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Course 30001 Reader: Non-destructive Testing Page 2

Course 30001 Reader:

Non-destructive Testing

This document contains the web-based learning materials for this course.

Contents

Introduction and objectives.................................................................................4

Overviews and applicability of NDT methods....................................................5Overview of defects in materials ........................................... ................................... 5Common defects in cast materials. ........................................ ................................. 11Common defects in forged or rolled materials. ...................................... ................ 12

Overview of NDT methods ...............................................................................13Visual inspection (VT)..................................................... ....................................... 13Radiographic testing (RT) ......................................... ......................................... .... 14

Magnetic Particle Inspection (MT)................................................. ........................ 16Liquid Penetrant Testing (PT) .................................... ........................................ .... 17Eddy Current testing (ET)................................................... .................................... 18Applicability of NDT methods on different material defects ................................. 19

NDT methods Visual inspection....................................................................21Inspection Inspection during welding........................................ ............................. 24Inspection after welding................................. ............................................ ............. 25Imperfections associated with welding.......................................... ......................... 28Inspection reporting and records.................................. ........................................... 41

NDT Methods Radiographic Testing.............................................................42Introduction..................................... ........................................... ............................. 42The radiographic process. ........................................ ....................................... ........ 43

Quality of radiograph......................... ........................................ ............................. 47Film interpretation. ........................................... ........................................... ........... 51Advantages and limitations of radiographic testing................................................ 75

NDT Methods Ultrasonic Testing..................................................................77Definition of ultrasound and properties of waves........................................ ........... 77Methods ...................................... ........................................... ................................. 77Performance of ultrasonic testing ....................................... .................................... 80Measurement of thickness and detection of defects................................................ 91Advantages and limitations of ultrasonic testing ..................................... ............... 94

NDT Methods Magnetic Particle Testing ......................................................95Application ........................................ ........................................... .......................... 95Method........................................ ........................................... ................................. 95Magnetization principles and methods ...................................... ............................. 95MT Performance............................................... ........................................... ........... 97Surface preparation........................................... .......................................... .......... 100Examination of welds .......................................... ........................................... ...... 100Non-relevant indications............. ............................................... ........................... 102Advantages of the MT method ........................................... .................................. 102Limitations of the MT method........ ........................................ .............................. 102Demagnetization ........................................ ........................................... ................ 102Acceptance criteria ....................................... ........................................ ................ 103Reporting ........................................ ........................................... ........................... 103

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NDT Methods Liquid penetrant testing .......................................................104Introduction..................................... ........................................... ........................... 104Penetrant Testing Materials. ........................................ ....................................... .. 104Method........................................ ........................................... ............................... 107Surface preparation........................................... .......................................... .......... 108Types of penetrant ............................................ ........................................... ......... 108Types of developer............................................ ........................................... ......... 112

Penetration and developing time.................................. ....................................... .. 112Evaluation of indications ..................................... ........................................... ...... 112Acceptance criteria ....................................... ........................................ ................ 114Reporting ........................................ ........................................... ........................... 114NDT procedure specifications and reports (examples)....................... .................. 114Advantages and Disadvantages of Penetrant Testing (PT)........ ........................... 132

NDT Methods Eddy Current Testing...........................................................135Introduction..................................... ........................................... ........................... 135Electromagnetic Effects............................................... ......................................... 137Eddy Current Generation and Detection.............. ........................................ ......... 137Factors affecting Eddy Currents ..................................... ...................................... 141

NDT-methods Alternating current field measurement ................................147

Introduction to ACFM .................................. ........................................... ............. 147Basic ACFM theory.......................................... .............................................. ...... 149Benefits and limitations ....................................... ........................................ ......... 154General applications ......................................... ........................................... ......... 155Comparison of ACFM ...................................... ........................................... ......... 162ACFM examples........................................ .............................................. ............. 164

Other NDT Methods .......................................................................................167Leak testing............ ........................................... ........................................... ......... 167Thermographic inspection ....................................... ........................................ ..... 168Plastic replica method.......................................... ........................................... ...... 168Acoustic emission.................................. ........................................... .................... 169

Probability of detection (POD) .......................................................................170American Society for Non-Destructive Testing ASNT...................................... .. 176Document No. CSWIP-ISO-NDT-11/93-R Requirements for the Certification of Personnel

Engaged in Non-Destructive Testing............ ........................................ ................ 179EN 473:2000 Qualification and Certification of Non-Destructive Personnel General

Principles ........................................... ............................................... .................... 181ISO 9712:1999 Non-Destructive Testing Qualification and Certification of Personnel

...................................... ........................................... ........................................... .. 183Personnel Certification in Non-Destructive Testing (PCN) United Kingdom PCN Scheme

...................................... ........................................... ........................................... .. 183Japanese Scheme for Certification of NDT Personnel.......................................... 184Nordtest Scheme for Examination and Certification of Non Destructive Testing Personnel...................................... ........................................... ........................................... .. 185

NDT standards ................................................................................................186General................................. ........................................... ...................................... 186Current NDT standards etc .................................. ........................................... ...... 186

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Introduction and

objectivesMany standards and codes require non-

destructive testing. In some cases the testing

methods to be used are specified. In cases

where more than one method is permissible,

the DNV surveyor/inspector may be called on

to specify the method. Whether the inspection

method is specified or optional, it is

important for the inspector to have sufficient

knowledge of the advantages and limitations

of common non-destructive testing methods,

and how they relate to different defects in

materials and welds.

The objective of the netbased training module is to acquaint the participants with thefundamentals of non destructive testing. The level of NDT knowledge shall be sufficient

to describe basic principles, advantages and disadvantages of the major non-destructive

testing methods, operator certification, interpretation of NDT reports and acceptance

criteria.

In particular the participants shall be familiar with:

The importance of visual inspection.

The application of radiographic testing and its dependence on weld joint location,

joint configuration, material thickness, etc. and principals of basic radiographic film

interpretation.

The use of ultrasonic testing and the basic steps in performing a pulse echo

examination.

The characteristics of magnetic particle testing, and the basic steps in performing

testing.

The use of liquid penetrant and the basic steps to performing testing.

The use of eddy current equipment and the basic steps for performing testing.

The use of alternating current field measurement equipment and the basic steps for

performing testing

Leakage tests, plastic replica technique, and acoustic emission methods.

The reliability of the inspection process, probability of detection.

Certification schemes and the required level for qualification and certification of

personnel performing NDT.

The necessity of documented procedures and knowledge of international standards.

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Overviews and applicability of NDT

methods

Overview of defects in materials

Common defects in connection with welds.

Reference is made to the figure below where some of the defects described are

illustrated.

1. POROSITY

2. SLAG INCLUSIONS

3. SLAG LINES

4. LACK OF FUSION

5. INCOMPLETE PENETRATION

6. UNDERCUT

7. UNDERFILL

8. OVERLAP

9. LAMELLAR TEARING

10. SURFACE CRACK

11. INTERNAL CRACK

13. LAMINATION

Weld joints showing the.most common defects referred to in section 2.1

Porosity:

Porosity is the result of gas being entrapped in

solidifying metal. The discontinuity formed is

generally spherical but may be cylindrical.

