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 .