A Phased Array Inspection Solution for the Assessment of Thin Stainless Welds

D P J Bell, I G Pettigrew and C R Bird

Doosan Power Systems Nuclear Technology Services, Porterfield Rd, Renfrew, PA4 8DJ, UK.

Tel: +44 (0)141 885 3473 Fax: +44 (0)141 885 3670

E-Mail: david.bell@doosan.com 1. ABSTRACT EDF Energy has the requirement to detect and size lack of penetration type defects in thin-walled stainless steel components. A high frequency ultrasonic phased array inspection technique has been successfully developed and applied to thin walled pipes and is currently used as an ENIQ qualified inspection on nuclear plant. The welds are manufactured with an autogenous Tungsten Inert Gas (TIG) welding technique. This type of technique can generate unique defects. The thin wall requires a high resolution very reliable, non-destructive evaluation method to provide accurate defect sizing. This paper discusses a one-stop phased array inspection system and with a highly reliable defect sizing capability for examining thin-walled pipe of different geometries and varied diameters. In addition this paper demonstrates the operator qualification process undertaken to validate the inspection method. Keywords: phased array, thin welds, through wall sizing 2. INTRODUCTION The nuclear plant component contains a number of welded load path joints. Progressing along the component there is a transition of materials from carbon steel to austenitic stainless steel. All of the welds within the stainless steel section are spigot, joining of thin tube section pipe to stainless steel cast ring, with the exception of one; a butt weld joining thin section pipe to thin section pipe (schematic shown in Figure 1). The welds to be examined on the component are of a range of diameters.

All of the welds are autogenous TIG welds. The weld acceptance criteria is based on BS 4677:1984 Specification for Arc Welding of Austenitic Stainless Steel Pipework for Carrying Fluids. The acceptance criteria have been modified to reflect the defects likely to be present and those of concern to the integrity of the welded joints. Root penetration is the depth of the weld extending into the joint. Lack of Side Wall Fusion (LoSWF) is the lack of joining of weld metal and parent metal at the fusion faces on the sides of the weld. Any lack of fusion that is evident at the external surface of the weld is deemed unacceptable. The angle of tilt for lack of side fusion will be inline with the weld preparation, namely 0 for autogenous TIG welds. The volume to be inspected for each weld extends through the full thickness of the material and to the fusion faces of the welds.

EDF Energys cautionary inspection requirement is to detect and size any lack of fusion or lack of penetration (LoP), to ensure that a specified minimum penetration

is achieved over the full circumference of the weld so that the overall load bearing area is acceptable for service.

A time of flight diffraction (TOFD) technique is not possible for this inspection due to one-sided access to six of the seven welds.

This paper discusses a commissioned phased array inspection system for examining thin-walled pipe of different geometries and varied diameters. In addition, the results from detection and through-wall sizing of defects in test specimens are demonstrated to give reassurance that the inspected components are acceptable for service.

3. DEVELOPMENT OF A PHASED ARRAY INSPECTION SYSTEM The inspections are performed on the outer diameter of the component. The components are in-situ in a maintenance cell vertically oriented with the weld caps intact. To achieve the inspection requirements a specific manipulator was designed taking into consideration the environmental constraints such as restricted access, available clearance for inspection around the component, minimising dose rate in the nuclear environment and nuclear plant compatible materials. A photograph of the installation environment and the manipulator configuration on a training simulator is shown in Figure 2. The manipulator was designed for access via port holes and is capable of being assembled, aligned, scanned and disassembled efficiently within the glove-box environment. The inspection system comprises a 15 MHz, 128 element linear ultrasonic phased array probe. A linear scan pattern is applied for defect detection and sizing by multiplexing 32 active elements at a fixed beam angle of 65. A double sampled resolution scan is performed to provide 0.1 mm axial resolution [1, 2]. The probe is housed in a bespoke water wedge to ensure the high frequency spectrum is maintained. The novel design of the water wedge incorporates a flexible membrane diaphragm to retain the water because in this particular inspection environment free flowing water is prohibited. The conformability feature of the membrane diaphragm enables application with the weld cap intact. This guarantees there is no deterioration of the weld integrity if the component is defect-free by unnecessary removal of the weld cap.

In the inspection system, the main parts of the manipulator consist of a hard-anodised aluminium track, probe carriage and probe arm. The track has been designed to accommodate the variation in diameter for each of the welds using different sets of feet on the ring; one half with two fixed and the other half with one adjustable. The probe carriage supports a centrally-mounted yoke on a suspension arm that enables a fixed axial position for the probe. The yoke provides stability for the water-filled probe holder whilst ensuring the probe does not skew during the inspection.

