OPUS

Joining Method Welding

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Welding is generally defined as the act of joining together two or more components by heating the parent metal surfaces with an electric arc or arcs, and producing coalescence (a solid connection) with or without the use of a filler metal.

Shielded Metal Arc Welding (SMAW) or Stick Welding.

The most commonly used pipe welding process is Shielded Metal Arc Welding (SMAW), also known as stick welding. This is an arc welding process which establishes an arc between a mineral-coated electrode and the parent metal (pipe). SMAW electrodes consist of a core material that matches the material being welded and a coating called flux. Electrodes are available in various diameters (1/16” to 5/16”) to match the thickness and type of metal being welded. The flux coating gives the electrode its welding characteristics.

Electrodes for steel pipe are coded with an American Welding Society (AWS) numbering system.  Common electrodes for welding steel pipe include E6010 and E7018 electrodes.  The “E” stands for electrode. The first digits indicate minimum tensile strength of the electrode in thousands of pounds per square inch (psi). For example, E60XX electrodes have a minimum tensile strength of 60,000 psi, while E70XX electrodes are rated at a minimum of 70,000 psi.

The next to last digit indicates the position the electrode can be used for welding.

1. EXX1X is for use in all positions. These electrodes produce a weld puddle that is sufficiently viscous as to allow it to “stick” to the side or bottom of a piece of pipe as it is being welded, allowing the welder to weld completely around the joint between two horizontal pipes. A common term for this type of electrode is “fast freeze”.
2. EXX2X is for use in flat and horizontal positions. These electrodes produce a puddle that is sufficiently viscous as to allow for welds along a horizontal line. However, welds attempted up a vertical line or underneath a pipe will result in the weld pool draining away due to gravity.
3. EXX3X is for flat welding. The weld puddle produced by these electrodes is too liquid to allow for welding in any position other than on a flat piece of metal or on the very top of a piece of pipe. These electrodes are good for high deposition rate welding. A common term for this type of electrode is “fast fill”.

The last two digits together describe the type of current to use with the electrode.  There are there current types.

DC+. Direct current electrode positive (DCEP), sometimes called direct current reverse polarity (DCRP). Welders like to say the “heat” is on the pipe being welded rather than on the electrode. DCEP produces deep weld penetration with less deposition of filler metal.

DC-. Direct current electrode negative (DCEN), sometimes called direct current straight polarity (DCSP). Welders like to say the “heat” of the weld is on the electrode rather than the pipe being welded. DCEN melts the electrode faster than DCEP and produces less weld penetration with more deposition of filler metal.

AC. Alternating current electrodes perform in between DCEN and DCEP.

• EXX10 DC+
• EXX11 AC or DC-
• EXX12 AC or DC-
• EXX13 AC, DC- or DC+
• EXX14 AC, DC- or DC+
• EXX15 DC+
• EXX16 AC or DC+
• EXX18 AC, DC- or DC+
• EXX20 AC ,DC- or DC+
• EXX24 AC, DC- or DC+
• EXX27 AC, DC- or DC+
• EXX28 AC or DC+

One end of the electrodes is left bare and free of flux, which enables the welder to clamp in an electrode holder, commonly called a stinger. The stinger is connected to the welding machine with a heavy electrical cable. A second cable, called a ground, connects the welding machine to the pipe, usually with a clamp.

When the welder touches the pipe with the end of the electrode, an electrical circuit is completed, and high current begins to flow between the electrode and the pipe. This is called “striking an arc”. The current is sufficiently strong to melt both the electrode and the pipe, and a pool of molten metal is created at the weld.  As the weld progresses, the electrode becomes shorter and shorter as the core metal is melted and flows through the arc to become filler metal that is added to the weld. Thus the Metal Arc “MA” portion of the Shielded Metal Arc Welding (SMAW) designation.

Before welding, the flux coating on the electrode protects it from rust and other contaminants.  During the welding process, the flux decomposes in the heat of the weld, forming liquid that covers the molten pool of metal. The flux also releases a gas that protects the molten pool of metal from contaminants in the atmosphere. As the weld puddle cools and solidifies, the liquid flux also cools and forms a protective layer over the weld that further protects it from contaminants.  This layer of flux must be removed before subsequent layers of weld can be added and must be removed from the final weld. The layer of flux “shield” the electrode and the weld, thus the “Shielded” portion of the Shielded Metal Arc Welding (SMAW) designation.

When the electrode becomes too short, the welder must stop the weld, clamp a new electrode in the stinger, remove the flux from the weld, and then continue welding.

