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Tube testing by electromagnetic ndt methods — 1
Tube testing by electromagnetic ndt methods--1 W. Stumm The wide variety of electromagnetic testing systems for tubes has been developed because of the multitude of materials, manufacturing methods and defects encountered in the tubemaking industry. The author describes methods and equipment available for detecting defects in tubes by the leakage-flux method. The second part of the article will cover eddy-current techniques.
In the magnetic leakage flux method of testing ferromagnetic tubes, the tube is magnetised at right angles to the expected direction of the defect. Hence, to detect longitudinal defects the pipes are magnetised circumferentially and to detect transverse defects they are magnetised longitudinally. If a defect is present, the magnetic flux distributed throughout the tube wall will be distorted, producing leakage flux outside the tube, and this can be detected and measured. The magnitude of the magnetic field is used to assess the defect. All testing systems which utilise this principle are comprised of basic components: the magnetising device; the probes; the evaluation equipment; the mechanical system to produce relative motion between the tube and the tester to effect scanning. The testing systems available at present differ in the manner of magnetisation and in the mechanical methods used for scanning. The other components of the various test systems from the same family are basically similar.
The Rotomat The Rotomat testing system employs all the components mentioned above. The tube to be tested is passed through a rotating magnetising yoke which produces a circumferential magnetic field in the tube wall. Sensitive probes, usually Hall probes, are located between the poles of the rotating yoke, and these sense any magnetic field which is being deflected by the presence of a flaw. As the tube is fed through the rotating head it is scanned in a longitudinal spiral path. The scanning head consists of a number of spring-loaded, pivoted fingers which slide on the surface of the tube (Fig.l). The position of the probes can be adjusted to suit tubes of different diameters. The magnetising yoke is adapted to other tube diameters by changing the pole shoes. Magnetisation by means of a rotating yoke and the axial passage of the tube through the test station are characteristic features which identify the Rotomat as a member of the family of leakage-flux testing systems. There are several The author works for Institut Dr F~irster, Grathwohlstrasse 4, D--7410 Reutlingen, W. Germany. UK agents are WellsKrautkramer, Blackhc-se Rd., Letchworth.
NON-DESTRUCTIVE TESTING. OCTOBER 1974
Fig.1 A close-up of the scanning head shows two banks of Hall probes on either side of the tubes' path
sizes of rotating head available to cover tube diameters from 2 0 - 4 5 0 mm. The standard versions of the systems have heads for 2 0 - 1 4 0 mm diameter, 90-235 mm diameter and 100-450 mm diameter, and intermediate sizes are also available. As far as the mechanical performance of the system is concerned, the rubbing speed of the probe shoes on the surface of the material is 1.5 ms "1 in the case of the 20-140 mm diameter range. With a 70 mm diameter tube the axial velocity when performing 100% testing of the surface is 0.7 ms -1 . Each of the probe shoes carries 4 Hall sensors which means that there are 8 independent measuring channels. In the electronic equipment, each measuring channel has its own filter amplifier and digital discriminators, one or two of which can be used to segregate the detected flaws according to their size. The output signals from the discriminators pass together to the evaluating equipment which either initiates colour marking of the defective section (one or two colours depending on the number of defect categories), or sorting of the defective tubes. It is also possible by means of additional equipment to use the length of the flaw as a means of assessing its seriousness. The defect signals obtained from the probes are displayed on an oscilloscope and
251
also recorded on a highspeed chart recorder which can either accept the peak value of all signals from all channels or the values from individual channels. The oscillograms of defect signals obtained during the testing of welded tube and polished sections of the flaw involved can be compared (Fig.2). External flaws with a depth of roughly 10% of the wall thickness give a clear signal. The use of the Rotomat for testing seamless tube has been described by Kr~chter. 2 A significant problem with the leakage-flux method for testing tubes is the difference in defect signal obtained from internal and external flaws (Fig.3). When using the normal probes which are used for detecting external defects, the signal obtained from the internal defects was much reduced For example, the signal obtained from a 20% internal flaw was about as large as that from an 8% external flaw. With,
1
Fig.4 The signal peaks on this oscillogram represent, from the left: an outer-surface defect of 10% wall thickness; an inner-surface defect of 10%; and an outer-surface defect of 20%. The tube has a diameter of 219 mm and a wall thickness of 10 mm
I
however, the use of specially modified probes which utilise the geometric pattern of the defect-leakage flux at the surface it is possible to reduce the difference between the signals obtained from external and internal flaws to less than 10%. For a 219 x 10 mm tube with 10% internal and external flaws and a 20% internal flaw, the two 10% internal and external flaws give practically identical signals with a good signal-to-noise ratio. The internal flaw of depth 20% of the wall thickness gives a signal approximately twice that of the 10% flaw which demonstrates the linearity of the indication obtained from artificial flaws (Fig.4).
