UT of SS

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Ultrasonic Testing Of Austenitic Stainless Steel

Presented byPresented byN.KuppusamyN.Kuppusamy

Singapore Chapter

NDT HORIZONNDT HORIZON

Module 18A

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INTRODUCTION

• IN THIS PRESENTATION WE ARE GOING TO DICUSS THE FOLLOWING:

• PROBLEMS ENCONTERED DURING ULTRASONIC TESTING• HOW TO OVERCOME THE PROBLEMS• CALIBRATION AND SENSITIVITY SETTIN G

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INTRODUCTION

The main practical implications of this are:• Welding procedure and preparation geometry have a strong

influence upon the capabilities of ultrasonic examination, so that careful consideration of these factors at the design stage can be very beneficial to the examination.

• Many technical aspects of the examinations are strongly influenced by the particular weld structure.– Only skilled, specially trained operators with a full knowledge

of the physical basis of the examination should be employed

Until recent years, austenitic steel welds were widely regarded as uninspectable by ultrasonics. Research and development have made it possible for a useful level of examination to be carried out in many situations. In general, though, the methods are more complicated and the capabilities more limited than for the examination of welds in ferritic steel.

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Problems involved in the Examination of Austenitic Welds • The term austenitic covers a variety of materials and material

combinations, including austenitic stainless steels and nickel chromium alloys such as "Inconel", "Incoloy", etc.

• The capabilities of ultrasonics for the examination of welds in austenitic materials are restricted compared to the ferritic casebecause of the presence of large elongated anisotropic grains(dendrites), often forming an ordered columnar structure, which are characterisitic of the austenitic weld metal.

• This type of grain structure can lead to anisotropic ultrasonic behavior contrasting with the isotropic behavior of homogenous welds made in carbon or low alloy steels.

SOLIDIFICATION BY THE FORMATION OF COLUMNAR GRAINS

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Fine and coarse grained steelat the same magnification

Fine grained steel Coarse grained steel

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Problems involved in the Examination of Austenitic Welds

• The size, the arrangement, and the elastic anisotropy of the different grains result in high scattering associated with mode conversion effects, beam distortion, and a variation of ultra-sound velocity with direction and position in the weld.

• The scattering of energy is observed as a relatively high noise level (grass) and high attenuation.

• The problems which occur in ultrasonic testing of austenitic welds differ according to the parent material production method (rolled, drawn, forged, or cast), the weld processes, andthe heat treatment as well as the composition of the parent and weld metals.

L waveShear wave

DistortionMode conversions

Mode conversion1

Mode conversion

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INTRODUCTION Cont’d …

• The capabilities for defect detection, positioning and size assessment are more limited than for ferritic weld examination.

• So, monitoring the occurrence of small defects can rarely be used for the quality control of welds, as is usual with ferritic welds.

• It may be necessary to use fracture mechanics to set less rigorous defect acceptance standards for the particular component. These acceptance standards should be compatible with the limitations of the ultrasonic techniques.

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Difference• The ultrasonic methods applied to austenitic welds follow basically

the same principles as Ultrasonic Examination of Welds. • Some important differences do exist, to detect, locate, characterize,

and to estimate the size of weld defects. • The most important of these differences are the following:

– Scattered energy from natural metallurgical discontinuitiesgenerates noise indications at higher amplitude than would be expected for the case of ferritic welds.

Shear wave

– The choice of wave mode (longitudinal; shear) and probe characteristics (sound field, frequency, bandwidth, etc.) should be optimized to allow a reliable separation of weld defect indications from noise indications.

Scattering

S/N

Beam profile

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Difference– The ultrasonic beam has to cross different regions in the parent metal

and in the weld itself. The velocity of sound may vary along this path and this may change the direction of the sound beam. Consequently, this may result in inaccuracy in determining reflector positions.

– Attenuation in the weld metal is generally more severe than for ferritic welds and can be more or less pronounced depending on the angle of the beam with respect to the preferred orientation direction of the grain structure.

Therefore, the ultrasonic technique should seek to minimize beam path length in the weld metal and, where possible, aim to take advantage of any directions of reduced attenuation in the weld.

