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Chapter 1Introduction

1

In this chapter:

Overview of differential

absorption (attenuation)

Variables involved in

radiographic testing

Brief definitions of

procedures, codes and

standards Preview of topics

RT_Vol6_Rev_RT Programmed Instruction 7/14/10 1:35 PM Page 1

Draft for Review Purposes Only - (c) ASNT


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2 Programmed Instruction: Radiographic Testing

The results of almost all NDT methods are indirect. The results are oftenin the form of signals, waveforms and images. In film radiography, we geta permanent image on a photographic film. The images are the shadows of

the discontinuities that are favorable to the radiation beam and to therecording film plane. The interpreter has to translate the images intounderstandable language so that everyone concerned with the job canunderstand and discuss the discontinuities in terms of causes andcorrections that can be achieved. Correct interpretation provides gooddecision making. Wrong interpretation may be very harmful.

The aim of the radiographic method is to produce the discontinuitysshadow in the most detectable form, with sharp details and with an abilityto see the discontinuity in the film. A discontinuity in the line of the

radiation beam and perpendicular to the direction of the film plane ismost effectively recorded. A radiographic image is the plan view of thediscontinuity. For example, the greater the thickness of the inclusion orvoid in the direction of the beam, the greater the amount of differentialabsorption or attenuation either low or high based on the atomicnumber of the inclusion.

The Meaning of Differential Absorption

The absorption (or attenuation) of radiation intensity by a material isbased on its thickness and atomic number.

Thickness ChangeWhen there is no change in thickness, the radiation absorption orattenuation is uniform and the transmitted intensity is uniform. Thismeans the film receives the same intensity or quantity of radiation, andthe film is uniformly exposed. The resulting radiograph has the sameoptical density indicating there is no change in thickness. (See Figure 1.1.)

If the absorber thickness increases, the absorption increases, and if theabsorber thickness is reduced, the absorption by the absorber decreases.The final radiograph has lesser density for higher thicknesses and greaterdensity for lower thicknesses. This lighter or darker density is one part ofradiographic interpretation.




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Atomic Number ChangeThe second factor which causes a change in density, producing a darker orlighter image, is the atomic number of the material. For example, theatomic number of iron (Fe) is 26 in the periodic table of elements, and itabsorbs some quantity of radiation. In a steel weld, if a tungsten inclusionis included or gets entrapped, both of the metals are present in the weld.The atomic number of tungsten (W) is 74; hence, it has greater absorptionof radiation in comparison with steel. Practically speaking, the tungsten

inclusion will attenuate all of the radiation intensity, leaving no intensityor quantity of radiation for the film plane. The effect on the radiograph isa totally lighter image compared to its surrounding area. (See Figure 1.2.)

Specimen

Film

Developed film

(radiograph)

Figure 1.1: Effect of change in specimen thickness on radiographic film, based on the principle

that as specimen thickness increases, absorption (or attenuation) increases.

Linear absorption

coefficient (cm1)

Atomic numberZ

Fe = 26 W = 74

Figure 1.2:The theoretical concept of radiation absorption (attenuation), based on the

principle that as the atomic number increases, absorption (attenuation) increases.

3Volume VI: Radiographic Interpretation




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Similarly, if the weld has entrapped gas or porosity, the radiation willeasily pass through, getting absorbed much less than steel, and theintensity received by the film plane is higher. The higher quantity of

radiation received by the film causes more darkness. (See Figure 1.3.)

The shadow of every inclusion independent of its atomic number isformed because of the straight-line propagation of radiation. Thus, theshadow of the discontinuity produced on the radiograph is either darkeror lighter. So now you know that the reason for this darker or lighterimage is the differential absorption (attenuation) of radiation by thematerial.

Keep in mind that a specimen or component can have both a change inthickness and atomic number. Both of these parameters will absorb (orattenuate) the radiation differentially, causing a lighter or darker image.

Question 1.1

Select the best answer with respect to the topic differentialabsorption:

The plate thickness of a steel butt joint is 20 mm (0.79 in.) and the

reinforcement is 3 mm (0.12 in.). The darkness (optical density) ofthe plate is 2.8. The weld density would be:

A. More than 2.8.B. Less than 2.8.

Answer is on p. 8.

Gas hole Tungsten

inclusion

Figure 1.3: Radiographic images produced by two different kinds of discontinuities.


Film




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Question 1.2

In the above weld, if tungsten and porosity are included, the density

of tungsten will be:

A. Less compared to the weld density.B. More compared to the weld density.

Answer is on p. 8.

Variables in Radiographic Testing

As we have seen, the energy absorption by a material is based on thicknessand atomic number. There are many variables in recording a radiographicimage. They include:

1. The radiation source its energy affects the contrast.2. The source dimension, the distance from the source to the

object and the distance from the object to the film these havean effect on geometrical unsharpness and distortion.

3. The test specimens size, shape and geometry these factorsdetermine the most suitable technique to project the image onto

the film so as to obtain the best possible image for interpretation.4. The type of film used and processing cycle these have an effect

on the sensitivity of the radiograph.5. The technique employed the technique must be chosen so as to

produce the best possible shadow of the discontinuities.6. Illumination and the illuminator in the inspection booth to

see the discontinuities by transmitting the light through theradiograph.

7. Procedures, codes and standards for acceptance or rejection toperform the job uniformly by all and to get uniform results byanyone who does the job.

8. Interpreters knowledge of various primary processes and theirassociated discontinuities, as well as their most probablelocation and shape to characterize the discontinuity and tomake a decision for acceptance or rejection.




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9. Use of collimators, filters and masks to prevent unwantedscatter radiation and reduce the effect of undercutting of theradiograph due to scatter radiation.

10. The penetrating thickness range for a given energy thethickness of a specimen may be too small, resulting in a loss ofcontrast, or the specimen thickness may be too great so that theradiation energy may be too small to penetrate it.

11. Mishandling of film before and after exposure this results inunwanted images or false indications called artifacts.

12. Operators ability to distinguish between relevantdiscontinuities and artifacts an important aspect ofradiographic interpretation.

In order to perform consistent interpretations, the interpreter must knowthe following:

1. The history of the part and the manufacturing process.2. The discontinuities associated with the process.3. The probable location of relevant discontinuities.4. The severity of relevant discontinuities and their effects during

service.5. The most suitable radiographic technique to record the best

possible image.

6. The location and the type of stress a component will undergo onceit is put into use; the orientation of discontinuities with referenceto the principle stress axis.

7. Sensitivity requirements.8. The difference between relevant discontinuities and artifacts.

Armed with knowledge of the above, the interpreter is in a position tocharacterize the discontinuity, assess the possible cause of thediscontinuity and make a decision for acceptance or rejection.

Procedures, Codes and Standards

Procedures are step-by-step instructions given to the radiographer toradiograph an object. Procedures enable uniform results, includingrepeatability of test results. Codes of practice are developed by expertsbased on past data, experimental results, etc.




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Preview of Volume 6

Volume 6 is a guide to the interpreter on how to interpret radiographs

within reason. Interpretation also aids production engineers to improvetheir process, minimize rejection and move toward 100% reliability of acomponent through its entire life cycle.

The user of Volume 6 will understand the art of interpreting radiographs,including the ability to distinguish different discontinuities and artifacts.After successful interpretation, you will be prepared to evaluatediscontinuities for acceptance/rejection per code.

The topics covered in this volume follow the outlines inANSI/ASNT

CP-105-2006.

The first part of this training manual describes the results of NDT thatis, the indications produced, types of discontinuities, and manufacturingprocesses along with associated discontinuities.

The second part of this manual explains in detail the various types ofdiscontinuities, their probable location and their image on radiographicfilm. Once the indications are interpreted, then they may be evaluatedwith reference to various codes and standards, presented in the form of

test reports.