i
•
.
Fig.2 Defects in the 1.2 mm thick wall of a weldea tube of 36 mm diameter give strong flux-leakage indications which can be compared w i t h photomicrographs: above - 0.14 mm crack; below 0.88 mm crack
Such close approximation between internal and external flaws can generally be obtained in practice but the surface roughness and distortion of the surface of the tube do set certain limits. For external and internal flaws with a depth of 10% of the wall thickness in a tube of polygonal crosssection, the signal-to.noise ratio is still just adequate for automatic evaluation (Fig.5). The testing of tubes of wall thickness up to 12.5 mm (casings) for use in oil drilling has shown that when the surface finish is good, external and internal defects of more than 5% of the wall thickness can
f
Fig.3 These oscillograms were got for a tube of 7 mm wall thickness with a number of artificial flaws, the signal peaks originate from alternately the outer and inner surface; starting on the left the defect pairs are 20%, 12% and 8% of the wall thickness. The left display is for a normal probe the middle for a semi-adapted probe and the right for an adapted probe
252
NON-DESTRUCTIVE
TESTI NG.
OCTOBER
1974
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are identical to those used in the Rotomat, are mounted on pneumatically controlled arms. The various types of Tubomat differ according to the method of magnetisation used. For the testing of large pipes using the central conductor magnetisation method the pipe is transported onto rotating rollers and the central conductor is inserted and located by two tripods at the pipe ends. The pipe is magnetised, the arms are lowered on to the surface of the pipe which is now rotating and is then moved axially at the same time in order to spirally scan the whole surface (Fig.7). The test results obtained are fully comparable with those obtained from the Rotomat.
Fig.5 The main signal peaks on this oscillogram represent (left) an inner-surface defect of 10% and an outer-surface defect of 10% of the 8 mm wall of the 76.1 mm diameter tube
Fig.6 Another method of scanning for flux leakage is to rotate the tube while the probe is held stationary at the end of an adjustable arm
Although the use of a central conductor for magnetization produces the ideal uniform magnetic field in the tube wall, it necessitates considerable mechanical complications in order to introduce the conductor into the pipe; furthermore, very high currents are necessary to achieve the optimum field strength for testing, particularly with large-diameter pipes. For tubes with thinner walls the yoke magnetization system is considerably simplified. By this method a small yoke with a probe located between the pole shoes is placed on the tube (Fig.8). The probe is located between the rollers which guide it on the surface of the tube (Fig.9). Because of the non-uniform distribution of the magnetic field the test results obtained are not comparable with those obtained from the Rotomat or the Tubomat when used with a central conductor. Nevertheless, for practical purposes the results are extremely satisfactory (Fig. 10). The methods used in the Rotomat to obtain comparable signals from internal and external defects are again applicable to this system. Length ot tube already tested / Contact Testarm ~ ~ a w s Loading table Contact Marker \~\~ jaws ~ ~ A d a p t e r " \
\
\
~
~
be reliably detected to API Standards. When the surface finish is poorer it is still possible to satisfy API Standards for a 10% defect depth. In order to satisfy the requirements of the many kinds of application it is necessary for the basic system to be capable of modification and extension. As already mentioned, different rotating heads are available for various diameter ranges. In addition, an automatic probe lifting mechanism is incorporated to protect the probes from ragged tube ends. The standard versions of the testing systems incorporate equipment for the automatic colour marking of defective sections. In order to facilitate rectification work, an additional sector marking system can be added which uses up to 16 spray guns, and an electronic control system to mark accurately the defective areas of the tubes in both the longitudinal and circumferential directions. An extra spray gun is employed to mark the tube to indicate whether the defects discovered are in the external or internal surfaces.
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detects
\Friction roller for rotation Conveyor chain
~pporting rollers
Discharc~ of tube Fig.7 The inspection of large tubes needsa special handling system and a number of test arms Testing yoke
Pneumatic litt
Marker
/-
~ ' ~
.
The Tubomat
In contrast to the Rotomat, the magnetizing and probe system in the Tubomat remains stationary while the tube
being tested rotates. The arrangement of the probes is the same in all versions of the Tubomat (Fig.6). The probes themselves, which together with the electronic equipment
NON-DESTRUCTIVE TESTING . OCTOBER 1974
rack
Loading rack Fig.8
Thin-walled tubes can be tested in this kind of arrangement
253
/
",//',, '4'
Fig.11 Probes can be oscillated as the tube turns to give a t h o r o u g h scan Fig.9
The probe is the light object between the two metal shoes
I Fig.10 A signal can be clearly distinguised from the noise for this 1.2 m m defect on the inner surface of a 10 m m wall of a 270 m m diameter tube
If transverse defects are suspected in tubes, due, for example, to the use of special manufacturing methods such as centrifugal casting, it is necessary to magnetize the tube in the longitudinal direction. This can be achieved by the appropriate arrangement of coils. To scan the tube surface it is once again necessary to produce relative spiral motion between the tube and the coils and probes. The tube is rotated and also moved axially in relation to the coils and probes. The scanning of the tube surface is facilitated if the probes are oscillated in the longitudinal direction. The resuiting scanning path follows a sine curve which considerably widens the zone of detection covered by each probe and also allows filtering methods to be used to isolate transverse defect signals from those arising from variations in the magnetization and other irregularities (Fig. 11). For a tube with an artificial defect (a sawcut 1.6 mm deep) results were obtained with the probe oscillating to and fro, and as the pipe passed spirally through the tester (Fig.12). The signal-to-noise ratio was excellent (Fig.13). The equipment is also able to detect the presence of porosity in centrifugally-cast tubes (Fig.14). The presence of porosity is not so much indicated by a high signal amplitude, but rather by the location of a large number of signals grouped together at a particular spot. Comparing the defect signal amplitude with the actual defect depth the resulting scatter indicates the limits within which the presence of transverse defects can be detected with such a magnetic testing system (Fig.15).
254
Fig.12 A transverse saw cut 1 x 1.6 m m in a centrifugally-cast tube is shown by: above left - p h o t o g r a p h y ; above right -sectioning; and below - recording from the oscillating-probe system
. . . . . . .
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Fig.13 A hot tear in a tube could equally well be detected: above left - photograph; above right -- section; below - test recordi ng
NON-DESTRUCTIVE
TESTING.
