INDUSTRY STANDARD OF THE PEOPLE S REPUBLIC OF CHINA. Nondestructive Testing of Pressure Equipments - Part 6: Eddy Current Testing

Transcription

1 ICS NB H 26 INDUSTRY STANDARD OF THE PEOPLE S REPUBLIC OF CHINA NB/T Replacing JB/T Nondestructive Testing of Pressure Equipments - Part 6: Eddy Current Testing 承压设备无损检测 第 6 部分 : 涡流检测 Issued on: April 2, 2015 Implemented on: September 1, 2015 Issued by: National Energy Administration of the People's Republic of China

2 Contents Foreword... I 1 Scope Normative References Terminologies and Definitions General Requirements Eddy Current Testing for Ferromagnetic Tubular Products Eddy Current Testing for Non-ferromagnetic Metal Tubular Products Far Field Eddy Current Testing for Ferromagnetic Heat Exchange Tube In-Service Eddy Current Testing for Non-Ferromagnetic Heat Exchange Tube In-Service Eddy Current Testing for the Parts of Probe coil Eddy Current Testing for Welds with Probe Coil Measurement of Cover Layer Thickness Evaluation of Testing Results Testing Record and Report Appendix A (Normative) Reference Sample for Eddy Current Testing over Flaw Characteristics of Ferromagnetic Tubes for Pressure Equipments In-service Appendix B (Normative) Type III Reference Sample for Eddy Current Testing over Non-ferromagnetic Tubes for Pressure Equipments In-service... 46

3 Foreword This Standard "Nondestructive Testing of Pressure Equipments" (NB/T 47013) comprises the following 13 parts: - Part 1: General Requirements; - Part 2: Radiographic Testing; - Part 3: Ultrasonic Testing; - Part 4: Magnetic Particle Testing; - Part 5: Penetrant Testing; - Part 6: Eddy Current Testing; - Part 7: Visual Testing; - Part 8: Leak Testing; - Part 9: Acoustic Emission Testing; - Part 10: Ultrasonic Time of Flight Diffraction Technique; - Part 11: Standard Practice for X-ray Digital Radioscopy; - Part 12: Magnetic Flux Leakage Testing; - Part 13: Pulsed Eddy Current Testing. This Part is Part 6 of NB/T 47013: Eddy Current Testing. This Part was drafted according to the rules given in "Directives for Standardization - Part 1: Structure and Drafting of Standards" (GB/T ). This Part supersedes "Nondestructive Testing of Pressure Equipments - Part 6: Eddy Current Testing" (JB/T ); compared with JB/T , this Part has the following main technical changes: - the content of eddy current testing for spare parts of probe coil was added; - the content of eddy current testing for weld of probe coil was added; - the content of measurement of cover layer thickness was added; - the testing content of eddy current thickness measurement was added; - the scope of eddy current testing was extended. This Part was proposed by and is under the jurisdiction of National Technical Committee on Boilers and Pressure Vessels of Standardization Administration of China (SAC/TC 262). Drafting organizations of this Part: Hefei General Machinery Research Institute, China Special Equipment Inspection and Research Institute, Beijing Institute of Aeronautical Materials, North China Electric Power Research Institute, Eddysun (Xiamen) Electronic Co., Ltd. and Zhejiang Provincial Special Equipment Inspection and Research Institute. Chief drafters of this Part: Guan Weihe, Shen Gongtian, Xu Kebei, Hu Xianlong, Lin Junming, Cheng Huayun, Hu Bin and Liu Fujun. The previous editions of the standards superseded by this Part are as follows: - JB and JB/T I

4 Nondestructive Testing of Pressure Equipments - Part 6: Eddy Current Testing 1 Scope 1.1 This Part of NB/T specifies the requirements for the eddy current testing methods and quality classification of pressure equipments. 1.2 This Part is applicable to the eddy current testing for the surface and near surface flaws of metallic tubular products, spare parts and welded joints having electric conductivity used for the pressure equipments in-fabrication and in-service, and is also applicable to the measurement of surface cover layer thickness of metallic substrate with magnetic method and eddy current method. 1.3 The eddy current testing for the supporting members and structural members relevant to pressure equipments may also be made reference to this Part. 2 Normative References The following documents for the application of this document are essential. Any dated reference, just dated edition applies to this document. For undated references, the latest edition (including any amendments) applies to this document. GB/T 5126 Eddy Current Inspection Method for Cold Drawn Thin Wall Tubes of Aluminum and Aluminum Alloy GB/T 5248 Copper and Copper Alloy - Seamless Tubes - Eddy Current Testing Method GB/T 7735 Steel Tubes - The Inspection Method on Eddy Current Test GB/T Non-destructive Testing - Terminology - Terms Used in Eddy Current Testing GB/T Method of Eddy Current Inspection for Titanium and Titanium Alloy Tubes GB/T Non-destructive Testing - Equipment for Eddy Current Examination - Part 3: System Characteristics and Verification NB/T Nondestructive Testing of Pressure Equipments - Part 1: General Requirements NB/T Nondestructive Testing of Pressure Equipments - Part 3: Ultrasonic Testing NB/T Nondestructive Testing of Pressure Equipments - Part 4: Magnetic Particle Testing NB/T Nondestructive Testing of Pressure Equipments - Part 5: Penetrant Testing 1

