1.0 INTRODUCTION
Welding is widely used in the fabrication of nearly all industrial components. Despite the best care taken during design, fabrication and inspection, many of the welded components fail especially at the weld and heat affected zones, drastically influencing the performance reliability and component availability. Majority of the failures are attributed to improper design of weld joint, selection of base materials and filler materials, welding processes, residual stresses, inspection procedures and operating parameters. One way to minimise the failures of welded components is to impart non destructive testing (NDT) procedures i) immediately after the fabrication to make sure the welded joint is defect-free and ii) during the service life of welded components to ensure that no unacceptable defects are present and grow [1]. Variety of NDT techniques exits and nearly every form of energy is used in NDT field to device methods for detection and evaluation of nearly all kinds of defects, be they surface or internal. While a few basic NDT methods such as penetrant, ultrasonic, radiography, visual testing are sufficient and routinely employed for the inspection of welds, use of advanced NDT techniques is resorted to when high sensitivity detection and quantitative characterisation of harmful defects is envisaged.
Often, signal and image processing methods are adopted to meet these objectives. While NDT data on defect dimensions helps fracture mechanics to assess the health of a welded component, it is fracture mechanics based analysis that acts as a driving force for developments in the science and technology of NDT by putting straight demand to detect very small defects as and when they are formed in the component
[2]. In this paper, after a brief review of conventional NDT methods, advances made in the field of x-radiography, eddy current and ultrasonic methods are discussed. Also use of acoustic emission and infrared thermography methods for on-line monitoring and control of welding processes and for integrity assessment of welded pressure vessels are discussed. The techniques used for evaluation of residual stresses in welded components are also covered.
2.0 CONVENTIONAL NDT TECHNIQUESNo weld is completely perfect. Despite the best care taken during design, fabrication and inspection, many of the welded components fail especially at the weld and heat affected zones due to cracks, residual stresses and variations in microstructure and mechanical properties, all of which are often grouped together and referred to as discontinuities or simply defects, if harmful. Weld defects can be classified broadly as cracks (crater, transverse, toe, under bead, fusion line and root cracks), cavities, porosities, slag inclusions, incomplete fusion or penetration, poor fusion, under cut, excessive reinforcement, imperfect shape or unacceptable contour, spatter, arc strikes etc. Cracks are considered more harmful and they are usually classified as longitudinal or transverse or toe or crater cracks depending on their orientation. Also based on the temperature of formation, they are termed as hot cracks or cold cracks. Porosity and slag inclusion are also harmful defects and they are caused by entrapment of gases and oxides and non-metallic solids in the weld metal during the solidification.A variety of NDT techniques are available for detection and characterisation of defects in welds. All NDT techniques are based on physical principles. Nearly every form of energy is used as probing medium in NDT. Likewise nearly every property of the materials to be inspected has been made the basis for some method or technique of NDT. In general, NDT methods involve subjecting the material (being examined) to some form of external energy source (X-rays, ultrasonic, thermal wave, electromagnetic fields etc.) and analysing the detected response signals (refracted energy, induced voltage and diffracted energy).
The essential parts of any NDT method are 1) application of a probing or inspection medium, 2) modification of the probing or inspection medium by defects or variations in the structure or properties of the material, 3) detection of this change by suitable detector or sensor, 4) conversion of this change into a suitable signal or image and 5) interpretation of the information obtained [2]. For example, in the case of X-ray film radiography of welds, 1) the X-rays are the probing or inspecting medium, 2) any defects in the weld being radiographed modify the intensity of the radiation reaching the film on the opposite side of the weld, 3) certain silver bromide emulsions are sensitive to X-rays and are used as a detector, 4) the emulsions are capable of recording variations in X-ray intensity and by proper developing, a permanent record is made, and 5) interpretation is then a process of explaining variations in density of the radiograph. Some of the widely used NDT techniques for the examination of welded structures include; visual, liquid penetrant, leak, magnetic particle, ultrasonic, eddy current, Gamma and X radiography, acoustic emission, potential drop, infrared thermography etc. As NDT measurements are indirect, artificial standard defects are used to set the instruments, sensors and test procedures for a desired performance. Further, since NDT operator decides the success of an inspection method, experienced or certified skilled personnel are specified for interpretation of NDT results.
