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    ULTRASONIC TESTING

    OF STEEL CASTINGS

    by

    J . D . LavenderResearch Manager. Quality Assurance Group

    Steel Castings Research & Trade Association

    Sheffield. England

    TABLE OF CONTENTSPage

    Preface .................................................................................................................. 2

    Calibration and Reference Blocks ................................................................. 7Longitudinal Wave Probes ............................................................................. 7

    Transverse Wave Probes ............................................................................. 10

    Measurement of Steel Thickness ................................................................. 12

    Radiographic Correlation ...................................................................................... 13

    Micro-Shrinkage ............................................................................................ 13

    Theory of Ultrasonic Flaw Detection ...................................................................... 3

    Calibration of the Ultrasonic Instrument ................................................................ 7

    Formation of Casting Defects-Ultrasonic and

    Flaws from Inadequate Feeding. Macro-. Filamentary-.

    Flaws from Hindered Contraction. Hot Tears. Cracks ................................... 14

    Flaws from Gas and Entrapped Air. Airlocks. Gas Holes .............................. 19

    Ultrasonic Attenuation - Carbon. Low Alloy and Austenitic Steels ........................ 22

    Influence of Structure on Ultrasonic Attenuation ........................................... 22

    Measurement of Ultrasonic Attenuation ........................................................ 24

    Sizing of Flaws and Acceptance Standards .......................................................... 27

    Beamspread and Maximum Amplitude Techniques ...................................... 27

    Surface Flaws ................................................................................................ 30

    Beamspread from Transverse Wave Probes ................................................. 31Production and Economics of an Ultrasonic Technique ........................................ 33

    References ............................................................................................................. 36

    by Steel Founders' Society of America, 1976

    CAST METALS FEDERATION BUILDING20611 CENTER RIDGE ROADROCKY RIVER, OHIO 44116

    Printed in the United States of America

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    J. D. Lavender was educated at Ecclesfield Grammar School, Nr. Shef-field. He received the Associateship of the Institution of Metallurgistsin 1954 and became a Fellow in 1972. He is a member of the Instituteof Physics and of the British Institute of Nondestructive Testing. He wasemployed by Brown-Firth Research Laboratories from 1940 to 1946 onradiography of non-f errous and ferrous alloys, X-ray crystallography andmetallography of low and high alloy steels.

    In 1954 he became foundry metallurgist with Firth-Vickers StainlessSteels in Sheffield, and in 1957 moved to the Steel Castings Research andTrade Association (S.C.R.A.T.A.) as a senior investigator of nondestructivetesting. He was made section head in 1964 and research manager ofquality assurance in 1972. Mr. Lavender has presented the S.C.RA.T.A.Exchange Lecture at the National Technical and Operating Conferenceof the Steel Founders Society of America in 1969 and 1975.

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    PREFACE

    Ultrasonic flaw detection is a method of non-destructive testing that is finding increasing ac-ceptance in the United States. This growth inthe application of ultrasonics is intimately tied tothe field of fracture mechanics and the scientifi-cally based approaches to designing against fail-ure. Ultrasonic flaw detection, as opposed to themore widely used radiography, permits the in-spector to pinpoint accurately the location of theflaw and to determine its shape and size. Thesefactors play an important role in fracture mechan-ics where the maximum safe stresses can be cal-culated for a given flaw size and location. Con-versely, for a given flaw type, size and operating

    stress field, the maximum flaw size that can betolerated safely can be determined. Thus theunique ability of ultrasonic inspection to assessflaw location and flaw geometry is vital to engi-neering approaches of fracture-safe design.

    Further insight into the growth of nondestruc-tive testing is gained by a historical review ofdevelopments. Radiography was developed earlyand achieved industrial status when a set of radio-graphs called, Gamma Ray Radiographic Stand-ards for Steam Pressure Service was issued in1938 by the Bureau of Engineering, U.S. Navy.Numerous ASTM specifications relative to radio-graphy in steel casting production have been is-sued since then. Ultrasonics, in contrast, receivedits first major boost towards industrial applicationfor steel castings in Britain when a study on itsuse and development possibilities was undertakenin 1958. ASTM specification A-609, StandardSpecification for Longitudinal Beam UltrasonicInspection of Carbon & Low Alloy Steel Castings

    was published in 1970. This specification wasfollowed in 1974 by the ASME Boiler& PressureVessel Code, Section V, T524.2, Angle BeamExamination of Steel Castings. Other specifica-tions of international importance are the West-inghouse Specification 600964, Ultrasonic Testingof Steel Castings, and the Central ElectricityGenerating Board United Kingdom Standard66011, Turbine Castings (chromium, molyb-denum, vanadium steel) .

    Increased acceptance and utilization of ultra-sonic inspection are to be expected for the future.These trends are apparent from the extensive

    activity going on now in the United States andabroad. Three standards, in addition to ASTM

    A-609, are currently considered. These are theBritish IS0 Standard-Draft Proposal for anInternational Standard for the Ultrasonic Inspec-tion of Steel Castings, the German standard-Introduction of Ultrasonic Testing and Stand-ards and General Conditions of Delivery for SteelCastings, and a new proposed ASTM specifica-tion which will be similar to Westinghouse Speci-fication 600964.

    This booklet is published to present basic in-formation on the nature of ultrasonic inspectionprinciples with specific guidelines on flaw detec-tion in steel castings. This information and thefavorable economic aspects of flaw detection byultrasonic means are presented for technical per-sonnel and managers of casting producers andparticularly the technical staff of casting userswho control the level to which ultrasonic inspec-tion will find acceptance in the future.

