Ultrasonic method of testing welded joints

Welded joints of austenitic steels are tested by ultrasonic sound caused to be incident on the joint at an oblique angle thereto. Information regarding the size and nature of reflectors in the joint can be derived from the amplitude of the echoes generated at the reflectors in response to the sound beam. To ensure a definite relationship between the amplitude of the echoes and the nature and size of the reflector, the frequency of the sound use for testing is below and upper frequency limit at which the wavelength of sound in the base material equals the largest peak-to-valley distance of the base metal weld interface in any of the consecutive areas of the interface on which the sound beam is incident at any time.

This invention relates to an ultrasonic method of testing welded joints, 
particularly joints of austenitic steel, wherein the sound waves 
consisting preferably of longitudinal waves are directed onto the joint at 
an oblique angle thereto and the amplitudes of the resulting echo pulses 
are measured as indications of the approximate size and nature of the 
reflector. The invention relates also to a seam weld which is adapted to 
be tested by the method. 
The testing of seam welds by transverse waves which are directed onto the 
joint at an oblique angle thereto permits a highly selective detection of 
faults in seam welds consisting of ferritic steels but does not permit 
such selective detection of faults in seam welds of austenitic steel 
because the coarser structure of the seam weld of austenitic steel gives 
rise to interference so that even large faults generally cannot be 
detected. The ultrasonic testing of austenitic welded joints has been 
greatly improved by the use of longitudinal waves which are directed onto 
the joint at an oblique angle thereto. In that case, the signal-to-noise 
ratio of the echo amplitudes has been further improved by the use of 
special sound transducer heads comprising suitable electroacoustic 
transducers. The selectivity with which faults in austenitic seam welds 
can be detected has been greatly improved by the use of such special sound 
transducer heads. On the other hand, it has not been possible to analyze 
the echo amplitudes for the size and nature of the fault because the 
amplitudes of the echoes which are received are highly independent from 
the size and nature of the reflector. 
It is an object of the invention to avoid these disadvantages and so to 
improve an ultrasonic method of testing welded joints, which is of the 
kind described first hereinbefore, that a proper dependence of the echo 
amplitudes on the nature and size of the reflector is ensured. 
This object is accomplished according to the invention in that the 
frequency of the sound used for testing is below an upper frequency limit, 
at which the wavelength of sound in the base material equals the largest 
peak-to-valley distance of the base metal-weld interface in any of the 
consecutive areas of said interface on which said sound beam is incident 
at a time. 
The invention is based on the recognition that owing to the known 
differences between the velocities of sound in the base material and in 
the weld the sound waves are refracted at the base metal-weld interface so 
that owing to the inevitable waviness of said interface the sound rays are 
deflected in different directions, depending on their angles of incidence. 
This phenomenon gives rise to interference and to partly considerable 
changes of the amplitudes from those which would be obtained in case of an 
undisturbed propagation of sound. Surprisingly it has been found that the 
interference which changes the amplitudes can be decreased to a 
nondisturbing value if the wavelength of the sound is not in excess of the 
largest peak-to-valley distance of the base material-weld interface in the 
area on which the sound beam is incident at a time. If the frequency of 
the sound used for testing is below the frequency limit that is determined 
by said peak-to-valley distance, the echo amplitudes will depend as 
desired on the size and nature of the reflector. As the sound transducer 
heads are operated at a frequency which is selected in consideration of 
the velocity of sound in the base material, the upper frequency limit 
which must not be exceeded if the object of the invention is to be 
accomplished is related to the wavelength of sound in the base material. 
Whereas strictly speaking the upper frequency limit depends on the 
wavelength of sound at the interface or in the weld, the velocity of sound 
at that interface and in the weld are not known in general. The selection 
of the upper frequency limit in dependence on the velocity of sound in the 
base material introduces an error but this is so small that it can be 
neglected in practice. 
It is believed that the fact that disturbing interferences which 
appreciably change the echo amplitudes do not arise unless the wavelength 
of sound exceeds the peak-to-valley distance of the base metal-weld 
interface in the area on which the sound is incident at a time is due to 
the presence of sound beams rather than discrete sound rays so that only 
the sum effect of the interferences rather than the individual 
interferences can be detected. Nevertheless, an increase of the wavelength 
of sound corresponding to a decrease of its frequency sound waves will 
further decrease the influence of the interference. For this reason, 
particularly favorable conditions will be obtained if the frequency of the 
sound used for testing is less than one-half of the frequency limit. The 
decrease of the frequency of the sound used for testing is limited by the 
desired resolution, which depends on the wavelength of the sound beam. 
To permit the use of sound at the desired high frequencies for testing, an 
excessive waviness of the base metal-weld interface in the area on which 
the sound beam is incident at a time must be avoided. To permit the use of 
sound at the conventional frequency for testing, the seam welds to be 
tested by the method should be so designed that the base metal-weld 
interface is substantially planar and its largest peak-to-valley distance 
in any area on which the sound beam is incident at a time does not exceed 
the wavelength of sound in the base material at conventional testing 
frequencies. As has been stated above, only the peak-to-valley distance of 
the interface in any area on which the sound beam is incident at a time is 
significant. For this reason the base metal-weld interface need not be 
exactly planar but may have a curvature if such curvature is so gentle 
that the peak-to-valley distance consisting of the difference in height 
between the lower portion defined by the sound beam and the apex does not 
exceed the wavelength of the sound beam.