OCTOBER
1974
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Fig.14 Porosity is clearly shown b y comparing the recording f r o m porous tube (above) w i t h t h a t f r o m sound t u b e
40 Fig.16 The test head (right) of the rotating-disc equipment can be raised and lowered to fit a production line c U
30
c
.o
• Hot teors
u "O c
• Cold weldin9 costin 9 spirol 20
•
m
Needle holes
U O~ O /
IO
5 Defect depth [ram]
IO
Fig.15 The limits of detectability for defects can be shown graphically
The D i s c o m a t It is advisable for the weld seam zone of welded tubes to be tested for defects as soon as possible after the weld has been made on the production line. The method is to magnetize the tube transversely to the direction of the weld as it passes through a magnetizing yoke (Fig.16). The weld zone is scanned by sensitive probes mounted on a disc rotating at high speed. If the disc carries five probes and rotates at 50 rps it produces 250 scans per second which, assuming the effective scanning width of each probe to be approxima-
Fig.17
tely 5 ram, is sufficient to give 100% scanning of the seam at the maximum welding speed which can be expected. The same equipment illustrates how the testing head with the rotating disc and the magnetising yoke can be raised, lowered, moved to the side or rotated about the tube axis to suit conditions on the particular welding production line (Fig. 16). Various types of probes can be mounted on the disc so that, as discussed below, not only internal and external defects can be detected but also geometrical irregularities in the seam such as offsetting of the edges. The electronic equipment is designed to permit the inclusion of additonal amplifier units for these functions. A tube was deliberately welded using a lower power than that specified and the tube was subsequently tested (Fig.17). At the end of the trace obtained the signal is seen slightly overshooting the range of the recorder, revealing a 1 mm diameter hole in the weld seam. The tube was then cut through at certain points and the resulting sections were examined under the microscope. Subsequently the pieces of pipe were expanded by a given percentage using a drift and again examined under the microscope. It can be seen that defects which open into the internal or external surfaces clearly coincide with the threshold of the automatic discriminator indicated in the trace. Off the other hand there are places where there is only a minute defect signal but which, when the pipe is expanded, open up into large defects. This is caused by the phenomenon known as cold welding in which a joint is made but because of the low energy used, a seam of insufficient strength is produced. Such defects do not cause sufficient leakage flux to make this a reliable method of detection. Direct methods of testing for such defects cannot be used and it is suggested that the heat transformation in the seam be measured in order to check for such cold-welded spots.
Faults in a t u b e of 48.3 mm diameter and 3.5 mm wall thickness were f o u n d b y the rotating disc method and photomicrographs made
of sections at the faults
N O N - D E S T R U C T I V E T E S T I N G . O C T O B E R 1974
255
Fig.18 Three temperature traces were made with different welding voltages and relative upsetting pressure. The voltages were 2.6, 1.6, 1.6 V, the relative pressure 1, 1,0.77. The last trace is for a faulty tube
~
Cold weldin9
Sparkin9
30
1
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Ex~nsi~ test
20-
Faulty ndirKj test
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o Faulty
15-
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I t 1.5 2.0 W¢ldin9 voltage [V]
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Fig.19 The temperature record clearly indicates the weld quality at a given welding voltage
A system can be used which uses a rotating head similar to that of the Discomat, but fitted with photodetectors instead of magnetic sensors for measuring the temperature and expansion of the weld seam. If measurements are
256
taken longitudinally at two displaced positions it allows the temperature uniformity to be checked in the whole seam area at the sides and in the depth. Temperature traces were taken from welds made with different supply voltages (Fig. 18). Also a graph was drawn of the measured values from these diagrams plotted against the welding supply voltage (Fig. 19). The results of additional destructive tests were also noted. It can be seen that there is a clear correlation between the weld temperature - and hence the heat transformation achieved in the seam - and the welding supply voltage. Above a certain value on the temperature trace the destructive testing reveals a good weld. ~ The Discomat and the conventional monitoring equipment in a welding production line can be combined to form a central monitoring position which can supply all necessary data for optimising the welding process. In addition to welding voltage, current, speed, the infrared detectors in conjunction with the Discomat with its various probes can indicate the heat transformation in the weld, the position of the weld under the testing head, edge offset, geometrical data on the weld and also indicate external and internal defects. A trained operator can use this display to determine the necessary corrective action to be taken in the production process. References
F6rster, F. Non-destructive testing for defects in the welded zone in pressure-welded tube under the special conditions of weld dressing. Paper presented at the International Welding Fair (Zagreb 1972) Krffcher, H. Research into the recognition of defects in boiler tubes using the eddy-current and flux-leakage methods. Proceedings of 6th ICNDT (Hanover 1970)
N O N - D E S T R U C T I V E TESTING . OCTOBER 1974
In the magnetic leakage flux method of testing ferromagnetic tubes, the tube is magnetised at right angles to the expected direction of the defect. Hence, to detect longitudinal defects the pipes are magnetised circumferentially and to detect transverse defects they are magnetised longitudinally. If a defect is present, the magnetic flux distributed throughout the tube wall will be distorted, producing leakage flux outside the tube, and this can be detected and measured. The magnitude of the magnetic field is used to assess the defect. All testing systems which utilise this principle are comprised of basic components: the magnetising device; the probes; the evaluation equipment; the mechanical system to produce relative motion between the tube and the tester to effect scanning. The testing systems available at present differ in the manner of magnetisation and in the mechanical methods used for scanning. The other components of the various test systems from the same family are basically similar.
