A Comparative Study of PAUT and TOFD responses with the changes in microstructure of various materials - A Step towards reliable detection

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1 A Comparative Study of PAUT and TOFD responses with the changes in microstructure of various materials - A Step towards reliable detection More info about this article: Abstract Dharmveer SINGH, Abhishek BANERJEE, Sumit DAS GE Power India Limited, Durgapur 71326, West Bengal, India dharmveer.singh@ge.com, abhishek.banerjee1@ge.com The ever-changing market and evolving power sector poses a constant need for cost reduction and reducing atmospheric emissions. This has led to a worldwide impetus to improve the power plant efficiency thereby reducing emissions and improving cost effectiveness. This necessitate the requirement for materials with improved elevated temperature and high strength properties. These new and critical materials need the reliable NDE techniques which can provide more accurate flaw detection. Among all non-destructive examination methods, Advanced Ultrasonic examinations (PAUT and TOFD) are some of the most reliable volumetric examinations along with conventional Ultrasonic Examination for the most used and advance materials. The change in microstructures of the materials affect acoustic properties of the material, resulting in change of attenuation, scattering, SNR etc. Heat treatment and mechanical work (cold or hot) modifies the microstructure of the material. This provides a correlation between heat treatment, mechanical work and responses from ultrasonic examination Ultrasonic testing suffers from some inherent drawbacks which can be correlated with the microstructure of the materials. Microstructural variations can cause varied ultrasonic response because of changes in attenuation, velocity, scattering, SNR. Further, it is well known that prior heat treatment and mechanical working also plays a key role in modifying microstructure of the material. Hence a correlation is often derived between the heat treatment and variation in ultrasonic response. The intent of this paper is to present the unique study of the variations in the ultrasonic responses with respect to the changes in microstructure of various material used in industry. Specimen was heat treated at different heat treatment cycles and this study was carried out to find the correlation between microstructural changes and acoustic response of the material. Further investigation was done to improve the detectability from PAUT and TOFD. Keywords UT (Ultrasonic Testing), PAUT (Phased Array Ultrasonic Testing), NDT (Non-Destructive Testing), SNR (Signal to Noise Ratio) 1. Introduction The prime objective of NDT is to detect and characterize flaws in a component to assure that the component meets the required quality standards and hence fit for its intended service. There are several NDT techniques which are used for volumetric examination like RT (Radiographic Testing), UT (Ultrasonic Testing), PAUT (Phased Array Ultrasonic testing), TOFD (Time of Flight Diffraction). However, application of PAUT, TOFD or a combination of both is increasingly being used in volumetric examination owing to its benefits of versatility, cost effectiveness and sensitivity in detection of subtle defects [1-2]. We know that among many other factors, material microstructure plays a very significant role in affecting the performance of ultrasonic systems. It is also a very well-established fact that the material microstructure can be altered greatly with different heat treatments cycles. Hence there is close interplay of factors like heat treatment, material microstructure and performance of PAUT, TOFD testing. Polycrystalline materials with grain structure will cause attenuation and backscatter which will 1

2 significantly alter the ultrasonic response of the system. Grain boundary which the interface between two adjacent grains exists in polycrystalline metals and will cause scattering of the propagating ultrasonic wave which causes attenuation as well as noise echo. Sometimes even the noise echoes can be strong enough to mask defect echoes from small indications [3]. Hence a better understanding of factors like heat treatment, subsequent microstructural changes and how it effects the performance of testing is of paramount interest to NDT personnel. It is the goal of this work to understand experimentally, the effects of different heat treatments upon the microstructure of the material and its interaction with PAUT and TOFDs response of the system. This study can help to understand the interaction of the factors which in turn can help to design better inspection system []. 2. Experimental Study A plain low carbon steel plate was identified for the study as specimen for this work. To start with, sample was prepared and analyzed with OES (Optical Emission Spectroscopy) to confirm the grade of the material. Artificial discontinuities were simulated by inserting SDHs through machining to get PAUT and TOFD responses. Various heat treatment cycles were attempted to create the changes in the microstructure and obtain a grain size variation. After each heat treatment, a detailed metallurgical and acoustic study with several changes in parameters of PAUT and TOFD was carried out in the plate to understand the correlation of these parameters with changes in microstructure and possibilities of optimization of the ultrasonic response. 2.1 Material: Material selected for this study was plain low carbon steel plate (ASTM Number SA516 Gr 7) which is supplied in as rolled condition. Great weldability, numerous applications in structural and power sector were the main reason to select this material. 2.2 Chemical Composition Determination by Optical Emission Spectroscopy (OES) OES confirms the material grade of SA 516 Gr. 7 after analyzing the chemical composition from a standard square sample of dimension 3 x 3 mm as given in Table-1 Element Percentage (%) Element Percentage (%) C.23 Mo.76 Si.216 Ni.15 Mn Cu.1 P.22 Al.2 S.19 As.61 Cr.21 B.2 Co.55 Ti.2 Nb.23 V.18 Sb. W.1 Sn.13 Zr.12 Table 1. Chemical Composition of SA516 Gr 7 Determined by OES 2.3 Sample Preparation for Ultrasonic Study 2

