Before going to the welding procedure in full detail let’s first understand what is welding? The term welding is used in mechanical engineering when we are joining two or more pieces of any material typically metal by application of heat, pressure or combination of both. This is simplest definition for welding. Mostly the term is used for metals but is also applicable for other materials like fusion welding of HDPE pipes and plastic materials i.e plastic welding. But our main focus in this welding procedure discussion is related to metallic welding.
Types of Welding
There are different types of welding done but most common Welding Types are Arc Welding, Oxyfuel Gas Welding and Resistance Welding. Lets talk few more details about types of welding.
Arc Welding: Arc Welding uses a welding power supply to create an electric arc between an electrode and the base material to melt the metals at the welding point. Arc welding is also known as stick welding. The process or method of arc welding is widely used because of its low capital and running costs. Arc welding is also termed as Shielded Metal Arc Welding smaw because It is performed by striking an arc between a coated-metal electrode and the base metal and flux – the coating of the metal electrode will form as shield to the molten metal protecting it from oxidization.
Tig Welding: This type of welding is also known as tungsten inert gas TIG welding which is also an an arc welding process that uses a non-consumable tungsten electrode to produce the weld. TIG welding has become a popular choice of welding processes when high quality, precision welding is required.
Mig Welding: Also termed as Gas Metal Arc Welding (GMAW) which is commonly used high deposition rate welding process where wire is continuously fed from a spool. MIG welding is therefore referred to as a semiautomatic welding process.
Advantages of Arc Welding SMAW
- High quality welds are made rapidly at a low cost.
- Can be used easily even to thick and wide work piece to be joined.
- Can be used from thinner to thicker materials.
Disadvantages of Arc Welding SMAW
- Consumes bigger electric current
- Dirty work finish
- Root pass is lower than TIG and MIG
- Prone to slag inclusions
- Weld deposits is prone to blue holes
Metal Inert Gas Welding
GMAW or MIG is an electric arc welding process which joins metals by heating them with an arc established between a continuous filler metal (consumable) electrode and the work. Shielding of the arc and molten weld pool is obtained entirely from an externally supplied gas or gas mixture both inert and reactive gases.
Advantages of GMAW:
- Produces High quality welds & much faster than with SMAW and TIG welding.
- No flux is used no slag entrapment in the weld metal.
- Very little loss of alloying elements as the metal transfers across the arc.
- Minor weld spatter is produced, and it is easily removed.
- Versatile and can be used with a wide variety of metals and alloys, such as Aluminum, Copper, Magnesium, Nickel, Iron and many of their alloys.
- The process can be operated in several ways, including semi- and full
- MIG welding is widely used by many industries for welding a broad variet
y of materials, parts, and structures.
Disadvantages of GMAW:
- IT cannot be used in the vertical or overhead welding positions due to the high heat input and the fluidity of the weld puddle.
- Has complex equipment compared to equipment used for the shielded metal-arc welding process, difficult to handle by arc welder.
Gas Tungsten TIG Arc Welding
Gas tungsten arc welding (GTAW) is an AW process that uses a non consumable tungsten electrode and an inert gas for arc shielding. The term TIG welding (tungsten inert gas welding) is often applied to this process (in Europe, WIG welding is the term—the chemical symbol for tungsten is W, for Wolfram). GTAW is applicable to nearly all metals in a wide range of stock thicknesses. It can also be used for joining various combinations of dissimilar metals. Its most common applications are for aluminum and stainless steel.
There are some limitations to TIG Welding as well i.e Cast irons, wrought irons, and of course tungsten are difficult to weld by GTAW and in steel welding applications, GTAW is generally slower and more costly than the consumable electrode AW processes, except when thin sections are involved and very-high-quality welds are require. Advantages of TIG GTAW welding in the applications to which it is suited include high-quality welds, no weld spatter because no filler metal is transferred across the arc, and little or no post weld cleaning is required because no flux is used in tig welding.
Oxygen Fuel Gas Welding (OFW)
Using this welding procedure metals are joined by heating with a fuel gas flame or flares with or without the application of pressure and with or without the use of filler metal. There are different types of Oxy-fuel Gas Welding:
- Oxy-Acetylene or Oxygen- Acetylene Gas Welding
- Oxy-Hydrogen or Oxygen- Hydrogen Gas Welding
- Methylacetone-Propadiene Gas Welding
- Pressure Gas Welding.
Advantages of Oxy-fuel Gas Welding
- Easy to use both welding and cutting by welder
- Controlled heat input
- Controlled bead size
- Convenient to use in welding thin sheets, tubes and small diameter pipes
Disadvantages of Oxy-Fuel Gas Weldinmg
- Cannot be use to weld on thick work piece.
- Expensive gas
This is another important type of welding in which the fusing temperature is generated at the joint by the resistance to the flow of an electrical current. High temperature is accomplished by clamping two or more sheets of metal between copper electrodes and then passing an electrical current through them. When the metals are heated to a melting temperature, forging pressure is applied through either a manual or automatic means to weld the pieces together.
Two common types of resistance welding are Spot and Seam welding. In spot welding metal to be joined is placed between two electrodes and pressure is applied. A charge of electricity is sent from one electrode through the material to the other electrode. In seam welding procedure welding is like spot welding except that the spots overlap each other, making a continuous weld seam.
What is WPS & PQR?
PQR: Procedure Qualification Record
WPQ : Welding Procedure Qualification
A Welding Procedure Specification (WPS) is a formal written document describing welding procedures, which provides direction to the welder or welding operators for making sound and quality production welds as per the code requirements . The purpose of the document is to guide welders to the accepted procedures so that repeatable and trusted welding techniques are used. A WPS is developed for each material alloy and for each welding type used. Specific codes and/or engineering societies are often the driving force behind the development of a company’s WPS.
Relation Between WPS & PQR
A WPS is supported by a Procedure Qualification Record (PQR or WPQR). A PQR is a record of a test weld performed and tested (more rigorously) to ensure that the procedure will produce a good weld. Individual welders are certified with a qualification test documented in a Welder Qualification Test Record (WQTR) that shows they have the understanding and demonstrated ability to work within the specified WPS.