The Rotomat The Rotomat testing system employs all the components mentioned above. The tube to be tested is passed through a rotating magnetising yoke which produces a circumferential magnetic field in the tube wall. Sensitive probes, usually Hall probes, are located between the poles of the rotating yoke, and these sense any magnetic field which is being deflected by the presence of a flaw. As the tube is fed through the rotating head it is scanned in a longitudinal spiral path. The scanning head consists of a number of spring-loaded, pivoted fingers which slide on the surface of the tube (Fig.l). The position of the probes can be adjusted to suit tubes of different diameters. The magnetising yoke is adapted to other tube diameters by changing the pole shoes. Magnetisation by means of a rotating yoke and the axial passage of the tube through the test station are characteristic features which identify the Rotomat as a member of the family of leakage-flux testing systems. There are several The author works for Institut Dr F~irster, Grathwohlstrasse 4, D--7410 Reutlingen, W. Germany. UK agents are WellsKrautkramer, Blackhc-se Rd., Letchworth.
NON-DESTRUCTIVE TESTING. OCTOBER 1974
Fig.1 A close-up of the scanning head shows two banks of Hall probes on either side of the tubes' path
sizes of rotating head available to cover tube diameters from 2 0 - 4 5 0 mm. The standard versions of the systems have heads for 2 0 - 1 4 0 mm diameter, 90-235 mm diameter and 100-450 mm diameter, and intermediate sizes are also available. As far as the mechanical performance of the system is concerned, the rubbing speed of the probe shoes on the surface of the material is 1.5 ms "1 in the case of the 20-140 mm diameter range. With a 70 mm diameter tube the axial velocity when performing 100% testing of the surface is 0.7 ms -1 . Each of the probe shoes carries 4 Hall sensors which means that there are 8 independent measuring channels. In the electronic equipment, each measuring channel has its own filter amplifier and digital discriminators, one or two of which can be used to segregate the detected flaws according to their size. The output signals from the discriminators pass together to the evaluating equipment which either initiates colour marking of the defective section (one or two colours depending on the number of defect categories), or sorting of the defective tubes. It is also possible by means of additional equipment to use the length of the flaw as a means of assessing its seriousness. The defect signals obtained from the probes are displayed on an oscilloscope and
251
also recorded on a highspeed chart recorder which can either accept the peak value of all signals from all channels or the values from individual channels. The oscillograms of defect signals obtained during the testing of welded tube and polished sections of the flaw involved can be compared (Fig.2). External flaws with a depth of roughly 10% of the wall thickness give a clear signal. The use of the Rotomat for testing seamless tube has been described by Kr~chter. 2 A significant problem with the leakage-flux method for testing tubes is the difference in defect signal obtained from internal and external flaws (Fig.3). When using the normal probes which are used for detecting external defects, the signal obtained from the internal defects was much reduced For example, the signal obtained from a 20% internal flaw was about as large as that from an 8% external flaw. With,
1
Fig.4 The signal peaks on this oscillogram represent, from the left: an outer-surface defect of 10% wall thickness; an inner-surface defect of 10%; and an outer-surface defect of 20%. The tube has a diameter of 219 mm and a wall thickness of 10 mm
I
however, the use of specially modified probes which utilise the geometric pattern of the defect-leakage flux at the surface it is possible to reduce the difference between the signals obtained from external and internal flaws to less than 10%. For a 219 x 10 mm tube with 10% internal and external flaws and a 20% internal flaw, the two 10% internal and external flaws give practically identical signals with a good signal-to-noise ratio. The internal flaw of depth 20% of the wall thickness gives a signal approximately twice that of the 10% flaw which demonstrates the linearity of the indication obtained from artificial flaws (Fig.4).
