Tuesday, 24 February 2009
Flux-cored arc welding
Flux-cored arc welding (FCAW) is a semi-automatic or automatic arc welding process. FCAW requires a continuously-fed consumable tubular electrode containing a flux and a constant-voltage or, less commonly, a constant-current welding power supply. An externally supplied shielding gas is sometimes used, but often the flux itself is relied upon to generate the necessary protection from the atmosphere. The process is widely used in construction because of its high welding speed and portability.
FCAW was first developed in the early 1950s as an alternative to shielded metal arc welding (SMAW). The advantage of FCAW over SMAW is that the use of the stick electrodes used in SMAW is unnecessary. This helped FCAW to overcome many of the restrictions associated with SMAW.
Two Types of FCAW
One type of FCAW requires no shielding gas. This is made possible by the flux core in the tubular consumable electrode. However, this core contains more than just flux, it also contains various ingredients that when exposed to the high temperatures of welding generate a shielding gas for protecting the arc. This type of FCAW is attractive because it is portable and generally has good penetration into the base metal. Also, windy conditions need not be considered. Some disadvantages are that this process can produce excessive, noxious smoke (making it difficult to see the weld pool); under some conditions it can produce welds with inferior mechanical properties; the slag is often difficult and time-consuming to remove; and operator skill can be a major factor.
Another type of FCAW uses a shielding gas that must be supplied by an external supply. This is known informally as "dual shield" welding. This type of FCAW was developed primarily for welding structural steels. In fact, since it uses both a flux-cored electrode and an external shielding gas, one might say that it is a combination of gas metal (GMAW) and flux-cored arc welding (FCAW). This particular style of FCAW is preferable for welding thicker and out-of-position metals. The slag created by the flux is also easy to remove. The main advantages of this process is that in a closed shop environment, it generally produces welds of better and more consistent mechanical properties, with fewer weld defects than either the SMAW or GMAW processes. In practice it also allows a higher production rate, since the operator does not not need to stop periodically to fetch a new electrode, as is the case in SMAW. However, like GMAW, it cannot be used in a windy environment as the loss of the shielding gas from air flow will produce visible porosity (small craters) on the surface of the weld.
One type of FCAW requires no shielding gas. This is made possible by the flux core in the tubular consumable electrode. However, this core contains more than just flux, it also contains various ingredients that when exposed to the high temperatures of welding generate a shielding gas for protecting the arc. This type of FCAW is attractive because it is portable and generally has good penetration into the base metal. Also, windy conditions need not be considered. Some disadvantages are that this process can produce excessive, noxious smoke (making it difficult to see the weld pool); under some conditions it can produce welds with inferior mechanical properties; the slag is often difficult and time-consuming to remove; and operator skill can be a major factor.
Another type of FCAW uses a shielding gas that must be supplied by an external supply. This is known informally as "dual shield" welding. This type of FCAW was developed primarily for welding structural steels. In fact, since it uses both a flux-cored electrode and an external shielding gas, one might say that it is a combination of gas metal (GMAW) and flux-cored arc welding (FCAW). This particular style of FCAW is preferable for welding thicker and out-of-position metals. The slag created by the flux is also easy to remove. The main advantages of this process is that in a closed shop environment, it generally produces welds of better and more consistent mechanical properties, with fewer weld defects than either the SMAW or GMAW processes. In practice it also allows a higher production rate, since the operator does not not need to stop periodically to fetch a new electrode, as is the case in SMAW. However, like GMAW, it cannot be used in a windy environment as the loss of the shielding gas from air flow will produce visible porosity (small craters) on the surface of the weld.
FCAW key process variables
Wire feed speed (and current)
Arc voltage
Electrode extension
Travel speed
Electrode angles
Electrode wire type
Shielding gas composition (if required) Note: FCAW wires that don't require a shielding gas commonly emit fumes that are extremely toxic; these require adequate ventilation or the use of a sealed mask that will provide the welder with fresh air.
Travel Angle.