5 continuously on whole or local part of the tested workpiece. Provided the tested workpiece is not permitted to preserve residual magnetism, the magnetizing apparatus shall be also employed with demagnetizing apparatus The mechanical transmission mechanism shall ensure the stable relative motion between the tested workpiece and the test coil with specified pattern, and shall not cause any damage to the surface of the tested workpiece The recording device shall be able to promptly and correctly record the output signal of testing instrument In the following cases, the sensitivity of eddy current testing equipment shall be inspected and reinspected with reference sample: a) Prior to testing and after testing every time; b) Suspicion of abnormal state about testing equipment; c) Change of the specification of tested object; d) In continuous testing, the inspection and reinspection are conducted once per 2h; e) Dispute between both contractors, or the inspection regarded as necessary. 4.3 Reference sample The reference sample is mainly used to adjust the testing sensitivity and testing parameters of eddy current testing instrument, to determine the acceptance level and to ensure the accuracy of testing results The reference sample shall have the specification, designation, heat treatment condition, surface condition and electromagnetic property same or similar to those of the tested workpiece The reference sample surface shall be free from foreign matters and the flaws influencing the calibration The artificial flaw processed on reference sample shall be measured by proper method, and shall meet the requirements of relevant standard or technical specification The size of artificial flaw on reference sample shall not be interpreted as the smallest detectable flaw size of testing equipment. 4.4 Tested workpiece The surface of tested workpiece shall be clean, no burrs, and free from powder dust and other contaminants, especially the ferromagnetic particle, that may influence the normal application of eddy current testing; otherwise, all of foreign materials shall be perfectly removed, and the surface of tested workpiece shall not be damaged in the removing process The parameters of the tested workpiece such as surface roughness shall meet the requirements of relevant product technical specification. 4.5 Testing environment The testing process shall comply with the relevant national and local laws, especially the laws and regulations in relation to accident prevention, electric safety, high-risk industry and environmental protection, and the standard users are liable to obey the laws and regulations relevant to safety The temperature and relative humidity at the testing site shall controlled within the allowable range of instrument, equipment and the tested workpiece The place nearby testing site shall have no magnetic field, vibration, corrosive gas and other interferences that may influence the normal operation of instrument and equipment. 3

6 some other characteristics of tested workpiece will influence the testing results The inside diameter (outside diameter) of test coil shall match the outside diameter (inside diameter) of tested tubular products, and its filling factor will influence the testing sensitivity The material selection and preparation of reference sample shall meet the requirements of this standard and the reference sample will influence the testing sensitivity The testing speed shall be consistent with or similar to the relative motion speed of sample and test coil during sensitivity debugging, and the testing speed will influence the testing sensitivity During eddy current testing for welds with probe coil, the cover layer of conductor will reduce the testing sensitivity, and the cover layer thickness and conductivity will influence the testing sensitivity During eddy current testing for welds with probe coil, the reduction degree of testing sensitivity by non-conductor cover layer relies on the distance between probe and tested workpiece The shape of tested workpiece and the approaching degree between probe and tested zone will influence the testing sensitivity The direction between coil and predicted flaw will influence the testing sensitivity The nature, size, and depth of flaw will influence the testing sensitivity The fringe effect of the tested workpiece will influence the testing sensitivity. 4.8 Safety requirements Current short circuit may result in electric shock and consequently cause harm to human body The temperature and relative humidity at the testing site may harm human body. 5 Eddy Current Testing for Ferromagnetic Tubular Products 5.1 Scope This chapter specifies the eddy current testing method for ferromagnetic tubular products such as seamless steel tube, nickel and nickel alloys tube, welded steel tube (excluding submerged arc welded steel tube) used for pressure equipments This chapter is applicable to the eddy current testing of steel tube with outside diameter not less than 4mm. 5.2 Reference sample Shape of artificial flaw on reference sample When feed-through coil is adopted, the shape of artificial flaw on sample shall be through hole When revolving steel tube/flat coil are adopted, the shape of artificial flaw on sample shall be through hole or slot When fan-type coil is adopted for weld testing, the shape of artificial flaw on sample shall be through hole Through hole When the eddy current testing technique with feed-through coil is used, three radial 5

