Electromagnetic testing emt-acfm chapter 10b

Charlie Chong/ Fion Zhang

Electromagnetic TestingMFLT/ ECT/ Microwave/RFTChapter 10B – ACFMAlternating Current Field Measurement 10th Feb 2015My ASNT Level III Pre-Exam Preparatory Self Study Notes


Expert at Works - The alternative NDT Method








ACFM- The alternative NDT Method


Experts at Work - The alternative NDT Method








Experts at Work - The alternative NDT Method


Experts at Work - LRUT






Expert at Works - The alternative NDT Method for surface breaking crack of significant dimensions


Experts at Works - The alternative NDT Method for surface breaking crack of significant dimensions



NDT Level III ExaminationsBasic and Method ExamsASNT NDT Level III certification candidates are required to pass both the NDT Basic and a method examination in order to receive the ASNT NDT Level III certificate.Exam SpecificationsThe table below lists the number of questions and time allowed for each exam. Clicking on an exam will take you to an abbreviated topical outline and reference page for that exam. For the full topical outlines and complete list of references, see the topical outlines listed in the American National Standard ANSI/ASNT CP-105, Standard Topical Outlines for Qualification of Nondestructive Testing Personnel.

ETElectromagnetic Testing135 Questions 4 hrs Papers Certification: NDT only

Fion Zhang at Shanghai2015 February

苏州太湖 2014



The Reading Magic


The Reading Magic – The Journey Start Here






Alternating Current Field Measurement ACFM - Reading Session Two

Note: ACFM is a trade mark owned by TSC Inspection Systems, Milton Keynes, England


Reading 1: Article on ACFM application for Interail (TSC)Keypoints:ACFM is a non-contact electromagnetic technique for the detection of small surface breaking cracks in metals. A coil generates an electromagnetic field across the surface of a metal sample, (this electromagnetic field induced a constant electric field on the metal penetrating to a depth depending on the frequency of the inducing coil.)

http://www.interailproject.eu/files/Workshop/05%20-%20TSC.pdf


The Me


In a basic alternating current field measurement system, a small probe is moved along the toe of a weld. The probe contains an exciter coil, which induces an AC magnetic field in the material surface aligned to the direction of the weld. This, in turn, causes alternating current to flow across the weld. – ASTM E2261-12


If the induced currents encounter a surface breaking crack they are forced to flow around or underneath the defect

Underneath the defect

Flow around

Linear Defect


Small sensor coils are placed a few millimeters above the surface. These detect the changes in the magnetic field caused by the crack.

Pick-up sensor coils

Flow aroundUnderneath the defect


More Reading on the principle of ACFMThe A.C. field measurement (ACFM) technique was developed from the A.C. potential drop (ACPD) technique which has been used for crack sizing and crack growth monitoring. ACPD has been used underwater even though electrical contact has to be maintained between the probe and the component being inspected. The ACFM technique is simpler in operation as it depends on the measurement of the near-urface magnetic fields rather than the surface electric fields, thus requiring no electrical contact. Theoretical work carried out at the Wolfson NDE Centre in the Mechanical Engineering Department of University College London determined the relationship linking these two fields. Thus existing models of electric fields around cracks can be used to size cracks using magnetic field measurements. This nonontacting sizing capability relies on the use of unidirectional input current in the region under inspection, similar to that required for the ACPD technique.

http://erikatchison.com/userfiles/file/Documents/ACFM%20Inspection%20Procedure.pdf


For the ACFM technique, the input current is induced into the specimen thus making the system fully non-contacting. In single probe ACFM operation the Crack Microgauge passes two signals to the ACFM crack detection and sizing software (QFMu). The first is the magnetic field strength measured in the direction parallel to the crack edge (Bx) and the second is the magnetic field strength measured in a plane perpendicular to the surface of the metal (Bz). The software (QFMu) then displays these signals in three forms; the Bx and Bz traces separately against a timebase, a dual digital meter display, and a polar plot display in which one component is plotted against the other. This latter form is known as a butterfly plot because of the characteristic trace produced by a defect.



The Probes for Rail Head ACFM


The Probes integrated set-up for Rail Head ACFM


The Probes integrated set-up for Rail Head ACFM


The Probes integrated set-up for Rail Head ACFM -The 75 KPH Trial on Rail Heads


The Probes integrated set-up for Rail Head ACFM The 75 KPH Trial on Rail Heads


Laboratory Trial: The 75 KPH Trial on Rail Heads








Laboratory Trial:The 75 KPH Trial on Rail Heads

Three artificial slots werecut into one of the four railheads– 20 x 3 mm at 30º– 20 x 5 mm transverse– 30 x 5 mm transverse


Laboratory Trial: The 75 KPH Trial on Rail Heads Results

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Laboratory Trial: Grinder Wheel Experiments at 80 kph










Laboratory Trial: Grinder Wheel Experiments at 80 kph Results


Conclusion

TSC have proven the ACFM system works at 80 kph, with defects just 12mm in length, and just a few mm in depth RCF like geometry

By varying the liftoff, it was possible to estimate noise due to vibration on track

System could detect RCF 12 x 2 mm in size, at 80 kph with a liftoff of 2.3±0.5 mm

This far exceeds original project specification


The actual application: The Test Assemblies fitted to the locomotive.








Reading 2: Brochure- Alternating Current Field Measurement (ACFM)

Alternating Current Field Measurement (ACFM) is an electromagnetic technique for the detection and sizing of surface breaking cracks. The main advantage of the technique is that it works through several millimeters of coatings. This means that paint and other protective coatings do not have to be removed and then reapplied.

Features Can be carried out while the vessel / pipe work is still in service Certain types of probes can inspect at elevated temperatures Minimal preparation before the test is required The system holds permanent records of all indications Minimal disruption to the plant High productivity of the ACFM equipment makes it very efficient


DiscussionSubject: Discussed on sentence “several millimeters of coatings.” w.r.t sensitivity, lift-off, POD.


Experts at Work


The ACFM Amigo U19 Crack Microgauge


Technique The ACFM Amigo U19 Crack Microgauge uses a probe to induce a uniform alternating current in the area under test and detects the resulting current flow near to the surface. The current is undisturbed if the area is free of surface breaking cracks. A surface breaking crack will redirect the current around the ends and faces of the crack.

The ACFM instrument measures these disturbances in the field and uses mathematical algorithms to estimate the crack depth.


The Current Perturbation and the Mathematical Algorithms

Bz

Bz

BX


Capabilities

1. No need to remove paint or thin coatings. 2. Detects and sizes both crack length & depth. 3. Offline analysis of data. 4. Provides a permanent record of indications. 5. Ongoing monitoring capability. 6. No chemical agents & therefore requires no COSHH assessment. 7. Provides an immediate evaluation of the weld area. 8. Quick & efficient method of inspection. 9. High temperature capability. 10.Works equally well on plain material or welds. 11.Will inspect ferritic & non-ferretic materials.


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Applications 1. Structural weld inspection 2. Offshore cranes 3. Storage tanks floor & roof ‘lap’ joints 4. Storage tank annular welds internal & external 5. Vessel nozzles

Limitations 1. Not recommended for short sections or small items 2. Locations of weld repairs & grinding can cause spurious indications 3. Crack dimensions need to be greater than 5-10mm long and 0.5mm deep 4. Multiple defects reduce the ability to depth size cracks


ExerciseSubject: Based on the information above discuss on the applicability of ACFM Amigo U19 Crack Microgauge in code compliance new construction of tank, jacket, piping and others.


Reading 3: The alternating current field measurement (ACFM) crack detection and sizing technique and its application to in-service inspection. A Raine technical software consultants Ltd.

Keypoints:

Used primarily for the detection of fatigue cracks in offshore structures both on the subsea and topside structural sections.

The inspection applications were then extended to the inspection of pressurised systems and process plant i.e. pressure vessels and pipe work.

Keywords:Abseiling –rope access

http://www.ndt.net/article/wcndt00/papers/idn644/idn644.htm


The Technique- Mathematical ModellingThe ACFM technique is an electromagnetic non-contacting technique which has been developed to be able to detect and size surface breaking defects in a range of different materials and through coatings of varying thickness. The basis of the technique is that an alternating current flows in a thin skin near to the surface of any conductor. When a uniform current is introduced into the area under test if the area is defect free the current is undisturbed. If the area were to have a crack present then the current would flow around the ends and the faces of the crack. A magnetic field is present above the surface associated with this uniform current and this will be disturbed if a surface breaking crack is present. It was realisedthat if these disturbances could be measured they should have some relationship to the defects that had caused them. University College London carried out studies into the mathematical modelling of these magnetic fields and their associated disturbances.

Keywords:Uniform AC currentCurrent disturbances on the uniform AC field, induced magnetic field disturbances, mathematical modelling of the disturbed magnetic fields.


A good correlation was produced between the theoretically predicted magnetic field disturbances and those measured and thus showed that it was possible to make quantitative measurements of the magnetic fielddisturbances and relate them to the size of the defects which produced them.

Special techniques are used to induce these electric currents and the components used are built into the ACFM probes Small detectors or sensors are also built into the probe, which measure the magnetic field disturbances.