    PETER F. WIESER

    Research DirectorBy direction of the

    Carbon and Low AlloyTechnical Research Committee

    H. J. SHEPPARD, ChairmanA. G. LINLEY P. J. NEFF

    F. H. HOHNL. H. LONG, JR

    A. J. WHITTLER. A. MILLER

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    THEORY OF ULTRASONIC FLAW DETECTION

    THE CHARACTERISTICS OF SOUND WAVES

    Sound is produced when a body vibrates and ispropagated only within a medium. Sound wavesare classified in terms of frequency, which is thenumber of vibrations per second or Hertz; thefrequency scale relating the sonic and ultrasonicranges is shown in Fig. 1.

    The basic formula, to which reference is madethroughout the whole study of ultrasonic examina-tion, is:

    The relationship between frequency and wave-length for the transmission of ultrasonic wavesin steel is given in Fig. 2.

    Sound waves must have a medium in which totravel and the velocities with which they aretransmitted through a particular medium dependson its elastic constants and on its density, as givenby the following formulae:Thin rod velocity

    Longitudinal wave velocity

    Transverse wave velocity

    wherec =wave velocity, mm/sE =Youngs modulus of elasticity, dynes/mm2G= shear modulus of elasticity, dynes/mm2=density, g/mm3=Poissons ratio

    Values of sound velocity, density and acousticimpedance of materials associated with ultrasonicexamination are given in Table I. The wavelengthsof longitudinal and transverse waves in steel aregiven in Table II.

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    COUPLANTS

    In order to transmit ultrasonic energy efficient-ly into the specimen, the probe must be coupledto the casting surface by means of a liquid, e.g.glycerine, oil, grease, water.

    The reflected energy which arises when a longi-tudinal wave meets a boundary between two mediais a function of their relative acoustic impedances(1 c1 and 2 c2 where =density and c=soundvelocity). The amount of energy reflected at a

    boundary is given by the formula (restricted tolongitudinal waves, normal to the boundary).

    Typical values of..c. are given in Table I. The useof values of.c. in this formula shows that at anair/steel interface, 100 % of the ultrasonic energyis reflected, while 88% is reflected at a water/steel interface.

    TYPES OF ULTRASONIC WAVES

    with ultrasonic examination :There are two wave types normally associated

    (a) Longitudinal waves (Fig. 3).(b) Transverse waves (Fig. 4).

    The longitudinal waves are produced by a volt-age applied to a piezo-electric transducer. Themechanical vibrations produced are related to thisvoltage, and are propagated as ultrasonic waves.

    Transverse waves are normally obtained by in-itially producing longitudinal waves which arethen refracted in accordance with Snell's Law asthey pass from a Plexiglass wedge into the steel.The design of the Plexiglass wedge, which formspart of the probe, is such that the longitudinalwave does not enter the specimen. Fig. 5 shows

    the conversions which occur when a longitudinalwave is transmitted from one isotropic medium(A) into a second isotropic medium (B).

    PRODUCTION OF ULTRASONIC WAVES

    are used to produce ultrasonic vibrations :There are two main types of transducers which

    (a) Naturally occurring, or artificially pro-duced anisotropic single crystals, e.g.quartz, lithium sulphate, Rochelle salt.

    (b) Ceramics, e.g. barium titanate, lead zir-conate.

    These transducers are frail and must therefore

    mechanical vibration, and a sturdy cover to pro-tect the whole device.

    This device is called a probe; several probesare illustrated in Figs. 6, 7 and 8.

    The above formula applies only to the far zone.The relationship between beam spread and fre-

    beam lies on the axis and decreases rapidly withincrease in angle from the probe axis; with fur-ther increase in angle, alternate maximum andminimum intensity values occur, which are calledside lobes, Fig. 10.

    be placed in a suitable housing which incorporatesa voltage lead, a suitable backing material to damp

    quency for 10, 15, and 23 mm diameter probes isgiven in Fig. 9. The maximum intensity of the

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    GEOMETRY OF THE ULTRASONIC BEAM

    In order to use the probes for flaw detection, apulse of sound waves from the probe must betransmitted at regular intervals through thesteel ; the time intervals between successive pulsesmust be long in comparison with the time takenfor the echoes to return to the probe.

    The ultrasonic beam, which is produced as eachpulse of energy is supplied to the transducer, hasa certain geometry. This geometry, which coversthe beam spread and the near and far zones, isdependent on the ultrasonic frequency, the trans-ducer diameter and on the way in which the trans-ducer is damped.

    (a) Beam spread

    The solid angle of beam spread 2 at 10% ofmaximum axial intensity is given bv the formula

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    (b) Near zone; transition zone; far zone

    The complex variations in sound intensity whichoccur close to the transducer cause interferenceeffects. This interference is confined to the nearzone, in which the beam is essentially parallel.Note that the near zone is affected by the Plexi-glass block, shear and combined double probes.The near zone is defined by a distance N, beyondwhich the beam diverges:

    This relationship for a number of media is plottedin Figs. 11a and 11b. In the far zone, the intensityof the beam follows the inverse square law. Adiagrammatic representation of the variationin intensity within the near and far zones is givenin Fig. 12; a transition zone separates these twozones.

    The interference and variation of intensitywithin the near zone makes flaw size estimation

    impossible and such measurements should onlybe made in the far zone: Claydon has carried outextensive work on this problem.

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    CALIBRATION OF THE ULTRASONIC INSTRUMENT

    CALIBRATION AND REFERENCE BLOCKS

    Ultrasonic instruments and their probe systemsare not built to a universally recognized specifica-tion. In order to have a means of comparing theprobe and instrument characteristics, it is ad-visable to use one or more reference blocks tocalibrate the instrument (Fig. 13). This calibra-

    tion is particularly important under working con-ditions to ensure that the correct sensitivity levelsare maintained. It may also be an advantage toprepare additional reference blocks of thickness,shape and surface finish appropriate to the needsof the operator.

