Now we can continue with Welding Processes, and in this post Piping & Fabrication will explain the Welding Method starting from SAW, GTAW, GMAW and FCAW.
Currently the most commonly used welding processes for fabrication of piping are SMAW, submerged arc welding (SAW), GTAW, GMAW, and flux core arc welding (FCAW). Some special applications may involve plasma arc welding (PAW) or electron beam welding (EBW), but their application to piping is still rare. However, any welding process which can be qualified under the requirements of ASME Section IX is acceptable. Detailed descriptions of these various processes and their variations may be found in the Welding Handbook.
This section will limit discussion to their application to piping. For shop work, the best efficiency in all welding processes is attained when the pipe axis is horizontal and the piece is rotated so that welding is always done in the flat position. This is referred to as the 1G position. Other positions are 2G (pipe vertical and fixed, weld horizontal); 5G (pipe horizontal and fixed, weld a combination of flat, vertical, and overhead); and 6G (pipe inclined at 45° and fixed). See ASME Section IX.
Shielded Metal Arc Welding. SMAW has been the mainstay for pipe welding for many years, but it is rapidly being displaced by newer, more efficient processes. It is a process where an arc is manually struck between the work and a flux-coated electrode which is consumed in the weld. The core wire serves as the filler material, and the flux coating disintegrates to provide shielding gases for the molten metal, scavengers, and deoxidizers for the weld puddle and a slag blanket to protect the molten metal until it is sufficiently cool to prevent oxidation. It can be used in all positions, for upward or downward progression, and for root pass welding depending on the flux composition. Each weld pass is about 1⁄8 in thick, and before subsequent passes are made the slag must be removed and the surface prepared by removing irregularities which could entrap slag during subsequent passes.
Submerged Arc Welding. Unlike SMAW, SAW is an automatic or semiautomatic process. For circumferential welds in pipe the welding head is fixed for flat welding and the work is rotated under the head (1G position). It is used most efficiently in groove butt welds in heavy wall materials with pipe sizes NPS 6 (DN 150) and larger. The arc is created between the work and a bare solid wire or composite electrode which is consumed during the operation. The electrode comes in coils. Shielding is accomplished by a blanket of granular, fusible material called a flux which covers the arc and molten metal by forming a slag blanket to prevent oxidation of the molten metal until it has sufficiently cooled. Particular wire-flux combinations are required to assure that the deposited weld has the needed chemical and physical properties. This process has the greatest deposition rate and accordingly is the preferred process wherever possible. Because of the high heat input, care must be taken to assure that the interpass temperature is controlled to minimize sensitization in austenitic stainless steels or loss of notch toughness in ferritic steels. High heat input can also result in excessive penetration, so this process cannot be used effectively for root pass welding unless the root is deposited against a backing
ring or sufficient backing is provided by two or more weld passes made by the shielded metal arc or a gas-shielded arc process.
Gas Shielded Arc Welding. The term gas-shielded arc welding applies to those welding processes where the arc and molten metal are shielded from oxidation by some type of inert gas rather than by a flux.
1. Gas tungsten arc welding: GTAW is a form of gas-shielded arc welding where the arc is generated between the work and a tungsten electrode which is not consumed. The filler metal must be added from an external source, usually as bare filler rod or preplaced consumable insert. The filler metal is melted by the heat of the arc, and shielding gases are usually argon or helium. Alloying elements are always in the filler material. GTAW is considered to be the most desirable process for making root welds of highest quality. Techniques using added filler metal or preplaced filler metal as inserts are equally effective in manual and automatic applications.
Automatic versions can be used in all positions provided sufficient clearance is available for the equipment. Automatic versions also require tighter fit-up requirements since the equipment is set to specific parameters and will not recognize variations outside of these limits, such as a welder would do in manual applications. In automatic GTAW, the welding head orbits the weld joint on a guide track placed on the pipe adjacent to the joint to be welded. The welding head contains motors and drive wheels needed to move the head around the track, a torch to create the arc, and a spool of filler wire. Welding current, voltage, travel speed, wire feed rate, and oscillation are controlled from an external source. These parameters may be varied by the operator as the welding head traverses the weld. Oscillation and arc energy can be adjusted to permit greater dwell time and heat input into the side walls. Automatic GTAW welds are usually deposited as a series of stringer beads to minimize the effects of high interpass temperature.
2. Gas metal arc welding: GMAW is a type of gas-shielded welding generally used in the manual mode but adaptable to automation. The filler wire is the electrode and is furnished in coils or spools of solid wire. Is is fed automatically into the joint, melted in the arc, and deposited in the weld groove. Alloying elements arc in the wire, and shielding gas may be argon, helium, nitrogen, carbon dioxide, or combinations thereof, depending on the application. Depending on the equipment and the heat input settings, filler metal can be transferred across the arc is several modes. In short-circuiting transfer, the electrode actually touches the work where it short-circuits, melts, and restarts the arc. This process has low heat input and accordingly low penetrating power. It can often result in lack of fusion. Because of the low heat input, however, it can be effectively used for open-butt root pass welding. In spray transfer, the heat input parameters are sufficiently high to transfer the molten electrode across the arc as small droplets. Argon or argon-rich gases are used for shielding, resulting in a very stable spatterfree arc. Because of the high arc energy, it is normally used in the flat (1G) position. For all-position welding, a procedure which superimposes high amplitude pulses of current on a low-level steady-state current at regular intervals is often used. This results in a discrete transfer of metal with lower heat input needed for all-position welding.
3. Flux core arc welding: FCAW is a variation of GMAW where a composite electrode is substituted for the solid wire. The electrode is a tubular wire containing a flux material. Depending on the application, the arc may be self-shielding, or shielding gases may be used. Because of its high deposition rate this process is rapidly being developed for shop and field welding of piping.
