ISO 19232–1, Non-destructive testing --- Image quality of radiographs --- Part 1: Image quality indicators wire type — Determination of image quality value ISO 19232–2, Non-destructive
Protection against ionizing radiation
Exposure to X-rays or gamma-rays poses significant health risks It is essential to adhere to legal regulations when using X-ray equipment or radioactive materials.
Local or national or international safety precautions when using ionizing radiation shall be strictly applied.
Surface preparation and stage of manufacture
Surface preparation is typically not required; however, if there are surface imperfections or coatings that hinder defect detection, it is essential to grind the surface smooth or remove the coatings.
Unless otherwise specified, digital radiography shall be carried out after the final stage of manufacture, e.g.
Location of the weld in the radiograph
Where the digital radiograph does not show the weld, high density markers shall be placed on either side of the weld.
Identification of radiographs
Each section of the object undergoing digital radiography must have symbols attached These symbols should be visible in the digital radiograph, ideally positioned outside the region of interest, to guarantee clear and unambiguous identification of each section.
Marking
Permanent markings on the object to be examined shall be made in order to accurately locate the position of each digital radiograph (e.g zero point, direction, identification, measure)
Where the nature of the material and/or its service conditions do not permit permanent marking, the location may be recorded by means of accurate sketches or photographs.
Overlap of digital images
When performing digital radiography with multiple detectors, it is essential that they overlap adequately to capture the entire area of interest This overlap must be confirmed by placing a high-density marker on the object's surface, which should be visible in each digital image If the radiographs are taken in sequence, the high-density marker must appear in all images to ensure consistency and accuracy.
Types and positions of image quality indicators (IQI)
The quality of image shall be verified by use of image quality indicators (IQIs) in accordance with ISO 19232-5 and ISO 19232-1 or ISO 19232-2
To verify the basic spatial resolution of the digital detector system, a reference image is required as outlined in Annex C The duplex wire value must be determined to ensure the system hardware meets the specified requirements based on the penetrated material thickness in Tables B.13 and B.14 While positioning the duplex wire IQI directly on the digital detector is necessary, its use for production radiographs is not mandatory and may depend on the agreement between contracting parties For production radiographs, the duplex wire IQI should be placed on the object, and the measured basic spatial resolution must not exceed the maximum values specified in the tables In single image inspections, the penetrated material thickness is defined as the single wall thickness, whereas for double wall double image inspections, it is based on the pipe diameter The basic spatial resolution for double wall double image inspections must align with the values in Tables B.13 and B.14, using twice the nominal single wall thickness as the penetrated material thickness.
If the geometric magnification technique (see 7.7) is applied with v 1,2, then the duplex wire IQI (ISO 19232-
5) shall be used on all production radiographs
The duplex wire IQI should be tilted between 2° to 5° relative to the digital rows or columns of the image If the IQI is set at a 45° angle to these lines, the resulting IQI number will decrease by one.
The contrast sensitivity of digital images shall be verified by use of IQIs, in accordance with the specific application as given in Tables B.1 to B.12 (see also ISO 19232-1 or ISO 19232-2)
For optimal results, single wire or step hole IQIs should be positioned on the source side of the test object, ideally at the center of the area of interest on the parent metal adjacent to the weld The IQI must maintain close contact with the object's surface and be located in a section of uniform thickness, which is defined by a consistent grey value (mean) in the digital image.
When utilizing an IQI, two scenarios must be considered: a) For a single wire IQI, the wires must be positioned perpendicular to the weld, ensuring that at least 10 mm of wire length is visible in a section with a uniform grey value or SNR N, typically found in the parent metal adjacent to the weld In accordance with sections 7.1.6 and 7.1.7, the IQI can be oriented with the wires across the pipe axis, avoiding projection into the weld image b) For a step hole IQI, it should be placed so that the required hole number is positioned close to the weld.
For exposures outlined in sections 7.1.6 and 7.1.7, the IQI type can be positioned on either the source or detector side If placement according to these guidelines is not feasible, the IQIs should be positioned on the detector side, and image quality must be assessed at least once by comparing exposures with one IQI on the source side and another on the detector side under identical conditions Additionally, if filters are utilized in front of the detector, the IQI must be placed in front of the filter.
For double wall exposures, when the IQI is placed on the detector side, the above test is not necessary In this case, refer to the correspondence tables (Tables B.9 to B.14)
Where the IQIs are placed on the detector side, the letter F shall be placed near the IQI and it shall be stated in the test report
The identification numbers and, when used, the lead letter F, shall not be in the area of interest, except when geometric configuration makes it impractical
To ensure consistent image quality in digital radiographs, it is essential that identical exposure and processing techniques are applied to similar test objects and regions If these conditions are met and no significant differences in image quality are anticipated, it may not be necessary to verify the image quality for every digital radiograph The level of image quality verification should be determined through mutual agreement between the contracting parties.
For pipes with a diameter of 200 mm or greater, at least three IQIs should be evenly distributed around the circumference, with the source positioned centrally This arrangement ensures that the IQI images accurately represent the entire circumference.
Minimum image quality values
Tables B.1 to B.14 outline the minimum quality standards for metallic materials, while alternative materials may have requirements established through mutual agreement between contracting parties These specifications should be determined in accordance with ISO 19232-4.
When using Ir 192 or Se 75 sources, contracting parties may agree to accept IQI values that are lower than those specified in Tables B.1 to B.12.
