Radiography of Thin Section Welds, Part 1: Practical Approach

G A Georgiou
Jacobi Consulting Ltd
London N1 3NL, UK
Telephone 020 7288 1601
Fax 0870 054 7372
E-mail george@jacobiconsulting.co.uk

C R A Schneider
TWI
Cambridge CB1 6AL, UK

Paper presented at BINDT Annual Conference 2002, Southport, UK, 17 Sept. 2002

Abstract

This paper describes some recent work on the capability of radiography to detect planar defects in thin section welds (thickness range 10-51mm). The paper concentrates on the practical aspects of the work, including the manufactureof defect specimens, the procedures used for X-ray and gamma radiography, the independent interpretations of the radiographs, the sectioning of the defect specimens and the practical analysis of the results. A separate paper at thisconference (Part 2) concentrates on the theoretical modelling and statistical analysis aimed at predicting radiographic capability for planar defects in thin welds.

The practical analysis here is based on 13 realistic planar defects revealed by sectioning. Each of these 13 defects has a number of radiographic exposures associated with it (i.e. different set up, such as beam angle, film to focusdistance, specimen thickness etc), making a total of 284 defect/radiograph combinations. One of the 13 defects was considered too complex and inappropriate for theoretical modelling (Part 2) and was left out of that analysis.

Specific conclusions are discussed in this paper, but overall the results for defect detectability support the expected trends that increasing penetrated thickness, increasing misorientation angle and decreasing typical gape allcontribute to making defect detection more difficult.

1. Introduction

During 1995-1999, TWI performed several detailed studies on the radiography of large planar defects in thick-section welds. ^[1] The work considered a number of issues, such as the capability of 1950s and 1960s radiography, the use of statistical models to predict defect detectability and the effect of human factors on defect detectability. Thethicknesses studied were in the range 50-114mm. The main application of the work was to quantify the capability of the construction radiography performed on the welds of the Magnox steel reactor pressure vessels, but the work also hadgeneric value in providing a better understanding of radiographic capability. The studies also confirmed that the Pollitt model was a valuable tool for predicting the detectability of planar defects in thick welds.

This current work extends the original programme to look at the detectability of planar defects in thinner section welds (thickness range 10-51mm). Both X-ray and gamma radiography was used. This new work, while intended forspecific applications in the nuclear power industry, should also be of generic value in improving the understanding of radiographic capability as well as evaluating Pollitt modelling for thin weld radiography. The Pollitt model isconsidered in a separate paper in these proceedings. ^[2]

2. Specimen and defect manufacture

Three defect specimens were manufactured at TWI and were intended to contain 12 realistic planar defects with through wall extent (TWE) in the range 2mm to 5mm and nominal lengths of 10mm. The intended defect types included lack ofsidewall fusion, centreline solidification cracking and HAZ hydrogen cracking. The methods used to produce these defects are well established at TWI, although not without difficulties and perhaps more so with such relatively smallintended defect sizes. In some cases more than one attempt was necessary to produce the desired defect type and the sectioning programme revealed a total of 21 defects. Out of these 21 defects, 13 planar defects were selected foranalysis ( Table 1).

Table 1 A summary of the final defect parameters for the 13 defects selected for analysis.

Key: *Defect tilt is measured positive anticlockwise

The sectioning programme revealed TWEs measuring about 1mm to 8mm. The lengths of the defects, as determined by NDT, were in the region of 7mm to 15mm. However, the lengths were never verified as sectioning was generally onlycarried out at one position along the length of each defect. Overall, the manufacturing methods used for the intended defect specimens were successful in producing defect types with the intended TWE and lengths. However, there were nosolidification cracks among the 13 planar defects selected, and the TWEs were generally rather smaller than intended, notwithstanding the range mentioned above. The three specimens were 10mm (ferritic), 17mm (austenitic) and 38mm(ferritic) thick respectively ( Table 1).

3. Summary of the radiography programme

Since only three specimens were manufactured, spacer plates were used to simulate different thicknesses between 10mm and 51mm. When spacer plates were used the majority (69%) was placed on the source side, but a substantialproportion (31%) was placed closest to the film. This was to simulate the effect of defects being at different depths within the welds.

The radiographic procedures used were largely based on Class B techniques (i.e. improved) in the current European standard, BS EN 1435, ^[3] with class B films (Fuji 80 or Fuji 100). It was decided at the outset to assess the radiographic performance using the 'minimum requirements' of this quality class, that is, using the minimum allowed focus- or source-to-filmdistances (FFD/SFD) and the highest allowed kV. This of course does not necessarily represent day-to-day practice, but represents the minimum performance that should be achieved in practice. However, whilst the majority of radiographswere taken using 'minimum requirements', a number were also taken using more favourable radiographic settings to provide a comparison.

A single-wall technique was used throughout with the Image Quality Indicators (IQI) always placed on the source side.

A total of 144 radiographic exposures were defined and taken using X-Ray (70% of the total) and Gamma (30% of the total) using both Iridium 192 and Selenium 75.

4. Results, analysis and conclusions

Two independent interpreters were chosen with suitable industrial qualifications (i.e. PCN level 2) and who had no involvement in any of the radiography programme.

A great deal of effort went into the mounting and masking of the radiographs ( Figure 1) in order to minimise the risk of the interpreters learning the locations of defects on the different specimens and realising that there were only 3 specimens. Further effort went into organising the order of theradiographs so that each interpreter received more challenging radiographs first in order to minimise the risk of the interpreter learning the defect locations from the easier exposures and then using this knowledge to help with themore challenging exposures.

