1

INDUSTRIAL RADIOGRAPHY

1.0 INTRODUCTION

Radiography is a non-destructive testing method to find out the internal discontinuities present in a component or assembly. It is based on differential absorption of penetrating radiation by the part being inspected. Radiographic inspection is used extensively on castings and weldments for steam power equipments (boiler and turbine components and assemblies) like superheaters, water walls, downcomer pipes, connecting tubes and headers and other high pressure systems and valves.

2.0 PRINCIPLE:

2.1 Studying the homogeneity of an opaque material using penetrating radiation is called radiography. The shadowgraph obtained in radiography is a radiograph. When penetrating radiation such as X-rays or Gamma-rays passes through a material, it gets absorbed or attenuated. The degree of absorption depends upon the thickness as well as physical density variation in the object. Higher the amount of radiation passing through the part to record in the film. The component, if any discontinuity of lesser density exists, more amounts of radiation will be emerging out. This penetrated radiation is let to fall on the photographic film.

2.2 Photographic films are very sensitive to X-rays and Gamma-rays. When the films are exposed, photo-chemical reaction takes place which can be converted to black and white image of internal structural. Variation of the part, by a process known as film processing.

More Absorption Lesser region

(Lighter region) (Darker region) Fig: SCHEMATIC SET UP OF RADIOGRAPHIC ARRANGEMENT2.3 The following discontinuities can easily be detected by radiography:

A. WELD DISCONTINUTIES:

B. CASTING DISCONTINUTIES1. Gas holes and porosities

1.Sand inclusions

2. Slag inclusions

2.Gas inclusions

3. Tungsten inclusion

3. Shrinkage

4. Lack of fusionA.

4.Hot tear

5. Lack of penetration

5.Crack

6. Crack

6.Cold shut

7. Undercuts

7. Unfused chaplets

8. Burn through

8.Chills

3.0 RADIATION SOURCES

3.1. X-Rays:

3.1.1. Conventional x-ray machines ranging from 100 KV to 420 KV and high energy x-ray machines such as linear Accelerators and Betatrons energy ranging from 2 Mev to 25 Mev are used for industrial radiography purposes. Portable equipment upto 300 KV are available for field radiography.

3.1.2. When fast moving electrons are suddenly stopped by a target x-rays are generated.

Fig: SET UP IN X-RAY BULB

To make the electron free from cathode and move towards the anode (target) current of the order of several kilo-voltage and to heat the filament and control the electron quantity, current of the order of mille-amperage is required. In other words, the quality of the x-rays produced is controlled by kilo-voltage and quantity by mille amperage. Associated with x-rays huge amount of heat is produced and it is very much essential to dissipate the heat to cool down the equipment to increase in oil bath and the oil bath in turn is cooled down by water circulation.TABLE IPENETRATING POWER OF X-RAYS OF VARIOUS KILO-VOLTAGE

KV

Penetration of range

mm of steelX-ray tubes

150

15 mm

250

40 mm

400

65 mm

KV

Penetration of range

mm of Steel

High Energy Sources:

2 MeV

6 mm to 250 mm

4 MeV

25 mm to 300 mm

3.2.0. Gamma-Ray Sources:

3.2.1. Certain naturally available elements with high atomic number like radium dis-integrated to another element by shedding out positively or negatively charged particles. This disintegration is always associated with production of gamma rays and this self-decaying process is known as radioactivity is induced artificially by bombarding the element with Neutrons. Such elements with the same atomic number but with different mass number are called ISOTOPES. These radioactive isotopes are gamma-rays generators.

3.2.2. Commonly used gamma-rays sources and their properties are given in Table II

TABLE II

Sl. No.Name of the IsotopeSymbolMain Energy MeVHalf Life PeriodApplication

01Cobalt-60Co601.33;1.175.3 yrs35 mm to 200 mm of steel

02Iridium-192Ir.1920.16-0.2975 days10 mm to 60 mm of steel

03Cesium-137Cs1370.6633 yrs10 mm to 75 mm of steel

04Thulium-170Tm1700.084-0.96127days2mm to 10 mm of steel or upto 25 mm of aluminum

For the thermal power station applications, Ir. 192 is commonly used for field radiography. Since Co.60 got greater hazard comparing to I.192 its application is usually restricted to closed enclosures. However for the examination of higher thickness components, Co.60 is also being used at times in field radiography.