The probe is scanned longitudinally along the weld with the front of the probe parallel and adjacent to the weld toe. Two components of the magnetic field are measured, the Bx along the length of the defect which responds to changes in surface current density and gives an indication of depth when the reduction is the greatest and Bz which gives a negative and positive response at either end of the defect caused by current generated poles. This gives an indication of length. Figure 1.


DiscussionSubject: Discuss on the sentence “Special techniques are used to induce these electric currents and the components used are built into the ACFM probes Small detectors or sensors are also built into the probe, which measure the magnetic field disturbances.”



Figure 1: Time Based ACFM Dispaly


A physical measurement of defect length indicated by the probe position is then used together with a software program to determine the accurate length and depth of the defect.

In order to aid interpretation the Bx and Bz components are plotted against each other and when a complete loop indication is produced this confirms the presence of a crack. This is called the Butterfly plot Figure 2 and because it is not sensitive to probe speed aids in the interpretation of the data collected and confirms defect indications.

During the application of the ACFM technique actual values of the magnetic field are being measured in real time. These are used together with mathematical model look-up tables so that there is no need for calibration of the ACFM instrument using a calibration piece with artificial defects such as slots.


Bz

Bx


DiscussionSubject: Discuss on the sentence “These are used together with mathematical model look-up tables so that there is no need for calibration of the ACFM instrument using a calibration piece with artificial defects such as slots.”


Designation: E2261/E2261M − 12Standard Practice for Examination of Welds Using the Alternating Current Field Measurement Technique

10. Alternating Current Field Measurement Reference Standards

10.1 Artificial Slots for the Operation Reference Standard: 10.1.1 The operation reference standard has specific artificial discontinuities. It is used to check that the instrument and probe combination is functioning correctly. It may also be used for standardization of the equipment for nonmagnetic materials. Unless otherwise specified by the client or equipment manufacturer, the artificial discontinuities for the operation reference standard are elliptical or rectangular slots. The slot geometry will be specified by the equipment manufacturer to be consistent with the crack size estimation model. Typical slot dimensions are as follows:10.1.1.1 Elliptical Slots—Two elliptical slots placed in theweld toe with dimensions 2.0 in. × 0.2 in. [50mm × 5mm] and0.8 in. × 0.08 in. [20 mm × 2 mm]. (Fig. 3, discontinuities Aand B.)..bra..bta bra…


In-service Inspection

In-service inspection can include many different areas of activity including the offshore industry, public safety, petrochemical, civil and mechanical engineering. All of these sectors have their unique problems in the forms of access; different forms of coating, from the protective to the cosmetic and the type of materials, which require inspection. In the majority of cases fatigue type cracks need to be detected, these are common in the offshore industry and the civil, mechanical engineering and public safety sectors. In the petrochemical industry environmental cracks such as stress corrosion cracking, hydrogen sulphide cracking and stress orientated hydrogen cracks are required to be detected.


The ACFM technique was originally developed for the inspection of carbon steel welds on subsea structures, which were usually nodal welds. A number of probes were developed, a general purpose weld inspection probe, a 30 degree angle probe for examining tight angle geometry's and a pencil probe specially designed to examine welds that had been subjected to grinding. This was used to inspect the bottom of the ground toe of the weld to determine if defects were present and then determine their length and depth or to confirm that the defect had been removed. During a trial organised by University College London where samples were produced to reproduce some of the difficult geometry's and access problems located in process plants, it was found that additional probes were required to gain access and detect and size the defects located within the samples. A range of mini and micro pencil probes has now been produced with straight and 90 degree access with increased sensitivity.

Keywords:Standard probes, pencil probes, angle probes, micro pencil probes, costumed make probes (thread probes).


In addition to this it was realised that the inspection of short lengths of weld also created problems in that the communication rate was too slow to produce a good representation of the weld result on the VDU screen. New software has now been produced that eliminates this problem including communication rates, which allows scanning speeds seven times faster than before.

Keywords:Communication rate.


This allows greater presentation on the screen for shorter lengths of welds and faster scanning speeds for the inspection of long lengths of weld. The technique was also used to inspect structures that had been coated with protective or anti fouling coatings so that the expensively applied coatings did not have to be removed and reapplied thus avoiding costly preparation and reinstatement. The topside inspection engineers also adopted the technique for the inspection of process and pressurised plant, structural steelwork and crane pedestals. The system was used in conjunction with rope access teams allowing inspection without scaffolding and proving the usefulness of two man operations and the Butterfly plot. Inspections could be carried out up to 50 metres between the ACFM operator and the probe pusher. The technique has also been applied to the inspection of drill threads on casing and drill tools. A special transportable system has been produced to automatically inspect the drill thread ends and classify them. This provides Go-NoGo reporting. The system is based on new ACFM array technology. A hand held probe has also been produced to inspect drill threads with the portable ACFM system.


Thread Inspection ACFM Probe


New materials are being used for components and coatings on offshore structures but the ACFM system has now been successfully applied to ferritic steels, austenitic stainless steels, aluminium, duplex, super duplex, moneland inconel. It has also been used to inspect through the following coatings, flame sprayed aluminium, epoxy coating, standard paints, ferrite based paints and copper coated threads. Some inspections have to be carried out when the plant is operational and ACFM has been used during inspections at - 0°C and up to 500°C. Because of the above advantages the ACFM technique has been used to inspect coated flare booms, epoxy coated pig traps, painted nozzle welds, pipe butt welds, pipe and saddle support welds and pressure vessel seam welds as well as the above mentioned inspections.


THEME PARK INSPECTIONTheme park rides are made up of several component parts. The structural section of the ride is very similar to the tubulars found in offshore structures with fairly long chord and brace node welds in the track and support areas and thus the problems of inspection are more of access than geometry. The foundation base sections have short fillet welds with access holes similar to those found in offshore sections.

Samples were made to examine UK technicians using the ACFM technique to inspect topside production plant and were found to have similar geometry to that found in the production of roller coasters. The carriages, axles and carts or trucks have a different problem. The majority of the welds on these components is short and has difficult access. This creates two problems one of end effect and the other of weld presentation. To reliably inspect these welds there is a requirement to have small probes with high sensitivity and little response to edge effect and hard wearing probe faces.


The communications rate between the ACFM instrument and the computer needs to be fast to obtain a meaningful length of weld on the screen of the computer. The alternative is to scan slowly.

Technical Software Consultants have addressed these problems with the introduction of the mini and micro pencil probes. Both of these probes have either straight or 90-degree access and have stainless steel probe faces. The mini and micro probes have slightly different sensitivity in that one can detect defects 0.04" deep and the other 0.02" These probes are particularly suited for the detection of shallow defects in tight access areas.

A new range of control software QFM 2 has also been produced which has additional features such as a faster communications rate allowing scanning speeds of up to 2"/second.This can be used for scanning long welds faster or producing longer images on the computer screen for short weld inspection. This software also allows automatic centralisation of the data display and the ability to select and print single scans of data.


Different values of lift off can also be selected in order to inspect through different thicknesses of coating.

The combination of these developments will allow the experience gained from critical offshore inspection to be applied to the inspection of the theme park components so that they can be carried out more efficiently and reliably.

In one theme park the track of one of the rides is made up of 300 ties each one having 70 welds of varying length and geometry. During the annual shutdown of this ride a number of these ties are cleaned, inspected using magnetic particle inspection techniques and then the ties are repainted. This normally takes three weeks, one for the cleaning, one for the inspection and one for refurbishment and repainting. This is one of the major problems, as the paint has to be matched as closely as possible with the original colours. During one inspection 30 ties were inspected with magnetic particle inspection.


During the next inspection the ACFM technique was used. No prior cleaning was required and 64 ties were inspected in four days and one day was used for repairs and re-inspection. No additional painting was required except for the localised painting where the repairs had taken place. In an industry where the customer expects all of the rides to be available when they visit the theme park the reduction in down time is very important. The ACFM technique has now being used in a number of theme parks and a mechanised system has been installed in one park where more than 1000 supports require inspection. Using the new system the inspection time/ support has been reduced from 8 minutes to 3minutes.

Keypoints: Advantages■ Speed of scanning,■ No removal of paint or coating.


Keypoints:

Two problems one of end effect and the other of weld presentation. To reliably inspect these welds there is a requirement to have small probes with high sensitivity and little response to edge effect and hard wearing probe faces.

The communications rate between the ACFM instrument and the computer needs to be fast to obtain a meaningful length of weld on the screen of the computer. The alternative is to scan slowly.

Different values of lift off can also be selected in order to inspect through different thicknesses of coating.

Multiple configured probes necessary.




Highway Construction

There are 240,000 welded steel bridges in the USA with an average age of 45 years and of these there are 58.9%, which are structurally defective. The major form of failure in these bridges is fatigue. Reference (1) In 1967 one steel bridge collapsed and 49 people were killed. The initial failure was a 1/8th deep fatigue crack in an eye bar. A second bridge failed in 1980, which was also caused by fatigue. In the USA there are 27,000 bridges classed as fracture critical and because of this there is a need to have an efficient and reliable NDT technique. In the opinion of the Federal Highways Authority NDT is still not used efficiently during operations and maintenance. Reference (2) In the UK there are similar problems with the increasing use of heavy road transport and the Euro regulations allowing heavier axle weight. The combination has caused not only fatigue problems but also bridge deck problems.




