Volume 17 - Nondestructive Evaluation and Quality Control Part 16 pptx

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

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 transducer (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 tube 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 near 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 fins 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 probes 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

For the detection of cracks and laplike discontinuities, the display on the flaw detection oscilloscope is gated The discontinuity can then be recorded in its position around the circumference Chart length can be made proportional to pipe length, thus facilitating discontinuity location and extent in relation to pipe length and variation in wall thickness around and along the pipe Alternatively, information can be monitored in a go/no-go method to provide a paint spray that identifies the locations of significant discontinuities

Resistance-Welded Pipe. The type of discontinuity usually responsible for the failure of resistance-welded pipe is incomplete fusion, with associated oxide film The nondestructive inspection of larger-diameter resistance-welded pipes is normally restricted to inspection of the weld region Systems similar to those used for the inspection of submerged arc welded seams can be employed, although the arrangements for tracking the probes with respect to the weld reinforcements are not applicable Because of problems associated with accurate weld tracking, it is necessary that small variations in weld-probe separation should only cause acceptably small variations in discontinuity detection sensitivity

Probe angles of 60 to 65° give satisfactory results, and coverage of the weld depth is achieved by using two probes on each side of the weld Such a system is also relevant to the ultrasonic inspection of submerged arc welded pipes

Nondestructive Inspection of Pipeline Girth Welds

The API specifications do not require that all girth welds be inspected; the use of radiography and the extent of coverage are optional Generally, it has been the practice to inspect 10% of the weld length Where the integrity of a pipeline is vital, as in high-pressure gas-transmission systems, it is advisable to consider a 100% inspection, especially when inspection is not a large proportion of the total cost of the pipeline Where operating conditions are less critical, however,

it may be possible to be less critical regarding the size and type of discontinuity permissible

Radiographic Inspection. Characteristic discontinuities in pipeline welds are slag, elongated piping in root, scattered piping and porosity, burn-throughs in the root, incomplete root penetration, incomplete sidewall fusion, and cracks, which often break the inner surface in the heat-affected zone Except for cracks and incomplete sidewall fusion, these discontinuities are amenable to detection by radiography Open cracks can be detected, but tighter cracks, even though favorably oriented, are detectable only by optimum practice Some cracks may not be revealed at all

Assuming good radiographic techniques, radiographic quality depends on the choice of conditions that control the contrast and definition of the radiograph; the detection of discontinuities improves with increasing contrast and fine definition Contrast can be assessed in terms of the thickness sensitivity, which can be conveniently estimated by image-quality indicators

Many factors other than good radiographic techniques influence radiographic contrast For pipeline radiography, radiation energy is probably the most important The absorption of radiation by steel decreases with increasing energy; the absorption coefficient at 150 kV is about three times that at 700 kV For optimum detection of small changes in thickness, the absorption should be as high as possible so that large differences in exposure, consistent with a reasonable amount of energy being transmitted to provide a realistic overall exposure, result at the film Gamma radiation from a 192Ir source is approximately equivalent to x-rays generated at 700 kV and therefore will not be absorbed sufficiently to give good contrast sensitivity For wall thicknesses typical of pipelines, x-rays generated at about 150 to 175 kV have reasonable absorption

For the detection and correct identification of discontinuities from the radiographic image, the delineation of the shadow must be sharp The principal sources of unsharpness in radiographic images are geometric unsharpness, resulting front the finite size of radiation sources; unsharpness in the film resulting from the kinetic energy of the radiation, grain size of the emulsion and degree of development; and unsharpness resulting from the intensifying screen Radiographs on pipelines are generally made under less-than-satisfactory conditions; nevertheless, it should be possible to avoid vibration and other forms of relative movement of the source and film during exposure so that the total geometric unsharpness is the penumbra effect With piping, geometric unsharpness is not large

An important side effect of unsharpness is that when the unsharpness is greater than the width of a flaw, the contrast resulting from the flaw is reduced from its theoretical value; the greater the unsharpness, the greater the contrast reduction For tight cracks, the contrast may be so reduced that the change in tone is below the threshold for detection On-site conditions may reduce the capability of radiography to detect flaws Under some conditions, it is difficult to maintain correct developer temperature The operator is often pressured to keep pace with the welding crews; also,

weather and working conditions may be adverse Suitable equipment and adequate planning should overcome these problems The more difficult problem is the repetitiveness of the procedure, which causes the operators to lose concentration and gradually to devote less attention to detail

For most pipe sizes it is necessary to make three exposures to cover the circumference of the weld because the length of weld that can be covered in one exposure (the diagnostic film length) is limited by fade at each end of the film Panoramic techniques have been used on some larger-diameter pipes The source is held in a spider arrangement and positioned on the pipe axis; the films are placed around the outer surface of the pipe at the weld In this manner, the entire weld can be radiographed in one exposure This exposure is shorter than one of the exposures required in the double-wall, single-image technique because the radiation has to propagate through only one wall of the pipe In practice, the radiation source can be manually positioned only inside pipes having a diameter of 762 mm (30 in.) or more; even then, conditions must

be good Crawler devices are available that are mechanically propelled through the pipe, with the exposure being operated from an external control (see the 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 cost Source containers are more compact and portable than x-ray generators and do not require a power supply

Panoramic x-ray radiography is barely feasible without crawler devices because of the difficulty of manually maneuvering the cumbersome x-ray sets and control units inside a pipe Because of the potential for increased use of x-ray radiography on pipelines, there has been considerable effort applied to development of x-ray crawlers

Ultrasonic Inspection. Welds are usually ultrasonically inspected by a pulse-echo reflection technique Before the inspection of a weld, the pipe should be checked for laminations that may divert the beam from its theoretical path

Discontinuities can be identified most reliably by accurate positioning of the source of the discontinuity echo, preferably during scanning from more than one direction Skilled operators may be able to gain additional information on type of discontinuity from the shape of the echo on the oscilloscope screen, but the display on battery-operated flaw detectors used in daylight is not sufficiently distinct for the technique to be employed on pipelines

The more significant discontinuities occur in the root of the weld, where discrimination between sources of echo reflection is more difficult Acceptable features such as full root penetration cause echoes comparable in magnitude to those from root underbead cracks or incomplete penetration Accurate positioning of the probe with respect to the weld centerline is necessary, and it has been suggested that the required accuracy can be achieved only by marking and machining the pipe ends before welding Positioning from the center of the weld cap is only approximate because the weld is not necessarily symmetrical about the centerline through the root Even with the premarking, it is difficult for an operator to locate the ultrasonic probe accurately and still be in a position to view the instrument screen Thinner-wall pipes reduce the differences in beam path and probe position for discrimination between the various features of the root Also, the weld must be examined with the probe-to-weld distance increased to avoid confusion between echoes from the weld and those from probe noise This increases the effect of beam spread and may lead to extraneous echoes from the cap reinforcement

The skip distance and beam-path length vary as the wall thickness varies Variations in wall thickness between nominally the same classes of submerged arc welded pipe range from 10 to 15%, but in seamless pipe a ±10% variation along the length or around the circumference at a given position along the length is common Although it is possible to measure wall thickness accurately by ultrasonics, it is not feasible to measure wall thickness concurrently with scanning the weld Surface roughness can cause considerable variations in beam angle Weld spatter can reduce the effectiveness of the coupling, and also alter beam angle by lifting part of the probe off the pipe

Ultrasonic inspection on girth welds was originally used to determine which welds to radiograph If the radiograph did not detect anything, it was the practice on most pipelines to accept the radiographic evidence and not that from ultrasonics Now that pipelines are being examined 100% by radiography, the role of ultrasonics has changed to that of detecting root underbead cracks that may escape detection by radiography and of providing supplementary evidence to aid in the interpretation of radiographic images of weld-root regions

