7TH International Conference on Nuclear EngineeringTokyo, Japan, April 19-23, 1999

ICONE-7079

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EFFECT OF ULTRASONIC SCATTERING ON INSPECTION OFWELDS IN AUSTENITIC STEELS

Michel Bith, C. Pecorari, T. Seldis, European Commission DG JRC;E. Neumann, BAM; P. Krarup, Force; F. Hardie, Mitsui Babcock;

E.B. Pers-Anderson, ABB TRC

ABSTRACT

There is a strong incentive to perform reliable ultrasonic inspection of austeniticstainless steel welds to detect and classify defects which could cause weld failure.

Reliable ultrasonic inspections for such welds are mainly hampered because ofsevere attenuation of the ultrasound. The sound is scattered and mode-converted atthe boundaries of the columnar grains of the austenitic weld metal.

This has several effects for evaluation of ultrasonic signals:

the amplitude of the ultrasonic echo is influenced by ultrasound scattering, phase-changes and mode-conversion in a characteristic way,

the spectrum of the ultrasonic pulse is distorted depending on the transfercharacteristics of the columnar grained weld metal,

the ultrasonic grain boundary backscatter is measured as noise decreasing thesignal-to-noise ratio during ultrasonic testing.

The required understanding of ultrasound scattering in columnar grained austeniticstainless steel weld metal has been achieved in the frame of this study by:

theoretical modelling of ultrasound columnar grain boundary scattering andvalidation of models with experiment,

metallurgical investigations of columnar grain structure,

inspection capability during defect assessment by ultrasound on welded austenitictest pieces.

A general examination procedure for austenitic stainless steel welds on the basis ofthe results is being written and be used in standards such as the currently intendedrevision of the International Institute of Welding (IIW) Handbook on the UltrasonicExamination of Austenitic Welds and as the draft CEN standard for ultrasonicexamination of welds in austenitic steels.

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1 Introduction

Specification of codes and regulations for non-destructive inspection of austeniticstainless steel welds with ultrasound, e. g. [1, 2], needs considering that:

ultrasound velocity and polarisation are direction dependent due to themacroscopic elastic anisotropy of the weld metal, e. g. [3],

ultrasonic scattering is occurring due to the polycrystalline character of thecolumnar grained weld metal, the ultrasound scattering being also directiondependent because of the anisotropy.

In contrast to ultrasound velocity and polarisation, ultrasound scattering mechanismsin polycrystalline anisotropic media are less understood.

2 Objectives and backgroundIn order to develop a general examination procedure for austenitic welds, the ultra-sound scattering mechanisms need to be better understood. This is the purpose ofthe present project undertaken by BAM (also co-ordinator), Mitsui Babcock, ABB-TRC, Force Institute and EC-JRC, and funded by the European CommissionStandards, Measurements & Testing (SMT) Programme.

3 Preparation of test welds and weld metal samples

Four industrially relevant material groups have been selected:

Group 1: Austenitic stainless Cr-Ni steelsAustenitic steel grades of this group are the commonly most used high alloysteels. These steels generally contain at minimum 12% chromium to improve thecorrosion resistance. Sufficient Ni, Mn, C, and N stabilise the austenitic structure.Austenitic stainless steels consist of an austenitic matrix, which may contain smallquantities of ferrite. The microstructure depends on the content of elementsstabilising ferrite and austenite and differs between fully austenitic and austenitic-ferritic. The specimen base metal is wrought stainless steel, and the weldingprocess is the submerged arc welding.

Group 2: Fully austenitic stainless steels with increased Ni-contentIn contrast to the steels grades of group 1 the austenitic steels with increased Ni-content consist of only one metallic phase with less complicated microstructure.The specimen base metal is centrifugally cast stainless steel, and the weldingprocess is the submerged arc welding.

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Group 3: Nickel-based alloysNickel based alloys differ from high alloy Cr-Ni steels by increased Ni content.They get more and more the pure Ni properties, e.g. low thermal conductivity. Itmay be assumed that this feature influences the grain growth by influencing thecooling rate. Nickel based alloys are alloyed with Mo, which forms segregations ofMo carbides and intermetallic phases.