Double wall, double image techniques, both class A and B (w 2t):
10 mm w 25 mm 1 wire or step hole value less for Ir 192;
5 mm w 12 mm 1 wire or step hole value less for Se 75
Single wall single image and double wall single image techniques, class A:
10 mm w 24 mm 2 wire or step hole values less for Ir 192;
24 mm w 30 mm 1 wire or step hole value less for Ir 192;
5 mm w 24 mm 1 wire or step hole value less for Se 75
Single wall single image and double wall single image techniques, class B:
10 mm w 40 mm 1 wire or step hole value less for Ir 192;
5 mm w 20 mm 1 wire or step hole value less for Se 75.
Personnel qualification
Personnel conducting non-destructive examinations under ISO 17636 must be qualified per ISO 9712 or an equivalent standard relevant to their industry Additionally, they must demonstrate that they have received further training and qualifications specifically in digital industrial radiology.
7 Recommended techniques for making digital radiographs
NOTE Unless otherwise explained, definitions of the symbols used in Figures 1 to 21 can be found in Clause 4.
Test arrangements
Normally digital radiographic techniques in accordance with 7.1.2 to 7.1.9 shall be used
The elliptical technique, illustrated in Figure 11, is not suitable for external diameters greater than 100 mm, wall thicknesses exceeding 8 mm, or weld widths greater than one-fourth of the external diameter For cases where the wall thickness to external diameter ratio is less than 0.12, two 90° displaced images are adequate; otherwise, three images are required Additionally, the distance between the two projected weld images should be approximately equal to one weld width.
For elliptical examinations where the diameter is less than or equal to 100 mm, the perpendicular technique as outlined in section 7.1.7 can be utilized, requiring three exposures spaced 120° or 60° apart.
For test arrangements depicted in Figures 11, 13, and 14, it is essential to minimize the beam inclination to avoid overlapping images Additionally, the source-to-object distance, denoted as \( f \), should be minimized for the technique illustrated in Figure 13, as specified in section 7.6 The Image Quality Indicator (IQI) must be positioned near the detector, marked with a lead letter F.
Contracting parties may agree on alternative digital radiographic techniques when beneficial, such as in cases involving the geometry of the piece or variations in material thickness An example of this is provided in section 7.1.9 Furthermore, thickness compensation using the same material can also be implemented.
NOTE In Annex A the minimum number of digital radiographs necessary is given in order to obtain an acceptable radiographic coverage of the total circumference of a butt weld in pipe
If the geometric magnification technique is not used, the detector shall be placed as close to the object as possible
When using rigid cassettes or planar digital detector arrays, as illustrated in Figures 2 b), 8 b), 13 b), and 14 b), the source-to-detector distance (SDD) must be determined based on the wall thickness (t), the maximum distance from the detector to the source side surface of the object (b), and the focal spot size or source size (d), as outlined in section 7.6, Formulae (3) and (4).
7.1.2 Radiation source located in front of the object and with the detector at the opposite side
Figure 1 — Test arrangement for plane welds and single wall penetration 7.1.3 Radiation source located outside the object and detector inside (see Figures 2 to 4) a) with curved detectors b) with planar detectors
Figure 2 — Test arrangement for single wall penetration of curved objects
Figure 3 —Test arrangement for single-wall penetration of curved objects (set-in weld)
Figure 4 —Test arrangement for single wall penetration of curved objects (set-on weld)
7.1.4 Radiation source centrally located inside the object and with the detector outside
Figure 5 —Test arrangement for single wall penetration of curved objects, planar detectors not applicable
Figure 6 —Test arrangement for single wall penetration of curved objects (set-in weld)
Figure 7 — Test arrangement for single wall penetration of curved objects (set-on weld)
7.1.5 Radiation source located off-centre inside the object and detector outside (see Figures 8 to 10) a) with curved detectors b) with planar detectors
Figure 8 — Test arrangement for single wall penetration of curved objects
Figure 9 —Test arrangement for single wall penetration of curved object (set-in weld)
Figure 10 —Test arrangement for single wall penetration of curved objects (set-on weld)
Figure 11 —Test arrangement for double wall penetration double image of curved objects for evaluation of both walls (source and detector outside of the test object)
NOTE The source-to-object distance can be approximated by the perpendicular distance f ’, calculated from b’
The test arrangement for evaluating double wall penetration involves a radiation source positioned outside the test object, with the detector placed on the opposite side This setup is illustrated in Figures 12 through 18, showcasing both curved and planar detectors for comprehensive analysis of the curved objects.
The test arrangement for evaluating double wall penetration involves a single image of curved objects, focusing on the wall adjacent to the detector This setup includes the use of an Image Quality Indicator (IQI) positioned near the detector, with configurations featuring both curved and planar detectors.
Figure 14 — Test arrangement for double wall penetration single image
Figure 15 —Test arrangement for double wall penetration single image of longitudinal welds
Figure 16 —Test arrangement for double wall penetration single image of curved objects for evaluation of the wall next to the detector
1 compensating edge a) Test arrangement without compensating edge b) Test arrangement with compensating edge
Figure 17 —Test arrangement for penetration of fillet welds
Figure 18 —Test arrangement for penetration of fillet welds 7.1.9 Technique for different material thicknesses (see Figure 19)
Choice of tube voltage and radiation source
7.2.1 X-ray devices up to 1 000 kV
To achieve optimal flaw sensitivity, it is essential to keep the X-ray tube voltage at a minimum while maximizing the signal-to-noise ratio (SNR) in digital images Figure 20 illustrates the recommended maximum X-ray tube voltage values based on penetrated thickness, which represent best practice standards for film radiography.