Surface Crack Detection. Root underbead cracks break the surface of the pipe in the bore and can be detected with liquid penetrant and magnetic particle inspection The weld area can be magnetized using a yoke powered by permanent magnets Both methods are sensitive under ideal conditions, but liquid penetrants require very clean surfaces Magnetic particle crack detection is therefore preferred for pipeline applications Interpretation of the indications is not a problem, except for the confusion that may arise from the tendency of sharp changes in root profile to give a slight crack indication

References cited in this section

12 R.F Lumb and G.D Fearnebaugh, Toward Better Standards for Field Welding of Gas Pipelines, Weld J., Vol 54 (No 2), 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, Destr Test., Vol 7 (No 1), Feb 1974, p 25-35

Non-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 Pipe Lines and Related Facilities," API 1104, American Petroleum Institute, 1968

24 R.F Lumb, Non-Destructive Testing of High-Pressure Gas Pipelines, Non-Destr Test., Vol 2 (No 4), Nov

1969, p 259-268

Note cited in this section

** Example 7 was prepared by L.D Cox, General Dynamics Corporation Examples 8, 9, and 10 were prepared by J.P Crosson, Lucius Pitkin, Inc

Nondestructive Inspection of Tubular Products

References

1 "Nondestructive Testing Terminology," Bulletin 5T1, American Petroleum Institute, 1974

2 H.C Knerr and C Farrow, Method and Apparatus for Testing Metal Articles, U.S Patent 2,065,379, 1932

3 W.C Harmon, "Automatic Production Testing of Electric Resistance Welded Steel Pipe," Paper presented

at the ASNT Convention, New York, American Society for Nondestructive Testing, Nov 1962

4 W.C Harmon and I.G Orellana, Seam Depth Indicator, U.S Patent 2,660,704, 1949

5 J.P Vild, "A Quadraprobe Eddy Current Tester for Tubing and Pipe," Paper presented at the ASNT Convention, Cleveland, American Society for Nondestructive Testing, Oct 1970

6 H Luz, Die Segmentspule ein neuer Geber für die Wirbelstromprüfung von Rohren, BänderBlecheRohre,

Vol 12 (No 1), Jan 1971

7 W Stumm, Tube-Testing by Electromagnetic NDT (Non-Destructive Testing) Methods: I, Non-Destr Test., Vol 7 (No 5), Oct 1974, p 251-258

8 F Förster, The Nondestructive Inspection of Tubings for Discontinuities and Wall Thickness Using

Electromagnetic Test Methods: I, Mater Eval., Vol 28 (No 4), April 1970, p 21A-25A, 28A-31A

9 F Förster, The Nondestructive Inspection of Tubings for Discontinuities and Wall Thickness Using

Electromagnetic Test Methods: II, Mater Eval., Vol 28 (No 5), May 1970, p 19A-23A, 26A-28A

10 P.J Bebick, "Locating Internal and Inside Diameter Defects in Heavy Wall Ferromagnetic Tubing by the Leakage Flux Inspection Method," Paper presented at the ASNT Convention, Cleveland, American Society for Nondestructive Testing, Oct 1974

11 H.J Ridder, "New Nondestructive Technology Applied to the Testing of Pipe Welds," Paper presented at the ASME Petroleum Conference, New Orleans, American Society of Mechanical Engineers, Sept 1972

12 R.F Lumb and G.D Fearnebaugh, Toward Better Standards for Field Welding of Gas Pipelines, Weld J.,

Vol 54 (No 2), Feb 1975, p 63-s to 71-s

13 M.J May, J.A Dick, and E.F Walker, "The Significance and Assessment of Defects in Pipeline Steels," British Steel Corporation, June 1972

14 W.C Harmon and T.W Judd, Ultrasonic Test System for Longitudinal Fusion Welds in Pipe, Mater Eval., March 1974, p 45-49

15 "Inspection, Radiographic," Military Standard 453A, May 1962

16 W Stumm, New Developments in the Eddy Current Testing of Hot Wires and Hot Tubes, Mater Eval.,

Vol 29 (No 7), July 1971, p 141-147

17 F.J Barchfeld, R.S Spinetti, and J.F Winston, "Automatic In-Line Inspection of Seamless Pipe," Paper presented at the ASNT Convention, Detroit, American Society for Nondestructive Testing, Oct 1974

18 T.W Judd, Orbitest for Round Tubes, Mater Eval., Vol 28 (No 1), Jan 1970, p 8-12

19 F Förster, Sensitive Eddy-Current Testing of Tubes for Defects on the Inner and Outer Surfaces, Destr Test., Vol 7 (No 1), Feb 1974, p 25-35

Non-20 K.J Reimann, T.H Busse, R.B Massow, and A Sather, Inspection Feasibility of Duplex Tubes, Mater Eval., Vol 33 (No 4), April 1975, p 89-95

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 Pipe Lines and Related Facilities," API 1104, American Petroleum Institute, 1968

24 R.F Lumb, Non-Destructive Testing of High-Pressure Gas Pipelines, Non-Destr Test., Vol 2 (No 4), Nov

In general, nondestructive inspection methods (NDI) are preferred over destructive inspection methods Sections can be trepanned from a joint to determine its integrity; however, the joint must be refilled, and there is no certainty that discontinuities would not be introduced during repair Destructive inspection is usually impractical, because of the 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 Evaluation" in this Volume

Acknowledgements

The contributions of the following individuals were critical in the preparation of this article: W.H Kennedy, Canadian Welding Bureau; Robert S Gilmore, General Electric Research and Development Center; and John M St John, Caterpillar, Inc Special thanks are also due to Michael Jenemann, Product Manager, NDT Systems, E.I Du Pont de Nemours & Company, Inc., for supplying the reference radiographs of welds shown in Fig 18 to 37 Finally, the efforts

of the ASM Committee on Weld Discontinuities and the ASM Committee on Soldering from Volume 6 of the 9th Edition

of Metals Handbook are gratefully acknowledged; material from the aforementioned Volume was used in this article

Nondestructive Inspection of Weldments, Brazed Assemblies, and Soldered Joints

Weldments

Weldments made by the various welding processes may contain discontinuities that are characteristic of that process Therefore, each process, as well as the discontinuities typical of that process, are discussed below 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 design or structural details, choice of the wrong type of weld joint for a given application, or undesirable changes in cross section These discontinuities, which are beyond the scope of this

article, are discussed in the Section "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 base

metal

• Porosity: Cavity-type discontinuities formed by gas entrapment during solidification

• Overlap: The protrusion of weld metal beyond the toe, face, or root of the weld

• Tungsten inclusions: Particles from tungsten electrodes that result from improper gas tungsten arc

welding procedures

• Backing piece left on: Failure to remove material placed at the root of a weld joint to support molten

weld metal

• Shrinkage voids: Cavity-type discontinuities normally formed by shrinkage during solidification

• Oxide inclusions: Particles of surface oxides that have not melted and are mixed into the weld metal

• Lack of fusion (LOF): A condition in which fusion is less than complete

• Lack of penetration (LOP): A condition in which joint penetration is less than that specified

• Craters: Depressions at the termination of a weld bead or in the molten weld pool

• Melt-through: A condition resulting when the arc melts through the bottom of a joint welded from one

side

• Spatter: Metal particles expelled during welding that do not form a part of the weld

• Arc strikes (arc burns): Discontinuities consisting of any localized remelted metal, heat-affected metal,

or change in the surface profile of any part of a weld or base metal resulting from an arc