Unless porosity is gross, it is not as critical a flaw as

sharp discontinuities that intensity stress. Porosity is

a sign that the welding process is not being properly

controlled or that the base metal is contaminated or

of vanable composition.

Uniformly scattered porosity is porosity uniformly distributed throughout a single pass

weld or throughout several passes of a multiple pass weld. Whenever uniformly

scattered porosity is encountered, the cause is generally faulty welding technique or

materials. Porosity is present in . a weld if the technique used or materials used or

conditions of the weld joint preparation lead to gas formation and entrapment. If welds

cool slowly enough to allow gas to pass the surface before weld solidification, there will

be little porosity discontinuities in the weld.

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Surface breaking pores

Uniformly Distributed porsity

Elon ated ores or wormholes

a) Poor (convex) weld bead profile

resulted in pockets of slag being

b) Smooth weld bead profile allows the

slag to be readily removed between runs

Radiograph of butt weld showing two slag

lines in the weld root. The influence of welder

technique on the risk of slag inclusions when

welding with a basic MMA (7018) electrode.

Cluster porosity is a localized grouping of pores

that results from im-proper initiation or termination

of the welding arc.

Linear porosity is porosity aligned along a joint

boundary, the root of the weld, or an interbead

boundary.

Piping porosity is a term for elongated gas

discontinuities. Piping porosity in fillet welds

extends from the root of the weld toward the

surface of the weld. Much of the piping porosity

found in welds does not extend to the surface.

Piping porosity in electroslag welds can become

very long.

Inclusions

Slag inclusions are nonmetallic solid material entrapped inweld metal or between weld metal and base metal. They

may be found in welds made by most arc welding

processes. In general, slag inclusions result from faulty

welding techniques and the failure of the designer to

provide proper access for welding within the joint.

Slag lines are elongated cavities usually parallel to

the axis of the weld, which contain slag or other

foreign matter.

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Lack o side wall usion

Lack of inter-run fusion

Excessively thick root face

Too small a root gap

Power input too low

Arc (heat) input too low

Lack of fusion

Lack of fusion is the result of improper welding

techniques, improper preparation of materials for

welding or improper joint design. Deficiencies causing

incomplete fusion include insufficient welding heat or

lack of access to all boundaries of the weld joint that

are to be fused during welding, or both.

Incomplete penetration

Incomplete penetration is joint penetration which is less

than that specified. Technically, this discontinuity may

only be present when the welding procedure

specification requires penetration of the weld metal

beyond the original joint boundaries. Inadequate joint

penetration may result from insufficient welding heat,

improper joint design (too much metal for the welding

arc to penetrate) or improper lateral control of thewelding arc.

Undercut

Undercut is generally associated with either improper welding techniques or excessive

welding currents, or both. It is generally located at the junction of weld and base metal

(at the toe or root). Undercut discontinuities create a mechanical notch at the weld

fusion boundary (see figure in the chapter on Visual Inspection).

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Lamellar tearing in t butt weld

Appearance of fracture

face of lamellar tear

Underfill/excess weld

Underfill is a depression on the face of a weld or root surface extending below the

surface of the adjacent base metal. It results simply from the failure of the welder or

welding operator to completely fill the weld joint as called for in the welding procedure

specification.

Overlap is the protrusion of weld metal beyond the toe, face, or root of the weld withoutfusion. It can occur as a result of lack of control of the welding process, improper

selection of welding materials or improper preparation of materials prior to welding.(see

figure in the chapter on Visual Inspection)

Excess weld

reinforcement is, in the

root of the weld, (see

figure at right) caused

by improper fitup

and/or welding

technique. On the top

(see figure in the

chapter on Visual

Inspection) it may be

caused by one or more

of the following factors:

too low travel speed,

too low current, poor

planning of the welding

sequence and bead size.

Cracks

Lamellar tearing (cracks) are generally terracelike separations

in base metal typically caused by thermally induced

shrinkage stresses resulting from welding.

Cracks occur in weld and base metal when

localized stresses exceed the ultimate strength of

the material. Cracking is generally associated

with stress amplification near discontinuities in

welds and base metal or near mechanical notches

associated with the weldment design. High residual stresses

are generally present and hydrogen embattlement is often

a contributor to crack formation. Cracks may be termed

longitudinal or transverse, depending on their orienta-

tion. When a crack is parallel to the axis of the weld it is

called a longitudinal crack regardless of whether it is a

centerline crack in weld metal or a toe crack in the heat-

affected zone of the base metal. Transverse cracks are

perpendicular to the axis of the weld.

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Brittle fracture in crmov steel pressure vessel probably

caused through poor toughness, high residual stresses and

hydrogen cracking

Crack in flange to drive shaft weld

Solidification crack along the centre line of the

weld

Longitudinal cracks in

submerged arc welds

made by automatic

welding processes are

commonly associated

with high welding

speeds and sometimesrelated to porosity

problems that do not

show at the surface of

the weld. Longitudinal

cracks in small welds

between heavy sections

are often the result of

high cooling rates and

high restraint.

Throat cracks are

longitudinal cracks

in the face of the

weld in the

direction of the

axis. They are

generally, but not

always, hot cracks.

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Cenre-line crack in weld ca weld

Root crack in weld between bulkhead and tanktop,

material: duplex capweld

Root cracks are

longitudinal cracks in

the root of the weld.

They are generally

forms of hot cracks.

Crater cracks occur in the crater formed by improper termination of a welding arc. They

are sometimes referred to as star cracks though they may have other shapes. Cratercracks are shallow hot cracks usually forming a multipointed star-like cluster.

Toe cracks are generally cold cracks. They initiate and propagate from the toe of the

weld where restraint stresses are highest. Toe cracks initiate approximately normal to

the base material surface. These cracks are generally the result of thermal shrinkage

strains acting on a weld heat-affected zone that has been embrittled by hydrogen or an

excessive cooling rate, or both.

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Surface or subsurface blowholes

Underbead and heat-affected zone cracks are generally cold cracks that form in the heat-

affected zone of the base metal. They are generally short but may join to form a

continuous crack.

Common defects in cast materials.Castings with wrong dimensions or indentations are usually the result of dimensional

errors in the pattern, incorrect design of pattern and mold equipment, or an uncontrolled

casting process. Such defects should be revealed by visual examination using proper

tools and measuring devices. The most obvious surface defects should also be

discovered at this stage.

The less obvious surface defects and internal defects may be revealed by use of other

NDT methods. The most common types of such defects are:

Segregation

Local concentration of alloying elements or harmful impurities with the result that

ingots have a heterogeneous structure, with maximum impurity concentrations in thelast regions to solidify, i.e. around any central pipe which may be formed. Smaller areas

of segregation elsewhere result from the entrapment of liquid zones between growing

solidifying crystals, as in the case of ingot corner segregation. Segregations may affect

the mechanical properties and weldability.

Shrinkage

Cavity voids resulting from solidification shrinkage. The growth of dendrites during the

freezing process may isolate local regions, preventing complete feeding from the

risers.

Pipe

The central shrinkage cavity in the feeder head of a casting.

Inclusions

Non-metallic materials in a solid metallic matrix. Common inclusions include particles

of refractory, sand inclusions, slag, deoxida-tion products, or oxides of the casting

material.

Gas porosity

Voids caused by entrapped gas,

such as air or steam, or by the

expulsion of dissolved gases during

solidification.