After accurate calibration, DDAs can provide sufficient image quality at significantly higher voltages than those shown in Figure 20
Imaging plates with high structure noise in the sensitive IP layer (coarse grained) should be applied with about
For class B testing, use X-ray voltages that are 20% lower than those shown in Figure 20 High definition imaging plates, which are exposed like X-ray films and exhibit low structural noise, can be utilized with the X-ray voltages from Figure 20 or even higher, provided that the signal-to-noise ratio (SNR) is adequately enhanced.
an improvement in contrast sensitivity can be achieved by an increase in contrast at constant SNRN [by reduction of tube voltage and compensation by higher exposure (e.g milliampère minutes)]; or
improvement in contrast sensitivity by an increase in SNRN [by higher exposure (e.g milliampère minutes)] at constant contrast (constant kilovolt level);
Increasing the tube voltage while maintaining a constant exposure, such as milliampère minutes, leads to reduced contrast but enhances the signal-to-noise ratio (SNRN) If the improvement in SNRN surpasses the reduction in contrast caused by the higher energy, the overall contrast sensitivity will improve.
U X-ray voltage 1 copper and nickel and alloys w penetrated thickness 2 steel
Figure 20 —X-ray voltage for X-ray devices up to 1 000 kV as a function of penetrated thickness and material
In certain applications involving radiography, where the object's thickness varies across its area, a technique modification utilizing a higher voltage may be necessary However, it is important to recognize that using an excessively high tube voltage can diminish the sensitivity for detecting defects.
The recommended penetrated thickness ranges for gamma-ray sources and X-ray equipment above 1 MeV are given in Table 2
Gamma-rays from Se 75, Ir 192, and Co 60 sources exhibit lower defect detection sensitivity in thin steel specimens compared to X-rays when using optimal technique parameters Nevertheless, the handling and accessibility benefits of gamma-ray sources make them a valuable option.
Table 2 gives a range of thicknesses for which each of these gamma-ray sources may be used when the use of X-ray tubes is difficult
By agreement between the contracting parties, the penetrated material thickness may be further reduced to
10 mm for Ir 192 and 5 mm for Se 75
For certain applications, wider material thickness ranges may be permitted, if sufficient image quality can be achieved
When producing digital radiographs with computed radiography (CR) using gamma-rays, the total travel time to and from the source must not exceed 10% of the total exposure time For digital detectors (DDAs), the capture time begins once the source is in position and concludes before the source is retracted.
Table 2 — Penetrated thickness range for gamma-ray sources and X-ray equipment with energy above
1 MeV for steel, copper and nickel base alloys
X-ray equipment with energy from 1 MeV to 4 MeV 30 w 200 50 w 180
X-ray equipment with energy from 4 MeV to 12 MeV w 50 w 80
X-ray equipment operating at energies above 12 MeV can penetrate varying thicknesses of materials For aluminium and titanium, the penetration thickness ranges from 10 mm to 70 mm for class A and 25 mm to 55 mm for class B Additionally, for class A, the penetration thickness can extend from 35 mm to 120 mm.
The maximum penetrated thicknesses as given in Table 2 may be exceeded if sufficient IQI sensitivity can be proven.
Detector systems and metal screens
7.3.1 Minimum normalized signal-to-noise ratio
For digital radiographic examinations, it is essential to achieve the minimum Signal-to-Noise Ratio (SNR N) values outlined in Tables 3 and 4, or the minimum grey values applicable to Computed Radiography (CR) only Annex D details the measurement procedure for SNR N and includes a conversion table for users who opt to utilize unnormalized measured SNR values rather than normalized ones.
Equivalent minimum grey values for computed radiography (CR) can replace minimum signal-to-noise ratio (SNR N) values, provided they are established using the method outlined in Annex D, tailored to the specific imaging plate (IP), scanner, and its settings, along with the necessary SNR N from Tables 3 and 4.
The SNR N value should be measured near the weld, close to the wire or step hole IQIs, in a section of uniform wall thickness and grey values In CR, grey values must be assessed in the weldment area near these IQIs Due to the impact of material roughness on image noise and SNR N, the values provided in Table 3 are merely recommendations Additionally, if SNR N measurements are taken near the weld in the heat-affected zone, the minimum SNR N values should be increased by a factor of 1.4, unless the weld cap and root are flush with the parent material.
In film radiography, the optical density in the heat-affected zone (HAZ) or parent material usually ranges from 3.5 to 4, indicating a signal-to-noise ratio (SNR N) that is approximately 1.4 times higher than that of the weld center, which should have an optical density of 2 or more It is advisable to measure the SNR N in the HAZ, as this area typically exhibits a consistent grey level, allowing for precise SNR N measurements.
Annex D describes the method for determination of equivalent minimum grey values (for CR only) in lieu of the required SNR N
Annex D includes a conversion table for users who favor unnormalized SNR measurements over SNR N The minimum unnormalized SNR is calculated based on the SRb of the detector and the necessary SNR N values found in Tables 3 and 4.
Users must establish minimum grey values or SNR N values for the acceptance of digital images, as outlined in Annex D For radiography using Digital Detector Arrays (DDAs), minimum SNR N values should be defined similarly to the minimum optical density required for film radiography In the absence of specific values, the standards set forth in Tables 3 and 4 must be met These tables provide the minimum SNR N values applicable to various radiation sources and material thicknesses.
