Volume 17 - Nondestructive Evaluation and Quality Control Part 16 pptx

This example demonstrates techniques applied to 9.5 mm ( in.) outside diameter stainless steel tubing with a 0.51 mm (0.020 in.) wall thickness. However, these techniques can be modified to enable detection of intergranular attack and/or root weld cracking in various materials and sizes. A 15-MHz Rayleigh (surface) wave transducer is machined with a 4.8 mm ( in.) radius to fit the 9.5 mm ( in.) outside diameter of the 0.51 mm (0.020 in.) wall thickness tubing to be inspected (Fig. 13a). The transducer is then positioned on the tubing, as shown in Fig. 13(b). No prior preparation of the sample was required. Mixed-mode shear wave is induced in the tubing to detect intergranular attack (Fig. 14) using a ring-pattern CRT display (Fig. 15). This transducer is also very sensitive and can be used for detecting root weld cracks (Fig. 16). The ultrasound instrument will be set up for monitoring discrete echoes from the root crack. The display produced is shown in Fig. 17. Fig. 13 15-MHz Rayleigh surface wave trans ducer (90° shear) used for detecting intergranular attack and root weld cracks. (a) Transducer with radius machined on transducer shoe to allow device to conform to tubing outside diameter. (b) Transducer positioned on tube outside diameter to couple to tu be using a lightweight oil couplant. Source: L.D. Cox, General Dynamics Corporation Fig. 14 Intergranular attack of 0.51 mm (0.020 in.) wall thickness, Fe-21Cr-6Ni- 9Mn stainless steel tubing inside diameter. (a) 60×. (b) 85×. Courtesy of L.D. Cox, General Dynamics Corporation Fig. 15 Mixed- mode shear wave used to detect intergranular attack showing oscilloscope screen display for (a) transducer in air, (b) transducer coupled to an acceptable tube having no defects due to intergranular attack, and (c) transducer coupled to tube rejected because of intergranular attack. Significant attenuation of the ultrasonic signal in (c) is due to scatter. Source: L.D. Cox, General Dynamics Corporation Fig. 16 Cross section of a tube having a crack at the root of the weld seam. Source: L. D. Cox, General Dynamics Corporation Fig. 17 Plots obtained on oscilloscope screen with ultrasonic device set up to monitor discrete echoes from a root crack: (a) transducer in air; (b) transducer coupled to tube devoid of root crack defects; and (c) transducer coupled to tube rejected due to presence of a root crack. Signal A in (b) and (c) is due to reflection from the transducer/tube OD contact point. Signal B in (c) is the root crack signal [when the transducer is indexed circumferentially, the A signal will be stationary (no change in time-of- flight) while the B signal will shift]. Source: L.D. Cox, General Dynamics Corporation Example 8: Eddy Current Inspection of Pitting and Stress-Corrosion Cracking of Type 316 Stainless Steel Evaporator Tubes in a Chemical Processing Operation. Eddy current inspection was performed on a vertical evaporator unit used in a chemical processing plant. The evaporator contained 180 tubes 25 mm (1 in.) in diameter. It was advised that the tube material was type 316 stainless steel. The shell side fluid was condensate and gaseous methylene chloride, while the tube side fluid was contaminated liquid methylene chloride. Eddy current inspection revealed 101 tubes that exhibited severe outer surface pitting and cracklike indications near each tube sheet. Several tubes exhibiting strong indications were pulled and examined visually and metallurgically. It was observed that the indications correlated with rust-stained, pitted, and cracked areas on the outer surfaces. The observed condition was most severe along the portions of the tubes located between the upper tube support and top tube sheet. Figures 18(a) and 18(b) show a pitted and cracked area before and after dye-penetration application. Fig. 18 Pitting and stress corrosion in type 316 stainless steel evaporator tubes. (a) Rust- stained and pitted area n ear the top of the evaporator tube. Not clear in the photograph, but visually discernible, are myriads of fine, irregular cracks. (b) Same area shown in (a) but after dye- penetrant application to delineate the extensive fine cracks associated with the rust-stained, pitted surface. (c) Numerous multibranched, transgranular stress- corrosion cracks initiating from the outer surface pits. 35×. Courtesy of J.P. Crosson, Lucius Pitkin, Inc. Metallographic examination revealed that the cracking initiated from the outer surface, frequently at pits, and penetrated the tube wall in a transgranular, branching fashion. The crack features were characteristic of chloride stress-corrosion cracking. In many cases, the cracking, rather than penetrating straight through the tube wall, veered off in a tangential direction at or about mid-wall, suggesting the possibility of a change in the residual stress-field from tube drawing. Figure 18(c) shows stress-corrosion cracking originating from pits on the outer