Classification of Welding Processes #FirstPart

Welding is a process of joining similar metals by application of heat with or without application of pressure and addition of filler material. The term “weldability” has been defined as the capacity of being welded into inseparable joints having specified properties such as definite weld strength, proper structure, etc. There are several factors that affect the weldability i.e. Melting Point, Thermal conductivity, Thermal Expansion, Surface condition, and Change in Microstructure.

There are basically two types of welding, Plastic Welding and Fusion Welding.

Plastic Welding: In Plastic welding or pressure welding, the pieces of metal to be joined are heated to plastic state and then forced together by external pressure.

Fusion Welding: In fusion welding or non pressure welding , the material at the joint is heated to molten state and allowed to solidify.

The welding process have been classified into the following 7 processes and these 7 processes further classified into sub processes.

1. Gas welding
i. Oxyacetylene
ii. Air-acetylene
iii. Oxy-hydrogen

2. Arc Welding
i. Carbon Arc
ii. Metal Arc
iii. Gas Metal Arc ( MIG)
iv. Gas Tungsten Arc ( TIG)
v. Atomic-hydrogen arc
vi. Plasma Arc
vii. Submerged Arc welding
viii. Flux-cored arc
ix. Electro-slag

3. Resistance welding
i. Butt welding
ii. Spot welding
iii. Seam welding
iv. Projection welding
v. Percussion welding

4. Thermite welding

5. Solid State welding
i. Friction
ii. Ultrasonic
iii. Diffusion
iv. Explosive

6. Newer Welding Processes
i. Electron Beam
ii. Laser

7. Related processes
i. Oxyacetylene cutting
ii. Arc cutting
iii. Hard facing
iv. Brazing
v. Soldering

GAS WELDING

Oxy-Acetylene Welding

Max temperature reached: 3300

Gas Welding Equipment

Gas Welding Equipment
1. Regulator
2. Gas hoses
3. Non-return valve
4. Check valve
5. Torches

Types of Torch
 Welding torch
 Cutting torch
 Rose-bud torch (used to heat metals for bending, straightening, etc)
 Injector torch(equal-pressure torch, merely mixes the two gasses. venturi effect).

Fuels
Acetylene (Acetylene is the primary fuel for oxy-fuel welding and is the fuel of choice for repair work and general cutting and welding, Acetylene generator as used in Bali by a reaction of calcium carbide with water)
Gasoline
Hydrogen (Hydrogen has a clean flame and is good for use on aluminium)
MAPP gas(methylacetylene-propadiene. It has the storage and shipping characteristics of LPG and has a heat value a little less than acetylene)
Butane, propane and butane/propane mixes
 Butane is like the propane gas, the two are both saturated hydrocarbons and do not react with each other so they are regularly mixed together to form Butane/Propane gas mixture.
 Propane does not burn as hot as acetylene in its inner cone, and so it is rarely used for welding
 Propane is cheaper than acetylene and easier to transport.
Propylene(Propylene is used in production welding and cutting. It cuts similarly to propane. When propylene is used, the torch rarely needs tip cleaning.

ARC WELDING

Temperature is 6000 to 7000’c

Comparison of AC and DC Arc Welding

ParticularsDirect CurrentAlternating Current
No-load voltageLow (higher safety)Too high, over 70V (Danger)
EfficiencyLowHigh (advantageous)
Prime costHigh 2 to 3 times of ACLow
Connected loadNormalHigh (disadvantage)
ElectrodesBoth bare (non-coated) cheaper electrode can be usedOnly coated electrodes i.e. expensive electrodes can be used
Welding of non-ferrous metalsSuitableNot suitable
Heat generationHigh in +ve pole and low in –ve poleSame at each pole

Electrode

In arc welding an electrode is used to conduct current through a work piece to fuse two pieces together. Depending upon the process, the electrode is either consumable, in the case of gas metal arc welding or shielded metal arc welding, or non-consumable, such as in gas tungsten arc welding.

Coating and Specification

All mild steel and low-alloy electrodes are classified with a 4 or 5-digit number prefixed by “E”.