NOTE 2 For details of SNR N measurement, see ISO 16371-1, ASTM E 2446 [10] (for CR) or ASTM E2597 [11] (for DDA) and Annex D
If the detector system and exposure conditions fail to meet the IQI sensitivities outlined in Tables B.1 to B.14, enhancing the visibility of single IQI wires or step holes can offset the exceeded unsharpness values or SR b values.
If the required values of D12 and W16 for a 5 mm thickness, class B, are not simultaneously met for a specific detector setup, the values D11 and W17 can offer equivalent detection sensitivity Compensation is restricted to a maximum increase of two single wires for two missing resolved duplex wire pairs However, if the necessary flaw sensitivity is demonstrated for the specific application and agreed upon by the contracting parties, this compensation may be extended to a maximum of three single wires for three missing resolved duplex wire pairs.
The contrast sensitivity of digital detectors (DDA) is influenced by the integration time and tube current (milliampères) used during the acquisition of radiographic images, as well as the distance and tube voltage Increasing the exposure time and/or tube current can enhance the visibility of single wire or step hole IQIs This principle also applies to computed radiography (CR), although it is constrained by the maximum achievable signal-to-noise ratio (SNR) due to the structural noise of the photostimulable luminescence (PSL) layer in imaging plates Additionally, the quality of the calibration procedure limits the maximum achievable SNR for DDA radiography.
The SRb of the detector is fixed by design and hardware parameters
When utilizing the magnification technique, the SRb should be obtained from the magnified image, along with the duplex wire IQI measurement taken from the object.
7.3.3 Metal screens for IPs and shielding
When using metal front screens, good contact between the sensitive detector layer and screens is required
To enhance image clarity, it is essential to use vacuum-packed imaging plates (IPs) or apply pressure Lead screens that do not maintain close contact with the IPs can lead to image blurriness Additionally, the level of intensification achieved with lead screens in direct contact with imaging plates is considerably lower compared to traditional film radiography.
Many IPs are very sensitive to low energy backscatter and X-ray fluorescence of back-shielding from lead
To enhance image quality and minimize edge unsharpness and reduced contrast-to-noise ratio (CNR), it is advisable to use steel or copper shielding directly behind the imaging plates (IPs) Additionally, placing steel or copper shielding between a backscatter lead plate and the IP can further improve image quality Modern cassette and detector designs may address this issue, potentially eliminating the need for extra shielding outside the cassette.
The protective layer between the lead and the sensitive layer of an IP significantly diminishes the intensification effect of electrons, which becomes noticeable only at higher energies The degree of intensification varies between 20% and 100% at typical X-ray energies, depending on the radiation energy and the design of the protective layer.
The small intensification effect from a lead screen in contact with an imaging plate (IP) can be offset by increasing exposure time or milliampère-minutes when lead screens are not utilized However, since lead screens can cause scratches on IPs if not properly separated during scanning, they should be employed for intermediate filtering of scattered radiation outside cassettes It is important to note that no intermediate filtering is advised for inspecting steel specimens with a thickness of less than 12 mm.
Tables 3 and 4 show the recommended screen materials and thicknesses for different radiation sources
Agreements on alternative screen thicknesses can be made between the contracting parties, ensuring that the desired image quality is maintained It is advisable to use metal screens in front of imaging plates (IPs), as they can also help minimize the effects of scattered radiation when paired with digital detectors (DDAs).
Alignment of beam
The radiation beam must be aimed at the center of the examined area and positioned perpendicularly to the object's surface, unless a different alignment is shown to better reveal specific imperfections In such instances, an alternative beam alignment is acceptable Additionally, other digital radiography methods may be mutually agreed upon by the contracting parties.
EXAMPLE For better detection of lack of side wall fusion, the beam direction should be aligned with the weld preparation angles.
Reduction of scattered radiation
In order to reduce the effect of scattered radiation, direct radiation shall be collimated as much as possible to the section under examination
For radiation sources such as Se 75, Ir 192, and Co 60, or X-ray sources exceeding 1 MW, a lead sheet filter can effectively reduce low-energy scattered radiation between the object and the detector or DDA The recommended thickness of the lead sheet ranges from 0.5 mm to 2 mm, depending on the required penetration Additionally, alternative materials like tin, copper, or steel may also serve as effective filters It is advisable to place a thin steel or copper screen between the lead sheet and the detector for optimal performance.
Table 3 — Minimum SNR N values (CR and DDA) and metal front screens (screens for CR only) for digital radiography of steels, copper and nickel based alloys
Radiation source Penetrated material thickness w mm
Minimum SNR N C Type and thickness of metal front screens mm Class A Class B
50 kV to 150 kV 70 120 0 to 0,1 (Pb)
150 kV to 250 kV 70 100 0 to 0,1 (Pb)
100 70 100 0,3 to 0,8 (Fe or Cu) + 0,6 to 2 (Pb)
70 0,3 to 0,8 (Fe or Cu) + 0,6 to 2 (Pb)
100 70 100 0,6 to 4 (Fe, Cu or Pb)
For effective shielding, a steel screen should be positioned between the iron and lead screens when multiple screens (Fe+Pb) are used Alternative materials such as copper, tantalum, or tungsten can replace Fe or Fe+Pb, provided that the image quality is validated When measuring the signal-to-noise ratio (SNR) in the heat-affected zone (HAZ) or parent material, the values must be multiplied by 1.4, unless the weld cap and root are flush with the parent material Additionally, lead screens can be fully or partially substituted with iron or copper screens, with the equivalent thickness for iron or copper being three times that of lead.
