Handbookon the

Ultrasonic Examination

Austenitic Welds

The International Institute of WeldingEdition 1986

HandbookOn theUltrasonicExamination ofAustenitic Welds

Compiled by

COMMISSION V — Testing, Measurement,and Control of Welds

of

THE INTERNATIONAL INSTITUTEOF WELDING

Published on behalf of

THE INTERNATIONAL INSTITUTEOF WELDINGbyTHE AMERICAN WELDING SOCIETY550 NW LeJeune Road, P.O. Box 351040, Miami, FL 33135

1986

CONTENTS

LIST OF WORKING GROUP MEMBERS 3FOREWORD 4

1.

2.2.12.2

2.3

3.3.13.23.2.13.2.23.33.43.53.5.13.5.23.5.33.5.43.5.53.63.73.8

4.4.14.24.2.14.2.1.14.2.1.24.2.1.34.2.1.44.2.1.4.14.2.1.4.24.2.1.4.34.2.1.4.44.2.24.2.34.2.3.14.2.3.24.2.3.34.2.44.2.4.14.2.4.2

5.5.15.25.35.3.15.3.25.3.35.3.45.3.55.3.6

General

IntroductionScope and ApplicationParticular Problems Involved in the Examination of

Austenitic WeldsPrinciple of the Method

Conditions to be Established prior to the ExaminationPersonnelRequired Information about the WeldsWelding samples availableAbsence of weld samplesSurface Preparation and MarkingCondition of the Parent MetalAgreements Before the Start of the ExaminationExtent of the examinationSensitivity requiredSpecial conditionsRegular check of equipmentReportingVisual InspectionSurface PreparationCouplants

Ultrasonic Propagation BehaviorStructure of Austenitic WeldsEffects of Austenitic Structures on Ultrasound PropagationUltrasound propagation in an anisotropic structureVariation of velocityBeam directionBeam deformationEffect of the weld fusion facesReflectionRefractionMode conversionInteraction with defectsAttenuation in weld metalInfluence of weld metal on pulse characteristicsPulse spectrumInfluence of attenuation of the ultrasonic pulseScattered ultrasoundPractical implications for ultrasonic testing on austenitic weldsDefect locationAmplitude assessment

Description of EquipmentIntroductionFlaw Detector, Cables and MatchingAngled Longitudinal Wave ProbesIntroductionGeneral propertiesSingle crystal probesTwin crystal probesSurface wave probesFocussing probes

5

55

56

6677777888899999

99

111112121314141414141416161617181818

18181919191920202223

6.6.16.26.36.46.56.5.16.5.26.5.36.5.3.16.5.3.26.5.3.36.5.3.4

6.5.46.5.56.5.6

7.7.17.27.37.3.17.3.27.47.4.17.4.27.4.37.4.47.4.57.57.67.6.17.6.27.6.37.6.47.77.7.17.7.27.7.2.17.7.2.27.7.2.37.8

8.

8.18.28.38.4

8.58.6

9.9.19.29.3

10.

Calibration and CharacterizationIntroductionCalibration BlocksSteps in Time Base SettingProbe Characterization BlockSteps in Characterizing the ProbesIntroductionProbe index for angled longitudinal wave probesDistance amplitude curve, beam angle and beam widthDistance amplitude curveBeam angleBeam widthAmplitude behavior for different reflectors

(for TRL or Focussing Probes)Estimation of dominant frequency and bandwidthDead zone and near fieldNominal signal to noise ratio

Development of an Examination ProcedureOutline of ActivitiesPreparation of Preliminary ProcedurePreparation of Reference BlockGeneralArtificial reflectorsSelection of ProbesWave typeProbe angleFrequencyTypeSize and geometry of probe and componentUse of Reference Blocks to Establish DAC-CurvesSensitivity SettingIntroductionSetting test sensitivityRecording levelAcceptance criteriaPreparation of Detailed ProcedureWritten procedure requirementsGeneral examination requirementsExamination coverageRate of probe movementScanning sensitivityAssessment of Procedure and Documentation

Inspectability of the Component Compared tothe Reference Block

IntroductionSurface ConditionsGeometrical ConditionsComparison of Attenuation Between Reference Block

and ComponentSignal to Noise RatioWeld Repairs

Reporting and EvaluationReportingEvaluationAdditional Investigations

General Recommendations and Comments

242424242526262829292929

30303031

31313133333334343536363737383838393939393939404040

40404040

404141

41414141

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1. GENERAL

Until recent years, austenitic steel welds werewidely regarded as uninspectable by ultrasonics.Research and development have made it possiblefor a useful level of examination to be carried outin many situations. In general, though, the meth-ods are more complicated and the capabilitiesmore limited than for the examination of welds inferritic steel. The main practical implications ofthis are

(1) Welding procedure and preparation geome-try have a strong influence upon the capabilitiesof ultrasonic examination, so that careful consid-eration of these factors at the design stage can bevery beneficial to the examination.

(2) Many technical aspects of the examinationsare strongly influenced by the particular weld

structure so that only skilled, specially trainedoperators with a full appreciation of the physicalbasis of the examination should be employed.

(3) The capabilities for defect detection, posi-tioning and size assessment are more limited thanfor ferritic weld examination. So, monitoring theoccurence of small defects can rarely be used forthe quality control of welds, as is usual with fer-ritic welds. Rather, it may be necessary to usefracture mechanics to set less rigorous defectacceptance standards for the particular compo-nent. These acceptance standards should be com-patible with the limitations of the ultrasonictechniques.

