More difficult to pass - Guided-Bend or X-Ray?

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Which is more difficult - passing a guided-bend test or passing a radiographic examination of the same weld?
Reply:Nancy,The answer depends in part on the code that is being used to judge the test results.  API, AWS, ASME are different codes, with different criteria for what consistutes a discontinuity or a defect.For clarity, a discontinuity is some form of inclusion or void in the test that is small enough to be deemed acceptable.  A defect is an inclusion or void that is larger than the upper limit(s) for what is considered acceptable by the standard being applied.All that said, speaking in general terms, I think that the radiographic examination is tougher to pass.  Small defects in the center of a bend strap may not show through to the surface.X-rays are difficult to interpret, and therefore I think it more likely that there could be false positive results that can't be easily confirmed or refuted.A visible discontinuity in a bend strap can be measured, and evaluated with a number of different tools, to determine the size of the discontinuity/defect, and therefore, I think is less likely to give false positive(failure) results than an X-ray.Neither tool comprehensive in judging a welding procedure or welder's qualifications to make a given weld.  Some welding applications have extremely high standards for mechanical properties.  Neither X-rays or guided bend tests give diffinitive results about a weldment's impact toughness or tensile strength.I hope this sheds some light on your question.  If not, please provide some more background information for your inquiry, and I'll try to give a more relevant response.Benson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:A_DAB_will_do,It's nice to get such a comprehensive response.  THANKS.  The purpose of my question was to see who's right - a colleague of mine and I disagree on the answer.  I say RT is more difficult, since our welders have no problem passing a bend test, but seem not to fair as well on RT.The Code we're asked to meet is usually ASME B31.3 (category D or normal fluids), which I don't always agree with since design pressure of our piping systems is often times under 15 PSI.  We weld stainless steel pipe (12" to 66" in diameter), usually thin wall (gauge piping), in the field (often outside), in position - not the easiest welding as you probably know.  RT keeps me up at nights.
Reply:I'm no expert on any of the requirements spelled out in any of the commonly used welding standards.  My perception is that ASME has some of the most stringent requirements; usually with good reason.  Do you have a choice of welder qualification method under ASME B31.3?I'm curious as to what type of defect you see most often in the radiographs?If you're welding thin walled stainless, I'm guessing that most of the welding is done with the TIG process.  I would hazard another guess and say that small tungsten inclusions, from accidentally dipping the tungsten in the weld pool, are a common occurrence.  I'd also bet that most of these small inclusions don't affect a bend test, but show up like shooting stars in the radiographs.I can also imagine a scenario where your welding large OD pipe, and having difficulty with oxide inclusions; because back purge has got to be difficult on med-large OD pipe?Or is your company using MIG for some/all of this work?.  I think there's a whole different subset of defects that you must encounter with MIG as compared to TIG?Benson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:Bend test is done on a coupon, right?X-ray done on the final field welded item?That -could- be a difference right there.Also, B31.3 code requirements do NOT apply to the following excluded categories:a.  piping systems designed for internal gage pressures at or above zero but less than 105 kPa (15 psi), provided the fluid handled is nonflammable, nontoxic, and not damaging to human tissue as defined in 300.2, and its design temperature is from -29 C (-20 F) through 186 C (366 F); b.  power boilers in accordance with BPV Code Section I and boiler external piping which is required to conform to B31.1; c.  tubes, tube headers, crossovers, and manifolds of fired heaters, which are internal to the heater enclosure; and d.  pressure vessels, heat exchangers, pumps, compressors, and other fluid handling or processing equipment, including internal piping and connections for external piping.  The best laid schemes ... Gang oft agley ...
Reply:I agree it depends on the code/spec.We have frequent problems passing thin sheet x-rays on aluminum to MIL-1595.  What happens is the oxides float to the surface and the middle of the weld.  There is a visible oxide line down the centerline. It shows up on the x-ray as a linear indication. The level III's reject for cracks.  I have to fight back to override their disposition.  It is hard because I am not a level III.  I have sectioned them through the "linears" and never found a single crack using a microscope.
Reply:Just the kind of thing I was thinking of in my original post.  I don't have my ASNT certifications, but I used to do X-ray radiography(film) and Digital CT on engine block castings and other automotive components.  It's very easy to see things that 'aren't there' in radiographs; particularly with film radiography.  In digital CT you can manipulate a 3D representation, and get any perspective you want on a part.  This helps you eliminate the ghosts.  In film radiography, you're looking at everything superimposed overtop of everything else.  It very easy to see things that aren't there due to the overlapping perspective....None of this is a condemnation of film radiography.  But it is a word of warning and a healthy dose of skepticism when it comes to the tool.   Originally Posted by obewanI agree it depends on the code/spec.We have frequent problems passing thin sheet x-rays on aluminum to MIL-1595.  What happens is the oxides float to the surface and the middle of the weld.  There is a visible oxide line down the centerline. It shows up on the x-ray as a linear indication. The level III's reject for cracks.  I have to fight back to override their disposition.  It is hard because I am not a level III.  I have sectioned them through the "linears" and never found a single crack using a microscope.
Reply:The problem with the sub 15-psi thing and 31.3 is that it's usually just easier for clients to arbitrarily choose to spec out 31.3, rather than look for a suitable alternative or come up with their own inspection criteria.Back to the topic at hand, I'm in the "it depends" category.  Sometimes you get away with sub-surface defects on bend tests, others can be missed based on bend orientation.  RT can be more discerning, and at the other end, you can miss quite a bit based on joint geometry, geometric unsharpness, lack of fusion defects, etc etc.  Six in one, half dozen in the other.
Reply:Nancy,This is difficult to give an answer. But typically I would say that a wrap around bend test would be the easiest."Religion is regarded by the common people as true, by the wise as false, and by the rulers as useful." -- Seneca the Younger