Industrial Radiography
Exposure Vaults & Cabinets
Exposure vaults and cabinets allow personnel to work safely in the area while exposures are taking place. Exposure vaults tend to be larger walk in rooms with shielding provided by high-density concrete block and lead.

Exposure cabinets are often self-contained units with integrated x-ray equipment and are typically shielded with steel and lead to absorb x-ray radiation.

Exposure vaults and cabinets are equipped with protective interlocks that disable the system if anything interrupts the integrity of the enclosure. Additionally walk in vaults are equipped with emergency "kill buttons" that allow radiographers to shut down the system if it should accidentally be started while they were in the vault.

Image Considerations

The most common detector used in industrial radiography is film. The high sensitivity to ionizing radiation provides excellent detail and sensitivity to density changes when producing images of industrial materials. Image quality is determined by a combination of variables: radiographic contrast and definition. Many variables affecting radiographic contrast and definition are summarized below and addressed in following sections.

Radiographic Contrast


Radiographic contrast describes the differences in photographic density in a radiograph. The contrast between different parts of the image is what forms the image and the greater the contrast, the more visible features become. Radiographic contrast has two main contributors: subject contrast and detector or film contrast.

Subject contrast is determined by the following variables:
- Absorption differences in the specimen
- Wavelength of the primary radiation
- Scatter or secondary radiation
Film contrast is determined by the following:
- Grain size or type of film
- Chemistry of film processing chemicals
- Concentrations of film processing chemicals
- Time of development
- Temperature of development
- Degree of mechanical agitation (physical motion)
Exposing the film to produce higher film densities will generally increase contrast. In other words, darker areas will increase in density faster than lighter areas because in any given period of time more x-rays are reaching the darker areas. Lead screens in the thickness range of 0.004 to 0.015 inch typically reduce scatter radiation at energy levels below 150, 000 volts. Above this point they will emit electrons to provide more exposure of the film to ionizing radiation thus increasing the density of the radiograph. Fluorescent screens produce visible light when exposed to radiation and this light further exposes the film.

Definition
Radiographic definition is the abruptness of change in going from one density to another. There are a number of geometric factors of the X-ray equipment and the radiographic setup that have an effect on definition. These geometric factors include:
- Focal spot size, which is the area of origin of the radiation. The focal spot size should be as close to a point source as possible to produce the most definition.
- Source to film distance, which is the distance from the source to the part. Definition increases as the source to film distance increase.
- Specimen to detector (film) distance, which is the distance between the specimen and the detector. For optimal definition, the specimen and detector should be as close together as possible. .
- Abrupt changes in specimen thickness may cause distortion on the radiograph.
- Movement of the specimen during the exposure will produce distortion on the radiograph.
- Film graininess, and screen mottling will decrease definition. The grain size of the film will affect the definition of the radiograph. Wavelength of the radiation will influence apparent graininess. As the wavelength shortens and penetration increases, the apparent graininess of the film will increase. Also, increased development of the film will increase the apparent graininess of the radiograph.

Film Processing
Processing film is a strict science governed by rigid rules of chemical concentration, temperature, time, and physical movement. Whether processing is done by hand or automatically by machine, excellent radiographs require the highest possible degree of consistency and quality control.

Manual Processing & Darkrooms
Manual processing begins with the darkroom. The darkroom should be located in a central location, adjacent to the reading room and a reasonable distance from the exposure area. For portability darkrooms are often mounted on pickups or trailers.
Film should be located in a light, tight compartment, which is most often a metal bin that is used to store and protect the film. An area next to the film bin that is dry and free of dust and dirt should be used to load and unload the film. While another area, the wet side, will be used to process the film. Thus protecting the film from any water or chemicals that may be located on the surface of the wet side.
Each of step in film processing must be excited properly to develop the image, wash out residual processing chemicals, and to provide adequate shelf life of the radiograph. The objective of processing is two fold. First to produce a radiograph adequate for viewing, and secondly to prepare the radiograph for archival storage. A radiograph may be retrieved after 5 or even 20 years in storage.