7 In the case that three through holes are drilled on the reference sample, their indication amplitude shall be basically identical. The minimum amplitude among them shall be selected as the trigger-alarm level of testing equipment. The inside diameter of test coil shall match the outside diameter of the tested tubular product with the filling factor greater than or equal to In the case that one through hole is drilled on the reference sample, the position of hole on reference sample shall pass through the testing equipment at 0, 90, 180 and 270 successively, and the minimum amplitude of signal shall be taken as the alarm level of testing equipment When eddy current testing technique with revolving steel tube/flat revolving coil is adopted, if reference sample is adopted with drill hole or slot, the signal amplitude obtained shall be taken as the alarm level of testing equipment Testing The tested steel tube shall be tested according to the parameters set during sensitivity adjustment The relative moving speed between the tested steel tube and test coil during testing shall be the same as or approximate to that between the reference sample and test coil under instrument debugging condition The instrument and probe shall be verified once per 2h in the testing according to the requirements of Provided the artificial flaw characteristic parameters of reference sample tube present evident change in the system verification, all the tested tubes since last system verification shall be retested. 5.5 Evaluation and treatment of testing results Acceptance level The acceptance level is classified into Level A and Level B (see Tables 2 and 3). The acceptance level shall be selected by supplier and purchaser via negotiation and shall be indicated in the contract Evaluation of testing results Acceptable steel tube: if the signal generated by steel tube when passing through the eddy current testing equipment is lower than the alarm level, this steel tube may be evaluated as acceptable steel tube Suspicious steel tube: if the signal generated by steel tube when passing through the eddy current testing equipment is equal to or higher than the alarm level, this steel tube may be identified as suspicious steel tube and the suspicious steel tube may be evaluated as unacceptable steel tube by eddy current testing. The disposal of suspicious steel tube shall meet the requirements of GB/T Eddy Current Testing for Non-ferromagnetic Metal Tubular Products 6.1 Applicable scope This chapter specifies the eddy current testing method for non-ferromagnetic metal 10

8 6.3.2 Test coil Appropriate test coil shall be selected according to the specification of tested tubular product The inside diameter of test coil shall match the outside diameter of tested tubular products, with filling factor greater than or equal to 0.6. The testing frequency range shall be 1kHz~125kHz Testing procedure Eddy current testing instrument and equipment shall operate in determined testing speed, the eddy current testing instrument shall be debugged to make the standard artificial flaw signal exactly alarm and the amplitude of corresponding artificial flaw signal be 30%~50% of the full screen scale of instrument The indication amplitude of three middle through holes shall be basically identical. The minimum amplitude among them shall be selected as the trigger-alarm level of testing equipment After the sensitivity is confirmed, the tested tubular product shall be tested according to the parameters set during sensitivity adjustment The testing speed during testing shall be the same as or approximate to that during sensitivity debugging, and shall not exceed the allowable testing speed upper limit of instrument During testing, the instrument shall be calibrated every other 2h according to the requirements stated in and If it is discovered that the variance in sensitivity data is larger than 2dB, the tubular products during the period from the last calibration to this calibration shall be retested. 6.4 Testing method for titanium and titanium alloy tubular products Reference sample The artificial flaw on reference sample shall be five radial through holes of equal diameter perpendicular to tube wall. The permissible deviation of through hole diameter is ±0.05mm, and the permissible deviation of verticality of through hole shall not be larger than 5, see Figure The size of artificial flaw, i.e., through hole size, shall be selected according to different outside diameters of tubular products, see Table 8. Tube outside diameter, D Table 4 Size of Artificial Flaw on Reference Sample In: mm 15 Hole size of artificial flaw 4 D< D< D< D< D< D< Artificial flaws shall be symmetrically spaced by 120 along circumference Upon agreement between contracting parties, sample with longitudinal slot specified in GB/T may also be selected Test coil Appropriate test coil shall be selected according to the specification of tested tubular

9 product The inside diameter of test coil shall match the outside diameter of tested tubular products, with filling factor greater than or equal to 0.6. The testing frequency range shall be 1kHz~125kHz Testing procedure Eddy current testing instrument and equipment shall operate in determined testing speed, the eddy current testing instrument shall be debugged to make the standard artificial flaw signal exactly alarm and the amplitude of corresponding artificial flaw signal be 50%~70% of the full screen scale of instrument The indication amplitude of three middle through holes shall be basically identical, with difference not greater than ±10% of the average amplitude. The minimum amplitude among them shall be selected as the trigger-alarm level of testing equipment After the sensitivity is confirmed, the tested tubular product shall be tested according to the parameters set during sensitivity adjustment The testing speed during testing shall be the same as or approximate to that during sensitivity debugging, and shall not exceed the allowable testing speed upper limit of instrument During testing, the instrument shall be calibrated every other 2h according to the requirements stated in and If it is discovered that the variance in sensitivity data is larger than 2dB, the tubular products during the period from the last calibration to this calibration shall be retested. 6.5 Evaluation of testing results The provision on adopting artificial flaw for the acceptance of through hole, flat bottom hole or slot shall be per the requirements of contract between supplier and purchaser or the requirements of relevant product standard The tubular product without alarm signal found during testing shall be evaluated as acceptable product by eddy current testing The tubular product with alarm signal found during testing shall be evaluated as unacceptable product by eddy current testing. 7 Far Field Eddy Current Testing for Ferromagnetic Heat Exchange Tube In-Service 7.1 Scope This chapter specifies the methods for eddy current testing and eddy current thickness measurement of ferromagnetic heat exchange tube in-service by using inner feed-through coil. The heat exchange tube installed on pressure equipment in manufacturing may also refer to it This chapter is applicable to the far field eddy current testing and far field eddy current thickness measurement for ferromagnetic steel tube with outside diameter of 12.5mm ~ 60mm and wall thickness of 0.70mm ~ 4mm. The ferromagnetic steel tubes other than the above specifications may also refer to the provisions of this chapter. 7.2 Flaw testing 16