Table 4 — Minimum SNR N values (CR and DDA) and metal front screens (screens for CR only) for the digital radiography of aluminium and titanium
Radiation source Minimum SNR N b Type and thickness of metal front screens
X-ray potentials > 150 kV to 250 kV 70 100 0,2 (Pb) a
X-ray potentials > 250 kV to 500 kV 70 100 0,2 (Pb) a
For measurements of 75, 70, and 100 with a lead equivalent of ≤0.3 (Pb), a 0.1 mm screen with an additional 0.1 mm filter can be utilized outside the cassette instead of the standard 0.2 mm lead Additionally, if the signal-to-noise ratio (SNR N) is assessed in the HAZ or parent material, these values should be multiplied by 1.4, unless the weld cap and root are flush with the parent material.
Each new CR test arrangement must be verified for backscattered radiation using a lead letter B, which should have a minimum height of 10 mm and a thickness of at least 1.5 mm, positioned directly behind each cassette If this symbol appears as a lighter image on the digital radiograph, it indicates the presence of backscattered radiation.
(negative presentation, i.e decreased linearized grey value), it shall be rejected If the symbol is darker
(increased linearized grey value), or invisible, the digital radiograph is acceptable and demonstrates good protection against backscattered radiation
To protect the detector from backscattered radiation, a lead sheet of at least 1 mm thickness or a tin sheet of at least 1.5 mm thickness should be positioned behind it Additionally, a steel or copper layer approximately 0.5 mm thick should be placed between the lead shield and the detector to minimize the effects of lead X-ray fluorescence radiation It is important to note that lead screens must not be in direct contact with the back side of the detector for radiation energies exceeding 80 keV.
Source-to-object distance
The minimum source-to-object distance, denoted as \$f_{min}\$, is influenced by the size of the source or focal spot, represented as \$d\$, as well as the distance from the object to the detector, referred to as \$b\$ It is essential that the focal spot size \$d\$ complies with the standards set by EN 12543 or EN 12679.
When the source size or focal spot size is defined by two dimensions, the larger shall be used
For exposure geometries, the distance \( f \) should be selected to ensure that the ratio of this distance to the source size or focal spot size \( d \) (i.e., \( f/d \)) meets or exceeds the values specified in Formulae (1) and (2), except for the cases illustrated in Figures 2 b), 8 b), 13 b), and 14 b).
7,5 2/3 f b d (1) for class A and for class B
(2) where b is expressed in millimetres
If the distance b is less than 1,2t, then the dimension b in Formulae (1) and (2) and Figure 21 shall be replaced by the nominal thickness, t
For determination of the source-to-object distance, f min , the nomogram in Figure 21 may be used This nomogram is based on Formulae (1) and (2)
For exposure geometries based on Figures 2 b), 8 b), 13 b), and 14 b), it is essential to select the distance \( f \) such that the ratio of this distance to the source size \( d \) (i.e., \( f/d \)) meets or exceeds the values specified in Formulae (3) and (4).
(3) for class A and for class B
(4) where t is the nominal thickness, in millimetres, to inspect; b is the object-to-detector distance, in millimetres
In class A, to meet the requirement for detecting planar imperfections, the minimum source-to-object distance, denoted as \$f_{min}\$, must match that of class B This alignment is essential to achieve a reduction in geometric unsharpness by a factor of 2.
In critical technical applications of crack-sensitive materials, more sensitive radiographic techniques than class B shall be used
Figure 21 — Nomogram for the determination of minimum source-to-object distance f min in relation to object-to-detector distance b and the source size d
The total unsharpness (\$u_T\$) in a digital detector system is influenced by both inherent unsharpness and geometric unsharpness (\$u_G\$) If not corrected through geometric magnification, these factors can significantly affect image clarity The relationship can be expressed as \$u_{det} = 2SRb\$.
Therefore, it is recommended that the distance f min be increased to compensate for any additional unsharpness of the detector system
When utilizing digital detectors that exhibit greater inherent unsharpness compared to X-ray film, it is advisable to follow conditions a) and b) to achieve comparable low total image unsharpness values, as outlined by ISO 17636-1.
To achieve optimal film radiography results, it is essential that the object is in contact with the detector, except when using the geometric magnification technique It is recommended to choose digital detectors with a basic spatial resolution (SR b) that is lower than the values specified in Formulae (6) and (7), which depend on the distance between the object and the detector.
To achieve an unsharpness level similar to that of film radiography (ISO 17636-1) for class B, it is necessary to increase the minimum focal length, \$f_{min}\$, beyond the values provided by Formulae (1) or (2) and Figure 21 This adjustment can be calculated using Formulae (8) and (9), provided that the conditions of Formula (6) or (7) are met.
To determine \$f_{min}\$, one can use Formula (1), (2), or Figure 21, particularly when the detector's basic spatial resolution (\$SR_b\$) is significantly lower than what is stipulated by Formula (6) or (7) Additionally, achieving the IQI visibility, as outlined in Tables B.1 to B.12, can be accomplished by enhancing the signal-to-noise ratio (SNR) in CP II.