    LONGITUDINAL WAVE PROBES

    The first operation (Figs. 14a-d) is used forsteel thickness measurements between 0 and 200mm and automatically sets the initial pulse at thezero point on the screen. It is necessary to re-calibrate each longitudinal wave probe for the

    zero point in a similar manner.1. Calibration of the time base in terms of steel

    thickness

    The probe is placed in position A on the testblock and by controlled movement of the horizontalshift (or delay if no horizontal shift is provided)and the fine time base control, a series of echoesare obtained at the screen positions of 25, 50, 75and 100 scale divisions (Fig. 14a). These move-ments of the two instrument controls automatical-ly set the zero point. If the echoes will not coin-cide with appropriate scale divisions, the time baseis not linear and a graphical calibration should be

    prepared, or the supplier of the instrument ap-proached if the error is excessive.

    The following operations (Figs. 14b-14d) mayrequire adjustment of the time base and othercontrols.

    For greater accuracy in the range of less than10 mm, a 4-5 MHz probe should be used, becausea higher frequency probe has a smaller dead zone.Place the probe in positions D and E, as shown inFigures 14d and 14e.

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    2. Check on linearity of amplifier

    The amplification is linear when the ratio ofthe height of any two consecutive echoes remainsconstant when the degree of sensitivity is altered(Fig. 14f). A quantitative value of amplifier lin-earity may be determined and reference shouldbe made to BS.4331: Part 1: 1968.

    In order to ensure that an apparatus has ade-quate penetrating power, operation 3 (a) shouldbe carried out.

    3. Assessment of-

    (a) Penetrating power

    The sensitivity control is set at a minimum andthe probe placed on the metallized surface of thePlexiglass block (Fig. 15a). The thickness of thePlexiglass is calculated to correspond to 50 mm ofsteel. A measure of the penetrating power canthen be obtained by slowly increasing the sensi-tivity control to a maximum and noting the num-ber of bottom echoes which appear. A low pene-trating power apparatus may only give 2-4 bot-tom echoes, a high penetrating power apparatus6 - 10 bottom echoes.

    (b) Relative sensitivity

    The sensitivity control is initially set at a mini-mum and the probe moved until an echo fromthe 1.5 mm diameter hole is obtained on thescreen. The height of this echo at a given settingof the sensitivity control constitutes a relativemeasure of sensitivity (Fig. 15b).

    The dead zone, which is dependent on the instru-ment and probe characteristics, should be assessedfor each probe, since flaw indications cannot bereceived in this zone.

    4. Check of the dead zone

    The distance from the probe to the Plexiglassat positions D and E is 10 mm and 5 mm, respec-tively. The ability to see one or both of theseechoes is a measure of the dead zone (Figs.16a, b).

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    5. Check on the resolving power

    The probe is moved in order to obtain 3 bottomechoes at 85, 91 and 100 divisions (Figs. 17a andb). Three separate echoes on the screen will in-dicate a probe system with good resolving power ;when the echoes are not separate, but merge to-gether giving a blurred effect, the equipment issaid to have bad resolution. A quantitative valueof resolution may be determined and referenceshould be made to BS.4331:Part 1:1968.

    TRANSVERSE WAVE PROBES

    The first three of the following operations areessential because the probe index and the beamspread must be found by practical measurementand not assumed to be correct from probe mark-ings and theoretical considerations. The resultscan then be used for the assessment and locationof flaws.

    1. Determination of probe index

    The probe index is given on the probe ; in orderto check that the position marked on the probeis correct, place the probe in position H (Fig. 18).Move the probe until maximum amplitude is re-ceived from the 100 mm curved surface. The cen-tral mark on the graduated scale will be the posi-tion at which the beam leaves the Plexiglass andenters the steel, i.e. the probe index.

    2. Determination of the angle of refraction

    Move the probe until maximum signal amplitudeis obtained from the Plexiglass cylinder (Fig. 19).The reference block has calibrated scales engravedat 40-70 and the relevant angle of refractionmarked on the probe should coincide with the cor-rect scale position. The probe index which hasbeen previously determined should be marked onthe probe in order to obtain the correct results.

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    3. Correction of the zero point

    The presence of the Plexiglass in the transversewave probe causes a time lag between the momentat which the signal leaves the transducer and themoment it leaves the wedge. This time lag mustbe corrected for and the zero point set on thecathode ray screen. Obtain an echo at a maximumamplitude on the screen from 100 mm radius (Fig.20). Adjust the echo by movement of the hori-zontal shift and the fine time base control in orderto obtain two echoes on the screen at 50 subdivi-sions and 100 subdivisions; the zero on the scalewill then correspond to the moment the beamleaves the Plexiglass wedge.

    It is necessary to recalibrate each transversewave probe for the zero point in a similar manner.

    A beam plot is used to verify the position ofthe beam in a cast section. It is used in conjunc-tion with a second Plexiglass slide on which adrawing is produced showing the geometric shapeof the component. Flaw size, shape and positionmay be assessed using this method, and referenceshould be made to the Test Procedure. The abovemethod may also be used to assess the beamspread from both single and combined doubleprobes. A horizontal polar diagram is shown in

    Fig. 21.The scale on this part of the block is calibrated

    every 25 mm and the hole diameter is 1.5 mm.Obtain an echo on the screen by placing the probeat a suitable distance from the hole, e.g. for a 45angle probe, the distance would be 25 mm. Rotatethe probe from one side to the other and note theecho amplitude at various angles of probe posi-tion. A horizontal polar diagram can be plotted,as previously described. It is suggested that inorder to plot a more accurate diagram, a similarcheck should be carried out at multiples of a givendistance.