Due to self decay process, the strength of the source reduces to half at the end of the each one half life period. This process is continuous and it becomes necessary to replenish the source at some time.

3.2. Properties of X-rays and Gamma- Rays:

3.2.1. Both are electromagnetic radiation traveling in straight line with the velocity of light. Their presence cannot be felt by any of our organ. They ionize the medium to which they pass through. Their photo chemical properties are advantageously used to expose films and fluorescent properties for fluoroscopy. But destruction of living cells poses radiation hazard.

3.2.2. X-ray can be generated as and when required and its energy can be controlled. Whereas the emission of gamma-rays are spontaneous and its energy level cannot be changed. Depending upon the thickness and type of material, the correct isotope is to be selected. The size of the gamma-ray equipment is small comparing to x-ray machine and is free from any external power and no cooling system is required. Hence isotopes are widely used in erection site and the facility to keep the source inside the pipe is an added advantage of isotope over X-ray machine. But gamma-ray source is a self decaying one and so a recurring expenditure on changing the source is unavoidable.

4.0 RECORDING MEDIA:

4.1.0. X-ray films are the recording media in radiography. Unlike photographic films, x-ray films are coated on both sides of a transparent flexible base of either cellulose triacetate or polyester, using proper adhesive.

4.2. Films are classified as slow speed, medium speed and fast speed depending upon the grain size of the emulsion.

Table III below gives the type of Industrial Radiographic films as per ASTM E 94/74. TABLE-III

Description

Film TypeSpeedContrastGraininess

01LowVery HighVery Low

02MediumHighLow

03HighMediumHigh

For the radiography of high pressure component either type I or type II films are used. The following are the brands of different films commonly used for radiography.

Type-I

01. Agfa Gevaert D2, D4, D502. Indu Not 55

03. Kodak M.

Type-II

01. Agfa Gevaert D7

02. Indu NDT 65;70

03. Kodak A, AA

5.0 INTENSIFYING SCREENS

5.1. All the radiation reaching the films will not inter-act with the emulsion to have photo-chemical reaction. 99% of the radiation falling on the film will just pass through it. To increase the photo-chemical reaction and thus to cut short the exposure time and to increase the radiographic contrast radiographic screens known as intensifying screens are pressed into intimate contact with the film during exposure. Screens are of two types, viz., metallic screens (usually lead screens) and fluorescent intensifying screens. 5.2. Lead Screens

It is the combination of filtration and intensification that makes lead screens the most widely used in industrial radiography. Low energy radiation is more readily absorbed by a lead screen than high energy radiation. Because scattered radiation from a test piece is always of a lower energy than the incident beam passing through a test piece, a lead screen will absorb a relatively high percentage of unwanted scattered radiation than the image forming radiation. Scattered radiations arise mainly from the test piece (internal scattering) and from the table or floor on which the film with its holder is placed during exposure (back scatter). Because of the need to filter out both internal scatter and back-scatter, two screens are normally used. The screen that faces the subject and source of radiation is referred to as the front screen and the screen behind the film towards the table or floor is referred to as the back screen; both screens absorb scattered radiation.

In practice the front screen is the thinner of the tow because the image forming radiation always must pass through the screen. The usual thickness of the front lead screen is 0.1 mm and back screen is 0.15 mm for Ir. 192.

5.3. Fluorescent Screens:

Fluorescent intensifying screens fluoresce of produce light when excited by X-rays or Gamma-rays. Certain compounds such as calcium tungstate or barium lead sulphate have the property of emitting light immediately upon excitation by short-wave length radiation. These screens are widely used in medical radiography and its use in pressure parts radiography is not permitted

6.0 FILM PROCESSING:-

6.1. By a chemical process the latent image formed during exposure is made visible and permanent. The films are to be handled in a dark room fitted with safe light. The safelight will give the required illumination and at the same time the particular coloured light will not affect the film. Generally lights with olive green or red orange filter is used as safelight. The film emulsion will get affected by high temperature. So the dark room should be air-conditioned.