• Underfill: A depression on the face of the weld or root surface extending below the surface of the

adjacent base metal

Metallurgical discontinuities include:

• Cracks: Fracture-type discontinuities characterized by a sharp tip and high ratio of length and width to

opening displacement

• Fissures: Small cracklike discontinuities with only a slight separation (opening displacement) of the

fracture surfaces

• Fisheye: A discontinuity found on the fracture surface of a weld in steel that consists of a small pore or

inclusion surrounded by a bright, round area

• Segregation: The nonuniform distribution or concentration of impurities or alloying elements that arises

during the solidification of the weld

• Lamellar tearing: A type of cracking that occurs in the base metal or heat-affected zone (HAZ) of

restrained weld joints that is the result of inadequate ductility in the through-thickness direction of steel plate

The observed occurrence of discontinuities and their relative amounts depend largely on the welding process used, the inspection method applied, the type of weld made, the joint design and fit-up obtained, the material utilized, and the working and environmental conditions The most frequent weld discontinuities found during manufacture, ranked in order

of decreasing occurrence on the basis of arc-welding processes, are:

Shielded metal arc welding (SMAW)

The type of porosity within a weld is usually designated by the amount and distribution of the pores Some of the types are classified as follows:

• Uniformly scattered porosity: Characterized by pores scattered uniformly throughout the weld (Fig 1a)

• Cluster porosity: Characterized by clusters of pores separated by porosity-free areas (Fig 1b)

• Linear porosity: Characterized by pores that are linearly distributed (Fig 1c) Linear porosity generally

occurs in the root pass and is associated with incomplete joint penetration

• Elongated porosity: Characterized by highly elongated pores inclined to the direction of welding

Elongated porosity occurs in a herringbone pattern (Fig 1a)

• Wormhole porosity: Characterized by elongated voids with a definite worm-type shape and texture (Fig

2)

Fig 1 Type of gas porosity commonly found in weld metal (a) Uniformly scattered porosity (b) Cluster

porosity (c) Linear porosity (d) Elongated porosity

Fig 2 Wormhole porosity in a weld bead Longitudinal cut 20×

Radiography is the most widely used nondestructive method for detecting subsurface gas porosity in weldments The radiographic image of round porosity appears as round or oval spots with smooth edges, and elongated porosity appears

as oval spots with the major axis sometimes several times longer than the minor axis The radiographic image of wormhole porosity depends largely on the orientation of the elongated cavity with respect to the incident x-ray beam The presence of top-surface or root reinforcement affects the sensitivity of inspection, and the presence of foreign material, such as loose scale, flux, or weld spatter, may interfere with the interpretation of results

Ultrasonic inspection is capable of detecting subsurface porosity However, it is not extensively used for this purpose except to inspect thick sections or inaccessible areas where radiographic sensitivity is limited Surface finish and grain size affect the validity of the inspection results

Eddy current inspection, like ultrasonic inspection, can be used for detecting subsurface porosity Normally, eddy current inspection is confined to use on thin-wall welded pipe and tubing because eddy currents are relatively insensitive to flaws that do not extend to the surface or into the near-surface layer

Magnetic particle inspection and liquid penetrant inspection are not suitable for detecting subsurface gas porosity These methods are restricted to the detection of only those pores that are open to the surface

Slag inclusions may occur when using welding processes that employ a slag covering for shielding purposes (With other processes, the oxide present on the metal surface before welding may also become entrapped.) Slag inclusions can

be found near the surface and in the root of a weld (Fig 3a), between weld beads in multiple-pass welds (Fig 3b), and at the side of a weld near the root (Fig 3c)

Fig 3 Sections showing locations of slag inclusions in weld metal (a) Near the surface and in the root of a

single-pass weld (b) Between weld beads in a multiple-pass weld (c) At the side of a weld near the root

During welding, slag may spill ahead of the arc and subsequently be covered by the weld pool because of poor joint

fit-up, incorrect electrode manipulation, or forward arc blow Slag trapped in this manner is generally located near the root Radical motions of the electrode, such as wide weaving, may also cause slag entrapment on the sides or near the top of the weld after the slag spills into a portion of the joint that has not been filled by the molten pool Incomplete removal of the slag from the previous pass in multiple-pass welding is another common cause of entrapment In multiple-pass welds, slag may be entrapped any number of places in the weld between passes Slag inclusions are generally oriented along the direction of welding

Three methods used for the detection of slag below the surface of single-pass or multiple-pass welds are magnetic particle, radiographic, and ultrasonic inspection Depending on their size, shape, orientation, and proximity to the surface, slag inclusions can be detected by magnetic particle inspection with a dc power source, provided the material is ferromagnetic Radiography can be used for any material, but is the most expensive of the three methods Ultrasonic inspection can also be used for any material and is the most reliable and least expensive method If the weld is machined

to a flush contour, flaws as close as 0.8 mm ( in.) to the surface can be detected with the 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 reflected-beam techniques, using an angle-beam (shear-wave) transducer

Tungsten inclusions are particles found in the weld metal from the nonconsumable tungsten electrode used in GTAW These inclusions are the result of:

• Exceeding the maximum current for a given electrode size or type

• Letting the tip of the electrode make contact with the molten weld pool

• Letting the filler metal come in contact with the hot tip of the electrode

• Using an excessive electrode extension

• Inadequate gas shielding or excessive wind drafts, which result in oxidation

• Using improper shielding gases such as argon-oxygen or argon-CO2 mixtures, which are used for GMAW

Tungsten inclusions, which are not acceptable for high-quality work, can only be found by internal inspection techniques, particularly radiographic testing

Lack of fusion and lack of penetration result from improper electrode manipulation and the use of incorrect welding conditions Fusion refers to the degree to which the original base metal surfaces to be welded have been fused to the filler metal On the other hand, penetration refers to the degree to which the base metal has been melted and resolidified to result in a deeper throat than was present in the joint before welding In effect, a joint can be completely fused but have incomplete root penetration to obtain the throat size specified Based on these definitions, LOF discontinuities are located on the sidewalls of a joint, and LOP discontinuities are located near the root (Fig 4) With some joint configurations, such as butt joints, the two terms can be used interchangeably The causes of LOF include excessive travel speed, bridging, excessive electrode size, insufficient current, poor joint preparation, overly acute joint angle, improper electrode 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 weld

Radiographic methods may be unable to detect these discontinuities in certain cases, because of the small effect they have

on x-ray absorption As will be described later, however, lack of sidewall fusion is readily detected by radiography Ultrasonically, both types of discontinuities often appear as severe, almost continuous, linear porosity because of the nature of the unbonded areas of the joint Except in thin sheet or plate, these discontinuities may be too deep-lying to be detected by magnetic particle inspection

Geometric weld discontinuities are those associated with imperfect shape or unacceptable weld contour Undercut, underfill, overlap, excessive reinforcement, fillet shape, and melt-through, all of which were defined earlier, are included

in this grouping Geometric discontinuities are shown schematically in Fig 5 Radiography is used most often to detect these flaws

Fig 5 Weld discontinuities affecting weld shape and contour (a) Undercut and overlapping in a fillet weld (b)

Undercut and overlapping in a groove weld (c) and (d) Underfill in groove welds

Cracks can occur in a wide variety of shapes and types and can be located in numerous positions in and around a welded joint (Fig 6) Cracks associated with welding can be categorized according to whether they originate in the weld itself or

in the base metal Four types commonly occur in the weld metal: transverse, longitudinal, crater, and hat cracks Base metal cracks can be divided into seven categories: transverse cracks, underbead cracks, toe cracks, root cracks, lamellar tearing, delaminations, and fusion-line cracks