Now lets see how well you already know the effects of welding and castingdiscontinuities on radiographic film in the form of a short quiz.




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Answers to Chapter 1 Questions

Question 1.1

Answer: B.

A If A is your choice, you have not understood the absorptionprinciple with regard to thickness. Remember thatabsorption is related to thickness. The more thickness, themore absorption and less intensity to the film. Therefore,your selection should have been B.

B If B is your choice, then you have understood the principle ofabsorption. We have discussed that a greater thickness will

absorb more radiation and the available intensity under athick area to the film is less than the thinner area. Thus, thedensity will be less than 2.8.

Questions 1.2

Answer: A.

A Your choice A is excellent. Tungsten has a higher atomicnumber and denser material compared to steel. Therefore,

tungsten has absorbed the energy, leaving no intensity forthe film to produce photographic density.

B B is your choice? Then you are incorrect. This is an importantarea where you have to understand how image densitycharacterizes a discontinuity. All less dense inclusions givedarker images compared to images produced by densermaterials. For example, lead letters and numbers aretypically used as radiographic markings. The lead is ahigh-density material with a high atomic number comparedto steel and thus absorbs essentially all of the radiation,

leaving no or little radiation for the film and projecting theshape of the letters, which can be read against a darkerbackground.




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Volume 6 Preview Quiz

Complete the following with respect to steel weld radiographs using

the term darker or lighter to describe the image of thediscontinuity that would result:

1. Porosity: __________.

2. A nonmetallic slag inclusion: __________.

3. A tungsten inclusion: __________.

4. Excess penetration: __________.

5. Underfill: __________.

6. Crack: __________.

Would an aluminum casting radiograph show the followinginclusions as darker or lighter? Choose one of these terms todescribe the effect of the discontinuity on radiographic film:

7. Inclusion of a steel wire: __________.

8. Microporosity: __________.

9. Nonmetallic inclusion: __________.

10. Thicker section in the casting: __________.

11. A crack: __________.




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10 Programmed Instruction Book: Radiographic Testing

Volume 6 Preview Quiz Key

1. darker

2. darker

3. lighter

4. lighter

5. darker

6. darker

7. lighter

8. darker

9. darker

10. lighter

11. darker




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11

In this chapter:

Types of radiographic

indications Brief review of artifacts

Detection of discontinuities

using radiography

Imaging factors enabling

radiographic interpretation

and evaluation

Review of image quality

indicators (IQIs)

Illumination levels

Characterizing discontinuitiesby type and origin

Understanding the difference

between discontinuities and

defects

Chapter 2

Indications, Discontinuities andDefects




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What Is an Indication?

Locations on a nondestructively tested object where the normal physical

structure has a discontinuity by way of separation of material, inclusion,crack, etc., are referred to as indications. The testing method produces anindication by way of signal, accumulation of powder particles, excessbleedout, difference in the darkness in radiographic film and so on.

Compared to other types of inspection, such as measurement ofdimensions, metallographic inspections, hardness tests or destructive testmethods where direct results are obtained, nondestructive inspectionproduces the results in the form of indications.

In summary, we can define an indication as an evidence of discontinuityformed at a certain location where the normal physical structure hasbroken. In radiography, an indication means a density change appearing ona radiograph.

In NDT all such indications are to be:

1. Interpreted.2. Evaluated for acceptance/rejection and reported.

The purpose of nondestructive testing of materials is to locate a relevantdiscontinuity. The presence of a discontinuity is indicated in the form ofan image or signals.

By applying certain physical principles on materials, and withoutdestroying the job in any manner, it is possible to locate discontinuities. Anormal break in physical structure is called a nonrelevant discontinuity.When such discontinuities are made possible for an operator to see orlocate, directly or indirectly by application of certain principles, they arecalled indications. The indication is interpreted and evaluated for

acceptance/rejection.

Radiographic IndicationsIn radiography, any darker or lighter image is an indication. Radiographictesting produces an indication by means of penetrating radiation when itis differentially absorbed by a change in thickness, atomic number ordensity. The net effect is a difference in intensity received by the film,




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which in turn produces a darker or lighter image that is to be interpretedand evaluated, as illustrated in Figure 2.1.

As shown in the above figure, not all indications appearing on theradiographic film need be discontinuities. Indications can be true or false,as defined below.

False indications are due to film artifacts, screen problems, fog,scatter, mottling, etc.

True indications are due to differential absorption that is, athickness change or change in atomic number.

Furthermore, a discontinuity and defect can be differentiated as follows:

Discontinuity a break in the test specimens structural continuity. Defect a condition that renders the specimen unsuitable for

intended service.

All true indications are formed due to the physical principle with whichthe test is being conducted, whereas all false indications do not obey thisprinciple and do not reappear after reprocessing!

When the interpreter views a radiograph, he or she sees darker or lighter

shadows. Some of the images are due to the presence of discontinuitiesand some may not have any relevance, meaning that they are notassociated with a possible defect. Such indications with no relevance to adefect or discontinuity are called false indications. So all indications areto be interpreted as true or false.

No discontinuity

no indication

Relevantdiscontinuity

true indication False indication

Nonrelevantdiscontinuity

true indication

No density

differenceA crack darker

density

Damaged lead

screen lighter

density

Bolt hole darker density

Figure 2.1: Radiographic effects of different types of discontinuities and indications.





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Moreover, true indications formed due to differential absorption(attenuation) are to be further classified as relevant and nonrelevantindications.

True relevant indications in radiographs are due to discontinuities suchas:

Incomplete penetration/lack of penetration. Incomplete root fusion. Lack of sidewall fusion. Root concavity/suck back or suck up. Inclusions. Porosity.

Undercut. Cracks. Excess penetration. Underfill. Undercut. Burn-through. Hot tear. Shrinkage. Arc strike.

All of the above indications are characterized by their:

Shape. Location. Difference in density (contrast). Thickness of the discontinuity by measuring the optical density.

True nonrelevant indications are also formed due to differentialabsorption; however, such indications are the result of changes ingeometry or differences in thickness due to design considerations.Examples of nonrelevant indications include:

A keyway. A hole drilled in a flange. Two different thicknesses joined in a welding process. A boss in a casting.




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In radiography, false indications are also called artifacts, which form dueto improper handling of film during any stage of the radiographic process.For example, when the film is being manufactured, a static charge may

occur. A static current so developed may expose the film causing a falseindication. Static charges may also be inadvertently produced in thedarkroom by the radiographer when the film is removed or inserted intothe cassette with a rubbing action.

False indications include:

Static marks branchlike, jagged dark lines or irregular dark spotsoriginating from rapid loading or unloading of film.

Pressure marks produced by extreme pressure on an area of film.

Chemical stain streaks on the film caused by inadequate removalof chemicals between processing stages or insufficient agitation ofthe film hanger.

Crimp marks caused by abrupt bending of film; typically crescentshaped.

Water mark circular pattern caused by water droplets drying onthe film surface.

Reticulation formation of a network of wrinkles or cracks in aphotographic emulsion.

Dichoric fog a stain visible under reflected or transmitted light

due to improper development. Frilling of emulsion loosening of the emulsion from the film base

due to warm or exhausted fixer solution, high temperature ofprocessing solutions or prolonged washing in warm water.

Scratches caused by abrasive materials or rough handling,including fingernails.

Damaged lead screens includes scratches on lead foil screens. Damaged cassette.

Detection of Discontinuities Based on LocationThe purpose of conducting NDT is to successfully locate the discontinuity.The discontinuity may be on the surface or subsurface or totally buriedinside the specimen. One NDT method may not detect all discontinuities.

Radiography is not sensitive to surface discontinuities but best for volumediscontinuities.