There are a number of road bridges produced from box girder construction, which have longitudinal as well as transverse cracking. Unfortunately the majority of these welds are coated and to clean and inspect would be very expensive and labour intensive. These box sections are about 40' long with both horizontal and vertical welds present. The problem of inspecting for and detecting fatigue cracks through coatings has been well known in the offshore industry for over thirty years and is now being tackled with the use of electromagnetic techniques such as the ACFM technique. The results obtained following inspection are a major factor in calculating the structural integrity of these welds and determining the valid life of the welded joint in terms of Probability of Failure and Reliability Index.


The problems of inspection of road bridges are not unrelated to that of the inspection of offshore structures in that the material is steel which is coated, the welds have difficult access and geometry and the inspection has to be reliable and repeatable. These were the same problems and background with which ACFM technique was presented. The ACFM technique has sincesuccessfully overcome these problems using the portable unit, two man rope access, specially developed probes and communication techniques and has been used to carry out inspection of coated steel structures such as offshore structures and bridge sections. One other problem which has arisen is the failure of overhead signs and light supports which has been subject to high cycle fatigue. This has caused fatalities in one county in the UK and also in one of the Northern States of the USA and has caused the inspection and design of these structures to be re-examined. Some of these designs such as the flagpole design where the weld is on the elbow may have to be changed. Because of their location it is not easy to remove the protective coating inspect and re-coat without causing some disruption to the traffic flow. Comparative trials has shown that there can be a 60% saving in time and cost when changing from magnetic particle inspection to a non contacting technique such as ACFM.



INSPECTION OF DUPLEX AND SUPER DUPLEX MATERIAL

Duplex and super duplex stainless steel has been developed to combine the high mechanical strength of carbon steels with the corrosion resistance of conventional stainless steels. In situations where corrosion resistance is required, they allow the use of thinner section components than before, thus giving great weight and cost reductions. Duplex steels are being increasingly used for process and production pipework operating at high temperatures and pressures. Although the fatigue crack propagation rate is similar to carbon steel the reduced stiffness gives higher fatigue loading and this combined with the thinner walls gives a reduced fatigue life. This makes the detection of fatigue cracking at an early stage more important.


The complex metallurgical structure of duplex steels produces problems with most NDT techniques. The complex structure is produced because duplex can occur as islands of ferrite in an austenite matrix or vice versa. If it is the former the material will perform as if it was an austenitic material and the current flow would penetrate several millimetres. If the structure were of austenite islands in a ferrite matrix then the penetration would be only a fraction of a millimetre. The ACFM technique performs equally as well under both conditions but has the added advantage that in the austenitic situation the technique has detected internal defects and defects occurring on the inner surface.

Keypoints: If it is the former the material will perform as if it was an austenitic material

and the current flow would penetrate several millimetres.

If the structure were of austenite islands in a ferrite matrix then the penetration would be only a fraction of a millimetre.


Duplex Steel


The Standard Penetration

δ = (πfμσ) -½

The governing factors: μ, σ

Where:δ = Standard Depth of Penetration (mm)π = 3.14f = Test Frequency (Hz)μ = Magnetic Permeability (H/mm)σ = Electrical Conductivity (% IACS)


When inspecting ferrite based duplex it was found that the depth-sizing model was quite accurate but with the austenite based duplex a 10% inaccuracy was detected. This was overcome by multiplying the value determined by a compensation factor. So far over 15000 welds have been examined using the ACFM technique with a good detection rate and a low number of false calls. New procedures are being developed using higher frequencies to allow discrimination between surface and subsurface defects.

Keywords:Ferrite based duplex (?)Austenite based duplex (?)


PRESSURE VESSEL INSPECTION: INSPECTION FOR TRANSVERSECRACKS IN A PRESSURE VESSEL SHELL PLATE

A section of 1" shell plate which had a 3/4" wide weld running across it was presented for inspection. A micro ACFM inspection probe was used to scan the toe of the weld using the normal scanning procedure. Several positive Bx indications were noted indicating the presence of transverse cracking. The probe was then used to scan along the centre of the weld cap from one end of the section to the other. During this scan nine strong transverse crack indications were noted. Their locations were noted along the length of the weld. Two more indications were also noted, one was located at 45° between defects 5 and 6 and this was located during the longitudinal weld toe scan and gave a weak indication. A second indication was noted between defects 4 and 5 also during the toe scan but this was only 4mm long (also transverse) and thus was not covered by the weld cap centre line scan. The defects were then sized using the normal ACFM procedure to produce length and depth information.


Pressure Vessels


COKE DRUM INSPECTIONThese drums are subject to thermal and mechanical fatigue due to the process of producing fine grade carbon then using drills to release the final product. Cracking occurs at the bottom section of these vessels but detection is difficult because of the very rough surface. The oil company involved normally allocated two weeks down time for the inspection of six drums using conventional inspection techniques such as magnetic particle inspection and manual ultrasonic inspection. One total inspection of a drum took eight hours with the ACFM technique and the results were compared with the other techniques and gave a good correlation. The oil company has specified the ACFM technique for the inspection of these drums and it is estimated that the total inspection will take four days. The ACFM technique is going to be used to detect and then monitor the crack growth during the life cycle of the coke drums.


Coke Drums


Coke Drums


Delay Coker

http://en.wikipedia.org/wiki/Delayed_coker


DETERMINATION OF PRESSURE VESSEL LIFEA number of pressure vessels were nearing the end of their determined life but the plant operator wanted to extend the operational life. Inspection of the internal welds had been carried out from the external surface with an ultrasonic technique and detects had been detected. The ACFM technique was then used from the inside to confirm the presence and determine the extent of the internal cracking. The technique was able to measure the length and the depth of the defects and from these measurements it was possible to determine the structural integrity of the vessels and allow them to remain in service. The ACFM technique is now used as the front line and verification inspection technique.

INSPECTION OF DAMAGED AREA OF A REFINERYA reactor located in a refinery in Minnesota had been damaged in a fire. Following conventional inspection ten defects were located but because of access problems it was not possible to examine them with manual ultrasonic techniques to determine their depth. In subzero temperatures the ACFM crack microgauge and a micro pencil probe was used to determine the depths of the defects. The only problem the inspector had with this inspection was that because the temperatures were so low the screen of the laptop froze.


REACTOR CATALYST BED SUPPORTThe catalyst bed support consisted of a 3" internal strip running around the total circumference of the vessel. Internal diameter cracks were suspected. The ACFM technique was used and only cracks at the toes of the welds were detected. These were sized and run or repair decisions were based on these results.

REACTOR INSPECTIONThese particular reactors had a number of nozzles, which were known to be subject to thermal and mechanical fatigue. Plates had been fabricated to reinforce these nozzles and these required regular inspection during the operating process. The external welds were inspected with the ACFM technique and all cracks detected were sized.


ENVIRONMENTAL CRACK DETECTION: CRACKING IN DISTILLATIONTOWERSThe distillation towers were manufactured from carbon steel and the internal surface was subject to a hydrogen sulphide atmosphere producing environmental cracking. The surface was very difficult to clean and the inspection was carried out using the ACFM technique to detect the onset of cracking and to determine the degree of damage by measuring the depth of the cracks.

ABSORBER TOWER INSPECTIONThe absorber towers in gas plants are manufactured from thick wall carbon steel. Due to the production process they are subject to hydrogen induced cracking on the tray support welds on the internal surfaces. These welds were inspected and crack depths measured.


RING TYPE JOINT FLANGES These particular flanges were manufactured from 21/4%Chrome Molybdenum steel which had a weld overlay produced from 347 stainless steel. Stress corrosion cracking was suspected in the weld overlay and decisions had to be made whether to continue operations or repair the cracks.

REFINERY COKE DRUM CRACKINGThe coke drum was manufactured from carbon steel and the internal surface was clad in 410 stainless steel. Cracking was occurring in the clad material and the ACFM technique was used to determine the depths of the cracking and to determine if the cracking had extended into the parent material. The ACFM technique was used to detect and measure the size of the cracks in terms of length and depth and a decision was made based on this information whether to repair or monitor.


HIGH TEMPERATURE INSPECTION AND MONITORING OF CRACKS:RE-CERTIFICATION OF A GAS PROCESS PLANT AT HIGH AND LOWTEMPERATURES

The ACFM technique has found many applications in refineries and process plant, but one of the most successful has been in weld inspection at high and low temperatures. Because the probes do not have an inner core they are not subjected to the Curie point limitations which affect some electromagnetic techniques. The standard probes can be scanned at medium temperatures up to 200°C on process plant and high temperature applications at 500 °C on pressure vessels. The same probes can also be used to inspect plant with surface temperatures of -20 °C. A gas process plant in Scotland was coming to the end of its first year in service and recertification was required. Because a major part of the plant required inspection the only way that it would be possible to carry out this inspection would be to do some of the inspection prior to and after the shutdown period. This would mean inspecting the plant live at high and low temperatures.


