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Welcome

WELCOME Welcome to learn more about "NDT Principles and Limitations". This document was originally the old Web course from 2002. The information is still valuable to read. Expect from this welcome section the text are not changed.

LEARNING OBJECTIVE After completion of this web based training and the following workshop you should have:

Basic NDT knowledge Knowledge about NDT application on different

objects such as; welds, castings, forgings Knowledge about typical defects in welds, castings and forgings Knowledge about the most commonly used NDT methods applied within DNV Class Knowledge about NDT procedures Knowledge of personnel qualification and certification requirements Knowledge about acceptance criteria Knowledge to identify non-conformities during evaluation of procedures and witness NDT

An active role in giving feedback is both necessary and expected.

WEB BASED TRANING This web-based training comprises of several parts (see left side menu). If you go through all the parts, it should take you approximately 16 hours. Please note that you must complete this module before attending the Workshop to follow.

We recommend that you divide your study into manageable parts. For example, two chapters per day on the main "NDT Methods" section is usually enough.

We also recommend that you use the "Forum". This forum is an important channel for exchanging viewpoints or for addressing questions to your colleagues. Your access to the forum is at the bottom of the left-hand menu. Take a look !

NOTE: It is a challenge to go through all the content in this web-based program at your desk at the same time as conducting your normal everyday business. It is essential that you allocate enough time to your study.

If necessary, talk to your line manager and explain to him/her the importance of this web-based element of your training. Remember that you will also have to pass a written test as a part of the workshop that comes later on, the answers to which you will only find in this web based training module.

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Introduction and objectives

Many 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 this net based training module is to acquaint the participants with the fundamentals 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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Overview of defects in materials

Chapter 2.1: Common defects in connection with welds.

Chapter 2.2: Common defects in cast materials

Chapter 2.3: Common defects in forged or rolled materials

2.1 Common defects in connection with welds.

Reference is made to fig. 2.1 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

Fig.2.1 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.

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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.

POROSITY

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.

ELONGATED PORES OR WORMHOLES

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 in weld 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.

SLAG INCLUSION

SLAG LINES

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.

LACK OF FUSION

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 the welding arc.

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INCOMPLETE PENETRATION

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 ( fig. 5.5, in ch. Visual Inspection).

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 without fusion. 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.( fig. 5.7, in ch. Visual Inspection)

Excess weld reinforcement is, in the root of the weld, ( fig. 5.5, in ch. Visual Inspection) caused by improper fitup and/or welding technique. On the top ( fig. 5.6 in ch. 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.

fig.5.5

Cracks Lamellar tearing (cracks) are generally terracelike separations in base metal typically caused by thermally induced shrinkage stresses resulting from welding.

LAMELLAR TEARING

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

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formation. Cracks may be termed longitudinal or transverse, depending on their orientation. 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.

BRITTLE FRACTURE IN STEEL PRESSURE VESSEL

Longitudinal cracks in submerged arc welds made by automatic welding processes are commonly associated with high welding speeds and sometimes related 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.

CRACK IN FLANGE TO DRIVE SHAFT WELD

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.

SOLIDIFICATION CRACK

CENRE-LINE CRACK IN WELD CAPWELD

Root cracks are longitudinal cracks in the root of the weld. They are generally forms of hot cracks.

ROOT CRACK IN WELD

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. Crater cracks 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.

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.

2.2 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 the last 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.

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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.

SURFACE OR SUBSURFACE BLOWHOLES

BLOW HOLES, PINHOLES

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 that develop 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 temper ature several days after casting.

QUENCH CRACKING

2.3 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

Chapter 3.1: Visual inspection (VT)

Chapter 3.2: Radiographic testing (RT)

Chapter 3.3: Ultrasonic testing (UT)

Chapter 3.4: Magnetic Particle Inspection (MT)

Chapter 3.5: Liquid Penetrant Testing (PT)

Chapter 3.6: Eddy Current Testing (ET)

3.1 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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3.2 Radiographic testing (RT).

Method: Radiographic image is produced by the passage of X-rays or gamma rays through the test object onto a film.

Application/advantages: 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.

3.3 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.

Application/advantages: 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

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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 involving nickel 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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3.4 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.

Application/advantages: 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:

Inspection of crankshafts with hand yoke BWM 220/12 and adjustable poles

Comments:

Magnetic particle testing is used for locating surface or near surface discontinuities in ferromagnetic materials. This method involves the es-tablishment 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 buildup. 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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3.5 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 after removal of excess liquid film from the surface of the test object.