FCAW advantages and applications
FCAW may be an "all-position" process with the right filler metals (the consumable electrode)
No shielding gas needed making it suitable for outdoor welding and/or windy conditions
A high-deposition rate process (speed at which the filler metal is applied) in the 1G/1F/2F
Some "high-speed" (e.g., automotive applications)
Less precleaning of metal required
Metallurgical benefits from the flux such as the weld metal being protected initially from external factors until the flux is chipped away
Low operator skill is required
Wire feed speed (and current)
Arc voltage
Electrode extension
Travel speed
Electrode angles
Electrode wire type
Shielding gas composition (if required) Note: FCAW wires that don't require a shielding gas commonly emit fumes that are extremely toxic; these require adequate ventilation or the use of a sealed mask that will provide the welder with fresh air.
Travel Angle.
FCAW advantages and applications
FCAW may be an "all-position" process with the right filler metals (the consumable electrode)
No shielding gas needed making it suitable for outdoor welding and/or windy conditions
A high-deposition rate process (speed at which the filler metal is applied) in the 1G/1F/2F
Some "high-speed" (e.g., automotive applications)
Less precleaning of metal required
Metallurgical benefits from the flux such as the weld metal being protected initially from external factors until the flux is chipped away
Low operator skill is required
Welding defects
What are Welding-defects?
They are excessive conditions, outside the acceptance limits, which risks to compromise the stability or the functionality of the welded structure. They are also called rejectable discontinuities. This means that the same type of discontinuity of a lesser degree, might be considered harmless and acceptable.
Are there acceptable Welding-defects? No, by definition a defect is rejectable. There can be acceptable discontinuities. The designer, or the purchaser, or the person in charge of the welding project is entitled to define the limits of acceptance. And these limits are valid only for the application and the usage involved.
Are there undetected defects? Hopefully not! No Welding-defects should go undetected, but undetectable discontinuities yes, that are acceptable, as defined by the designer.
What should be done when Welding-defects are detected? One should reject the items and put them temporarily on hold. One should determine the cause and try to implement a corrective action to avoid future recurrence. Then an authorized professional should determine if the defects are repairable or not. If yes by which procedure. Standard procedures may be approved for routine application.
What is the difference between discontinuities and Welding-defects?
A discontinuity is an objective lack of material, an interruption in the physical consistence of a part. Examples are cracks, seams, laps, porosity or inclusions. It may or may not be considered a defect depending if it its presence endangers or not the integrity, the usefulness and the serviceability of the structure.
By knowing what is likely to produce Welding-defects one should learn how to avoid them. It is essential to distinguish discontinuities from harmful defects. Production without defects saves worktime, materials, repair costs, decrease in productivity. Excessive defect production indicates some basic condition affecting the operation which should be investigated and corrected.
Causes for rejection and how to avoid Welding-defects.
Avoidance of Welding-defects starts with correct design and preparation. This may look as an obvious statement but somehow it is a more frequent than desired situation. There is no point in trying to correct by welding for misalignment or for improper set up of the workpiece. There is no gain in time, really, only an increased probability of producing Welding-defects and of spending time and resources in trying to repair the welded item.
Also the use of recommended tools and fixtures should be implemented with no excuses admitted for temporary unavailability. The required means, in good operational condition, should be used with the correct parameters, according to the approved procedure. If the welding procedure is incapable of ensuring defect free implementation, then it should be improved upon.
The welder or the machine operator should be proficient in the process selected and all physical accessories assigned should be ready for use. Among them, aspirators of fumes, fan to circulate air, screens to protect other workers nearby, etc.
If electrodes need be dried, so they should be. Cleaning of fixtures and workpiece should be performed before setting up. A last touch up may be repeated just before welding.
Types of Welding-defects.
DIMENSIONAL Welding-defects can be assessed by visual inspection and by measuring with simple weld gages. They derive from improper set up or by distortion which should be controlled in a proper fixture, or by a different welding sequence. In general they should be corrected by employing proper means before welding. MISALIGNMENT is a setup problem.
Other appearance features which may cause rejection of these Welding-defects are excessive bead convexity and reinforcement, or the opposite condition, namely considerable concavity and undersized welds. Here the welder's technique should be improved.