Automatic Processor Evaluation
The automatic processor is the essential piece of equipment in every x-ray department. The automatic processor will reduce film processing time when compared to manual development by a factor of four. To monitor the performance of a processor, apart from optimum temperature and mechanical checks, chemical and sensitometric checks should be performed for developer and fixer. Chemical checks involve measurement of pH values for developer and replenisher, fixer and replenisher, measurement of specific gravity and fixer silver levels. Ideally pH should be measured daily and it is important to record these measurements, as regular logging provides very useful information. The daily measurements of pH values for developer and fixer can then be plotted to observe the trend of variations in these values compared to normal pH operating levels to identify problems.
Sensitometric checks may be carried out to evaluate if the performance of films in the automatic processors is being maximized. These checks involve measurement of basic fog level, speed and average gradient made at 1° C intervals of temperature. The range of temperature measurement depends on the type of chemistry in use, whether cold or hot developer. These three measurements: fog level, speed, and average gradient, should then be plotted against temperature and compared with the manufacturer's supplied figures.

Viewing Radiographs
Radiographs (developed film exposed to x-ray or gamma radiation) are generally viewed on a light-box. However, it is becoming increasingly common to digitize radiographs and view them on a high resolution monitor. Proper viewing conditions are very important when interpreting a radiograph. The viewing conditions can enhance or degrade the subtle details of radiographs.

Viewing Radiographs

Before beginning the evaluation of a radiograph, the viewing equipment and area should be considered. The area should be clean and free of distracting materials. Magnifying aids, masking aids, and film markers should be close at hand. Thin cotton gloves should be available and worn to prevent fingerprints on the radiograph. Ambient light levels should be low. Ambient light levels of less than 2 fc are often recommended, but subdued lighting, rather than total darkness, is preferable in the viewing room. The brightness of the surroundings should be about the same as the area of interest in the radiograph. Room illumination must be arranged so that there are no reflections from the surface of the film under examination.

Film viewers should be clean and in good working condition. There are four groups of film viewers. These include: strip viewers, area viewers, spot viewers, and a combination of spot and area viewers. Film viewers should provide a source of defused, adjustable, and relativity cool light as heat from viewers can cause distortion of the radiograph. A film having a measured density of 2.0 will allow only 1.0 percent of the incident light to pass. A film containing a density of 4.0 will allow only 0.01 percent of the incident light to pass. With such low levels of light passing through the radiograph the delivery of a good light source is important.
The radiographic process should be performed in accordance with a written procedure or code, or as required by contractual documents. The required documents should be available in the viewing area and referenced as necessary when evaluating components. Radiographic film quality and acceptability, as required by the procedure, should first be determined. It should be verified that the radiograph was produced to the correct density on the required film type, and that it contains the correct identification information. It should also be verified that the proper image quality indicator was used and that the required sensitivity level was met. Next, the radiograph should be checked to ensure that it does not contain processing and handling artifacts that could mask discontinuities or other details of interest. The technician should develop a standard process for evaluating the radiographs so that details are not overlooked.
Once a radiograph passes these initial checks it is ready for interpretation. Radiographic film interpretation is an acquired skill combining, visual acuity with knowledge of materials, manufacturing processes, and their associated discontinues. If the component is inspected while in service, an understanding of applied loads and history of the component is helpful. A process for viewing radiographs, left to right top to bottom etc., is helpful and will prevent overlooking an area on the radiograph. This process is often developed over time and individualized. One part of the interpretation process, sometimes overlooked, is rest. The mind as well as the eyes need to occasionally rest when interpreting radiographs.
When viewing a particular region of interest, techniques such as using a small light source and moving the radiograph over the small light source, or changing the intensity of the light source will help the radiographer identify relevant indications. Magnifying tools should also be used when appropriate to help identify and evaluate indications. Viewing the actual component being inspected is very often helpful in developing an understanding of the details seen in a radiograph.
Interpretation of radiographs is an acquired skill that is perfected over time. By using the proper equipment and developing consistent evaluation processes, the interpreter will increase his or her probability of detecting defects.

Contrast and Definition

The first subjective criteria for determining radiographic quality is radiographic contrast. Essentially, radiographic contrast is the degree of density difference between adjacent areas on a radiograph.