i
•
.
Fig.2 Defects in the 1.2 mm thick wall of a weldea tube of 36 mm diameter give strong flux-leakage indications which can be compared w i t h photomicrographs: above - 0.14 mm crack; below 0.88 mm crack
Such close approximation between internal and external flaws can generally be obtained in practice but the surface roughness and distortion of the surface of the tube do set certain limits. For external and internal flaws with a depth of 10% of the wall thickness in a tube of polygonal crosssection, the signal-to.noise ratio is still just adequate for automatic evaluation (Fig.5). The testing of tubes of wall thickness up to 12.5 mm (casings) for use in oil drilling has shown that when the surface finish is good, external and internal defects of more than 5% of the wall thickness can
f
Fig.3 These oscillograms were got for a tube of 7 mm wall thickness with a number of artificial flaws, the signal peaks originate from alternately the outer and inner surface; starting on the left the defect pairs are 20%, 12% and 8% of the wall thickness. The left display is for a normal probe the middle for a semi-adapted probe and the right for an adapted probe
252
NON-DESTRUCTIVE
TESTI NG.
OCTOBER
1974
J !
are identical to those used in the Rotomat, are mounted on pneumatically controlled arms. The various types of Tubomat differ according to the method of magnetisation used. For the testing of large pipes using the central conductor magnetisation method the pipe is transported onto rotating rollers and the central conductor is inserted and located by two tripods at the pipe ends. The pipe is magnetised, the arms are lowered on to the surface of the pipe which is now rotating and is then moved axially at the same time in order to spirally scan the whole surface (Fig.7). The test results obtained are fully comparable with those obtained from the Rotomat.
Fig.5 The main signal peaks on this oscillogram represent (left) an inner-surface defect of 10% and an outer-surface defect of 10% of the 8 mm wall of the 76.1 mm diameter tube
Fig.6 Another method of scanning for flux leakage is to rotate the tube while the probe is held stationary at the end of an adjustable arm
Although the use of a central conductor for magnetization produces the ideal uniform magnetic field in the tube wall, it necessitates considerable mechanical complications in order to introduce the conductor into the pipe; furthermore, very high currents are necessary to achieve the optimum field strength for testing, particularly with large-diameter pipes. For tubes with thinner walls the yoke magnetization system is considerably simplified. By this method a small yoke with a probe located between the pole shoes is placed on the tube (Fig.8). The probe is located between the rollers which guide it on the surface of the tube (Fig.9). Because of the non-uniform distribution of the magnetic field the test results obtained are not comparable with those obtained from the Rotomat or the Tubomat when used with a central conductor. Nevertheless, for practical purposes the results are extremely satisfactory (Fig. 10). The methods used in the Rotomat to obtain comparable signals from internal and external defects are again applicable to this system. Length ot tube already tested / Contact Testarm ~ ~ a w s Loading table Contact Marker \~\~ jaws ~ ~ A d a p t e r " \
\
\
~
~
be reliably detected to API Standards. When the surface finish is poorer it is still possible to satisfy API Standards for a 10% defect depth. In order to satisfy the requirements of the many kinds of application it is necessary for the basic system to be capable of modification and extension. As already mentioned, different rotating heads are available for various diameter ranges. In addition, an automatic probe lifting mechanism is incorporated to protect the probes from ragged tube ends. The standard versions of the testing systems incorporate equipment for the automatic colour marking of defective sections. In order to facilitate rectification work, an additional sector marking system can be added which uses up to 16 spray guns, and an electronic control system to mark accurately the defective areas of the tubes in both the longitudinal and circumferential directions. An extra spray gun is employed to mark the tube to indicate whether the defects discovered are in the external or internal surfaces.
