Sunday, February 27, 2011

Comparision of major NDT methods for Nondestructive testing of Welds

Detection of
Magnifying glass
Weld sizing gauge
Pocket rule
Straight edge
Workmanship standards
Surface flaws - cracks, porosity, unfilled craters, slag inclusions Warpage, underwelding, overwelding, poorly formed beads, misalignments, improper fitup
Low cost.
Can be applied while work is in process, permitting correction of faults.
Gives indication of incorrect procedures.
Applicable to surface defects only.
Provides no permanent record.
Should always be the primary method of inspection, no matter what other techniques are required.
Is the only "productive" type of inspection.
Is the necessary function of everyone who in any way contributes to the making of the weld.
Commercial X-ray or gamma units made especially for inspecting welds, castings and forgings.
Film and processing facilities.
Fluoroscopic viewing equipment.
Interior macroscopic flaws - cracks, porosity, blow holes, nonmetallic inclusions, incomplete root penetration, undercutting, icicles, and burn through.
When the indications are recorded on film, gives a permanent record.
When viewed on a fluoroscopic screen, a low-cost method of internal inspection
Requires skill in choosing angles of exposure, operating equipment, and interpreting indications.
Requires safety precautions. Not generally suitable for fillet weld inspection.
X-ray inspection is required by many codes and specifications.
Useful in qualification of welders and welding processes.
Because of cost, its use should be limited to those areas where other methods will not provide the assurance required.
Special commercial equipment.
Magnetic powders - dry or wet form; may be fluorescent for viewing under ultraviolet light.
Excellent for detecting surface discontinuities -
especially surface cracks.
Simpler to use than radiographic inspection.
Permits controlled sensitivity.
Relatively low-cost method.
Applicable to ferromagnetic materials only.
Requires skill in interpretation of indications and recognition of irrelevant patterns.
Difficult to use on rough surfaces.
Elongated defects parallel to the magnetic field may not give pattern; for this reason the field should be applied from two directions at or near right angles to each other.
Liquid Penetrant
Commercial kits containing fluorescent or dye penetrants and developers.
Application equipment for the developer.
A source of ultraviolet light - if fluorescent method is used.
Surface cracks not readily visible to the unaided eye.
Excellent for locating leaks in weldments.
Applicable to magnetic and nonmagnetic materials. Easy to use. Low cost.
Only surface defects are detectable.
Cannot be used effectively on hot assemblies.
In thin-walled vessels will reveal leaks not ordinarily located by usual air tests. irrelevant surface conditions (smoke, slag) may give misleading indications.
Special commercial equipment, either of the pulse-echo or transmission type.
Standard reference patterns for interpretation of RF or video patterns.
Surface and subsurface flaws including those too small to be detected by other methods.
Especially for detecting subsurface lamination-like defects.
Very sensitive.
Permits probing of joints inaccessible to radiography.
Requires high degree of skill in interpreting pulse-echo patterns. Permanent record is not readily obtained.
Pulse-echo equipment is highly developed for weld inspection purposes.
The transmission-type equipment simplifies pattern interpretation where it is applicable.