Group 4: Duplex steelsDuplex stainless steels have a balanced ferritic-austenitic microstructure obtainedby controlled chemical analysis and heat treatment with a limited ferrite content of40 to 60 %. The ferritic matrix of the base material contains lathy or also globularaustenitic grains. The so-called duplex structure exists also in the weld metal andtherefore columnar grain-growth as observed in austenitic materials can beavoided.

Specimens containing test welds and weld metal samples have been prepared for:

experimental validation through attenuation measurement,

microstructure characterisation of austenitic stainless steel weld metal,

capability assessment of ultrasonic testing for austenitic welds.

The specimen manufacture is detailed in the mid-term assessment report of this SMTproject [4].

4 Microstructure of the austenitic stainless steel welds

By scanning electron microscopy, by X-ray diffraction, and by acoustic microscopythe following facts have been determined:

columnar grains are made up of dendrites and residual interdendritic meltingareas. The same crystallographic basis vectors characterize both. This featurecan therefore be used to define a columnar grain as being an area of constantdendritic orientation.

though the main dendritic growth orientation depends on the temperature gradientduring solidification, only a few grains within a bead are observed to have exactlyequal orientation. Growth orientations of adjacent grains in a bead can differ by30 degrees and sometimes even more.However there seems to be no mechanism, which controls the axe orientationdifferent grains in a certain direction. Rather it is more probable that duringsolidification the axes of different grains distribute randomly. This enables theweld metal to be described (macroscopically) as transversely isotropic.

Average values of columnar grain length up to 8 mm and columnar grain width upto 2 mm have been measured.Grains of adjacent beads are intergrown epitaxially, in a defined manner in mostcases, all three crystallographic directions being unchanged. However dendriticconfiguration may be changed without changing the crystallographic orientation.Depending on the direction of the maximum temperature gradient, thecrystallisation rate of the subsidiary dendrite branches may increase and one ofthese directions may become the main branch as the preferred growth direction

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In the direction of welding the grains may be tilted up to 20 degrees (laybackangle).

Neighbouring columnar grains with different crystallographic orientations form anacoustic boundary between them. It can be concluded that the acoustic boundariesare predominantly responsible for ultrasonic scattering (reflection and refraction),whereas grain boundary precipitations, segregations, polyphase weld metal onlyhave negligible effect.

5 Modelling of ultrasonic propagation in austenitic stainless steel weld

5.1 Ultrasonic grain scattering

To describe the microscopic process of backscattering the reflection andtransmission coefficients of plane elastic waves at the interface between cubiccolumnar grains, the textured austenitic weld metal is consisting of, are calculated foran example. It is the special case, where one of the cube axes of both columnargrains (the columnar grain direction) always is perpendicular to the plane of soundpropagation.Backscattering at a single grain boundary is between 5 and 10% and only at large(wave vector) incidence angles exceeds 10%. However, it is not possible to calculatethe scattering coefficient by a simple summation of all grain boundary contributions tobackscattering, because all kinds of interactions and interferences within thescattering structure must be taken into account.The transfer matrix model [5] has been applied to the ultrasound propagation in alayered structure, representing the columnar grains, yielding the reflection andtransmission coefficients.A simple model of the cubic columnar grains of the austenitic weld metal - anaustenitic cubic plate immersed in water with one of the cube axes normal to theplate surface - has been treated numerically. The complex transmission andreflection coefficients (modulus and phase) have been calculated as a function of theplane wave incidence angle and of the rotation angle of the crystallographic co-ordinate system about the plate surface normal. Additional parameters are theultrasound frequency and the plate thickness. The coefficients are only slightlydependent of rotation angle, plate thickness and frequency, but are highly sensible tothe variation of the angle of incidence. Consideration of the energy coefficients yieldsa perfect complementary behaviour of the reflection and transmission coefficient,emphasising the fact that the total sum of reflected and transmitted energy equals theincident energy. This confirms the concept of dealing with plane waves while thenecessity of introducing bounded beams is not given up to now.

The transfer matrix now is calculated for a system of layers with varying thicknessand varying crystallographic rotation angles from layer to layer by taking into accountthe local transfer matrix of a given layer and the boundary conditions to theneighbouring layer. Coupling this system to an upper and lower isotropic material (inthis case water) yields the general equation for the field variables and the reflectionand transmission coefficient, which is going to be solved.

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As a first cross check, perfect agreement with the results for the single plate hasbeen obtained, if more layers but with equal crystallographic rotation angles aretaken into account.

A second approach was undertaken: by solution of the stochastic wave equation thescattering (attenuation) coefficient in polycrystalline austenitic weld metal, whichconsists of cubic columnar grains, is achieved as the imaginary part of thepropagation constant [6].

5.2 Ultrasonic ray tracing

Three-dimensional (plane wave) beam propagation in the weld metal has beenmodelled assuming that only the columnar grain boundaries are acting on ultrasound,and validated using a series of test blocks.

6 Experimental validation of the modelling results

The theoretical model of ultrasonic wave propagation in polycrystalline materials withtexture predicts that the angular dependent longitudinal wave attenuation steadilyincreases as the angle between the wave vector and the texture direction varies from0 to 90 degrees. Up to date, however, experimental investigations found that theattenuation of longitudinal waves having a relative maximum in the proximity of thegrain growth direction.

The experimental work focuses on this contradiction. To assure reliable longitudinalwave attenuation measurements versus the texture direction a novel approach called"Scanning Technique" has been developed to map the ultrasonic field of animmersion transducer by recording the A-scans received by a hydrophone [7]. Boththe incident ultrasonic beam and the field transmitted through the plate are scannedin raster fashion with 77 points per line and per column, respectively. The scannedarea was approximately 18 times larger than the transducer cross section, and allowsall the energy carried by the beam to be recovered. This feature of the scanningtechnique makes corrections for beam diffraction and beam steering unnecessary.

Two distinct data processing procedures are applied to the maps of pressure fielddata acquired during the measurements on X5 CrNi 18 11 cast austenitic steelplates:

Simulated Finite Beam:This procedure simulates a receiving transducer having the same area and axisas the transmitter. To this end, all the A-scans recoreded by the hydrophonewithin the surface of the simulated receiver are integrated over the transducer'ssurface. The result is a single, synthesised A-scan. Subsequently, this A-scan isFourier transformed into the angular frequency domain and conventional datacorrections are then applied to the integrated signal.

Energy Approach:According to this data processing procedure, each and every A-scan acquired by

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the hydrophone is Fourier transformed into the angular frequency domain. Thetransformation yields a complex pressure field at discrete angular frequencies.Then, the complex pressure field is further decomposed into a two-dimensionalspectrum of plane waves in the k-space domain. The amplitudes of the plane-wave components of the incident and transmitted pressure field are corrected forthe appropriate angular dependent acoustical impedance mismatches at thewater/solid/water interfaces. Finally, the energy of the incident and transmittedfield is evaluated by integrating the intensities of each component over the wholek-space.

The following has been shown:

The longitudinal wave attenuation versus the texture direction obtained with thesimulated finite beam approach exhibits a local maximum near the grain growthdirection. The relevance of this exercise relies on the fact that the raw dataacquired by the hydrophone exhibits, after a conventional signal processing, thesame odd behaviour versus the texture direction as it was found in previousinvestigations.

The longitudinal wave attenuation versus the texture direction obtained with theenergy approach does not display any relative maximum in the proximity of thegrain-growth direction. Following this approach the theoretical predictions of thelongitudinal wave attenuation versus the texture direction are recovered at least ina qualitative sense.

The main conclusions of this part of the work are:

The energy approach suggests that interference effect between the plane-wavecomponents of the beam as well as inappropriate corrections for the acousticalimpedance mismatches are responsible for the observed maximum in previousinvestigations.