3.0 ADVANCES IN NDT TECHNIQUES
3.1VISUAL TECHNIQUESVisual techniques play an important role in quick assessment of the quality of welds and to identify various defects like undercut, lake of penetration etc. The basic design of the borescopes, which has been in use for many decades for visual examination, has been modified accommodating the state-of-the-art advances in video, illumination, robotic, optical and computer technologies. Developments in image processing, artificial intelligence, video technology and other related fields have significantly improved the capability of visual techniques [3]. Present day demand for higher performance and faster production exceed the abilities of visual tests by humans. Consequently, visual tests made by human eye are being replaced by automated visual testing using optical instruments and unstaffed inspection stations. Such aspects are usually referred to as machine vision. Geometrical imperfections such as improper weld ripples, convexity and concavity need to be detected in inaccessible regions. Replica technique is ideal for such applications. In this technique, the profile of a defect region is replicated using silicon rubber compound and the dimensions of the defect region are measured using profile projector which can achieve an accuracy of + 5 microns. The hardware for the replica technique essentially consists of manual injection device, mixing guns, silicone rubber compound, injection head, spring loaded CRS plugs and recovery cork screw head. Replica technique has been adopted for examination of a number of joints in the reheater of Prototype Fast Breeder Reactor (PFBR). Similarly, for measuring concavity and convexity of weld joints videoimagescope is preferred. It is an advanced version of the flexible fiberscope in which a CCD chip is used for imaging. Compared to fiberscopes, videoimagescopes provide high resolution and brighter images. With the introduction of advanced image management functions and measurement capabilities, accurate measurements of internal profiles of tube are possible. A variety of viewing tips with varying field of view are available which can be used depending on the nature and type of application. Similarly, images can be suitably enhanced through image processing functions such as contrast stretching and edge enhancement. Using the 3-D graphic measurement system, it is possible to measure the length, width etc. on the images very accurately. Commercially available Olympus videoimagescope model IVC-6 has been used for measuring concavity and convexity of a number of joints of Reheaters of PFBR [4].
3.2 RADIOGRAPHYAs the X-ray absorption coefficient depends strongly on material density, radiography is particularly effective at detecting volumetric defects, which contain either extra mass or missing mass (such as slag inclusions or porosity). The benchmark for radiographic inspection of welds is still high-quality film radiography and good radiographic practice is now enshrined by a series of national standards, covering factors such as choice of voltage, film–source distances, intensifiers, image quality indicators, film density, film processing, etc. There have been a number of advances in radiography over the past 10–15 years including more reliable microfocus tubes, real-time radiography and the application of image processing techniques to sharpen the image and to increase the contrast. For better definition of defects and delectability of small defects like micro-cracks in thin components and complex geometries, high resolution micro-focal X radiography has an edge over the conventional radiography. One of the important applications of micro-focal radiography is evaluation of tube to tube sheet weld joints of PFBR steam generators (made by welding between pull out of tube sheet and the tube).The most significant recent development in radiography has been the real-time radiography. Real time radiography or fluoroscopy differs from conventional radiography in that the X ray image is observed on a fluorescent screen rather than recorded on a film. Fluoroscopy has the advantages of high speed and low cost of inspection. Present day real time systems use image intensifiers, video camera and monitor. The principal advantages of real-time radiography are that it is well suited to automation and the images of the component under inspection are available directly without time delays due to film exposure and processing. Furthermore, as the images are provided in digital form, image processing and automatic defect interpretation softwares can be readily incorporated into the inspection system. On-line monitoring of welding is another possibility by real time radiography. Direct examination of the welds in real time saves films and time and is found to be cost effective in the long run [5]. The use of microfocal units in conjunction with image intensifying system greatly enhances the versatility and sensitivity of the real time radiography, by way of zooming or projection magnification. With the advent of image processing systems, the sensitivity that can be achieved is comparable to film sensitivity. The stored or digitized X-ray image can be subjected to image processing and enhancement techniques such as contrast stretching, edge enhancement, special filtering, differentiation, averaging, and pattern recognition for enhanced detection of defects and also for obtaining quantitative information. The versatility of image processing is that this can be performed in real time as well as on film images. Figures 1(a) and 1(b) show typical radiograph of a weld joint. Figure 1(a) gives the raw image wherein penetrameter wires are not clearly seen. After contrast stretching and image enhancement (Fig. 1(b)), the lack of penetration can be seen and the wire penetrameters can be identified thereby increasing the sensitivity.