A ten-week inspection programme was organised to include the two-week shutdown period. Pressure vessels, separators, saddle welds, girth welds, pipe work and fractionating columns were inspected over this period using the same probe with a 25 metre cable length and a U9 crack microgaugeinstrument. Six defects were detected and these ranged from 10mm- 70mm long x 4mm deep. The operating temperatures of the plant ranged from 225°C to - 20°C.

Because of access problems it was not possible always to locate the crack microgauge near to the worksite. This problem was overcome by using a 25-metre probe cable from the unit to the probe and a 30-metre cable between the unit and the laptop. The probe operator communicated with the crack microgauge operator using a head-up display system and two way audio communications. This enabled the inspection of fractionating columns and other high parts of the plant to be inspected without the use of scaffolding. The ability of the technique to be used to inspect through coatings as well as at high and low temperatures meant that critical areas which could not be inspected on line were inspected during the shutdown period and the remainder was inspected on line.


During the first inspection ACFM was used as the front line inspection tool with verification of defects being carried out by local coating removal and MPI followed by grinding to remove the defects. Good correlation was found between the techniques and verification is not now carried out and the ACFM technique has been used during every maintenance shutdown period.

Keywords: Through coating High & low temperature application No curie temperature limitation Remote interpretation.


STAINLESS STEEL PRESSURE VESSELA particular client had a requirement to monitor stainless steel pressure vessels operating at 500°C. It was not possible to shut down the plant so a number of probes were developed to be able to operate continuously at these high temperatures. The probes for inspection were manually deployed whereas the probes for monitoring were continuously mounted on the vessels beneath the lagging and automatically monitored remotely.

PROCESS PIPE WORKThis was another case of crack monitoring. Cracks had been observed in the circumferential welds of a pipeline operating at 750°F and the plant operator did not want to shut down the process plant. Manual ACFM inspection was used to determine the size of the cracks and the fracture mechanics engineer was able to determine the structural integrity of the welds. The plant was allowed to continue operating and the cracks were monitored on a regular basis.


FABRICATION OF PIPE WORKA new 347 stainless steel pipe line was been fabricated and in the past the process has been to produce the root weld allow the weld to cool down then inspect with dye penetrant. Any repairs were then carried out and the next pass laid down after re-heating. At each inter-pass stage the weld was allowed to cool, then inspected and then reheated to allow welding to continue. The ACFM system together with a pencil probe was used instead of the dye penetrant inspection as a quality control tool. No cooling below the re-eating temperature was required and the weld production increased. The specialised welding time had been reduced from 12 hours /weld to 2 hours /weld because of the reduced inspection time and heat cycle time. No repairs ere necessary during the fabrication.

Keypoints:High temperature application.Pencil probe for weld bead inspection.


COMMENTSThe ACFM technique's reputation has grown based on initial work carried out by the Department of Mechanical Engineering, University College London in its work on detection and sizing of surface breaking fatigue cracks. This showed that the ACFM technique had a high level of Probability of Detection and characterisation of defect length and depth. This enabled the technique to establish itself in the offshore and mechanical and civil engineering world as well as the petrochemical industry for the detection of surface breaking fatigue cracks. From there it has progressed and shown that it is equally successful in the detection and sizing of environmental cracks. The technique has also been used to inspect nonmagnetic materials such a stainless steel and titanium and to inspect through coatings making it a useful and adaptable inspection technique for the new millennium.


UCL


Reading 4: ASTM Designation: E2261/E2261M -12 Standard Practice for Examination of Welds Using the Alternating Current Field Measurement Technique

Keypoints: 3.2.1 exciter— a device that generates a time varying electromagnetic field, usually a coil energized with alternating current (AC); also known as a transmitter.

3.2.2 detector— one or more coils or elements used to sense or measure a magnetic field; also known as a receiver.


3.2.3 uniform field— as applied to nondestructive testing with magnetic fields, the area of uniform magnetic field over the surface of the material under examination produced by a parallel induced alternating current, which has been passed through the weld and is observable beyond the direct coupling of the exciting coil.

3.2.4 graduated field— as applied to nondestructive testing with magnetic fields, a magnetic field having a controlled gradient in its intensity.

Keypoints:The Uniform Field:The uniform magnetic fieldThe uniform electric field?


3.3 Definitions of Terms Specific to This Standard:

3.3.1 alternating current field measurement system—the electronic instrumentation, software, probes, and all associated components and cables required for performing weld examination using the alternating current field measurement technique.

3.3.2 operational reference standard—a reference standard with specified artificial slots, used to confirm the operation of the system.

3.3.3 Bx—the x component of the magnetic field, parallel to the weld toe, the magnitude of which is proportional to the current density set up by the electric field.

3.3.4 Bz—the z component of the magnetic field normal to the inspected base metal/heat affected zone surface, the magnitude of which is proportional to the lateral deflection of the induced currents in the plane of that surface.

3.3.5 X-Y Plot—an X-Y graph with two orthogonal components of magnetic field plotted against each other.


3.3.6 time base plots— these plot the relationship between Bx or Bz values with time.

3.3.7 surface plot— for use with array probes. This type of plot has one component of the magnetic field plotted over an area, typically as a color contour plot or 3-D wire frame plot.

3.3.8 data sample rate— the rate at which data is digitized for display and recording, in data points per second.

3.3.9 configuration data— standardization data and instrumentation settings for a particular probe stored in a computer file.

3.3.10 twin fields— magnetic fields generated in two orthogonal directions by use of two exciters

NOTE 1— Different equipment manufacturers may use slightly different terminology. Reference should be made to the equipment manufacturer’s documentation for clarification.


The surface plot


The Bx, Bz time base plots


4. Summary of Practice

4.1 In a basic alternating current field measurement system, a small probe is moved along the toe of a weld. The probe contains an exciter coil, which induces an AC magnetic field in the material surface aligned to the direction of the weld. This, in turn, causes alternating current to flow across the weld.

The depth of penetration of this current varies with material type and frequency but is typically 0.1 mm [0.004 in. ] deep in magnetic materials and 2 ~ 7 mm. [0.08 - 0.3 in] deep in non-ferrous materials.

Any surface breaking discontinuities within a short distance of either side of the scan line at this location will interrupt or disturb the flow of the alternating current. The maximum distance from the scan line to a target discontinuity, potentially detectable at a specified probability of detection, is determined by the probe assembly size, but is typically 10 mm [0.4 in ]. Measurement of the absolute quantities of the two major components of the surface magnetic fields (Bx and Bz) determines the severity of the disturbance (see Fig. 1) and thus the severity of the discontinuity.

Keywords: absolute quantities


FIG. 1 Example Bx and Bz Traces as a Probe Passes Over a Crack (The orientation of the traces may differ depending upon the instrumentation.)


Discontinuity sizes, such as crack length and depth, can be estimated from key points selected from the Bx and Bz traces along with the standardization data and instrument settings from each individual probe. This discontinuity sizing can be performed automatically using system software.

Discontinuities essentially perpendicular to the weld may be detected (in ferritic metals only) by the flux leakage effect. However confirmation of such transverse discontinuities (and detection of the same in non-ferritic metals) requires scans with the induced magnetic field perpendicular to the direction of the weld.

Keypoints:

■ Primary induced magnetic field perpendicular to the direction of the weld.■ Induced eddy current parallel to the weld■ Electrical perturbation, the defect run transverse to the weld in the same

direction as the primary induced magnetic field.


4.2 Configuration data is loaded at the start of the examination. System sensitivity and operation is verified using an operation reference standard. System operation is checked and recorded prior to and at regular intervals during the examination. Note that when a unidirectional input current is used, any decay in strength of the input field with probe lift-off or thin coating is relatively small so that variations of output signal (as may be associated with a discontinuity) are reduced. If a thick coating is present, then the discontinuity size estimation must compensate for the coating thickness. The coating thickness requiring compensation is probe dependent. This can be accomplished using discontinuity-sizing tables in the system software and an operator-entered coating thickness or automatically if the equipment measures the coating thickness or stand-off distance during the scanning process. Using the wrong coating thickness would have a negative effect on depth sizing accuracy if the coating thickness discrepancy is too large. Data is recorded in a manner that allows archiving and subsequent recall for each weld location. Evaluation of examination results may be conducted at the time of examination or at a later date. The examiner generates an examination report detailing complete results of the examination.


5. Significance and Use

5.1 The purpose of the alternating current field measurement method is to evaluate welds for surface breaking discontinuities such as fabrication and fatigue cracks. The examination results may then be used by qualified organizations to assess weld service life or other engineering characteristics (beyond the scope of this practice). This practice is not intended for the examination of welds for non-surface breaking discontinuities.

Keywords:This practice is not intended for the examination of welds for non-surface breaking discontinuities.