For the first time, the predictions of the theoretical models of ultrasonic wavepropagation in polycrystalline materials with texture have been validated applyinga novel experimental approach.

In order to prepare attenuation measurements of transverse waves with electro-dynamical transducers, preliminary attenuation measurements of longitudinal waveshave been performed on the specimens of Group 1, Group 3 and Group 4.

Attenuation of Group 4 weld metal (Duplex) is the same as in ferritic weld metal.Especially, no dependency of attenuation on the beam-to-grain angle could be found.

This is not the case with both other weld metal groups. An example of measuredlongitudinal wave attenuation in Group 1 material shows the relation between thebeam and the grain angle.

A set of 12 specimens cut in different depths from a X 6 Cr Ni 18 10 austenitic weldfor repeated sound field measurements under varying conditions has beenmanufactured and used for sound field measurements.

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The first series of tests carried out on 6 specimens with the weld fusion face parallelto the coupling surfaces, both with the electrodynamic and the piezoelectric probe,show that the beam skewing, beam spreading, and distortion is low, as the shape ofthe sound field patterns are revealing. They are circular as obtained with a test blockof ferritic steel.

From the preliminary investigations no hints on a scattering effect of the longitudinalwaves have been obtained yet in the area of the weld material. This could be due tothe size of the piezoelectric transducer and would probably be visible with the use ofthe electrodynamic sensors where it now may be obstructed by noise especially forthe thick specimens.

In conclusion of this project part, the attenuation of the weld metal of the four steelgroups has been measured, revealing qualitatively fairly good agreement with thetheoretical predictions, which are calculated with the plane wave.However, corrections for impedance mismatch and phase differences due to beamdivergence are necessary to get quantitative agreement with the theoreticalpredictions. The attenuation of the Duplex steel is as low as the attenuation of ferriticweld metal and no direction dependence could be observed.

7 Ultrasonic inspection capability

Manual and automated detection and sizing has been performed to assess andcompare the intrinsic capability of different ultrasonic testing techniques currentlyused by the industrial partners.

Automated and manual ultrasonic inspection was carried out by the three industrialpartners on the test specimens representing the four groups of stainless steel underinvestigation in this study. Capability of the ultrasonic testing was determined usingthe PISC [8] type A sharp notches, which were introduced in the weld area.

Results of the Round Robin Test show that:

for flaw detection, the frequency is a very important parameter to be consideredwhen inspecting austenitic welds. There is a slight improvement in detection ratewhen using a 1 MHz transducer rather than a 2 MHz transducer. The detectionrate improves dramatically when using a 0,5 MHz transducer instead of a 1 MHztransducer. This was clearly demonstrated, when inspecting welds of the groups1 and 2,

groups 3 and 4 demonstrated a better weld inspectability than groups 1 and 2.However, the weld thickness shall also to be consider: the fact that the testspecimens in groups 3 and 4 have only about half the thickness of the testspecimens in groups 1 and 2, also influences the inspectability difference,

for inclined longidudinal wave probes, straight and tilted notches in the weld area,with different through wall size are highly selective for probe efficiency evaluation,

there is a wide spreading of length sizing, which could also be partly due to theprogressive shape of the notch ends,

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the 3 mm side drilled holes, which were introduced in the weld, the base metaland in the along the weld-base metal interface, are essential for conducting anefficient ultrasonic inspection of austenitic welds.

8 Examination procedure

A proposal for a standard on ultrasonic examination of welds in austenitic steels hasbeen already submitted to the European standardisation bodies CEN [1].

Requirements for the examination technique to minimise ultrasonic back scatteringhave been defined:

wave modes to be used,

probe techniques (piezoelectric transducer or EMAT, single, twin, or multi-transducer),

frequency,

beam angles and beam scanning directions.

9 Conclusions

The intention of this project is to improve examination procedures for austeniticstainless steel welds by better understanding of ultrasound scattering in the columnargrained austenitic stainless steel weld metal. Consequently, the following researchworks were performed:

theoretical modelling of ultrasound columnar grain boundary scattering andvalidation of models with experiment,

metallurgical and acoustic microscopy investigations of columnar grain structure,

ultrasonic inspection capability on welded austenitic test pieces.

Four groups of materials comprising the full scale of industrially relevant stainlesssteels were investigated: austenitic stainless Cr-Ni steels, fully austenitic stainlesssteels with increased Ni-content, Nickel-base-alloys, and Duplex steels (Ferritic-austenitic steels).Theoretical concepts have been followed to model the ultrasonic amplitude, which isattenuated due to grain boundary scattering. Other inhomogeneities in the weldstructure, e. g. different degrees of segregations within grains and beads, or grainboundary precipitations, only seem to play a minor role in causing ultrasoundscattering.Validation of the theoretical models is by measuring attenuation of the ultrasonicbeam in austenitic weld metal plates. A novel approach called Scanning Techniquehas been developed to measure the longitudinal wave attenuation versus the texturedirection. By applying this technique, the results show that the longitudinal waveattenuation does not display any relative maximum in the proximity of the grain-growth direction. For the first time, the predictions of the theoretical models ofultrasonic wave propagation in polycrystalline materials with texture have beenvalidated applying this novel experimental approach.

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Test specimens with reference reflectors and realistic defects in the weld area, wereprepared and inspected using various ultrasonic testing techniques. The RoundRobin Test results demonstrate the importance of the transducer frequency on theweld inspectability.A proposal for a standard on ultrasonic examination of welds in austenitic steels hasbeen already submitted to the European standardisation bodies CEN and is underreview in the corresponding Technical Committee.

Acknowledgement

This research project, referenced as Project No. SMT4-CT95-2012, is funded by theCommission of the European Communities under the Standards, Measurements &Testing Programme, 1996 - 1999, DG XII, Brussels.

References

[1] CEN/TC 121/SC 5B/WG 2 N 102, WI 121188, Non-destructive examination ofwelds. Ultrasonic methods. Examination of welds in austenitic steels, March1998, Secretariat DS, Kollegievej 6, DK-2920 Charlottenlund[2] Handbook on the ultrasonic examination of austenitic and dissimilar welds,Working Group `Ultrasonic Testing of Austenitic Welds' of Subcommission VC`Ultrasonically Biased Weld Inspection Topics' of Commission V `Quality Control andQuality Assurance of Welded Products' of the International Institute of Welding (IIW),Draft of the second edition, March 1998

[3] Neumann, E.; Hirsekorn, S.; Huebschen, G.; Just, T.; Schmid, R.: UltrasonicTesting of Austenitic Claddings, Dissimilar Metal Welds, and Austenitic Welds,Theory - Practice Regulations, Renningen - Malmsheim, Expert-Verlag, 1995,ISBN 3-8169-1078-5

[4] Neumann, E., Pers-Anderson, E. B., Bith, M., Fraser, F. E., Jeppesen, L.: Mid-term assessment report of the SMT4-CT95-2012 project Effect of ultrasonicscattering on inspection of austenitic welds.

[5] Nayfeh, A. N.: Wave propagation in layered anisotropic media, Elsevier ScienceB. V., Amsterdam, 1995, ISBN 0-444-89018-1

[6] Munikoti, V. K.; Neumann, E.: Extension of Ahmed & Thompson theory to generalelastic plane quasi-wave propagation in textured polycrystalline material, Review ofProgress in Quantitative Nondestructive Evaluation 17 (1997), 1657-1664, Edt. D. O.Thompson, D. E. Chimenti, ISBN 0-306-45901-9

[7] Seldis, T.; Pecorari, C.; Bith, M.: Measurement of longitudinal wave attenuationin austenitic steels, 1st International Conference on NDE in Relation to StructuralIntegrity for Nuclear and Pressurised Components, 20-22 October, 1998, Amsterdam

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[8] Nichols, R. W.; Crutzen, S: Ultrasonic Inspection of Heavy Section SteelComponents, The PISC II Final Report, Elsevier Applied Science Publ. Ltd. London(1988) ISBN1-85166-155-7