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Difference

– Beam divergence can also be directionally dependent. The beam profile is usually different from that measured in parent plate (whether ferritic or austenitic) so that size estimation methods which depend on a knowledge of the beam profile, such as the so-called dB drop methods, are not always suitable on austenitic welds.

– Conventional instruments are used for examinations, but in most cases, special probes need to be applied

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4001,5001,5005,0001

2501,0007503,0002

251001002005

Grey Iron(mm)

SG Iron(mm)

Coarse Grained

Steel(mm)

Fine Grained

Steel(mm)

Frequency(MHz)

Typical maximum test ranges for compression mode

• These are typical ranges. In practice, maximum range will depend on the probe design, equipment, pulse strength, probe diameter and specific material grain structure.

• For shear waves, which have approximately half the wavelength, the maximum shear wave ranges are approximately equal to a compression wave of twice the frequency in the table above. For example 2 MHz shear has a similar test range to 4 MHz compression.

• The improved penetration at low frequencies is obtained at the expense of reduced sensitivity to smaller discontinuities

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Testing of Austenitic Welds

• Austenitic welds provide special challenges due to the nature of the coarse weld metal grains

• Austenitic stainless steels are the ‘300’ series, such as the 304, 309, 316, and 321 grades.

• The ‘400’ series are not austenitic, but are generally welded with type 300 welding filler, so they have a weld metal which is substantially austenitic.

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Testing of Austenitic Welds

• The word ‘austenitic’ means that the material has not undergone a transition to the ferritic form, which is what happens with normal carbon and low alloy steels.

• The consequence is that the grains of austenitic weld metal are very large and have a different attenuation and acoustic velocity to the parent metal.

• In contrast, conventional weld metals have a much finer grain size, and similar acoustic velocity to the parent metal.

• The large grain size of austenitic weld metal means it has a very high attenuation.

Austenitic welds provide special challenges due to the nature of the coarse weld metal grains

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Testing of Austenitic Welds . . .

• Complicating the issue is that these large grains are ‘anisotropic’.

• This means that they have different properties in different directions within each grain.

• This means that each grain boundary can be an interface.

• The degree of bending of the beam can be as much as 40° for shear waves and 20° for compression waves.

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Effects of Austenitic Structures on Ultrasound Propagation

• The effect of austenitic structures on the behavior of ultrasound depends largely on grain size.

• Small grains, as found in rolled plate, have no adverse effect on sound propagation.

• On the other hand, coarse grain cast structures and those in thewelds have marked effects, leading to increased scatter and attenuation, variations in sound velocity, and often to beam-distortion.

Simple apparatus to demonstrate ultrasound propagation behavior

• The effects are primarily due to the anisotropic nature of the austenitic grains.

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Velocity change with refracted angle

Elastic anisotropy leads to variations in the propagation velocity of ultrasound waves. In general, the propagation velocity depends on the angle between the wave front and the major axes of the columnar grains.

Longitudinal Wave (CL)

Shear wave (horizontally polarized -CTH)

Shear wave(Vertically Polarized-CTV)

For the specimens with their axes machined parallel to the surface of the original weld sample the amplitude of the transmitted signal varies systematically

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Conditions to be stabilized1. Personnel – already discussed2. Required information About the Welds.

To obtain an effective ultrasonic technique with optimum flaw detection capabilities (e.g. choice of optimum probe parameters), it is necessary to collect maximum information on the weld characteristics.1. Weld geometry - Selection of probe parameters depends on

weld preparation and on the degree of penetration.2. Welding process - Knowledge of the weld process and

procedure will contribute to an estimate of the likely grain structure

3. Heat-treatment - Information on the heat treatment cycle is useful to provide an estimate of the parent material grain size. Using this information, the likely transparency can be assessed.

4. Representative Weld samples (in terms of weld preparation and process, procedure, geometry, heat treatment, heat input, etc) to assess the possible beam deviations resulting from the angle of the weld fusion faces

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Conditions to be stabilized3. Surface Preparation and Marking• Full volumetric inspection requires dressing of the weld crown. • When the weld is fully ground, the operator will not be able to visually

identify the region to be examined (or a repaired zone) unless these areas are clearly marked.