It is entirely possible to radiograph a particular subject and, by varying factors, produce two radiographs possessing entirely different contrast levels. With an x-ray source of low kilovoltage, we see an illustration of extremely high radiographic contrast, that is, density difference between the two adjacent areas (A and B) is high. It is essential that sufficient contrast exist between the defect of interest and the surrounding area. There is no viewing technique that can extract information that does not already exist in the original radiograph.
With an x-ray source of high kilovoltage, we see a sample of relatively low radiographic contrast, that is, the density difference between the two adjacent areas (A and B) is low.

Definition

Besides radiographic contrast as a subjective criteria for determining radiographic quality, there exists one other, radiographic detail. Essentially, radiographic definition is the abruptness of change in going from one density to another. For example, it is possible to radiograph a particular subject and, by varying certain factors, produce two radiographs which possess different degrees of definition.

In the example to the left, a two-step step tablet with the transition from step to step represented by Line BC is quite sharp or abrupt. Translated into a radiograph, we see that the transition from the high density to the low density is abrupt. The Edge Line BC is still a vertical line quite similar to the step tablet itself. We can say that the detail portrayed in the radiograph is equivalent to physical change present in the step tablet. Hence, we can say that the imaging system produced a faithful visual reproduction of the step table. It produced essentially all of the information present in the step tablet on the radiograph.
In the example on the right, the same two-step step tablet has been radiographed. However, here we note that, for some reason, the imaging system did not produce a faithful visual reproduction.

The Edge Line BC on the step tablet is not vertical. This is evidenced by the gradual transition between the high and low density areas on the radiograph. The edge definition or detail is not present because of certain factors or conditions which exist in the imaging system.
In review, it is entirely possible to have radiographs with the following qualities:

  • Low contrast and poor detail
  • High contrast and poor definition
  • Low contrast and good definition
  • High contrast and good definition

One must bear in mind that radiographic contrast and definition are not dependent upon the same set of factors. If detail in a radiograph is originally lacking, then attempts to manipulate radiographic contrast will have no effect on the amount of detail present in that radiograph

Radiographic Density

Film speed, gradient, and graininess are all responsible for the performance of the film during exposure and processing. As these combine with processing variables a final product or the radiograph is produced. In viewing the radiograph, requirements have been established for acceptable radiographs in industry. The density of a radiograph in industry will determine if further viewing is possible.

Density considerations date back to early day radiography. Hurder and Drifield have been credited with developing much of the early information on the characteristic curve and density of a radiograph. Codes and standards will typically require densities of a radiograph to be maintained between 1.8 to 4.0 H&D (Hurder and Drifield) for acceptable viewing. As density increases, contrast will also increase. This is true above 4.0 H&D, however as density reaches 4.0 H&D an extremely bright viewing light is necessary for evaluation.

Density, technically should be stated "Transmitted Density" when associated with transparent-base film. This density is the log of the intensity of light incident on the film to the intensity of light transmitted through the film. A density reading of 2.0 H&D is the result of only 1 percent of the transmitted light reaching the sensor. At 4.0 H&D only 0.01% of transmitted light reaches the far side of the film.

Controlling Radiographic Quality

One of the methods of controlling the quality of a radiograph is through the use of image quality indicators (IQI). IQIs provide a means of visually informing the film interpreter of the contrast sensitivity and definition of the radiograph. The IQI indicates that a specified amount of material thickness change will be detectable in the radiograph, and that the radiograph has a certain level of definition so that the density changes are not lost due to unsharpness. Without such a reference point, consistency and quality could not be maintained and defects could go undetected.

Image quality indicators take many shapes and forms due to the various codes or standards that invoke their use. In the United States two IQI styles are prevalent; the placard, or hole-type and the wire IQI. IQIs comes in a variety of material types so that one with radiation absorption characteristics similar to the material being radiographed can be used.

Hole-Type IQIs

ASTM Standard E1025 gives detailed requirements for the design and material group classification of hole-type image quality indicators. E1025 designates eight groups of shims based on their radiation absorption characteristics. A notching system is incorporated into the requirements allowing the radiographer to easily determine if the penetrameter is the correct material type for the product. The thickness in thousands of an inch is noted on each pentameter by a lead number 0.250 to 0.375 inch wide depending on the thickness of the shim. Military or Government standards require a similar penetrameter but use lead letters to indicate the material type rather than notching system as shown on the left in the image above.

