Cameron Sinclair joined TWI in October 1990 having spent four years in the nuclear industry as an NDT development engineer. Having practical experience in application of NDT methods and interpretation of results, he is currently working in the Applications Section of the NDT Research Department.
Whilst employed in the nuclear industry, he worked extensively on development of high integrity, automated NDT procedures. Eddy current inspection of steam generator tubing and ultrasonic inspection of pressure vessel nozzle to shell welds are his main interests.
Expanding applications of duplex stainless steels have increased requirements for suitable inspection and testing techniques for these materials. A Group Sponsored Project (GSP) is underway at TWI to investigate the effectiveness of non-destructive testing of duplex stainless steel, as project leader Cameron Sinclair reports.
Duplex stainless steel, with an approximately 50:50 ferrite to austenite phase balance, is a material that over the last 20 years or so has found a great many applications where its high resistance to corrosion and good mechanical strength can be exploited. Examples include pressure vessels and pipelines in the petrochemical industry, and large forgings such as nozzles in the nuclear industry.
To ensure quality in such components, NDT is performed during manufacture, prior to service and whilst in service. Therefore, as new applications for materials such as duplex steels are developed new NDT techniques must also be developed.
NDT Techniques
Three NDT techniques are widely applied as listed below with brief details of their mode of operation for readers not familiar with NDT methods.
Magnetic particle inspection (MPI)
This technique makes use of the material's magnetic properties, i.e. its permeability. It can therefore be applied only to materials that can be magnetised. Where there is a discontinuity in the material, the lines of magnetic flux tend to be forced around the feature. If the discontinuity is near to the surface, some flux breaks the surface. It is this leaking flux that attracts magnetic particles and thus reveals the presence of discontinuities.
Radiographic testing (RT)
This technique makes use of the difference in absorptivity (of X- or gamma-rays) between the material under test and voids in that material. In the presence of a flaw, less radiation is absorbed by the specimen, so more radiation impinges on and darkens the radiographic film. This means that buried flaws can be detected, but planar flaws may only be revealed if their plane is aligned with the radiation beam. Visual evidence of the weld can be produced i.e. a radiograph.
Ultrasonic testing (UT)
This technique makes use of the reflection of sound waves at discontinuities in a material. Several beam angles can be used to make the technique sensitive to flaws in several planes and, using pulse-echo techniques, the range to a given reflector can be measured. This allows the depth of the flaw to be evaluated.
Application
Duplex stainless steels contain sufficient ferrite to make MPI possible. However, the ferrite may not be uniformly distributed across a weld in this material. The net effect is that magnetic flux leakage might occur at ferrite to austenite interfaces creating spurious flaw indications. A condition where this effect can be most noticeable is where the weld cap has been removed and an MPI indication appears along the parent metal to weld metal interface.
RT can be applied to welds in duplex steels using the same techniques that have been tried and tested over many years in other steels. Because the wavelength of the radiation is very much smaller than the grain size of the material there is little change in the scattered energy between ferritic and austenitic steels.
Further, RT relies on the transmission of energy with as little interaction with the material as possible. Therefore, RT is unaffected by changes in the solidification structure of the weld. The technique can however be disruptive to production because of its inherently hazardous nature.
UT in constructional ferritic steels has proved to be an excellent alternative to RT. It can provide information on the throughwall extent of a flaw and is more sensitive to crack-like flaws than RT. UT practitioners require thorough and standardised training, as detection, classification and sizing of flaws can be highly subjective. Further, particularly in the presence of noise, a great deal of concentration is required to ensure that the probes are scanned correctly and no reportable signals on the A-scan display are missed.
However, UT lends itself to automation, which can take much of the burden off the operator. There is a great diversity of instrumentation available, allowing the purchaser only to procure that which is necessary to meet a given specification. Automation can significantly increase the advantages of UT over RT in applications that warrant the expenditure. Unfortunately, in austenitic and duplex stainless steels, UT can be severely limited in practice.
UT of duplex steel
Welds in duplex stainless steel, exhibit a characteristically anisotropic solidification structure. That means that the material does not have uniform elastic properties. In addition, the grains comprising this structure are relatively large. A typical macrosection of a weld in a duplex steel shows a characteristic fan like distribution of grains ( Fig.1). