Welding is extensively used in 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 stress, inspection procedures and operating parameters. Non destructive testing (NDT) is the best way to minimise the failures of welded components to ensure that no unacceptable defects are present. 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.
NDT Advancements in Field of Welds
ACOUSTIC EMISSION TECHNIQUE
Acoustic 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 hydro testing of as-fabricated welded vessels and also in service during their hydro testing. 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. 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) Micro fissuring,
(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. Therefore, in-process AE monitoring can be used both as an examination method and also as a means for providing feedback control.
ALTERNATING CURRENT POTENTIAL DROP TECHNIQUES
Ultrasonic 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.
INFRARED THERMOGRAPHY (IRT) TECHNIQUE :
Measurements for this NDE technique are derived from changes in thermal resistance that arises in the flow of heat through the components. These changes can be detected by inferred cameras that are sensitive to surface temperature differences of less than 0.1 degrees Celsius. Precisely, IRT let one “see” heat . 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.
X-RAY DIFFRACTION (XRD) TECHNIQUE FOR RESIDUAL STRESSES :
Residual 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 non uniform heat distribution taking place during the welding process. Several nondestructive techniques are presently available for the residual stress measurements.
Some of these techniques include:
(i) Ultrasonic Testing.
(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. 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 inter planar 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 peakpositions. 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).
Advances in NDT techniques for inspection of welds for detection and quantitative Characterisation of defects, residual stresses and micro structural 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. 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.