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    4. Assessment of sensitivity

    Obtain an echo at a convenient height on thescreen by placing the probe 25 mm away fromthe 1.5 mm hole (Fig. 22). Move the probe back50, 75 and 100 mm away from the hole and notethe new echo amplitude. Note that to carry out asimilar measurement using a 70 probe, the abovefigures, excepting the diameter of the hole, mustbe multiplied by 2.75. For a 60 probe the figuresmust be multiplied by 2.

    The sensitivity of the equipment can now bespecified in terms of echo height and distancefrom the 1.5 mm hole.

    MEASUREMENT OF STEEL THICKNESS

    Ultrasonics, using longitudinal waves and con-ventional flaw detection equipment, can readily beused for thickness measurement. A frequency

    range of 2-5 MHz is recommended.

    The accuracy of the technique, which should beof the order of 2 - 5% steel thickness, depends ontwo main factors:

    (a) the surface roughness, and

    (b) whether or not the two surfaces are parallel.The value of 2 - 5% accuracy will fall off wheresteel thicknesses less than 6mm are to be meas-ured.

    The first technique which is described is theone recommended by the Procedure and may beused with any ultrasonic instrument provided the

    instrument is correctly calibrated.

    1. Direct calibration method

    This technique is widely used because of itssimplicity, and depends on the time base linearityof the instrument. It consists of initially cali-brating the instrument on the A2 calibration block(Fig. 13), details of which are given in the pre-ceding pages under the heading, Calibration ofthe Time Base in Terms of Steel Thickness.

    Once this operation is complete, a check may bemade by the use of the B.S.C.R.A. reference block,ignoring the echo from the drilled hole. A rule

    measurement in two directions on this block canbe directly related to the position of each bottomecho on the graduated cathode ray screen.

    The next two techniques (2 and 3) are specificto certain types of equipment.

    2. The resonance method

    In this technique, a unit attached to the ultra-sonic equipment enables a pulse at a continuously

    variable repetition frequency to be transmittedinto the specimen. From the received signal, thethickness of material can be obtained.

    3. The use of a calibrated time marker

    This technique depends on two factors, firstly,a linear time base on the instrument and, secondly,the sound velocity of the longitudinal wave in thematerial. Since this sound velocity is a variablefactor, the method is not recommended, but isdescribed in order to illustrate the use of the timemarker.

    For a given material, the distance on the screen

    between the initial pulse and the reference echo,or between consecutive reference echoes, repre-sents the time interval for the pulse to travel thetotal distance from the transducer to the bottomof the material and back to the transducer. Cer-tain commercial instruments have a calibratedtime marker circuit coupled to the time base,which enables accurate time measurements to bemade.

    The thickness of the material is then calculatedfrom the formula:

    s = v x ts = thickness, mm

    v = sound velocity, mm/sect = time interval, sece.g. number of bottom echoes = 6

    number of time markers = 20

    Assume the marker time interval is 5 micro sec-onds (1 micro second = 10-6 seconds).The total time for sound to travel 12 times thesteel thickness is therefore 100 micro seconds.

    Time required totravel the thicknessof the material

    Velocity of longi-

    tudinal wave = 5.85 106 mm/sec*

    =100/12 micro seconds

    = 8.33 micro seconds

    s = v x ts = (5.85x106) (8.33x10-6) mms = 48.7mm

    thickness ofmaterial = 48.7 mm

    Reference is also made to ultrasonic thicknessmeasuring devices, but direct reading methodsusing conventional flaw detection equipment is tobe preferred.

    * In order to obtain correct results, this method requiresaccurate knowledge of the velocity of sound in the mate-rial undergoing testing.

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    FORMATION OF CASTING DEFECTSULTRASONIC AND RADIOGRAPHIC CORRELATION

    The use of non-destructive testing for the ex-amination of steel castings has increased consider-ably during the past few years. The most strikingadvance has been in the application of ultrasonic

    examination as a quality control technique in thesteel foundry. Ultrasonic examination to assessthe presence and nature of internal flaws in steelcastings has many advantages over radiography,these including such factors as capital and run-ning costs, safety in operation, portability and,a most important point, ultrasonic examination isindependent of section thickness. The Atlas ofSome Steel Castings Flaws, therefore, places aparticular emphasis on the use of ultrasonics forthe detection of casting flaws. Its purpose is toillustrate and describe typical casting flaws anddiscuss how they may be detected by ultrasonics.In order to clarify the ultrasonic indications, ap-

    propriate radiographs, micro-sections, and dia-grams have been included.

    Ultrasonic examination requires an experiencedoperator to obtain the maximum economic bene-fits. It is essential that the operator be given everyfacility so that he can estimate the possible loca-tion of flaws before carrying out an examination,i.e. he should be provided with a drawing showinglocation of feeder heads, gating systems, etc. andshould have all the information on the history ofthe casting.

    FLAWS DUE TO INADEQUATE FEEDING(SHRINKAGE FLAWS)

    1. Description of flaws

    Shrinkage flaws are cavities formed duringsolidification, which occur as a result of liquid tosolid contraction. The flaws are not normally as-sociated with gas but a high gas content will mag-nify their extent.

    Shrinkage flaws may occur in steel castingswhere there is a localized variation in sectionthickness ; they may, however, occur in parallelsections where penetration of the liquid feed metalis difficult. The shrinkage flaws which occur insteel castings may be considered as falling into

    three types, namely :(a) Macro-shrinkage

    (b) Filamentary shrinkage

    (c) Micro-shrinkage

    (a) Macro-shrinkage

    A large cavity formed during solidification isreferred to as a macro-shrinkage flaw. The mostcommon type of this flaw is piping, which occursdue to lack of sufficient feed metal. In good de-sign, piping is restricted to the feeder head.