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Blow holes inholes

uench crackin !

Crack

A discontinuity formed in the surface, with length and depth substantially greater than

the width. The origin of cracks varies. Hot cracks

are fractures caused by internal stresses thatdevelop after solidification and during cooling from

an elevated temperature (above 65QC). A hot

crack is less visible (less open) than a hot tear and

usually exhibits less evidence of oxidation and

decarburization. Stress cracks result from high

residual stresses after the casting has cooled to

below 650 C. Stress cracks may form at room

temperature several days after casting.

Common defects in forged or rolled materials.

Many of the defects typical for cast materials will still appear as defects after forging or

rolling of e.g. a faulty ingot.

Lamination

is as excessive large laminar, non-metallic inclusion embedded in the material.

Laminations are usually caused by shrinkage cavities present in the upper section of an

ingot enlarged by the forging or rolling process.

Inclusions

In rolled and forged, materials inclusions are elongated in the work direction. Such

elongated inclusions are the main cause of the anisotropy of rolled steel plates.

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Overview of NDT methods

Visual inspection (VT).

Method

The test object is subjected to examination by the experienced eye of an inspector

assisted by vieing aids and measuring gauges.

Application/advantages

The method may be used on all objects cast, rolled, forged and welded. Visual

inspection before, during and after welding may detect an aid in the elimination of

discontinuities that might become defects in the final weldment

Limitations

It is limited to what the eye can see.

Principle

Comments

Visual inspection is the basic non-destructive inspection method. Its ability to prevent

defects is perhaps the most important feature of visual inspection, and more than for any

other method its success is in direct proportion to the knowledge and experience of the

inspector. The method should be applied as early as possible in a production process.

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Radiographic testing (RT)

Method

Radiographic image is produced by the passage of X-rays or gamma rays through the

test object onto a film.


Radiographic testing can be used on all metals to detect defects with an appreciable

dimension parallel to the radiation beam, on or below the surface of the object.

Radiographic testing is most applicable on three dimensional defects. Dependant on

radiation energy, radiographic testing can be used on material thickness up to 100 mm

Fe or more.

Limitations

Defects such as cracks perpendicular to the radiation beam cannot be detected by

radiographic testing. Radiography is readily used on flat plates. Lack of accessibility

due to object/weld configuration may, however, preclude the use of this method.

Due to radiation hazard operators must have an authorized knowledge of radiation

protection.

Principle

Comments

The applicability of radiography for weld inspection depends a great deal upon the weld

joint location, joint configuration and material thickness.

Radiography uses X- or gamma radiation that will penetrate through the part and

produce an image on a film or plate. The density of the material in a discontinuity (air in

the case of a crack, incomplete fusion, or porosity) is usually lower than that of the solid

metal. Different density material attenuate the radiation differently and consequently

produce optical density differences on a film or plate. The selection of the radiation

source (energy of the emitted rays) for a particular thickness of weld is a critical factor.

If the energy of the source is too high or too low for a given thickness of material, then

low contrast and poor radiographic sensi-tivity result.

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Ultrasonic testing (UT)

Method

Ultrasonic pulses are directed into a test object. Echoes and reflections indicate

presence, absence, and location of flaws, interfaces, and/or defects.


Ultrasonic testing is a sensitive NDT-method, which can be used on metals or non-

metals. Best results are obtained when the sound beam is perpendicu-lar to the defect.

Defects may be detected at depths ranging from 5 mm to 10 m in steel.

Limitations

Operation of ultrasonic equipment requires experienced personnel. False indications

may arise from multiple reflections and geometric complexity. Small and thin objects

and coarse-grained materials may be difficult to test. For example, welds involvingnickel base alloys and austenitic stainless steels tend to scatter and disperse the sound

beam: penetration of the sound beam into these materials is limited and interpretation of

the results may be difficult.

Principle

Comments

The ultrasonic method uses the transmission of mechanical energy in waveform at

frequencies above the audible range. Reflections of this energy by discontinuities are

detected. In the pulse-echo technique, which is most commonly used, a transducer

transmits a pulse of high frequency sound into and through the material and the

reflected sound is received from a discontinuity or the opposite surface of the test

object. The reflected sound is received as an echo which, together with the ori-ginal

pulse, is displayed on the screen of a cathode ray tube. The method can be used to detect

both surface and subsurface discontinuities.

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Inspection of crankshafts with

hand yoke BWM 220/12 and adjustable poles

Magnetic Particle Inspection (MT)

Method

When an object is magnetized, iron powder applied to the surface will accumulate over

regions where the magnetic field is disturbed as a result of surface flaws.


MT is a simple and fast method to detect surface defects in ferromagnetic materials.

Limitation

The MT is applicable only to ferromagnetic materials. It is for example not applicable to

stainless weld deposit on ferromagnetic base material. Trained operators are necessary

to avoid misin-terpretations.

Principle

Comments

Magnetic particle testing is used for locating surface or near surface discontinuities in

ferromagnetic materials. This method involves the establishment of a magnetic field

within the material to be tested. Discontinuities at or near the surface set up a

disturbance in the magnetic field. The pattern of discontinuities is revealed by applying

magnetic particles to the surface, either by dry powder or suspended in a liquid (wet

method). The leakage field attracts the magnetic particles, and thus the discontinuities

may be located and evaluated by observing the areas of particle build-up. These

magnetically held particles form an indication of the location, size and shape of the

discontinuity. Some of the factors which determine the detectability of discontinuities

are the magnetizing current, the direction and density of the magnetic flux, the method

of magnetization and the material properties of the object to be tested.

The electric current used to generate the magnetic field may be alternating (AC) or

direct (DC). The primary difference is that magnetic fields produced by DC are far more

penetrating than those produced by AC.

Compared to liquid penetrant inspection, the MT has the following advantages: it will

also reveal those discontinuities that are not surface open cracks (cracks filled with

carbon, slag or other contaminants) and therefore not detectable by liquid penetrant.

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Liquid Penetrant Testing (PT)

Method

The surface to be examined is covered with liquid that penetrates surfaceopen cracks.

The liquid in cracks bleeds out to stain powdercoating applied to the surface afterremoval of excess liquid film from the surface of the test object.


PT is a sensitive method to detect defects like cracks and pores that are open to the

surface of the material. PT may be used on both ferromagnetic and non-ferromagnetic

materials.

Limitations

PT can only be used on clean surfaces and can only detect defects open to the surface.

Principle

Comments

The method is particularly useful

on nonmagnetic materials where

magnetic particle inspection

cannot be used. The liquid

penetrant method is used

extensively for exposing surface

discontinuities in nonmagneticmaterials such as aluminum,

magnesium and austenitic steel

weld-ments. It is also useful for

locating cracks or other

discontinuities, which may cause

leaks in containers and pipes.

There are two varieties of the

penetrant method, both using a

similar pe-netrant. One uses a

visible dye, usually red for color

contrast, and the other a

fluorescent dye. The main

difference is in the visibility of

the indication: very small

indications are less likely to be overlooked if they are revealed by a fluorescent glow in

a near darkness rather than a red indication viewed in normal light.