SMAW is typically the first process welders are taught, even though it is not the easiest process to learn. The primary advantages of SMAW are its applicability on most metals, its portability, and its minimum requirements in equipment, set-up time, and protection from the environment. Its primary weakness is the high degree of manual dexterity required of the welder. Not all welders are proficient at using SMAW. Another disadvantage of SMAW is the high amount of spatter caused by the metal transfer through the arc. Some filler metal is vaporized and lost in the process, and the melting flux creates a large amount of smoke. The pool itself appears fairly turbulent due to the relative violence of the arc. Combined with the smoke and spatter, the turbulent weld pool makes it difficult for the welder to see exactly what is going on and requires that protective leather clothing be worn and proper ventilation supplied.

Gas Tungsten Arc Welding (GTAW) or Tungsten Inert Gas (TIG) Welding or Heliarc 

The most expensive welding process which can be used on piping is Gas Tungsten Arc Welding (GTAW).

GTAW is an arc welding process where an arc is established between a nonconsumable tungsten electrode, and the work. Thus the “Tungsten Arc” portion of the Gas Tungsten Arc Welding (GTAW) designation.  No flux is involved as in SMAW, and shielding from the atmosphere is provided by an external gas envelope. Thus the “Gas” portion of the Gas Tungsten Arc Welding (GTAW) designation.  The gas is drawn from cylinders supplied by a local welding supply house, and delivered to the weld through a hose that runs along with the electrical cable from the welding machine. The electrode is held in an electrode holder designed for GTAW welding, and gas is supplied through the holder so that the entire electrode and weld puddle are shielded during the welding process.

Since the tungsten electrode is nonconsumable (it does not melt), welds are possible without adding filler material.  If filler is to be added, the welder dips a filler wire into the welding arc and weld pool.  The filler is made of the same metal as the metal being welded, thus allowing GTAW to be used in any type of metal.

TIG welding, as welders usually call GTAW, stands for Tungsten Inert Gas, and is an inaccurate name for the process.  Occasionally, and active gas rather than an inert gas is used for shielding. When this occurs, the process is knows as Tungsten Active Gas, or TAG.  The name Heliarc derives from the original patents on the process, in which helium was used as a shielding gas. Though most welders still call the process TIG or Heliarc, the term Gas Tungsten Arc Welding (GTAW) is preferred because it accurately describes the process no matter what shielding gas is used.

The GTAW process may be applied to weld all metals used for piping because welding occurs under a gas shield of inert gas, usually argon. Since there is no flux or metal across the arc, the weld pool is very calm and stable making it clearly and easily visible to the welder. This allows him or her to see exactly what is occurring under the arc, in the weld pool and at the surfaces being welded. This clarity and ease of visibility make GTAW the most precise welding process, giving it a reputation for making top quality welds. The lack of spatter also allows for less protective clothing than required for SMAW.

Weld quality is the strongest feature of the GTAW process. Its weaker features are that it is expensive and the process is slow. GTAW also requires the highest skill level, as the welder must use both hands to manipulate both the tungsten holder and the filler metal. GTAW should only be used where it is necessary. It is most commonly used for making the root pass (i.e., the first pass) and possibly a second pass on welds that must be perfect or near-perfect on the inside of the pipe joint, or where GTAW is the only suitable process for welding. The GTAW process may also be implemented using a consumable insert as a backing ring. The insert must be completely melted and fused into the root edges of the pipe joint. Inserts are typically used without adding filler metal and the back-side portion is drawn up into the weld pool forming a weld bead on the inside of the joint. The GTAW process may be used with automatic equipment (e.g., orbital welding) where electrode travel, position, manipulation, feed, and arc level are machine controlled. Fit-up requirements are very important with automatic welding equipment. The use of automatic equipment is becoming popular where quality welds are desired.

Gas Metal Arc Welding (GMAW) or Metal Inert Gas (MIG) Welding

Gas Metal Arc Welding (GMAW) is an arc welding process where an arc is established between a continuous solid bare consumable electrode, and the work. Shielding (used to protect the weld pool from the atmosphere) is provided by an external gas envelope.  This process is also known as Metal Inert Gas (MIG) Welding, which is accurate as long as an inert gas is used for shielding.  When an active gas is used (rarely), the process becomes Metal Active Gas, or MAG.  The term Gas Metal Arc Welding (GMAW) is preferred because it describes the process regardless of the shielding gas used.