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The presence of a discontinuity in the sample is recorded on the film forinterpretation. There are two important factors for the discontinuity to getrecorded on the film.

1. Orientation of the discontinuity in relation to the radiation beamand film. A discontinuity in line with the radiation beam andperpendicular to the film plane is best recorded.

2. The thickness separation of the discontinuity or its volume. If thethickness separation of a discontinuity is parallel to the film andperpendicular to the radiation beam direction, the absorption(attenuation) of radiation by the separation of the planardiscontinuity is negligible. Hence, there is no change in intensity orquantity of radiation received by the film. Such planar

discontinuities will not have a difference in density (that is, nocontrast) and therefore can not be detected.

Let us assume there are three gas packets included in a casting and theirdiameters are 2 mm (0.08 in.), 4 mm (0.16 in.) and 6 mm (0.24 in.). Herewe say the radiation is penetrating all three holes and the separation inthickness is 2, 4 and 6 mm respectively. The volume of these voids isgreatest for 6mm and least for 2mm. Radiation intensity easily passesthrough the 6 mm void, rendering more intensity and quantity ofradiation to the film and causing more darkness compared to the 2 mm

gas pocket.

A lamination in a plate has two dimensions and no thickness, and is not avolumetric discontinuity. Therefore, laminations can not be detected byradiographic technique.

Any discontinuity closest to the film or recording plane is well definedwith least geometrical unsharpness, but the same discontinuity farthestfrom the film plane has maximum geometrical unsharpness. Ifgeometrical unsharpness is maximum, the definition of the shadow imageis very poor. Poor definition renders a poorly sensitive radiograph andthereby the interpreter may miss the discontinuity. Hence, any surfacecrack farthest from the film plane will not get recorded on the film.

For example, lets say that you take a light source and locate the source faraway from the wall. If you place your fingers close to the wall and far awayfrom the source, you get a well-defined shadow. On the other hand, if you




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place your fingers closer to the light source and farther from the wall, youwill notice that the shadow is blurred. The blurred shadow has maximumimage unsharpness.

How does radiographic testing compare with other nondestructive testingmethods in detecting discontinuities?

The ultrasonic method does not indicate the volume of thediscontinuities but indicates the separation or interface.

Magnetic particle testingis best for surface-breaking cracks onferromagnetic materials only.

In order for the liquid penetrant testingmethod to detect adiscontinuity, it must be open to the surface.

Probability of DetectionThe probability of detecting a discontinuity using radiographic testing isbased on the discontinuitys size, width, length, depth and orientation withrespect to the test surface and to the radiation beam.

The probability of detection is high if the radiograph has:

High contrast. Adequate density.

Sharp definition. Least distortion.

Interpretation and EvaluationLets look at how the above factors are related to radiographicinterpretation.

1. High ContrastIn industrial radiography, it is the job of the interpreter to detect adiscontinuity that is either darker or lighter than the gray background. If

radiography has been rendered with no change in darkness, even fordiscontinuities in the test object, the interpreter will report this conditionas no discontinuity is found because he or she is not able to perceivecontrast. Therefore, our aim in industrial radiography is to produce filmwith high contrast.




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Factors that may affect contrast are:

Subject Contrast

Change in thickness. The atomic number for example, the presence of an inclusion

with a high or low atomic number. Radiation energy or quality of radiation.

Film Contrast Type of film used for example, high-contrast film or low-contrast

film. (See Figure 2.2.) Type of intensifying screens used. Processing condition.

Scatter radiation.

Differenceindensity=

co

ntrast

Log relative exposure

Fast film

Slow film

Fast film (low contrast)

A small change in exposure produces a

small change in density = low contrast.

Fine details can not be seen because of

low contrast (i.e., no density difference).

Slow film (high contrast)

A small change in exposure produces a

large density difference = high contrast.

Fine details can be seen (i.e., greaterdensity difference).

If you are asked to take a radiograph of

a weld with these two films, and

assuming there is a fine, shallow crack

on the source side of the weld, both

films will receive the same amount of

exposure due to the change in

thickness at the crack location.

However, the slow film will produce a

greater density difference (contrast)

even for a small change in exposure,

whereas the same small amount ofexposure on the fast film produces only

a very small difference in density, that is,

poor contrast. The interpreter will find

the crack using slow film; however, on

the fast film the crack cannot be seen

because of poor contrast.

Figure 2.2: Differences between two types of film fast and slow in terms of contrast.





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2. Adequate DensityThe degree of darkness in a radiograph is called optical densityor simplydensity. Density is a measurable quantity and is equal to:

log10 Ii/It

where

Ii = the incident intensity of light.It = the transmitted intensity of light.

If all the incident light is transmitted, then the ratio is one and the densityis equal to zero. The darker area of film transmits less intensity, whereas

the lighter area transmits more light, as shown in Figure 2.3. Therefore,the density will vary according to the degree of darkness. The density ofthe radiograph must be 2 to 4, as density less than 2 and more than 4 doesnot produce enough contrast to see the discontinuity.

INCIDENT LIGHT

TRANSMITTED

LIGHT

High contrast film Low contrast film

DENSITY

100 100 100 100

100 10 1 0.1

0 1 2 3

Figure 2.3: Relationship between incident and transmitted light and density (top); effect of film contrast

on the appearance of discontinuities (bottom).





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3. Sharp DefinitionDefinition is the sharpness of the image or discontinuity details. Sharpnessis affected by geometry. Since all sources have a dimension and all

specimens have thickness, there will be always some amount of imageunsharpness that cannot be eliminated. Geometrical unsharpness isexpressed as:

where

F = the longest dimension of the source.

d = the object-to-film/detector distance.D = the source-to-object distance.

It is the aim of a radiographer to produce the image details with minimumunsharpness (Ug). The easiest way to get the best definition that is,minimum Ug is that the D shall be as large as possible (taking practicalconsiderations into account). For any size source and any distance of anobject to the film, we can get a well-defined shadow (umbra) with largesource-to-object distance, as shown in Figure 2.4. For example, our sun isvery big compared to us, but still we get a very sharp shadow because of

the large distance between the sun and us.

UFd

Dg=

Figure 2.4: Relationship between source-to-object distance (D), object-to-film/detector distance (d) and

image unsharpness (Ug).

Source

Test object

Film/

detector

F

D

d

(a) (b) (c)

Minimum Ug





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Figure 2.4(b) shows greater geometrical unsharpness (penumbra) whenthe source-to-film/detector distance remains unchanged from Figure2.4(a) but the object-to-film/detector distance (d) is increased. On the

other hand, Figure 2.4(c) shows a smaller geometrical unsharpness whenthe object-to-film/detector distance (d) is the same as in Figure 2.4(a) butthe source-to-object distance (D) is increased.

Optimum geometrical sharpness of the image is also obtained when theradiation source is small. Figure 2.5 illustrates the decrease in geometricalunsharpness with a decrease in source size.

4. Least DistortionWhen we take a radiograph, not all of the discontinuities will either lieperpendicular to the film or be in a position favorable to the radiationbeam. As the radiation beam is diverging, the ray that hits thediscontinuity will throw the shadow in an angle to the radiation beam. Forthis reason, the discontinuity in the specimen may be outside the

Source

Test object

Umbra

Penumbra

Figure 2.5: Effect of source size on image sharpness.





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projected area of the beam and the radiograph will not show the exactshape of the discontinuity. The angular position of the discontinuity,which is not exactly perpendicular, results in distortion. Thus, if the plane

of the test object and the film/detector plane are not parallel, imagedistortion will occur, as shown in Figure 2.6(a).

As an analogy, when we stand in the morning before the sun, our shadowis long and distorted. However, when the sun comes overhead, the noonshadow is projected and the beam is perpendicular with no distortion inthe image.

