Introduction
Electrogas welding is a variation of the MIG/MAG processes and the FCAW Welding process. As with the Electroslag process, Electrogas welding uses retaining shoes to confine the weld pool in the vertical position.
The formation of the protective atmosphere and the transfer of the metal are identical to the MIG/MAG process.
Additional protection may or may not be used by injecting a gas or a mixture of gases from an external source.
The mechanical aspects of the electrogas process are similar to those of the electroslag process and, once started, it continues until the weld is completed.
For the beginning of the operation, a consumable electrode in the form of wire, solid or tubular, is fed into a cavity formed by the faces of the chamfer of the parts to be welded and by the retaining shoes. An electric arc starts between the electrode and a plate located at the bottom of the joint.
The heat of the arc fuses the bevel faces and the electrode which is fed continuously.
The molten metals from the molten filler metal and base metal form a molten pool below the arc and solidify.
The electrode can oscillate horizontally across the joint, particularly in thicker joints, in order to more evenly distribute heat and filler metal.
As the weld solidifies one or both of the shoes will move up along with the weld head to continue the weld. Although the axis of the is vertical, the welding position is the flat position, with vertical displacement.
Many have difficulty differentiating electroslag from electrogas welding. To make it easier to remember, remember that electrogas has "Gas" in the name and is like a GMAW in electroslag.
Basically, the components of electrogas welding are:
With power sources such as variable voltage (constant current), variable displacement (constant current), the vertical displacement can be controlled by varying the electric arc.
The mechanical aspects of the electrogas process are similar to those of the electroslag process and, once started, it continues until the weld is completed.
How welding occurs in the electrogas process
Welding is normally done in a single pass.For the beginning of the operation, a consumable electrode in the form of wire, solid or tubular, is fed into a cavity formed by the faces of the chamfer of the parts to be welded and by the retaining shoes. An electric arc starts between the electrode and a plate located at the bottom of the joint.
The heat of the arc fuses the bevel faces and the electrode which is fed continuously.
The molten metals from the molten filler metal and base metal form a molten pool below the arc and solidify.
The electrode can oscillate horizontally across the joint, particularly in thicker joints, in order to more evenly distribute heat and filler metal.
As the weld solidifies one or both of the shoes will move up along with the weld head to continue the weld. Although the axis of the is vertical, the welding position is the flat position, with vertical displacement.
Many have difficulty differentiating electroslag from electrogas welding. To make it easier to remember, remember that electrogas has "Gas" in the name and is like a GMAW in electroslag.
Welding Equipment
The basic equipment for electrogas welding is similar to conventional electroslag welding. The fundamental difference is the introduction of shielding gas from the arc and molten metal pool, when shielding gas is needed (in electrogas welding with flux cored wire, shielding gas is not always necessary.Basically, the components of electrogas welding are:
- Direct current power source
- Water cooled shoes to contain the molten solder;
- A welding gun;
- Device for feeding;
- A mechanism for oscillating gun in welding;
- Equipment to supply shielding gas when used.
Power source
The power source can be either a constant voltage or a constant current type. When a constant voltage unit is used, the vertical displacement can be controlled manually or by a device, such as a photoelectric cell, that detects the height of the rise of the molten pool.With power sources such as variable voltage (constant current), variable displacement (constant current), the vertical displacement can be controlled by varying the electric arc.
Wire feeder
It is similar to the one used in automatic MIG/MAG and FCAW processes. The feeder must be able to supply the electrode at high speeds and straighten the wire by taking its straight end.Welding Gun
The welding gun for electrogas welding performs the same functions as those for MIG/MAG and tabular wire welding.It guides the electrode to the desired position at the joint gap and transmits welding current to the electrode, and in some applications it provides shielding gas around the electrode and arc.
The main difference between an electrogas gun and those of MIG/MAG welding or flux cored wire is the limitation in the dimension parallel to the root opening between sheets, as the nozzle of the gun must adapt to this narrow opening. The width of the pistol is often limited to 10mm so that it can have adequate horizontal travel.
The main difference between an electrogas gun and those of MIG/MAG welding or flux cored wire is the limitation in the dimension parallel to the root opening between sheets, as the nozzle of the gun must adapt to this narrow opening. The width of the pistol is often limited to 10mm so that it can have adequate horizontal travel.
Retention Shoes
As with electroslag welding, shoes are used to retain the weld pool. Usually both shoes move up as the weld progresses. In some welds, one of the shoes may be a stationary gasket.To prevent the weld pool from incorporating the copper of the shoes, these are cooled with water so they do not melt.
For welding steel with flux cored wire, CO2 is the commonly used shielding gas. The mixture of 80% argon and 20% CO2 is normally used for welding steel with solid electrodes.
Some tubular electrodes are of the self-shielded type. When molten, the fluxes generate a shielding gas to protect the filler metal and molten weld metal.
Economic viability depends on sheet thickness and joint length. The process is mainly used for welding carbon steels and alloy steels, but is also applicable to austenitic stainless steels, and other metals and alloys that are weldable by the MIG/MAG processes. The base metal thickness can vary over a range of 10 to 100 mm. Usually, when the thickness is greater than 75 mm, the electroslag welding process is more recommended than the electrogas process.
The larger the joint to be welded, the greater the efficiency of this process. For field welding, for example vertical joints of large storage tanks, the process eliminates the labor and cost of manual welding. The welding variables of the electrogas process are similar to those of the electroslag process.