Fig 6 Identification of cracks according to location in weld and base metal 1, crater crack in weld metal; 2,

transverse crack in weld metal; 3, transverse crack in HAZ; 4, longitudinal crack in weld metal; 5, toe crack in base metal; 6, underbead crack in base metal; 7, fusion-line crack; 8, root crack in weld metal; 9, hat cracks in weld metal

Weld metal cracks and base metal cracks that extend to the surface can be detected by liquid penetrant and magnetic particle inspection Magnetic particle inspection can also detect subsurface cracks, depending on their size, shape, and proximity to the surface Although the orientation of a crack with respect to the direction of the radiation beam is the dominant factor in determining the ability of radiography to detect the crack, differences in composition between the base metal and the weld metal may create shadows to hide a crack that otherwise might be visible Ultrasonic inspection is generally effective in detecting most cracks in the weld zone

Transverse cracks in weld metal (No 2, Fig 6) are formed when the predominant contraction stresses are in the

direction of the weld axis They can be hot cracks, which separate intergranularly as the result of hot shortness or localized planar shrinkage, or they can be transgranular separations produced by stresses exceeding the strength of the material Transverse cracks lie in a plane normal to the axis of the weld and are usually open to the surface They usually extend across the entire face of the weld and sometimes propagate into the base metal

Transverse cracks in base metal (No 3, Fig 6) occur on the surface in or near the HAZ They are the result of the high residual stresses induced by thermal cycling during welding High hardness, excessive restraint, and the presence of hydrogen promote their formation Such cracks propagate into the weld or beyond the HAZ into the base metal as far as is needed to relieve the residual stresses

Underbead cracks (No 6, Fig 6) are similar to transverse cracks in that they form in the HAZ because of high

hardness, excessive restraint, and the presence of hydrogen Their orientation follows the contour of the HAZ

Longitudinal cracks (No 4, Fig 6) may exist in three forms, depending on their positions in the weld Check cracks

are open to the surface and extend only partway through the weld Root cracks extend from the root to some point within the weld Full centerline cracks may extend from the root to the face of the weld metal

Check cracks are caused either by high contraction stresses in the final passes applied to a weld joint or by a hot-cracking mechanism

Root cracks are the most common form of longitudinal weld metal crack because of the relatively small size of the root pass If such cracks are not removed, they can propagate through the weld as subsequent passes are applied This is the usual mechanism by which full centerline cracks are formed

Centerline cracks may occur at either high or low temperatures At low temperatures, cracking is generally the result of poor fit-up, overly rigid fit-up, or a small ratio of weld metal to base metal

All three types of longitudinal cracks are usually oriented perpendicular to the weld face and run along the plane that bisects the welded joint Seldom are they open at the edge of the joint face, because this requires a fillet weld with an extremely convex bead

Crater cracks (No 1, Fig 6) are related to centerline cracks As the name implies, crater cracks occur in the weld crater

formed at the end of a welding pass Generally, this type of crack is caused by failure to fill the crater before breaking the arc When this happens, the outer edges of the crater cool rapidly, producing stresses sufficient to crack the interior of the crater This type of crack may be oriented longitudinally or transversely or may occur as a number of intersecting cracks forming the shape of a star Longitudinal crater cracks can propagate along the axis of the weld to form a centerline crack

In addition, such cracks may propagate upward through the weld if they are not removed before subsequent passes are applied

Hat cracks (No 9, Fig 6) derive their name from the shape of the weld cross section with which they are usually

associated This type of weld flares out near the weld face, resembling an inverted top hat Hat cracks are the result of excessive voltage or welding speed The cracks are located about halfway up through the weld and extend into the weld metal from the fusion line of the joint

Toe and root cracks (No 5 and 8, Fig 6) can occur at the notches present at notch locations in the weld when high

residual stresses are present Both toe and root cracks propagate through the brittle HAZ before they are arrested in more ductile regions of the base metal Characteristically, they are oriented almost perpendicular to the base metal surface and run parallel to the weld axis

Lamellar tearing is the phenomenon that occurs in T-joints that are fillet welded on both sides This condition, which

occurs in the base metal or HAZ of restrained weld joints, is characterized by a steplike crack parallel to the rolling plane The crack originates internally because of tensile strains produced by the contraction of the weld metal and the surrounding HAZ during cooling Figure 7 shows a typical condition

Fig 7 Lamellar tear caused by thermal contraction strain

Fusion-line cracks (No 7, Fig 6) can be classified as either weld metal cracks or base metal cracks because they occur

along the fusion line between the two There are no limitations as to where along the fusion line these cracks can occur or how far around the weld they can extend

Discontinuities Associated With Specialized Welding Processes

The preceding section has dealt mainly with the discontinuities common to conventional arc-welding processes In addition, there are certain more specialized welding methods that may have discontinuities unique to them These methods include electron beam, plasma arc, electroslag, friction, and resistance welding In general, the types of discontinuities associated with these processes are the same as those associated with conventional arc welding; however, because of the nature of the processes and the joint configurations involved, such discontinuities may be oriented differently from those previously described, or they may present particular problems of location and evaluation

Electron Beam Welding

In electron beam welding, as in all other welding processes, weld discontinuities can be divided into two major categories:

• Those that occur at, or are open to, the surface

• Those that occur below the surface

Surface flaws include undercut, mismatch, underfill, reinforcement, cracks, missed seams, and LOP Subsurface flaws include porosity, massive voids, bursts, cracks, missed seams, and LOP Figure 8 shows poor welds containing these flaws, and a good weld with none of them

Fig 8 Electron beam welds showing flaws that can occur in poor welds and the absence of flaws in a good weld

with reinforcement

Surface discontinuities such as undercut, mismatch, reinforcement, and underfill are macroscopic discontinuities related to the contour of the weld bead or the joint As such, they are readily detected visually or dimensionally Surface discontinuities such as cracks are usually detected visually using liquid penetrant inspection or using magnetic particle inspection if the material is ferromagnetic

When liquid penetrants are used to inspect a weld for surface discontinuities such as cracks, missed seams, and LOP, the surface to be inspected must be clean and the layers of metal smeared from machining or peened from grit- or sandblasting must be removed Generally, some type of etching or pickling treatment works well, but the possibility of hydrogen pickup from the treatment must be considered

Occasionally, special inspection procedures must be employed to detect some types of surface discontinuities Missed seams and LOP are often difficult to detect because they are frequently associated with complex weld joints that prevent direct viewing of the affected surface

Because of this difficulty, missed seams are often detected using a visual witness-line procedure, in which equally spaced parallel lines are scribed on both sides of the unwelded joint at the crown and root surfaces Missed seams, which result from misalignment of the electron beam with the joint such that the fusion zone fails to encompass the entire joint, are detected by observing the number of witness lines remaining on either side of the weld bead By establishing the relationship between the width of the fusion zone and the spacing of the witness lines, reasonably accurate criteria for determining whether the joint has been contained within the weld path (and therefore whether missed seams are present) can be developed

Lack of penetration discontinuities occur when the fusion zone fails to penetrate through the entire joint thickness, resulting in an unbonded area near the root of the joint These discontinuities are best detected by etching the root surface and observing the macroscopic shape and width of the fusion zone for full and even penetration

An alternative method for inspecting complete weld penetration is that of immersion pulse-echo ultrasonic testing The planet gear carrier assembly shown in Fig 9(a) consists of three decks (plates) and eight curved spacer sections The assembly, which is made from SAE 15B22M boron-treated structural steel, is held together by 16 welds Weld integrity is monitored by ultrasonic methods