3 SDHs of 1.5 mm diameter at ½ T and ¼ T at plate of dimension (29x15x25mm) was used as artificial reflectors for ultrasonic study to provide more versatility and the same reflection for different beam angles (T = Thickness of the plate). A detailed schematic view is presented as below: PAUT/TOFD PROBE 1/ T 1/2 T 25 mm SDH 29mmm 15 mm Fig.1.Schematic View of Sample 2. Heat Treatment The plate was exposed to multiple heat treatment cycles at different stages to produce microstructural variation in the sample. Heat Treatment was performed by resistance coil heating and the sample was insulated sufficiently on both sides to prevent heat loss and sharp temperature gradient. Calibrated thermocouples and temperature recorders were used to monitor the heat treatment parameters. Inside and outside surface thermocouples were attached to the sample by means of capacitor discharge method. Sufficient care was taken so that the sample was cooled under insulated condition to maintain extremely slow cooling rate. Annealing heat treatment was used since it causes changes in microstructure and corresponding mechanical properties of the material by three processes which takes place at different stages of annealing: recovery, recrystallization and grain growth. In the first heat treatment cycle, sub critical annealing performed and for the subsequent two more cycles, full annealing was performed with increase in the holding time. The annealing conditions as shown in Table 2. Heat Treatment Cycle Type of Heat Treatment Annealing temperature (ºC) Soaking Time (min) Process / Subcritical 65 ±1 18 Annealing Full Annealing 95 ±1 185 Full Annealing 95 ±1 37 Table 2. Annealing conditions used for heat treatment Temperature ( C) Rate of heating: 8-9( C/hr) Rate of cooling: 2-3( C/hr) Temperature ( C) Rate of heating: 8-9( C/hr) Rate of cooling: -5( C/hr) Time (min) Time (min) 3

4 95 37 Rate of cooling: -5( C/hr) Temperature ( C) Rate of heating: 8-9( C/hr) Time (min) Fig.2 Annealing regimes used for heat treatment of the sample 2.5 Microstructural Investigation and Hardness Survey Samples were cut and prepared for microstructural investigation and hardness survey for as rolled condition and after each annealing treatment Sample Preparation Samples in the longitudinal and transverse directions were cut from the plate using a band saw machine and then the samples were polished using emery papers of different grit, starting with coarse and then finer grit was used. Final polishing was done in a wheel polishing machine using diamond suspension as abrasive. The sample was then etched using 2% Nital (98% Ethyl alcohol + 2% Nitric Acid) Microstructural Study Samples were studied using an optical microscope at 2X magnification. The results of microstructural investigations are presented below: a. As rolled condition Fig. 3 Longitudinal Direction (2X) Fig. Transverse Direction (2X)

5 b. After Heat Treatment -1 () Fig. 5 Longitudinal Direction (2X) Fig. 6 Transverse Direction (2X) c. After Heat Treatment -2 () d. After Heat Treatment -3 () Fig. 7 Longitudinal Direction (2X) d. After Heat Treatment -3 () Fig. 8 Transverse Direction (2X) Fig. 9 Longitudinal Direction (2X) Fig. 1 Transverse Direction (2X) 5