surface of the tube. The results of the examination indicated that the subject tube failures occurred by way of stress-corrosion cracking as a result of exposure to a wet-chloride-containing environment. Therefore, a change in tube material was recommended to avoid future failures and loss of service. Example 9: Eddy Current Inspection of a Pitted Type 316 Stainless Steel Condenser Tube. Eddy current inspection was performed on approximately 200 stainless steel tubes in a main condenser unit aboard a container ship. The stainless steel tubes comprised the upper two tube rows in the condenser. The tube material was reported to be type 316 stainless steel; this was confirmed by subsequent chemical analysis. The remaining tubes were 90Cu-10Ni. Recurring leaks had occurred in the stainless steel tubes, but no leaks had occurred in the copper-nickel tubes. Eddy current indications typical of inner surface pitting were observed in 75% of the stainless steel tubes inspected. A tube exhibiting a strong indication was pulled from the condenser and examined visually and metallographically. Visual examination of the outer surface revealed occasional patches of rust-colored deposit at the locations of the eddy current indications. No apparent defects of any type were observed on the outer surface. Subsequent splitting of the tube revealed several areas of severe pitting corrosion attack on the inner surface at locations corresponding to the eddy current indications. The corrosion progressed in such a way as to hollow out the wall thickness, and at several locations the pits had completely penetrated the wall thickness. The pitting corrosion attack tended to be close to the bottom of the tube and essentially in line along the tube sample length. Figure 19(a) shows a severely pitted location. Metallographic examination revealed the attack to be broad and transgranular in nature without any corrosion product build-up at or around the pits. Figure 19(b) shows the manner in which the pitting had penetrated into and beneath the inner surface. Fig. 19 Pitted type 316 stainless steel condenser tube. (a) Inner surface of main condenser tube showing extensive but localized pitting corrosion attack. 1×. (b) Longitudinal section passing through a pitted area showing extensive pitting that had progressed beneath the inner surface of the main condenser tube. 55×. Courtesy of J.P. Crosson, Lucius Pitkin, Inc. The results of the examination revealed that the subject stainless steel condenser tube had failed as a result of pitting corrosion attack, which initiated at the inner surface and progressed through the tube wall. That the pitting was essentially on the bottom of the tubes was strong evidence of deposit-type pitting corrosion attack. Deposit attack occurs when foreign material carried by the tube side fluid settles or deposits on the inner surface, generally at the bottom of the tube. The deposit shields the tube surface, creating a stagnant condition in which the fluid beneath the deposit becomes deficient in oxygen compared to the free-flowing fluid around the deposit. The difference in oxygen content results in the formation of an oxygen concentration cell in which the smaller, oxygen-deficient sites become anodic with respect to the larger oxygenated cathodic sites. As a result, pitting corrosion attack occurs at the anodic sites. In stainless steel, the condition is further aggravated by the fact that type 316 stainless steel performs best in a service where the fluid is oxidizing and forms a passive film on the surface of the tube. If there is an interruption in the film, as may be caused by chemical breakdown through decomposition of organic materials or mechanically by abrasion, and if the damage film is not reformed, pitting corrosion will initiate and grow at the damaged site. In main condenser service, certain deposits, such as shells, sand, or decomposing sea life, can initiate breakdown of the passive film. Example 10: Eddy Current Inspection of a Magnetic Deposit Located on a Steel Tube at Tube Sheet Joint in a Centrifugal Air-Conditioning Unit. In this case, defective tubes were not detected. However, the results of the eddy current inspection were directly influenced by a previous tube failure in the unit. Eddy current inspection of the condenser bundle of a centrifugal air-conditioning unit revealed several tubes with indications typical of tube-wall wear at locations corresponding to the tube supports. One of the tubes exhibiting indications was pulled and visually examined. A tightly adherent magnetic deposit was observed at the area of the tube in contact with the first tube support. Splitting the tube revealed the deposit to be tightly packed in the fins, as shown in Fig. 20. This tube was of tru-finned rather than skip-finned design; that is, the tube did not have smooth support saddles where it was in contact with the tube support plate. Instead, the tube was finned from end to end. Therefore, although the test instrument parameters were selected to phase out the magnetically induced indications from the steel tube supports, the magnetic deposit, which was tightly embedded between the fins, was closer to the internal test probe and caused an indication that was interpreted as tube-wall wear. Fig. 20 Magnetic corrosion product embedded in the tube fin s at the tube support of a steel tube. The corrosion product caused by eddy current indication characteristic of tube-wall wear at the support. Courtesy of J.P. Crosson, Lucius Pitkin, Inc. Through further investigation it was determined that a previous tube failure had caused the Freon on the shell side to become contaminated with water. This condition proved corrosive to the steel supports and shell and subsequently caused the magnetic corrosion deposit observed at the tube support. Oil-Country Tubular Products The application of nondestructive inspection to the tubular products of the oil and gas distribution industry is extensive and is vital to successful operation. The American Petroleum Institute has, with international cooperation and international acceptance, developed tubing and pipe specifications that include many rigorous requirements for nondestructive inspection (Ref 22). Inspection installations range from simple magnetic particle installations to complex assemblies of machinery whose continuous productivity is completely dependent on the reliability and accuracy of its nondestructive inspection equipment. The larger installations may use ultrasonic, eddy current, flux leakage, or radiographic equipment, singly or in combination, and can be supplemented by magnetic particle inspection. The inspections of pipe or casing can be performed during manufacture, when it is received on site, while it is in service, or when it must be inspected for reuse or resale. When inspection is included in the manufacturing operation, tests are usually performed immediately after the pipe is produced and again after processing has been completed. Industry has promoted the development and use of highly sophisticated equipment for the in-service inspection of pipe in diameters of 75 mm (3 in.) and larger. In one of several different pipe crawlers available commercially, the probe travels through the gas lines and, by means of flux leakage measurements, reports on the condition of the pipe. Another type of in-service inspection unit, which is shown in Fig. 21, includes tight-fitting seals so that it can be propelled through the pipelines by the oil or gas being carried. The traveling unit includes not only the test instrumentation and a tape recorder but also a power supply so that it is completely self-sufficient, requiring no connection outside the pipe. The sections of this unit are connected by universal joints to permit passage around bends. When the unit completes its cycle, total information on the condition of the pipe is immediately available. Fig. 21 Self-contained flux leakage inspection unit used in oil and gas pipeline for in-service inspection One of the most important inspection procedures in this industry involves the inspection of girth welds joining the ends of pipes to each other or to fittings and bends. Although radiographic tests are widely favored for this application (Ref 23), supplementary tests are needed to detect the tightly closed flaws not detected by radiography (Ref 12). This industry also uses automated inspection of small tubular pipe couplings. One machine separates acceptable couplings from rejectable couplings automatically and requires no operator. The couplings are fed into the machine from a cutoff lathe. After automatic inspection to API specifications, rejectable couplings are diverted to a reject receptacle. Nondestructive Inspection of Steel Pipelines (Ref 24) The nondestructive inspection of welds in steel pipe is used to eliminate discontinuities that could cause failure or leakage. Most steel pipes for gas transmission are made by the hot rotary forging of pierced billets or by forming plate or strip and then welding by either the submerged arc or the resistance process. Pipes are usually made to one of the API specifications, with supplementary requirements if necessary. Submerged Arc Welded Pipe. The shrinkage of liquid metal upon solidification results in primary piping in the ingot, which can cause laminations oriented in the plane of the plate or strip rolled from the ingot. Laminations can also result from secondary piping and from large inclusions. Laminations can nucleate discontinuities during welding or propagate to form a split in the weld. They cannot be detected by radiography, because of their orientation, but they can be detected by ultrasonics or can be seen when they occur as skin laminations. The API specifications do not require nondestructive inspection of the plate or strip before welding, but ultrasonic inspection is mandatory in some customer requirements. Generally, the periphery of the plates must be examined to ensure that edges