6.2. The film processing includes 5 steps: 6.2.1. Developing:-

In developer the exposed silver bromide is reduced to silver. Developer is an alkaline solution of various compounds and the reducing action is slow at temperature below 18o C. At temperature above 30oC, the film emulsion will get damaged. Hence the developer temperature is to be maintained at 20oC by refrigeration system and the recommended developing time is 5 minutes at 20oC. Prolonged development will fog the image. Development in higher temperature will result in poor contrast.

6.2.2. Rinsing

Once the developing is over, the film turn to black colour and at the same time is not transparent due to the existence of undeveloped emulsion on the film. The unexposed emulsion will again react with light when it is taken out in ordinary light and turn black. Hence, it is essential to remove the unexposed emulsion. Before going for the removed neutralized by rinsing. So, after developing, film is immersed in another tank containing ordinary water or water with glacial acetic acid for few minutes to stop the developing action.

6.2.3. Fixing:-In fixer, the unexposed silver bromide is dissolved away any only the converted silver which is black in colour will remain on the film which will represent the internal structural variation of the part. The fixer solution is acidic in nature.

6.2.4. Washing:-

The film coming out of fixer will have chemicals carried over, which should be thoroughly washed, otherwise the film will get discoloured after sometime, when in storage. To remove the chemicals, films are thoroughly washed in running water.

6.2.5. Drying:-

After washing the films in running water for about 20 to 25 minutes, films are dried. Dust free hot air of temperature 100oF to 120oF is used for drying the film. Drying cabinet or rooms are used for drying the film. Once the film is dried, the radiograph is ready for evaluation.

7.0 IMAGE QUALITY INDICATORS (PENETRAMETERS)

7.1. Penetrameters are used to determine the ability of the radiographic process to record images of small flaws or the sensitivity. Penetrameters are of known size and shape and have the same attenuation characteristics as the material in the test piece. Penetrameters preferably are located in regions of maximum test piece thickness and greatest test piece to film distance, and near the outer edge of the central beam of radiation. The image of the penetrameter that appears on the finished radiograph is evaluated during interpretation to assure that the desired sensitivity, definition and contrast have been achieved in the developed image. Penetrameters of different designs have been developed by various standards-making organizations. Applicable codes specifications or purchase agreements usually determine the type of penetrameters to be used.

7.2. The three basic types of penetrameters in used are:

1. Plague type

2. wire type and

3. Step Wedge type7.2.1. Plague type penetrameter consists of strips of material of uniform thickness with holes drilled through them. ASTM, ASME and AWE specifications for such type penetrameters.

Designation

The holes are T1, 2T and 4T in diameter, where T is the thickness of the plague. Various degrees of image quality can be measured by using plaque type penetrameter of different thickness. Sensitivity is usually expressed in terms of penetrameter thickness (as a percentage of test piece thickness) and resolution is determined by the smallest hole size visible in the radiography. For instance, an image quality level of 2 2T indicates that the thickness of the penetrameter equals 2% of section thickness and the 2T hole is visible. If image quality of1-1T were required, a radiograph would be acceptable if the outline of a 1% penetrameter and the 1T hole in that penetrameter were distinguishable.

7.2.2. Wire type penetrameter:

A typical wire penetrameter is the standard of DIN 54109, which consists of sixteen of wires of each of three metals-steel, aluminium and copper. The wire diameters decreases in geometric progression from 3.2 mm (wire no.1) to 0.1 mm wire number 16.

TABLE-IV

NUMBERS(mm)

13.2

22.5

32.0

41.6

51.25

61.0

70.8

80.63

90.5

100.4

110.32

120.25

130.20

140.16

150.12

160.10

The standard of a given material is contained in the plastic envelopes, suitably marked for identification. Each envelop contains seven wires viz.

DIN 62 FE 1 ISO 7

DIN 62 FE 6 ISO 12

DIN 62 FE 10 ISO 16

Fig: Construction of a DIN penetrameter.

In contrast to the ASTM system, the DIN system does not provide contrast sensitivity, the sensitivity varies with test piece thickness within each thickness range.

% sensitivity = of smallest wire seen on the Radiograph

Thickness under the penetrameter x 100

Lesser the value, higher the sensitivity.