6 As rolled sample shows banded microstructure with alternating bands of ferrite and pearlite aligned parallel to the rolling direction (Fig. 3 & Fig. ). After the first heat treatment cycle which was performed below lower critical temperature there was minor increase in the grain size, however the banded microstructure was still evident (Fig. 5 & Fig. 6). After the full annealing heat treatment performed above Upper Critical Temperature, an increase in grain size was observed and there was homogenization of the banded structure as evident from microstructure in the transverse direction (Fig. 7 & Fig. 8). In the third heat treatment cycle, full annealing was performed with increase in soaking time from the second heat treatment cycle, this resulted in more homogenization of the structure, since it s a diffusion based process complete homogenization may take extremely long annealing time, partial homogenization was noticed (Fig 8 & Fig 1) at the second and third heat treatment cycle which was performed above Upper Critical Temperature Grain size measurement and Hardness Survey Grain size measurement was performed as per ASTM E-112 in the longitudinal and transverse direction using image analyzer, in the as rolled state and after each annealing treatment. Hardness measurement was also done at each stage using Vickers s hardness testing machine. Grain Size measurement and hardness was measured consecutively for three times and average was reported. The results are shown as below: ASTM Grain Size Number(G) Longitudinal Transverse ,5 1,1 1 9, ,5 9,9 1 9, 9, 12 1 As rolled As rolled Hardness (Hv1) Sample Condition Fig. 11 Grain Size variation with Heat Treatment Sample Condition Fig 12 Hardness variation with Heat Treatment As evident from Fig. 11 the ASTM Grain Size Number (G) reduced subsequently after annealing heat treatment. The reason behind this reduction in G is the corresponding grain growth that resulted from annealing treatment. The major increase in grain size happened in i.e. after full annealing treatment performed above the Upper Critical Temperature. However, there was not much of a change noticed in grain size after the soaking time was increased in annealing treatment (after HT-). The sudden drop in the hardness value (Fig. 12) after the first sub critical annealing () can be attributed to the recovery process that happens during annealing which reduces the dislocation density in the material. However, there was no major change in the hardness values after &. The results obtained after hardness and grain size survey are in conformance with the microstructural changes than happened due to the annealing treatments given to the sample. 6

7 2.6 Ultrasonic Study Attenuation Study A calibrated Vernier Caliper was used to measure the thickness of the block and attenuation study was performed using backwall echo patterns of longitudinal wave obtained using phased array probe of MHz and Ultrasonic Testing machine Omniscan MX2. Attenuation study was performed in as rolled condition and after performing each heat treatment. Decrease in ultrasound intensity with distance i.e. attenuation is expressed in decibels per unit length and can be found by the following relation: = 2 2 Where A m and A n = amplitudes of mth and nth back reflections (n>m), and T is the thickness [5]. In this study, the first and the second backwall reflection was used to calculate the attenuation coefficient. The variation of attenuation coefficient in the as rolled condition and different stages after heat treatment is presented in the below (Fig. 13) As Rolled,18,16,1,12,1,98,96,9,92 Fig. 13 Variation in attenuation coefficient with heat treatment Attenuation (db/mm) The attenuation that takes place in the material is having two components, absorption and scattering. Energy loss due to absorption is a combined effect of mechanisms like dislocation damping, hysteresis losses, thermos elastic effects, etc. Loss due to scattering is predominantly dependent upon ratio of grain size and wavelength. Attenuation in the material is strongly influenced by the microstructure of the material and the same has been studies in this work. It is established that heat treatment plays an important role to modify the microstructure which causes a change in the attenuation within the material. The parameters playing a role may include grain size, grain boundaries, the relative amounts of phases, the layer thickness of the pearlite [6-7]. As illustrated in Fig. 13, attenuation was found to increase with heat treatment which can be attributed to the factor of increase in grain size due to heat treatment. However, there was a minor decrease in attenuation coefficient noticed after the final heat treatment which can be due to partial removal of banding structure and the onset of homogenization of the structure, however this needs to be investigated in detail in future work to study the effect of homogenization on attenuation within the material PAUT and TOFD testing and evaluation The purpose of the study was to investigate the changes in response of PAUT and TOFD due to heat treatment and study the possibilities of overcoming the challenges of higher noise and lower SNR due to microstructural changes. To have a better understanding of all these factors and their 7

8 interdependence, multiple scans were performed with variation in beam path, indication depth (SDH at different depths), frequency, filter, averaging. The same number of scans were repeated for both PAUT and TOFD after each heat treatment to understand the effect of microstructural changes upon PAUT and TOFD response and to get the optimum response with changes in UT parameters PAUT and TOFD setup Omniscan MX2 machine was used to perform the testing and Cutting Oil was used as a coolant medium in all the scans. Manual encoded scans were recorded and the data evaluation was done using Tomoview Software. To minimize the possibilities of variation in manual scan same set of data was recorded multiple times and once uniform results were obtained the same was used for analysis. Scanning setup is as per attached Fig. 1 Fig.1 PAUT and TOFD Scanning Scanning variables can be well understood from the attached details (Table 3, Table): Probe Frequency Wedge 5L6A12/ 2.25L6A12 5L6A12/ 2.25L6A12 5L6A12/ 2.25L6A12 5L6A12/ 2.25L6A12 5MHz/ 2.25MHz 5MHz/ 2.25MHz 5MHz/ 2.25MHz 5MHz/ 2.25MHz SA12- N55S SA12- N55S SA12- N55S SA12- N55S Angle (Deg) Scan Half Skip Probe Index Full Skip 5 Linear Linear Linear +1-3 to 67 Sectorial Probe Frequency Wedge Angle PCS C563- SM C563- SM 5 MHz 5 MHz 1 MHz 1 MHz C53- SM C53- SM ST1-6L-IHC ST1-7L-IHC ST1-6L-IHC ST1-7L-IHC 6 Deg Deg Deg Deg Table 3. PAUT scanning parameters Scanning was performed as per attached scan plans: Table. TOFD scanning parameters Fig. 15. PAUT sample scan plan 8