Tables B.13 and B.14 provide the maximum total image unsharpness values and SR b requirements for sufficient image quality for class A and class B, respectively
When using the elliptic technique specified in 7.1.6 or the perpendicular technique specified in 7.1.7, b shall be replaced by the external diameter D e of the pipe in Formulae (1) and (2) and in Figure 21
In the double wall penetration technique, when the source is positioned outside the object and the detector is placed on the opposite side, the minimum distance between the source and the object is solely influenced by the wall thickness, rather than the diameter of the pipe.
To optimize radiographic examination, it is advisable to avoid the double wall technique by positioning the radiation source within the object being examined, ensuring a more effective direction of examination The minimum source-to-object distance should not exceed a 20% reduction If the source is centrally located inside the object with the detector placed externally, and the Image Quality Indicator (IQI) requirements are satisfied, this reduction may be increased to a maximum of 50% Any further reductions can be negotiated between the contracting parties, provided the IQI standards are maintained.
Geometric magnification technique
A significant challenge in utilizing Computed Radiography (CR) and Digital Detector Array (DDA) systems for weld radiography is the large pixel size (≥50 µm) of most digital detectors, which contrasts with the fine grain size of film that provides superior spatial resolution However, this issue can be addressed by leveraging the distinctive capability of DDAs to enhance the signal-to-noise ratio (SNR) in images and, if necessary, applying geometric magnification.
NOTE Geometric magnification is different from digital magnification (zoom) of displayed images Only geometric magnification provides a reduction in image unsharpness
To meet the necessary IQI-sensitivity and SRb requirements outlined in Tables B.1 to B.14, one option is to enhance the image signal to noise ratio, as detailed in section 7.3.2 of CP II.
An alternative approach involves utilizing the geometric magnification technique, which increases the distance between the image receptor (IP) or digital detector array (DDA) and the object This method is enhanced by employing an X-ray tube with a small focal spot or a gamma-ray source with a minimized source size.
Finally, after employing both methods, if the required IQI values are still not visible, that CR system or the DDA cannot be used for that examination
The proper selection of magnification in production radiographs is validated by the use of a duplex wire IQI, which should be placed closer to the detector if the condition \(2SR > d\) (where \(d\) is the source or focal spot size) is met If this condition is not satisfied, the duplex wire IQI must be positioned on the source side of the object For optimal magnification value selection, it is advisable to position duplex wire IQIs on both sides of the object; however, only one should be visible in the final production radiographs after determining the correct magnification factor and focal spot size.
Automated defect recognition can be disrupted by Image Quality Indicators (IQIs) in digital images To ensure consistent image quality in production radiographs without IQIs, it is essential to periodically validate the images against reference images that utilize wire IQIs, step hole IQIs, or duplex IQIs.
The image unsharpness u Im can be estimated from the magnification v, the geometric unsharpness u G and the
The basic spatial resolution of the detector at a magnification of 1 is denoted as SR b The source-to-object distance is represented by f, while geometric unsharpness is indicated by u G The focal spot size, in accordance with EN 12543 or EN 12679, is referred to as d Geometric magnification, calculated as the ratio of SDD to f, is represented by v Lastly, the required maximum image unsharpness for class A or B testing is denoted as u Im, as specified in Table B.13 or Table B.14.
To minimize image unsharpness to the acceptable levels outlined in Tables B.13 or B.14, it is essential to either increase the magnification or decrease the focal spot size This reduction in unsharpness must be verified using a duplex wire IQI placed on the object, as previously detailed.
The magnification factor varies between the source and detector sides of an object, necessitating the selection of a magnification value for the object's center It is important that the variation in magnification between these sides remains within ±25% In certain cases, smaller magnification values may be appropriate.
CP II as described in 7.3.2 is used.
Maximum area for a single exposure
The quantity of digital radiographs required for a thorough assessment of flat and curved welds, with the radiation source positioned off-centre, must be determined based on established technical specifications.
The penetrated thickness ratio at the outer edge of a uniformly thick evaluated area must not exceed 1.1 for class B and 1.2 for class A.
The SNR N values from any changes in penetrated thickness must meet or exceed the levels specified in Tables 3 and 4 Additionally, GVs can be utilized for CR as detailed in Annex D.
The examination area encompasses both the weld and the heat-affected zones, typically requiring an inspection of approximately 10 mm of the parent metal on each side of the weld.
Recommendations for the number of digital radiographs are indicated in Annex A which gives an acceptable examination of a circumferential butt weld.
Processing
7.9.1 Scan and read-out of image
To achieve optimal image quality, detectors and scanners must be utilized according to the manufacturer's guidelines It is essential that digital radiographs remain free from artifacts caused by processing, handling, or other factors that could hinder accurate interpretation.
If using DDAs, the detector calibration procedure, as recommended by the manufacturer, shall be applied
The detector shall be calibrated with a background image (without radiation) and at least with one gain image
Multi-gain calibration enhances the achievable signal-to-noise ratio (SNR) and linearity, although it requires additional time To reduce noise during calibration, all calibration images must be captured with an exposure dose at least double that of the inspection radiographs If properly documented, calibrated images can be treated as original images for quality assurance Regular calibration is essential, especially when exposure conditions change significantly.
Bad pixels are underperforming detector elements of DDAs They are described in ASTM E2597 [11]
When utilizing Digital Detector Arrays (DDAs), it is crucial to create a bad pixel map following the manufacturer's guidelines, and this map must be documented Interpolating bad pixels is a necessary procedure in radiography with DDAs It is advisable to use only DDAs that do not contain cluster kernel pixels (CKP) within the region of interest (ROI).