The inspection is performed with a single probe axial index offset standoff with an electronically rastered focused beam providing sufficient inspection volume coverage. Fixed axial positioning eliminates errors introduced by variable coupling conditions. The manipulator uses an encoder fixed on a probe carriage running around the outside of the track to record the circumferential position. The mechanical movement is supplied manually in the circumferential direction when applied in both environments. The probe is traversed around the pipe providing 360 coverage. To

ensure full coverage the probe is scanned 365 providing approximately 10 mm circumferential overlap.

Figure 1: Weld geometries; spigot weld and butt weld.

Ultrasonic data is acquired using a 32/128-channel digital phased array flaw detector at 2 mm intervals with respect to the manipulator track. Inspection control and data acquisition are achieved by use of a suitably-interfaced laptop computer running Ultravision software.

This inspection system is used for the inspection of all the required weld geometries.

(a) (b)

Figure 2: (a) Location of in-situ inspection and (b) manipulator installation on training simulator of the in-cell environment.

Figure 3: Linear phased array probe housed in a water wedge.

3.25 mm

Spigot Butt

Membrane diaphragm

4. SITE DEPLOYMENT OF THE INSPECTION SYSTEM The material is stainless steel with the associated attenuation caused by austenitic grain structure. The ultrasonic frequency provides sufficient material grain noise contrast between that of the parent material and the weld material, and resolution to use maximum amplitude sizing method. The back scattered energy from the weld clearly shows the weld pool shape and indicates when the weld is fully penetrated (shown in Figure 4) adding to the confidence that the weld is defect free. Similarly, the technique can reliably detect low amplitude signals from the ID (Internal Diameter) of the pipe. This rumble of noise from the ID of the component provides useful information about the condition of the ID e.g. detection of root concavity and of tack welds. Signals obtained from defect facets or buried edges are compared in depth location with the back-wall signals allowing accurate measurement of defect height.

Compressive stresses are applied during manufacture of these welds. The uncertain nature of these stresses results in cases where the fusion faces are sufficiently compressed to reduce the response from LoSWF defects and in turn provide no relationship to amplitude from known reflectors such as EDM slots. Hence, an amplitude-based sizing criteria was eliminated as a reliable sizing method. It is fundamental that the incidence plane is perpendicular to the weld direction to maximise detection of defects. To achieve this requirement the track alignment is configured so that it is within +/- 4 mm of the weld centre-line. A geometry overlay provides a quality control method to aid the operator in measuring any manipulator or weld irregularities. Welding offset/drift could be asymmetric or irregular, and may contribute to the overall judgement of track alignment on a scan (Figure 5). An example of a poorly aligned manipulator is shown in a sinusoidal plot of the weld root. Figure 6 provides an illustration of misalignment and the potential effect on defect detection on a fully defective weld where LoP indications are only detected around half of the circumference and the remaining half does not exhibit the indications.

The flexible membrane enables the surface profile to be mapped around the circumference as illustrated in Figure 7.

Figure 4: B-scan of a fully penetrated weld. Skew is 90 and beam is plotting in that direction

(pointing +ve Z). Interface noise

Figure 5: C-scan (top-down) image of track alignment within +/- 4 mm tolerance. Weld root contains

no LoP-type indications. The weld run varies unsystematically around the weld centre-line.

Figure 6: C-scan (top-down) image of track alignment out with tolerance showing a sinusoidal

response.

Figure 7: D-scan (end-elevation) image showing interface noise mapping surface profile at OD.

Interface noise nominally 1.5 mm penetration.

The welds under examination are designed to have a 0 fusion face. The weld pool is larger near the OD (Outside Diameter) and narrows towards the ID. Hence any

Feature: spiral weld

No LoP LoP

upper limit

lower limit weld centre-line

lack of fusion is biased towards the ID of the pipe thickness. The interface noise may mask low amplitude responses from the edge of the unfused land and therefore full-skip responses are used in addition to that of the half skip data. If indications of similar size are present on both half-skip and full-skip regions, sizing is considered most accurate from using the half-skip response. Data is acquired up to skip-and-a-half range.

