Development and Production of Welded Flaw Specimens For Non-Destructive Testing

Transcription

1 Development and Production of Welded Flaw Specimens For Non-Destructive Testing By Gonçalo Duarte Fazenda da Silva, Instituto Superior Técnico ABSTRACT It is mandatory that the personnel who performs non-destructive testing have been trained and is certified. This is the only way to ensure valid and reliable results. The present paper addresses the development of welding procedures for systematic production of flawed specimens to be used for training and certification of NDT personnel. Another very important aspect in executing NDT is the proper equipment calibration. For that purpose calibration standards are used. The present paper also addresses the development of a database of NDT calibration blocks that will be used as a query. KEY-WORDS: Non-Destructive Testing; Calibration Block; Calibration; Discontinuity; Data Base Welding and Its Defects in Industry Welding is a critical technique for joining materials in many industries, such as the construction industry, petroleum, petrochemical, aeronautics, manufacturing processes, among others. It is an efficient, economic and reliable process. However, during the process of welding different defects may occur, such as porosity, inclusions, lack of fusion, deficits or excesses of penetration, cracks, etc., all of which can be due to poor gas protection [2], poor welding technique, unclean surfaces, wrong welding parameters, etc. These defects can cause failures and accidents, endangering the normal functioning of parts and components, and risking people s lives. [3] INTRODUCTION The non-destructive testing (NDT) [1] are techniques to examine objects, materials or systems without damaging them and, therefore, without affecting their future usefulness. These techniques are extremely valuable because, as they do not permanently change the state of the component, they allow to save time and money in evaluation, in problem solving, in prevention of occurrence of failures and product research. They are widely used in many different industries like oil/petrochemical, chemical, electromechanical, aeronautic, aerospace, among others. The conventional NDT techniques include penetrant testing (PT), magnetic testing (MT), radiography testing (RT), ultrasonic testing (UT) and eddy current testing (ET). Figure 1: Crack in a rail section It is necessary to do the inspection of welds to assure an adequate control of the manufacturing process. Non-destructive testing is the most important technique used for this purpose [4][5][6] Penetrant Testing [7], [8] This non-destructive testing method builds on the principle of Visual Inspection. It increases the seeability of small superficial discontinuities that the human eye might not be able to detect alone. It is used in metals, ceramic materials, plastics, etc. 1

2 The calibration blocks used in PT have the same purpose, to check the sensitivity [9] of the consumable (penetrant and developer) and to compare between brands of liquids. Figure 4: General Principles of RT Figure 2: Calibration block for Penetrant Testing Magnetic Testing [10], [11] Magnetic particle inspection can detect both superficial and sub-superficial discontinuities in ferromagnetic materials. A test specimen is magnetized with a strong magnetic field created by a magnet or special equipment. If the specimen has a discontinuity, it will interrupt the magnetic field flowing through the specimen and a leakage field will occur. Finely milled iron particles coated with a dye pigment are applied to the test specimen. These particles are attracted to leakage fields and will cluster to form an indication directly over the discontinuity. This indication can be visually detected under proper lighting conditions. The calibration standards for the magnetic particle testing are used to check the magnetic field created and to check the indicating sensitivity of the inspection agent. The energy of the radiation affects its penetrating power. Higher energy radiation can penetrate thicker and more dense materials. The radiation energy and/or exposure time must be controlled to properly image the region of interest. The film darkness (density) will vary with the amount of radiation reaching the film through the test object. Figure 5: Effects of radiation energy The RT has certain limitations concerning the detection of defects. Figure 6: Discontinuity orientation: (a) wrong angle; (b) right angle Figure 3: Calibration block for Magnetic Particle Testing [12] Radiography Testing [13], [14] The part is placed between the radiation source and a piece of film. The part will stop some of the radiation. Thicker and more dense area will stop more of the radiation. The image quality indicators (IQI) are essential for a precise evaluation of the integrity of the sample. Figure 7: Image Quality Indicator 2