Currently the most commonly used welding processes for fabrication of piping are SMAW, submerged arc welding (SAW), GTAW, GMAW, and flux core arc welding (FCAW). Some special applications may involve plasma arc welding (PAW) or electron beam welding (EBW), but their application to piping is still rare. However, any welding process which can be qualified under the requirements of ASME Section IX is acceptable. Detailed descriptions of these various processes and their variations may be found in the Welding Handbook.
This section will limit discussion to their application to piping. For shop work, the best efficiency in all welding processes is attained when the pipe axis is horizontal and the piece is rotated so that welding is always done in the flat position. This is referred to as the 1G position. Other positions are 2G (pipe vertical and fixed, weld horizontal); 5G (pipe horizontal and fixed, weld a combination of flat, vertical, and overhead); and 6G (pipe inclined at 45° and fixed). See ASME Section IX.
Shielded Metal Arc Welding. SMAW has been the mainstay for pipe welding for many years, but it is rapidly being displaced by newer, more efficient processes. It is a process where an arc is manually struck between the work and a flux-coated electrode which is consumed in the weld. The core wire serves as the filler material, and the flux coating disintegrates to provide shielding gases for the molten metal, scavengers, and deoxidizers for the weld puddle and a slag blanket to protect the molten metal until it is sufficiently cool to prevent oxidation. It can be used in all positions, for upward or downward progression, and for root pass welding depending on the flux composition. Each weld pass is about 1⁄8 in thick, and before subsequent passes are made the slag must be removed and the surface prepared by removing irregularities which could entrap slag during subsequent passes.
Submerged Arc Welding. Unlike SMAW, SAW is an automatic or semiautomatic process. For circumferential welds in pipe the welding head is fixed for flat welding and the work is rotated under the head (1G position). It is used most efficiently in groove butt welds in heavy wall materials with pipe sizes NPS 6 (DN 150) and larger. The arc is created between the work and a bare solid wire or composite electrode which is consumed during the operation. The electrode comes in coils. Shielding is accomplished by a blanket of granular, fusible material called a flux which covers the arc and molten metal by forming a slag blanket to prevent oxidation of the molten metal until it has sufficiently cooled. Particular wire-flux combinations are required to assure that the deposited weld has the needed chemical and physical properties. This process has the greatest deposition rate and accordingly is the preferred process wherever possible. Because of the high heat input, care must be taken to assure that the interpass temperature is controlled to minimize sensitization in austenitic stainless steels or loss of notch toughness in ferritic steels. High heat input can also result in excessive penetration, so this process cannot be used effectively for root pass welding unless the root is deposited against a backing
ring or sufficient backing is provided by two or more weld passes made by the shielded metal arc or a gas-shielded arc process.
Gas Shielded Arc Welding. The term gas-shielded arc welding applies to those welding processes where the arc and molten metal are shielded from oxidation by some type of inert gas rather than by a flux.
1. Gas tungsten arc welding: GTAW is a form of gas-shielded arc welding where the arc is generated between the work and a tungsten electrode which is not consumed. The filler metal must be added from an external source, usually as bare filler rod or preplaced consumable insert. The filler metal is melted by the heat of the arc, and shielding gases are usually argon or helium. Alloying elements are always in the filler material. GTAW is considered to be the most desirable process for making root welds of highest quality. Techniques using added filler metal or preplaced filler metal as inserts are equally effective in manual and automatic applications.
Automatic versions can be used in all positions provided sufficient clearance is available for the equipment. Automatic versions also require tighter fit-up requirements since the equipment is set to specific parameters and will not recognize variations outside of these limits, such as a welder would do in manual applications. In automatic GTAW, the welding head orbits the weld joint on a guide track placed on the pipe adjacent to the joint to be welded. The welding head contains motors and drive wheels needed to move the head around the track, a torch to create the arc, and a spool of filler wire. Welding current, voltage, travel speed, wire feed rate, and oscillation are controlled from an external source. These parameters may be varied by the operator as the welding head traverses the weld. Oscillation and arc energy can be adjusted to permit greater dwell time and heat input into the side walls. Automatic GTAW welds are usually deposited as a series of stringer beads to minimize the effects of high interpass temperature.
2. Gas metal arc welding: GMAW is a type of gas-shielded welding generally used in the manual mode but adaptable to automation. The filler wire is the electrode and is furnished in coils or spools of solid wire. Is is fed automatically into the joint, melted in the arc, and deposited in the weld groove. Alloying elements arc in the wire, and shielding gas may be argon, helium, nitrogen, carbon dioxide, or combinations thereof, depending on the application. Depending on the equipment and the heat input settings, filler metal can be transferred across the arc is several modes. In short-circuiting transfer, the electrode actually touches the work where it short-circuits, melts, and restarts the arc. This process has low heat input and accordingly low penetrating power. It can often result in lack of fusion. Because of the low heat input, however, it can be effectively used for open-butt root pass welding. In spray transfer, the heat input parameters are sufficiently high to transfer the molten electrode across the arc as small droplets. Argon or argon-rich gases are used for shielding, resulting in a very stable spatterfree arc. Because of the high arc energy, it is normally used in the flat (1G) position. For all-position welding, a procedure which superimposes high amplitude pulses of current on a low-level steady-state current at regular intervals is often used. This results in a discrete transfer of metal with lower heat input needed for all-position welding.
3. Flux core arc welding: FCAW is a variation of GMAW where a composite electrode is substituted for the solid wire. The electrode is a tubular wire containing a flux material. Depending on the application, the arc may be self-shielding, or shielding gases may be used. Because of its high deposition rate this process is rapidly being developed for shop and field welding of piping.
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