10 7.2.2 Reference sample Processing requirements of artificial flaw The depth expression of artificial flaw shall be the depth of the deepest point of flaw, and denoted by the percentage of nominal wall thickness of tube The size accuracy of the depth of artificial flaw shall be within ±20%, and that of other directions shall be within ±0.25mm Type and dimensions of artificial flaw Round bottom hole - use spherical drill with diameter of 10mm and processing depth of 50% (see Figure 9 flaw A) Through hole - the diameter of through hole is 1.25 times the wall thickness (see Figure 9 flaw B) Circumferential narrow slot - slot depth is 20% of the wall thickness, and slot width is 3mm (see Figure 9 flaw C) Circumferential wide slot - slot depth is 20% of wall thickness, slot width is greater than or equal to 2 times the nominal tube diameter (see Figure 9 flaw D) Single side flaw Single side flaw of Type I reference sample - circumferential single side flaw of 60% depth, axial length of 15mm and circumferential amplitude of 180 ~ 270, see Figure 9 a) flaw E; Single side flaw of Type II reference sample - milling groove of 50% depth, axial length larger than or equal to 2 times the tube outside diameter, see Figure 9 b) flaw E. 18

11 In order to provide more data for further analysis, the auxiliary frequency may be used for testing The system shall be verified by testing the artificial flaw of reference sample tube through far field eddy current testing to comply with the required sensitivity Testing After the sensitivity is verified, the tested tube shall be tested according to the parameters set during sensitivity debugging The internal and external surfaces of tested ferromagnetic steel tube shall be cleaned to meet the testing requirements prior to testing During testing, the testing speed shall be the same or close to that during sensitivity debugging, and during the whole testing process, the moving speed of probe shall be as stable as possible. If such mechanical transmission modes as push-pull apparatus of probe are used as an auxiliary testing apparatus, they shall not create any damage to the inner wall of tested tube. The testing speed of probe inside the tube shall be determined by the instrument and parameters to be used, generally not greater than 10m/min During testing process, the instrument and probe shall be verified once every other 2h according to the requirements of In case of obvious change of the characteristic parameter of artificial flaw of reference sample tube during system calibration, the tube tested after system calibration shall be retested Evaluation of testing results The signals appeared in testing, except those can be proved to the unrelated ones due to workpiece structure or other causes, shall be evaluated The testing results shall be comprehensively evaluated according to the amplitude and phase of flaw response signal Suitable auxiliary testing method may be adopted to test the areas with flaw again, e.g. using special video endoscope to confirm The result evaluation of far field eddy current testing of ferromagnetic steel tube in-service shall be determined upon negotiation with the owner according to the requirements of relevant technical specifications and standards. 7.3 Far field eddy current thickness measurement With special thickness measurement software, the stimulating signal obtained from eddy current testing may be converted into digital signal in real time according to the pre-established mathematical model, via observing, the real-time wall thickness value of the tested tube bundle may be obtained directly and the thinnest wall thickness value of each tested tube may be obtained. Due to the average effect of eddy current, the value of eddy current thickness measurement is the average thickness value of the circumferential section of the tube Probe Absolute probe shall be adopted for far field eddy current thickness measurement The selection of probe shall be determined according to the dimensions of the tested workpiece and actual situation, testing probe with large filling factor shall be preferably selected because testing sensitivity is mainly considered Reference sample Reference sample shall meet the requirements of