UNDERCUT consists in a groove formed into the base metal, adjacent to the weld bead. It derives from improper manipulation of torch or electrode. Further training and improved skill of the welder should save future performance.
CRACKS are Welding-defects never permitted, because they are seen as stress raisers, and capable to grow until fracture. Different forms and positions of cracks can hint at their origin, and should be investigated before trying to correct for their appearance. Except for cases of lack of experience of the welder, who may be unable to end a weld bead without crater cracking, other instances derive mostly from limited weldability of the materials, and should be dealt with by whoever has metallurgical experience, by means of special procedures invoking pre-heat and post heat and other tricks which the welder cannot be expected to provide.
Fine cracks that cannot be seen by visual inspections, are the object of specialized inspection techniques. .
More Welding-defects...
POROSITY is a condition caused by gases remaining entrapped in the melt. This pertains generally to internal Welding-defects, which can be detected either by sectioning (which is a destructive test) or by special non destructive testing like radiography or ultrasonic testing. If this condition is determined, one should eliminate the cause, be it the material, or humidity in the electrode sheathing, or gases from excessive heat and turbulence in the melt or incorrect manipulation (improve skill).
Among other important Welding-defects one should strive to eliminate the following. INCOMPLETE FUSION, which is generally assessed by sectioning the joint (mostly a test piece) and finding the unmelted base metal that outlines the original joint shape.And INADEQUATE PENETRATION that means that the weld bead extends from its face only to a limited distance, less than what is required by the procedure.
NON METALLIC INCLUSIONS (in Shielded Metal Arc Welding) usually refer to Welding-defects in the form of slag being trapped in the melt, generally meaning insufficient skill of the welder. It could also mean oxide inclusion, in all types of welding, or tungsten metal, in Gas Tungsten Arc Welding. All these conditions can be detected either in a section or by non destructive inspection techniques.
RESISTANCE WELDING has its own set of unacceptable Welding-defects per Specification, to be determined at the stage of schedule approval by destructive testing. Shear Strength, a common Specification requirement, is determined by mechanical testing of spot welded coupons. The shape and dimensions of the weld nugget, and the appearance of Welding-defects in the form of internal cracks or inadequate fusion can only be determined in a section by destructive examinations of cut up specimens, ground, polished and etched, under a microscope.
At the production stage one should put the utmost care in manufacturing acceptable joints because it is the least expensive solution. Most of the Welding-defects are visible. The welder should be encouraged to inspect his/her own weld. Inexperienced welders should be asked to seek advice from more skilled fellow workers, and to look for help for repair in order to avoid just a cover up.
Failures in service can come up for reasons different from manual welder's skill. But this is another subject .
They are excessive conditions, outside the acceptance limits, which risks to compromise the stability or the functionality of the welded structure. They are also called rejectable discontinuities. This means that the same type of discontinuity of a lesser degree, might be considered harmless and acceptable.
Are there acceptable Welding-defects? No, by definition a defect is rejectable. There can be acceptable discontinuities. The designer, or the purchaser, or the person in charge of the welding project is entitled to define the limits of acceptance. And these limits are valid only for the application and the usage involved.
Are there undetected defects? Hopefully not! No Welding-defects should go undetected, but undetectable discontinuities yes, that are acceptable, as defined by the designer.
What should be done when Welding-defects are detected? One should reject the items and put them temporarily on hold. One should determine the cause and try to implement a corrective action to avoid future recurrence. Then an authorized professional should determine if the defects are repairable or not. If yes by which procedure. Standard procedures may be approved for routine application.
What is the difference between discontinuities and Welding-defects?
A discontinuity is an objective lack of material, an interruption in the physical consistence of a part. Examples are cracks, seams, laps, porosity or inclusions. It may or may not be considered a defect depending if it its presence endangers or not the integrity, the usefulness and the serviceability of the structure.
By knowing what is likely to produce Welding-defects one should learn how to avoid them. It is essential to distinguish discontinuities from harmful defects. Production without defects saves worktime, materials, repair costs, decrease in productivity. Excessive defect production indicates some basic condition affecting the operation which should be investigated and corrected.