• A good practice is to mark the surface by punch marks. These marks may be made after completion of the weld to indicate the real fused zone of the weld.

• The material surfaces must allow free movement of the probe(s) and provide satisfactory conditions for the transmission of the ultrasonic waves.

• Therefore, the surface roughness should generally not exceed 20 µm and the waviness should not exceed ± 0.5 mm over any area of 50 mm x 50 mm.

• This is necessary to avoid disturbance of the ultrasonic beam which could reduce the sensitivity and result in errors in defect location.

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Conditions to be stabilized3. Surface Preparation and Marking• Where there is access to only one surface of the weld, any weld

reinforcement must be ground off if the whole weld volume is to be examined and if shear waves cannot be applied. This is because mode conversion losses on reflection reduce the effectiveness of examination in the second traverse with angled compression waves.

• Furthermore, on each side of the weld for a minimum of 5/4 skip (1¼skip) distances, the surface should be free from weld spatter, loose scale, machining and grinding particles, dirt, paint, or other foreign matter.

4. Condition of the Parent Metal• To allow adequate penetration of ultrasonic waves into the weld, the

parent metal should be transparent to ultrasound and free from large flaws.

• As a guide, difficulties in penetrating parent material can become severe for grain sizes larger than ASTM 3 (average grain size 0.125 mm.)

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Conditions to be stabilized5. Agreements before the Start of the Examination• To avoid misunderstanding and arguments about the examination to be

carried out, a number of conditions or directives need to be established prior to the examination. 1. Extent of the Examination 2. Sensitivity Required 3. Special Conditions 4. Regular Check of Equipment 5. Reporting

6. Visual Inspection• The visual appearance of the welded joint should be recorded with

particular reference to visible defects and the shape of the weld, • e.g. surface curvature, degree of root penetration, backing ring,

different parent metal thicknesses, extent of the reinforcement,presence of undercut, weld finish, and alignment of parts.

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Conditions to be stabilized7. Couplants• A couplant, usually a liquid or semi-liquid, is required between the face

of the probe and the surface being examined to permit transmission of the acoustic energy from the transducer to the material under test.

• Typical couplants include water, oil, grease, and glycerine. • The couplant used should form a film between the probe and the test

surface. • It should not be injurious to the material to be tested - or disturb

subsequent surface treatment.• This is of particular importance in examining austenitic materials where

coupling residues may cause problems such as stress corrosion cracking in service.

• Couplants containing halogens and sulpher are to be avoided.

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Testing of Austenitic Welds . . .

• The best conditions for examination of austenitic weld metal occur when:– the compression mode is used, as its longer wavelength

results in lower attenuation, and less beam bending occurs, and

– the beam travels either parallel or at approximately 45° to the grain orientation for minimum bending.

– Effective testing needs to be a cooperative effort between the welding engineer and ultrasonic specialist and should be well planned.

– Do not be frightened by austenitic materials – just follow basic principles.

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Testing of Austenitic Welds . . .• Compression probes are generally used when ultrasonic

testing of austenitic welds• Compression angle probes are constructed with very short

pulse lengths, with a wedge angle less than the first critical angle, such that there is a shear and compression wave generated in the material. The shear wave travels at a slower velocity than the compression mode, and can generally be disregarded after careful interpretation.

• It is also risky to skip off far walls to test a weld, as the process of skipping off a surface will cause the compression wave to split off yet another shear component, which will confuse interpretation, as well as weakening the primary compression mode.

1. Shear wave2. Compression wave3. Reflected shear wave4. Mode converted compression wave5. Mode converted shear6. Reflected compression wave

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Testing of Austenitic Welds . . .

• Note that this does not make it impossible to test austenitic welds by skipping, but it does raise the complexity.

• There have been some very innovative special test procedures developed that have used the mode converted shear, or other waves to their advantage, but we will leave those to the experts to worry about. Many of these procedures use separate transmit and receive probes (tandem tests).