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Eddy Current testing (ET)

The Eddy Current testing method include also the following testing methods :

Alternating Current Field Measurement (ACFM)

Electro Magnetic Array (Lizard EMA) (not presented in the course notes)

1. ET is widely used in the industry as an alternative to MT. The equipment type is

often recognised as Hocking impedance plane inspection. The method is based on

manually probe-scanning without recording devices of defect indications. Normally

the method is conducted as dry based inspection (i.e topside above water).

http://www.hocking.com/

2. ACFM provided by Technical Software Consultants, UK (TSC) is a computerised

system with both automatic and manually probe-scanning options. The system

provides recording devices for post interpretation of defect indications. The system is

capable to operate both as dry and wet based inspection. ( i.e underwater and above

water).

http://www.tscuk.demon.co.uk/tschome.htm

3. Lizard EMA provided by Newt International Ltd, UK is a computerised eddy current

system with both automatic and manually probe scanning options. The system

provides recording devices for post interpretation of defect indications. The system is

capable to operate both as dry and wet based inspection. ( i.e underwater and above

water).

http://www.lizard.co.uk/

These methods of detection can find fine surface breaking defects through non-

conductive coatings. In addition they can be used to size defects both for length and

depth. They are used mainly for detection of surface breaking defects. General-purpose

equipment can also be used for coating thickness measurement and material sorting

given appropriate calibration samples.

ET advantages

1. Can be used through good quality non-conducting coatings

2. Can assess crack depth as well as length (immediately)

3. Quicker than MT (>2m/Hr)

4. Can be used on all conducting materials5. Gives an electronic and written report (ACFM, Lizard EMA)

6. Can replay the scan for off-line assessment (ACFM, Lizard EMA)

ET disadvantages

1. Can be more difficult than MT on tight geometry

2. Cannot assess sub surface defects

3. Depth of the defect will be along the surface of the defect not Through thickness

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Applicability of NDT methods on different material

defects

Applicability of different NDT-methods vs. defects in welded joints

Note: For non-magnetic materials liquid penetrant testing is used instead of magnetic particle inspection.

Applicability at different NDT-methods vs. defects in casting

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Generally accepted methods for detection imperfections

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NDT methods Visual inspection

Viewing aids and measuring gauges.

Proper working light is imperative during all visual inspection. The color of the light

should be such that there is good contrast between any imperfections and theirbackground. It should be possible to vary the direction of the light to reveal

imperfections in slight relief.

To give a reasonable idea of what the unaided eye can see, it may be remembered that a

normal eye under average viewing conditions can see a disc approx. 0,25 mm and a

line approx. 0,025 mm wide. The normal eye cannot focus on objects closer than about

150-250 mm. The function of hand lenses is to enable the eye to view an object from a

very short distance. For this purpose a hand lens with a magnification 2 2,5 is

suitable.

To inspect a weld that is not directly visible but is within viewing distance of the eye, a

dental mirror may be used. For more remote welds, intrascopes, fiber optic or portableTV-cameras may be used.

Standard workshop tools are used to inspect welds, such as straight edge, ruler,

protractor, caliper (internal, external or vernier), height/depth gauge and contour gauge.

Two typical gauges to be used for measuring the sizes of butt welds and fillet welds are

shown in figure 5.1. Another measuring gauge which can be used for measuring of weld

reinforcement on butt welds, fillet weld leg length and angle for edge preparation is

shown in figure 5.2.

Fig. 5.1 Measurement of weld profiles

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Fig. 5.3 Alignment of butt welds

Fig. 5.2 Instrument for measuring weld profiles

Inspection before welding.

Before welding, the inspector should:

have knowledge of the applicable standard and specification to be used

have knowledge of the welding procedure to be used and the welders qualifications

where appropriate

be provided with the working drawings

The inspector should then carry out checks on the following items:

Parent metal

The parent metal should be checked for correct specifications, dimensions, flatness,

surface condition etc.

Weld preparation, fit-up and assembly

The shape and dimensions of the weld preparation, including backing material are to bechecked using appropriate measuring devices. The fusion faces and adjacent material

are to be checked for cleanness.

The methods of assembly are often specified in the procedure or specification. It may be

necessary to note the position of tack welds for subsequent checks. Tack welds to be

incorporated in subsequent runs should be cleaned. When preheat is specified, this is to

be applied before tacking. Minimum size of the tack welds may also be specified.

Regarding fit-up, the gap between the

components should be uniform, see A,

B and C on Fig. 5.3, however, some

non-uniformity may be acceptable.

Linear and angular misalignment (D

and E) should also be within tolerance,

however, it might be necessary to

preset the components to take care of

the distortion caused by the welding.

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Welding consumables

Consumables are to be checked to ensure that correct item is being used and that it is in

good condition.

Manual metal-arc electrodes

Type coding and/or makers identification and diameter are to be as called for by the

welding procedure. Taken from sealed packets, the covering shall not be flaked or

broken off and there shall be no sign of electrode having been damp and subsequently

dried out, such as crystallized salts on the covering or rusty core wire. Storage ovens

and heated quivers shall be used as applicable. (No unauthorized returns to packet by

economy-minded storekeepers!)

Submerged-arc wires and fluxes

Identification and matching of wire to flux are, to be checked. The flux shall not be

contaminated (caused by over-enthusiastic recovery) or damp.

Gas-shielded welding

Correct composition and diameter of wire, correct spooling for equipment in use, no

contamination by rust or grease, correct shielding gas and flow. In the case of mixtures

correct ingredients and proportions are important items.

Safe wire feeding is important for keeping a stable arc and preventing lack of fusion.

Protection of the arc from draught is also important.

Gas-cutting

The type and amount of fuel gas shall match the equipment in use. A correct cutting

speed is necessary to obtain a satisfactory surface of the cut.

Preheating

Rapid cooling after welding may lead to cracking, and the cooling rate may need to be

reduced by preheating. The faces to be welded and the adjacent metal, are usually

heated to a temperature in the range of 50 250 C immediately before welding.

Preheat temperature is normally to be re-established at the start of each run. There may

be adverse metallurgical effects if the required preheating temperature is not correct.

Two common methods of measuring the temperature are:

Surface pyrometer, the accuracy of this and other instruments should be checked

regularly

Temperature indicating crayon (often referred to as the trademark of a major

supplier, Tempilstick).

A check should be made that the preheat temperature is maintained at the specified

distance from the Joint, usually approx. 75 mm or six times the plate/wall thickness.

Electrical parameters

The welding procedure will normally specify the current and voltage to be used. When

assessing the tolerances for this, the following should be taken into consideration:

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The static and dynamic characteristics vary for the different makers of machines.

Increased fluctuations may be caused by loose connections (a loose welding return

often causes arc strikes which may be harmful to the material).

Meter readings may also for other reasons fluctuate substantially during normal

welding.

Meters on the equipment are not always trustworthy unless they have recently beencalibrated.

It is difficult to assess tolerances for current and voltage. Generally, a small deviation in

the volt reading is not so important, more important is that the heat input is sufficient to

keep balance between the melt and solid material and to keep good control of the melt.

A clamp meter is practical to control the current.

Inspection Inspection during welding.