3.3 ULTRASONICSUltrasonics is now the major NDT technique used for validation of welded structures in many pre-service as well as in-service applications. Ultrasonics is a preferred technique over X-radiography in in-service inspections is due to inherent limitations in radiography and to actual benefits in applying ultrasonics. Ultrasonic waves are scattered by both planar and volumetric defects, making the ultrasonic technique useful for detecting and sizing both types of defects. Even closed cracks are detectable by ultrasonics if appropriate procedures are used. Unlike radiography, ultrasonics also readily gives depth information concerning a defect. Ultrasonics also offers benefits over radiography in terms of cost savings through increased productivity and safety. In the last few decades, ultrasonics has developed from a purely manual technique, to a manual technique with computer-assisted processing, to the use of automatic scanners and more recently to the development of fully automated systems incorporating multiple piezoelectric transducers for weld assessment. Studies clearly establish the fact that the probability of detecting a defect with ultrasonics increases with the degree of sophistication of the system. Ultrasonic methods are also widely used for measurement of residual stresses and also for characterisation of microstructures [6]. For these studies, ultrasonic velocity is preferred to attenuation measurements.The use of ultrasonics to establish the integrity of welded structures requires not only reliable defect detection but also sufficiently accurate defect location and sizing using amplitude dependent techniques (e.g. 20 dB drop, 6 dB drop, or comparison with the amplitude expected from a drilled hole). However, these techniques are known to be inaccurate. The incorporation of computer-assisted processing into ultrasonic systems has allowed the easy implementation of potentially better methods for defect detection and sizing such as time-of-flight-diffraction (TOFD). TOFD has the ability to capture high-resolution, low amplitude signals and perform real time processing to carry out crack tip diffraction examination (Fig.2) [7]. It lends itself ideally to fast volumetric detection applications where inspection results need to be of sufficient quality to enable decisive on line action. This speed is achieved by virtue of the fact that a wide beam, tandem array of transducers and scanned parallel to the weld, are usually sufficient to achieve full coverage and scan rates upto 50mm/s without the need for comprehensive raster scanning and probe skewing. There are also efforts to develop multi probe system (instead of two probe system) capable of inspecting the given weld region of the pressure vessel as well as building a prototype nozzle scanner based on the same technique. The multiprobe system is capable of detecting and locating defects in the given weld region throughout the full depth of the pressure vessel and to a width of t/L on either side of the weld region. It uses sixteen probes controlled by a computer with automated data acquisition and processing. It is intended to size defects within ± 2mm and size all defects greater than 5mm in depth below the interface between the stainless steel cladding and the carbon steel plate.Important advances in defect sizing for weld inspection have also been made possible by the incorporation in automated ultrasonic systems of ultrasonics imaging based on synthetic aperture focussing (SAFT) and variants such as SUPERSAFT. When the transducer is located directly above a defect, the time delay to receive the defect echo is minimal. As the transducer moves away from this position, the time delay increases in a non-linear fashion. The curve defined by tracing the peak amplitude (in each aperture element) as the transducer moves parallel to the surface is a function of the speed of the sound in the material and the geometry of the transducer and the target. The first synthetic aperture processing step is to choose a collection of aperture elements to be processed as a unit, herein after referred as the ‘aperture’. The essence of SAFT processing is to introduce a time shift to each individual A-scans which varies with time delay introduced by the test system geometry, to sum these individual aperture elements point by point across their length, and then to place the result at the center of the chosen aperture. Reflections coming from defect are constructively added and other signals such as grain noise and electronic noise are destructively summed, resulting in good signal to noise ratio for the defect [8]. When scanning is done in one direction (X), we get only two dimensional amplitude distribution corresponding to the area below the scanned line and perpendicular to the scanned surface thus performing a side view (B Scan). The microstructures of austenitic welds cause special concerns for ultrasonic testing. These materials strongly attenuate ultrasonic waves, cause high background noise due to scattering from the large grains present, and result in skewing of the ultrasonic beam unless the propagation is along principal