Fig 9 Planet gear carrier assembly (a) showing the four welds in one stock that connect the three decks The

top and top-center welds are tested by the top ultrasonic transducer, and the bottom and bottom-center welds are inspected by the bottom transducer (b) Close-up of upper transducer in position to test the welds Courtesy of John M St John, Caterpillar, Inc

The parts are mounted on a turntable on a locating fixture so that the welds along the outside diameter are accessible Two transducers, located above and below the assembly, are used Figure 9(b) shows the upper transducer in position to test the welds An overall view of the tank, turntable, controls, and reject/accept light panel is shown in Fig 10 The use of such a system involves little downtime and enables a high quality level to be maintained More information on ultrasonic test methods is presented later in this article

Fig 10 Overall view of the ultrasonic unit used to test the electron beam welded assembly shown in Fig 9

Courtesy of John M St John, Caterpillar, Inc

Subsurface discontinuities are generally considerably more difficult to detect than surface discontinuities because observation is indirect The two most reliable and widely used NDI methods are radiography and ultrasonics

Volume-type discontinuities such as porosity, voids, and bursts are detected by radiographic inspection, provided their cross sections presented to the radiating beam exceed 1 to 2% of the beam path in the metal Discontinuities that present extremely thin cross sections to the beam path, such as cracks, missed seams, and LOP, are detectable with x-rays only if they are viewed from the end along their planar dimensions

Ultrasonic inspection can detect most volume discontinuities as well as planar discontinuities Planar discontinuities are best detected normal to the plane of the discontinuities, but missed seams and LOP often appear as continuous porosity when viewed looking down from the crown to the root of the weld in the plane of the discontinuity

Because of the inherent dependence of both radiographic and ultrasonic inspection on the shape and orientation of flaws and because each of the two methods can generally detect those flaws that the other misses, it is most advisable to complement one method with the other Furthermore, to increase the likelihood of properly viewing a flaw, one of the methods should be employed in at least two (preferably perpendicular) directions

Plasma Arc Welding

Discontinuities that occur in plasma arc welds include both surface and subsurface types, as shown in Fig 11

Fig 11 Plasma arc welds showing flaws that can occur in poor welds and the absence of flaws in good

reinforced weld

Surface discontinuities such as irregular reinforcement, underfill, undercut, and mismatch that are associated with weld bead contour and joint alignment are easily detected visually or dimensionally Lack of penetration is also detected visually through the absence of a root bead Weld cracks that are open to the surface are detected with liquid penetrants Surface contamination, which results from insufficient shielding-gas coverage, is detected by the severe discoloration of the weld bead or adjacent HAZ

Subsurface discontinuities are generally more prevalent in manual than in automatic plasma arc welding and are detected primarily by radiographic or ultrasonic inspection

Porosity is by far the most commonly encountered discontinuity Radiographic inspection is limited to detecting pores greater than approximately 1 to 2% of the joint thickness Visibility is greater if both the crown and root beads are machined flush Ultrasonic inspection can detect porosity if the joint is machined flush and joint thickness exceeds approximately 1.3 mm (0.050 in.)

Tunneling, as shown in Fig 11, is a severe void along the boundary of the fusion zone and the HAZ This discontinuity results from a combination of torch alignment and welding conditions (particularly travel speed) Tunneling is readily detectable by radiographic inspection

Lack of fusion discontinuities occur in either single-pass or multiple-pass repair welds (Fig 11) These discontinuities result from insufficient heat input to permit complete fusion of a particular weld bead to the part Incomplete fusion can

be detected by radiographic or ultrasonic inspection Depending on the orientation of the discontinuity, one method may have an advantage over the other, so both should be used for optimum inspection

Subsurface weld cracks, regardless of their cause, are detectable by radiographic and ultrasonic inspection

Subsurface contamination in plasma arc welding results when copper from the torch nozzle is expelled into the weld This

is caused by excessive heat, usually produced in manual repair welding when the torch nozzle is placed too close to the weld, particularly in a groove The resulting contamination, which may be detrimental, is undetectable by conventional NDI methods The only way of detecting copper contamination is by alerting the operator to watch for copper expulsion, which then must be machined out

Electroslag Welding

Electroslag welding involves the use of copper dams over the open surfaces of a butt joint to hold the molten metal and the slag layer as the joint is built up vertically Wire is fed into the slag layer continuously and is melted by the heat generated as current passes through the highly resistant slag layer

Generally, electroslag welds are inspected with the same nondestructive examination (NDE) methods as other section welds With the exception of procedure qualification, all testing is nondestructive because of the sizes used Techniques such as radiography and ultrasonic inspection are most often used, while visual, magnetic particle, and liquid penetrant testing are used also Internal defects are generally more serious Radiography and ultrasonic tests are the best methods for locating internal discontinuities

heavy-Because of the nature of the process, LOF is rare If fusion is achieved on external material edges, then fusion is generally complete throughout Cracking may occur either in the weld or the HAZ Porosity may either take the form of a rounded

or a piped shape; the latter is often called wormhole porosity Ultrasonic inspection is probably the quickest single method for inspecting any large weldment If defects should occur, they appear as porosity or centerline cracking Ultrasonic inspection is effective for locating either type of defect; however, only well-qualified personnel should set up the equipment and interpret the test results

Electroslag welding results in large dendritic grain sizes because of the slow cooling rate Inexperienced personnel often use high sensitivity and actually pick up the large coarse grains; when such welds are sectioned, usually no defects are present Inspectors must learn to use low sensitivity to obtain good results when inspecting electroslag welds Magnetic particle inspection is not a particularly good inspection method, because the areas examined by this technique are primarily surface or near surface This is only a small percentage of the total weld; the only useful information is either checking the ends for craters, cracks, or centerline cracking or possibly for lack of edge fusion on the weld faces Usually,

a visual examination gives the same result unless the defect is subsurface Visual examinations are only effective for surface defects, which are not common in this process

Friction Welding

If impurities are properly dispelled during upsetting, friction or inertia welds are generally free of voids and inclusions Incomplete center fusion can occur when flywheel speed is too low, when the amount of upset is insufficient, or when mating surfaces are concave Tearing in the HAZ can be caused by low flywheel speed or excessive flywheel size Cracks can occur when materials that are prone to hot shortness are joined The penetration of a split between the extruded flash into the workpiece cross section is most prevalent during the welding of thin-wall tubing using improper conditions that

do not allow for sufficient material upset

The area where LOF generally occurs is at or near the center of the weld cross section Because this is a subsurface discontinuity, detection is limited to radiographic or ultrasonic inspection; ultrasonic inspection is more practical The longitudinal wave test (either manual-contact or immersion method) with beam propagation perpendicular to the area of LOF gives the most reliable results This test can be performed as long as one end of the workpiece is accessible to the transducer

Penetration of the split between the extruded flash on the outer surface of a tube is readily detected by liquid penetrant or magnetic particle inspection after the flash has been removed by machining A split between the weld flash on the inner surface of the tube can be detected by ultrasonic inspection using the angle-beam technique with manual contact of the transducer to the outside surface of the tube The transducer contacts the tube so that the sound propagates along the longitudinal axis of the tube through the weldment

Resistance Welding

Resistance welding encompasses spot, seam, and projection welding, each of which involves the joining of metals by passing current from one side of the joint to the other The types of discontinuities found in resistance welds include porosity, LOF, and cracks Porosity will generally be found on the centerline of the weld nugget Lack of fusion may also

be manifested as a centerline cavity Either of these can be caused by overheating, inadequate pressure, premature release

of pressure, or late application of pressure Cracks may be induced by overheating, removal of pressure before weld quenching is completed, improper loading, poor joint fit-up, or expulsion of excess metal from the weld