12 8.1 Scope This chapter specifies the methods for eddy current testing and eddy current thickness measurement of non-ferromagnetic heat exchange tube in-service using inner feed-through coil. The heat exchange tube installed on pressure equipment in manufacturing may also refer to it This chapter is applicable to the eddy current testing and eddy current thickness measurement for non-ferromagnetic tube with outside diameter of 10mm~200mm and wall thickness of 0.75mm~8.0mm. The tubes other than the above specifications may also refer to the provisions of this chapter. 8.2 Flaw testing Testing instrument The eddy current testing instrument shall at least be provided with two independent optional frequencies of range of 1kHz ~ 1MHz The eddy current testing instrument shall also be provided with the functions to measure and recognize the uniform thinning of tube thickness, in addition to the testing of common flaws such as tube crack, corrosive pits and fold, etc. When the instrument is able to test both differential and absolute channels, the above contents may be accomplished simultaneously. Otherwise, the testing for differential and absolute channel shall be performed separately The instrument shall include the processing elements of excitation, signal processing, phase adjustment, filtering and frequency mixing, etc., and shall also have the corresponding alarm apparatus, impedance planar indication and reliable recording device The combination of inside inserted probe and eddy current testing instrument shall be able to generate induction current in tube wall to test the damage situation of tube, and be able to effectively eliminate the influence of interference signal of tube plate, support plate and noise, etc The outside diameter of probe shall be determined according to the inner diameter of the tested tube to endure the filling factor as large as possible Reference sample The shape of artificial flaws on reference sample shall be through hole, flat bottom hole and circumferential slot When Type I reference sample is used to adjust the testing system, the positions and sizes of artificial flaws on the reference sample are as follows, see Figure 11: a) A is one through hole penetrating tube wall. For tube with outside diameter is less than or equal to 20mm, the hole diameter shall be 1.3 mm; for tube with outside diameter greater than 20mm, the hole diameter shall be 1.7mm. b) B is four flat bottom holes, with the hole diameter of 4.8mm, and circumferentially arranged on the same cross section by 90 spacing each, and drilled from the outer surface by 20% of the wall thickness. c) C is one 360 circumferential slot with the width of 3.2mm and depth of 20% of wall thickness, cut from the outer wall surface of tube (selectively used for verifying absolute probe); d) D is one 360 circumferential slot with slot width of 1.6mm, depth of 10% of wall thickness, cut from the inner wall surface of tube (selectively used for 22

13 testing requirements prior to testing During testing, the testing speed shall be the same or close to that during sensitivity debugging, and during the whole testing process, the moving speed of probe shall be as stable as possible. If such mechanical transmission modes as push-pull apparatus of probe are used as an auxiliary testing apparatus, they shall not create any damage to the inner wall of tested tube. The testing speed of probe inside the tube shall be determined by the instrument and parameters to be used, generally not greater than 20m/min During testing process, the instrument and probe shall be verified once per 2h according to the requirements of In case of obvious change of the characteristic parameter of artificial flaw of reference sample tube during system calibration, the tube tested after system calibration shall be tested again The data of all the flaw signals and suspicious signals shall be well recorded Evaluation of testing results The signals appeared in testing, except those can be proved to the unrelated ones due to workpiece structure or other causes, shall be evaluated The testing results shall be comprehensively evaluated according to the amplitude and phase of flaw response signal Suitable auxiliary testing method may be adopted to test the areas with flaw again, such as using special video endoscope to confirm The result evaluation of eddy current testing of non-ferromagnetic tube in-service shall be determined upon negotiation with the owner according to the requirements of relevant technical specifications and standards. 8.3 Eddy current thickness measurement With special thickness measurement software, the analog signal obtained from eddy current testing may be converted into digital signal in real time according to the pre-established mathematical model, via observing, the real-time wall thickness value of the tested tube bundle may be obtained directly and the thinnest wall thickness value of each tested tube may be obtained. Due to the average effect of eddy current, the value of eddy current thickness measurement is the average thickness value of the circumferential section of the tube Probe Absolute probe shall be adopted for eddy current thickness measurement The selection of probe shall be determined according to the dimensions of the tested workpiece and actual situation, testing probe with large filling factor shall be preferably selected because testing sensitivity is mainly considered Reference sample Reference sample shall meet the requirements of The reference sample used for eddy current thickness measurement may be designed by reference to Figure 10. The reduction of inner wall or outer wall of standard sample tube depends on the condition of the tested tube Instrument debugging Suitable frequency shall be selected to ensure that eddy field can penetrate tube wall and have good linear attenuation linear attenuation The selected phase shall enable the signals of different wall thickness to have good 26

14 linear relation when being reflected on horizontal axis projection, and the direction of the signal generated by tube plate must be in the direction of thickening signal The selection of gain shall ensure the signal amplitude at the proper position of screen, which is convenient for observing The thickness measurement system shall be calibrated and mathematical model for measurement shall be established before eddy current thickness measurement is started each time Testing After the instrument is debugged, the tested pipe shall be tested according to the parameters set during debugging The internal and external surfaces of the tested tube shall be cleaned to meet the testing requirements prior to testing During the whole testing process, the moving speed of probe shall be as stable as possible Evaluation of testing results The wall thickness of tested tube shall be recorded and analyzed timely during testing process The result evaluation of eddy current thickness measurement of non-ferromagnetic tube in-service shall be determined upon negotiation with the owner according to the requirements of relevant technical specifications and standards. 9 Eddy Current Testing for the Parts of Probe coil 9.1 Scope This chapter specifies the eddy current testing method for testing the regular or irregular parts made of conductivity material using probe coil. 9.2 Testing requirements The tested area shall be free from lubricating grease, oil, rust or other substances hindering testing; the surface of non-magnetic tested piece shall be free from magnetic powder. If the above-mentioned requirements are not met, such substances shall be cleaned up, without damaging the surface of the tested part The tested surface shall be smooth and with roughness not greater than 6.3μm; the ratio of the signal obtained from the artificial flaw of reference sample to the noise signal obtained from the tested surface shall not be less than 3: The thickness of the non-conductive coating at the tested position generally does not exceed 150μm, otherwise non-conductive membrane of similar thickness shall be used to cover the artificial flaw of the reference sample for compensation adjustment of sensitivity testing. 9.3 Eddy current testing system Eddy current testing system shall at least consist of eddy current instrument, probe and reference sample. Eddy current testing system may be automatic, semi-automatic or manual testing system Eddy current instrument 27