axes. Considerable progress has already been made on detailed modelling of wave propagation in austenitic materials and on using neural networks for defect recognition and automated decision making. Various options exist for the improved generation and detection of ultrasound in welding applications, e.g. by the use of phased arrays, laser techniques and other specialist probes. Increasingly, electromagnetic transducers (EMATs) are finding application in the non-contact generation of horizontally polarised shear waves which have a number of advantages in weld testing. Phased array angle beam EMAT systems capable of exciting narrow band, obliquely propagating bulk waves with controlled direction and focusing are available [7]. The phased array EMATs provide a greater flexibility in both the selection of wave modes and angles of propagation. Experimental investigations were carried out using the SH waves generated by EMATs for defect sizing by TOFD method. Two 8-segmented EMATs were designed for their use as transmitter and receiver in pitch-catch mode. Test and instrument parameters were optimised for high sensitive detection of diffracted signals. Besides signal averaging, cross-correlation and analytical signal processing using Hilbert transform were adopted to enhance SNR and to improve accuracy in the transit time measurements and in turn the defect sizing. Experimental studies were carried out on fatigue cracks and machined notches in carbon steel and stainless steel specimens. Typical rf signal from a 28 mm deep fatigue crack in 56 mm thick carbon steel sample is shown in Fig.3. The back wall and diffracted echoes are clearly seen. The beam entry point was determined by the back-wall echo arrival time and analytic signal method was implemented for precise transit time measurements. From the transit time measurements, using distance between EMATs and angle of insonificaiton, defect depths were calculated and a correlation coefficient of 0.99 was observed between actual and calculated defect depths. EMATs are particularly attractive for high temperature and radioactive components and also for components with limited access, primarily due to the fact that EMATs do not need couplant to transfer ultrasonic energy. Two major limitations in using EMATs for a number of practical applications are the physical size of the source of magnetic field (a few tens of millimeters) and the low transduction efficiency (due to weak Lorenz and magnetostrictive driving forces) as compared to piezoelectric transducers.
3.4 EDDY CURRENT TESTINGConventional eddy current (EC) testing of welds is affected by surface roughness, microstructural variations, delta-ferrite, lift-off, edge-effect etc. Presence of these disturbing variables significantly influences the defect detection as well as sizing. In order to realize quick detection and accurate on-line depth evaluation of defects in the presence of such disturbing variables, an artificial neural network (ANN) based approach has been developed. This uses a three-layer feed forward error back-propagation type network with one hidden layer and one output node that gives the defect depth in user defined units. In this method, the digitized real and imaginary components of EC probe impedance are given as input to an optimized neural network. The network output is evaluated and displayed continuously. The performance of the network has been evaluated on stainless steel plates and welds for detection and depth evaluation of surface-breaking machined notches in the presence of disturbing variables. Systematic optimization studies have revealed that a 12-5-1 architecture is optimum and can detect weld defects as small as 0.4 mm deep with an accuracy of +0.04 mm (Fig. 4) A network trained with holes and notches has detected with 100 % success both notches and holes. However, for accurate depth evaluation, it has been necessary to use separate networks [9]. The on-line ANN approach has been successfully applied to thin walled stainless steel tubes with periodic wall thickness variations for detection and accurate quantification of depth of defects.Defects, which are described as three-dimensional functions of the space co-ordinates, cannot be completely reconstructed by scanning an eddy current probe coil over a defect in one direction and even by using sophisticated signal processing methods. On the other hand, a definite benefit exists if raster-scan imaging is made and the results are presented in the form of a gray level or pseudo color image. Eddy current imaging (ECI) is a recently emerging trend in the field of eddy current imaging. There are many advantages that follow the image format. It is rather comfortable to interpret the images of defects, as compared to the dynamic impedance display. Another advantage of the image format lies in the ability of the human eye and brain to readily discern irregularities in an image by comparing different regions. Further, the processed image and the defect details provide an objective and documentable information that could, e.g. be used to monitor the growth of defects with time. More important, process automation is