It is apparent from FIG. 1 that in a welded joint the interface 1 between 
the base material 2 and the weld 3 does not exactly conform to the 
theoretical configuration of the seam but has a waviness which is due to 
the penetration of the several weld beads. As a result, a sound beam 4 
which is directed onto the joint at an oblique angle thereto is refracted 
at the interface 1 because the sound travels at different velocities in 
the base material 2 and the weld 3. This refraction of several sound rays 
is diagrammatically indicated in FIG. 1, which shows that owing to the 
waviness and the non-planarity of the interface the several sound rays are 
deflected in different directions, depending on the angle of incidence of 
each sound ray on the interface. This phenomenon results in an 
interference of the sound rays so that the amplitudes of the sound rays 
are changed. As a result, the amplitude of an echo which has been 
generated in response to the sound and is subsequently received depends 
not only on the size and nature of the reflector but also on that of the 
interference. Because the influence of these interferences during the test 
cannot be estimated, the echo amplitudes will not permit any conclusion to 
be drawn concerning the nature and size of the reflector if relatively 
strong interferences occur. 
The occurrence of disturbingly large interferences can be avoided if the 
wavelength of the sound beam 4 is selected in consideration of the 
waviness of the interface 1. The largest peak-to-valley distance W of the 
interface 1 in any area on which the sound beam 4 is incident at a time 
defines an upper frequency limit for the sound used for testing. If the 
depth-to-valley distance W of the interface 1 in any such area is less 
than the wavelength of sound and preferably less than one-half of the 
wavelength of sound, the disturbing influence of the interference will be 
negligible so that the echo amplitudes will be an indication of the nature 
and size of the reflector. Because the velocity of sound depends on the 
material and establishes a direct relationship between the frequency and 
wavelength of sound, the largest peak-to-valley distance W determines an 
upper frequency limit for the sound used for testing. The wavelength is 
related to the base material 2 as the velocity of sound in the base 
material is generally known whereas the actually significant velocity of 
sound at the base metal-weld interface or in the weld is not known. 
Whereas this gives rise to an error, the latter is not significant 
particularly at frequencies below the upper frequency limit. 
When it is desired to use sound at the usual frequencies for testing, the 
peak-to-valley distance of the interface 1 in any area on which the sound 
beam is incident at a time must be sufficiently small. This can be 
accomplished by the provision of base metal-weld interfaces which are as 
planar as possible and by the selection of suitable welding conditions. 
FIG. 2 shows a bell seam, which is particularly suitable for an ultrasonic 
test because the interfaces 1 are planar except for a waviness which in 
any area on which the sound beam is incident at a time has a largest 
peak-to-valley distance not in excess of the wavelength of sound at the 
frequencies which are usual for testing. In order to form such interface 
1, the root zone is removed, in accordance with FIG. 2, so that the 
original lands 5 are entirely eliminated and the constrictions otherwise 
formed in the base metal-weld interfaces are avoided. 
When it is desired to form a double-V seam, as shown in FIG. 3, weld ribs 6 
are deposited on the flat end faces of the base material 2. These weld 
ribs 6 constitute the lands required to form the seam weld. The weld ribs 
6 have no influence on the interference if the weld ribs exhibit the same 
acoustic behavior as the remaining weld. It will be understood that such 
weld deposits can be used also for other types of seams. 
Because only the largest peak-to-valley distance W within any area of the 
base metal-weld interface on which the sound beam 4 is incident at any 
given time is significant, the base metal-weld interfaces need not be 
planar but may have any curvature within the largest peak-to-valley 
distance which is determined by the upper frequency limit. The teaching of 
the invention to select the frequency of the sound used for testing in 
consideration of the waviness of the base metal-weld interface may be used 
to advantage where different velocities of sound give rise to a refraction 
of the sound beam at the base metal-weld interface and this refraction 
would result in interferences which change the sound amplitudes and the 
amplitude of the resulting echoes so that the latter may not properly 
indicate the size and nature of the reflector.