6. Basis of Application

6.1 Personnel Qualification:

6.1.1 If specified in the contractual agreement, personnel performing examinations to this practice shall be qualified in accordance with a nationally or internationally recognized NDT personnel qualification practice or standard such as ANSI/ASNT-CP-189 or SNT-TC-1A or a similar document and certified by the employer or certifying agent, as applicable. The practice or standard used and its applicable revision shall be identified in the contractual agreement between the using parties.

6.2 Qualification of Nondestructive Evaluation Agencies—if specified in the contractual agreement, NDT agencies shall be qualified and evaluated as described in Practice E543, with reference to sections on electromagnetic examination. The applicable edition of Practice E543 shall be specified in the contractual agreement.


7. Job Scope and Requirements

7.1 The following items may require agreement by the examining party and their client and should be specified in the purchase document or elsewhere:

7.1.1 Location and type of welded component to be examined, design specifications, degradation history, previous nondestructive examination results, maintenance history, process conditions, and specific types of discontinuities that are required to be detected, if known.

7.1.2 The maximum window of opportunity for work. (Detection of small discontinuities may require a slower probe scan speed, or cleaning of surface, or both, which will affect productivity.)

7.1.3 Size, material grade and type, and configuration of welds to be examined. If required by type of equipment chosen, thickness of coating and variation of coating thickness.


Example of Cleaning Requirements:

The surface must be cleaned sufficiently to allow smooth probe travel and to allow features such as grinds or seam welds to be seen. This requires removal of marine fouling and flaking paint or corrosion for which use of a wire brush, hand scraper or water jet is normally sufficient. It should be noted that cleaning to bright metal is not required. However in some locations, particularly in warmer waters, marine growth can be hard and thick. In such cases it will be necessary to use grit blast cleaning. The system operator shall confirm that the surface condition is acceptable prior to carrying out the inspection, using information supplied by the diver and the divers video camera.



Underwater Structures


Underwater Structures


7.1.4 A weld numbering or identification system.

7.1.5 Extent of examination, for example: complete or partial coverage, which welds and to what length, whether straight sections only and the minimum surface curvature.

7.1.6 Means of access to welds, and areas where access may be restricted.

7.1.7 Type of alternating current field measurement instrument and probe; and description of operations reference standard used, including such details as dimensions and material.

7.1.8 Required operator qualifications and certification.

7.1.9 Required weld cleanliness.

7.1.10 Environmental conditions, equipment and preparations that are the responsibility of the client; common sources of noise that may interfere with the examination.


7.1.11 Complementary methods or techniques may be used to obtainadditional information.

7.1.12 Acceptance criteria to be used in evaluating discontinuities.

7.1.13 Disposition of examination records and reference standards.

7.1.14 Format and outline contents of the examination report.


8. Interferences

8.1 This section describes items and conditions, which may compromise the alternating current field measurement technique.

8.2 Material Properties:

8.2.1 Although there are permeability differences in a ferromagnetic material between weld metal, heat affected zone and parent plate, the probe is normally scanned along a weld toe and so passes along a line of relatively constant permeability. If a probe is scanned across a weld then the permeability changes may produce indications, which could be similar to those from a discontinuity. Differentiation between a transverse discontinuity signal and the weld signal can be achieved by taking further scans parallel to the indication, or using an array probe. The signal from a discontinuity will die away quickly. If there is no significant change in indication amplitude at 0.8 in. [20 mm] distance from the weld then the indication is likely due to the permeability changes in the weld.


Keywords:the probe is normally scanned along a weld toe and so passes along a line of relatively constant permeability

Weld toe

Probe


Weld profile

Weld toe


8.3 Magnetic State:

8.3.1 Demagnetization—It must be ensured that the surface being examined is in the non-magnetized state. Therefore the procedure followed with any previous magnetic technique deployed must include demagnetization of the surface. This is because areas of remnant magnetization, particularly where the leg of a magnetic particle examination yoke was sited, can produce loops in the X-Y plot, which may sometimes be confused with a discontinuity indication.


8.3.2 Grinding marks— magnetic permeability can also be affected by surface treatments such as grinding. These can cause localized areas of altered permeability across the line of scan direction. The extent and pressure of any grinding marks should always be reported by the probe operator, since these can give rise to strong indications in both Bx and Bz, which may be confused with a discontinuity indication. If a discontinuity is suspected in a region of grinding, further scans should be taken parallel but away from the weld toe and perpendicular across the region of grinding. The indication from a linear discontinuity will die away quickly away from the location of the discontinuity so that the scan away from the weld toe will be flatter. If there is no significant change in indication amplitude at 0.80 in. [20 mm] distance from the weld then the indication is likely due to the effect of the grinding. The indication from a region of grinding will be the same for the perpendicular scan.


8.4 Residual stress, with accompanying permeability variations, may be present with effects similar to those due to grinding, but are much smaller.

8.5 Seam Welds:

8.5.1 Seam welds running across the line of scanning also produce strong indications in the Bx and Bz, which can sometimes be confused, with a discontinuity indication. The same procedure is used as for grinding marks with further scans being taken away from the affected area. If the indication remains constant then it will not have been produced by a linear discontinuity.


Longitudinal Seam welds running across the line of scanning


8.6 Ferromagnetic and Conductive Objects:

8.6.1 Problems may arise because of objects near the weld that are ferromagnetic or conductive which may reduce the sensitivity and accuracy of discontinuity characterization when they are in the immediate vicinity of the weld.

8.7 Neighboring Welds: 8.7.1 In areas where welds cross each other, there are indications, which may be mistaken for discontinuities. (See 8.5.)


FIGURE 13. Magnetic flux density signals from transverse discontinuity compared to parallel discontinuity and seam weld: (a) chart recorder plot of Bx measurements; (b) chart recorder plot of Bz measurements; (c) butterfly shaped plot of magnetic flux density.

Legend1. Transverse discontinuity.2. Parallel discontinuity.3. Seam weld.



8.8 Weld Geometry:8.8.1 When a probe scans into a tight angle between two surfaces the Bx indication value will increase with little change in the Bz value. In the representative plot of Fig. 2, this appears as a rise in the X-Y plot. If the equipment is capable of measuring lift-off, the lift-off will also change.


FIG. 2 Example X-Y Plot Produced by Plotting the Bx (vertical)and Bz (horizontal) Together (The orientation of the plot may differ depending upon the instrumentation.)

Bx

Bz


Tight Geometries


Tight Geometries


Experts at Work


Tight Geometries


Tight Geometries


8.9 Crack Geometry Effects:

8.9.1 A discontinuity at an angle to the scan—a discontinuity at an angle to the scan will reduce either the peak or the trough of the Bz as the sensor probe only passes through the edge of one end of the discontinuity. This produces an asymmetric X-Y plot. Additional scans may be made along the weld or parent plate to determine the position of the other end of the discontinuity.

8.9.2 A discontinuity at an angle to the surface—the effect of a discontinuity at a non-vertical angle to the probe is generally to reduce the value of the Bzsignal. The value of the Bx signal will not be reduced. This has the effect of reducing the width of the X-Y plot in the representative plot of Fig. 2.

8.9.3 Line contact or multiple discontinuities—when contacts occur across a discontinuity then minor loops occur within the main X-Y plot loop produced by the discontinuity. If more than one discontinuity occurs in the scan then there will be a number of loops returning to the background.

Keywords: X-Y Plot is butterfly plot.


Crack Geometry

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discontinuity at an angle to the surface—The value of the Bx signal will not be reduced (?). This has the effect of reducing the width of the X-Y plot in the representative plot of Fig. 2.

Bx

Bz

Fig. 2

The value of the Bx signal will not be reduced (?). (of the butterfly vector plot)

It does affect the Bx plot.

Reducing


FIGURE 14. Magnetic flux density from cracks oriented at different angles to scan direction: (a) Bx measurement; (b) Bz measurement.

Legend1. 0 degrees.2. 30 degrees.3. 45 degrees.4. 90 degrees.

(a)


FIGURE 14. Magnetic flux density from cracks oriented at different angles to scan direction: (a) Bx measurement; (b) Bz measurement.

Legend1. 0 degrees.2. 30 degrees.3. 45 degrees.4. 90 degrees.

(b)


Typical ACFM Displays






Keywords: X-Y Plot is butterfly plot.











8.9.4 Transverse discontinuities—if a transverse discontinuity occurs during the scan for longitudinal discontinuities (or if a longitudinal discontinuity occurs during the scan for transverse discontinuities) then the Bx may rise instead of falling and the Bz signal will remain the same as for a short longitudinal discontinuity. The X-Y plot will then go upwards instead of down in the representative plot of Fig. 2. This flux leakage effect is, however, related to the opening of the discontinuity, so it may not be seen for tightly closed discontinuities. To confirm the presence of transverse discontinuities, further scans should be made with the probe orientated to give an induced field perpendicular to the weld, or through use of an array probe with twin fields.


8.9.5 Alternating current field measurement end effect – the field from the standard weld probe is able to propagate around the end of a weld and this can result in sloping changes in the Bx and Bz traces. A discontinuity indication may be obscured or distorted if the discontinuity or any active probe element is close to the weld end. The distance over which this effect occurs depends on probe type, but can be up to 2 in. [50 mm] for large probes. Smaller probes should be used in these situations as they have less susceptibility to edge effect.