If we radiograph our area of interest with a restricted cone angle, then wehave reduced the distortion. Once again, it is important that the

film/detector plane shall be parallel to the surface of the specimen andperpendicular to the radiation source to minimize the effect of distortion.As shown in Figure 2.6(b), image distortion will also result if the radiationbeam is not directed perpendicular to the film/detector plane, even if thefilm/detector plane and test object plane are parallel.

Point source

Test

object

Film/detector

Film/detector

in tilted plane

Axis of test

object

perpendicular

to film plane

Figure 2.6:Two instances of image distortion resulting from proper alignment of point source,

test object plane and film/detector plane.

(a) (b)





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A large source-to-object distance not only reduces Ug but also produces animage with minimum distortion. To achieve both the effects of minimumUg and least distortion, it is always recommended to use as large a

source-to object distance as possible, considering the exposure time andavailable distance.

Interpretation and EvaluationInterpretation of indications is the cream of NDT. Here lies the fate of thetest specimen in terms of accept or reject criteria. A wrong interpretationmay lead to acceptance of a defective sample or rejection of a good sample,just as a wrong diagnosis by a doctor may kill a patient. To do effectiveinterpretation in any NDT method and in radiography in particular, it isimportant that the interpreter understand the physical principles of

radiography, along with its advantages and limitations. Radiographersmust first judge the quality of the radiograph before starting theirinterpretation.

The quality of a radiograph is based on factors we have alreadyconsidered, namely:

Sensitivity sensitivity is based on contrast and definition andexpressed in % of material thickness.

Contrast based on subject contrast and film contrast.

Definition or sharpness of the image based on many factors ofwhich Ug is of prime importance. By controlling Ug we can obtain awell-defined radiograph.

Adequate density in order to achieve good contrast. Freedom from artifacts the area under interest shall not have any

image or shadow to cause confusion to the interpreter. Such shadowsalso can mask the actual discontinuities. Improper handling of filmfrom start to end of the radiography process may cause artifacts;therefore, film shall be carefully handled.

Only a satisfactory radiograph shall be interpreted and evaluated. If theradiograph is not meeting the above requirements, it shall be replacedwith a fresh radiograph that meets the requirements.

Now let us start our job of interpretation and then evaluate any indicationbefore making a decision.




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To do an effective interpretation, prepare a checklist as follows:

1. The films shall be viewed only in a darkened area.

2. Switch off all lights that may reflect from the film surface, which isa highly polished surface.

3. Verify that the film corresponds to the part for which theradiograph was taken and record any surface imperfection if anyhas been obtained by visual inspection.

4. Verify the technique used and that it meets the procedurerequirement.

5. Verify that the image quality indicator (IQI) designation isproperly placed and properly located per procedure.

Image Quality Indicators (IQIs)Before characterizing a discontinuity by radiography for a given testobject, it is very important to qualify the radiograph based on sensitivity.Here the sensitivity means the smallest discontinuity that the method candetect.

The interpreter can qualify the radiograph for contrast and definition onlywith a tool called an image quality indicator or IQI. If the radiograph hasproduced a difference in density due to a small change in thickness(generally 2% of the material thickness), then the radiograph has been

produced with the required contrast. If a 1T, 2T or 4T hole in a hole-typeindicator, or the designated wire in a wire-type indicator, is appearing onthe radiograph, then the geometry of the exposure has been set to producethe best definition.

With a plaque- or hole-type penetrameter or IQI, we specify the qualitylevel as 2-2T. The first numeral 2 is for contrast that is, the ability todifferentiate 2% of material thickness in terms of a difference in density.The 2T means that the hole diameter of 2% of the material thicknessshould be defined.

Question 2.1

Lets put your understanding of IQIs to the test. For example, a 1 in.(2.54 cm) ground-finished weld is radiographed. The required IQIper procedure is ASTM 20. Let us assume that three radiographers(A,B and C) produce radiographs of the same test piece individually.




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In As radiograph, all the holes of the IQI can be seen.

In Bs radiograph, only the 2T and 4T holes can be seen.

And in Cs radiograph, only the 4T hole can be seen.

Of the above three radiographers , who has produced a radiographwith the highest sensitivity?

Answer is on p. 39.

Using the above example, we can calculate the equivalent penetrametersensitivity abbreviated as EPS (alpha) for each radiograph using the

following equation:

EPS (alpha) = n(H/2)1/2

where

n = the percentage sensitivity required as per procedure.H = the hole designation as 1, 2 or 4 in terms of thickness of IQI.

Question 2.2

Using the EPS formula, calculate the equivalent penetrametersensitivity for each radiograph in Question 2.1.

Answer is on p. 39.

Question 2.3

Now that you have determined the equivalent penetrametersensitivity for each radiograph, what is your conclusion of the above

radiographs produced by the three different radiographers in termsof the percentage of material thickness?

Answer is on p. 39.




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Now you can conclude the purpose of an IQI and how to go aboutqualifying a radiograph with an IQI. Knowing the specimen thickness, it isimportant to select the IQI for the required sensitivity.

OK, back to our checklist.

6. Check the illuminator for the required level of illumination so as toread the film varying from 2 to 4 density.

The basic tool used to interpret a radiograph is a good illuminator. Anilluminator is a high-intensity light source capable of transmitting enoughlight to view the discontinuity. The amount light that is transmitted inorder to see discontinuities is based on the darkness of the film. As

discussed previously, the degree of darkness is called optical densityor,simply, density. As previously discussed, density is dimensionless and issimply a number, expressed by the following formula:

density = log10 (Ii/It)

where

Ii = the incident intensity of light.It = the transmitted intensity of light.

If 100% of the initial intensity of 1000 units is transmitted, the density iszero and we perceive a transparent object.

If 10% is transmitted, then we get log10 (1000/100) = 1.

If 1% is transmitted, then we get a density of 2.

If 0.1% of the initial intensity is transmitted, then the density is 3.

So if the density of a blowhole discontinuity a type of porosity is 4,then it has transmitted only 0.01% of the initial intensity. This can be seenmore visually in Figure 2.7.

Because the silver deposit on the film is formed from individual grains,the transmitted light is scattered. Consequently, the spatial intensity of thetransmitted light depends greatly upon such factors as the distribution and




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size of the grains and upon the condition of the incident light. Themeasured density of the film will also depend upon the optics of the

instrument used.

7. The illuminator shall be equipped with a collimator or mask inorder to restrict the light so that it only falls on and passes throughto the area of interest. This is because excess light from alow-density area will produce glare, which may causediscontinuities to be missed. Masking also improves the contrastratio of the discontinuity density to its surroundings. This leads toa higher probability of detection.

8. Use a calibrated densitometer to measure the density for

acceptance and evaluation of radiographs.

Before the invention of electronic devices, the film density was comparedwith a density strip. Today, we have photocells to measure the lightintensity that is incident and transmitted, which permits us to calculatethe optical density of radiographs.

The instrument that measures the density is called a densitometer. Thisinstrument must be calibrated initially before measuring the density. Thedensitometer is calibrated with a known available density, called a densitystrip. This density strip is a standard calibrated strip and the density ofeach step is printed nearby and traceable to a national standard. Note: Thedensity in the density strip itself may vary due to improper storage, aging,etc. Thus, the density strip has a certain validity period. Also, any changein initial intensity of light due to voltage fluctuations, dirt and foreignmatter will wrongly report the density.

Incident intensity of light on radiograph (1000 units)

1000 100 10 1 0.1

Transmitted light

0 1 2 3 4

Density

Figure 2.7: Calculation of density values based on incident light intensity of 1000 units.





