NDE is a general term used to identify all methods that permit evaluation of welds and adjacent areas without destroying their usefulness. Here the following basic NDE methods will be discussed:
It should be noted that NDE does not eliminate the need for destructive testing but rather complements it. The general knowledge presented here should be of valuable assistance to the reader as it provides an overview of the examination methods without unnecessary details.
The integrity of most welds is verified principally by visual examination. Even for weldments with joint specified for inspection throughout by other NDE methods, VT still constitutes an important part of practical quality control. The most extensively used of any method of NDE, VT is easy to apply, quick, and often requires no special equipment other than good eyesight and some relative simple and inexpensive tools.
Despite the many advantages of VT, a major disadvantage is the need for an inspector who has considerable experience and knowledge in many different areas which encompass visual welding examination. The inspector must be familiar with materials, drawings, codes, specifications, weld procedures, performance qualification, procedure qualification requirements, and workmanship standards, and all aspects of good shop practice. Some codes and specifications require that the welding inspector be qualified and certified by examination.
Certain tools are sometimes necessary for some aspects of VT. Various measuring scales and gauges are used for checking the dimensions of the welds. There are many different types of fillet weld gauges used to determine the size of fillet welds. Other gauges can be used to verify root opening, weld reinforcement, and weld bevel angle. Measuring devices are used to check root openings, clearance dimensions of materials, backing materials, and alignment and fit-up of the work pieces. Temperature indicators verify preheat and interpass temperatures. Borescopes, video scopes, flashlights and mirrors are used in areas of limited accessibility. The development of flexible fiber optic inspection systems enables the inspector to visually inspect areas inaccessible to other devices.
PT is a sensitive method of detecting and locating discontinuities, provided the discontinuities are clear and open to the surface. The method employs a penetrating liquid dye which is applied to the properly cleaned surface to be examined and which enters the discontinuity. After a suitable swell time, the excess penetrant is removed from the surface and the part is dried. A developer is then applied which acts as a blotter, drawing the penetrant out of the discontinuity. The penetrant drawn from an opening on the surface indicates the presence and location of a discontinuity.
There are two basic classification of the penetrant method, both using a similar principle. One uses a visible dye and the other uses a fluorescent dye which is only visible with exposure to UV light. Visible penetrant is usually red in color to provide a contrast against the white developer background. Normal white light is usually sufficient to view the discontinuities.
Fluorescent penetrants provide a greenish yellow indication against a dark background when viewed in darkened area under a black (ultraviolet) light source. The fluorescent method is inherently more sensitive due to the fact that human eye can more easily discern a fluorescent indication.
PT is widely applicable on magnetic and non-magnetic materials, but it is particularly useful on nonmagnetic materials such as aluminum, magnesium, and austenitic stainless steels where MT examination cannot be used. It is also useful for locating cracks or other discontinuities which may cause leaks in containers and pipes.
There are two common methods of recording a PT indication for evaluation. A photo may be taken of the discontinuities exposed by the examination. Another method involves the application of clear plastic tape over the indication. When the tape is lifted off the test surface, the indication will adhere to the tape and may be transferred to the inspection report for future reference. These techniques also apply to MT method.
PT method is relatively inexpensive. The process is simple and the operators find little difficulty in learning to apply it properly. The success of this method, like most other examination methods, depends on the visual acuity of the inspector. It should be pointed out that some substances in penetrants can have deleterious effect on either welds or base metals and can affect the service life of the weld or application of the product. Penetrants are difficult to remove completely from discontinuities, and if corrosive to the material, or otherwise not compatible with the product application, they should be avoided.
This method is used for locating surface or near surface discontinuities in ferromagnetic materials. MT is based on the principles that magnetic lines of force will be distorted by a change in material continuity, i.e. discontinuity creating a magnetic field or flux leakage.
A weldment can be magnetized by passing an electric current through the weld area (direct magnetization) or by placing the weldment in a magnetic field (indirect magnetization). When the magnetic field has been established within the workpiece, magnetic particles (medium) are applied to the surface to be examined. The magnetic particles can be dry or suspended in a liquid. Discontinuities can be further enhanced using fluorescent magnetic particles and observing them under black light. After removal of excess particles, the remaining particles trapped in the leakage field of a discontinuity reveal the location, shape, and size of a detectable discontinuity. These indications usually are distinguished by their appearance as sharp, well defined lines of medium against the background of the weld or heat-affected zone surface.
MT can be very beneficial as an in-process evaluation technique. Assurance of a sound weld before the weld is completed may prevent costly repair of the final product. In-process MT has become more of a common practice due to the portability of modern lightweight equipment. This advantage aids in reducing production time.
The cost of MT is considerably less expensive than radiography (RT) and ultrasonic (UT) – both in terms of the equipment cost and the cost of training the personnel. Using MT, the inspector obtains an instant visible indication of the size and orientation of the discontinuity, and allows the inspector to judge if the discontinuity is acceptable or rejectable. Compared to PT, this method has the advantage of revealing discontinuities that are not open to the surface, and therefore not detected by PT. MT is generally faster, requires less surface preparation, and is therefore usually more economical than PT (neglecting equipment costs).