• Where possible, it is preferable to keep it as simple as possible by working within the half skip section of the beam path and ignore the echoes that follow!

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CalibrationCalibration is different to conventional angle shear wave probes• Calibration is best carried out with a V2 (AS2083 Block 5)

block made of austenitic steel.Range• It is possible to calibrate the range with a zero compression

probe, as the acoustic velocity is the same for the angle and zero compression beams. Calibrate the range as if using a zero compression probe.

Zero• Set the zero as you would for a conventional angle probe using

the 50 mm radius of the V2 Block. Be careful that you calibrate against the shortest reflection, as the longer one may be a shear wave.

Sensitivity and DAC• This is best done using a block similar to the IOW Block

(AS2083 Block 2). Be careful when setting sensitivity, because the shear wave will also be striking holes and reflecting back to the probe. Make sure by calculation and measurement that you are striking the correct hole with the right mode!

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Calibration Block for examination of Austenitic Welds• This block is one design that may be useful for setting sensitivity and

has a series of inclined surfaces and reference holes for verifying equipment performance.

• Reference blocks welded with similar materials and welding procedures are invaluable aids to effective testing and final validation.

Validation should also be made on a reference blockcontaining a weld of similar geometry and welding procedure, with reference reflectors located at key point in the cross section to demonstrate that the combination of weld and test procedure are compatible. This will demonstrate if there are any major anomalies due to beam bending or variations in attenuation.

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Validation block for Austenitic steels• This block allows the performance to be verified when scanning through

both weld and parent metal.• Austenitic plate does not have the same limitations as welds or castings

due to the grain size reduction in rolling.• Fortunately, austenitic steel plate has very similar ultrasonic properties

to carbon and low alloy steel plate, so can be tested similarly, with an adjustment to calibration for the slight variation in acoustic velocity.

• In certain situations, you can use conventional zero and compression probes to undertake a useful examination of an austenitic weld by approaching the fusion faces through the parent metal.

• This is of course a limited inspection, but may be useful in some instances if you do not need a volumetric scan of the weld metal and have limited probes.

SKETCH TO CKECK

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Summary

Wave Type• The use of conventional shear wave probes should always be

considered first. If the signal-to-noise ratio is insufficient for an effective inspection,

• Special probes like refracted longitudinal angle beam probes, including the surface wave type, need to be used.

Selection of ProbesThe major variables to be considered when selecting a probe are(1) wave type(2) angle(3) frequency(4) type of probe(5) size and geometry of probe and component

Signal-to-Nolse RatioFor a reasonable ultrasonic examination, the noise level should be at least 6 dB below the recording level for the whole sound path

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Summary

Probe Angle• In general, the probe angle should be selected to be suitable

for the defect orientation expected.• This means that if possible, the incident angle should be chosen

to strike a defect perpendicularly for maximum echo amplitude.• For weld examination,it istherefore necessary to know the weld

geometry priorto and after welding; • Particularly the details of the weld root if this is not machined

or ground flush.

Recording levelIn the absence of any experience and of any prescribed criteria, 50% of the DAC for reference reflectors indicated through the weld might be a good first approach. In certain cases, it may be necessary to go down to 25%. However, the recording level in the appropriate depth zone should be at least 6 dB higher than the maximum noise level in that depth zone.

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Probe Angle

The entire volume of weld and fusion zone should be covered with at least two angles of incidence is shown in the Figure.

1. Surface and near surface area can be tested with 70º (2MHz) probe.

2. Rest of the weld volume and Root can be tested with 45º probe.

If the weld penetration is not ground flush, geometrical or false echoes may be generated with a 45° probe, and

for that reason, a probe witha larger angle (60º) should be used to evaluate the root along with 45º probe.

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Probe Angle

• The choice of probe angle largely depends on the weld preparation and weld surface condition.

• It should also be remembered that examination at more than halfskip distance is not very effective due to mode conversion effects at the backwall.

• To reach the lower part of the weld with large angle probe requires a long sound path and this can also reduce the effectiveness of the examination

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Special TechniquesSurface wave technique• A derivative of the angled longitudinal wave probe is the "surface

wave" probe. • The surface wave is generated at the first critical angle of incidence,

as shown in the Figure, and propagates along the surface as a compression wave.