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    (b) Filamentary shrinkage

    This is a coarse form of shrinkage but ofsmaller physical dimensions than a macro-shrink-age cavity. The cavities may often be extensive,branching and interconnected. Occasionally theflaw may be dendritic. Theoretically, filamentaryshrinkage should occur along the center line of thesection but this is not always the case and on someoccasions it does extend to the casting surface.Extension to the casting surface may be facili-tated by the presence of pin or worm holes.

    (c) Micro-shrinkage

    This is a very fine form of filamentary shrink-age due to shrinkage or gas evolution duringsolidification. The cavities occur either at thegrain boundaries, (intercrystalline shrinkage), or

    between the dendrite arms (interdendritic shrink-age).

    Typical locations at which shrinkage cavitiesare most likely to occur are illustrated in Fig.23. Localized changes of section thickness repre-sent hot spots which cannot be fed adequately inmost cases. Shrinkage cavities will form, there-fore, unless special precautions are taken. In-

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    scribing circles in the junction and in the adjacentsections, and determining the ratio of the circlediameters, provides a relative measure of the hotspot severity and hence susceptibility to shrinkageformation. If the technique is applied to L, T, V,and Y sections, it will be seen that crosses andacute angle junctions are to be avoided in favor ofT and L junctions.

    2. Detection of flaws

    (a) Macro-shrinkage

    The technique used for the detection of thisflaw is dependent on the casting section thickness.Where the section exceeds 3 inches a normalsingle probe is satisfactory, while for sectionthicknesses of less than 3 inches it is advisable touse a combined double probe. The presence of theflaw is shown by a complete loss in back wall echo,together with the appearance of a new flaw echo.The position of the flaw echo on the oscilloscopescreen indicates the depth of the flaw below thesurface. An angle probe should be used to confirmthe results obtained from the scan using a normal

    probe.

    (b) Filamentary shrinkage

    The presence of filamentary shrinkage is bestdetected with a combined double probe if the sec-tion thickness is less than 3 inches. The flawechoes which are obtained reveal the extent ofthe flaw and also the flaw depth. It is suggestedthat an initial scan on expansive rough surfacesis best carried out with a large combined doubleprobe (say 23 mm in diameter) ; final assessmentand flaw depth measurement are achieved by asmall diameter combined double probe (say 10 - 15

    mm in diameter).

    (c) Micro-shrinkage

    A fine grass type of oscilloscope indication istypical of this flaw. The extent to which the flawoccurs may be assessed by a consideration of thenumberof back wall echoes which occur at anygiven frequency. Where it is difficult to obtainone or a number of back wall echoes at 4 - 5MHzdue to random scattering of the ultrasonic beam,and since this could give the impression of a largecavity type of flaw, it is advisable to reduce thefrequency to 1 - 2 MHz. This reduction in fre-

    quency will indicate that no large cavity does infact exist if a back wall echo can be obtained.

    FLAWS ASSOCIATED WITH HINDEREDCONTRACTION DURING COOLING

    1. Description of flaws

    (a) Hot tears

    These are cracks which are discontinuous andgenerally of a ragged form, resulting fromstresses developed near the solidification tempera-ture when the metal is weak. The stresses arisewhen the contraction is restrained by a mold or

    core or by an already solid thinner section. Typicalexamples are illustrated in Fig, 24. Hot tearsoccur at or near to changes in section, e.g. re-entrant angles and joints between sections. Theyare not fully continuous and commonly exist ingroups, often terminating at the surface of thecasting.

    (b) Cracks or stress cracks

    These are well defined and approximatelystraight cracks formed when the metal is com-pletely solid. They are revealed as clearly defined,smooth, dark lines.

    2. Detection of flaws

    (a) Hot tears and cracks

    The location of a hot tear can rarely be deter-mined accurately using a normal probe because ofthe orientation of the flaw. The most satisfactorytechnique is the use of a transverse wave probe.Since the tears usually exist in groups, the extent

    is assessed by passing the beam underneath theflaw (Fig. 25). The location of a hot tear is foundmost easily by magnetic crack detection and itsdepth by the ultrasonic angle probe. A cold crackmay be detected in a similar manner (Fig. 26).

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    FLAWS DUE TO GAS AND ENTRAPPED AIR

    1. Description of flaws

    (a) Airlocks

    When molten metal is poured into a mold, airmay be entrained in the metal stream which mayappear in the subsequent casting as a cavity orseveral cavities just below and parallel to thecasting surface.

    (b) Gas holes

    These flaws are discrete cavities usually greaterthan 1/16 inch diameter, which are caused by theevolution of dissolved gases from the metal duringsolidification. A particular type of gas hole is ablow hole, which is due to gas evolved from themold or core rather than the metal. A wormholeis another form of gas hole, which occurs as atube-like cavity usually normal to the casting sur-face and almost extends to the surface.

    2. Detection of gas cavity flaws

    (a) Airlocks

    Since airlocks tend to lie just below the castingsurface and parallel to it, their presence is bestdetected by a normal or combined probe (Fig. 27).

    (b) Gas holes

    A combined double probe is also recommendedfor the detection and assessment of gas holes, i.e.blowholes and wormholes. The gas hole type offlaw, where there are many similar flaws asso-ciated, is typified by a loss in back wall echo witha number of flaw echoes (Figs. 28 and 29). It isoften advantageous to use angle probes to assessthe size and severity of such flaws,

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    ULTRASONIC ATTENUATIONCARBON, LOW ALLOY AND AUSTENITIC STEELS

    INFLUENCE OF STRUCTURE ONULTRASONIC ATTENUATION

    1. Carbon, low alloy and martensitic steels

    These steels undergo a transformation duringcooling in the solid state, i.e. from austenite toferrite.