What is said about welding consumables, preheating and electrical parameters in the

previous chapter also applies during welding. During welding the following may be

important to pay attention to:

Interpass temperature

For the case of multi-run welds, check that the conditions specified in the welding

procedure for interpass temperature are applied. Time lapse between root run and the

following pass (in some cases referred to as hot pass) may be important and is in some

cases specified in the procedure.

Back gouging

When back gouging is specified, check that the back of the first run is gouged out by

suitable means to sound metal normally followed by grinding before welding is started

on the gouged-out side. The shape and surface of the resultant groove should be such as

to permit complete fusion and a proper shape of the run to be deposited.

Tack welds and interrun cleaning

All recognized specifications call for cracked tack welds to be ground out. In some pipe

joints proper tack welds must be ground out to the original preparation before carrying

out the root run in the area.

It should be checked that each run of weld metal is cleaned before it is covered by a

further run, particular attention should be paid to the junction between the weld metal

and fusion faces. Weld profiles with excess overlap or undercut at their edges may lead

to poor fusion or defects in later runs. Slag must also be removed before restriking the

arc after stopping.

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Inspection after welding.

After the weld runs are completed, the weld is to be cleaned and inspected for shape and

surface defects. The assembly should also be checked against the manufacturing

drawings and applicable specifications or codes.

The weld contour and transition to the base material may in some cases be veryimportant from a fatigue point of view.

Cleaning and dressing

It should be checked that all slag has been removed. Dressing may be specified from a

design aspect or may be necessary to facilitate testing by certain methods. When

dressing of the weld face is required, ensure that overheating of the material due to the

grinding action is avoided. Furthermore, ensure that due consideration is given

regarding the di-rection of the grinding pattern versus the stress direction. Use of the

same grinding equipment for different materials may in some cases lead to corrosion

problems.

Weld contour and shape of welds

Butt welds

Fig. 5.4 Incompletely filled groove can be measured and is normally not acceptable.

Root concavity may be acceptable in moderation.

Fig. 5.5 Undercut and excess penetration

Fig 5.6 Too much weld metal can adversely affect fatigue strength.

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Fig. 5.7 Overlap caused by weld metal flowing onto the parent metal without fusing

to it. Often difficult to identify positively.

Fig. 5.8 Insufficient weld metal reduces the weld strength.

Fillet welds

Fig. 5.10 Leg lengths are the primary dimension of fillet welds, unless otherwise

stated the leg lengths are intended to be equal.

Fig. 5.11 Throat thickness, actual dimension is Tl. Dimension measura-ble by visual

inspection of finished joint is T2.

Fig. 5.12 Concave and convex weld faces

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Fig. 5.13 Undercut and overlap

Weld repairs

Repairs required after visual inspection are normally to be completed and the area

reinspected prior to testing by other methods.

When the weld does not meet the requirements, one of the following actions may be

specified:

1. Report fault to authority for decision

2. Scrap fabrication

3. Re weld surface defects after grinding out faulty material, oxide, slag, etc.

4. Grind all faulty areas back to sound parent metal as per original specifications for

edge preparation, taper weld metal at ends of fault to allow adequate access and re

weld to original procedure.

5. Cut out (by thermal or mechanical process) all weld metal, re-prepare and re-weld

according to original procedure.

Where no guidance is given, a combination of 3) and 4) is assumed.

Intermediate inspection may be necessary during the process of repair-ing the defects to

ensure that the work is correctly carried out and that the defect is exposed and removed.

Various NDT-methods may also be used in addition to visual inspection to ensure that

the defects are removed.

Not only weld defects and correct weld reinforcement should be paid attention to, other

surface defects may also be important, such as:

Torn surface, caused by removal of temporary attachments.

Arc marks, caused by insecure connection of welding return.

Stray flash, caused by electrode accidentally coming into contact with work away

from weld region.

Such defects may be harmful in high-stressed areas, and they are usually rectified by

being ground back to sound metal.

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Inspection reporting and records.

To be able to verify that the fabrication and inspection is performed according to the

governing procedures, specifications or codes, the inspector may need to make up a

check list to ensure that visual inspection of all relevant items at each stage offabrication has been carried out. When required, welds that have been inspected and

approved should be suitably marked or identified.

The report should state how the

inspection was performed, i.e. if

artificial light, hand lenses or

other equipment have been used.

If other NDT methods are

utilized, a report for visual

inspection should normally be

available and accepted beforefurther NDT is carried out.

A careful inspection and

description of a defect can be of

considerable assistance to experts

trying to diagnose the cause and

possible remedies. Photographs or

accurate sketches or both may in

many cases be helpful.

It should also be kept in mind that

if special problems areexperienced during fabrication, a

comprehensive reporting may be

very important for future

inservice inspection.

Concerning reporting, see also

part "NDT Procedures and

reports".

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NDT Methods Radiographic Testing

Introduction.

Radiographic testing can be applied to most materials depending on material type and

thickness. All materials absorb radiation, some more than others. Steel absorbs more

than aluminum, copper more than steel, tungsten more than copper etc., depending on

atomic number and specific weight. As a rule we say that the more dense a material is,

the more radiation it will absorb and the thicker a material is, the more radiation will be

absorbed.

The applicability of radiographic testing for weld inspection depends a great deal upon

the weld joint location, joint configuration and material thickness. The radiographic

method is an excellent method for examining buttwelds for volumetric defects (three

dimensional) like pores, slag inclusions, slag lines, incomplete penetration etc. The

radiographic principle is shown in Fig. 6.1. The film must be located as close as

possible to the back surface of the object.

To detect two dimensional defects like cracks and lack of fusion, the radiation beam

must be parallel to the defects.

Fig. 6.1 Radiographic examination of butt weld

Typical example of radiographic testing steel products

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T ical x-ra tube

Gamma ray projector Crancking unit Extension cables

The radiographic process.

Radiographic testing can be performed by using two types of radiation:

x-rays, which are produced electrically

gamma-rays, which are produced by (nuclear decay of) radioactive material

X-rays are generated by high velocity electrons hitting a tungsten anode. The anode will

emit x-rays whose energy level and spectrum can be controlled by adjusting the

acceleration voltage (kilo Volts) in the x-ray tube.

A radioactive source

(for example Cobalt

60 or Iridium 192)

cannot be turned off

and special

shielding containers

of lead or uranium

have to be used for

storage and control

of the source.

Typical gamma-ray equipment

Sketch radioaktiv

source

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In tables 6.1 and 6.2 some data on x-ray machines and gamma ray sources and their

applications are listed.

Table 6.1 Typical x-ray machines and their applications.

Table 6.2 Radioactive materials for industrial radiography (Iridium 192 and Cobalt

60 most commonly used)

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The penetrating power of the radiation increases with its energy. The energy of Iridium

192 radiation corresponds to a x-ray voltage of appr. 800 kV. For Cobalt 60 the

corresponding x-ray voltage is appr. 3000 kV. (Due to radioactive decay the activity of

radioactive isotopes decreases with time. After one half-life the activity measured in

Curie or Becquerel is reduced to one half.)

When using the x-ray machine as exposure source, the energy penetrating the test object

may be controlled both by the high voltage and by the exposure time. When using

radioactive sources (gamma rays), only the exposure time is controllable. This makes a

x-ray apparatus better suited for radiographic testing.