Application/advantages: 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 nonmagnetic materials such as aluminum, magnesium and austenitic steel weld-ments. It is also useful for locating cracks or other discontinuities, which may cause leaks in

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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.

3.6 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/(open in new window)

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(open in new window)

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/ (open in new window)

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 materials 5 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

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

Table 4.1 Applicability of different NDT-methods vs. defects in welded joints:

Table 4.2 Applicability at different NDT-methods vs. defects in casting

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Table 4.3 Generally accepted methods for detection imperfections

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Visual Inspection

Introduction

Visual inspection is the basic non-destructive testing method both during production processes and also during field and inservice inspection.

This chapter deals with visual field inspection with emphasis on welding inspection, for which the following may be stated: Visual inspection should be performed before, during and after fabrication of any weldment. If properly carried out, visual inspection may greatly reduce defects in the final weldment.

The main sections:

Chapter 5.1: Viewing aids and measuring gauges.

Chapter 5.2: Inspection before welding Parent metal Weld preparation, fit-up and assembly Welding consumables Preheating Electrical parameters

Chapter 5.3: Inspection during welding lnterpass temperature Back gouging Tack welds and interrun cleaning

Chapter 5.4: Inspection after welding. Cleaning and dressing Weld contour and shape of welds Weld repairs

Chapter 5.5: Imperfections associated with welding TIG welding MMA welding MIG welding Submerged Arc Welding Oxy-acetylene Welding

Chapter 5.6: Inspection reporting and records

Chapter 5.7: Quiz related to Visual Inspection

5.1 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 their background. It should be possible to vary the direction of the light to reveal imperfections in slight relief.

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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 portable TV-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. 5.2 Instrument for measuring weld profiles

5.2 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:

5.2.1 Parent metal The parent metal should be checked for correct specifications, dimensions, flatness, surface condition etc.

5.2.2 Weld preparation, fit-up and assembly The shape and dimensions of the weld preparation, including backing material are to be checked 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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Fig. 5.3 Alignment of butt welds

5.2.3 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.

5.2.4 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. Pre heat 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.

5.2.5 Electrical parameters The welding procedure will normally specify the current and voltage to be used. When assessing the

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

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 been calibrated.

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.

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5.3 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 pro-cedure.

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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5.4 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.

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The weld contour and transition to the base material may in some cases be very important 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.

Fig. 5.7 Overlap caused by weld metal flowing onto the parent metal without fusing to it. Often difficult to identify positively.

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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

Fig. 5.13 Undercut and overlap

Weld repairs

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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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5.5 Imperfections associated with welding.

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5.6 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 of fabrication 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 before further 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 are experienced during fabrication, a comprehensive reporting may be very important for future inservice inspection.

Concerning reporting, see also part "NDT Procedures and reports" (se left side menu).

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Radiographic Testing

Chapter 6.1: Introduction

Chapter 6.2: The radiographic process

Chapter 6.3: Quality of radiograph

Chapter 6.4: Film interpretation

Chapter 6.5: Advantages and limitations of radiographic testing

Chapter 6.6: Quiz related to Radiographic Testing

6.1 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

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Typical example of radiographic testing steel products

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6.2 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.

Typical 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

In tables 6.1 and 6.2 some data on x-ray machines and gamma ray sources and their applications are listed.

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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)

The penetrating power of the radiation increases with its energy. The energy of Iridium 192 radiation

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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.)

The electromagnetic spectrum

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.

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.

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*) Tin or lead alloyed in the brass will increase these factors. Table 6.3 Radiographic material thickness relative to aluminum or steel

Aluminum 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 (aluminum 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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6.3 Quality of radiograph.

6.3.1 Geometrical unsharpness

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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)

6.3.2 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).

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 10100 times depending on the screen type.

Lead salt intensifying screens combine the properties 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.

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Lead screens

6.3.3 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.

Sketch radiographic film

6.3.4 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). (fig. 6.3 and 6.4).

Calculation of sensitivity in per cent

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.

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 6.4: Film inter-pretation.

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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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Fig. 6.4 DIN and IIW penetrameters

Radiographic Sensitivity

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6.4 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.

Interpreting of radiographs

6.4.1 Identification The radiographs must be marked in such a way that no doubt can arise as to which part of the object it represents. The identification has to be beyond dispute concerning the position and orientation of the film.

Identification system

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.

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Identification, traceability between the object being tested and the film

6.4.2 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. The density 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.

Example: Calculation of density

6.4.3 Sensitivity The radiographs should be checked for sensitivity level to prove that the recommended radiographic technique is used.

Radiographic sensitivity

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.

6.4.4 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 sect. 6.4.1 6.4.4 shall be fulfilled before an evaluation of the quality/homogenity of the object is made. If one or more of these requirements is not fulfilled the inspector may find it necessary to repeat the radiographs with an improved technique.

6.4.5 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

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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.

Radiographs and sketchs of weld defects

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) in thickness 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.

Radiograps and sketches of weld defects

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6.5 Advantages and limitations of radiographic testing.

6.5.1 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

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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 for airport security checks.

6.5.2 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 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 with or 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.

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

http://www.agfa.com (open in new window) http://www.yxlon.com (open in new window) http://www.ndt.net (open in new window)

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Ultrasonic Testing

Chapter 7.1: Definition of ultrasound and properties of waves

Chapter 7.2: Methods

Chapter 7.3: Performance of ultrasonic testing

Chapter 7.4: Measurement of thickness and detection of defects

Chapter 7.5: Advantages and limitations of ultrasonic testing

Chapter 7.6: Quiz related to Ultrasonic Testing

7.1 Definition of ultrasound and properties of waves

7.1.1 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.

7.1.2 Properties of waves

The following relationship exists between the parameters frequency (f), wave length (l) and propagation velocity (v) in a propagating sound wave:

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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

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7.2 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 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 Ultrasonic thickness

7.2.1 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 (fig. 7.1).

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Fig. 7.1 The pulse-echo principle

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7.3 Performance of ultrasonic testing

7.3.1 Ultrasonic equipment

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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.

7.3.2 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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Fig. 7.2 Probes (transducers) for ultrasonic equipment. Left: normal probe 0, right: angle probe 70.

The normal probe ( 0) generates longitudinal waves and transmits t hem (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.

Ultrasonic inspection

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.

Fig. 7.3 Application of a normal probe

The angle probe is constructed to transmit transverse waves at a defined angle into a test object. Ultrasonic inspection

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.

Fig. 7.4 Application of the angle probe

Table 7.1 gives the angles of refraction in different materials for the most common types of angle probes having an angle of incidence of 35 - 80 wit h respect to steel. The acoustic velocity in cast iron depends on various factors, the quoted values being average figures.

Table 7.1 Angles relative to steel

The double crystal probe (which is a special normal probe) consists of two separated piezo-electric crystals, transmitter and receiver (fig. 7.5). Because the initial pulse has to pass an acoustic delay block before reaching the contact surface of the material, the initial pulse will not interfere with defects immediately below the contact surface. In other words, the deadzone will be greatly reduced. (fig. 7.1)

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Fig. 7.5 Principle of the double crystal probe (TR or SE probe)

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 (see also section 7.4).

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.

7.3.3 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

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7.3.4 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 for this 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.

7.3.5 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. (fig. 7.6).

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Fig. 7.6 Calibration blocks, range calibration

Range calibration V1/V2

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 fig. 7.7.

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Fig. 7.7 - Construction of reference curves

Construction of reference curve

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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).

7.3.6 Acceptance criteria

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

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Fig. 7.8 - 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.

Table 7.2 Calibration reference block requirements

7.3.7 Defect sizing

A method which is suitable for determining the size of large defects with normal probes and 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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7.4 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 % (fig. 7.9) Echoes appearing between full thickness echoes indicate lamination or other types of defects.

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Fig. 7.9 Thickness measurement using multiple echo-technique

Multiple Echo Technique Ultrasonic thickness

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, the coating 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 (fig. 7.9)

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.

Ultrasonic thickness (single crystal probe) Ultrasonic thickness (twin crystal probe)

When using double crystal probes for measurement of pipe wall thickness, be aware of correct probe position related to the axis of the pipe.

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7.5 Advantages and limitations of ultrasonic testing

The principal application of ultrasonic techniques consist of flaw detection and thickness measurement.

7.5.1 Advantages of ultrasonic tests:

1. Capable of detecting planar defects not detectable by radiography.

2. High sensitivity, permitting detection of minute defects.

3. Great penetrating power, allowing examination of extremely thick sections, e.g. up to 10 m of steel.

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

5. Fast response, permitting rapid and automated inspection.

6. May be performed with access to only one surface of the object.

7.5.2 Limitations of ultrasonic tests:

Test conditions which may limit the application of ultrasonic methods usually relate to one of the following factors:

1. Unfavourable geometry of test object; for example, size, contour, complexity and defect orientation.

2. Undesirable material structure; for example grain size, structure porosity, or inclusion content. Examples of materials difficult to test by ultrasonics are austenitic steel and welds involving nickel base alloys. Penetration of sound into these materials is limited and interpretation of results may be difficult. (It should be noted that austenitic materials are now widely used for fabrication of chemical tankers and -installations as well as nuclear reactors.)

3. Coupling and scanning problems, surface conditions etc.

4. When using normal probes, defects located less than 4-5 mm below the test objects surface is difficult to detect. (This is due to the equipment dead zone, the width of the pulse, and the probes near zone where interference will affect the measurements).

5. Due to high sensitivity false or irrelevant indications may occur.

6. Requirement to operators qualifications. (Ref. appendix II).

Information on typical ultrasonic equipment is given on below web links : www.krautkramer.com (open in new window) www.panametrics.com (open in new window)

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Magnetic Particle Testing

Chapter 8.1: Application

Chapter 8.2: Method

Chapter 8.3: Magnetization principles and methods

Chapter 8.4: MT Performance

Chapter 8.5: Surface preparation

Chapter 8.6: Examination of welds

Chapter 8.7: Non-relevant indications

Chapter 8.8: Advantages of the MT method

Chapter 8.9: Limitations of the MT method

Chapter 8.10: Demagnetization

Chapter 8.11 Acceptance criteria

Chapter 8.12: Reporting

Chapter 8.13: Quiz related to Magnetic Particle Testing

8.1 Application Magnetic particle inspection may be applied to detect surface defects in ferromagnetic materials.

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Welding inspection on reactor tubes with hand yoke and isolating transformer

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8.2 Method

The test object is magnetized Magnetic powder (iron powder or iron oxide) is applied to the surface during magnetization.

The powder will accumulate where a surface flaw causes a leakage in the magnetic field.

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8.3 Magnetization principles and methods

Direct magnetization is induced when current is passing directly through the test object, e.g. by applying prods. (Fig. 8.1)

Indirect magnetization is induced when placing the test object in a magnetic field, e.g. by means of a yoke (electromagnet). (Fig. 8.2)

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Inspection of crankshafts with hand yoke and adjustable poles

The principle of circular magnetization is shown in Fig. 8.1 and longitudinal magnetization in Fig. 8.2.

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Fig. 8.1 Circular magnetization methods

Fig. 8.2 Longitudinal (or axial) magnetization

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8.4 MT Performance

Wet particles (iron particles suspended in liquid) are recommended below 60 C.

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Inspection of turbine blades with hand yoke and adjustable poles

Dry particles are recommended between 60C and 300 C. Fluorescent particles may be advantageously used.

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It is recommended to use contrast color to provide adequate contrast when using non-fluorescent particles. The thickness of the layer should not exceed 75 um. The contrast color must not be electrically conductive.

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The highest detection sensitivity for surface cracks is obtained by applying alternating current (AC) magnetization and wet powder.

The use of permanent magnets is not recommended due to the magnetic field configuration which may mask defects in a large region around the poles. (The part of the field perpendicular to the surface will hamper the mobility of the magnetic particles, and thereby disturb the indications. Only the region between the poles with dominating field tangential to the surface may be reliably tested, fig. 8.3.)

Prods, when applied, shall be tipped with lead or aluminium to avoid copper deposits and hard spots from burns on the part being examined.

Fig. 8.3 Magnetic field configuration of a permanent magnet

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8.5 Surface preparation

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Prior to magnetic particle inspection, the surface to be examined and all adjacent areas within at least 25 mm shall be dry and free of all dirt, grease, lint, scale, welding flux and spatter, oil, or other extraneous matter that could interfere with the examination.

Rough surfaces hamper the mobility of magnetic powder due to mechanical trapping which in turn produces false indications. Such areas should be surface ground.

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8.6 Examination of welds

Recommended field strength, perpendicular to the defect, is in the range of 2,4 kA/m (30 Oersted) to 4,0 kA/m (50 Oersted). The field strength should be checked by a proper instrument (e.g. Hall probe). Field Strength Meter

Maximum sensitivity is obtained when the direction of the magnetic field is perpendicular to the defect. Prof. Berthold Test Block

As a rule of thumb the ratio current/prodseparation shall be in the range of 3 to 5 A/mm.

The prods and yoke shall be positioned as indicated in fig. 8.4 to obtain full coverage of a weld.

