P. Genocchio1,a, e. SanGuineti1,b F. BuSSchaert2,c

(InstItut de soudure)

Dissimilar metal weld (DMW) between austenitic and ferritic materials have always been considered a challenge in terms of welding and NDE inspectability.In particular, Ultrasonic Testing (UT) is demanding because of the distortion of sound beam due to the anisotropic inhomogeneous material.For this reason, Radiographic Testing (RT) has been always regarded as the main choice for the volumetric inspection of DMW.On the other hand, RT method has its own weaknesses and suffers of poor Probability of Detection (PoD) for certain types of defects such as side-wall lack of fusion (LoF).According to ATV experience, particular configuration of DMW with narrow gap preparation can lead to the presence of false indications between ferritic base material and austenitic weld metal.This type of indications are called “ghost-indications” and their detection increases dramatically False-Call Rate (FCR).In order to overcome these issues, a new inspection approach based on Radiographic and Ultrasonic Phased Array methods has been developed with enhanced PoD and reduced FCR.

1. IntroductIon

In the subsea oil and gas field, the use of dissimi-lar welds in critical joints has initiated the drive to improve the capabilities of the non-destructive tes-ting (NDT) methods. The joint between pup pieces and body of weld over-laid carbon steel or solid Corrosion Resistant Alloy (CRA) often require the use of weld filler material such as Alloy 59. Depending on the weld metal volume, bevel geo-metry, thickness and the filler grade, the inspection of such girth welds vary in difficulty from possible to severely limited possibilities.Historically, Radiographic Technique (RT) has been selected as the main volumetric inspection tech-nique in subsea valves field.The reason behind that choice was the possibility for NDT experts from end user and contractors to review the results and preserve a hard copy of the inspection findings for future evaluations and com-parison.However, the main advantage of ultrasonic tes-ting (UT), in comparison to radiography, is the ability to better find and characterize planar flaws as the interpretation is based on signal amplitudes, orientations and distances (based on times of flight); whereas in radiography the interpretation is based on absorption (which can be severe) and the ability to evaluate crack-like flaws is limited.In the past, manual ultrasonic technique was not easily recordable and the challenges related to the application of Ultrasonic Technique to dissimilar metal welds required skilled operators for the data interpretations [1].Nowadays, with the development and implemen-tation of advanced UT such as Phased Array, the

capabilities of ultrasonic technique have been signi-ficantly increased.Phased array ultrasonic technology is a recent evolu-tion of the conventional ultrasonic technology and in particular the main difference is in the probes capa-bilities.Traditional single crystal or twin crystal probes are designed to generate an ultrasonic beam along a cer-tain single angle when placed on the component and the beam may be designed to concentrate the acous-tic pressure at a certain point (termed focal point). With a single phased array probe, the ultrasonic beam can be “steered” to cover a wide range of angles and can also be “focussed” to concentrate the energy to a desired region [2].This advantage makes them really flexible and enhances the capabilities to overcome the problems related to the presence of austenitic material and heavy thickness.

2. Background

ATV has been involved on Total Project for the deli-very of 6” and 8” Subsea Slab Gate Valves.Typical joint configuration was the following:•  Body  side: ASTM A694-F60 with  Inconel 625 (ERNiCrMo-3) weld overlay on inner profile;•  Pup side: ASTM A182-F51.The procedure was qualified according to ASME IX and Project specification with fully mechanised Hot Wire GTAW process.Welding consumable was ER-NiCrMo13 (Alloy 59) and thickness 35mm (F60 side) and 23mm (F51 side).According to industrial practice and Project specifi-cations, the NDT technique selected as volumetric inspection was RT. The technique applied was the following:

1. ATV Spa – Advanced Technology Valves, Via Ombriano2, Colico, Italy

2. Total S.A., Avenue Larribau 64018 Pau Cedex, France a pgenocchio@atvspa.com, b esanguineti@atvspa.com, c freddy.busschaert@total.com

A new ApproAch to the inspection of dissimilAr metAl weld bAsed on rt And pAUt: methods And presentAtion of pAUt sUccess stories

conjonctureétudes  et  recherche

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3 I mai-juin 2018 I Soudage et techniqueS connexeS

A new ApproAch to the inspection of dissimilAr metAl weld bAsed on rt And pAUt

Figure 2. Macro picture from welding qualification.

Figure 1. Typical body to pup weld confiugration.

Figure 3. Example of linear indications.

Figure 4. Sketch of PAUT reference block.

Figure 5. Example of PAUT indication confirmed by Macro.

Radiographic technique: Single wall Single imageSource: X-RayFilm Type: Class C3Geometric unsharpness: 0.2Density: 2.5-4.0During the review of the films, the systematic pres-ence of longitudinal linear indications on F60 fusion line for the almost entire length of the weld has been detected.

NDE III Level involved in the evaluation were unable to reach an agreement on the interpretation of the indications.