15 Eddy current instrument shall have impedance planar indication and time base indication mode It shall be able to change correspondingly to the eddy current generated by continuous induce according to the adjustment of testing frequency, response signal phase and gain Probe During eddy current testing, eddy current probe with appropriate size, shape and frequency shall be selected according to the tested object and testing requirements Shielded or non-shielded differential or absolute eddy current probe may be adopted for eddy current testing Eddy current probe shall not generate interference signal to the change of the applied pressure The probe mark shall at least include serial No., frequency and other information In order to prevent abrasion of probe, wearproof protective layer may be applied on the probe top during testing The abrasion of probe shall be inspected at any time during testing process, once the abrasion affects testing, it shall no longer be used Sample Standard sample Standard sample is used for testing the performance of eddy current instrument, its overall dimensions and artificial flaw depth shall meet the requirements of Figure Artificial slot flaw may be processed and fabricated by linear cutting, the width shall be 0.05mm, and the depth of A, B and C slot flaws are 0.2 mm, 0.5 mm and 1.0 mm respectively, and the depth dimensions tolerance is ± 0.05 mm Standard sample shall be processed and fabricated with 2024 aluminium materials of T3 state or aluminium materials with similar conductivity Reference sample Reference sample is used to establish testing sensitivity, verify the working state of instrument and evaluate flaw, its conductivity, heat treatment state, surface state, structure and the location of artificial flaw shall be the same as or similar to the tested workpiece, and the materials of reference sample may be selected according to Table The reference sample used for parts and partial area eddy current testing may be fabricated by reference to Figure 15, and the number and depth of artificial flaws may be determined according to the testing acceptance requirements Reference sample may be made of actual parts The surface roughness of reference sample shall meet the requirement that the ratio of artificial flaw signal on reference sample to noise signal shall not be less than 5: Prior to initial use of reference sample, the width and depth dimensions of artificial flaw shall be tested, and reference sample may be put into service if they meet the manufacture requirements. 28

16 signal and lift-off signal is generally large. Generally, surface crack response signal has high frequency As for the area with abnormal response signal, the location of the corresponding signal on parts surface shall be observed carefully, and the crack direction and length or the size of other types of flaws shall be determined according to the scan mode as shown in Figure Evaluation of testing results As for the signals found in testing which may be caused by relevant interference factors, for example, lift-off, edge and step interference signals, etc., the response signal shall be regarded as being caused by flaw, and the direction, length or area and type shall be evaluated according to As for surface flaws, the depth of flawing causing this response signal may be evaluated by comparing the response signal amplitude to the amplitude of response signal of artificial flaw with relevant depth on reference block. The phase of flaw response signal may be regarded as the reference information for evaluating the depth of surface flaw The conclusion of whether the tested parts are qualified or not shall be drawn according to the technical specifications of relevant products or the acceptance criteria agreed by the client When acceptance criteria is not given in product specifications and relevant technical agreement, it is allowed to only give quantitative evaluation for the found flaw, rather than draw the conclusion of whether the tested parts is qualified or not. 10 Eddy Current Testing for Welds with Probe Coil 10.1 Scope The chapter specifies the eddy current testing technique for surface opening and near-surface plane flaw of ferromagnetic materials (weld, heat affected zone and base metal). Testing of non-ferromagnetic materials may also refer to it Testing may be carried out on various forms of accessible weld surfaces Pre-testing preparation The tested area shall be free from lubricating grease, oil, rust and other materials that disturb testing Before testing, type of filler metal, position and scope of to-be-tested weld, geometric shape of weld surface, surface condition, type and thickness of coating shall be known Influence factors Eddy current testing is related to the approaching degree of probe and tested surface, the geometric shape and surface condition of tested weld surface shall ensure proper contact between probe and tested surface. When eddy current testing for weld is carried out, it shall be considered that the irregular shape, weld spatter, weld beading, corrosive material, paint peeling, etc. of weld surface may interfere the sensitivity of testing by changing the distance between probe and tested surface and causing noise As for workpieces with conductive coating such as hot-spray aluminum and lead, I the testing is carried out with eddy current method, testing effect may be influenced and 31