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With no film in the port, the densitometer is calibrated to zero density.From the explanation given in point 7 above, if all the light is incident onthe light-sensing device, the instrument will show zero density. This is

called zero calibration. After doing an initial calibration, the film is placedin the port, where the light to the light-sensing device is obtained afterpassing through the film. If the films degree of darkness is very high, itmay transmit only 0.01 percent of the initial intensity. Then the density ofthe radiograph will be 4, as calculated in point 6 above.

9. Characterize the discontinuity; separate the images of actualdiscontinuities from artifacts.

In industrial radiography, the indications are formed with differences in

optical density. The image so formed may be either a true indication dueto discontinuities present or may be a false indication due to improperhandling of the film or poor processing conditions called artifacts. Theinterpreter, given sufficient knowledge, shall be in a position to separateartifacts from the actual indications of relevant discontinuities. Once thisexercise is done, by seeing the shape of the image of the discontinuity, itslocation in the specimen (e.g., weld bead), and its optical density, theinterpreter should be able to name the discontinuity.

10. Once the discontinuities are characterized, make a decision for

acceptance or rejection based on the recommended codes andstandards.

11. Prepare a test report indicating all discontinuities and make adecision per the code recommended in the procedure. Allindications are to be interpreted and evaluated. That is, theinterpreter must ask how or why the indications have formed. Inradiography, interpretation means to give the meaning of or toexplain the characteristics of the radiographic image. This isdifferent than evaluation. To evaluate an image means todetermine whether the discontinuity shall be permitted or not forthe service life of the part.

Evaluation of discontinuities involves the following:

Characterizing the nature of the discontinuity. Determining the location of the discontinuity.




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Assessing the extent, length, width, size, grouping or isolation,randomness of and spacing between discontinuities.

Making a decision as to acceptance/rejection based on the above

with reference to code.

Characterizing a DiscontinuityAll indications must be characterized properly, before making a decisionfor acceptance/rejection. Thus, NDT personnel must be in a position tocategorize them as true or false indications.

As mentioned before, indications are classified as true (either relevant ornonrelevant) or as false (artifacts). Artifacts can actually mask a trueindication of a discontinuity, which may endanger the service life of the

component. Therefore, all such artifacts should be avoided. In fact, noartifact is permitted on the area of interest and, if found, the area ofinterest must be re-radiographed for interpretation and evaluation. In thiscase, the interpreter asks for a re-shoot.

To accomplish this, all false indications shall be removed and aftercleaning, the job shall be reprocessed. Any true indication will definitelyappear, while a false indication will not reappear.

A radiograph which has not produced the required sensitivity or density

must also be rejected. A radiograph with too much scatter will undercutthe area of interest and, hence, all radiographs showing scatter radiationmust be re-radiographed.

Other reasons for unsatisfactory radiographs include:

Too much variation in density within a radiograph. Radiographic technique that does not conform to the procedure.

A sound knowledge in welding and casting processes is of paramount

importance for evaluating discontinuities. From the shape of the shadow,the interpreter should be able to characterize a discontinuity. For example,in welding, the following discontinuities appear as a straight, dark image:

Incomplete penetration. Lack of sidewall fusion. Longitudinal crack.




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Incomplete fusion.

Slag and sand inclusions, on the other hand, present a diffused image

anywhere and in any position with any shape.

In casting, the following discontinuities are possible:

Hot tear a jagged crack located at the junction of thick and thinsections.

Various types of shrinkage branching or spreading darkness.

Shift of Image Due to DistortionThe interpreter must be able to determine from the radiograph whether

the source location was proper with respect to the orientation of thediscontinuity. For example, the most probable location of macroshrinkagein a casting is at the abrupt change in thickness. From the radiographicshooting sketch, the interpreter should be in a position to ascertainwhether the image would have formed or been missed with relation to thesource and film positions.

Heres a different example. The recommended technique to take acircumferential seam in a pipe weld of less than 3.5 in. (8.89 cm) outsidediameter (OD) is a double wall exposure and double wall viewing. This

technique is possible only by offsetting the source to get both sides of theweld image in one film by producing an elliptical shadow. Here weintentionally get a distorted image. This distortion may dislocate thediscontinuity on the film and the interpreter, while interpreting such aradiograph, must keep in mind this effect and then characterize thediscontinuity.

Reference to CodeAfter characterizing the discontinuity, the interpreter makes a decision foracceptance/rejection based on the referencing code. There is no common

code of acceptance or common procedure for all jobs. A pipe inside aplant is interpreted with a particular code and a cross country pipe isinterpreted with a different code. A butt joint under cyclic load has one setof acceptance norms, while the same type of weld under static load has adifferent set of norms. Therefore, it is important to have the latest code ofacceptance for each radiograph being interpreted.




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Comprehension Check 2.1

1. A weld radiograph shows a darker image. This image is called:

A . An indication.B. A discontinuity.C. A defect.

2. The ability to detect the smallest discontinuity is called:

A. Definition.B. Contrast.C. Sensitivity.

D. Distortion.

3. Radiographic quality is measured by % sensitivity. Theradiograph must have:

A. Highest contrast.B. Adequate density.C. Sharpest definition.D. Least distortion.

Which one of the above in your opinion will determine the sensitivity

of a radiograph?

A. Both A and B.B. Both C and D.C. All of the above.

4. The relative absorption of a high-density material inclusion willrender:

A. Darker density.B. Lighter density.




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5. As the divergent cone angle of the radiation beam increases:

A. Distortion increases.

B. Sharpness increases.C. Intensity increases.

Answers begin on p. 40.

Discontinuities

Discontinuities can be classified into three general categories. They are:

Inherent. Processing. Service.

Lets start with inherent discontinuities.

Inherent DiscontinuitiesInherent discontinuities originate during the solidification process (liquidto solid state).

All engineering metals, like iron, aluminum, copper, etc., are obtainedfrom the earth. They are not obtained in pure form that is, ready to use but with certain impurities. Metal containing natural impurities iscalled ore. Ore is further purified in a process called ore benefaction.Then the ore is melted in a furnace by adding coke and lime. The moltenmetal is drawn and poured into cavities. There it solidifies into ingots (seeFigure 2.8).

The molten metal, before solidification, may contain nonmetals like slag,coke, lime and refractory bricks. These nonmetallic inclusions may get

entrapped into the solid ingots. Also, it is possible for the metal to evolvegases and such gases also get entrapped within the ingots. All suchdiscontinuities formed during solidification are called inherentdiscontinuities.

The casting process is from a liquid to solid state to form the requiredshape, like valves, pump bodies, etc. Discontinuities originating in the




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casting process are also called inherent discontinuities, as the process isfrom a liquid to solid state. Inherent discontinuities are naturally formedwithout any external force, that is, formed on their own and hence calledinherent.

Based on design requirements, the ingots have to be turned into the

required shape by application of external force, in order to get the desiredmechanical and metallurgical properties as well as the required strength.Castings can take very high compressive loads (flattened, squeezed orpressed) but are not the best for tensile loads (drawn or stretched bytension). So we need other manufacturing processes whereby we can getthe required tensile strength, impact resistance and hardness values.

To achieve this, the ingots are further cut into billets and blooms. Billetsand blooms are only a part of an ingot and get their name by their size. Abloom has the same width and thickness, whereas billets are primarily

used in the rolling process. These billets and blooms are further worked byforging or rolling to get the desired mechanical properties and shapes.More discussion of billets and blooms and the primary manufacturingprocesses are dealt with in the next chapter in this volume.

Figure 2.8: Ingot. Photograph courtesy of Bay Forge Fomas India Ltd., Chennai, India.





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Processing DiscontinuitiesWhen the ingots, billets and blooms are further processed to get thedesired form through such processes as rolling, forging and welding,

discontinuities may be formed that are called primary processingdiscontinuities. After such primary processing, when the product isfurther processed for removal of thermal stress or grain refinement bygrinding, machining, painting, handling and so on, other discontinuitiesmay occur. These are called secondary processing discontinuities.