When a beam of x-rays or gamma rays strikes an object, some of the radiation is

absorbed, some scattered and some transmitted. A thicker portion of material will

absorb more rays than a thinner portion. The film under the thin portion will become

darker because more rays will penetrate to the film and give a higher exposure.Discontinuities (pores, slag inclusions etc.) are normally light compared to the base

material and explain why discontinuities produce dark spots or lines on the radiograph.

An experienced inspector or interpreter will recognize the type of discontinuity from its

image (shape, size etc.) on the radiograph.

Sometimes discontinuities may produce light spots on the radiograph, due to heavy

metal inclusions e.g. tungsten inclusions from the tungsten electrode used with shielding

gas welding.

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For determination of exposure times, special calculators are provided with the

equipment. These calculators normally give exposure times referred to steel. If other

materials than steel are to be tested, the calculat-ed exposure times have to be adjusted

according to table 6.3.

*) Tin or lead alloyed in the brass will increase these factors.

Table 6.3 Radiographic material thickness relative to aluminium or steel

Aluminium is taken as the standard metal at 50 kV and 100 kV, and steel at the higher

voltages and gamma rays. The thickness of another metal is multiplied by the

corresponding factor to obtain the approximate equivalent thickness of the standard

metal (aluminium or steel). The exposure applying to this thickness of the standard

metal is used.

Example: To radiograph 0.5 inch of copper at 220 kV, multiply 0,5 inch by the factor

1.4, obtain an equivalent thickness of 0.7 inch of steel. Thus, use the exposure required

for 0.7 inch of steel.

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Quality of radiograph.

Geometrical unsharpness

One important variable related to radiography is the geometrical unsharpness Ug. The

factor is calculated from the following formula:

where

b. = object thickness + object to film distance

d. = effective width of the focal spot (given in the equipment documentation for the x-

ray or gamma ray source)

f. = film to source distance

For high quality radiographs, a small value of Ug is desired (IIW allows Ug = 0,2 mm

for best quality).

Fig. 6.2 Geometrical unsharpness (clarification)

Intensifying screens

To improve the intensifying efficiency of the photographic process, socalled

intensifying screens are used.

Note that screens in general should be placed close to the film (vacuum-packed).

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Lead intensifying screens are usually thin lead foils (0.02 0.15 mm) glued to a

cardboard support. Lead screens may have an intensifying effect of 5 times, depending

on the radiation energy. They have the further advantage of absorbing the longer

wavelength scattered radiation, thereby producing better contrast in the radiographic

image.

Certain chemical salts have

the property of

fluorescence (they emit

light) under the excitation

of x-rays. Placing a sheet

of this salt next to the film

will increase the sensitivity

of the radiograph by 10

100 times depending on

the screen type.

Lead salt intensifying

screens combine theproperties of the two

screen types mentioned

above: they are highly intensifying and absorb scattered radiation at the same time.

Codes and specifications normally require lead screens to be used.

Radiographic films

Radiographic film is

classified according to its

sensitivity to radiation(often termed the speed of

the film). In USA four

sensitivity groups (14)

are usually specified, while

European manufacturers

specify three groups (G1

G3). High-speed films

are coarse grained and give

low contrast radiographs,

while slow-speed films are

fine grained and give better contrast and cleaner radiographs.

Standards and codes specify the films to be used, normally medium to fine grained

films.

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Image quality indicator (I.Q.I.)

In order to determine the sensitivity of a radiograph, a penetrameter or image quality

indicator (I.Q.1.) is used. (fig. 6.1). Each radiograph must show the image of a

penetrameter in order to be of any value. Code requirements will specify type, size and

position of the I.Q.I. If possible, the penetrameter shall always be placed on the source

side of the object. The most frequently used types of I.Q.I. are ASME (hole

penetrameter), IIW or DIN (wire step penetrameters).

The smallest hole or thinnest visible wire indicates the sensitivity in per cent of the base

metal or weld thickness. Depending on the code requirements the sensitivity shall

normally be 1.52.0 per cent.

ASME standards normally

specify a sensitivity

requirement of 22T. The

first number is the

penetrameter thickness in per

cent of the object thickness.

The last number (2T) is the

hole diameter where T is the

thickness of the

penetrameter. Each ASME

IQI has three holes IT, 2T

and 4T and the highest

sensitivity requirements is

1IT and the lowest is 4

4T.

Example 1:

Wall thickness: 10 mm steel

DIN/ISO 10-16 Fe: 4 visible wires, thinnest is 0,2 mm (table 6.3)

Sensitivity in per cent: 0.2 mm 100/10 mm = 2%

Example 2:

Wall thickness: 10 mm steel

ASME requirement: 2 2T

Sensitivity: The image of the plate and the hole 2T (with diameter twice the thickness of

the I.Q.I.) is visible. The sensitivity is then app. 2 per cent. (ref. ASME V).

If all wires of the DIN/ISO penetrameter in Example 1 were visible (thinnest wire is 0.1

mm) the sensitivity would be I per cent.

The material of the I.Q.I. should belong to the same material group as the object (Steel,

Aluminum, Copper etc.).

The IIW-penetrameters are available only in steel. DIN penetrameters are available in

Steel, Aluminum and Copper, and ASME penetrameters in all commonly used

materials.

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The diameters of penetrameter wires are shown in table 6.4

Ex.: BZ No. 16 corresponds to a wire diameter of 0,1 mm, No. 15 to 0,125 mm etc.

The radiographic sensitivity depends on correct density, good definition and high

contrast. On page 33 are indicated parameters and remedies for improving the quality of

radiographs. See also section on Film inter-pretation.

Table 6.4 Diameters of penetrameter wires

Note that the wire diameters of the IIW 0,1 0,4 are the same as DIN 10 16. This is

also the case for IIW 0,25 1,0 and DIN 6 12. DIN penetrameters are identified by

Bildgutezahl (BZ) given in brackets in table 6.3.

Image Quality indicator, ASME hole penetrameter and DIN wire penetrameter.

Fig. 6.3 ASME penetrameter

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Interpreting of

radiographs

Fig. 6.4 DIN and IIW penetrameters

Film interpretation.

Viewing of the radiographs is the most important part of

radiographic inspection. The interpreter must be familiar with

the radiographic method and techniques, welding processes

etc.

The interpretation and evaluation shall be in accordance with

valid specifications, codes or standards.

Identification

The radiographs must be marked in such a way that no doubt can arise as

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to which part of the object it represents. The identification has to be beyond dispute

concerning the position and orientation of the film.

Lead letters and numbers, measuring tape and direction arrows should be fixed to the

Section being radiographed and should appear on the radiograph. Position/orientation

should be marked on a suitable sketch or drawing to show the necessary details.

Identification, traceability between the object being tested and the film

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Density

The density of the radiograph shall be correct

according to the procedure or specification.

Generally, a density less than 1 is underexposed

whiles a density above 4 is overexposed. Thedensity could be measured with a direct reading

densitometer or by means of density strips, i.e.

filmstrips with fixed density. The density should be

between 1,5 3,5 on a radiograph of a

homogeneous part of the object unless otherwise

specified.

Sensitivity

The radiographs should be checked for sensitivity level to prove that the recommended

radiographic technique is used.

For radiographic sensitivity, see page 52.

The sensitivity shall be within the limit stated in the procedure or specification,

normally 1,5 2,0 per cent of the radiographed cross section, see section 6.3.4.