8.10 Instrumentation:

8.10.1 The operator should be aware of indicators of noise, saturation or signal distortion particular to the instrument being used. Special consideration should be given to the following concerns:

8.10.1.1 The excitation frequency of operation should be chosen to maximize discontinuity sensitivity whilst maintaining acceptable noise levels.

8.10.1.2 Saturation of electronic components is a potential problem in alternating current field measurement because signal amplitude can increase rapidly as a probe is scanned into tight angle geometry. This could cause the Bx indication to rise above the top of the range of the A/D converter in the instrument. Data acquired under saturation conditions is not acceptable and appears as a flattening of the Bx response in the representative plots of Fig. 1 at the maximum possible signal value. If saturation conditions are observed, the equipment gain should be reduced until the Bx value no longer appears to saturate and the inspection repeated. After adjusting the equipment gain, an equipment operation check as described in 11.2 is recommended, except that the loop size will be smaller. Note that this gain adjustment does not affect the discontinuity sizing capability.


8.10.2 Instrument-induced Phase Offset—The measurements of magnetic field are at a chosen and fixed phase so that unlike during conventional eddy current examination the phase angle does not need to be considered. The phase is selected at manufacture of the probes and is stored in the probe file and is automatically configured by the instrument.


The Eddy Current Impedance Plane Responses


8.11 Coating Thickness

8.11.1 If a coating thickness exceeds the specified range for uncompensated operation then the discontinuity size estimation must compensate for the coating thickness. This can be accomplished by manually entering a coating thickness and using discontinuity tables in the system software. Otherwise, using the wrong coating thickness would reduce the depth sizing accuracy. Alternatively, the compensation may be performed automatically if the equipment measures the stand-off distance or coating thickness during the scanning process.

Keypoints:■ Uncompensated operation■ Compensated operation?■ Stand-off measurement ( same as coating thickness) and compensated

for different coating thickness in the equipment software algorithms.


9. Alternating Current Field Measurement System9.1 Instrumentation

9.1.1 The electronic instrumentation shall be capable of energizing the exciter at one or more frequencies appropriate to the weld material. The apparatus shall be capable of measuring the Bx and Bz magnetic field amplitudes at each frequency. The instrument will be supplied with a portable personal computer (PC) that has sufficient system capabilities to support the alternating current field measurement software, which will be suitable for the instrument and probes in use and the examination requirements. The software provides control of the instrumentation including set-up, data acquisition, data display, data analysis and data storage.


The software provides algorithms for sizing the discontinuities. (See 11.2.2) The software runs on the PC and, on start up, all communications between the PC and the instrument are automatically checked. When the software starts up it automatically sets up the instrument connected in the correct mode for alternating current field measurement examination.

Configuration data for each probe is stored on the PC and is transmitted to the instrument whenever a probe is selected or changed. For non-magnetic materials, if configuration data is not available from the equipment manufacturer, a standardization may be performed on reference blocks prior to the material examination. Equipment operation is also checked by scanning over a reference standard. (See 11.2.2) Once the instrumentation is set up for a particular probe, the software can be used to start and stop data acquisition. During data acquisition at least two presentations of the data are presented on the PC screen in real time. (See 4.1). Data from the probe is displayed against time (with Fig. 1 as an example) and also as an X-Y plot (with Fig. 2 as an example).


FIG. 1 Example Bx and Bz Traces as a Probe Passes Over a Crack (The orientation of the traces may differ depending upon the instrumentation.)


FIG. 2 Example X-Y Plot Produced by Plotting the Bx (vertical)and Bz (horizontal) Together (The orientation of the plot may differ depending upon the instrumentation.)

Bx

Bz


The data from the probe can also be displayed against position (see Fig. 1) if an encoder is used with the probe. Depending upon equipment type, manual or automatic position markers may be incorporated with the data. Once collected the data can be further analyzed offline using the software to allow, for example, discontinuity sizing (see 11.2.2) or annotation for transfer to examination reports. The software also provides facilities for all data collected to be electronically stored for subsequent review or reanalysis, printing or archiving.

9.2 Driving Mechanism:9.2.1 When a mechanized system is in operation, a mechanical means of scanning the probe, or probes in the form of an array, along a weld or surface area at approximately constant speed may be used.

Keywords:Encoder


9.3 Probes:

9.3.1 The probes selected should be appropriate for the form of examination to be carried out dependent on length of weld, geometry, size of detectable discontinuity and surface temperature.

9.3.1.1 Standard weld probe— Commonly used for weld examination whenever possible as it has its coils positioned ideally for discontinuity sizing.

9.3.1.2 Tight access probe— designed specifically for occasions where the area under examination is not accessible with the standard weld probe. It is not as accurate as the weld probe for sizing in open geometries such as butt welds.


9.3.1.3 Grind repair probe—designed for the examination of deep repair grinds. It has the same basic geometry as a standard probe but is more susceptible to produce indications from vertical probe movement.

9.3.1.4 Mini-probe—designed for restricted access areas such as cut outs and cruciforms and has a reduced edge effect. It may be limited to shallow discontinuities only and is more sensitive to lift off. This probe may be in the form of a straight entry or 90°.


9.3.1.5 Micro-probe—designed for high-sensitivity discontinuity detection in restricted access areas and has the same limitations as a mini-probe. This probe may be in the form of a straight entry or 90°.

9.3.1.6 Array probe— made up of a number of elements; each element is sensitive to a discrete section of the weld width. The elements may be oriented with their axes aligned longitudinally or transversely with respect to the weld toe. The array probe may have two orthogonal直交的 field exciters to allow examination for longitudinal and transverse discontinuities in a single scan. The array probe is generally used either for scanning a weld cap in one pass or for covering a section of plate.

9.3.1.7 Edge effect probe—designed to reduce the edge effect when carrying out examination only near the ends of welds. (A mini probe may also be used for the same examination.)


Scanning Pattern General Considerations - All WeldsThe following notes on probe deployment describe how to inspect welded components for fatigue cracks where it is assumed that defects will closely follow the weld line. The technique relies on recognition of the signal from a probe scan along the length of a defect, so an ACFM probe is always scanned along a line parallel to the weld. For this reason, defects that lie at an angle of more than about 25o to the weld may not be detected. If inspection for transverse defects is required, refer to the procedure in section "Transverse Cracks" on page 14. Standard weld probes should be used for all welds where access allows. A pencil probe should only be used for inspecting ground out regions or other geometries where a weld probe cannot gain access. The recommended scanning speed is about 10mm per second. The standard probe scans a width of approximately 20mm. Scans should always be made along both weld toes and, if wider than 20mm, the weld cap should also be covered by making a number of passes sufficient to cover the weld cap width taking into account the coverage of the probe.



9.4 Data Displays:

9.4.1 The data display should include Bx and Bz indications as well as an X-Y plot.

9.4.2 When multi-element array probes are being used, the facility to produce color contour maps or 3D-wire frame plots representing peaks and troughs should be available.


3D-wire frame plots


9.5 Excitation Mechanism:

9.5.1 The degree of uniformity of the magnetic field applied to the material under examination is determined by the equipment manufacturer. Representative magnetic field distributions are a uniform magnetic field and a graduated magnetic field. The geometry of the slots used in the operation reference standard and the discontinuity sizing model must be consistent with the excitation field.

Keywords:Uniform magnetic fieldGraduated magnetic fieldDiscontinuity sizing model


10. Alternating Current Field Measurement Reference Standards10.1 Artificial Slots for the Operation Reference Standard:

10.1.1 The operation reference standard has specific artificial discontinuities. It is used to check that the instrument and probe combination is functioning correctly. It may also be used for standardization of the equipment for nonmagnetic materials. Unless otherwise specified by the client or equipment manufacturer, the artificial discontinuities for the operation reference standard are elliptical or rectangular slots. The slot geometry will be specified by the equipment manufacturer to be consistent with the crack size estimation model. Typical slot dimensions are as follows:


10.1.1.1 Elliptical Slots—Two elliptical slots placed in the weld toe with dimensions 2.0 in. × 0.2 in. [50mm × 5mm] and 0.8 in. × 0.08 in. [20 mm × 2 mm]. (Fig. 3, discontinuities A and B.)

10.1.1.2 Rectangular Slots— Three rectangular slots with depth 0.08 in. [2 mm] and lengths of 0.4 in. and 0.8 in. [10 mm and 20 mm] (Fig. 3, discontinuities C and D) and with depth 0.16 in. [4 mm] and length of 1.6 in. [40 mm] (Fig. 3, discontinuity E.)

10.1.2 These slots shall be less than 0.008 in. [0.2 mm] wide.

10.1.3 Artificial discontinuity depths are specified by giving the deepest point of the discontinuity. Discontinuity depths shall be accurate to within +/- 10% of the depth specified, measured, and documented. The discontinuity length shall be accurate to within +/- 0.040 in. [+/- 1.00 mm] of the dimension specified.