Service-Induced DiscontinuitiesAll manufactured products are not 100% perfect. There is no casting orweld without any discontinuities in any manufacturing process.Components with allowable discontinuities are put into service. Therefore,

nondestructive testing is to be carried out to assess the condition from theraw material stage throughout the life cycle of the component before it isdiscarded from use.

Service-related discontinuities are caused by the following conditions:

Due to service conditions, there can be metal loss due to corrosion,erosion, wear and cavitation (reduction in wall thickness) to namea few.

As a result of extreme temperatures, the material may lose its desired

mechanical properties through yield stress or ultimate stress, forexample, causing failure much below the calculated stress values atnormal temperature. Continuous exposure to the environment, withalternating high and low temperatures, results in the material losingits wall thickness along with other desirable mechanical andmetallurgical properties. Loss of metal and other desirable propertiesmay lead to premature failure. All metals at room temperature showa particular tensile and yield stress. When the metal is stressed athigh temperature, the stress values will be considerably reduced;consequently, the metal yields at a lower stress value and the partwill be deformed from its original shape. This deformation itself isconsidered failure. Alternatively, at too low a temperature, all metalsbecome brittle and fracture is sudden and catastrophic.

With repeated cycles of loading, the material may fail at lower stressvalues due to fatigue (endurance limit). All engineering products,ranging from pins to rockets, are subjected to stress or some form ofload. Engineers design a component with sufficient strength to




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sustain the applied load. If the load increases more than thisstrength, the component fails. It is the purpose of nondestructivetesting to find stress-related discontinuities and prevent failure.

Abuse may also be a factor in the service life of a part or component.

Defects

All NDT methods are based on the application of some form of energyinto the object, and the object modifies the energy. The amount and typeof modification allows the interpreter to infer the quality of thecomponent; the nature, distribution and location of discontinuities; theextent of wall loss, etc.

From the point of view of an engineer, certain small or very smalldiscontinuities may not endanger the components service life and may beconsidered acceptable from a functional aspect of the component. Basedon the type of stress imposed on the component, the engineer has todecide whether such discontinuities, however small, may lead to failure.

With regard to the actual functional aspect of the component, allimperfections detected by NDT are classified as either a discontinuity ordefect. All NDT methods detect discontinuities. These detected

discontinuities are interpreted, evaluated and characterized as per theapplicable code. If the code warrants rejection based on suchdiscontinuities, they are called defects. If it allows them, they continue tobe referred to as discontinuities. For the engineer, not all discontinuities aredefects, but all defects are discontinuities.

In radiography with conventional techniques, the depth of the discontinuitycan not be determined. However, whenever a discontinuity is detected, it isimportant to know the following:

Location in the tensile member side or the compressive memberside. The location of a discontinuity is very important from a servicepoint of view. A surface defect and a near-surface defect are moredetrimental for failure since the surface is the part where maximumtensile load occurs and which is in constant contact with theatmosphere. Also, for moving parts, the surfaces of each component




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are in contact with each other. Loss due to wear occurs only at thesurface.

Size size is very important as a discontinuity shall be below the

critical size. Mere size alone is not an acceptable factor; the locationof a discontinuity with respect to the principal stress axis isimportant for acceptance/rejection, as are the grouping and spacingof discontinuities.

Extent length, grouping, separation from edge to edge, etc. Nature a sharp notch, linear, planar, volumetric or spherical, etc. Shape the shape of the discontinuity is another important factor to

be considered. A sharp notch such as an undercut will definitely leadto fatigue failure. Likewise, a sharp corner is considered as a sourceof fatigue failure.

Discontinuities may be active that is, they grow in size during service;other discontinuities are passive in that they do not change in size, shapeor location. An initially acceptable discontinuity may develop into a defectdue to prolonged use; hence, some parts require periodic inserviceinspection.

Whenever a discontinuity is detected, it is also important to know the typeof load imposed on the structure or that a component is subjected to. Adiscontinuity under static or compressive load may be safe, whereas the

same discontinuity under cyclic loading or located at or near a tensile partmay develop further into a crack.

Here is an example from industry: Lets imagine that an undercut in acircumferential joint in a pressure vessel is found in a radiograph. Theundercut is accepted up to 0.8 mm (0.03 in.) per pressure piping code,while the same undercut is not acceptable in a longitudinal seam in thesame component. Why? Here we have to study the type of stress imposedon the two joints. With a circumferential weld, the internal pressure isworking from inside to outside, while outside the shell or pipe, theatmospheric pressure is acting. This is a compressive stress and so noundue stress will concentrate on the undercut. Now considering thelongitudinal seam, the undercut is not permitted because a long seamundergoes two types of stresses: one is compressive, just like acircumferential seam, and the other is tensile stress, pulling the weld seamaway. It implies that any sharp discontinuity on the tensile member isharmful.




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Chapter 2 Summary

1. Every NDT method has a particular physical principle by which

test results are obtained. They are then interpreted andevaluated as per acceptance code.

2. Indications produced by NDT methods are considereddiscontinuities and not defects.

3. A discontinuity may be big or small; it may be on the surface ortotally buried inside the specimen.

4. The discontinuity may have length, width and depth, meaning it

has volume, or it can be simply a planar discontinuity withoutthickness compared to its length and width.

5. When we evaluate the discontinuity in terms of its size, length,location and nature, it may not be acceptable to the service lifeof the component. Such discontinuities, which are notacceptable by the referencing code, are considered defects.

6. True indications can further be divided into relevant andnonrelevant.

7. Nonrelevant indications are formed due to geometry (i.e., shapeof the object) and have no relevance to a discontinuity. Suchindications can be easily compared to the geometry of the part.

8. All relevant indications are discontinuities. For example, adiscontinuity under a particular type of stress may bedetrimental; hence, it is classified as a defect. The samediscontinuity under certain other conditions of load or stressmay be inconsequential for its service and, hence, called only a

discontinuity.

9. The discontinuitys size, shape, location, grouping and nature arevery important for acceptance or rejection. If the discontinuityssize, shape, location or extent do not meet the acceptancestandards, then the discontinuity is called a defect.




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10. There is no perfect material or component in any of themanufacturing processes.

11. Many apparent discontinuities that are readily seen visually maynot reduce the service life of the component.

12. Failure may also result in a nearly perfect material, with nodiscontinuities, due to poor design factors.




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Answers to Chapter 2 Questions

Question 2.1

Answer: A.

A has produced a radiograph with very high sensitivity compared toB and C. B has produced an acceptable radiograph compared to C.Cs radiograph has not been produced with the required definition.

Question 2.2

A gets a score of (alpha) = 2(1/2)1/2 = 1.4%.

B gets a score of sensitivity as 2(2/2)1/2 = 2%.

C gets a score of sensitivity as 2(4/2)1/2 = 2.8%.

Question 2.3

We can express our conclusions this way: A is able to obtain detailsas small as 1.4% of material thickness, while B can get details ofchange in thickness as high as 2% or above. Anything less than 2%

of the specimen thickness is not able to be resolved by thisradiographer.

C has produced a radiograph with poor definition, which alsoaffects the sensitivity. Thus, C can only report any change inthickness of 2.8% or higher.




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1. Answer: A.

A. A is the correct choice. All indications have to be interpreted that is, the radiographer needs to ask how and why thediscontinuity was formed. Once it is termed a discontinuity,the interpreter has to further evaluate it in accordance withcodes for acceptance/rejection.

B. If your choice is B, it is wrong. In radiography, all indicationshave to be interpreted. This is the first step. The second stepis to evaluate and the third step is to make a decision, percode, for acceptance/rejection.