    During early investigations it became evidentthat heat treatment of carbon and low alloy steelvitally affected the choice of an optimum test fre-quency for penetration and test sensitivity. Aninvestigation of the effect of heat treatment onultrasonic attenuation in 0.3% carbon steel waves

    revealed a sharp decrease in grain coarseness afterheat treatment above 1580F (860C) (Fig. 30).It may also be interpreted from the graph thatwhen the material has not been heat treated abovethe AC3 temperature, the attenuation is frequencydependent, that is, little attenuation occurs at 1MHz with a sharp increase occurring as the fre-quency is increased from 2 - 10 MHz. Practicalexperience in the examination of cast componentsto various steel specifications has led to the pro-duction of a series of histograms, Figs. 31a and31b indicating which steels may and may not beexamined by ultrasonic techniques.

    Steels are graded into the following types: low

    carbon (less than .08% C), plain carbon, low alloyand 13% chromium steels. After heat treatment,these steels exhibit a small grain size comparedwith the wavelength of the ultrasonic beam

    Two methods of assessing the attenuation char-acteristics of a casting are outlined in Table III.This assessment is necessary to determine wheth-er a given steel casting is acceptable for ultrasonicexamination. In a situation where values in TableIII are not obtained, the steel should be re-heattreated and re-checked in order to ascertain thesignificance of the original heat treatment givento the casting.

    The possibility of using ultrasonic attenuationvalues to assess mechanical properties has beenstudied. Grain size related properties were foundto correlate with the attenuation characteristics.

    A reasonable correlation was obtained betweenCharpy Izod impact energy (Figs. 32a, 32b), the15 ft*lbs transition impact temperature (Fig. 32c)and ultrasonic attenuation. It may be possible tocorrelate other related properties affected by grainstructure, e.g. fracture toughness.

    (> G).

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    2. Austenitic steels

    These steels do not have a tansformation dur-ing cooling in the solid state. Their grain size isequal to, or larger than the wavelength of the ul-trasonic beam. This fact introduces a loss of atransmitted pulse due to scattering which severelyreduces the possibility of ultrasonic flaw detection.Flaw detection is also affected by the anisotropyof the cast structure, the sound velocity, attenua-tion and elastic moduli, of individual grains which

    all vary with the direction of grain growth.The influence of grain orientation alone is shown

    in the columnar structure of an 18% Chromium,12% Nickel Austenitic Steel, Figs. 33a and 33b.

    A variation in transmission and sound velocityoccurs at different orientations to the columnaraxis (Table IV) . If the grain size of cast austeniticsteels is less than 2 - 4 mm it may be possible toobtain some information when using 1 MHzprobes.

    The effects of orientation, grain size and metalcomposition on ultrasonic attenuation require fur-ther research in order to establish the reason for

    this phenomenon.

    MEASUREMENT OF ULTRASONICATTENUATION

    Measurement of ultrasonic attenuation in caststeel components will provide useful informationin the assessment of the grain size and therebyon the effect of heat treatment. The grain size ofas-cast steel is large and high attenuation iscaused by the ultrasonic beam being scattered bythe large grains. An increase in background noise(grass) occurs which reduces the sensitivity ofthe ultrasonic technique. In the fully heat treated

    condition, when the temperature of the heat treat-ment exceeds the transformation temperature,the as-cast grain structure is recrystallized. Thegrain size of fully heat treated material is smalland its ultrasonic attenuation is low,

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    The predominant factor in attenuation measure-ment is the relationship between the ultrasonicwavelength and the grain size. These are relatedas follows:

    High attenuation occurs when < DLow attenuation occurs when > D

    where = wavelength

    D = grain diameter

    The operator must consider the following fac-tors before carrying out the ultrasonic attenuationmeasurements :

    (a) The frequency of the transducer; as highan ultrasonic frequency as practicableshould be used i.e. between 4 and 6 MHz,for longitudinal waves and 2 - 5 MHz fortransverse waves.

    (b) More critical results are obtained by theuse of transverse waves than longitudinalwaves.

    (c) The path distance when longitudinal wavesare used should be between 50 - 200 mm.

    (d) The path distance for transverse wavesshould be between 10 and 100 mm.

    (e) The roughness of the input surface.

    (f) The roughness of the back surface.

    (g) The analysis of the cast steel.

    (h) The nominal heat treatment.

    (j) The position of ingates, risers, feeder headsand test bars.

    (k) The couplant.

    (m) The examination must be carried out onsound material. In excess of 50 mm thepresence of microshrinkage is unlikely toaffect the results.

    Three methods of measurement are applicable:

    1. Method 1

    This is a practical shop floor method which usescontact coupling with longitudinal and transversewaves. Fig. 34 is comparative and will locate vari-ations of heat treatment within a single casting,

    Attenuation is compared from a series of dBvalues.

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    2. Method 2

    This is a practical shop floor method which usescontact coupling with longitudinal waves. Fig. 35will give a quantitative value. It may be used tocompare one casting with another provided thesame probe, the same thickness and the samegeometric configuration is assessed on each cast-ing. The attenuation may be expressed in dB/mm.

    3. Method 3

    This is a water immersion technique more suit-able for laboratory conditions. Fig. 36 will givean accurate assessment of attenuation. It is usualto produce a standard machined test piece.

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    SIZING OF FLAWS AND ACCEPTANCE STANDARDS

    Customers may specify their own or otherstandards for ultrasonic inspection. Internation-ally, however, the only standards that are widelyrecognized are ASTMA609 Standard Specifi-cation for Longitudinal Beam Ultrasonic Inspec-tion of Carbon & Low Alloy Steel Castings, andthe ASME Boiler & Pressure Vessel Code, Sec-tion V, T524.2, Angle Beam Examination ofSteel Castings.