FIG. 3 Flat Plate Sample Serial Number XXX Showing Size and Location ofReference Slots (Plan View and Side View. Not to Scale)

Weld cap


FIG. 3 Flat Plate Sample Serial Number XXX Showing Size and Location ofReference Slots (Plan View and Side View. Not to Scale)


10.2 Reference standards having artificial or simulated discontinuities are not required for standardization when the technique is to be used to examine carbon steel welds or if configuration data is available for the examination material.

10.3 Materials other than carbon steel:

10.3.1 If the technique is to be used on materials other than carbon steel, then it may be necessary to standardize the probes on this material if configuration data is not available from the equipment manufacturer, refer to manufacturer’s instructions.


10.2 Reference standards having artificial or simulated discontinuities are not required for standardization when the technique is to be used to examinecarbon steel welds or if configuration data is available for the examination material.


NOTE 2 — If this is not done then the sizes of the indications may be toosmall (so that small discontinuities may be missed) or too large (so thatspurious indications may be called), or the Bx indication may saturatemaking the examination invalid. This standardization is done using a slotof reasonable size located at a weld toe of a representative sample. Thegain settings are altered, either automatically or manually according toequipment type, until a loop of reasonable size is produced in the X-Y plotwhile background noise indications are kept low. When the technique is tobe used to size the depths of discontinuities detected in material for whichconfiguration data is unavailable, then a reference standard should bemanufactured from the material with at least two slots of differing depth.This provides an adjustment coefficient that modifies the estimated depthfrom the sizing model.


10.4 Reference standards having artificial or simulated discontinuities for welds in materials other than carbon steel shall not be used for discontinuity characterization unless the signals from the artificial discontinuities can be demonstrated to be similar to the signals for discontinuities detected. To be considered similar, a direct comparison should be performed between responses to the simulated discontinuities and real cracks. This comparison should involve at least one limited sizing trial or a probability of detection (POD) study.


10.5 Manufacture and Care of the Operation Reference Standards:

10.5.1 Drawings — for each operation reference standard and standard, there shall be a drawing that includes the as-built measured slot dimensions, material type and grade, and the serial number of the actual operation reference standard or weld standard.

10.5.2 Serial Number — each operation reference standard shall be identified with a unique serial number and stored so that it can be obtained and used for reference when required.

10.5.3 Slot Spacing — the slots should be positioned longitudinally to avoid overlapping of indications and interference from end effects.

10.5.4 Proper machining practices shall be used to avoid excessive cold-working, over- heating, and undue stress and permeability variations.

10.5.5 Blocks should be stored and shipped so as to prevent mechanical damage.


11. Equipment Operation Check11.1 Instrument Settings:

11.1.1 Operating Frequency—using the appropriate operation reference standard the procedure in 11.2.2 below is intended to help the user select an operating frequency. Demonstrably equivalent methods may be used.

The standard operating frequency depends upon the equipment to be used and typically is in the range of 5 to 50 kHz.

A higher operating frequency will give better sensitivity on good surfaces. If the system available is not capable of operating at the frequency described by this practice, the inspector shall declare to the client that conditions of reduced sensitivity may exist.


11.1.2 Standardization — For non-magnetic materials where configuration data is not available, the equipment may need to be standardized. Standardization is performed by loading manufacturer supplied configuration data, performing standardization measurements, and saving the resulting data and instrument settings as user configuration data.

The standardization measurements are performed using the appropriate operation reference standard (see 10.1). The probe is placed at the toe of the weld with the nose of the probe parallel to the longitudinal direction of the weld. The probe is then scanned across the operation reference standard and over a reference slot as specified by the equipment manufacturer.

The signal for the scanned slot is then selected and the gain is adjusted manually or automatically based on the measured signal and a reference signal for the discontinuity. Care must be used to ensure that the reference slot is the same as the discontinuity for the reference signal. This information can then be saved as user configuration data.


11.2 Test System Check and Procedure:

11.2.1 The test system shall consist of an alternating current field measurement instrument, the PC, the probe and the operation reference standard.


11.2.2 The equipment operation check will be performed using theappropriate operation reference standard (see 10.1). The probe is placed at the toe of the weld with the nose of the probe parallel to the longitudinal direction of the weld. The probe is then scanned across the operation reference standard and over the appropriate reference slot, which depends upon the probe type and as specified by the equipment manufacturer producing a standardized data plot. Discontinuity indications are created When:

(1) the background level Bx value is reduced and then returns to the nominal background level (see Fig. 1) and this is associated with

(2) a peak or positive (+ve) indication followed by a trough or negative (-ve) indication (or a trough followed by a peak, depending on direction of scan) in the Bz values.

The resultant effect of the changes in Bx and Bz is a loop in the X-Y plot shown, for example, as the downward loop of Fig. 2.


The presence of a discontinuity is confirmed when all three of these indications are present, that is, changes in the Bx and the Bz values and a loop in the X-Y plot. The loop should fill approximately 50 % of the Bx direction and 175 % of the Bz direction of the X-Y plot (that is, the loop is larger than the display in the Bz direction). The scanning speed or data sampling rate can then be adjusted if necessary, depending on the length and complexity of weld to be examined.

11.2.2.1 Once the presence of the discontinuity has been confirmed by the Bx and Bz indications the discontinuity should be sized.


Keywords:Operation reference standard


11.2.2.2 Discontinuity sizing is performed in the examination software and uses look-up tables of expected responses versus discontinuity sizes. These tables are based upon mathematical models that simulate the current flow around the discontinuities and the resultant change in surface magnetic field. The operator either positions cursors on the displayed data or enters background and minimum values of Bx along with the Bz length and any coating thickness to allow the software to estimate discontinuity length and depth.

11.2.2.3 If the discontinuity sizing values differ from those expected from the operation reference standard then the instrument and probe settings should be checked. Each probe should have a unique probe file, the validity of which has been checked against the discontinuity sizing tables. The instrument settings can be checked using the software package.


11.2.3 Each alternating current field measurement unit and probe to be used during the examination should be checked with the operation reference standard. Discontinuity sizing estimation results obtained should be the same as the measured dimensions of the slots in the block. If the dimensions differ by more a specified margin (for example, 10 %), then check that the correct probe files and gain have been used. If the correct probe files and gain have been used then there is a fault with the system, which will have to be determined. Do not use for examination unless standardization validity is confirmed within the specified margin between the estimated and measured slot dimensions.

Keywords: Each alternating current field measurement unit and probe to be used

during the examination should be checked with the operation reference standard.

Do not use for examination unless standardization validity is confirmed within the specified margin between the estimated and measured slot dimensions.


Keywords: Each alternating current field measurement unit and probe to be used

during the examination should be checked with the operation reference standard.

Do not use for examination unless standardization validity is confirmed within the specified margin between the estimated and measured slot dimensions.

NOT


11.3 Frequency of System Checks:

11.3.1 The system should be checked with all of the probes to be used during the examination prior to examining the first weld.

11.3.2 System operation should be checked at least every four hours with the probe in use or at the end of the examination being performed. If the discontinuity responses from the operation reference standard have changed by a specified margin (for example, 10 %), the welds examined since the last operations reference standard check shall be re-examined after following the procedure in 11.2.3.


12. Examination Procedure

12.1 If necessary, clean the weld surface to remove obstructions and heavy ferromagnetic or conductive debris.

12.2 Following the guidelines in 9.3, select a suitable probe for the examination task, then, using the installed software, select a data file and a probe file.

12.2.1 The probe is placed at the toe of the weld with the nose of the probe parallel to the longitudinal direction of the weld.


12.2.2 The probe is then scanned along the weld. Discontinuity indications are created when the following three points are indicated:

12.2.2.1 The background level Bx value is reduced and then returns to the nominal background level, Fig. 1.

12.2.2.2 This is associated with a peak, or positive (+ve) indication followed by a trough, or negative (-ve) indication (or a trough followed by a peak, depending on direction of scan) in the Bz values. Fig. 1.

12.2.2.3 The resultant effect of the changes in Bx and Bz is a downward loop in the X-Y plot, which is shown as a downward loop in the example plot of Fig. 2.

Keywords:Downward loop


12.2.3 The presence of a discontinuity is confirmed when all three of these indications are present, that is, the Bx, the Bz and a loop in the X-Y plot. The scanning speed or data sampling rate can be adjusted if necessary, depending on the length and complexity of weld to be examined.


12.3 Compensation for Material Differences:

12.3.1 To compensate for the small differences in readings caused by variations in permeability, conductivity or geometry for a given material, the data may be centered on the display area. For larger differences, the equipment settings should be adjusted, and/or a more suitable probe configuration should be used, in accordance with the manufacturer’s instructions.


12.4 Compensation for Ferromagnetic or Conductive Objects:

12.4.1 Techniques that may improve alternating current field measurement results near interfering ferromagnetic or conductive objects include:

12.4.1.1 Comparison of baseline or previous examination data with the current examination data.

12.4.1.2 The use of special probe coil configurations.

12.4.1.3 Use of higher or lower frequency probes may suppress non-relevant indications.

12.4.1.4 The use of a complementary method or technique.


12.5 Size and record all discontinuity indications as described in Section 14.

12.6 Note areas of limited sensitivity, using indications from the operation reference standard as an indicator of discontinuity detectability.