17 Weld testing probe Specially designed weld testing probe shall be used to test the weld of ferromagnetic material. Probe may be assembled in differential types, orthogonal type, tangent type or other equivalent types, so as to minimize the influence of such changes as material conductivity, magnetic permeability and lift-off effect on probe in weld and heat affected zone. The diameter of probe shall be selected according to geometric shape of tested workpiece. Probe shall still be able to operate normally after coating with nonmetallic wear-proof thin layer on its contact surface. If probe is with package structure, the package shell and the surface of calibration block shall always contact with each other in the process of calibration. Probe shall be able to operate normally at any selected frequency within the range of 1kHz~1MHz Verification of testing equipment Equipment shall be calibrated by qualified laboratory at least once a year Periodical checking and adjustment for testing equipment shall be carried out so as to verify the functions of instruments. Checking and adjustment for the functions of instruments shall also be carried out in case of the emerging mistake or change of testing equipment during the testing on site Calibration block Calibration block shall be made of the material the same as or similar to the tested workpiece. Unless otherwise agreed by related parties of the contract, artificial slot in depths of 0.5mm, 1.0mm and 2.0mm shall be processed on the calibration block with wire-electrode cutting method. The tolerance of slot depth shall be ±0.1mm. The recommended width of slot shall be less than or equal to 0.2mm (See Figure 18). Non-conductor elastic gasket with known thickness may be used for simulating coating, or actual coating may be directly sprayed on the calibration block. Integral multiple of 0.5mm is recommended for the thickness of elastic gasket Non-conductor elastic gasket with known thickness may be used for simulating cover layer, or actual coating may be directly sprayed on the calibration block. Integral multiple of 0.5mm is recommended for the thickness of elastic gasket. 33

18 probe. Prior to calibration, a layer of non-conductor elastic gasket shall be covered on the slot surface, the gasket thickness shall be greater than or equal to the coating thickness of tested workpiece. The signal amplitude of 1mm-deep slot shall be adjusted to about 80% of the height of full screen. Then, the level of sensitivity shall be adjusted to clear up the influence caused by the geometrical shape of workpiece. Calibration and checking shall be carried out periodically, and shall be carried out at least at the beginning and the end of testing as well as whenever the working condition changes. Calibration shall be recorded every time. The balance point shall be adjusted to the center of display screen after calibration Scanning The weld surface and heat affected zone shall be scanned with the selected probe. Testing probe shall move along the direction which is perpendicular to the main trend of possible flaws as long as it is permitted by the geometrical shape of the tested workpiece. But if the trend is unclear or may divide into different trends, scanning shall be at least carried out at two mutually perpendicular directions. Testing may be carried out in two parts: heat affected zone (see Figures 19~21) and weld surface (see Figure 22 and Figure 23). It shall be noticed that the reliability of testing highly depends on the direction between coil and tested surface. And it shall be ensured that surface of various conditions in heat affected zone is scanned by the probe at the best angle. The sensitivity of differential probe may be influenced by the included angle between flaw and coil. Therefore, control of angle shall be paid attention to in the process of testing Testing procedures for welds of other materials Eddy current testing method is also applicable to the testing for welds of workpieces of aluminium, stainless steel and other metallic materials. Generally, the requirements of testing for these welds are the same as the requirements given in But the frequency, probe, calibration block, scanning mode, etc. shall be selected in accordance with the performance of actual materials, which may bring a huge difference between the selected material and the recommended ferromagnetic material. Thus, testing process specification shall be formulated according to practical experiences based on proper instrument, probe and calibration block, and special process specification shall be formulated in detail. The limiting factors in all kinds of cases shall be specified in detail Testing Prior to testing, the surface conditions of tested workpiece, including the coating type and thickness and the shape of weld, shall be confirmed to ensure that they meet the testing requirements The movement speed of probe shall be maintained as constant and steady as possible as possible in the entire testing process In the process of scanning, it shall be ensured that the testing direction is perpendicular to the trend of predicted flaws, meanwhile, probe angle shall be under well control The maximum scanning speed is determined according to the selected instrument and selected parameters. Generally, it shall not exceed 50mm/s. 35