Film quality evaluation

The radiograph shall be sharp and free from scratches, stains, unsharpness, fog and

imperfections due to processing. Where a continuous length of weld (object) is to be

radiographed (100 per cent) the separate radiographs should overlap sufficiently to

ensure that no portion of the weld remains unexamined.

All requirements in the sections above shall be fulfilled before an evaluation of the

quality/homogenity of the object is made. If one or more of these requirements is not

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fulfilled the inspector may find it necessary to repeat the radiographs with an improved

technique.

Material homogenity evaluation and grading

The evaluation and grading shall be carried out according to given standards or

specifications, considering:

type of defect

amount of defect

classification according to standard and specification (accepted/not accepted) or

grading in classes.

The radiographs should be examined on an illuminated diffusing screen (viewing box)

in a darkened room and the illuminated area should be masked to the minimum required

area for viewing of the radiographic image. The brightness of the screen should be

adjustable so as to allow satisfactory reading of the radiographs.

Some radiographs and sketchs of weld defects

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Some typical standards or recommendations are:

ASME V/VIII ASME Boiler and Pressure Vessel Code; Non Destructive

Examination

ASTM E 155 Reference Radiographs for Inspection of Aluminum and magnesium

Castings

ASTM E 446 Reference Radiographs for Steel Castings up to 2 (51 mm) inthickness

Radiographic standards for steel castings

ISO5817/EN 25817 Arc-welded joints in steels - Guidance on quality levels for

imperfections.

EN 26520 Classification of imperfections in metallic fusion welds with explanations.

ISO10042/EN 30042 Arc-welded joints in aluminium and its weldable alloys -

Guidance on quality levels for imperfections.

Advantages and limitations of radiographic testing.

Advantages

A radiograph will detect volumetric discontinuities such as porosity, inclusions, and

even cracks if the crack opening runs parallel to the radiation beam.

The radiogramme or film provides a 'visual' indication of flaws

A radiograph is an excellent and permanent record of the testing, with built-in

evidence (penetrameter) to verify the sensitivity of the film.

Well established standards and codes of practice

Can be used on almost any material

A radiograph will show surface discontinuities such as undercut, in-adequate

penetration, excessive penetration and burn through. These defects can also be

detected visually. Note: RT should not replace visual inspection for surface

inspection.

For visible testing of materials or processes, the film may be substituted by a

fluorescent screen. This enables the operator to see defects in materials, unwanted

particles in a substance etc.. The same method is often used in hospitals and forairport security checks.

Limitations

X-rays and gamma rays are hazardous radiations. Irradiation of the human body will

increase the risk for developing cancer and genetic defects. Such radiation cannot be

detected by any of the human senses and proper instruments have to be used to check

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the radiation level. Due to the radiation danger, limitations may be imposed upon

time and place of radiography activities.

Access to both sides of the test object is necessary to produce a radiograph.

The shapes of the test object may make it difficult to produce a radiograph with

useful information.

Discontinuities such as cracks, laminations, lack of fusion, etc., must be aligned withor parallel to the radiation beam to be detected clearly.

Choice of radiation energy for a particular thickness of weld is a critical factor.

Location of defect in test objects cross section is difficult to determine.

Information typical x-ray systems is given on below web links:

http://www.agfa.com/

http://www.yxlon.com/

http://www.ndt.net/

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NDT Methods Ultrasonic Testing

Definition of ultrasound and properties of waves

Ultrasound

Sound waves with a frequency of 20kHz or more, i.e. above the normal range of the

human ear, are generally referred to as ultrasonic waves. In practical use 50 kHz to 50

MHz is used for material testing. To a certain extent ultrasonic waves possess properties

similar to those of light waves, i.e. they may be refracted, focused and reflected.

For the testing of materials, piezo-electric crystals formed as thin plates are used for

generating ultrasonic waves. If an alternating voltage is applied to the crystal, the plate

will vibrate with the frequency of this voltage, i.e. it emits sound waves. Conversly, a

sound wave striking the plate produces a voltage at its electrodes. Common piezo-

electric transducers are made of quarts and barium titanate.

Properties of waves

The following relationship exists between the parameters frequency (f), wave length (l)

and propagation velocity (v) in a propagating sound wave:

When ultrasonic waves are used for material testing, the following applies:

shorter wavelengths will detect smaller defects

the penetrating power increases with the wavelength

longer wavelengths should be used on coarse grained material

Frequencies may therefore be selected as follows:

small defects: high frequency (2-4 MHz)

large defects: low frequency (0,5-2 MHz)

fine grained material: high frequency

coarse grained material: low frequency

Methods

When testing materials with ultrasonic waves, high-frequency sound waves propagate in

homogeneous solid bodies as directed beams, with very little attenuation. At interfaces

between media with different acoustic properties, such as air and metal, the waves are

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almost completely reflected. This makes it possible to detect cracks, inclusions and

other flaws by means of ultrasonic waves.

Ultrasonic testing of materials may be performed by the following methods:

a. The reflection (pulse-echo) method

b. The transmission method

c. The immersion method

The most important method is the pulse-echo technique which will be emphasized in

this section.

Ultrasonic inspection of buttweld in piping system using angle probe.

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Fi . 7.1 The ulse-echo rinci le

Ultrasonic thickness measurement of piping using D-meter and single crystal 0degree

probe.

The reflection (pulse-echo) method

When an ultrasonic pulse is transmitted to the object, the time

delay between the initial pulse and the echo from the back wall,

or from a flaw inside the object, can be measured.

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Performance of ultrasonic testing

Ultrasonic equipment

For indication and measurement of thickness, distances and defect sizes, an ultrasonic

apparatus containing transmitter, receiver and indicating screen is required. Relevant

requirements for such equipment are:

The ultrasonic equipment should cover a frequency range of at least 1,0 - 6,0 MHz.

The ultrasonic equipment is to be fitted with a calibrated gain regulator with

maximum 2 dB gain per step.

Test range: applicable to the test

The ultrasonic equipment is to be equipped with a flat screen extending to the front

of the apparatus so that a reference curve can be drawn directly on the screen (see

calibration 7.3.5).

The ultrasonic equipment must be able to operate with both combined and separate

transmitter and receiver probes (fig. 7.5).

The ultrasonic equipment should allow echoes with amplitudes of 5% of full screen

height to be clearly detectable under test conditions.

Probes

When testing materials with ultrasound, two types of probes may be used; the normal

probes (0) (longitudinal waves) and the angle probes (transverse waves).

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Application of a normal probe

Probes (transducers) for ultrasonic equipment.

Left: normal probe 0, right: angle probe 70.

The normal probe (0)

generates longitudinal

waves and transmits them

(via a couplant such as oil,

grease or water) into a test

object in a direction

normal to the surface to

which the probe is applied.

The pulse propagates in a

straight direction, but due

to beamspread, the

soundfield will become

cone-shaped. The angle of

beamspread is related to

probe diameter and

frequency. In fig. 7.3 the

principle of application of

a normal probe is shown.

Note that the echo height

on the screen decreases as

the length of the soundpath

increases.

Normal probes are to cover a

frequency range of 0,5 - 6 MHz.