3. InspectIon

Since it was not possible to discriminate between real and false indication by only means of RT ins-

pection results, Phased Array Ultrasonic Technique (PAUT) has been implemented.Reference block for PAUT inspection have been prepared on welding qualification test ring with ø1,6mm Side Drilled Hole and notches as reference reflectors.Applied technique includes the use of linear phased array probes and creeping waves probes from inside and outside surface.The procedure was reviewed and approved by Institut de Soudure.PAUT inspection detected the presence of isolated small indications not linked to the indications seen on radiographs.In order to clarify the real nature of such indications, the weld has been cut-out from the weld and macro sections have been prepared in the area of RT and PAUT indications.The indications detected by RT have not been found on macros.Macros confirmed the presence of defects associa-ted to PAUT indications.

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4. InterpretatIon of InspectIon results

When dissimilar metal weld, and in particular mate-rials with different x-ray absorptions, are involved, the interpretation of x-ray films may be quite chal-lenging.The main difficulty arise at the interface between Ni-alloy weld metal and carbon steel base material.Linear indications not linked to real discontinuities may be visible.This type of indication has been defined in scien-tific literature as “ghost” or “enigma” indications and have been encountered in GTAW welding of Ni-alloys [3].The sudden variation in radiographic density caused by different material absorption and steep fusion line and the austenitic grain structure causing by x-ray diffraction may lead to the presence of indica-tions.In order to confirm thus assumption, ATV has perfor-med a simulation on CIVA 11 software.The simulation has been performed with the same parameters as Radiographic inspection performed during valve production.Three main physical phenomena are involved in X-ray inspection: direct radiation, radiation scatte-ring and radiation diffraction.Direct radiation and radiation scattering effect have been considered during simulation.The results does not show the presence of any linear indications.The difference between densities profile may hence be related to the effect of x-ray diffraction.According to relevant literature, it has been reco-gnized it is difficult to discriminate such false indica-tions from real lack of fusion.When this type of indication are involved, Ultraso-nic testing is more suitable as volumetric inspection technique.Ultrasonic testing has in general an higher Proba-bility of Detection for planar defect lying on bevel, such as lack of fusion.

However, it has to be noted that the propagation of the ultrasonic wave through the austenitic material is obstructed by its anisotropic metallographic struc-ture. The presence of austenitic structure, when compa-red to ferritic ones, leads to:•  an increase in the signal “noise” caused by increased backscattered energy;•  an increase in attenuation caused by increased backscattered energy;•  an increase in distortion of sound wave caused by large austenitic anisotropic grains.It is difficult to predict the entity of this problems because their influence depends from welding process, base materials, filler metals and weld thickness.The approach to the UT inspection of dissimilar joints mandatory requires the use of dedicated refe-rence block taken from a weld mock-up which is representative of the one to be investigated.Reference reflectors shall be representative of the most likely defects to be encountered in terms of size and location.The selection of probes, inspection setup and volume coverage is crucial for the reliability of UT results.On the other hand, UT has poor Probability of Detec-tion for rounded indications, but rounded indications are considered from fatigue point of view as less dangerous than planar indications for joint integrity.The application of PAUT technique allowed ATV to significantly increase the reliability of Hot Wire GTAW welding process for valve butt welding.

In the following projects with Total, the presence of not acceptable welds has decreased from the initial 40% to 0,02%.

5. conclusIon

Dissimilar metal weld between austenitic and ferri-tic materials have always been considered a chal-lenge in terms of welding and NDE inspectability.In particular, Ultrasonic Testing is demanding because of the distortion of sound beam due to the anisotropic inhomogeneous material.For  this  reason,  Radiographic  Testing  has  been always regarded as the main choice for the volume-tric inspection of DMW.On the other hand, RT method has its own weaknesses and suffers of poor Probability of Detection or certain types of defects such as side-wall lack of fusion.In this paper ATV and Total have presented their shared experience on the PAUT inspection of DMW.The proposed approach is based on the kind of defects expected from the applied welding process and on their detectability from RT and UT.The application of PAUT technique allowed ATV to increase the welding process reliability.In the following project with Total, the presence of not acceptable indications has been significantly reduced.

references

[1] IIW Commission V, Handbook on the Ultrasonic Examination

of Austenitic and Dissimilar Welds, 1986.

[2] Ed Ginzel, Introduction to Phased Array Ultrasonic Techno-

logy Applications, Olympus.

[3] J.L. Walley, A.C. Nunes, J.L. Clounch, C.K. Russell, Study

of Radiographic Linear Indications and Subsequent Micros-

tructural Features in Gas Tungsten Arc Welds of Inconel 718, 

NASA.

Figure 6. Sketches of simulated RT setup.

Figure 7. Simulated image from CIVA software.

Figure 8. Comparison between simulated optical densities profile on real X-ray film (green line) and simulated image (red line).

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