19 The cover layer thickness measurement range of magnetic thickness tester is generally less than 2,000μm Influence factors: a) The cover layer thickness; b) The magnetism of substrate metal; c) The thickness of substrate metal; d) The fringe effect; e) The workpiece curvature; f) The surface roughness; g) The machining direction of substrate metal; h) The residual magnetism; i) The environmental magnetic field; j) The external adherent dust; k) The cover layer conductivity; l) The testing pressure Instrument calibration Standard calibration membrane Non-conductive membrane with even thickness, or test piece composed of substrate metal and the cover layer which has a known and even thickness and also firmly bonded with the substrate metal, may be adopted as standard calibration membrane The substrate metal of standard calibration membrane shall be with surface roughness and magnetic property which are similar to those of the tested piece. It is recommended to calibrate the zero reading of instrument in the area without cover layer on the tested piece as well as the corresponding reading of instrument in the area covered with standard thickness membrane As for tested object with distinct directionality on the substrate metal magnetism, the probe head shall be rotated by 90 so as to check against instrument calibration The substrate metal thickness of tested piece and standard calibration membrane shall be the same. Metallic material with enough thickness may also be superposed respectively upon the substrate metal of standard calibration membrane and tested piece to keep the measurement reading free from the influence of substrate metal thickness When calibration fails to be carried out horizontally because of the curve of tested piece, curvature of standard membrane with cover layer or curvature of substrate placed with calibration membrane shall be the same as that of the tested piece Testing procedure Before the use of instrument every time, at the end of measurement and at regular intervals (at least once per hour) in service, verification for instrument calibration shall be carried out on measurement site, to ensure normal performance of instrument Instrument calibration shall be carried out with the superposing method mentioned in , or with standard calibration membrane with the same thickness and magnetic property as the test piece Measurement in discontinuous area, such as the area close to edge, footstep, pore space and corner shall be avoided, otherwise, effectiveness of measurement shall be confirmed. 40

20 i) The measurement probe placement; j) The deformation of cover layer on test piece Instrument calibration Standard calibration membrane Plastic mebrane, or test piece composed of substrate metal with non-conductive cover layer, may be adopted as standard calibration membrane. Plastic membrane is conducive to the calibration on curved surface, and is easier to get than the standard calibration membrane with cover layer. While it shall be ensured that plastic membrane is firmly bonded to the substrate Electrical properties of substrate metal in standard calibration membrane shall be the same as or similar to those of the substrate metal of test piece. Prior to measurement, the reading of cover layer thickness obtained from uncoated substrate metal of test piece shall be compared with the reading of cover layer thickness obtained from substrate metal of tested piece, to confirm the applicability of standard calibration membrane The substrate metal thickness during measurement and calibration shall be as same as possible. A metallic piece with enough thickness and the same or similar electrical properties may be superposed respectively upon substrate metals of standard calibration membrane and tested piece to keep the measurement reading free from the influence of substrate metal thickness. This method is not applicable if both sides of substrate metal of tested object are covered with cover layer When calibration fails to be carried out horizontally because of the curve of tested piece, curvature of standard membrane with cover layer or curvature of substrate placed with calibration membrane shall be the same as that of tested piece Measurement procedure Before the use of instrument every time, at the end of testing and at regular intervals (at least once per hour) in service, verification for instrument calibration shall be carried out on testing site, to ensure normal performance of instrument Instrument calibration shall be carried out with standard calibration membrane mentioned in Measurement in the area, such as the area close to edge, pore space, interior corner, etc., shall be avoided, otherwise, effectiveness of calibration for measurement shall be confirmed Measurement shall be carried out on plane surface; but if measurement can only be carried out on curved surface of test piece, effectiveness of calibration for measurement shall be confirmed Reading times Number of measurement points shall be determined according to the size of tested object, evenness of cover layer thickness and testing requirements. At least five measurement points shall be selected at the corners and center of tested area. The density of selected measurement points shall be increased as for the measurement of cover layer with uneven thickness Surface cleanliness Any external material on the surface of test piece, such as dust, oil, corrosion products, etc., shall be removed prior to measurement. Cover layer shall not be damaged by cleaning work. 42

21 Evaluation of testing results The measurement uncertainty of cover layer thickness shall be within 10% of the cover layer thickness or within 1.5μm, whichever is larger. 12 Evaluation of Testing Results 12.1 Signal analysis Unless otherwise proved to be uncorrelated signal caused by workpiece structure or other reasons, all signals appeared during testing shall be analyzed based on amplitude, phase or both Evaluation and treatment of testing results The specification on quality acceptance levels of tubular products shall be in accordance with the contract between the supplier and purchaser or the requirements of relevant product standards Result evaluation of equipment in-service shall be determined with interested parties according to the requirements of relevant technical specifications and standards Evaluation and treatment of testing results for tested workpiece shall meet the requirements in each chapter of this standard. 13 Testing Record and Report 13.1 The relevant information and data during the testing process shall be recorded in detail according to the actual situation of site operation. The testing record not only shall meet the requirements of NB/T , but also shall at least include the following content Condition of the tested workpiece: a) The name of workpiece; b) The serial number of workpiece; c) The specification of workpiece; d) The material of workpiece; e) The welding method and heat treatment condition of workpiece; f) The coating type and thickness (when necessary) Testing area and scope Testing types: a) The eddy current inspection; b) The eddy current thickness measurement Testing instrument and equipment: a) The model of testing instrument; b) The serial number of testing instrument; c) The probe (type, size, etc.); d) The type of test block Edition of testing process specification, and serial number of process card Testing situation of flaws: 43