Image quality levels are typically designated using a two part expression such as 2-2T. The first term refers to the IQI thickness expressed as a percentage of the region of interest of the part being inspected. The second term in the expression refers to the diameter of the hole that must be revealed and it is expressed as a multiple of the IQI thickness. Therefore, a 2-2T call-out would mean that the shim thickness should be two percent of material thickness and that a hole that is twice the IQI thickness must be detectable on the radiograph. This presentation of a 2-2T IQI in the radiograph verifies that the radiographic technique is capable of showing a material loss of 2% in the area of interest.It should be noted that even if 2-2T sensitivity is indicated on a radiograph, a defect of the same diameter and material loss may not be visible. The holes in the penetrameter represent sharp boundaries, and a small thickness change. Discontinues within the part may contain gradual changes, and are often less visible. The penetrameter is used to indicate quality of the radiographic technique and not intended to be used as a measure of size of cavity that can be located on the radiograph.

Wire Penetrameters

ASTM Standard E747 covers the radiographic examination of materials using wire penetrameters (IQIs) to control image quality. Wire IQIs consist of a set of six wires arranged in order of increasing diameter and encapsulated between two sheets of clear plastic. E747 specifies four wire IQIs sets, which control the wire diameters. The set letter (A, B, C or D) is shown in the lower right corner of the IQI. The number in the lower left corner indicates the material group. The same image quality levels and expressions (i.e. 2-2T) used for hole-type IQIs are typically also used for wire IQIs. The wire sizes that correspond to various hole-type quality levels can be found in a table in E747 or can be calculated using the following formula.

Where:
F = 0.79 (constant form factor for wire)
D = wire diameter (mm or inch)
L = 7.6 mm or 0.3 inch (effective length of wire)
T = Hole-type IQI thickness (mm or inch)
H = Hole-type IQI hole diameter (mm or inch)

Placement of IQIs

IQIs should be placed on the source side of the part over a section with a material thickness equivalent to the region of interest. If this is not possible, the IQI may be placed on a block of similar material and thickness to the region of interest. When a block is used, the IQI should the same distance from the film as it would be if placed directly on the part in the region of interest. The IQI should also be placed slightly away from the edge of the part so that atleast three of its edges are visible in the radiograph.


Exposure Calculations

Properly exposing a radiograph is often a trial and error process, as there are many variables that affect the final radiograph. In this section we make the assumptions of a generic (and fixed characteristic) x-ray source, fixed film type, and fixed quality of development.
The applet below estimates the density of the radiograph based on material, thickness, geometry, energy (voltage), current, and, of course, time. The effect of the energy and the physical setup are shown by looking at the film density after exposure. It should be noted that the applet will differ considerably from industrial x-ray configurations, and is designed for demonstration of variables in an x-ray system.

Radiograph Interpretation - Welds
In addition to producing high quality radiographs, the radiographer must also be skilled in radiographic interpretation. Interpretation of radiographs takes place in three basic steps which are (1) detection, (2) interpretation, and (3) evaluation. All of these steps make use of the radiographer's visual acuity. Visual acuity is the ability to resolve a spatial pattern in an image. The ability of an individual to detect discontinuities in radiography is also affected by the lighting condition in the place of viewing, and the experience level for recognizing various features in the image. The following material was developed to help students develop an understanding of the types of defects found in weldments and how they appear in a radiograph.

Discontinuities
Discontinuities are interruptions in the typical structure of a material. These interruptions may occur in the base metal, weld material or "heat affected" zones. Discontinuities, which do not meet the requirements of the codes or specification used to invoke and control an inspection, are referred to as defects.

General Welding Discontinuities

The following discontinuities are typical of all types of welding.
Cold lap is a condition where the weld filler metal does not properly fuse with the base metal or the previous weld pass material (interpass cold lap). The arc does not melt the base metal sufficiently and causes the slightly molten puddle to flow into base material without bonding.

Porosity is the result of gas entrapment in the solidifying metal. Porosity can take many shapes on a radiograph but often appears as dark round or irregular spots or specks appearing singularly, in clusters or rows. Sometimes porosity is elongated and may have the appearance of having a tail This is the result of gas attempting to escape while the metal is still in a liquid state and is called wormhole porosity. All porosity is a void in the material it will have a radiographic density more than the surrounding area.