C. Deciding that a radiographic image is a defect as the firststep is also the wrong choice. To term an image as a defect,you need to interpret the indication and then name thediscontinuity. Only then can you make a decision foracceptance/rejection as per code. If the indication is foundto be true and rejectable, then it is called a defect.

2. Answer: C.

A. Wrong choice. Definition is the sharpness of the image and if

there is no contrast, you cannot see the discontinuity.B. Contrast is only the ability to see the discontinuity. The

image of a discontinuity may have contrast, but if there is nodefinition, we can not characterize the discontinuity.

C. This is the correct choice. A and B are partly correct, but ifboth are achieved, then we have sensitivity. Therefore,sensitivity is based on both contrast and definition.

D. Distortion is an unwanted requirement for a goodradiograph.

3. Answer: C All of the above is the best choice. Highest contrastand adequate density produce the best radiographic contrast.Sharpest definition and least distortion would produce theminimum image unsharpness and minimum distortion. C and Dwill be affected by the geometrical setup. All the desiredproperties must be met.




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4. Answer: B.

A. Darker density sorry, your choice is wrong. We have seen

that high-density material like lead and tungsten absorbs orattenuates relatively larger amounts of radiation, leaving thefilm with little or no radiation to cause photographic density.

The processed radiograph will show lower density.B. Lighter density excellent. So if you see a lighter image in a

radiograph, you will interpret it as a heavy metal inclusion,including the probable metal if you know the manufacturingprocess.

5. Answer: A.

A. Correct. You are following the geometry of the beam. Whenan object is viewed from an angle, we see a distorted image.When you see your shadow in front of the sun in the earlymorning or in the evening, your shadow is distorted. If thetrue shape of the discontinuity is to be projected on the film,the cone angle shall be as small as possible. You canconclude that the image of a discontinuity lying in the pathof the central cone of radiation is projected with its trueshape and the same discontinuity at the extreme edge of the

radiation cone will be distorted.B. The sharpness increases you are incorrect. The sharpness

will increase when the source-to-object distance is increasedand object-to-film distance is decreased.

C. The intensity increases is also incorrect. The intensity willdecrease relatively as the distance increases at the extremecone of radiation.




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Chapter 2 Review

True or false:

1. All NDT methods detect discontinuities.

2. Any discontinuity is a defect.

3. Discontinuities formed during solidification are called inherent.

4. All true discontinuities are defects.

5. All indications found in the radiographic film are true and

considered defects.

6. A true indication formed due to the principle of that particularmanufacturing method is always a defect.

7. A high atomic number attenuates more radiation and, hence, inthe film the indication appears lighter than the background.

8. All defects are discontinuities but not all discontinuities aredefects.

9. All acceptable discontinuities are defects.

10. All unacceptable discontinuities are defects.

11. In radiography, rejectable indications are to be evaluated as percode.

Multiple choice:

12. The evidence of a discontinuity that requires interpretation andevaluation is called:

A. A defect.B. An indication.C. An imperfection.D. A discontinuity only.




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13. In order to detect a discontinuity in a finished radiograph by theinterpreter, the radiograph shall have:

A. High density.B. Good contrast.C. Best film.D. Ideal processing.

14. If a discontinuity is finely defined with the exact contour of thediscontinuity, then the radiograph has:

A. High contrast.B. High definition.

C. Latitude.D. Density.

15. The quality of the radiograph is represented by:

A. Contrast.B. Definition.C. Percent (%) sensitivity.D. The technique employed.

16. The sensitivity of the radiograph is based on:

A. Radiographic sensitivity.B. Radiographic definition.C. Radiographic technique.D. Both A and B are correct.

17. The interpreter sees the outline of the 20 ASTM plaque- or hole-type IQI placed on the source side of the specimen, but theholes are not seen. This radiograph has no:

A. DefinitionB. Contrast.C. Density.D. All of the above.




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18. The subject contrast is based on:

A. Energy radiation.

B. Thickness of the specimen.C. Atomic number of the included material.D. All of the above.

19. If it is desired to have a high-sensitivity radiograph, the choice offilm will be:

A. A fast f ilm.B. A f ine-grained film.C. A slow film.

D. A film with wide latitude.E. Both B and C.

20. The sensitivity of the radiograph is expressed with a tool calleda(n):

A. DensitometerB. IQI.C. Illuminator.D. Survey meter.

21. A densitometer is an instrument to measure:

A. Density.B. Sensitivity.C. The discontinuity size.D. The depth of the discontinuity.

22. Which one of the following indications is to be evaluated foracceptance/rejection?

A. A pressure mark.B. A developer stain.C. A slag inclusion which has width and length.D. A poor image caused by a bad lead screen.




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23. If a high-density material is included in the metal, the image ofthis inclusion would appear:

A. Darker.B. Lighter.C. Sharper.D. All of the above.

24. Acceptance/rejection of a discontinuity is based on:

A. The operators ability.B. The companys practice.C. Codes intended for the job.

D. Past experience.

25. When the radiograph of a casting is being interpreted, theinterpreter needs:

A. The test object by his or her side.B. A well-illuminated area.C. A radiographic shooting sketch.D. All of the above.

26. Why it is necessary to interpret all of the indications in aradiograph?

A. All indications need not be defects for rejection.B. All indications need not be true and some of them may be

false.C. It is necessary to interpret all true indications and evaluate

them.D. All of the above are true.




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27. A false indication is found in the area of interest in a weldradiograph. Which ofthe following is true?

A. A false indication may mask the true relevant indication.B. All false indications are not defects and can be ignored.C. The radiograph is not acceptable and hence a fresh

radiograph is required for an interpretation.D. Both A and C are correct.

28. A discontinuitys shadow or image in a radiographic film is:

A. Of the same size as the discontinuity.B. Enlarged and the enlargement depends on the location of

the discontinuity in the part.C. Smaller than the size of the discontinuity.D. A cross section view.

29. Discontinuities produced in the casting process are classified as:

A. Inherent.B. Primary processing.C. Secondary processing.D. Service.

30. Which one of the following discontinuities is best detected byradiographic testing?

A. A lamination in a rolled plate.B. A lamellar tear in a flange plate of a T joint.C. A slag inclusion.D. A cavity in a casting.E. Both C and D are correct.




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Chapter 2 Review Key

1. True

2. False

3. True

4. False

5. False

6. False

7. True

8. True

9. False

10. True

11. True

12. B

13. B

14. B

15. C

16. D

17. A

18. D

19. E




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20. B

21. A

22. C

23. B

24. C

25. C

26. D

27. D

28. B

29. A

30. E




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Chapter 3

Manufacturing Processes andAssociated Discontinuities

In this chapter:

Types of iron ore converters

Production of ingots, blooms

and billets

Different types of casting

processes

Overview of casting

discontinuities

Forging, rolling and extruding

processes and respective

discontinuities Various kinds of welding

processes

Overview of welding

discontinuities

49




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Casting Processes

Employing a casting process is by far the cheapest and most widely used

metal shaping technique. A casting process, in short, involves melting themetal and pouring it in a pair of molds, which have been formed usingpatterns. The cavities in the cast component are produced by the use ofcores, which in turn are made from core boxes. When we say casting,generally it means sand casting unless otherwise specifically mentioned,like investment casting, centrifugal casting, etc.

Therefore, sand is the medium in which a mold is made. The variousmolding processes are:

Green sand molding. Skin-dried sand molding. Dry sand molding. Carbon dioxide process of molding. Shell molding and its variants. Investment casting.

Except for investment casting, a refractory material that is, sand isused. (A detailed discussion of sand casting is presented later in thischapter.)