12.7 Using a discontinuity characterization standard, evaluate relevant indications in accordance with acceptance criteria specified by the client, if applicable.

12.8 If desired, examine selected areas using an appropriate complementary method or technique to obtain more information, adjusting results where appropriate.

12.9 Compile and present a report to the client.


13. Examination Considerations

13.1 Scanning Speed:

13.1.1 The scanning speed is chosen using the appropriate data sampling rate to obtain reasonable fidelity with the details of the scanned object given the length of the shortest discontinuity required to be found. A typical scan speed is 1 in. [25 mm]/second. This will produce a regular scan on the PC screen.

If short welds are to be examined then a faster data sampling rate should be used.

If long welds are to be examined and the whole weld needs to be seen on the PC screen then a slower data-sampling rate should be used.

The weld length and speed of scanning will govern the data-sampling rate selected. With the introduction of faster software or hardware it is possible to select respective data sampling rates to produce faster scanning rates.


13.1.2 Acquire and record data from the operation reference standard at the selected examination speed.

13.1.3 Acquire and record data from the welds to be examined. Maintain as uniform a probe speed as possible throughout the examination to produce repeatable indications.


13.2 Width of Scan:

13.2.1 The scan width is determined by the size of the probe and should be considered when performing an inspection. The sensitivity of the probe to a discontinuity decreases with distance. This distance is a factor that affects the number of scans that must be performed in order to provide full coverage when inspecting the weld. Note that even if a scan width is larger than the width of the weld cap, both toes of the weld should be scanned separately in most cases.

13.3 Continuous Cracking:

13.3.1 Prior to the scanning of a weld, checks should be made that the discontinuity is not continuous by scanning the probe from 2 in. [50 mm] away from the weld towards the toe. If a discontinuity is present the Bx indication on the computer screen will dip as the probe approaches the weld toe. If this form of indication occurs then this procedure shall be repeated at intervals along the toe of the weld.


13.4 Scanning Direction:

13.4.1 The probe should always be scanned parallel to the weld toe (except when confirming transverse discontinuities or discontinuities in regions of grinding) and this will give recognizable indications from longitudinal discontinuities in the weld area. Scanning in this direction will also give recognizable indications from transverse discontinuities and discontinuities inclined to the toe of the weld. The operator should be familiar with these types of indications.

Keywords:The probe should always be scanned parallel to the weld toe


13.5 Circumferential Welds:

13.5.1 The scanning pattern for a circumferential weld is shown in Fig. 4. Overlapping scans are required to ensure no discontinuities are missed if they occur at the end of a scan. The number of overlapping scans will vary depending on the component diameter. The overlap should be between 1 in. [25 mm] and 2 in. [50 mm] depending on the diameter of the tube or pipe. All detection shall be complete before any sizing operation is performed. Remember to check for continuous discontinuities before scanning.


FIG. 4 Scanning Pattern for a Circumferential Weld


13.6 Linear Welds:

13.6.1 The scanning pattern is similar to that for circumferential welds except that an edge effect may occur at the end of the weld or if the weld ends at a buttress. In the case of the end of the weld an edge-effect probe should be used but for the buttress a mini- or micro-probe should be used. These probes can also be used as an alternative to the edge effect probe. The standard weld probe should be used for sizing if at all possible. Recourse to other techniques, possibly including conventional eddy current techniques, may be necessary in these situations.


Bx background and minimum values to be used near a plate edge


Bx minimum

Bx background

substrate


13.7 Attachments, corners and cutouts:

13.7.1 The scanning patterns for the attachment welds and gussets are shown in Fig. 5, Fig. 6, and Fig. 7 where lines A1-A6, B1-B3 and C1 and 2 are the probe scan lines and positions 1-10 are the incremental positions along the weld length. The corners are difficult to scan and the micro- or mini-robes should be used where possible.

13.8 Cut outs and cruciform geometries:

13.8.1 The examination of this geometry is difficult due to the access problems; the scanning patterns and identification of the areas are shown in Fig. 8, Fig. 9, Fig. 10 and Fig. 11. The 90° mini- or micro- probe is essential for examining the cut-out areas.


FIG. 5 Scanning Pattern for an Approach to an Attachment


FIG. 6 Scanning Pattern for the End of an Attachment


FIG. 7 Scanning Pattern Across an Attachment (Crack in the Toe End)


FIG. 8 Scans of the Main Weld


FIG. 9 Scans of the Horizontal Weld into a Cut Out


FIG. 10 Nomenclature for Vertical Welds


FIG. 11 Scans of Vertical Cut Out Weld and Cut Out Surface


13.9 Ground-out Areas:

13.9.1 The repair or groundout area is usually 0.5 in. [12.5 mm] wide, and the grind repair probe is designed for the examination of these areas. The probe should be scanned into one end of the groundout area and the scan continued through the other end. Areas with discontinuities should be noted and sized for length and depth with the grind repair probe.


14. Discontinuity Sizing Procedure

14.1 The depth and length of the discontinuity are estimated from measurements taken of the Bx signatures plus the distance between terminal peak/trough of the Bz signature with compensation provided by either a user entered coating thickness or a real-time thickness compensation function.

14.2 Length:

14.2.1 Once an area containing a discontinuity has been located, a repeat scan is taken through the discontinuity. The Bz length of the discontinuity is determined by locating the extreme ends of the discontinuity using the peak (+ve) and trough (-ve) Bz locations. These positions should be just inside the actual ends of the discontinuity. This Bz length is used with the discontinuity sizing tables to determine the true length and depth of the discontinuity. The length of the detected discontinuity may be measured directly by the system software using properly placed manual markers or a position encoder. If the markers are placed manually, then the scan speed should be kept constant.


14.3 Depth:

14.3.1 The depth of the discontinuity is calculated using the Bx minimum and Bx background values and the Bz length of the discontinuity measured from the Bz data. Once these values have been put into the discontinuity sizing table, together with the coating thickness, if the equipment does not provide for lift-off compensation, then the discontinuity depth will be estimated by the software. Alternatively, if the equipment provides a lift-off value, the coating thickness can be determined automatically and the depth can be determined from the equipment software and discontinuity sizing table.


15. Report

15.1 Reporting Requirements—a list of reporting requirements is given in Table 1. Reference should be made to the Client reporting requirements (7.1.14). The items listed below should be included in the examination report. All information below should be archived, whether or not it is required in the report.

15.1.1 Owner, location, type and serial number of component examined.

15.1.2 Size, material type and grade, and configuration of welds examined. If required by type of equipment chosen, thickness of coating and variation in coating thickness.

15.1.3 Weld numbering system.


15.1.4 Extent of examination, for example, areas of interest, complete or partial coverage, which welds, and to what length.

15.1.5 The names and qualifications of personnel performing the examination.

15.1.6 Models, types, and serial numbers of the components of the alternating current field measurement system used, including all probes.

15.1.7 For the initial data acquisition from the operation reference standard, a complete list of all relevant instrument settings and parameters used, such as operating frequencies, and probe speed. The list shall enable settings to be referenced to each individual weld examined.


15.1.8 Serial numbers of all of the operations reference standards used.

15.1.9 Brief outline of all techniques used during the examination.

15.1.10 A list of all areas not examinable or where limited sensitivity was obtained. Indicate which discontinuities on the operations reference standard would not have been detectable in those regions. Where possible, indicate factors that may have limited sensitivity.

NOTE 3 — Factors that influence sensitivity to discontinuities include but are not limited to: operating frequency, instrument noise, instrument filtering, digital sample rate, probe speed, coil configuration, probe travel noise and interference described in Section 8.


More on: NOTE 3 — Factors that influence sensitivity to discontinuities include but are not limited to:

operating frequency, instrument noise, instrument filtering, digital sample rate, probe speed, coil configuration, probe travel noise and interference described in Section 8.


15.1.11 Specific information about techniques and depth sizing for each discontinuity.

15.1.12 Acceptance criteria used to evaluate discontinuities.

15.1.13 A list of discontinuities as specified in the purchasing agreement with the thickness of the coating over these discontinuities if the equipment does not measure and compensate for lift-off.

15.1.14 Complementary examination results that influenced interpretation and evaluation.


15.2 Record data and system settings in a manner that allows archiving and later recall of all data and system settings for each weld. Throughout the examination, data shall be permanently recorded, unless otherwise specified by the client.

15.2.1 Report form. An example report form is shown in Table 2.


16. Keywords

16.1 alternating current field measurement; electromagnetic examination; ferromagnetic weld; non-conducting material; weld




SUMMARY OF CHANGESCommittee E07 has identified the location of selected changes to this standard since the last issue (E2261 - 07) that may impact the use of this standard. (November 1, 012)

(1) Updated the units statement in 1.4. (2) Updated 4.1 and 8.9.4, and added definition 3.3.10 to clarify conditions for

examination for discontinuities perpendicular to welds. (3) Updated 8.3.2 and 13.4.1 to clarify procedures for examining in the vicinity of

grinding areas.


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Bx background and minimum values to be used near a plate edge


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