The MT method is limited to ferromagnetic materials. Welded joints made between metals of dissimilar magnetic characteristics may create irrelevant magnetic particle indications even though welds themselves are sound. Most weld surfaces are acceptable for MT after the removal of slag, spatter or other extraneous material which may mechanically hold the test medium.
RT is a method of NDE that utilizes radiation to penetrate a weld and reveal information about its internal condition. When a weld is exposed to penetrating radiation, some radiation will be absorbed, some scattered, and some transmitted through the weld onto recording device (See Figure 13). Most conventional RT techniques used today involve exposures that record a permanent image on a photographic film, although other image recording methods are also used.
The basic process of radiographic examination involves two general steps:
The essential elements needed to carry out these two operations are:
Two types of radiation sources commonly used in weld inspection are X-ray machines and radioactive isotopes. X-radiation is produced by machines which range from portable, low energy units capable of radiographing relatively thin objects, to mammoth linear accelerators and betatrons capable of radiographing thick steel welds up to 20 in. of steel. Gamma radiation is emitted by radioisotopes, the two most common being Cobalt 60 which will penetrate to approximately 5 in. of steel, and Iridium 192 which is limited to a steel thickness of approximately 3 in.
The radiographic process is dependent upon varying amount of radiation being absorbed by different areas of the weld. The differences in the absorption occurring during the exposure process account for the dark and light regions on the radiograph. The interpretation of a radiograph involves identifying the images resulting from various light and dark regions on the film. The dark regions represent the easily penetrated parts of the weld (i.e., thin sections and most discontinuities) while the lighter areas represent the more difficult areas to penetrate (i.e. thick sections).
A significant limitation of radiographs is that discontinuities must be favorably aligned with the radiation beam to be reliably detected. This is usually not a problem for discontinuities such as porosity or slag since they are usually round in cross-section and align with a beam from any direction. This is not the case with planar discontinuities such as cracks, incomplete fusion, and laminations. There are several other limitations associated with radiography:
However, there are some advantages as well:
UT is becoming one of the most widely used methods of NDE. Its primary application is the detection and characterization of internal discontinuities. It is also used to detect surface discontinuities, to define bond characters, and to measure thickness. In this method, high-frequency sound waves are introduced into the material to detect surface and subsurface discontinuities. The sound waves travel through the material with some loss of energy (attenuation) and are reflected at interfaces. The reflected sound beam is detected and analyzed to define the presence and location of discontinuities.
UT is usually performed with either longitudinal waves (straight beam) or shear waves (angle beam). In longitudinal beam testing (commonly used to examine plate material), sound in the form of ultrasonic vibrations is introduced into part perpendicular to the entry surface by straight beam search unit. When the entry surface and the back surface are parallel, a back reflection will appear on the display screen. A discontinuity lying between the front and back surfaces will also be displayed on the display screen. By measuring the height of the reflection on the display screen, from a real or artificial discontinuity of a known size, a reference level can be established such that reflections from discontinuities of unknown sizes may be evaluated.
The angle beam technique is used for the examination of welds. Ideally, only discontinuities should appear on the display screen during the angle beam inspection. This is not always the case, however, since the geometrical boundaries of the part often reflect sound in same manner as a discontinuity. Therefore care must be taken during UT of joints with complex geometries (such as welds with backing bars) to assure that the indications are the result of the presence of discontinuities and not simply due to the configuration of the joint.
It is generally desirable to have the sound beam intercept the plane of the discontinuity at or near 90o so that the maximum amount of sound is reflected to the transducer. However, cracks that are not oriented perpendicular to the UT beam can be detected because their surfaces are not smooth and sound is reflected from the facets that are approximately perpendicular to the beam. Selection of test surface for scanning with the search unit depends upon accessibility. Scanning surface selection is also based on the weld shape and structure. Since it is important to intercept the discontinuity at or near 90o, it is common for more than one angle unit to be used to examine a particular weld.
The principal advantages of UT over other NDE methods for metal parts are:
Some disadvantages of UT include:
Table 2 relates the examination methods to various types of discontinuities. Table 3 relates the joint types to the applicable NDE methods.
Inspection Methods | |||||||
Discontinuities | RT | UT | PT | MT | VT | ET | LT |
Porosity | A | O | A | O | A | O | A |
Slag inclusions | A | O | A | O | A | O | O |
Incomplete fusion | O | A | U | O | O | O | U |
Incomplete joint penetration | A | A | U | O | O | O | U |
Undercut | A | O | A | O | A | O | U |
Overlap | U | O | A | A | O | O | U |
Cracks | O | A | A | A | A | A | A |
Laminations | U | A | A | A | A | U | U |
Notes:
Legend:
Inspection Methods | |||||||
Joints | RT | UT | PT | MT | VT | ET | LT |
Butt | A | A | A | A | A | A | A |
Corner | O | A | A | A | A | O | A |
Tee | O | A | A | A | A | O | A |
Lap | O | O | A | A | A | O | A |
Edge | O | O | A | A | A | O | A |
Legend:
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