ASM Metals HandBook Vol. 17 - Nondestructive Evaluation and Quality Control

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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

192

Ir 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, Non-

Destr. Test., Vol 7 (No. 1), Feb 1974, p 25-35

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, "Automati

c 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

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 Discont

inuities and Wall Thickness Using

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

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, Non-

Destr. Test., Vol 7 (No. 1), Feb 1974, p 25-35

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

1969, p 259-268

Nondestructive Inspection of Weldments, Brazed Assemblies, and Soldered Joints

Introduction

THE SELECTION of a method for inspecting weldments, brazed assemblies, and soldered joints for flaws (referred to as

discontinuities in welding terminology) depends on a number of variables, including the nature of the discontinuity, the

accessibility of the joint, the type of materials joined, the number of joints to be inspected, the detection capabilities of the

inspection method, the level of joint quality required, and economic considerations. Regardless of the method selected,

established standards must be followed to obtain valid inspection results.

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 tun

gsten 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 o

f 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)

Slag inclusions

Porosity

LOF/LOP

Undercut

Submerged arc welding (SAW)

LOF/LOP

Slag inclusions

Porosity

Flux cored arc welding (FCAW)

Slag inclusions

Porosity

LOF/LOP

Gas metal arc welding (GMAW)

Porosity

LOF/LOP

Gas tungsten arc welding (GTAW)

Porosity

The commonly encountered inclusions--as well as cracking, the most serious of weld defects--will be discussed in this

section.

Gas porosity can occur on or just below the surface of a weld. Pores are characterized by a rounded or elongated

teardrop shape with or without a sharp point. Pores can be uniformly distributed throughout the weld or isolated in small

groups; they can also be concentrated at the root or toe of the weld. Porosity in welds is caused by gas entrapment in the

molten metal, by too much moisture on the base or filler metal, or by improper cleaning of the joint during preparation for

welding.

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. 1

c). Linear porosity generally

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

• Elongated porosity: Characterized by highly elongated pore

s 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.

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-CO

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.