7.2.3. Step wedge penetrameters:-

Usually have either an arithmetic or geometric progression of step thickness. A plain step wedge penetrameter is useful only for determining the ability of a radiograph to resolve variations in test piece thickness it cannot be used to evaluate the effect of geometric unsharpness. However, if a plain step wedge is modified by drilling holes in each step, it becomes sensitive to geometric unsharpness. This type of design is used by the British Welding Research Association (BWRA) and French Navy (AFNOR)

8.0 DIFFERENT TECHNIQUES OF RADIOGRAPHY:

8.1. In a simple radiographic technique, source is kept at one side and film is kept at opposite sides of the object as shown in Figure-1. This is a single image technique. Welds similar to plate weld or in big cylinder can be radiographed with this technique. When the length of the weld is more, the weld is divided into small segments to suit the film length available and processing capability. Sufficient overlap is to be given at the extremities so that no area of the weld will go untested. We should see that density difference in the radiograph is within the range of 15% and +30%.

8.2. The inspection of complex shapes most often requires multiple exposures, usually with different viewing directions. The selection of views for each exposure depends primarily on the shape of the section of the test piece to be inspected with that exposure and the probable orientation of suspected flaws. There are other three major inspection techniques for tubular section or pipes, viz., the double wall double image technique, double wall single image technique and panoramic.

8.2.1. Double Wall Double Image Technique:-

This technique is applicable mainly to sections of no more than 3 O.D. This technique produces a radiograph in which the images of both walls of a tabular section are super-imposed on one another. The beam of radiation is directed toward one side of the section and the recording surface is placed on the opposite side, usually tangent to the section.

Fig. Set up a Double Wall Double Image Technique

Two exposures, 90 apart are required to provide complete coverage when the ratio of OD to ID is 1.4 or less. When the exposure required to provide complete coverage can be determined by multiplying that ratio by 1.7 and rounding off to the next higher integer. The circumferential displacement between shot is found by dividing 180 by the number of shots if the number is even. When an odd number of exposures are required for complete coverage, the angular spacing between shots can be determined by dividing 360 by the number of shots as an alternative to dividing 180 by the number. This alternative cannot be used when the number of shots is even because of half of the resulting radiographs would be mirror images of the remaining radiographs and sections of the outside circumference would not receive adequate coverage.

Variation of the double wall, double image technique, sometime called the Corona or Offset technique is often used the inspection of circumferential butt welds in small diameter tubing and pipe (Fig. Above). In the corona technique, the central beam is directed at an acute angle to the run of the tube (see Fig. Above) so that the weld is projected on the film as an ellipse rather than a straight band. The offset angle of the radiation beam must be large enough that the image of the upper section of the weld zone does not overlap the image of the lower portion, but not so large as to introduce an unnecessary degree of distortion. Also the larger the offset angle, the greater the probability that the technique will fail to detect incomplete fusion at the root of a plain butt weld. The correct number of exposure, and the circumferential location of corresponding view can be determined for the corona technique in the same manner as for the basic double wall, double image technique.

8.2.2. Double Wall, Single Image Technique

Double wall, single image technique is applicable mainly to hollow cylinders and tubular sections exceeding 3 in outer diameter. This technique produces a radiographic image of only the section of the wall that is closest to the recording plane, although the radiation penetrates both walls. The source is positioned relatively close to the section, so that the blurring caused by geometric un-sharpness in the image of the cylinder wall closest to the source makes that image completely in-distinguishable. Only the image of the wall section closest to the film is sharply defined. Exposures are calculated on the basis of double the wall thickness of the hollow section as they are for the double wall, double image technique. The area of coverage is limited by geometric unsharpness and a distortion at the extremities of the resolved image for the hollow cylinders that are less than about 15 inches in outside diameter. For larger cylinder, film size is the usual limiting factor. There must be enough overlap between adjacent exposures to ensure that the entire outside circumference is clearly recorded.

Fig: Schematic set up of a double wall single image technique.

8.2.3. Panoramic Exposures:

This technique can be used to radiograph the entire circumference of a pipe in a single exposure or a several small components arranged in a semi-circle by keeping the source at the centre. By providing a hole near the weld in a thick walled pipe (Gamma hole), the gamma-ray source can be inserted inside the pipe to the centre and the entire weld can be radiographed in one exposure. By this method the disadvantage of giving longer exposure due to the double wall penetrating and several exposures to cover full circumference can be avoided.