Weld Appearance. On the surface of a resistance spot welded assembly, the weld spot should be uniform in shape and relatively smooth, and it should be free of surface fusion, deep electrode indentations, electrode deposits, pits, cracks, sheet separation, abnormal discoloration around the weld, or other conditions indicating improper maintenance of electrodes or functioning of equipment However, surface appearance is not always a good indicator of spot weld quality, because shunting and other causes of insufficient heating or incomplete penetration usually leave no visible effects on the workpiece

The common practice for monitoring spot weld quality in manufacturing operations is the teardown method augmented with pry testing and visual inspection (Ref 1) In visual inspection, the operator uses the physical features of the weld surface, such as coloration, indentation, and smoothness, for assessing the quality of the weld In pry testing, a wedge-shaped tool is inserted between the metal sheets next to the accessible welds, and a prying action is performed to see if the sheets will separate in the weld zone The teardown method consists of physically tearing apart the welded members with hammers and chisels to determine the presence and size adequacy of fused metal nuggets at the spot weld site The specifications require that the parent metal be torn and that the weld nugget remain intact The destructiveness and/or inadequacy of these common inspection methods has long been recognized, and as a result, nondestructive methods have been extensively studied The pulse-echo ultrasonic inspection of spot welds is now feasible and is discussed in the section "Ultrasonic Inspection" in this article The use of acoustic emission is also discussed below

Diffusion Bonding (Ref 2, 3, 4, 5)

Diffusion bonding is a metal joining process that requires the application of controlled pressures at elevated temperatures and usually a protective atmosphere to prevent oxidation No melting and only limited macroscopic deformation or relative motion between the faying surfaces of the parts occur during bonding As such, the principal mechanism for joint formation is solid-state diffusion A diffusion aid (filler metal) may or may not be used Terms that are also frequently used to describe the process include diffusion welding, solid-state bonding, pressure bonding, isostatic bonding, and hot press bonding Diffusion bonding has the advantage of producing a product finished to size, with joint efficiencies

approaching 100% Details of the process are given in the article "Fundamentals of Diffusion Bonding" in Welding,

Brazing, and Soldering, Volume 6 of the ASM Handbook

Discontinuities in Diffusion Bonds. In the case of fusion welds, the detection of discontinuities less than 1 mm (0.04 in.) in size is not generally expected In diffusion bonding, in which no major lack of bonding occurs, individual discontinuities may be only micrometers in size To understand how discontinuities form in diffusion-bonded structures, it

is first necessary to consider the principles of the process

As illustrated in Fig 12, metal surfaces have several general characteristics:

• Roughness

• An oxidized or otherwise chemically reacted and adherent layer

• Other randomly distributed solid or liquid products such as oil, grease, and dirt

• Adsorbed gas, moisture, or both

Because of these characteristics, two necessary conditions that must be met before a satisfactory diffusion bond can be made are:

• Mechanical intimacy of metal-to-metal contact must be achieved

• Interfering surface contaminants must be disrupted and dispersed to permit metallic bonding to occur (solvent cleaning and inert gas atmospheres can reduce or eliminate problems associated with surface contamination and oxide formation, respectively)

Fig 12 Characteristics of a metal surface showing roughness and contaminants present Source: Ref 5

For a given set of processing parameters, surface roughness is probably the most important variable influencing the quality of diffusion-bonded joints The size of the discontinuities (voids) is principally determined by the scale of roughness of the surfaces being bonded The degree of surface roughness is dependent on the material and fabrication/machining technique used It has been shown that bonding becomes easier with finer surface roughness prior

to bonding Figure 13 compares the surface roughness values produced by a variety of fabrication methods More detailed

information on surface roughness can be found in the article "Surface Finish and Surface Integrity" in Machining, Volume 16 of ASM Handbook, formerly 9th Edition Metals Handbook

Fig 13 Surface roughness produced by common production methods The ranges shown are typical of the

processes listed Higher or lower values can be obtained under special conditions

The mechanism of bond formation in diffusion bonding is believed to be the deformation of the surface roughness in order to cause metal-to-metal contact at asperities, followed by the removal of interfacial voids and cracks by diffusional and creep processes For conventional diffusion bonding without a diffusion aid, the three-stage mechanistic model shown

in Fig 14 describes bond formation In the first stage, deformation of the contacting asperities occurs primarily by yielding and by creep deformation mechanisms to produce intimate contact over a large fraction of the interfacial area At the end of this stage, the joint is essentially a grain boundary at the areas of contact with voids between these areas During the second stage, diffusion becomes more important than deformation, and many of the voids disappear as the grain-boundary diffusion of atoms continues Simultaneously, the interfacial grain boundary migrates to an equilibrium configuration away from the original plane of the joint, leaving many of the remaining voids within the grains In the third stage, the remaining voids are eliminated by the volume diffusion of atoms to the void surface (equivalent to diffusion of vacancies away from the void) Successful completion of stage three is dependent on proper surface 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 distribution of discontinuities be determined However, many conventional NDE methods and equipment are not adequate for discontinuity determination

Fluorescent penetrants provide excellent detection capability for lack of bonding as long as the interfacial crack breaks the surface They are completely ineffective for defects that have no path to the surface

Conventional film radiography is not suitable for detecting the extremely small defects involved in diffusion bonding, but the use of x-ray microfocus techniques coupled with digital image enhancement offers an improvement in resolution Discontinuities as small as 50 m (0.002 in.) have been detected

Conventional ultrasonic testing has some applications, although only significant lack of bonding or clusters of smaller defects can be reliably detected High-resolution flaw detection involving frequencies approaching 100 MHz appears to have distinct advantages over conventional testing Scanning acoustic microscopy also appears to offer excellent 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 weld on the basis of its fitness for purpose under the service conditions imposed on the structure

The appropriate method of inspection is different for each function If evaluation is a viable option, discontinuities must

be detected, identified, located exactly, sized, and their orientation established, which limits inspection to a volumetric technique

Weld discontinuities constitute the center of activity with the inspection of welded constructions The most widely used inspection techniques used in the welding industry are visual, liquid penetrant, magnetic particle, radiographic, ultrasonic, acoustic emission, eddy current, and electric current perturbation methods Each of these techniques has specific advantages and limitations Existing codes and standards that provide guidelines for these various techniques are based on the capabilities and/or limitations of these nondestructive methods

Selection of Technique. A number of factors influence selection of the appropriate nondestructive test technique for inspecting a welded structure, including discontinuity characteristics, fracture mechanics requirements, part size, portability of equipment, and other application constraints These categories, although perhaps unique to a specific inspection problem, may not clearly point the way to the most appropriate technique It is generally necessary to exercise engineering judgment in ranking the importance of these criteria and thus determining the optimum inspection technique

Characteristics of the Discontinuity Because nondestructive techniques are based on physical phenomena, it is

useful to describe the properties of the discontinuity of interest, such as composition and electrical, magnetic, mechanical, and thermal properties Most significant are those properties that are most different from those of the weld or base metal

It is also necessary to identify a means of discriminating between discontinuities with similar properties

Fracture mechanics requirements, solely from a discontinuity viewpoint, typically include detection,

identification, location, sizing, and orientation In addition, complicated configurations may necessitate a non-destructive assessment of the state of the stress of the region containing the discontinuity In the selection process, it is important to establish these requirements correctly This may involve consultation with stress analysts, materials engineers, and statisticians

Often, the criteria may strongly suggest a particular technique Under ideal conditions, such as in a laboratory, the application of such a technique might be routine In the field, however, other factors may force a different choice of technique

Constraints tend to be unique to a given application and may be completely different even when the welding process

and metals are the same Some of these constraints include:

• Access to the region under inspection

• Geometry of the structure (flat, curved, thick, thin)