possible because computers perform raster-scan imaging as well as data/image processing. An ECI system has been built around a PC at the author’s laboratory to scan the object surface and create impedance grey level images (Fig. 5). Eddy current images of welds, notches, corrosion pits and cracks in austenitic stainless steels have been obtained using the ECI system. The precise location of the weld centre line, in the inner vessel of PFBR, is required as feedback information for remote operation of robots for detailed inspection of the welds by ultrasonic techniques. ECI method has been developed to precisely locate the weld centre line. Figure 6 shows the 3-D profile image of the weldment. Due to predominant variations in the electrical conductivity and magnetic permeability (due to the presence of delta ferrite) of the weld metal, this region is distinctly brought out. The changes in the material properties affect the probe impedance. The change in the impedance varies from the base metal - weld interface to the weld - base metal interface and reaches a peak at the centre of the weld. This peak is clearly observed in the 3-D profile (Fig. 6). Thus the precise location of the weld centre line is found from this profile by measuring the distance along Y-axis from the origin i.e. starting point of the scanning. The accuracy of detection of the weld centre line is found to be + 0.1 mm. ECI also has been performed on welds for detection and sizing of defects in welds. However, three major problems have been noticed. They are 1) blurring of images as probe diameter is large, 2) noisy images due to the influence of disturbing variables and 3) time intensiveness. In order to realize fast and automated detection and enhanced characterization of surface defects in austenetic stainless steels, an intelligent imaging scheme has been developed by synergistically combining neural network and image processing methods [10]. This scheme involves:The imaging scheme has been validated on austenetic stainless steel plates and welds consisting of machined defects as well as natural defects. The imaging scheme has been successfully applied to welds for automatic detection and evaluation of longitudinal as well transverse notches. It has been observed that the scheme has been able to reliably detect all the defects present in the imaged regions and evaluate their length, width, depth and orientation, suppressing the dominance of welds variations and presence of magnetic delta-ferrite have not degraded the performance of the scheme (Fig. 7). Defects deeper than 0.4 mm have been detected and characterized with a ten-fold reduction in imaging time. Further, the scheme has clearly brought down the computer memory requirements for storage of image data, due to the fact that only the image data of fine-scan imaging regions has been stored.
3.5 ACOUSTIC EMISSION TECHNIQUEAcoustic Emission Technique (AET) is an important NDT technique. Its origination lies in the phenomenon of rapid release of energy within a component in the form of a transient elastic wave resulting from dynamic changes like deformation, crack initiation and propagation, leakage etc. It is a real time technique which can detect initiation and growth of cracks, plastic deformation, fatigue failure, leaks etc. AET is used during hydrotesting of as-fabricated welded vessels and also in service during their hydrotesting. AET is also used for on-line inspection of welded vessels and pipe lines for monitoring their structural integrity. In addition to this, of late AET is being considered for on line weld monitoring during fabrication for simultaneous detection of defects as the welding progresses [11]. The defects so found can be immediately rectified thus avoiding the completion of defective weld, saving time and money. AET has been successfully used for on line monitoring of welds prepared by TIG, submerged arc, electroslag welding etc. However, non slag forming welding methods are most suitable for AE monitoring. The defects that can be detected, located and quantitatively evaluated by AE monitoring during welding are: (1) Weld cracking associated with phase transformation, (2) Nucleation and growth of cracks during welding and subsequent cooling e.g., delayed cracking, (3) Porosity and slag inclusions, (4) Microfissuring, (5) Hot and cold cracking and (6) Reheat cracks. Once weld defects are located, they are further probed using other NDT techniques for in-depth analysis.AE technique has been used for on-line monitoring during resistance spot welding. Figure 8 shows the typical pattern of AE signals generated during resistant spot welding. By judicious analysis of the signals generated during different periods of the welding cycle, it has been possible to identify good and bad wleds and also the shear strength of the nugget can be estimated using AE parameters. The AE generated can be related to the weld quality parameters such as strength and size of the nugget, the amount of expulsion and the amount of cracking. Therefore, in-process AE monitoring can be used both as an examination method and also as a means for providing feedback control.