    Two steps are important in working to thesestandards. The first step consists of specifyingrejection levels for discontinuities. The secondstep consists of specifying one of the followingmethods of recording discontinuities :

    1) Method based upon flat bottom holes andback wall echoes produced from test blocks

    2) Method based upon back wall echoes fromtest blocks

    3) The DGS system, using theoretical curvesbased upon flat bottom holes and back wallechoes

    4) Method based upon back wall echo/defectecho ratio, to quantify the actual flaw sizeand shape. This method uses either the 6or 20 dB technique or the maximum echoamplitude from the flaw.

    To establish the ultrasonic technique completelya knowledge of its accuracy in both sizing andindicating the flaw type is essential. The flawsize, type and accuracy are dependent upon thecasting thickness, or whether the casting wallsare planar (linear) or rounded. A review of theallowable flaw sizes in current specifications(Table V) shows the averaged value of the mini-mum flaw size to be detected by magnetic particle,dye penetrant, radiographic and ultrasonic tech-niques. The flaw type is specified in terms of itsshape (linear, planar, or rounded).

    The critical flaw size to be detected is 10 mmfor a linear flaw, and 4 mm for a rounded flaw.The ultrasonic technique should be able to detecteach of these flaw sizes with an accuracy of20%, i.e. between 8 - 12 mm for a 10 mm linearand between 3-5 mm for a 4 mm rounded flaw.The accuracy would be severely reduced and maynot be practical should the critical flaw size bereduced to a value of, for example, 1 mm. A sec-ond important aspect is how far apart two flaws

    would be located before they could be resolved:the value is of the order of 10 mm. However, fur-ther research is essential to determine the accu-racy of the ultrasonic technique in many differentsituations.

    The currently available ultrasonic methods ofassessing flaw size are the maximum amplitudeand beam spread techniques.

    The use of flat bottomed hole test blocks or theGerman DGS system are irrelevant in this con-text since they express the value of an equivalentflaw size. The actual flaw may be very muchgreater than the flat bottomed hole to which the

    flaw is stated to be equivalent. They should beused, if at all, for equipment calibration and set-ting sensitivity levels.

    The use of the methods which are described andthe accuracy of their use depend primarily on thestructure of the cast component and its ultrasonicattenuation. This factor determines the test fre-quency MHz of the resolving power of the method.

    Other important factors to be considered are theequipment characteristics, setting sensitivity,casting production, shape, surface roughness andthe ability of the ultrasonic operator.

    Beam Spread and MaximumAmplitude Techniques

    probes with the 6 and 20 dB echo and maximumamplitudes of the beam, Figs. 37a and 37b.

    The geometry and intensity within the beamin both the horizontal and vertical directions areused to define the flaw depth and to quantify the

    flaw in terms of shape and size.

    The above techniques use straight and angle --

    The two methods (6 dB and 20 dB) use, initially,the axis or maximum intensity of the beam andthen a selected sound pressure level or isobar tolocate the flaw boundary. The flaw is located, max-imized and recorded, the probe is then movedtowards the edge of the flaw until its echo losesheight to a prescribed amount, i.e. 6 or 20 dB (1/2to 1/10 in. amplitude). The position of the edgeof the flaw relative to the probe is plotted by ref-erence to the known beam profile. The distance atwhich the flaw lies and a datum mark on the probeis, in turn, related to a fixed datum on the com-ponent.

    The maximum amplitude technique simply plotsa defect in relation to either the maximum echoheight or a series of different maximum echoheights at various positions across the detectedflaw. The method is simple and its degree of ac-curacy may be completely adequate for cast steelexamination. A simple comparison of the tech-niques is given in Figs. 38a and 38b.

    The sensitivity setting for ultrasonic examina-tion is to establish a 1-3 mm grass structurelevel on the screen, when the probe is positionedon the cast surface. Once a flaw is detected, itssize, position and type are ascertained by probe

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    upon the cast surface. The depth of the tear, thatis, the Z axis is obtained by measurement of thebeam direction. Due to both the orientation and

    position of the flaw, longitudinal wave probes willbe of little value.

    Visual aids such as those relating to the sub-stitution technique will help the operator to posi-tion and diagnose flaw echoes.

    A further important procedure for the flaw sizeestimation is transverse wave probe manipula-tion. The probe has four degrees of freedom: aswivel movement about the flaw; a circular rotarymovement around the flaw; a lateral movementalong the flaw and a 90 transverse movementtowards and away from the flaw. The extent, sizeand position of each flaw echo on a graduated

    cathode ray screen as each probe movement iscarried out should be noted. These flaw indicationecho changes relate directly to the flaw size andposition in the cast section.

    Figs. 40a and 40b give an example of probe ma-nipulation techniques for the identification and

    movement, using one of the techniques described size estimation of a flaw. In this context a linearabove. planar flaw (crack) being bi-directional is only

    detectable with lateral probe movement along theA two dimensional x - Y axis picture of the flaw flaw, a rounded flaw (gas cavity) is detectable

    may be drawn on the cast surface by assessing over a very small probe movement from any direc-the ratio of the flaw echo to the back echo on the tion.oscilloscope screen. A complete loss of back wallecho with corresponding increase in height of the It should be appreciated that while it is desir-

    flaw echo, indicates a significant flaw. Reference able to scan at a high sensitivity level, minorshould be made to Figs. 39a and 39b. The third flaws may be detected, probe movement will as-dimension, depth or Z axis is obtained from the certain if their size is significant.position of the flaw echo on the graduated screen.