to be welded are free of laminations. Pulse-echo ultrasonic inspection has been used for most plate-inspection specifications. This method cannot distinguish laminations near the surface remote from the probe, because the echoes from the laminations cannot be inspected from the back echo. Some specifications require that the plate be inspected from both surfaces or that transmission methods be used. Manual scanning, although time consuming, is feasible because the echo pattern from laminations persists on the oscilloscope screen and is easy to interpret. However, if laminations are fragmented or at an oblique angle to the surface of the plate, there is no distinct flaw echo, but merely a loss of back echo. In most pulse-echo equipment, no account is taken of this loss; therefore, transmission methods are preferable for plate inspection. Such methods normally require mechanization with automatic recording of the results, and inspection systems based on these methods have been installed in some plate mills. In both transmission and pulse-echo inspection, the probe area represents the area of the plate under inspection at any instant. Shear-wave angle probes are used to detect lamination, but the method is not reliable, particularly for the thin plates used in pipe manufacture. Laminations can also be detected by ultrasonic Lamb waves, which can inspect a zone extending across some or all of the plate. Lamb waves have been used on steel sheet, but they cannot be excited in plates more than 6.4 mm ( in.) thick using standard equipment. Special equipment is now available for plate up to 13 mm ( in.) thick. The main flaws that occur in submerged arc welds are incomplete fusion and incomplete penetration between the inside and outside weld beads or between the base metal and the filler metal, cracks, undercut or underfill, and overflow. The API standards require full-length inspection of welds by radiography or ultrasonics. Fluorescent screens or television screens are permitted for radiography, and they are often used because they are less expensive than radiographic film, although less discriminating. Fluoroscopy is inherently less sensitive to the more critical flaws, cracks, incomplete sidewall fusion, and incomplete penetration. Ultrasonic inspection is more sensitive to serious flaws and can be automated. The arrangements of transmitter and receiver probes in ultrasonic inspection of submerged arc welded pipe for detection of longitudinally and transversely oriented discontinuities are shown in Fig. 22. The region of the oscilloscope time base corresponding to the weld region is analyzed electronically, and echoes above the amplitude of the reference derived from the calibration block actuate a relay that can operate visible or audible warnings, paint sprays, or pen recorders. In some installations, only one probe is used on each side of the weld, and detection of discontinuities that are oriented transversely to the weld is not possible. To ensure correct lateral positioning of the probes on the weld, they are mounted on a frame, which is then moved along the weld; alternatively, the pipe can be moved past stationary probes. Fig. 22 Diagram of arrangements of probe s in the ultrasonic inspection of submerged arc welded pipe for the detection of (a) longitudinally oriented and (b) transversely oriented discontinuities Accurate positioning of the probes over the welds is difficult because the width, shape, and straightness of the weld bead vary. The inspection area is limited in order to reduce confusion between echoes from flaws within the weld and the boundaries of the weld reinforcement. Acoustic coupling can be reduced by the probes riding up on weld spatter, by drifting of the scanning frame, by loss of coupling water, or by loose mill scale. General practice is to use automatic ultrasonic inspection methods and to radiograph those regions of the pipe suspected of containing discontinuities. If radiography does not reveal an objectionable flaw, the ultrasonic indication is ignored and the pipe is accepted. This procedure would accept cracks or laminations parallel to the plate surface that, because of their orientation, cannot be detected by radiography. As an alternative approach, regions that give an ultrasonic flaw indication should be inspected radiographically and by manual ultrasonics. If the original ultrasonic indication was from a discontinuity shown by the radiograph to be acceptable within the specification or if the manual ultrasonic inspection revealed that the indication was a spurious echo arising from a surface wave or from local weld shape, then the pipe was accepted; if the radiograph showed an objectionable flaw, then the pipe was rejected. If there was no explanation for the echo, it was assumed to have arisen from a discontinuity adversely oriented for radiography. Seamless Pipe. There are two sources of flaws in roll forged seamless pipe: inhomogeneities and the manufacturing process. Inhomogeneities