Refractory materials are those materials, which will withstand highstresses at high temperatures. The following are the most commonrefractory materials:

Silicon dioxide or silica SiO2 Aluminum oxide or alumina Al2O3 Iron oxide Fe2O3

Iron is always found as an ore or in a combined state, so it must be

refined. One element that is more active than iron and is cheap and easyto get is carbon. When carbon combines with ore, we get iron and carbonmonoxide. The reaction is:

Fe2O3 + C = Fe + CO

Programmed Instruction: Radiographic Testing50




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Iron ore is mined and concentrated to remove the soil and otherundesirable materials; it is then shipped to the refinery. There it is mixedin layers in a huge blast furnace with coke and limestone. The limestone

acts as flux to remove the impurities formed in the refining process.

Air, heated to 1100 F (593 C), is blown from the bottom of the blastfurnace, through the iron ore, coke and limestone layers of the charge. Atthis temperature, the carbon in the coke will react with the oxygen in theiron ore and start to burn off the oxygen from the iron oxides.

This, in turn, causes the temperature to rise above the melting point of theiron, to about 3000 F (1649 C). After about 5 to 6 hours, the molten ironis drawn off through a tap hole at the bottom of the furnace and poured

into ingots called pigs.

The pig iron is not yet steel and contains many impurities, such as siliconand sulfur. The carbon content of this metal is about 4% at this stage. Ifcast iron is the desired end product, the pigs can be remelted and useddirectly. However, the addition of alloying elements to the pig iron is acommon practice to give the metal better properties.

Pig iron contains too much carbon and other undesirable materials in itfor use as a strong metal. To acquire sufficient strength, it must be

converted into steel. This is done in one of several types of converters.Although they differ in design, all the converters do the same thing thatis, they burn off the carbon in the iron.

There are four types of converters:

Open hearth converter. Bessemer converter. Electric arc furnace. Oxygen lance converter.

These are described in more detail on the following pages.

Volume VI: Radiographic Interpretation 51




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Open Hearth ConverterPigs from the blast furnace are mixed with scrap steel and melted. Hot airis blown across the surface and the excess carbon and other impurities are

burned out of the melt. After about 11 hours, the desired alloyingelements with an exact amount of carbon are added and the steel is drawnoff, poured into large brick wall crucibles and cast into ingots.

Bessemer ConverterHot air is blown into the molten pig. Scrap steel, the desired amount of

carbon and alloying elements are added. To refine the steel, only 20 to 30minutes are required. Afterward, the converter is tilted and the steel ispoured into crucibles to create ingots.

Fuel supply(operating)

Preheated air

Fuel supply(idle)

Waste gases

Cold air inlet

Hearth

Molten steel

Slag

Stack

Flame

Checkers Checkers

Figure 3.1: Cross section of open hearth furnace (converter).

Figure 3.2: Bessemer converter.

Blowing Charging Pouring





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Electric Arc FurnaceAn electric arc furnace requires a very heavy current on the order of25 000 amps to produce the highest quality steel. This is achieved through

careful control of atmosphere and composition.

Oxygen Lance ConverterPure oxygen is blown with high pressure into the molten metal. Oxygenburns out the carbon very rapidly. After the carbon is burned, the desiredamounts of carbon and alloying elements are added to get the desiredquality of steel. The furnace tilts for loading and pouring. Steel pours

through the tap hole from under a slag covering.

Heavy brick wall

Electrodes

Arc

Molten metal

Figure 3.3: Electric arc furnace.

Oxygen lance

Tap hole

Molten steel

Slag

Figure 5.4: Basic oxygen furnace (converter).





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Upon completion of the conversion process, the steel is ready to be formedinto any of the several thousand products of the steel industry.


Fill in the blanks with the best word from each word pair:

1. Most of castings are made with a __________ mold.(metal/sand)

2. Sand is a(n) __________ material. (refractory/investment)

3. Pig iron is produced from a __________. (blast furnace/sand

mold)

4. Pig iron contains too much __________ and other impurities.(coke/ carbon)

5. Pig iron from a blast furnace is made into the desired quality ofsteel by a(n) __________. (converter/investment mold)

6. The purpose of blowing hot air into an open hearth furnace is to__________ carbon and other impurities. (reduce/remove)

7. The best type of converter to produce high-quality steel is a__________ (electric arc furnace/open hearth converter)

8. After removal of excess carbon and other impurities, the desiredamount of carbon and other alloying elements are added andthen drawn and poured in a mold to get __________.(ingots/carbon dioxide)

Answers are on p. 77.

Ingots, Blooms and BilletsAs we have seen, iron ore is melted in a furnace to get pig iron. Pig ironcontains a lot of impurities and excess carbon, unfit to use ashigh-strength steel for desired engineering applications. Thus, to improvethe strength and to get desired mechanical properties, excess carbon is




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burnt off and desired alloys are added to turn the iron into steel. The steelis then drawn and made into ingots.

Ingots (either fixed size or continuous pour) are further cut into thedifferent sizes and shapes called billets, blooms or slabs, depending on theshape of the final product needed. In addition to casting, rolling, forgingand welding, if a definite shape, like angles or channels, is required, theyare extruded. These shapes are further worked in rolling mills or forgingshops to get the desired shape. Forged shapes are shown in Figure 3.5.

These forms and shapes are defined as follows:

Billet A section of the ingot that is suitable for rolling operations. Bar Solid shapes that can be hot or cold rolled in rounds, squares

or flats in sizes ranging from 0.75 in. (1.9 cm) to 12 in. (30.5 cm)thick.

Bloom A slab of steel or other metal whose width and thicknessare equal.

Plate A large flat slab thicker than 0.25 in. (0.64 cm). Shapes Angles, channels, etc. Angles and channels are particular

shapes used in engineering industries. They are of definite shape and

Figure 3.5: Forged ingot and rough forged ingots. Photograph courtesy of Fomas Bay Forge,

Chengleput, Tamilnadu.





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extruded from ingots. Extrusion is the process of squeezing themetal with the help of a die, just like a grease gun. In a grease gun,the grease is squeezed out based on the shape and size of the orifice.

(See Figure 3.6 for a rough guide to shapes.)

Sand CastingWhenever huge castings are to be made, we produce the casting with asand mold. A pattern box is made to the shape of the casting, taking intoaccount the shrinkage that normally takes place from the liquid to solidstate. Then the sand mold is prepared using the pattern box and sand isrammed around it; binders are added to the sand to form a desired shape.(See Figure 3.6.)

Molten metal is slowly poured through gates, without turbulence, so thatthe molten metal fills up the mold. On the other side of the gate, a vent isattached called a riser to allow the gas that has evolved during the casting

process to escape. The gas, along with nonmetallic slag and otherimpurities, tries to escape through the riser, making the casting free fromgas voids and slag.

The rate of solidification depends on the thickness of every section of thecasting. To have a uniform rate of solidification, chills are kept in themold.

Pouring cupGating system MoldSprue

Riser

Molten metal in

form (mold)

Figure 3.6: Basic sand casting process.





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Chaplets and core supports are also placed in the mold both to supportthe core and allow all the sections of the casting to solidify at the sametime to prevent the formation of hot tears. (See Figure 3.7.)

Chaplets, Chills and CoresChaplets or chill blocks are metal blocks placed in the mold for localizedheat dissipation. They may be placed at an intersection or joint wherethere is a comparatively large volume of metal to cool, like a thick flangein an I-section casting. This helps produce a uniform rate of cooling withbetter microstructure without formation of hot tears or otherdiscontinuities due to forces created by a nonuniform rate of cooling. If

the chill is not totally melted, there will be a gap between the chill orchaplet, without it being fused. This discontinuity can be seen in theradiograph as a ring-like pattern with dark density showing a gap betweenthe solidified molten metal and the chill.

A core is placed in molds wherever it is necessary to preserve the space itoccupies in the mold so as to create a void or a gap in the finished casting.A core is generally made with sand and with some binders and core oil,and placed in the casting where a cavity is sought. Cores are supported bychaplets; molte

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