For this inspection, sizing of LoP-type indications is based on using tip-diffracted signals (see Figure 8). The calibration reflector is a smooth 0.5 mm EDM slot in which the phased array technique can resolve the top and bottom edges of the reflector. An indication present at the weld root, with no measureable throughwall height that is not resolved, is conservatively considered to be 0.5 mm and is recorded as 0.5 mm for reportage. The typical indication of a low amplitude vertical response (no strong corner echo) from unfused land on the far side of the weld grain structure is shown in Figure 9. This type of flaw is typically multi-faceted and the maximum amplitude sizing technique can be applied. In this case the weld has attenuated the energy of the beam and the unfused land is sufficiently compressed with no air gap between the fusion faces to create a strong response from the change in impedance.

The procedure has also taken into consideration the possibility of a filled weld. To include this situation test pieces were constructed with a 30 fusion face and a specular response is obtained from the full-skip response. In the absence of tip signals or multi-faceted responses from this type of indication, 6 dB drop sizing is called for in the procedure. This is a less favourable option since this sizing technique is less accurate.

Detection of offset welds were also part of the inspection requirements. The procedure also encompasses this cause of flaw. This means that the weld has been fully penetrated but offset from the weld centre-line/fusion face ends. An example of weld offset is provided in Figure 10.

Ovality of a pipe when scanned with a circular track can result in beam skew relative to lack of fusion. This means the beam is no longer aligned with radial-axial plane in the weld fusion faces. The skew in the beam may cause the defect to be detected at a lower sensitivity but this does not affect the inspection capability because the technique is amplitude independent.

Figure 8: B-scan image of defective indication in a test piece. Saturated corner echo at ID with a lower

amplitude tip-diffracted response above corner echo in the half-skip region. Mode converted signal from corner displaced to the RHS at longer range. Corner echo repeat at skip-and-a-half range.

Mode converted signal

Corner

Tip

Skip-and-a half

Full-skip

Half-skip

Figure 9: B-scan images of detection of welding offset. Low amplitude signals at the RHS of the weld

noise. Adding 6 dB soft gain provides better resolution.

Figure 10: B-scan image of the pool of weld grain back scattered energy stopping mid-wall in the half-

skip region plotting to the LHS of weld centre-line. This is typical of an offset weld.

Corner at ID and tip on

full-skip leg

6 dB soft gain applied during

analysis

6 dB soft gain applied during

analysis

5. PRACTICAL TRIALS Blind trials were performed on more than 100 welds test pieces to provide evidence for defect detection and sizing capability. These were manufactured by modifying the welding process to simulate and create welding defects. Therefore the size and location of defects within the welds was relatively random and the quality of the welds was unknown because the welds were formed realistically. The random nature of the weld defects ensured that the trials were truly blind. Inspection trials were undertaken and the results were reported. After reporting the inspection results, the welds were destructively examined to determine the actual depth of the defects.

A range of spigot and butt weld geometries of nominal through wall thickness of 3.25 mm with different diameters of pipe (geometry shown in Figure 1) were manufactured. In total more than 100 defective welds were ultrasonically and destructively examined. The varieties of weld types examined were distributed amongst the various weld configurations.

The destructive examination required stage-grinding of the defect areas. At each stage in the grinding process, the new surface was examined to detect any lack of penetration. The depth of this penetration from the OD was then measured by both microscope and Vernier. The grind outs were created with an axial circular disc providing a grind gradient at each grind position. A single point measurement could be taken at the point where the grind out crossed the lack of penetration because the depth of penetration was relatively constant over the coupon length of 15 mm.

A viewgraph of the blind trials results is presented in Figure 11. This graph shows a comparison between the Phased array measurements (Y axis) and the true sizes of the corresponding defects, measured by Vernier (X axis). Lines at 0, 1 and 2 standard deviations are incorporated for comparison. The results are promising with a strong relationship of measurements falling on or around the 1:1 correlation line. The falling of points above and below indicates random error rather a systematic deficit.

The blind trials demonstrated a mean sizing capability better than 0.35 mm with a confidence level exceeding 98%. The inspections undertaken during the trials detected all defects which had a though wall height greater than 0.5 mm [2].

Figure 11: 1:1 Correlation viewgraph of phased array inspection result with respect to destructive

examination result for a single weld configuration.