J\
detects
\Friction roller for rotation Conveyor chain
~pporting rollers
Discharc~ of tube Fig.7 The inspection of large tubes needsa special handling system and a number of test arms Testing yoke
Pneumatic litt
Marker
/-
~ ' ~
.
The Tubomat
In contrast to the Rotomat, the magnetizing and probe system in the Tubomat remains stationary while the tube
being tested rotates. The arrangement of the probes is the same in all versions of the Tubomat (Fig.6). The probes themselves, which together with the electronic equipment
NON-DESTRUCTIVE TESTING . OCTOBER 1974
rack
Loading rack Fig.8
Thin-walled tubes can be tested in this kind of arrangement
253
/
",//',, '4'
Fig.11 Probes can be oscillated as the tube turns to give a t h o r o u g h scan Fig.9
The probe is the light object between the two metal shoes
I Fig.10 A signal can be clearly distinguised from the noise for this 1.2 m m defect on the inner surface of a 10 m m wall of a 270 m m diameter tube
If transverse defects are suspected in tubes, due, for example, to the use of special manufacturing methods such as centrifugal casting, it is necessary to magnetize the tube in the longitudinal direction. This can be achieved by the appropriate arrangement of coils. To scan the tube surface it is once again necessary to produce relative spiral motion between the tube and the coils and probes. The tube is rotated and also moved axially in relation to the coils and probes. The scanning of the tube surface is facilitated if the probes are oscillated in the longitudinal direction. The resuiting scanning path follows a sine curve which considerably widens the zone of detection covered by each probe and also allows filtering methods to be used to isolate transverse defect signals from those arising from variations in the magnetization and other irregularities (Fig. 11). For a tube with an artificial defect (a sawcut 1.6 mm deep) results were obtained with the probe oscillating to and fro, and as the pipe passed spirally through the tester (Fig.12). The signal-to-noise ratio was excellent (Fig.13). The equipment is also able to detect the presence of porosity in centrifugally-cast tubes (Fig.14). The presence of porosity is not so much indicated by a high signal amplitude, but rather by the location of a large number of signals grouped together at a particular spot. Comparing the defect signal amplitude with the actual defect depth the resulting scatter indicates the limits within which the presence of transverse defects can be detected with such a magnetic testing system (Fig.15).
254
Fig.12 A transverse saw cut 1 x 1.6 m m in a centrifugally-cast tube is shown by: above left - p h o t o g r a p h y ; above right -sectioning; and below - recording from the oscillating-probe system
. . . . . . .
I
I
= I II
I
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fig.13 A hot tear in a tube could equally well be detected: above left - photograph; above right -- section; below - test recordi ng
NON-DESTRUCTIVE
TESTING.