Cluster porosity is caused when flux coated electrodes are contaminated with moisture. The moisture turns into gases when heated and becomes trapped in the weld during the welding process. Cluster porosity appear just like regular porosity in the radiograph but the indications will be grouped close together.

Slag inclusions are nonmetallic solid material entrapped in weld metal or between weld and base metal. In a radiograph, dark, jagged asymmetrical shapes within the weld or along the weld joint areas are indicative of slag inclusions.

Incomplete penetration (IP) or lack of penetration (LOP) occurs when the weld metal fails to penetrate the joint. It is one of the most objectionable weld discontinuities. Lack of penetration allows a natural stress riser from which a crack may propagate. The appearance on a radiograph is a dark area with well-defined, straight edges that follows the land or root face down the center of the weldment.

Incomplete fusion is a condition where the weld filler metal does not properly fuse with the base metal. Appearance on radiograph: usually appears as a dark line or lines oriented in the direction of the weld seam along the weld preparation or joining area.

Internal concavity or suck back is condition where the weld metal has contracted as it cools and has been drawn up into the root of the weld. On a radiograph it looks similar to lack of penetration but the line has irregular edges and it is often quite wide in the center of the weld image.

Internal or root undercut is an erosion of the base metal next to the root of the weld. In the radiographic image it appears as a dark irregular line offset from the centerline of the weldment. Undercutting is not as straight edged as LOP because it does not follow a ground edge.

External or crown undercut is an erosion of the base metal next to the crown of the weld. In the radiograph, it appears as a dark irregular line along the outside edge of the weld area.

 

Offset or mismatch are terms associated with a condition where two pieces being welded together are not properly aligned. The radiographic image is a noticeable difference in density between the two pieces. The difference in density is caused by the difference in material thickness. The dark, straight line is caused by failure of the weld metal to fuse with the land area.

Inadequate weld reinforcement is an area of a weld where the thickness of weld metal deposited is less than the thickness of the base material. It is very easy to determine by radiograph if the weld has inadequate reinforcement, because the image density in the area of suspected inadequacy will be more (darker) than the image density of the surrounding base material.

 Excess weld reinforcement is an area of a weld that has weld metal added in excess of that specified by engineering drawings and codes. The appearance on a radiograph is a localized, lighter area in the weld. A visual inspection will easily determine if the weld reinforcement is in excess of that specified by the engineering requirements.

Cracks can be detected in a radiograph only when they are propagating in a direction that produces a change in thickness that is parallel to the x-ray beam. Cracks will appear as jagged and often very faint irregular lines. Cracks can sometimes appear as "tails" on inclusions or porosity.

Discontinuities in TIG welds
The following discontinuities are peculiar to the TIG welding process. These discontinuities occur in most metals welded by the process including aluminum and stainless steels. The TIG method of welding produces a clean homogeneous weld which when radiographed is easily interpreted.
Tungsten inclusions. Tungsten is a brittle and inherently dense material used in the electrode in tungsten inert gas welding. If improper welding procedures are used, tungsten may be entrapped in the weld. Radiographically, tungsten is more dense than aluminum or steel; therefore, it shows as a lighter area with a distinct outline on the radiograph.

Oxide inclusions are usually visible on the surface of material being welded (especially aluminum). Oxide inclusions are less dense than the surrounding materials and, therefore, appear as dark irregularly shaped discontinuities in the radiograph.


Discontinuities in Gas Metal Arc Welds (GMAW)

The following discontinuities are most commonly found in GMAW welds.
Whiskers are short lengths of weld electrode wire, visible on the top or bottom surface of the weld or contained within the weld. On a radiograph they appear as light, "wire like" indications.
Burn-Through results when too much heat causes excessive weld metal to penetrate the weld zone. Often lumps of metal sag through the weld creating a thick globular condition on the back of the weld. These globs of metal are referred to as icicles. On a radiograph, burn through appears as dark spots, which are often surrounded by light globular areas (icicles).