9.0 GEOMETRIC UNSHARPNESS:

Radiographic definition varies according to the geometric relationships among source size, source to object distance and object to image distance. When radiation from a finite size produces a shadow that portion of the image that is in shadow for radiation emanating from all points on the surface of the source is a region of complete shadow, known as the umbra. Portions of the image that are in shadow for radiation from some portion are regions of partial shadow, known as penumbra. The degree of geometrical unsharpness is equal to the width of the penumbra.

Geometrical unsharpness (Penumbra)

Fig. Effect of shadow formation by different parameter. Mathematically, the geometric unsharpness Ug is determined from the laws of similar triangles as illustrated in figure above and can be expressed as

s x t

Ug=

f

Where S is the size of the source or focal spot, t is the object to image distance and f is the source to object distance. The amount of geometric unsharpness can be reduced by lengthening the source to object distance, reducing the size of the source of focal spot or reducing the object to image distance.

In applications, where the maximum unsharpness must be kept below a specific known value (specified by the governing code) so as to resolve certain types and sizes of flaws, the radiographer can determine the minimum source to object distance for a given part from the equation given above.

10.0 Density:

Another factor controlling the quality of the radiograph is density. The quantitative measure of blackening a photographic emulsion is called density.

Io

Density =log10

It

WhereD = Density

Io= Intensity of light incident on the film

It= Intensity of light transmitted through the film.

DensityCapacity Io / It

01

0.32

0.64

1.010

2.0100

3.01000

4.010000

Density can be measured using a densitometer or can be compared with a pre-calibrated density strip. The minimum density recommended for radiographs with x-rays is 1.8 and with gamma-rays is 2.0 maximum can be 3.5 to 4.0. Higher density illuminator is required for viewing up the radiographs above density 2.0

11.0 INTER PRETATION OF RADIOGRAPHS:

11.1 A qualified interpreter must

a. Define the quality of the radiographic image which includes a critical analysis of the radiographic procedure and the image developing procedure.

b. Analyse the image to determine the nature and extent of any abnormal condition in the test piece.

c. Evaluate the test piece by comparing interpreted information with standards or specification and

d. Report inspection results accurately, clearly and within proper administrative channels.

11.2 Viewing or Radiographs

Viewing of radiographs should be done in an area with subdued lighting to minimize distracting reflections from the viewing surface. The viewing apparatus should have an opal glass or plastic screen large enough to accommodate the largest film to be interpreted. The screen should be illuminated from behind with light of sufficient intensity to reveal variations in photographic density up to a nominal film density of at least 3.0. There may be a need for a smaller, more intensely illuminated viewer for evaluation small areas of film having densities up to 4.5 or more. Viewing screens of high intensity illuminator should be cooled by blowers or other suitable apparatus to prevent excessive heat from damaging films and to extend lamp life.

12.0 RADIOGRAPHIC APPEARANCE OF SPECIFIC TYPE OF FLAWS:

12.1. Porosity (Gas Holes)

The porosity may be spherical, elongated or worm hole shapes and in pattern that are random, clustered or linear on a radiograph, the spherical voids have the appearance of rounded dark area, while the non-spherical voids have an elongated dark area with smooth outline.

12.2. Tungsten Inclusion

Tungsten inclusions are lighter than the surrounding areas and may be rounded or irregular.

12.3. Incomplete penetration:

Appears as a dark straight line through the centre of the weld. The width of the indication is determined by the root gap and amount of weld penetration.

12.4. Slag Inclusion:

Appears darker than the surrounding area and may be irregular in shape or elongated in the direction of the deposited weld bead.

12.5. Lack of Fusion:

Appears as a dark indication usually elongated and varying in width.

12.6. Crack:

Appears as a dark jagged or straight line.

12.7. Icicles (Tear Drops)

Appears as individual, rounded lighter indications with an occasional small dark spot in the centre of a drop.

12.8. Burn Through:

Appears as an individual darkened area of elongated or rounded contour which may be surrounded by a lighter ring.

12.9. Under cut:

Appears as a relatively straight and narrow dark line and can be located on either or both sides of the root opening locations.

13.0 EVALUATION

13.1. A flaw detected by radiography is meaningless unless compared to a known condition. Evaluation consists of comparing the interpreted image with the least acceptable condition in terms of the type, size, quantity and severity level of any flaws that are found. The result of this comparison is a judgement to accept or reject the part.

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