3 Ultrasonic Testing [15], [16] Ultrasonic waves are introduced into a material where they travel in a straight line and at a constant speed until they encounter a surface. On the surface interfaces some of the wave energy is reflected and some is transmitted. The amount of reflected or transmitted energy can be detected and provides information about the size of the reflector. The travel time of the sound can be measured and this provides information on the distance that the sound has traveled Figure 8: General Principles of UT The UT uses different types of calibration blocks, each type with a specific purpose like (thickness measurement calibration, angle beam calibration, index calibration, velocity calibration, DAC construction, etc ) In order to provide the inspector useful information, the signals generated from the piece must be compared to results with known values. To do this, reference standards are made of the same material of the component or a very similar one. Figure 11: Reference Standards for ET Welded Test Specimens Used in NDT Personnel Training and Certification The welded specimens used in the practice training consist of plates, pipes, halfrounded, corner joints or T joints that have defects such as porosity, lack of fusion, cracks, inclusions, among others [21]. Figure 9: Calibration Block for UT [17] Eddy Current Testing [18], [19] Eddy currents are induced electric currents that have a circular path and are generated by a probe within which is a coil-shaped conductor. An alternating current in the coil passes a frequency determined by the operator forming a dynamic magnetic field around the coil. When a conductive material is placed in the dynamic magnetic field, Eddy currents will be induced in the material. These currents will generate its own magnetic field. Figure 10: Generation of Eddy Currents [20] Figure 12: Welded Test Specimens Many papers and studies were developed in order to enhance the different welding processes (on certain materials such as aluminum [22], tungsten [23], austenitic stainless steels [24]; about welding parameters optimization [25] [26] [27]; under the influence of protective gases [28] [29] [30]; about the causes of defects [31] [32]; about programming assistance in welding process [33] [34], etc...). Others consisted of studying the effects that machined defects in a component can cause [35]. About the production of welds with real defects, the case is different because there is not much information about it. This thesis aims to bridge this gap, with the elaboration of welding procedures for the production of welds with realistic defects. 3

4 MAG PROCEDURE For MAG welding, carbon steel plates were used with dimensions 350x150x12mm. The geometry for the joint was a Single-V with an angle of 30º. joint in order to have a gap between the bead and the wall. Finally, the filler pass is done and the plate is ready. Figure 14: MAG plate Table 1: Parameters A 1mm diameter consumable wire was used and a gas protection which consisted of a mixture 82% Ar + 18% CO2. First, the root pass was done. Next step, to cause porosities, the shield gas should be shut while the weld is being done. NDT ON MAG PLATE Radiography testing and ultrasonic testing were made in the plates. Figure 12: Porosities About the lack of fusion, it should be done as in the figure below. Figure 15: Radiography on MAG plate porosities (up) and lack of fusion (down) Figure 13: Lack of Fusion The pass nº2 should be done in one side of the joint and the pass nº3 should cover the nº2 avoiding to fusion the other side of the Figure 16: Ultrasonic on MAG plate porosities (up) and lack of fusion (down) 4

5 After the UT, the inspection sheets were filled. wanted the inclusion, particles of slag are deposited. SMAW PROCEDURE For SMAW welding, the same carbon steel plates were used. The geometry for the joint was a Single-V with an angle of 30º. Figure 18: Slag deposition To finish, the filler pass is done. Table 2: Parameters The electrodes used were 3mm diameter. It is recommended to do the filling with larger diameters. In this case all the steps were done with electrodes of diameter 3 mm because there wasn t larger ones in store. After the root pass, to cause porosities, the electrode coating was removed and left the soul. This way the welding would not have gas protection and the desired porosity would appear. Figure 19: SMAW plate NDT ON SMAW PLATE Again, radiography testing and ultrasonic testing were made in the smaw plates. Figure 17: Coat removal (up) and Porosities (down) To create the lack of fusion, the same principle used in MAG was used in SMAW (see Figure 13). For the slag inclusion, in the area where we Figure 20: Radiography on SMAW plate porosities (up), inclusion slag (center) and lack of fusion (down) 5

6 penetration in the crack. For the tungsten inclusion, one side of the plate was filled to the end but the other one wasn t. In this side, it was made a small channel or cavity to put the tungsten inclusion. Figure 21: Ultrasonic on SMAW plate slag inclusion After the UT, the inspection sheets were filled. TIG PROCEDURE For TIG welding, aluminium plates were used with dimensions 300x150x10mm. The geometry for the joint was a Double-V with an angle of 30º. Figure 23: Tungsten inclusion Finally, on this side of the plate was given the filler pass. Table 3: Parameters In the process of TIG welding it was used, as filler metal, a 3mm diameter rod with 95% Al and 5% Si composition and the tungsten electrode was 3mm diameter. First, in one side of the plate, a short bead was made with 50mm approximately. After that, heat was given with the torch to make the bead crack. Figure 24: TIG plate NDT ON TIG PLATE As in MAG and SMAW, radiography testing and ultrasonic testing were made in the TIG plates. Figure 22 Crack The welding pass right after the crack should have a lower amperage to minimize 6