Causes for rejection and how to avoid Welding-defects.
Avoidance of Welding-defects starts with correct design and preparation. This may look as an obvious statement but somehow it is a more frequent than desired situation. There is no point in trying to correct by welding for misalignment or for improper set up of the workpiece. There is no gain in time, really, only an increased probability of producing Welding-defects and of spending time and resources in trying to repair the welded item.
Also the use of recommended tools and fixtures should be implemented with no excuses admitted for temporary unavailability. The required means, in good operational condition, should be used with the correct parameters, according to the approved procedure. If the welding procedure is incapable of ensuring defect free implementation, then it should be improved upon.
The welder or the machine operator should be proficient in the process selected and all physical accessories assigned should be ready for use. Among them, aspirators of fumes, fan to circulate air, screens to protect other workers nearby, etc.
If electrodes need be dried, so they should be. Cleaning of fixtures and workpiece should be performed before setting up. A last touch up may be repeated just before welding.
Types of Welding-defects.
DIMENSIONAL Welding-defects can be assessed by visual inspection and by measuring with simple weld gages. They derive from improper set up or by distortion which should be controlled in a proper fixture, or by a different welding sequence. In general they should be corrected by employing proper means before welding. MISALIGNMENT is a setup problem.
Other appearance features which may cause rejection of these Welding-defects are excessive bead convexity and reinforcement, or the opposite condition, namely considerable concavity and undersized welds. Here the welder's technique should be improved.
UNDERCUT consists in a groove formed into the base metal, adjacent to the weld bead. It derives from improper manipulation of torch or electrode. Further training and improved skill of the welder should save future performance.
CRACKS are Welding-defects never permitted, because they are seen as stress raisers, and capable to grow until fracture. Different forms and positions of cracks can hint at their origin, and should be investigated before trying to correct for their appearance. Except for cases of lack of experience of the welder, who may be unable to end a weld bead without crater cracking, other instances derive mostly from limited weldability of the materials, and should be dealt with by whoever has metallurgical experience, by means of special procedures invoking pre-heat and post heat and other tricks which the welder cannot be expected to provide.
Fine cracks that cannot be seen by visual inspections, are the object of specialized inspection techniques. .
More Welding-defects...
POROSITY is a condition caused by gases remaining entrapped in the melt. This pertains generally to internal Welding-defects, which can be detected either by sectioning (which is a destructive test) or by special non destructive testing like radiography or ultrasonic testing. If this condition is determined, one should eliminate the cause, be it the material, or humidity in the electrode sheathing, or gases from excessive heat and turbulence in the melt or incorrect manipulation (improve skill).
Among other important Welding-defects one should strive to eliminate the following. INCOMPLETE FUSION, which is generally assessed by sectioning the joint (mostly a test piece) and finding the unmelted base metal that outlines the original joint shape.And INADEQUATE PENETRATION that means that the weld bead extends from its face only to a limited distance, less than what is required by the procedure.
NON METALLIC INCLUSIONS (in Shielded Metal Arc Welding) usually refer to Welding-defects in the form of slag being trapped in the melt, generally meaning insufficient skill of the welder. It could also mean oxide inclusion, in all types of welding, or tungsten metal, in Gas Tungsten Arc Welding. All these conditions can be detected either in a section or by non destructive inspection techniques.
RESISTANCE WELDING has its own set of unacceptable Welding-defects per Specification, to be determined at the stage of schedule approval by destructive testing. Shear Strength, a common Specification requirement, is determined by mechanical testing of spot welded coupons. The shape and dimensions of the weld nugget, and the appearance of Welding-defects in the form of internal cracks or inadequate fusion can only be determined in a section by destructive examinations of cut up specimens, ground, polished and etched, under a microscope.
At the production stage one should put the utmost care in manufacturing acceptable joints because it is the least expensive solution. Most of the Welding-defects are visible. The welder should be encouraged to inspect his/her own weld. Inexperienced welders should be asked to seek advice from more skilled fellow workers, and to look for help for repair in order to avoid just a cover up.
Failures in service can come up for reasons different from manual welder's skill. But this is another subject .
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