An analysis of the interpreters' report forms show that each interpreter reported at least Class A IQI sensitivity level ^[3] in nearly all cases. Moreover, for a substantial proportion of the cases, each interpreter was reporting at least Class B IQI sensitivity level (40% for interpreter X and 67% for interpreter Y). These results reflect the factthat interpreter Y was recording more IQI wires than interpreter X.

4.2 Comparison of interpreters: Overall detection of defects

From the 144 mounted radiographs viewed by each interpreter, the actual positions of the 13 defects selected were in the viewing window a total of 284 times. Each interpreter was asked to record indications as either easily visible(EV), barely visible (BV), based on a sample radiograph with notches, or not detected (ND). Interpreter Y detected more defects than interpreter X, by a margin of 4%. For this analysis, the number of detected defects recorded was withrespect to actual defect positions, so this was not just a case of an interpreter simply recording more defects because of human behaviour. The higher detection level of Interpreter Y was consistent with reporting a higher average IQIsensitivity level (see above).

4.3 Comparison of defect detection at different FFD/SFD

Whilst the vast majority of radiographic exposures were taken at the minimum FFD/SFD settings (i.e. 350mm to 760mm), some were repeated at longer FFD/SFD (i.e. 1000mm) to assess the difference in radiographic performance.

When all reported indications on the radiographs were considered there were about 10% fewer indications reported by each interpreter using minimum FFD/SFD compared with longer FFD/SFD settings. However, if the analysis was confinedto the 13 selected planar defects there appeared to be only very slightly fewer of these reported at minimum FFD/SFD than at longer FFD/SFD.

4.4 Comparison of defect detection between X-Ray and Gamma

When taking all reported indications into account, each interpreter reported substantially more indications (between 30% and 40%) for X-ray radiography than for gamma radiography. This was consistent with more IQI wires beingreported for X-ray than for gamma.

However, if the analysis was confined to the 13 selected planar defects, the interpreters reported only marginally more of these using X-ray radiography than when using gamma radiography, but there were also a few specific caseswhere the opposite was observed.

4.5 Comparison of defect detection for different defect positions

Spacer plates were used to simulate different defect positions through the specimen thickness. This change of defect position would also change the geometric unsharpness for each defect.

To simplify the comparison, only the corresponding normal incidence radiographs were selected. When all the reported indications were taken into account, slightly more indications were reported when the spacer plates were closest tothe source (i.e. lower geometric unsharpness) as might be expected. However, there were also a few indications reported with the spacer plates closest to the film (i.e. higher geometric unsharpness) that were not reported with thespacer plates closest to the source.

When the analyses were confined to the 13 selected planar defects, the defect detection results showed there was hardly any difference between the two geometric set-ups.

4.6 Defect detection as a function of other parameters

Other parameters such as penetrated thickness, the presence of the weld cap, the defect TWE, the misorientation angle and the typical gape were considered for their influence on defect detectability. The parameters were consideredboth singly and in pairs, as it is impossible to show all the relevant parameters in this way on just one graph.

The results, in general, support the expected trends that increasing penetrated thickness, increasing misorientation angle and decreasing typical gape all contribute to making defect detection more difficult. This is consistent withsimilar trends observed in the earlier thick-section weld study. ^[4]

4.7 Comparison of radiographic NDT and sectioning

Overall, the sectioning programme revealed defects at each of the sectioning positions as determined by radiographic NDT (see Figure 2 for example), except for a couple of cases In many cases the lengths, TWEs and orientations were also in good agreement.

Some particular problems were encountered with the 38mm specimen, where the parent material contained a number of small inclusions, which caused some difficulties with radiographic interpretation (see Figure 3

Fig. 3. A photograph of defect W5-2C (cf. Table 1) revealed by sectioning and detected by radiography

4.8 Realism of manufactured defects

The methods used by TWI for manufacturing and promoting realistic defects have been well established, but in some cases the methods used were difficult to control precisely. In this work the problems were made more difficult becauserelatively small defects were required.

The gape values for the 13 selected planar defects were compared with the comprehensive database of Oxford. ^[5] However, in making comparisons it was important to note that in the current work gape values were provided at only one slice position. In addition, only three gape measurements were provided (i.e. at the top, middle and bottomof the defect) as well as the maximum gape value. From these values a judgement was made as to the typical gape value (i.e. average gape).

In order to assess the realism of defects in the specimens here, the graphs of typical gape values vs. TWE, provided by Oxford, ^[5] were reproduced for lack of fusion and hydrogen cracking in the heat affected zone (HAZ). The typical gape value and maximum gape value for each of the 13 selected defects were added to the Oxford graphs.

The TWI defects show a reasonable overlap with the Oxford data, considering the limitations of the sectioning data for the TWI defects. There are however, some instances where the typical gape values from this study were relativelyvery small (e.g. typical gapes of the order 1 (m to 3µm) compared to gapes for similar TWE values in the Oxford data.

Acknowledgements

We wish to thank Dr R K Chapman and Mr G S Woodcock of British Energy plc for their support. The paper is published by permission of the Industry Management Committee (IMC) of the nuclear licensees, who also funded the work. The IMCcomprises members of British Nuclear Fuels Ltd and British Energy plc.

References

1. Part 2: Modelling