3.6 ALTERNATING CURRENT POTNETIAL DROP TECHNIQUESUltrasonic and alternating current potential drop (ACPD) methods are the only two established NDT techniques used for measuring crack depth in welds. Unlike ultrasonic inspection, which is used for both detection and sizing, ACPD is used almost exclusively for crack sizing. The ACPD method is only applicable to surface breaking cracks and requires electrical contact with the specimen. The surface current introduced into the specimen by the ACPD technique induces a magnetic field in free space above the specimen surface. Mapping of the perturbation of this magnetic field provides an alternative means of measuring crack depth and crack length without the requirement for a contacting probe. This technique is also termed as alternating magnetic field measurement (ACFM). ACFM offers the capability of both detection and sizing of surface breaking defects without the need for calibration and without the requirement for cleaning to the base metal. This technique is finding increasing application, particularly in weld inspection in offshore platforms.
3.7 INFRARED THERMOGRAPHY (IRT) TECHNIQUEMeasurements for this NDE technique are derived from changes in thermal resistance that arise in the flow of heat through the components. These changes can be detected by infrered cameras that are sensitive to surface temperature differences of less than 0.1 degrees Celsius. Precisely, IRT let one "see" heat [1, 2]. It is non-contact and fairly simple and it offers speed and high resolution plus the advantage of full-field imaging. IRT is also capable of providing very detailed images of situations invisible to the naked eye. By taking a thermograph of site electrical panels, thermographers develop and read a "heat picture" which reveals components that are overloaded or may become faulty. Unlike normal component operating conditions, faulty components exhibit readily detectable temperature increases over the ambient temperature profile. IRT verifies that electrical connections are properly made and maintained. IRT also detects hot spots that might be overlooked by visual inspections. IRT can be used to characterize defects in welds and voids in materials such as gaps in adhesive layers or air bubbles as these they have a much higher thermal resistance than the surrounding material. IRT has been used for the on-line monitoring of weld pools as part of intelligent processing of materials.3.8 X-RAY DIFFRACTION (XRD) TECHNIQUE FOR RESIDUAL STRESSESResidual stresses are introduced in industrial components during welding process and also during the service life of the welded component due to loading conditions. For example, the stresses are introduced during welding process due to nonuniform heat distribution taking place during the welding process. Several destructive and nondestructive techniques are presently available for the residual stress measurements. Destructive techniques cannot be applied on finished components and are time consuming and uneconomical. Therefore NDT techniques are preferred for residual stress measurements [12]. Some of these techniques include: (i) Ultrasonic (ii) X Ray Diffraction (XRD), (iii) Acoustic Barkhausen Noise (ABN) and (IV) Magnetic Barkhausen Noise (MBN). Additionally, semi destructive hole drilling strain gauge technique is also employed for measurement of residual stresses. Ultrasonic technique of evaluating residual stresses is based on the measurement of changes in the velocity of ultrasonic waves due to stress and by establishing the acousto-elastic constant. Several methods using ultrasonic waves of various types such as longitudinal, transverse and surface waves have been tried with varying degree of success for weldments. MBN and ABN techniques are based on Barkhausen effect and applicable only to ferromagnetic metals and alloys. Barkhausen effect takes place when a magnetic field is swept in the material along a hysteresis loop. MBN is due to irreversible change in magnetic domain movements during hysteresis and ABN is due to elastic deformation associated with magnetic domain rotation during irreversible changes in magnetization. MBN signals can be acquired by sensor coil or by Hall type probe and ABN signals are acquired by piezoelectric transducers. Both MBN and ABN signals are strong functions of stress condition and hence stresses can be assessed by analysing the MBN and ABN signals. XRD technique measures the change in the interplanar spacing of the lattice in the presence of stresses in a material. It is well known that peak intensity of diffracted X ray beam occurs when Bragg's law is satisfied. In the presence of elastic macro-stresses, there is shift in the diffraction peak positions. The magnitude of the shift gives a measure of the stress and the direction of the shift depends on the nature of the stresses i.e. whether they are tensile or compressive. XRD technique has been used