• Condition of the surface (smooth, irregular)

• Mode of inspection (preservice, in-service, continuous, periodic, spot)

• Environment (hostile, underwater, and so on)

• Time available for inspection (high speed, time intensive)

The terms accuracy, sensitivity, and reliability are used loosely in NDE Often, they are discussed as one term to avoid distinguishing among the specific aspects of these terminologies

Accuracy is the attribute of an inspection method that describes the correctness of the technique within the limits of its

precision In other words, the technique is highly accurate if the indications resulting from the technique are correct This does not mean that the technique was able to detect all discontinuities present, but rather that those indicated actually exist

Sensitivity, on the other hand, refers to the capability of a technique to detect discontinuities that are small or that have

properties only slightly different from the material in which they reside Figure 15 schematically illustrates the concepts

of accuracy and sensitivity in the context of detection probability In general, sensitivity is gained at the expense of accuracy because high sensitivity increases the probability of false alarms

Fig 15 Detection probability for a true positive indication High sensitivity increases the likelihood of false

indications The minimum NDE parameter size required to establish fitness-for-purpose must lie to the right of the transition region, or reliability threshold, to achieve satisfactory reliability

Reliability is a combination of both accuracy and sensitivity Three factors influence reliability: inspection procedure,

including the instrumentation; human factors (inspector motivation, experience, training, education, and so on); and data analysis Uncalibrated equipment, improper application of technique, and inconsistent quality of accessory equipment (transducers, couplant, film, chemicals, and so on) may affect accuracy and, in some cases, sensitivity Poor inspector technique, unfamiliar response, lack of concentration, and other human factors can combine to reduce reliability Data analysis, or the lack of it, can influence reliability as well; generally, inspection is performed under conditions in which detection probability is less than 100% and is not constant with discontinuity severity Consequently, statistics must be employed to establish the level of confidence that may be attached to the inspection results

High sensitivity with low accuracy may be far worse, from the viewpoint of reliability, than low sensitivity with high accuracy, especially if the sensitivity level is adequate for detecting the weld discontinuities in question As a general rule, the transition region of the detection probability curve indicates the degree of reliability If this region occurs with the limits encompassed by inspection capabilities, which are smaller than the values required for evaluating the fitness-for-purpose of the welds being inspected, reliability is satisfactory If, on the other hand, the region occurs at values higher than those required, reliability is unsatisfactory The transition region can be viewed as the reliability threshold More detailed information on the reliability of nondestructive test data can be found in the Section "Quantitative Nondestructive Evaluation" in this Volume

Visual Inspection

For many noncritical welds, integrity is verified principally by visual inspection Even when other nondestructive methods are used, visual inspection still constitutes an important part of practical quality control Widely used to detect discontinuities, visual inspection is simple, quick, and relatively inexpensive The only aids that might be used to determine the conformity of a weld are a low-power magnifier, a borescope, a dental mirror, or a gage Visual inspection can and should be done before, during, and after welding Although visual inspection is the simplest inspection method to use, a definite procedure should be established to ensure that it is carried out accurately and uniformly

Visual inspection is useful for checking the following:

• Dimensional accuracy of weldments

• Conformity of welds to size and contour requirements

• Acceptability of weld appearance with regard to surface roughness, weld spatter, and cleanness

• Presence of surface flaws such as unfilled craters, pockmarks, undercuts, overlaps, and cracks

Although visual inspection is an invaluable method, it is unreliable for detecting subsurface flaws Therefore, judgment of weld quality must be based on information in addition to that afforded by surface indications

Additional information can be gained by observations before and during welding For example, if the plate is free of laminations and properly cleaned and if the welding procedure is followed carefully, the completed weld can be judged on the basis of visual inspection Additional information can also be gained by using other NDI methods that detect subsurface and surface flaws

Dimensional Accuracy and Conformity. All weldments are fabricated to meet certain specified dimensions The fabricator must be aware of the amount of shrinkage that can be expected at each welded joint, the effect of welding sequence on warpage or distortion, and the effect of subsequent heat treatment used to provide dimensional stability of the weldment in service Weldments that require rigid control of final dimensions usually must be machined after welding Dimensional tolerances for as-welded components depend on the thickness of the material, the alloy being welded, the overall size of the product, and the particular welding process used

The dimensional accuracy of weldments is determined by conventional measuring methods, such as rules, scales, calipers, micrometers, and gages The conformity of welds with regard to size and contour can be determined by a weld gage The weld gage shown in Fig 16 is used when visually inspecting fillet welds at 90° intersections The size of the fillet weld, which is defined by the length of the leg, is stamped on the gage The weld gage determines whether or not the size of the fillet weld is within allowable limits and whether there is excessive concavity or convexity This gage is designed for use

on joints between surfaces that are perpendicular Special weld gages are used when the surfaces are at angles other than 90° For groove welds, the width of the finished welds must be in accordance with the required groove angle, root face, and root opening The height of reinforcement of the face and root must be consistent with specified requirements and can

be measured by a weld gage

A Minimum allowable length of leg

B Maximum allowable length of leg

C 1.414 times maximum allowable throat size (specifies maximum allowable convexity)

D Maximum allowable length of leg when maximum allowable concavity is present

E A plus B plus nominal weld size (or nominal length of leg)

F Minimum allowable throat size (specifies maximum allowable concavity)

T Additional tolerance for clearance of gage when placed in the fillet

Fig 16 Gage for visual inspection of a fillet weld at a 90° intersection Similar gages can be made for other

angles Dimension given in inches

Appearance Standards. The acceptance of welds with regard to appearance implies the use of a visual standard, such

as a sample weldment or a workmanship standard Requirements as to surface appearance differ widely, depending on the application For example, when aesthetics are important, a smooth weld that is uniform in size and contour may be required

The inspection of multiple-pass welds is often based on a workmanship standard Figure 17 indicates how such standards are prepared for use in the visual inspection of groove and fillet welds The workmanship standard is a section of a joint similar to the one in manufacture, except that portions of each weld pass are shown Each pass of the production weld is compared with corresponding passes of the workmanship standard

Fig 17 Workmanship standard for visual comparison during inspection of single-V-groove welds and fillet

welds Dimensions given in inches

Discontinuities. Before a weld is visually inspected for discontinuities such as unfilled craters, surface holes,

undercuts, overlaps, surface cracks, and incomplete joint penetration, the surface of the weld should be cleaned of oxides and slag Cleaning must be done carefully For example, a chipping hammer used to remove slag could leave hammer marks that can hide fine cracks Shotblasting can peen the surface of relatively 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 method of detecting surface and near-surface flaws in ferromagnetic materials It consists of three basic operations:

• Establishing a suitable magnetic field in the material being inspected

• Applying magnetic particles to the surface of the material

• Examining the surface of the material for accumulations of the particles (indications) and evaluating the serviceability of the material

Capabilities and Limitations. Magnetic particle inspection is particularly suitable for the detection of surface flaws

in highly ferromagnetic metals Under favorable conditions, those discontinuities that lie immediately under the surface are also detectable Nonferromagnetic and weakly ferromagnetic metals, which cannot be strongly magnetized, cannot be inspected by this method With suitable ferromagnetic metals, magnetic particle inspection is highly sensitive and produces readily discernible indications at flaws in the surface of the material being inspected Details on the application

of this method are available in the article "Magnetic Particle Inspection" in this Volume

The types of weld discontinuities normally detected by magnetic particle inspection include cracks, LOP, LOF, and porosity open to the surface Linear porosity, slag inclusions, and gas pockets can be detected if large or extensive or if smaller and near the surface The recognition of patterns that indicate deep-lying flaws requires more experience than that required to detect surface flaws