The energy normally used in the electrogas process is direct current, reverse polarity. Power sources used for electrogas welding are usually in the range of 750 to 1000A for 100% duty cycle (continuous use).
In this process, the heat from the electric arc must be applied evenly across the joint with sheets from 30 to 100 mm thick, the welding gun is oscillated horizontally over the weld pool to achieve a uniform deposition of the metal and the melts complete both sides. Horizontal swing is not usually necessary for sheets less than 30 mm thick.
All discontinuities found in welds made by the two processes can be found in welds made with electrogas welding. However, the cause of some discontinuities, such as lack of fusion, may be different in electrogas welding.
Welds made with the electrogas welding process under normal operating conditions result in high quality welds free of harmful discontinuities.
The welding inspector must be aware of these discontinuities:
However, when electrode oscillation is used, the slag can partially solidify near one shoe while the arc is near the other shoe. When the bow returns, the slag can be incorporated if it is not remelted.
However, if something interferes with the shielding gas coating, porosities can result. Other causes of porosity can be excessive air currents, water leakage from retaining shoes and electrodes, or contaminated shielding gas.
Cracks can form at high temperatures, along with, or immediately after, solidification. They are located near the center of the weld (Hot Cracking).
Cracks in the weld can be avoided by modifying the solidification characteristic of the weld. This can be accomplished by altering the shape of the weld pool, through appropriate changes in welding variables.
The arc voltage should be increased, and the amperage and travel speed decreased. Often, increasing the root gap between sheets can help, although this may not be cost-effective.
If cracks are caused by high carbon or high sulfur in the steel, base metal penetration should be kept low to minimize base metal dilution in the weld. In addition, a high manganese electrode can be used for welding high sulfur steels.
In addition to these discontinuities, we must observe:
Types and functions of consumables – electrodes and gases
There are two types of electrodes used in electrogas welding, namely:- Tubular wire (as internal flow);
- Solid wire;
For welding steel with flux cored wire, CO2 is the commonly used shielding gas. The mixture of 80% argon and 20% CO2 is normally used for welding steel with solid electrodes.
Some tubular electrodes are of the self-shielded type. When molten, the fluxes generate a shielding gas to protect the filler metal and molten weld metal.
Features and Applications
Electrogas welding is used for joining thick sheets that must be welded in a vertical position or that can be positioned vertically for welding. Welding is usually done in a single pass.Economic viability depends on sheet thickness and joint length. The process is mainly used for welding carbon steels and alloy steels, but is also applicable to austenitic stainless steels, and other metals and alloys that are weldable by the MIG/MAG processes. The base metal thickness can vary over a range of 10 to 100 mm. Usually, when the thickness is greater than 75 mm, the electroslag welding process is more recommended than the electrogas process.
The larger the joint to be welded, the greater the efficiency of this process. For field welding, for example vertical joints of large storage tanks, the process eliminates the labor and cost of manual welding. The welding variables of the electrogas process are similar to those of the electroslag process.
The energy normally used in the electrogas process is direct current, reverse polarity. Power sources used for electrogas welding are usually in the range of 750 to 1000A for 100% duty cycle (continuous use).
In this process, the heat from the electric arc must be applied evenly across the joint with sheets from 30 to 100 mm thick, the welding gun is oscillated horizontally over the weld pool to achieve a uniform deposition of the metal and the melts complete both sides. Horizontal swing is not usually necessary for sheets less than 30 mm thick.
Discontinuities induced by the electrogas welding process
Electrogas welding is basically a MIG/MAG or cored wire welding process.All discontinuities found in welds made by the two processes can be found in welds made with electrogas welding. However, the cause of some discontinuities, such as lack of fusion, may be different in electrogas welding.
Welds made with the electrogas welding process under normal operating conditions result in high quality welds free of harmful discontinuities.
The welding inspector must be aware of these discontinuities:
Slag Inclusions
The process is usually in one pass, so slag removal is not required. The solidification speed of the solder is relatively low. There is a large amount of time available for the molten slag to float to the surface of the molten pool.However, when electrode oscillation is used, the slag can partially solidify near one shoe while the arc is near the other shoe. When the bow returns, the slag can be incorporated if it is not remelted.
Porosidade
Tubular electrodes contain deoxidizing and dehydrating elements in the core. A combination of shielding gas and slag-forming compounds from the electrode core usually produces a healthy, porosity-free weld.However, if something interferes with the shielding gas coating, porosities can result. Other causes of porosity can be excessive air currents, water leakage from retaining shoes and electrodes, or contaminated shielding gas.
Trincas
They do not occur under normal welding conditions. The relatively slow heating and cooling of the weld considerably reduces the risk of cold (hydrogen) cracking developing.Cracks can form at high temperatures, along with, or immediately after, solidification. They are located near the center of the weld (Hot Cracking).
Cracks in the weld can be avoided by modifying the solidification characteristic of the weld. This can be accomplished by altering the shape of the weld pool, through appropriate changes in welding variables.
The arc voltage should be increased, and the amperage and travel speed decreased. Often, increasing the root gap between sheets can help, although this may not be cost-effective.
If cracks are caused by high carbon or high sulfur in the steel, base metal penetration should be kept low to minimize base metal dilution in the weld. In addition, a high manganese electrode can be used for welding high sulfur steels.
In addition to these discontinuities, we must observe:
- The high deposition rate of this process implies a high risk of lack of fusion;
- Electrogas welding, like electroslag welding, has the problem of overheating: the coarse granulation of the weld and adjacent regions has deficient properties in terms of toughness. Heat treatment may be required after welding.
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