Typical values are

1 MHz, 2 MHz, 4 MHz and 6 MHz.

Most commonly frequencies used

are 2MHz and 4MHz.

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The angle probe is constructed to transmit transverse waves at a defined angle into a test

object.

Ultrasonic inspection of nozzle weld connection using angle probe.

Typical angles are 35, 45, 60, 70 and 80. The most commonly used angels are 45,

60, 70. On materials with sound-velocities different from steel, the angle will change

according to Snells Law. For instance, a probe of 60 in steel will give 56 in

aluminium, 37 in copper and 35 in cast iron (Table 7.1).

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The angle probes are to cover a frequency range of 2 - 6 MHz. Typical values are 2

MHz and 4 MHz.

Application of the angle probe

The table below gives the angles of refraction in different materials for the most

common types of angle probes having an angle of incidence of 35 - 80 with respect to

steel. The acoustic velocity in cast iron depends on various factors, the quoted values

being average figures.

Angles relative to steel

The double crystal probe (which is a special normal probe)

consists of two separated piezo-electric crystals,

transmitter and receiver. Because the initial pulse has to

pass an acoustic delay block before reaching the contact

surface of the material, the initial pulse will notinterfere with defects immediately below the contact

surface. In other words, the deadzone will be greatly

reduced.

Principle of the double crystal probe (TR or SE probe)

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An ultrasonic pulse from the transmitter crystal will propagate via the delay block into

the material, and reflected pulses from defects will reach the receiver crystal resulting in

an echo on the screen. The delay block and separate transmitter-receiver configuration,

make the double crystal probe useful for detecting defects immediately below the

contact surface and for measuring thicknesses within the range 1 - 30 mm. It is of

importance to notice that with a double crystal probe, the first echo is always used for

detection.

Usually the double crystal probe is constructed with the piezo-electric elements at an

angle (1 - 5) to the normal. This will increase the detection efficiency close to the

surface of the material and prevent multiple echoes from reaching the receiver. A

double crystal probe with focused beam will be efficient for detecting pitting corrosion.

Note: The surface must be metallic clean when using double crystal probes.

On a surface with a small radius of curvature, such as pipes with a small diameter, it

may be necessary to adjust the probe shoe to attain sufficient contact between the

material and the probe.

Procedure

Ultrasonic examination must be performed in accordance with a written procedure.

Each procedure must include at least the following information, as applicable:

Type of instrument

Type of transducers

Frequencies

Calibration details

Surface requirements Type of couplants

Scanning techniques

Recording details

Reference to applicable welding procedures

Coupling medium and contact surface

A satisfactory couplant, in either fluid or paste form, should be used to transfer the

ultrasound from the probe into the material. Oil, grease, or glycerine are well suited forthis purpose. A cellulose gum (wall paper paste) is particularly suitable as it can be

removed with water after inspection is completed. The contact surface should be free

from weld spatter and any other substance which may impede the free movement of the

probe or disturb the transmission of ultrasound to the material. Light grinding of the

surface and the weld may be necessary.

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Calibration

The calibration of the apparatus and probes are of decisive importance for the testing

result.

For the calibration of the equipment range scale and the angular determination of angle

probes, an IIW calibration block (V1 or V2) should be used

Calibration blocks, range

calibration

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Range calibration using V2 block, 25 mm radius.



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Acceptance criteria often define a defect by specifying the size/height of the defect echo

in relation to a calibrated reference curve. As the sound velocity will vary with the

material tested (i.e. beam angle, range calibration, sound beam profile, etc., varies with

the material) it is imperative that the calibration blocks are of the same material as the

test object. For construction of a reference curve, see figure below.

Construction of reference curves

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Construction of reference curve, 3/4 skip distance from reference reflector



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ASME Boiler and Pressure Vessel Code Section V, Article 5, describes a method or

standard which is frequently used for ultrasonic testing of welds in steel constructions.

In the reference block (fig. 7.8) made from the production material (or of a material

with similar acoustic and metallurgical properties) a drilled hole is used as a reference

reflector for establishing the reference curve.

The diameter and hole location are dependent on the thickness of the plate, and are

given in the ASME-standard. By placing the probe in different positions on the

reference block and marking the corresponding echo height, one can establish a

distance-amplitude curve on the screen. Defects will be accepted or rejected depending

on the echo height compared to the reference curve and the length of the defect.

Root defect detected, echo amplitude evaluation against reference curve

A more detailed description for the calibration of the ultrasonic apparatus is given in

VERITAS Classification Notes No. 7 "Ultrasonic Inspection of Weld Connections".

(Note, this document is currently under revision).

Acceptance criteria

Before starting the ultrasonic examination, it is important to define the code or standardthe examination should follow. The soundness of the materials/welds must comply with

the criteria in the defined code or standard.

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Reference block for construction of reference curve

L = length or reference block given by probe angle and material range to be covered.

T = thickness of reference block.

B = width of reference block, minimum 40 mm.

P = position of drilled hole.

Calibration reference block requirements

Defect sizing

A method which is suitable for determining the size of large defects with normal probesand angleprobes is the 6 dB-drop method, also called the half value-method. When a

defect is detected, the probe is moved towards the edge of the defect until the defect

echoheight it reduced by 6 dB (or 50 %), and the center of the probe is marked as the

edge of the defect. By moving the probe around the defect in this fashion, the extent of

the defect can be plotted. The same technique can be used with angle probes.

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Measurement of thickness and detection of defects

Material thickness (T) may be measured by using normal probes. Calibration has to take

place on similar materials as the test object to avoid errors due to different sound

velocities. By reading the distance to echo number n and divide by n, the thickness can

be measured within approximately 1 - 2 %

Echoes appearing between full thickness echoes indicate lamination or other types of

defects.

Thickness measurement using multiple echo-technique

Range calibration using 0 degree probe, 20 mm calibration.

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Ultrasonic thickness measurement of pipespool using ultrasonic apparatus and 0degree

twin crystal probe

In some cases the back wall of the test object may be so corroded (pittings) that the

transmitted sound is reflected from the pittings into the material. Thus very little

ultrasonic energy is reflected back to the probe and thickness measurement is

impossible. In such cases double crystal probes should be used.

Possible errors

If thickness measurements are to be carried out on an object with a coated surface, thecoating may give rise to measurement errors. To avoid such errors please note:

When using single crystal probes, measure the material thickness between first and

second echo

When using double crystal probes, the coating must be removed before measurement

is carried out.

When using corrometers, D- or K-meters, it is likewise imperative that the coating is

removed before measurements are carried out.

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Ultrasonic thickness measurement of piping using D-meter and single crystal 0 probe.

Ultrasonic thickness measurement of cast steel nozzle using D-meter and twin crystal

0degree probe.

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When using double crystal probes for measurement of pipe wall thickness, be aware of

correct probe position related to the axis of the pipe.

Advantages and limitations of ultrasonic testing

The principal application of ultrasonic techniques consist of flaw detection andthickness measurement.

Advantages of ultrasonic tests:

Capable of detecting planar defects not detectable by radiography.

High sensitivity, permitting detection of minute defects.

Great penetrating power, allowing examination of extremely thick sections, e.g. up to

10 m of steel.

Accuracy in the measurement of flaw position and estimation of flaw size.

Fast response, permitting rapid and automated inspect

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