 
Radiograph Interpretation - Castings

The major objective of radiographic testing of castings is the disclosure of defects that adversely affect the strength of the product. Casting are a product form that often receive radiographic inspection since many of the defects produced by the casting process are volumetric in nature and, thus, relatively easy to detect with this method. These discontinuities of course, are related to casting process deficiencies, which, if properly understood, can lead to accurate accept-reject decisions as well as to suitable corrective measures. Since different types and sizes of defects have different effects of the performance of the casting, it is important that the radiographer is able to identify the type and size of the defects. ASTM E155, Standard for Radiographs of castings has been produced to help the radiographer make a better assessment of the defects found components. The castings used to produce the standard radiographs have been destructively analyzed to confirm the size and type of discontinuities present. The following is a brief description of the most common discontinuity types included in existing reference radiograph documents (in graded types or as single illustrations).

RADIOGRAPHIC INDICATIONS FOR CASTINGS

Gas porosity or blow holes are caused by accumulated gas or air which is trapped by the metal. These discontinuities are usually smooth-walled rounded cavities of a spherical, elongated or flattened shape. If the sprue is not high enough to provide the necessary heat transfer needed to force the gas or air out of the mold, the gas or air will be trapped as the molten metal begins to solidify. Blows can also be caused by sand that is too fine, too wet, or by sand that has a low permeability so that gas can't escape. Too high a moisture content in the sand makes it difficult to carry the excessive volumes of water vapor away from the casting. Another cause of blows can be attributed to using green ladles, rusty or damp chills and chaplets.

Sand inclusions and dross are nonmetallic oxides, appearing on the radiograph as irregular, dark blotches. These come from disintegrated portions of mold or core walls and/or from oxides (formed in the melt) which have not been skimmed off prior to introduction of the metal into the mold gates. Careful control of the melt, proper holding time in the ladle and skimming of the melt during pouring will minimize or obviate this source of trouble.
Shrinkage is a form of discontinuity that appears as dark spots on the radiograph. Shrinkage assumes various forms but in all cases it occurs because molten metal shrinks as it solidifies, in all portions of the final casting. Shrinkage is avoided by making sure that the volume of the casting is adequately fed by risers which sacrificially retain the shrinkage. Shrinkage can be recognized in a number of characteristic by varying appearances on radiographs. There are at least four types: (1) cavity; (2) dendritic; (3) filamentary; and (4) sponge types. Some documents designate these types by numbers, without actual names, to avoid possible misunderstanding.
Cavity shrinkage appears as areas with distinct jagged boundaries. It may be produced when metal solidifies between two original streams of melt, coming from opposite directions to join a common front; cavity shrinkage usually occurs at a time when the melt has almost reached solidification temperature and there is no source of supplementary liquid to feed possible cavities

Dendritic shrinkage is a distribution of very fine lines or small elongated cavities that may vary in density and are usually unconnected.

Filamentary shrinkage usually occurs as a continuous structure of connected lines or branches of variable length, width and density, or occasionally as a network.

Sponge shrinkage shows itself as areas of lacy texture with diffuse outlines, generally toward the mid-thickness of heavier casting sections. Sponge shrinkage may be dendritic or filamentary shrinkage; filamentary sponge shrinkage appears more blurred because it is projected through the relatively thick coating between the discontinuities and the film surface.

Cracks are thin (straight or jagged) linearly disposed discontinuities that occur after the melt has solidified. They generally appear singly and originate at casting surfaces.

Cold shuts generally appear on or near a surface of cast metal as a result of two streams of liquid meeting and failing to unite. They may appear on a radiograph as cracks or seams with smooth or rounded edges.

Inclusions are nonmetallic materials in a supposedly solid metallic matrix. They may be less or more dense than the matrix alloy and will appear on the radiograph, respectively, as darker or lighter indications. The latter type is more common in light metal castings.

Core shift shows itself as a variation in section thickness, usually on radiographic views representing diametrically opposite portions of cylindrical casting portions. 

Hot tears are linearly disposed indications that represent fractures formed in a metal during solidification because of hindered contraction. The latter may occur due to overly hard (completely unyielding) mold or core walls. The effect of hot tears, as a stress concentration, is similar to that of an ordinary crack; how tears are usually systematic flaws. If flaws are identified as hot tears in larger runs of a casting type, they may call for explicit improvements in technique.

Misruns appear on the radiograph as prominent dense areas of variable dimensions with a definite smooth outline. They are mostly random in occurrence and not readily eliminated by specific remedial actions in the process.