Prefix “E” = Electrode

First two (or three) digits = Tensile strength (psi) (stress relieved or as welded)

Third (or fourth) digit = Position of welding

                1 = all positions (flat, horizontal, vertical, overhead)

                2 = horizontal and flat positions only

Fourth DigitType of CoatingWelding Current
1Cellulose PotassiumAC or DC Reverse or Straight
2Titania SodiumAC or DC Straight
3Titania PotassiumAC or DC Straight or Reverse
4Iron Powder TitaniaAC or DC Straight or Reverse
5Low Hydrogen SodiumDC Reverse
6Low Hydrogen PotassiumDC or DC Reverse
7Iron Powder Iron OxideAC or DC
8Iron Powder Low HydrogenDC Reverse or Straight or AC
0*See reference below

Example

* When the fourth digit is O, the type of coating and current to use are determined by the third digit. For example, E-6010 indicates a cellulose sodium coating operates on DC reverse, while E-6020 has an iron oxide coating and operates on AC or DC.

Electrode Types

 E6010 This electrode is used for all position welding using DCRP. It produces a deep penetrating weld and works well on dirty,rusted, or painted metals
 E6011 This electrode has the same characteristics of the E6010, but can be used with AC and DC currents.
 E6013 This electrode can be used with AC and DC currents. It produces a medium penetrating weld with a superior weld bead appearance.
 E7018 This electrode is known as a low hydrogen electrode and can be used with AC or DC. The coating on the electrode has a low moisture content that reduces the introduction of hydrogen into the weld. The electrode can produce welds of x-ray quality with medium penetration. (Note, this electrode must be kept dry. If it gets wet, it must be dried in a rod oven before use.)

Purpose of Coated Electrodes

1. To facilitate the establishment and maintenance of the arc.
2. To protect the molten metal from the Oxygen and nitrogen of the air
3. To protect the welding seam from rapid cooling
4. To provide the means of introducing alloying elements not contained in the core wire.

Precautions in Arc Welding

1. Because of the intensity of heat and light rays from the electric arc, the operator’s hand face and eyes are to be protected while arc is in use
2. Heavy gloves are worn
3. Hand shield or a helmet with window of coloured glass should be used to protect the face
4. The space for the electric arc welding should be screened off from the rest of the building to safeguard other workmen from the glare of the arc.

RESISTANCE WELDING

1. Resistance welding is a process used to join metallic parts with electric current. There are several forms of resistance welding, including spot welding, seam welding, projection welding, and butt welding.
2. The heat generated is expressed by the equation
i. E = I²·R·t
where E is the heat energy, I is the current, R is the electrical resistance and t is the time that the current is applied.

Spot Welding

Projection Welding

Seam Welding

Butt Welding

Percussion Welding

Percussion Welding uses electrical energy stored in a condenser to produce an intense momentary power discharge to provide the localized heating at the interface. It is suitable for joining dissimilar metals that are not weldable by flash butt welding, or when flash is not desirable at the weld joint.

Process steps in Percussion Welding

1. The two materials to be welded are positioned with a preset air gap between them
2. A burst of RF energy ionizes the air gap.
3. Capacitor banks discharge, creating an arc that heats the two materials to a weldable temperature.
4. When the materials reach the proper welding state, electromagnetic actuators accelerate them together. The molten masses combine, metal to metal, and are forged together. As the weld cools, a complete alloy bond is formed.

MIG welding

Gas Metal Arc Welding (GMAW) is frequently referred to as MIG welding. MIG welding is a commonly used high deposition rate welding process. Wire is continuously fed from a spool. MIG welding is therefore referred to as a semiautomatic welding process.

MIG Welding Benefits

1. All position capability
2. Higher deposition rates than SMAW
3. Less operator skill required
4. Long welds can be made without starts and stops
5. Minimal post weld cleaning is required

Shielding Gases

 Argon
 Argon – 1 to 5% Oxygen
 Argon – 3 to 25% CO2
 Argon/Helium
 CO2 is also used in its pure form in some MIG welding processes. However, in some applications the presence of CO2 in the shielding gas may adversely affect the mechanical properties of the weld.

TIG Welding

TIG Welding

 Gas Tungsten Arc Welding (GTAW) is frequently referred to as TIG welding.
 TIG welding is a commonly used high quality welding process.
 TIG welding has become a popular choice of welding processes when high quality, precision welding is required.
 In TIG welding an arc is formed between a non consumable tungsten electrode and the metal being welded.
 Gas is fed through the torch to shield the electrode and molten weld pool.
 If filler wire is used, it is added to the weld pool separately.