Fig. 8.4 Positions of prods or yoke for a 100 % coverage of a weld

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Welding inspection on tubes, longitudinal and transversal crack indication with cross yoke

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8.7 Non-relevant indications Non-relevant indications that do not result from presence of flaws may occur. Examples of such indications are:

When applying a too strong magnetic force, particle buildups may occur around sharp corners, at rough surfaces, small undercuts etc.

Changes in magnetic properties may give indications, i.e. between steel and mill scale, between different base metals or between weld metal and base metal. A well known example is non-relevant indications between non-ferromagnetic weld metal and ferromagnetic base metal.

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8.8 Advantages of the MT method A superior method for detection of surface cracks.

The method is fast and simple to carry out.

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8.9 Limitations of the MT method

The method is only applicable to ferromagnetic materials.

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Misinterpretations may occur depending on the test object surface, differences in chemical composition of welds and base materials, object geometry etc.

Limitation of temperature range (during welding).

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8.10 Demagnetization Reasons for demagnetization :

All ferromagnetic metals, after having been magnetized, will to some extent retain a residual magnetic field. Demagnetization may be necessary if :

the magnetic field will interfere with the operation of instruments sensitive to magnetic fields.

during machining or cleaning operations chips may adhere to the surface and interfere with subsequent operations like painting or dimensioning.

the test object is to be used for parts/components where remains from the magnetization is undesirable (e.g. bearings).

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8.11 Acceptance criteria

The criteria are usually specified in the relevant standard/code.

Linear surface discontinuities (cracks, linear porosity) are usually not allowed.

Undercut may be accepted within specific limits in depth and length. In addition to the magnetic particle examination, determination of the undercut depth must be performed by visual inspection.

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8.12 Reporting

Like other NDT methods the main purpose of an MPI report is to identify the object examined and to state exactly the location of the defects found. Photos and sketches are helpful enclosures to the MPI report.

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Liquid penetrant testing

Chapter 9.1: Introduction

Chapter 9.2: Penetrant Testing Materials

Chapter 9.3: Method

Chapter 9.4: Surface preparation

Chapter 9.5: Types of penetrant

Chapter 9.6: Types of developer

Chapter 9.7: Penetration and developing time

Chapter 9.8: Evaluation of indications

Chapter 9.9: Acceptance criteria

Chapter 9.10: Reporting

Chapter 9.11: Advantages and Disadvantages of Penetrant Testing (PT)

Chapter 9.12: Quiz related to Liquid Penetrant Testing

9.1 Introduction.

Liquid penetrat testing is a method that is used to reveal surface breaking flaws by bleedout of a coloured or fluorescent dye from the flaw.

The technique is based on the ability of a liquid to be drawn into a "clean" surface breaking flaw by capillary action. After a period of time called the "dwell", excess surface penetrant is removed and a developer applied. This acts as a "blotter". It draws the penetrant from the flaw to reveal it's presence. Coloured (contrast) penetrants require good white light while fluorescent penetrants need to be used in darkened conditions with an ultraviolet "black light".

The method is suitable for surface examination of all non-porous, non- absorbing materials. For ferromagnetic materials, magnetic particle testing is recommended.

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9.2 Penetrant Testing Materials.

The penetrant materials used today are much more sophisticated than the kerosene and whiting first used by inspectors near the turn of the 20th century. Today's penetrants are carefully formulated to produce the level of sensitivity desired by the inspector. To perform well, a penetrant must possess a number of important characteristics. A penetrant must:

Spread easily over the surface of the material being inspected to provide complete and

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even coverage. Be drawn into surface breaking defects by capillary action. Remain in the defect but remove easily from the surface of the part. Remain fluid so it can be drawn back to the surface of the part through the drying and

developing steps. Be highly visible or fluoresce brightly to produce easy to see indications. Must not be harmful to the material being tested or the inspector.

All penetrant materials do not perform the same and are not designed to perform the same. Penetrant manufactures have developed different formulations to address a variety of inspection applications. Some applications call for the detection of the smallest defects possible and have smooth surface where the penetrant is easy to remove. In other applications the rejectable defect size may be larger and a penetrant formulated to find larger flaws can be used. The penetrants that are used to detect the smallest defect will also produce the largest amount of irrelevant indications.