    Surface FlawsTransverse wave probes must be employed when

    the beam is required to enter the casting at an In order to detect near surface flaws it is es-angle relative to the orientation of the flaw. Fig. sential to use a combined longitudinal and a com-39c shows the detection of a hot tear in an L sec- bined transverse double probe which are obtain-tion using a 45 transverse wave probe where the able with different focussing systems. This en-flaw position is favorable and results in a maxi- ables maximum reflection to be obtained from amum reflection when the probe is in position B. flaw at different depths, so helping the operatorThe X - Y axis is measured by probe movement to assess the flaw size at any given position in the

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    cast section. Cross noise occurs with combineddouble probes which may mask the reflection fromnear surface flaws. New electronic systems toovercome this problem have been investigated.

    Beam Spread From Transverse Wave Probes

    (a) Determine the probe index

    (b) Calibrate the time base in terms of steelthickness.

    (c) Move the probe until maximum amplitude

    is obtained from the hole (Fig. 41). Markthe position of the probe index on the block.

    (d) Move the probe forward until the amplitudeis reduced to approximately 1/8 (20 dB) ofthe original value. Mark the position ofthe probe index on the block (Fig. 42).

    (e) Repeat (d) above move the probe back-ward (Fig. 43).

    (f) Repeat the exercise with holes at differentdepths.

    (g) Plot the beam spread on a Plexiglass sheet(Fig. 44).

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    PRODUCTION AND ECONOMICS OF AN ULTRASONIC TECHNIQUE

    One of the main advantages of using ultrasonicsin place of radiography is a decrease in the costof and an increase in the speed of inspection ; thisis of particular value to steel foundries not havingx-ray facilities for radiography of thick sectionsin excess of 50 - 100 mm (approx. 2 - 4 in.). Thesefacts were demonstrated in the early 1960s dur-ing work on the application of ultrasonic methods

    to the examination of steel castings.

    The standard of acceptance for the integritycastings, usually specified by the user, are the

    ASTM radiographic standards. The substitutionof ultrasonics therefore requires, in many in-stances, that flaws detected by the operator shouldbe assessed in ultrasonic flaw severity in terms ofASTM radiographic standards. This has partlybeen achieved by a current project and the resultsdescribed later indicate that direct correlationmay be obtained between ultrasonic examinationand the ASTM radiographic standards for theshrinkage flaws in the 50 - 100 mm (approx. 2 - 4

    in.) thickness range.

    The ultrasonic/radiographic substitution tech-nique is completely dependent on the productionof a written procedure for the examination of thecasting, This procedure is outlined in Fig. 45. Themain features are the use of longitudinal andshear wave beam spread diagrams (Figs. 46-46d)

    calibration values and the use of a full size castingdrawing. Full details of the scanning procedureand calibrations are essential in order to ensurecomplete coverage. It is no longer possible to useincompetent untrained operators who do notunderstand the full significance of the highlyskilled and important job which they undertake.Neither should the casting be examined by slop-ping couplant all over the cast surface and expect-ing the examination to be complete within 20 min-utes. The technique requires trained, competentoperators who fully understand steel casting pro-duction and who have the full co-operation ofmanagement .

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    A successful exercise of substituting radiog-raphy by ultrasonics was carried out on a valvecasting weighing 2 tons; its dimensions were asfollows: 500 mm (20 in.) diameter x 1200 mm(4 ft) long, barrel thickness 80 mm (3 1/2 in.)valve seat 200 mm (8 in.). A photograph and sec-tional diagram of the valve casting is shown inFig. 47.

    Originally, full radiography was specified andused. The technique is detailed in Table VI, Figs.48a - 48c.

    The second stage was to radiograph the valveseat and butt weld ends only, substituting anultrasonic technique for the barrel section. In thefinal stage the butt weld ends only were radio-graphed. This required a further ultrasonic tech-nique to be substituted for the more difficult valveseat examination, Table VII, Figs 49a - 49d.

    The commercial implication of the substitutionwas evidently favorable, Table VIII. The cost forfull radiography was $560 ($1120). By substitut-ing ultrasonic testing on the barrel the cost wasreduced to $340 ($680), a substantial decrease.A further lowering in cost to $240 ($480) was

    also possible by using an ultrasonic technique onthe valve seat. This latter substitution is a moredifficult examination due to the complex shape ofthe valve seat and makes further demands on theultrasonic operator.

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    This example shows that the substitution ofultrasonics for radiography can result in substan-tial economies. In this connection the followingfactors are essential :

    (a) The NDT Manager must be fully conver-sant with ultrasonic technology, ASNT Level III.

    (b) The ultrasonic operator should be trainedand certified to ASNT Level II- steel castings.

    (c) A written ultrasonic procedure, Fig. 45,must be produced by an ASNT Level III person

    which, together with the ultrasonic technique,shows that the whole volume of the casting hasbeen completely examined. Where changes in sec-tion occur full use must be made of the wholerange of longitudinal and shear wave probes.

    An ultrasonic test report must be produced upontest completion and cover the following:

    1) Procedures (techniques, calibration, refer-ence blocks, sensitivity setting, acceptancestandards)

    2) Results (location and size of recordableflaws, description of flaw type where pos-

    sible)3) Conclusions (acceptability of flaws relative

    to standards)

    REFERENCES

    1. Atlas of Some Steel Casting Flaws as shown by NonDestructive Testing, Steel Castings Research andTrade Association, Sheffield, England, 1968.

    2. Recommended Procedure for the Ultrasonic Examina-tion of Steel Castings, Steel Castings Research andTrade Association, Sheffield, England, 1971.

    3. ASTM A609, Standard Specification for LongitudinalBeam Ultrasonic Inspection of Carbon & Low AlloySteel Castings.

    4. ASME Boiler & Pressure Vessel Code, Section V,

    T524.2, Angle Beam Examination of Steel Castings.

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