GMAW uses a continuous coil of wire which is fed through a gun, commonly called a MIG gun. Once the welder pulls the trigger on the gun, gas begins to flow and the wire is fed at a rate that matches the current output of the welding machine. There are four methods by which the wire is melted and added to the weld puddle.

The most common is called short circuit transfer. In this method, the gun is held near the weld and the trigger is pulled.  The wire is fed and strikes the base metal, causing a short circuit. High amperage flows during the period of short circuit, and causes the wire to melt. A glob of melted wire is deposited in the weld puddle and an electrical arc is established between the base metal and the remaining wire, still being fed from the gun. The arc adds heat to the glob of melted wire and helps to flatten it into the weld puddle.  Since the wire is fed at a constant rate, as soon as the wire is melted up to perhaps a quarter of an inch from the base metal, the feed rate of the wire begins moving the end of the wire toward the base metal again.  The length of the arch becomes shorter and shorter until the wire again strikes the base metal, and the process repeats.  This process happens approximately 100 times per second and is characterized by a distinct buzzing sound caused by the repeating process.

Short circuit transfer is the easiest weld to learn and perform, but can be difficult to set up. The puddle has fast-freeze characteristics, and an improperly set up machine can produce welds that look proper but have little penetration and thus, little strength.  Like SMAW with its metal arc characteristic, GMAW has a high rate of spatter and a quantity of the filler material is lost.

Globular transfer in GMAW occurs with the end of the wire forms a ball of melting metal larger in diameter than the wire.  The globs may drop off the wire or get pulled off when the wire enters the weld puddle.  Globular transfer is to be avoided as the spatter rate is extremely high and weld quality can be low.

The third method of GMAW transfer is the spray-transfer mode. In this mode, wire speed and amperage are adjusted so that the wire is melted at exactly the same rate as it is fed. Spray transfer occurs at higher amperages and voltages than short circuit or globular, so the metal droplets are much smaller or even vaporized. Thus, metal transfers across the arc in a much cleaner and less violent manner.  As such, spatter is greatly diminished and deposition rate is high.  While short circuit transfer typically uses argon gas for shielding, spray transfer typically requires a mix of gasses which are a bit more expensive.  Because of the high amperage and voltage used, the weld puddle is very liquid. This process is good for flat welding, some horizontal, and occasionally for vertical welding downhill, but is typically not practical for pipe welding except for controlled conditions such as those in a fabrication shop.

Pulsed-spray GMAW transfer is an enhanced method of spray transfer in which pulses of power control exactly when and how much molten metal is transferred across the arc.  This allows for a more controlled weld and allows for spray-transfer in all position. Special power sources are needed for this process.

No matter which transfer process used, the gun may be held and guided around the pipe by hand, or the gun may be held in a fixture while the pipe is rotated underneath.

Due to the low voltage used with short-circuit transfer, thorough training of welders is required to avoid creation of imperfections in the welds known as lack-of-fusion defects. The potential for the occurrence of lack-of-fusion defects, which are difficult to expose using visual or radiographic inspection, is the weakest feature of the GMAW process. GMAW is not as portable as SMAW as bottles of gas are required and distance from the power source is limited by how far wire can be pushed through the feed cable. 

GMAW’s strongest feature is that welding can be completed without interruption due to the continuous wire supplied to the welding gun. With a properly set up machine, GMAW welding is very easy to learn and perform.

A variation of GMAW is Fluxcore Arc Welding (FCAW).  FCAW uses wire which is tubular and filled with flux. FCAW does not require shielding gas but does leave a coating of flux on the weld that may have to be removed.  The two strongest features of fluxcore welding are that it is easier to learn than short-circuiting transfer and it is less expensive to use. It is easier to learn because it does not easily form incomplete fusion defects. Fluxcore wire is three to four times more expensive than solid wire; however, it has a higher flow rate and reduces cost because of a savings in labor and savings in shielding gas.

Submerged Arc Welding (SAW) 

Submerged Arc Welding (SAW) is an arc welding process where an arc is established between one or more continuous bare-solid or cored-metal electrodes and the work. The welding arc or arcs and molten puddle are shielded by a blanket of granular, fusible material. Filler metal is obtained from the electrodes, and on occasion, from a supplementary welding wire. 

The SAW process is the least expensive of the welding processes available. To use the process, equipment must be used to rotate the pipe and maintain the liquid weld pool in the flat position, requiring some capital investment. The SAW process is usually accomplished with the welding equipment in a fixture rather than in the welder’s hands. Because of the sand-like flux, SAW is normally reserved for use with 8 inch and above pipe sizes.

 



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