• It is also referred to by other names, e.g. head wave, lateral wave, fast surface wave at the first critical angle. Unlike Rayleigh waves, the surface waveis not damped by couplants on the component surface, nor does the beam follow undulations in the surface.

• A surface wave probe generates– compression waves at large angles between 70° and 90°– shear waves according to Snell's Law

Probe

33º S wave

Primary Surface waves

Secondary Surface waves

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Special TechniquesSurface wave technique• The surface wave sound velocity is identical to that of compression waves.• Although it generates a beam with "complicated“ characteristics, it is a very

useful probe for detecting surface defects.• A surface wave probe can also be considered for inspection of the weld

root. The 33° shear wave component of the surface wave probe converts to a secondary surface wave at the backwall of the component, as shown in the Figure.

• In this case, the weld penetration echoes can largely be eliminated (because it does not follow contour).

• The advantage of a 70° probe with a strong surface wave component is useful for thin materials and near surface evaluation of thick materials.

Probe

33º S wave

Primary Surface waves

Secondary Surface waves

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Special Techniques

Tandem TechniquesSpecial tandem techniques as shown in the figure can be used as directed by experts.Longitudinal – Longitudinal waves tandem technique (or)Longitudinal – Shear wave tandem technique can be used.

R

T

T R

45ºL

70ºL70ºL

70ºL

31ºS

31ºS

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Rate of probe movement

• The rate of probe movement shall not exceed 50 mm/sec, unless the examination capability has been verified at the higher scanning speed.

• This reduced speed is because of the low signal-to-noise ratio compared to ferritic welds.

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FrequencyFrequencies from 1 mHz to 5 mHz can be used, depends up on thickness and graininess of the material.

– For thickness over 25mm 2 & 1 mHz probes are recommended for course grained materials.

– For thickness up to 25mm 4 & 2 mHz are predominently used.

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Type of ProbeVarities of probes are available for selection. Following probes can be selected for testing. The selection is often orbitrary.

– Single Crystal (Advantage of regularly decreasing DAC)0º Longitudinal wave probe for on the weld scanning.45º, 60º, 70º Longitudinal wave probes for weld scanning (limited to half skip)45º, 60º, 70º shear wave probes for weld scanning (can be used up to full skip & useful for low thickness materials).Surface wave probes for top & bottom near surface testing.

– Twin Crystal (eliminates dead zone)45º, 60º, 70º Longitudinal wave probes for weld scanning (limited to half skip)45º, 60º, 70º shear wave probes for weld scanning (can be used up to full skip & useful for low thickness materials).

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size and geometry of probe and component

• The probe should be of a size which allows manual examination with good acoustic coupling.

• Coupling involves an interaction between probe size, roughness, and the geometry of the component.

• If the probe dimension W fails to meet the requirement:R > W2/4, (dimensions in mm) orthe coupling gap exceeds 0.5 mm.

– The probe shoe should be adapted to the geometry of the component.

• R = Radius of the curved surface under examination and • W = Probe dimension (length or width).

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Multi TRL Probe Concept

• In the case of thick welds, more probes are necessary.

• The advantage in more probes is that the probe angle can be an optimized selection for flaw detectability in each depth zone.

• All compression wave TR probes have a very low sensitivity in the first part of the range behind the acoustic zero point (mechanical zero).

• This causes a very small dead zone.

• The surface wave probe fully eliminates this effect as shown in the Figure.

100MM

40MM

20MM50

IIIIII

IV

TR Creep 70 TRL 60 TRL 45 TRL

f10 f25 f50 f75

PROBE 1 2 3 4

( f = Transit Distance at Focus)

f

f

f

DEPTH (THICKNESS)

f

f

f

f f f

1 2 3PROBE

0 dB

-6 dB

80%

60%

40%

20%

0

I II III IV ZONE

0 5 20 40 100mmDEPTH (THICKNESS)

4

AM

PLIT

UD

E

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End of Session(End of Tech-Talk)