in the ingot such as primary and secondary ingot pipe can be carried into the roll-forged product and can cause flaws in a similar manner to the formation of laminations in steel plate. Such flaws are likely to have a major dimension oriented in the plane of the pipe wall. In manufacturing, the rolls and the mandrel can cause surface discontinuities such as tears and laps, and such discontinuities will have substantial orientation normal to the pipe wall. In addition, pipes and tubes produced by working pierced billets are prone to eccentric wall thickness, with the eccentricity varying along the length of the pipe. The API specifications that cover seamless line pipe require neither nondestructive inspection nor wall thickness measurement away from the pipe ends. In some mills, destructive inspections are carried out on samples cut from each pipe end to determine the presence of primary pipe flaws. In some API standards (casing, tubing, and drill pipe), nondestructive inspection is optional, but in other standards (high-strength casing and tubing) nondestructive inspection of the full pipe length is mandatory. Magnetic particle, ultrasonic, or eddy current inspection methods are permitted. Magnetic particle inspection methods have little or no sensitivity to discontinuities that do not show on the surface and are likely to detect laminar discontinuities resulting from ingot piping. Although surface laps are amenable to magnetic crack detection, it would be difficult to apply the inspection method to internal-surface discontinuities. Eddy current inspection methods can be used to inspect seamless tubing. Very rapid inspection rates are possible with the encircling-coil system. When pipe is passed through a coil fed with alternating current, the resistive and reactive components of the coil are modified; the modification depends on dimensions (and therefore indirectly on discontinuities), electrical conductivity and magnetic permeability, and the annulus between the pipe and the coil (and therefore the outside diameter of the pipe). The analysis to determine which effect is causing any modification is complex. Eddy current methods are extensively used for the inspection of small, nonferrous tubes, but ferrous material causes complications from magnetic permeability. The initial permeability is affected by residual-stress level. Roll-forged pipe may have varying amounts of residual cold work, depending on the original soaking conditions and the time taken to complete forging. The effect can be alleviated by applying a magnetically saturated field; equipment that can produce a magnetically saturated field has been installed in steel tube mills. However, saturation becomes more difficult as pipe diameter increases. Radiographic inspection methods, employing either x-ray or γ-ray transmission, can be used with a scintillation counter to estimate the wall thickness of pipe. The accuracy of scintillation counters depends on the size of the count for a given increment of thickness; the count increases with the time the increment is in the beam. As a result, the count, and therefore the accuracy, increases with decreasing scanning rate. When large-diameter pipes are scanned at realistic rates, eccentricity is usually averaged out. Ultrasonic inspection methods can detect discontinuities oriented both in the plane of, and normal to, the pipe wall. Discontinuities in the plane of the wall can be detected by using a compression-wave probe scanning at normal incidence. For discontinuities normal to the wall, the beam is converted to shear wave and propagated around or along the tube. The pipe is rotated and moved longitudinally relative to the probes, thus giving a helical scan. The reliability of mechanized scanning is a function of acoustic coupling, and optimum results are achieved with immersion coupling. The efficiency of acoustic coupling through large columns of water is lower but much more consistent than that through the thin liquid films used in contact scanning. Immersion methods also eliminate probe wear and the requirement for specially contoured probes to accommodate each pipe size. Alternatively, immersion coupling by a column of water flowing between the probe and the pipe can be used. With this method, probe-rotation scanning is possible. Advantage can be taken of the smaller inertia of the probes to increase the scanning rate, and therefore the speed of inspection, by about an order of magnitude. When an ultrasonic beam propagates radially through the pipe wall, the time interval between successive back echoes reflected from the bore surface is directly proportional to the wall thickness. If the first back echo is used to trigger a high- speed electronic counter whose frequency is such that it will produce a count of 100 during the time taken to receive four echoes in 25 mm (1 in.) thick plate and if a subsequent back echo is used to stop the counter, a count proportional to the wall thickness is produced. By changing the frequency of the counter oscillator, it is possible to change the thickness range inspected or to accommodate different materials. Information from the counter can be fed to a chart recorder, thus continuously recording the wall thickness. Lamination would be recorded as an abrupt localized reduction in wall thickness. [...]