9 Fig. 16. TOFD sample scan plan Signal to Noise Ratio (SNR): SNR 6 2 As Rolled SNR Filter 2 Filter 5 Filter Fig 17 Variation in SNR with heat treatment for ½ T SDH (2MHz, 5 ) Fig 18 Variation in SNR with heat treatment for ½ T SDH (2MHz, 6 ) 2.7 Experimental Result Noise levels at different heat-treated stages were compared. Impacts of averaging and filtering were analyzed for finding the way to improve SNR. Following results were observed a. Attenuation increased from as rolled stage up to HT2 while it decreased from HT2 to HT3. b. Noise level of PAUT and TOFD signals was increased from as rolled to HT1 while then it shown decremental effect from HT1 to HT2 and finally very less change from HT2 to HT3. c. Amplitude of the reflected signal shows the same trend. It increased from as rolled stage to HT1 and then decreased from HT1 to HT2 while very less change from HT2 to HT3. d. The variation in SNR is studied with the responses from the reflections from ½ T SDH with respect to the noise level. Variations in PAUT is described by Fig. 17 and Fig. 18 while variations in SNR in TOFD will be an extended work. e. Averaging and filtering has crucial role to play in improving detectability. 2.8 Scope of future work a. Change in orientation of grains in both longitudinal and transverse directions and their impact on the signals for TOFD and PAUT with more details. b. Effect of heat treatment in modifying the banded structure of the rolled steel plate can be investigated in more details through SEM and TEM analysis. c. Analysis of SNR of TOFD and PAUT with different heat treatment cycles 3. Conclusion 9

10 192 scans of PAUT and 112 scans of TOFD at a specimen with different heat-treated conditions provided great amount of information. Changes in grain size were noticed in subsequent heat treatment cycles. This results in more noise level in the indications of PAUT and TOFD of the same specimen compared to the indication observed at as rolled condition a 5 X 6 Averaging b 1 X 7 Averaging c 5 X 7 Filter Noise Level Noise Level Noise Level d e f Noise Level X 6 Filter Noise Level Noise -2MHz, 5 Deg, FS Fig. 19. a, b, c, d are the responses for TOFD where N is Frequency in MHz and M is angle in (N x M) format. e. represents noise level for PAUT and f. represents amplitude for all heat treatments This is far evident and as expected. When specimen was heat treated above upper critical temperature, even though grain size increases, onset of homogenization of the banded structure was also noticed. The probable reason for reduction in noise level and attenuation of PAUT and TOFD after HT2 may be due increase in homogeneity of the structure and it can be an extended scope of this work. The selection of appropriate filtering and averaging is the key for improving SNR which reduces due to increase in grain size in due course of different heat treatment cycles.. Acknowledgment We sincerely thank Supriyo Roy, Supervisor Metallurgical Laboratory, and Subhendu Saha, Engineer - Metallurgical Laboratory for carrying out the spectroscopy and metallography of the samples. 5. Reference 1. Characterization of Material Properties by Ultrasonics, P.P. Nanekar and B. K. Shah 2. Future of Volumetric Examination in Power Sector Phased Array Ultrasonic Testing (PAUT) and Time of Flight Diffraction(TOFD), Dharmveer Singh, Abhishek Banerjee Indication Level Amp -1/2 SDH -5 MHz, FS 5 8 1

11 3. Effects of material microstructure and surface geometry on ultrasonic scattering and flaw detection, Yanming Guo. Analysis of relation between Ultrasonic Testing and Microstructure : A Step Towards Highly Reliable Fault Detection, A Khaira, S Srivastav, A Suhane 5. Ultrasonic Attenuation and Velocity in Steel Standard Reference Blocks, M.G.S. Ali, N.Z. Elsayed, Ahmed M. Eid 6. Modelling of Microstructural Banding during Transformations in Steel, Dipl.-Ing. Eric A. Jägle 7. Ultrasonic characterisation of hot-rolled and heat-treated plain carbon steels, C H Gür and Y Keleş 11