DDAs without CKPs and CR are to be used for inspection when the basic spatial resolution (SR b) of the detector is less than or equal to the requirements outlined in Table B.13 or B.14 In cases where a magnification technique is employed, the SR b must be determined from the measured image as specified in Annex C, with the duplex IQI placed directly on the test object (refer to section 7.7) The SR b value must not exceed the limits set in Table B.13 or B.14 If the detector or image SR b surpasses these specified values, the CP II, as detailed in section 7.2.3, may be utilized.
To effectively inspect flaw sizes comparable to the image resolution using DDAs or imaging plates, a significant increase in the required signal-to-noise ratio (SNR N) is essential This inspection process should be conducted based on an agreement between the contracting parties The necessary enhancement in SNR N can offset the locally increased unsharpness that may arise from poor pixel interpolation.
The evaluation for bad pixels shall be performed periodically
NOTE By analogy to the CP II the increased SNR N also compensates also for the local unsharpness caused by bad pixel interpolation This is considered as CP III
The digital data from the radiographic detector must be assessed using a linearized grey value representation that correlates directly with the radiation dose to determine SNR, SR b, and SNR N To achieve optimal image display, it is essential that contrast and brightness are interactively adjustable Additionally, the software should include optional filter functions, profile plots, and tools for SNR and SNR N for effective image evaluation For precise image analysis, operators should utilize a zoom factor ranging from 1:1, where one pixel of the digital radiograph corresponds to one monitor pixel, to 1:2, where one pixel of the digital radiograph is represented by four monitor pixels.
7.9.4.2 Further means of image processing applied on the stored raw data (e.g high pass filtering for image display) shall be documented, be repeatable and be agreed between the contracting parties
7.9.4.3 If further image processing (e.g high pass filtering) is used when evaluating single wire or step hole
IQI values, then the same filter parameters shall be used for both weld evaluation and IQI value determination.
Monitor viewing conditions and storage of digital radiographs
The digital radiographs shall be examined in a darkened room The monitor setup shall be verified with a suitable test image
To ensure effective image evaluation, the display must meet specific minimum criteria: a brightness level of at least 250 cd/m², the capability to show a minimum of 256 shades of grey, a light intensity ratio of at least 1:250, and a resolution of no less than 1 million pixels with a pixel size smaller than 0.3 mm.
Original images, or regions of interest, must be preserved at their full resolution as provided by the detector system Prior to storing these raw data, only essential image processing related to detector calibration—such as offset correction, gain calibration for equalization, and bad pixel correction (refer to ASTM E2597 [11] for further details)—should be applied to ensure artifact-free images.
The data storage shall be redundant and supported by suitable back-up strategies to ensure long-time storage using lossless data compression only
A test report must be generated for each exposure or set of exposures, detailing the digital radiographic technique employed and any special circumstances that could enhance the understanding of the results.
The test report must contain essential details such as the name of the examination body, the object and material being tested, the heat treatment applied, the geometry of the weld, and the material thickness Additionally, it should specify the welding process used, outline the examination requirements for acceptance, and include information on the digital radiographic technique and class, along with the required IQI sensitivity as per ISO 17636 standards.
The article outlines the essential requirements for compliance with ISO 17636-2:2012, including the test arrangement, magnification, marking systems, and detector position plans It specifies the radiation source, focal spot size, and equipment identification, along with details on detectors, screens, filters, and spatial resolution For digital detectors (DDAs), it highlights the necessary signal-to-noise ratio (SNR) and parameters such as gain, frame time, and calibration procedures For computed radiography (CR), it emphasizes scanner specifications like pixel size, scan speed, and laser intensity Additional requirements include tube voltage, exposure time, source-to-detector distance, and image quality indicators The examination results must include software data, image quality indicator readings, and image-processing parameters Any deviations from ISO 17636 must be documented, along with the responsible person's certification and signature, and the dates of exposure and test report.
Recommended number of exposures which give an acceptable examination of a circumferential butt weld
The minimum number of exposures required is presented in Figures A.1 to A.4, which are valid for pipes with an external diameter exceeding 100 mm
When the wall thickness deviation of the joint under examination does not exceed 20%, as indicated by single exposure measurements (\$ \Delta t/t \$), Figures A.3 and A.4 should be utilized This method is advisable only when the likelihood of transverse cracks is minimal or when the weld has been assessed for such defects using alternative non-destructive testing techniques.
When t/t is less than or equal to 10 %, Figures A.1 and A.2 are used In this case, it is likely that transverse cracks are also detected
If the object is examined for single transverse cracks, then the required minimum number of digital radiographs increases compared with the values in Figures A.1 to A.4
The minimum number of exposures \( N \) required for single wall penetration, with the source positioned externally, is illustrated in Figure A.1 This analysis considers a maximum allowable increase in penetrated thickness \( \Delta t / t \) of 10% (class B) due to inclined penetration The results are presented as a function of the ratios \( t / D_e \) and \( D_e / f \).
The minimum number of exposures \( N \) required for off-centre penetration with the source positioned inside, as well as for double wall penetration, is determined by the maximum allowable increase in penetrated thickness \( \Delta t / t \) due to inclined penetration in the evaluated areas, set at 10% (class B) This relationship is expressed as a function of the ratios \( t / D_e \).