Type 1 - Fluorescent Penetrants Type 2 - Visible Penetrants

Fluorescent penetrants contain a dye or several dyes that fluoresce when exposed the ultraviolet radiation. Visible penetrants contain a red dye that provides high contrast against the white developer background. Fluorescent penetrant systems are more sensitive than visible penetrant systems because the eye is drawn to the glow of the fluorescing indication. However, visible penetrants do not require a darkened area and an ultraviolet light in order to make an inspection. Visible penetrants are also less vulnerable to contamination from things such as cleaning fluid that can significantly reduce the strength of a fluorescent indication. Penetrants are then classified by the method used to remove the excess penetrant from the part.

The methods are: Water Washable Post Emulsifiable, Lipophilic or Hydrophilic Solvent Removable

Water washable penetrants can be removed from the part by rinsing with water alone. These penetrants contain some emulsifying agent (detergent) that makes it possible to wash the penetrant from the part surface with water alone. Water washable penetrants are sometimes referred to as self-emulsifying systems. Post emulsifiable penetrants come in two varieties, lipophilic and hydrophilic. In post emulsifiers, lipophilic systems, the penetrant is oil soluble and interacts with the oil-based emulsifier to make removal possible. Post emulsifiable, hydrophilic systems, use an emulsifier that is a water soluble detergent which lifts the excess penetrant from the surface of the part with a water wash. Solvent removable penetrants require the use of a solvent to remove the penetrant from the part.

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9.3 Method

The main steps of the method are as follows :

Precleaning of the surface to be tested Drying of the surface Application of penetrant by spraying, brushing or dipping Penetration time Removal of excess penetrant

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Drying of the surface by normal evaporation or by careful blowing with a fan or hair dryer

Application of developer as a thin layer by dipping, spraying, or by use of dusttank Developing time Inspection of the test object Post cleaning (if required)

Principle of Liquid Penetrant Testing

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9.4 Surface preparation

The surface to be examined must be dry and free from paint, dirt, grease, lint, scale, welding flux, weld spatter, oil or other extraneous matter that could obscure surface openings or otherwise interfere with the examination (machining and grinding may close surface cracks mechanically).

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9.5 Types of penetrant

Three types of penetrant exist in both visible (most commonly red) and fluorescent color. Ordinarily, fluorescent examination is the most sensitive.

Water washable penetrant Water washable penetrants are most frequently used and are sensitive enough for ordinary weld examination. For rough surfaces this is the only suitable type of penetrant.

These penetrants may be removed from the surface by water washing. A none dusting clean cloth or free flowing water may be used.

Post emulsifying penetrant Post emulsifying penetrants are mainly used on smooth surfaces. For such surfaces this type of penetrant has a higher sensitivity than the water washable penetrant.

After the necessary penetration time a thin continuos layer of emulsifier is to be added to the top of the penetrant. The emulsifier will interact with the penetrant.

The resulting liquid from this interaction is water washable.

After an emulsifying time, depending of the type of surface, the liquids used and the temperature, all surface penetrant may be washed away without disturbing the penetrant inside the surface discontinuities.

Solvent removable penetrant For low temperature examination and for examination of smooth surfaces the solvent removable penetrant is recommended.

Excess penetrant is removed from the surface by wiping with a dry absorbing (non-dusting) cloth followed by re-wiping the surface using a clean cloth damp with a solvent remover.

Testblock

Normal temperature range for liquid penetrant examinations: 15C 50C (60 F 125F) Above and below this temperature range liquids suitable for high/ low temperature examination

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are to be used. A non-standard temperature requires a procedure qualification with a comparator block. For details see ASME Boiler and Pressure Vessel Code Sec. V, art. 6.

Comparator block

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9.6 Types of developer

Nonaqueous wet developer, which is a powder suspended in a volatile solvent.

Spraying with nonaqueous developer from a min. distance of 30 cm gives the best result for field work.

Dry developer, which is a dry powder, less suitable for field use.

Aqueous wet developer, which may be either a powder suspended in water or a powder water solution. The aqueous wet developer is suitable for high temperature examination.

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9.7 Penetration and developing time

It is important for the test to use sufficient penetration and developing time. Recommended times are given in table 9.7.

Table 9.7 Recommended penetration and developing times

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9.8 Evaluation of indications

Discontinuities at the surface will be indicated by bleeding-out of the penetrant, however, local surface irregularities such as machining marks may produce false indications.

Insufficient removal of excess surface penetrant may also produce red/ fluorescent shadows or false indications.

To evaluate indications, use a thin brush dipped in a solvent. Carefully remove just the colored developer. Apply a new thin layer of developer. If the indication reappear, a disc