... processing and joint processing Fig 14 Three-stage mechanistic model of diffusion welding (a) Initial asperity contact (b) First-stage deformation and interfacial boundary formation (c) Second-stage grain-boundary migration and pore elimination (d) Third-stage volume diffusion and pore elimination Source: Ref 5 Successful nondestructive evaluation of diffusion-bonded joints requires that the maximum size and. .. manipulation, and excessive arc blow Lack of penetration may be the result of low welding current, excessive travel speed, improper electrode manipulation, or surface contaminants such as oxide, oil, or dirt that prevent full melting of the underlying metal Fig 4 Lack of fusion in (a) a single-V-groove weld and (b) double-V-groove weld Lack of penetration in (c) a single-V-groove and (d) a double-V-groove... soft metals and hide flaws A stiff wire brush and sandblasting have been found to be satisfactory for cleaning surfaces of slag and oxides without marring Additional information on the uses and equipment associated with the visual examination of parts and assemblies is available in the article "Visual Inspection" in this Volume Magnetic Particle Inspection Magnetic particle inspection is a nondestructive. .. straight-beam technique of ultrasonic inspection, provided the instrument has sufficient sensitivity and resolution A 5- or 10-MHz dual-element transducer is normally used in this application If the weld cannot be machined, near-surface sensitivity will be low because the initial pulse is excessively broadened by the rough, as-welded surface Unmachined welds can be readily inspected by direct-beam and. .. Feb 1975, p 63-s to 71-s 19 F Förster, Sensitive Eddy-Current Testing of Tubes for Defects on the Inner and Outer Surfaces, NonDestr Test., Vol 7 (No 1), Feb 1974, p 2 5-3 5 21 V.S Cecco and C.R Bax, Eddy Current In-Situ Inspection of Ferromagnetic Monel Tubes, Mater Eval., Vol 33 (No 1), Jan 1975, p 1-4 22 "Specification for Line Pipe," API 5L, American Petroleum Institute, 1973 23 "Standard for Welding... high cost and the inability of such methods to accurately predict the quality of those joints that were not inspected This article will review nondestructive methods of inspection for weldments (including diffusion-bonded joints) and brazed and soldered joints More detailed information on the techniques discussed can be found in the Sections "Inspection Equipment and Techniques," and "Methods of Nondestructive. .. possibilities in diffusion bond inspection Eddy current and thermal methods are relatively unsatisfactory for most applications Methods of Nondestructive Inspection The nondestructive inspection of weldments has two functions: • • Quality control, which is the monitoring of welder and equipment performance and of the quality of the consumables and the base materials used Acceptance or rejection of a... "Joint Evaluation and Quality Control" in Welding, Brazing, and Soldering, Volume 6 of the ASM Handbook Discontinuities resulting from the welding process include: • • • • • • • • • • • • • • • Undercut: A groove melted into the base metal adjacent to the toe or root of a weld and left unfilled by weld metal Slag inclusions: Nonmetallic solid material entrapped in weld metal or between weld metal and. .. Explanations of welding processes, equipment and filler metals, and welding parameters for specific metals and alloys are available in Welding, Brazing, and Soldering, Volume 6 of the ASM Handbook Discontinuities in Arc Welds Discontinuities may be divided into three broad classifications: design related, welding process related, and metallurgical Design-related discontinuities include problems with... section "In-Motion Radiography" of the article "Radiographic Inspection" in this Volume) On pipelines where the rate of welding is low and the investment on crawlers is not justified, x-ray sets can be clamped onto the outside of the pipe and radiography implemented by the double-wall, single-image technique Gamma radiography has been favored for pipeline radiography because of its convenience and lower . Fig. 4 Lack of fusion in (a) a single-V-groove weld and (b) double-V- groove weld. Lack of penetration in (c) a single-V-groove and (d) a double-V-groove weld Radiographic methods may be. pipe flaws. In some API standards (casing, tubing, and drill pipe), nondestructive inspection is optional, but in other standards (high-strength casing and tubing) nondestructive inspection. article, are discussed in the Section "Joint Evaluation and Quality Control& quot; in Welding, Brazing, and Soldering, Volume 6 of the ASM Handbook. Discontinuities resulting from the welding