6. OPERATOR QUALIFICATION Qualification of operators can be split into two separate areas. These are Data Acquisition/ Mechanical Set-up and Data Analysis Training. Pre-requisites, qualifications and examination criteria will be discussed in this section for both areas. The overall aim of the training courses assessments and subsequent on the job experience are to produce operators who can then be identified as Suitably Qualified and Experienced Personnel (SQEP) for this type of inspection. The training courses were designed and implemented as part of the ENIQ process. 6.1. Data Acquisition/Mechanical Set-up Training Prior to formal training the operators as a pre-requisite must possess the following qualifications and experience:

PCN Level 2 Manual UT Inspection; PCN or CSWIP Level 2 Phased Array Weld Inspection; Two years experience with audited manual/Auto UT experience (EDF Energy

site experience). It is recommended that the operator has used phased array techniques using Tomoview or Ultravision for at least three months prior to this job specific training.

Doosan Power Systems have created a one week training course in order to formally train operators in the mechanical set-up of the manipulator and in using the phased array data acquisition software. The training course allows the operators to be suitably qualified to perform the assembly; fitting and operation of the manipulator, including the positioning of the system at the scan start positions, and monitoring all movements of the manipulator. The individuals are trained to set up, calibrate and operate the phased array system for the inspections, directing the scan setup and scan start and end points, and for monitoring the On-Line Display to check the data quality as the scans progress. The training consists of a formal training pack giving step by step instructions for the set-up and successful completion of a scan. This material is taught by a suitably experienced engineer in a classroom environment and tutorial type homework questions are set each night which can be answered using the training material provided and the procedure. In addition to the theory element the operators practice the mechanical set-up of the system and the use of the data acquisition software. A specially designed mock-up has been produced to allow test-pieces to be scanned in the actual conditions that will be encountered on site (shown in Figure 2b). After the formal course the operators were formally supervised on the job by an experienced engineer until they demonstrated sufficient competence to the approval of EDF Energy. 6.2. Data Analysis Training Qualification of operators as Data Analysts is by nature of the work more critical. In addition to the pre-requisites required for the Mechanical Set-up and Data Acquisition the operator had to pass a data analysis training course.

Doosan Power Systems produced a two week long training course. The course was assessed via a written examination and a practical data analysis assessment. The examination criteria was discussed with various leading training providers in the field of NDT to ensure a fair process was adopted which also reflects the high threshold of competence required by analysts to report on critical welds. The theory exam consisted of two sections; the first is multiple choice covering the analysis technique and procedure; the second section containing real-life screenshots of data where the candidates were required to comment on what they observe. The practical examination consisted of 5 sets of data acquired from inspections of a pool of blind trial testpieces. Candidates were required to analyse the full welds reporting the size and presence of the defects. 6.3. Reproducibility of Results In order to demonstrate that the analysis technique is reproducible by other operators than those who have been involved in the technique development a pilot course was run in-house. Figure 12 shows the results from one of the blind trial test pieces.

Blind Testpiece Weld 5

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

2.20

2.40

2.60

2.80

3.00

3.20

3.40

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440 460 480

CIRC POSITION

LOP

(MM

)

WALL THICKNESS (MM)

Experienced Engineer

Operator in Training

Figure 12: Comparison of results from analysis of a blind trial test piece weld between a SQEPd

engineer and an operator having completed the training course It can be seen from the above figure that the operator in training has produced similar results to that of the experienced analyst. Again analysts although technically qualified, will still be closely supervised on the actual job by an experienced engineer until they have demonstrated sufficient competence to the approval of the Inspection Leader and EDF Energy Inspection Group Technical Assessment Panel that they can perform the inspection and analysis satisfactorily.

7. CONCLUSIONS 1) A nuclear compatible phased array inspection system has been developed and

qualified via ENIQ methodology for the inspection of thin-walled austenitic welds.

2) A novel wedge with flexible membrane diaphragm used to house the phased array probe has been designed and successfully applied in a controlled nuclear plant application.

3) The inspection frequency provides sufficient material grain noise contrast between that of the parent material and the weld material to detect offset welds.

4) The inspection system mechanically and ultrasonically identifies weld ovality. 5) A phased array inspection technique had been developed with a mean defect

sizing capability of 0.35 mm with a confidence level of better than 98%. 6) The acquisition and accurate analysis of inspection data has been shown to be

reproducible by other operators who are suitably qualified for the job through the relevant Doosan Power Systems training course.

8. REFERENCES 1. Pettigrew I.G. and Bird C. R., Phased Array Inspection of Thin Welds, 8th International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurised Components, Berlin, 2010. 2. Bird C. R and Pettigrew I.G., Qualification of the Phased Array Inspection of Thin Welds, World Conference on NDT, Durban, 2012. 9. ACKNOWLEDGEMENTS The authors of this paper wish to acknowledge the considerable support of EDF Energy.