Nonrelevant indications that have no bearing on the quality of the weldment may be produced These indications are magnetic particle patterns held by conditions caused by leakage fields Some of these conditions are:

• Particles held mechanically or by gravity in surface irregularities

• Adherent scale or slag

• Indications at a sharp change in material direction, such as sharp fillets and threads

• Grain boundaries Large grain size in the weld metal or the base metal may produce indications

• Boundary zones in welds, such as indications produced at the junction of the weld metal and the base metal This condition occurs in fillet welds at T-joints, or in double-V-groove joints, where 100% penetration is not required

• Flow lines in forgings and formed parts

• Brazed joints Two parts made of a ferromagnetic material joined by a nonferromagnetic material will produce an indication

• Different degrees of hardness in a material, which will usually have different permeabilities that may create a leakage field, forming indications

Operational Requirements. The magnetic particle inspection of weldments requires that the weld bead be free of scale, slag, and moisture For maximum sensitivity, the weld bead should be machined flush with the surface; however, wire brushing, sandblasting, or gritblasting usually produces a satisfactory bead surface If the weld bead is rough, grinding will remove the high spots

Weldments are often inspected using the dry particle method A powder or paste of a color that gives the best possible contrast to the surface being inspected should be used

The type of magnetizing current used depends on whether there are surface or subsurface discontinuities Alternating current is satisfactory for surface cracks, but if the deepest possible penetration is essential, direct current, direct current with surge, or half-wave rectified alternating current is used

The voltage should be as low as practical to reduce the possibility of damage to the surface of the part from overheating

or arcing at contacts Another advantage of low voltage is freedom from arc flashes if a prod slips or is withdrawn before the current is turned off

The field strength and flux density used must be determined for each type of weldment An overly strong field will cause the magnetic particles to adhere too tightly to the surface and hinder their mobility, preventing them from moving to the sites of the flaws Low field strengths result in nondiscernible patterns and failure to detect indications

Inspections can be made using the continuous-field and residual-field methods In the continuous-field method, magnetic particles are placed on the weldment while the current is flowing In the residual-field method, the particles are placed on the weldment after the magnetizing current is turned off Residual magnetic fields are weaker than continuous fields Consequently, inspections using the residual-field method are less sensitive

The need for the demagnetization of weldments after magnetic particle inspection must be given serious consideration Where subsequent welding or machining operations are required, it is good practice to demagnetize Residual magnetism may also hinder cleaning operations and interfere with the performance of instruments used near the weldment

Liquid Penetrant Inspection

Liquid penetrant inspection is capable of detecting discontinuities open to the surface in weldments made of either ferromagnetic or nonferromagnetic alloys, even when the flaws are generally not visible to the unaided eye Liquid penetrant is applied to the surface of the part, where it remains for a period of time and penetrates into the flaws For the correct usage of liquid penetrant inspection, it is essential that the surface of the part be thoroughly clean, leaving the openings free to receive the penetrant Operating temperatures of 20 to 30 °C (70 to 90 °F) produce optimum results If the part is cold, the penetrant may become chilled and thickened so that it cannot enter very fine openings If the part or the penetrant is too hot, the volatile components of the penetrant may evaporate, reducing the sensitivity

After the penetrating period, the excess penetrant remaining on the surface is removed An absorbent, light-colored developer is then applied to the surface This developer acts as the blotter, drawing out a portion of the penetrant that had previously seeped into the surface openings As the penetrant is drawn out, it diffuses into the developer, forming indications that are wider than the surface openings The inspector looks for these colored or fluorescent indications against the background of the developer Details of the mechanics of this method are given in the article "Liquid Penetrant Inspection" in this Volume

Radiographic Inspection *

Radiography is a nondestructive method that uses a beam of penetrating radiation such as x-rays and -rays (see the article "Radiographic Inspection" in this Volume) When the beam passes through a weldment, some of the radiation energy is absorbed, and the intensity of the beam is reduced Variations in beam intensity are recorded on film or on a screen when a fluoroscope or an image intensifier is used The variations are seen as differences in shading that are typical of the types and sizes of any discontinuities present Radiography is used to detect subsurface discontinuities as well as those that are open to the surface

Penetrameters. Most U.S codes and specifications for radiographic inspection require that the sensitivity to a specific flaw size be indicated on each radiograph Sensitivity is determined by placing a penetrameter (image-quality indicator) that is made of substantially the same material as the specimen directly on the specimen, or on a block of identical material of the same thickness as the specimen, prior to each exposure

The penetrameters are usually placed on the side of the specimen nearest the source, parallel and adjacent to the weld at one end of the exposed length, with the small holes in the penetrameter toward the outer end Penetrameter thickness is usually specified to be 2% of the specimen thickness (through the weld zone), although 1 and 4% penetrameter thicknesses are also common American Society for Testing and Materials and American Society of Mechanical Engineers plaque-type penetrameters have three small drilled holes, whose diameters equal one, two, and four times the penetrameter thickness Image quality is determined by the ability to distinguish both the outline of the penetrameter and one or more of the drilled holes on the processed radiograph; for example, 2-2T sensitivity is achieved when a 2% penetrameter and the hole that has a diameter of twice the penetrameter thickness are visible Also, the image of raised numbers that identify the actual thickness (not the percentage thickness) of a penetrameter should appear clearly, superimposed on the image of the penetrameter

Surface discontinuities that are detectable by radiography include undercuts (Fig 18 and 19), longitudinal grooves, concavity at the weld root (Fig 20 and 21), incomplete filling of grooves, excessive penetration (Fig 22), offset or mismatch (Fig 23 and 24), burn-through (Fig 25), irregularities at electrode-change points, grinding marks, and electrode spatter Surface irregularities may cause density variations on a radiograph When possible, they should be removed before a weld is radiographed When impossible to remove, they must be considered during interpretation

Fig 18 External undercut, which is a gouging out of the piece to be welded, alongside the edge of the top or

external surface of the weld Radiographic image: An irregular darker density along the edge of the weld image The density will always be darker than the density of the pieces being welded Welding process: SMAW Source: E.I Du Pont de Nemours & Company, Inc

Fig 19 Internal (root) undercut, which is a gouging out of the parent metal, alongside the edge of the bottom

or internal surface of the weld Radiographic image: An irregular darker density near the center of the width of the weld image and along the edge of the root pass image Welding process: SMAW Source: E.I Du Pont de

Nemours & Company, Inc

Fig 20 External concavity or insufficient fill, which is a depression in the top of the weld, or cover pass,

indicating a thinner-than-normal section thickness Radiographic image: A weld density darker than the density

of the pieces being welded and extending across the full width of the weld image Welding process: SMAW Source: E.I Du Pont de Nemours & Company, Inc

Fig 21 Internal concavity (suck back), which is a depression in the center of the surface of the root pass

Radiographic image: An elongated irregular darker density with fuzzy edges, in the center of the width of the weld image Welding process: GTAW-SMAW Source: E.I Du Pont de Nemours & Company, Inc

Fig 22 Excessive penetration (icicles, drop-through), which is extra metal at the bottom (root) of the weld

Radiographic image: A lighter density in the center of the width of the weld image, either extended along the weld or in isolated circular drops Welding process: SMAW Source: E.I Du Pont de Nemours & Company, Inc

Fig 23 Offset or mismatch with LOP, which is a misalignment of the pieces to be welded and insufficient filling

of the bottom of the weld or root area Radiographic image: An abrupt density change across the width of the weld image with a straight longitudinal darker-density line at the center of the width of the weld image along the edge of the density change Welding process: SMAW Source: E.I Du Pont de Nemours & Company, Inc