OCTOBER
1974
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Fig.14 Porosity is clearly shown b y comparing the recording f r o m porous tube (above) w i t h t h a t f r o m sound t u b e
40 Fig.16 The test head (right) of the rotating-disc equipment can be raised and lowered to fit a production line c U
30
c
.o
• Hot teors
u "O c
• Cold weldin9 costin 9 spirol 20
•
m
Needle holes
U O~ O /
IO
5 Defect depth [ram]
IO
Fig.15 The limits of detectability for defects can be shown graphically
The D i s c o m a t It is advisable for the weld seam zone of welded tubes to be tested for defects as soon as possible after the weld has been made on the production line. The method is to magnetize the tube transversely to the direction of the weld as it passes through a magnetizing yoke (Fig.16). The weld zone is scanned by sensitive probes mounted on a disc rotating at high speed. If the disc carries five probes and rotates at 50 rps it produces 250 scans per second which, assuming the effective scanning width of each probe to be approxima-
Fig.17
tely 5 ram, is sufficient to give 100% scanning of the seam at the maximum welding speed which can be expected. The same equipment illustrates how the testing head with the rotating disc and the magnetising yoke can be raised, lowered, moved to the side or rotated about the tube axis to suit conditions on the particular welding production line (Fig. 16). Various types of probes can be mounted on the disc so that, as discussed below, not only internal and external defects can be detected but also geometrical irregularities in the seam such as offsetting of the edges. The electronic equipment is designed to permit the inclusion of additonal amplifier units for these functions. A tube was deliberately welded using a lower power than that specified and the tube was subsequently tested (Fig.17). At the end of the trace obtained the signal is seen slightly overshooting the range of the recorder, revealing a 1 mm diameter hole in the weld seam. The tube was then cut through at certain points and the resulting sections were examined under the microscope. Subsequently the pieces of pipe were expanded by a given percentage using a drift and again examined under the microscope. It can be seen that defects which open into the internal or external surfaces clearly coincide with the threshold of the automatic discriminator indicated in the trace. Off the other hand there are places where there is only a minute defect signal but which, when the pipe is expanded, open up into large defects. This is caused by the phenomenon known as cold welding in which a joint is made but because of the low energy used, a seam of insufficient strength is produced. Such defects do not cause sufficient leakage flux to make this a reliable method of detection. Direct methods of testing for such defects cannot be used and it is suggested that the heat transformation in the seam be measured in order to check for such cold-welded spots.
Faults in a t u b e of 48.3 mm diameter and 3.5 mm wall thickness were f o u n d b y the rotating disc method and photomicrographs made
of sections at the faults
N O N - D E S T R U C T I V E T E S T I N G . O C T O B E R 1974
255
Fig.18 Three temperature traces were made with different welding voltages and relative upsetting pressure. The voltages were 2.6, 1.6, 1.6 V, the relative pressure 1, 1,0.77. The last trace is for a faulty tube
~
Cold weldin9
Sparkin9
30
1
~25 t_
. ~
E
Ex~nsi~ test
20-
Faulty ndirKj test
E
o Faulty
15-
IO
I I.O
I t 1.5 2.0 W¢ldin9 voltage [V]
I 2.5
Fig.19 The temperature record clearly indicates the weld quality at a given welding voltage
A system can be used which uses a rotating head similar to that of the Discomat, but fitted with photodetectors instead of magnetic sensors for measuring the temperature and expansion of the weld seam. If measurements are
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taken longitudinally at two displaced positions it allows the temperature uniformity to be checked in the whole seam area at the sides and in the depth. Temperature traces were taken from welds made with different supply voltages (Fig. 18). Also a graph was drawn of the measured values from these diagrams plotted against the welding supply voltage (Fig. 19). The results of additional destructive tests were also noted. It can be seen that there is a clear correlation between the weld temperature - and hence the heat transformation achieved in the seam - and the welding supply voltage. Above a certain value on the temperature trace the destructive testing reveals a good weld. ~ The Discomat and the conventional monitoring equipment in a welding production line can be combined to form a central monitoring position which can supply all necessary data for optimising the welding process. In addition to welding voltage, current, speed, the infrared detectors in conjunction with the Discomat with its various probes can indicate the heat transformation in the weld, the position of the weld under the testing head, edge offset, geometrical data on the weld and also indicate external and internal defects. A trained operator can use this display to determine the necessary corrective action to be taken in the production process. References
F6rster, F. Non-destructive testing for defects in the welded zone in pressure-welded tube under the special conditions of weld dressing. Paper presented at the International Welding Fair (Zagreb 1972) Krffcher, H. Research into the recognition of defects in boiler tubes using the eddy-current and flux-leakage methods. Proceedings of 6th ICNDT (Hanover 1970)
N O N - D E S T R U C T I V E TESTING . OCTOBER 1974