The minimum number of exposures \( N \) required for single wall penetration, with the source positioned externally, is illustrated in Figure A.3 This analysis considers a maximum allowable increase in penetrated thickness \( \Delta t/t \) of 20% (class A) due to inclined penetration, and it is presented as a function of the ratios \( t/D_e \) and \( D_e/f \).
The minimum number of exposures \( N \) required for off-centre penetration with the source positioned inside, as well as for double wall penetration, is illustrated in Figure A.4 This analysis considers a maximum allowable increase in penetrated thickness \( \Delta t/t \) of 20% (class A) due to inclined penetration in the evaluated areas, depending on the ratios \( t/D_e \).
B.1 Single wall technique; IQI on source side
Table B.1 — Wire IQI Table B.2 — Step and hole IQI
Image quality class A Image quality class A
IQI value Nominal thickness t mm
The IQI values range from 1.2 to 1.8 W for heights between 18 and 2.0 H, and from 2.0 to 3.5 W for heights above 17 For heights above 3.5 to 6 H, the IQI value is above 3.5 to 5.0 W, while for heights above 6 to 10 H, it is above 5.0 to 7 W The values continue with heights above 10 to 15 H at above 7 to 10 W, and above 15 to 24 H at above 10 to 15 W For heights above 24 to 30 H, the IQI value is above 15 to 25 W, and above 30 to 40 H at above 25 to 32 W The pattern continues with heights above 40 to 60 H at above 32 to 40 W, and above 60 to 100 H at above 40 to 55 W For heights above 100 to 150 H, the IQI value is above 55 to 85 W, and above 150 to 200 H at above 85 to 150 W The values further extend to heights above 200 to 250 H at above 150 to 250 W, and above 250 to 320 H at above 250 W Finally, for heights above 320 to 400 H, the IQI value is above 250, and for heights above 400 H, it reaches 18.
Table B.3 — Wire IQI Table B.4 — Step and hole IQI
Image quality class B Image quality class B
IQI value Nominal thickness t mm
The IQI values range from 1.5 to 350, with corresponding wattage (W) and height (H) classifications For instance, an IQI value of 1.5 to 2.5 corresponds to W19 and H2, while values above 2.5 to 4 are classified as W17 and H3 As the IQI value increases, wattage and height classifications change, with W16 and H4 for values above 4 to 6, and W15 and H5 for values above 6 to 8 Continuing this trend, W14 and H6 apply for values above 8 to 12, and W13 and H7 for values above 12 to 20 Higher IQI values lead to lower wattage classifications, such as W12 for values above 20 to 30, down to W5 for values above 350 This structured classification helps in understanding the relationship between IQI values, wattage, and height.
B.2 Double wall technique; double image; IQI on source side
Table B.5 — wire IQI Table B.6 — Step and hole IQI
Image quality class A Image quality class A
IQI value Penetrated thickness w mm
The IQI values indicate a range of wattage and height classifications For values from 1.2 to 2 watts, the height is categorized as 4 As wattage increases from 2 to 3.5, the height rises to 5, and for 3.5 to 5 watts, the height is 6 At wattages above 5 to 7, the height is 7, while for 7 to 12 watts, it is 8 The classification continues with 12 to 18 watts at height 9, and for 18 to 30 watts, the height is 11 As wattage exceeds 30 to 40, the height is 10, and for 40 to 50 watts, it is 9 Heights decrease progressively with higher wattages, reaching 8 for 50 to 60 watts, 7 for 60 to 85 watts, and 6 for 85 to 120 watts The trend continues with heights of 5 for 120 to 220 watts, 4 for 220 to 380 watts, and finally, a height of 3 for wattages above 380.
Table B.7 — Wire IQI Table B.8 — Step and hole IQI
Image quality class B Image quality class B
IQI value Penetrated thickness w mm
The IQI values range from 1.5 to 1.5 W for H1, 2.5 W for H2, and 4 W for H3, with corresponding values of 19, 18, and 17 respectively For H4, the range is 4 to 6 W with a value of 16, while H5 spans 6 to 8 W with a value of 15 H6 covers 8 to 15 W with a value of 14, and H7 ranges from 15 to 25 W with a value of 13 The values continue with H8 at 20 to 35 W (value 12), followed by ranges of 38 to 45 W (value 11), 45 to 55 W (value 10), 55 to 70 W (value 9), 70 to 100 W (value 8), 100 to 170 W (value 7), 170 to 250 W (value 6), and above 250 W (value 5).
B.3 Double wall technique: single or double image; IQI on detector side
Table B.9 — Wire IQI Table B.10 — Step and hole IQI
Image quality class A Image quality class A
IQI value Penetrated thickness w mm
The IQI values range from 1.2 to 1.2 W for heights between 18 to 2 H, and from 2 to 5 H, the values increase to 4 W For heights from 5 to 9 H, the IQI value is 5 W, while for heights between 9 to 14 H, it rises to 6 W As heights increase from 10 to 15 H, the IQI value reaches 7 W, and for heights from 15 to 22 H, it is 8 W The values continue to decrease with increasing height, with 9 W for heights above 22 to 38, 10 W for heights above 38 to 48, and further reductions to 9 W for heights above 48 to 60, 8 W for heights above 60 to 85, and 7 W for heights above 85 to 125 The IQI values further decline to 6 W for heights above 125 to 225, 5 W for heights above 225 to 375, and finally, 4 W for heights above 375.
Table B.11 — Wire IQI Table B.12 — Step and hole IQI
Image quality class B Image quality class B
IQI value Penetrated thickness w mm