(4) The limited capabilities imply that it is pru-dent to supplement ultrasonic examination withradiography and surface examination techniquesto a greater extent than is necessary with ferriticwelds.

2. INTRODUCTIONThis Handbook does not contain defect indica-

tion acceptance criteria.

2.1 Scope and Application

This Handbook gives recommendations for theultrasonic examination of austenitic welds by man-ual scanning techniques which use the pulse-echo method and A-Scan presentation. The samerecommendations can be extended to mechanicalscanning techniques if special procedures areprepared for the data recording system.

The Handbook advises on how to devise proce-dures for the detection, location, and evaluationof ultrasonic indications of weld defects. Tech-niques involving the use of advanced instrumen-tation for signal processing might give betterdetection and identification of flaws but are notcovered in this document.

The applications covered are limited to buttwelds with weld metal of similar composition tothe parent material. For dissimilar weld metals orfor non butt weld geometries, procedures may bebased on the general advice of this Handbook,provided attention is paid to the requirements ofthe specific geometry and material.

The recommendations given are primarily aimedat post fabrication rather than in-service inspec-tion. Many sections of the Handbook will be rele-vant to both situations, but consideration of thespecific problems of in-service inspection is out-side the scope of this document.

2.2 Particular Problems Involved in theExamination of Austenitic Welds

The term austenitic covers a variety of materialsand material combinations, including austeniticstainless steels and nickel chromium alloys suchas "Inconel", "Incoloy", etc. The capabilities ofultrasonics for the examination of welds in aus-tenitic materials are restricted compared to theferritic case because of the presence of largeelongated anisotropic grains (dendrites), oftenforming an ordered columnar structure, which arecharacterisitic of the austenitic weld metal. Thistype of grain structure can lead to anisotropicultrasonic behavior contrasting with the isotropicbehavior of homogenous welds made in carbon orlow alloy steels.

The size, the arrangement, and the elasticanisotropy of the different grains result in highscattering associated with mode conversion ef-fects, beam distortion, and a variation of ultra-sound velocity with direction and position in theweld. The scattering of energy is observed as arelatively high noise level (grass) and high atten-uation.

The problems which occur in ultrasonic testingof austenitic welds differ according to the parentmaterial production method (rolled, drawn, forged,or cast), the weld processes, and the heat treat-

ment as well as the composition of the parent andweld metals.

The guidelines given in this Handbook takeaccount of the above factors to indicate howoptimum test procedures can be prepared.

2.3 Principle of the Method

The ultrasonic methods applied to austeniticwelds follow basically the same principles asthose described in the Handbook on the Ultra-sonic Examination of Welds. Some importantdifferences do exist, however, which influence theultrasonic method to be used and have implica-tions for the capability of ultrasonics to detect,locate, characterize, and to estimate the size ofweld defects. The most important of these differ-ences are the following:

(1) Scattered energy from natural metallurgicaldiscontinuities generates noise indications athigher amplitude than would be expected for thecase of ferritic welds. The choice of wave mode(longitudinal, shear) and probe characterisitics(sound field, frequency, bandwidth, etc.) shouldbe optimized to allow a reliable separation of weld

defect indications from noise indications (see sec-tion 4.2).

(2) The ultrasonic beam has to cross differentregions in the parent metal and in the weld itself.The velocity of sound may vary along this path(see section 4.2) and this may change the direc-tion of the sound beam. Consequently, this mayresult in inaccuracy in determining reflector posi-tions.

(3) Attenuation in the weld metal is generallymore severe than for ferritic welds and can bemore or less pronounced depending on the angleof the beam with respect to the preferred orienta-tion direction of the grain structure. Therefore,the ultrasonic technique should seek to minimizebeam path length in the weld metal and, wherepossible, aim to take advantage of any directionsof reduced attenuation in the weld.

(4) Beam divergence can also be directionallydependent. The beam profile is usually differentfrom that measured in parent plate (whether fer-ritic or austenitic) so that size estimation methodswhich depend on a knowledge of the beam pro-file, such as the so-called dB drop methods, arenot always suitable on austenitic welds.

(5) Conventional instruments are used for exam-inations, but in most cases, special probes needto be applied (see section 5.3).

3. CONDITIONS TO BEESTABLISHED PRIOR TO THEEXAMINATION

Since austenitic weldments present greater dif-ficulties for ultrasonic testing than ferritic ones,the preparation of ultrasonic procedures requiresmore attention. In "general, it is considered ofgreat importance that operators should be in-formed about relevant details of weld fabrication,as given in section 3.3.2. Particular written proce-dures which specify the examination conditionsand detailed ultrasonic techniques must be pre-pared in conjunction with metallurgists and NDToperators and need to take account of informationthat can be considered as fabrication know-how.

3.1 Personnel

The personnel applying this type of examina-tion should be chosen from the best teams of

ultrasonic operators and be qualified at least tolevel two or equivalent. Their organization shouldsubmit them regularly to requalification tests par-ticularly oriented to austenitic materials. As wellas having good qualifications and sufficient expe-rience in ultrasonic techniques, the personnelconcerned should be trained on representativesamples of austenitic welds to acquire specificexperience in distinguishing defects from noiseand spurious indications. For this they need thefollowing:

— to be familiar with refraction/reflection rulesand mode conversions

— to have experience in the use of specializedprobes (e.g. angled longitudinal wave probes)and flaw detectors (e.g. selectable bandwidth)

— to have received appropriate instruction inthe metallurgy of this type of material inorder to be capable of determining the bestapproach for the examination, including howto relate defects and indications in the weldand in the parent metal

— to know the capacity of other NDT methods

6