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
2. Arc Welding
|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
|6. Newer Welding Processes
i. Electron Beam
7. Related processes
Max temperature reached: 3300
Gas Welding Equipment
Gas Welding Equipment
2. Gas hoses
3. Non-return valve
4. Check valve
Types of Torch
Rose-bud torch (used to heat metals for bending, straightening, etc)
Injector torch(equal-pressure torch, merely mixes the two gasses. venturi effect).
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)
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.
Temperature is 6000 to 7000’c
Comparison of AC and DC Arc Welding
|Particulars||Direct Current||Alternating Current|
|No-load voltage||Low (higher safety)||Too high, over 70V (Danger)|
|Prime cost||High 2 to 3 times of AC||Low|
|Connected load||Normal||High (disadvantage)|
|Electrodes||Both bare (non-coated) cheaper electrode can be used||Only coated electrodes i.e. expensive electrodes can be used|
|Welding of non-ferrous metals||Suitable||Not suitable|
|Heat generation||High in +ve pole and low in –ve pole||Same at each pole|
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 Digit||Type of Coating||Welding Current|
|1||Cellulose Potassium||AC or DC Reverse or Straight|
|2||Titania Sodium||AC or DC Straight|
|3||Titania Potassium||AC or DC Straight or Reverse|
|4||Iron Powder Titania||AC or DC Straight or Reverse|
|5||Low Hydrogen Sodium||DC Reverse|
|6||Low Hydrogen Potassium||DC or DC Reverse|
|7||Iron Powder Iron Oxide||AC or DC|
|8||Iron Powder Low Hydrogen||DC Reverse or Straight or AC|
|0*||See reference below|
* 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.
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.
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.
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.
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
Argon – 1 to 5% Oxygen
Argon – 3 to 25% CO2
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.
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 Benefits
Superior quality welds
Welds can be made with or without filler metal
Precise control of welding variables (heat)
Free of spatter
Argon + Hydrogen
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
Electron Beam 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.
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
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):
High distortions and wide welds as a result of high heat input.