7 make the best choice in one single site while saving time and costs. Figure 25: Radiography on TIG plate crack (up) and tungsten inclusion (down) CALIBRATION BLOCKS DATABASE One of the most important things in the practice of NDT is the calibration of the equipment. If there isn t a good calibration, quality and accuracy of the method can be questioned. Calibration blocks are very important because with them it is possible to calibrate the equipment to different methods, different materials, different geometries, etc... Due to the great importance of calibration blocks for performing non-destructive testing, a database with standard blocks for the conventional NDT (UT, MT, RT, PT and ET) was developed. The purpose of this database is to facilitate the search for information, and acquisition of these blocks. Figure 26: Ultrasonic on TIG plate crack (up) and tungsten inclusion (down) After the UT, the inspection sheets were filled. NDT CALIBRATION BLOCKS MARKET The standard blocks for non-destructive testing are of great importance and, depending on the test, have different purposes. There are many companies involved in the market of reference blocks of nondestructive testing, which can make the task of someone interested in purchasing these products quite difficult due to the variety of options (examples are OLYMPUS, ZETEC, PHTool, Flawtech, ATS, Karl Deutsch, MPM Products, GE, etc). For facilitating the acquisition process, a database gathering information about the calibration blocks of the different tests in a single space was developed, so that the user has all the information available to Figure 27: Calibration Block Database Main Menu It s quite easy to use the database. First the user has to choose the NDT technique. After that, he must choose the block he s interested in from a list. A page with information about it will appear (image of the block, normative standards associated, description, selling companies and a drawing of the block). 7

8 quick access to the information for acquiring calibration standards. REFERENCES [1] ALMEIDA, Filomena Pinto, BARATA, João, BARROS, Pedro, ENSAIOS NÃO DESTRUTIVOS, ISQ Edições Técnicas Figure 28: Calibration Block Database Page with information of the V2 Block CONCLUSIONS The main conclusions drawn from this study are as follows: Successful developementof procedures for the production of welded flaw specimens for MAG and SMAW welding processes for steels (plates dimensions 350x150x12mm) and tungsten inert gas (TIG) for aluminum (plate dimensions 300x150x10mm). The procedures aim to fill a gap in the national market about the production and supply of this type of test pieces, making it easier the process of training and certification of NDT technicians. The proceeds will be able to decrease the dependence of companies interested in training NDT personnel and companies that give training on production and seller companies of test specimens. Radiography tests were executed in all the plates which revealed the presence of defects in the pre-defined locations. Ultrasonic tests were also made. Through the method of 6dB drop was possible to find and to size the defects. After the ultrasonic testing, the NDT Inspection sheets of all the plates were successfully filled. The specimens are ready to serve in the future in the practical training of ultrasound technicians. A database that contains information about the calibration blocks of conventional non-destructive testing (PT, MT, RT, UT, ET) was developed. Information about the calibration blocks of the different NDT techniques is now available. Users have now an easy and [2] P. Kah, J. Martikainen, Influence of Shielding Gases in the Welding of Metals, International Journal of Advanced Manufacturing Technology (2013) 64: , DOI /s [3] C.D. LUNDIN, Discontinuities in Welds Cause and Effect, J. MATERIALS FOR ENERGY SYSTEMS, AMERICAN SOCIETY FOR METALS VOL. 5, NO. 3, DECEMBER 1983 [4] ALMEIDA, Filomena Pinto, BARATA, João, BARROS, Pedro, ENSAIOS NÃO DESTRUTIVOS, ISQ Edições Técnicas [5] R. J. Ditchburn, S. K. Burke and C. M. Scala: NDT of welds: state of the art, NDT&E International 1996, PII: SO (96) [6] Abdul-Hannan Ali, Daniel Balint, Andrew Temple, Pat Leevers, The reliability of defect sentencing in manual ultrasonic inspection, NDT&E International51(2012) , DOI /j.ndteint [7] Penetrant Testing - org/educationresources/communitycolleg e/penetranttest/cc_pt_index.htm (April 2012) [8]Penetrant Testing - r_20.jsp?menu_pos=6 (April 2012) [9] System Performance Check - CommunityCollege/PenetrantTest/QualityPr ocess/performancecheck.htm (April 2012) [10] Magnetic Particle Testing - 8

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