Mottling is a radiographic indication that appears as an indistinct area of more or less dense images. The condition is a diffraction effect that occurs on relatively vague, thin-section radiographs, most often with austenitic stainless steel. Mottling is caused by interaction of the object's grain boundary material with low-energy X-rays (300 kV or lower). Inexperienced interpreters may incorrectly consider mottling as indications of unacceptable casting flaws. Even experienced interpreters often have to check the condition by re-radiography from slightly different source-film angles. Shifts in mottling are then very pronounced, while true casting discontinuities change only slightly in appearance.

Radiographic Indications for Casting Repair Welds

Most common alloy castings require welding either in upgrading from defective conditions or in joining to other system parts. It is mainly for reasons of casting repair that these descriptions of the more common weld defects are provided here. The terms appear as indication types in ASTM E390. For additional information, see the Nondestructive Testing Handbook, Volume 3, Section 9 on the "Radiographic Control of Welds."

Slag is nonmetallic solid material entrapped in weld metal or between weld material and base metal. Radiographically, slag may appear in various shapes, from long narrow indications to short wide indications, and in various densities, from gray to very dark.

Porosity is a series of rounded gas pockets or voids in the weld metal, and is generally cylindrical or elliptical in shape.

Undercut is a groove melted in the base metal at the edge of a weld and left unfilled by weld metal. It represents a stress concentration that often must be corrected, and appears as a dark indication at the toe of a weld.

Incomplete penetration, as the name implies, is a lack of weld penetration through the thickness of the joint (or penetration which is less than specified). It is located at the center of a weld and is a wide, linear indication.

Incomplete fusion is lack of complete fusion of some portions of the metal in a weld joint with adjacent metal; either base or previously deposited weld metal. On a radiograph, this appears as a long, sharp linear indication, occurring at the centerline of the weld joint or at the fusion line.

Melt-through is a convex or concave irregularity (on the surface of backing ring, strip, fused root or adjacent base metal) resulting from complete melting of a localized region but without development of a void or open hole. On a radiograph, melt-through generally appears as a round or elliptical indication.

Burn-through is a void or open hole into a backing ring, strip, fused root or adjacent base metal.

Arc strike is an indication from a localized heat-affected zone or a change in surface contour of a finished weld or adjacent base metal. Arc strikes are caused by the heat generated when electrical energy passes between surfaces of the finished weld or base metal and the current source.

Weld spatter occurs in arc or gas welding as metal particles which are expelled during welding and which do not form part of the actual weld: weld spatter appears as many small, light cylindrical indications on a radiograph.

Tungsten inclusion is usually denser than base-metal particles. Tungsten inclusions appear most linear, very light radiographic images; accept/reject decisions for this defect are generally based on the slag criteria.

Oxidation is the condition of a surface which is heated during welding, resulting in oxide formation on the surface, due to partial or complete lack of purge of the weld atmosphere. Also called sugaring.

Root edge condition shows the penetration of weld metal into the backing ring or into the clearance between backing ring or strip and the base metal. It appears in radiographs as a sharply defined film density transition.

Root undercut appears as an intermittent or continuous groove in the internal surface of the base metal, backing ring or strip along the edge of the weld root.

 

Real-time Radiography

 
Real-time radiography (RTR), or real-time radioscopy, is a nondestructive test (NDT) method whereby an image is produced electronically rather than on film so that very little lag time occurs between the item being exposed to radiation and the resulting image. In most instances, the electronic image that is viewed, results from the radiation passing through the object being inspected and interacting with a screen of material that fluoresces or gives off light when the interaction occurs. The fluorescent elements of the screen form the image much as the grains of silver form the image in film radiography.

The image formed is a "positive image" since brighter areas on the image indicate where higher levels of transmitted radiation reached the screen. This image is the opposite of the negative image produced in film radiography. In other words, with RTR, the lighter, brighter areas represent thinner sections or less dense sections of the test object.

Real-time radiography is a well-established method of NDT having applications in automotive, aerospace, pressure vessel, electronic, and munition industries, among others. The use of RTR is increasing due to a reduction in the cost of the equipment and resolution of issues such as the protecting and storing digital images. Since RTR is being used increasingly more, these educational materials were developed by the North Central Collaboration for NDT Education (NCCE) to introduce RTR to NDT technician students.

 
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