to measure the residual stresses before and after post weld heat treatment (PWHT), in autogenous butt weld joints in 2.25 Cr-1 Mo steel tubes. The tubes are used in the steam generator assemblies of PFBR. Hot liquid sodium flows in the shell region (outside the tubes) and water inside the tubes. A leak in the tube will lead to the generation of hydrogen due to the reaction of sodium with water with dangerous consequences. Tube-to-tube sheet weld joints are the weakest regions where a leakage path can be formed with relative ease. Apart from the requirement in the quality control procedures that the weld joints should be free from unacceptable defects that may lead to leak paths, it is also considered essential to use a NDT technique to assess the residual stress (RS) pattern and to evaluate the PWHT to be used for removing the residual stresses whose presence, otherwise, may lead to the failure of the tube to tube sheet weld joint. The equipment used for the measurement of the residual stresses is a portable X-ray stress analyzer (Rigaku Strainflex MSF) and Sin2? multiplex method was used for the stress measurements. Figure 9 shows the residual stress variations across are of the weld joints prepared with pre heat. The variations on both the outside surface and the inside surface are shown. The tensile stress maximum occurs at the weld center line both on the outside and the inside surface. On the inside surface, the zero crossing of RS from tensile to compressive occurs about 2 cm away from the weld center line, as compared to 1cm on the outside surface. The maximum compressive stress level on the inside surface is also much higher than that on the outside surface. The asymmetry in the variation of RS both on the outside and the inside surfaces is attributed to the restraint offered by the tube sheet block on one side where the compressive stresses were found to be higher. Variation of stress distribution at different depths below the top surface is also shown in Fig. 9. These measurements were done after removing 50 to 150 micron layers on the outside surface. Surface removal was done by electropolishing only at the point of measurement so as to avoid any stress relaxation due to layer removal. It is seen that, within a depth of only 150 microns, the stresses approach zero values. This shows that the RS is restricted only to a thin layer at the top [13]. With the help of portable equipments, it is also possible to apply the technique on large objects and carry out the measurements in field and quickly. It can be used for quantitative analysis of macro and micro residual stresses separately.
4.0 SUMMARYIn this paper, advances in NDT techniques for inspection of welds for detection and quantitative characterisation of defects, residual stresses and microstructural variations are highlighted. Since the probing medium and the interactions are different, capabilities and limitations of various NDT techniques for defect detection and evaluation differ. Hence, selection of NDT technique for a specific inspection application is very important. To site an example, for detection of fatigue cracks in stainless steel welds, it is not surprising that eddy current and liquid penetrant testing are superior to ultrasonic testing and radiography. At the same time, the later techniques are capable of reliably detecting deep-seated volumetric defects, which go undetected by the former techniques. Prior to selection of an NDT technique, it is essential to obtain information about the location and type of defects such as nature, probable size and orientation using chemical composition, material properties, microstructure, fabrication procedure, operating environment and history details. Sensitivity, detectability, accessibility, speed and past experience play a major role in the selection of a technique for a specific application. It may sometimes be necessary to use a combination of two or more techniques, in the best complementary way, to carry out NDT testing in a reliable manner. For such situations, detailed mock up studies are essential prior to actual inspection to optimise instrument parameters, design and selection of sensors, calibration defects, and to prepare procedures for recording and evaluation of test data, to ultimately arrive at the desired sensitivity and reliability. The summary of applicability and capability of various NDT techniques for assessment of defects in welded components is given in Table.1 as a guide to choose an appropriate technique. Continuous developments are taking place in NDT techniques with concurrent advances in micro-electronics, computers, optics, materials and sensors. Today NDT is matured enough to take up nearly all kinds of challenging jobs in welded structures as regards to quick detection and sizing of harmful defects, almost as and when they form or before they grow to critical sizes causing catastrophic failure of components.
