Improper joint preparation
Material too thick. Joint preparation and design must provide access to bottom of groove.
Improper weld technique.
1-Keep arc on leading edge of weld puddle.
2-Reduce travel speed.
Insufficient heat input.
Increase amperage. Select larger electrode and increase amperage
Advantages Of Shield Metal Arc Welding
The shield arc welding have main further that represent the advantages as below
- Strong and tight joining rather than the other methods
- Cost effectiveness (have low capital and running cost)
- The arc welding Simplicity of welded structures design
- May be mechanized and automated or manual in some duty
Disadvantages Of Arc Welding
The using of the shield metal arc welding lead to some defects as
- Internal stresses, distortion and change for the microstructure in the weld region
- Have harmful effects as radiation and fumes
Application Of The Shield Arc Welding
The arc welding it can be used in many applications as:
- Buildings and bridges structures
- Automotive, ship and aircraft constructions
- Pipe lines
- Tanks and vessels (2.2)
To introduce what the polarity means and the different type of the polarity, first of all the project will show what is the difference between AC and DC?
AC And DC Current
What Is The AC Current?
It means alternating current. In this current the electric charge flow periodically reverses direction. The electric charge in this case will move for instance backward then it will reverse the direction again to move forward, then backward then forward over and over again.
The waveform for alternating current is a sine wave. But sometimes in certain applications the waveform differs like square wave or triangular wave.
What Is The DC Current?
It means direct current because the electric charge in direct current moves in a constant direction, and that is the difference between DC and AC. The DC could be produced by such sources like thermocouples, batteries and solar cells.
We can use a rectifier which is a current-switching arrangement to obtain direct current from an alternating current. The concept of rectifier is containing electromechanical elements or electronic elements which allow current to flow only in one direction.
AC And DC Welding Machines:
The welder or welding machine is the power source that we used in arc welding. There are three basic types of weldingmachines used now which are rectifiers, motor- generators and transformers
Motor generator welding machines (direct current welding machines):
The motor generator could be powered by diesel, electrical or gasoline motor. We can use the gasoline and diesel motors in places where the electricity is not available or hard to reach it like deserts. With these machines we could generate either direct or alternating current. There are new and old machine types of these machines, the older type require reversing the cable connections in order to change the polarity. But in the new machines you can change the polarity from welding in the direct current mode to alternating current mode by turning a switch.
Theadvantages of a (dc) welding generator:
- The main advantage of a direct-current (dc) welding generator is that you can weld with either reverse or straight polarity.
- Almost we can weld all ferrous and non-ferrous metals.
- We can use DC in practically all welding operations so we can consider that DC is most universal in application.
- The output of generators like rectifier and transformer a sets is not affected by normal variations in power line voltage.
- Diesel driven generators form self-contained units.
Disadvantages of a (dc) welding generator:
The initial cost is high
The machine operation is noisy
The maintenance cost is high
AC transformers welding machines:
The static-transformer type is the arc welding machines that we used in alternating current. As shown in the figure 2.18
The advantages of these types ofmachines are lightest, smallest and least expensive welders made. We can use 200, 300, and 400 ampere ratings for manual operation in industrialapplications. But in light industrial, job/shop and garage welding we could use machines with a 150-ampere rating. These types of transforms equipped with arc- stabilizing capacitors.
The advantage of a (AC) transformer welding machine:
The main advantage of (AC) transformer welding machine is the freedom from arc blow. The arc blow causes the arc to wander while you are using large coated electrodes or welding in corners on heavy metal. The arc blow often occurs when welding with direct current machines.
What is the polarity?
Polarity is the direction of the current flow in a circuit. There are two types of polarity reverse polarity and straight polarity. In the straight polarity the work piece is positive and the electrode is negative. The electrons move from the electrode to the work piece. In the reverse polarity the work- piece negative and the electrode is positive. The electrons move from the work piece to the electrode ac and dc current was briefly covered.
The effect of polarity:
The polarity affects the amount of heat which is going to the base metal. You can direct the amount of heat to where it is needed as per the application you working in by changing the polarity. If you want to direct the majority of heat toward the work piece, you must use straight polarity. That type of polarity being used in some welding situations when it is desirable to have more heat on the work piece because of its size to melt the base metal than the electrode. We can use straight polarity for all mild-steel, bare, or lightly coated electrodes. If you want to concentrate the heat on the electrode, you can use reverse polarity because in this type less heat is concentrated at the work piece which allow filler metal to cool faster, giving it greater holding power because of this reverse polarity is being used when you are making vertical and overhead welds. Reverse polarity could be used in the welding of nonferrous metals such as bronze, aluminum and nickel.
Heat Affected Zone
Heat affected zone definition.
The heat affected zone (HAZ) represented by the area of base material subjected to the high heat in order to achieve the welding process. The area which subjected to change in its microstructure and mechanical properties. SMAW process give a highly heat concentration in specified region called (fusion zone), produced in the region of heat affected zone. To calculated the amount of heat in the HAZ the following formula used.
Q =60 VI / 1000S
Where Q = heat input (kj/mm), V = voltage (V), I = current (A), and S= welding speed (mm/ min). (2.10)
The molten occur in the fusion region not in the heat affected zone. So that the HAZ heated to a high enough temperature for sufficient period that grain growth occurred. Heat affected zone is that location of the base metal which in it the mechanical properties and the microstructure have been affected by the heat of welding process.
The heat affected zone is subjected to a complex thermal cycle (sudden heating followed by rapid cooling) in which all temperatures from the melting range of the metal down to much lower temperatures are involved and HAZ therefore consists of a series of graded structures ringing the weld bead. HAZ usually contains a variety of microstructures different from the remaining part from the base metal.
The width of HAZ varies according to the welding process and technique; in arc welds it extends only a few mm from the fusion boundary,
The HAZ in most metal welding process of normal structure welded in one run with coated electrodes that to classify it to four basic region.
Classification And Designation
The classification of electrodes is done by one of the three following categories:
1 - Strength of the deposited weld metal
2 - Current (ac or dc)
3 - Types of coating
The identification of electrodes is by numbers and letters as shown in the next table. Typical coated - electrode dimensions are in the range of 150 to 460 mm (6 to 18 in.) in length, and 1.5 to 8 mm (1/16 to 5/16) in diameter.
Specifications for electrodes and filler metals (including dimensional tolerances, quality control procedures, and processes) are published by the American welding society (AWS) and the American national standards institute (ANSI). Some specifications appear in the aerospace materials specifications (AMS) by the society of automotive engineers (SAE). Electrodes are sold by weight and are available in a wide variety of sizes and specifications.
Electrodes Types And Choices
When the choice of a suitable electrode is needed, many considerations are taken such as
- Quality of weld
- Used current whether AC or DC
- Crack resistant
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