TIG Welding

TIG Welding Benefits

 Superior quality welds
 Welds can be made with or without filler metal
 Precise control of welding variables (heat)
 Free of spatter
 Low distortion

Shielding Gases

 Argon
 Argon + Hydrogen
 Argon/Helium
 Helium is generally added to increase heat input (increase welding speed or weld penetration). Hydrogen will result in cleaner looking welds and also increase heat input, however, Hydrogen may promote porosity or hydrogen cracking.

TIG Welding Limitations

 Requires greater welder dexterity than MIG or stick welding
 Lower deposition rates
 More costly for welding thick sections

Special Welding Processes

 Submerged Arc Welding
 Plasma Arc Welding
 Thermit Welding
 Electron Beam Welding
 Friction Welding
 Diffusion Welding

Submerged Arc Welding

 Submerged arc welding (SAW) is a common arc welding process. It requires a continuously fed consumable solid or tubular (flux cored) electrode.
 The molten weld and the arc zone are protected from atmospheric contamination by being “submerged” under a blanket of granular fusible flux consisting of lime, silica, manganese oxide, calcium fluoride, and other compounds.
 When molten, the flux becomes conductive, and provides a current path between the electrode and the work. This thick layer of flux completely covers the molten metal thus preventing spatter and sparks as well as suppressing the intense ultraviolet radiation and fumes that are a part of the SMAW (shielded metal arc welding) process.

SAW

Equipment Used

 Power supply
 Start plate
Copper mold
 Electrode
 Guide tube
 Wire feed
 Power source
 SAW head
 Flux handling
 Protective equipment

Advantages

 High deposition rates (over 100 lb/h (45 kg/h) have been reported).
 High operating factors in mechanized applications.
 Deep weld penetration.
 Sound welds are readily made (with good process design and control).
 High speed welding of thin sheet steels up to 5 m/min (16 ft/min) is possible.
 Minimal welding fume or arc light is emitted.
 Practically no edge preparation is necessary.
 The process is suitable for both indoor and outdoor works.
 Distortion is much less.
 Welds produced are sound, uniform, ductile, corrosion resistant and have good impact value.
 Single pass welds can be made in thick plates with normal equipment.
 The arc is always covered under a blanket of flux, thus there is no chance of spatter of weld.
 50% to 90% of the flux is recoverable

Limitations

 Limited to ferrous (steel or stainless steels) and some nickel based alloys.
 Normally limited to the 1F, 1G, and 2F positions.
 Normally limited to long straight seams or rotated pipes or vessels.
 Requires relatively troublesome flux handling systems.
 Flux and slag residue can present a health & safety issue.
 Requires inter-pass and post weld slag removal

Plasma Arc Welding

 Plasma Arc Welding is the welding process utilizing heat generated by a constricted arc struck between a tungsten non-consumable electrode and either the work piece (transferred arc process) or water cooled constricting nozzle (non-transferred arc process).

Plasma is a gaseous mixture of positive ions, electrons and neutral gas molecules.

Plasma arc welding (PAW)

 Plasma arc welding (PAW) is an arc welding process similar to gas tungsten arc welding (GTAW). The electric arc is formed between an electrode (which is usually but not always made of sintered tungsten) and the workpiece.
 The key difference from GTAW is that in PAW, by positioning the electrode within the body of the torch, the plasma arc can be separated from the shielding gas envelope.
 The plasma is then forced through a fine-bore copper nozzle which constricts the arc and the plasma exits the orifice at high velocities (approaching the speed of sound) and a temperature approaching 20,000 °C.
 Plasma arc welding is an advancement over the GTAW process.
 This process uses a non-consumable tungsten electrode and an arc constricted through a fine-bore copper nozzle.
 PAW can be used to join all metals that are weldable with GTAW (i.e., most commercial metals and alloys).

Advantages of Plasma Arc Welding (PAW):

 Requires less operator skill due to good tolerance of arc to misalignments;
 High welding rate;
 High penetrating capability (keyhole effect);

Disadvantages of Plasma Arc Welding (PAW):

 Expensive equipment;
 High distortions and wide welds as a result of high heat input.

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Author: Aliva Tripathy

Taking out time from a housewife life and contributing to AxiBook is a passion for me. I love doing this and gets mind filled with huge satisfaction with thoughtful feedbacks from you all. Do love caring for others and love sharing knowledge more than this.

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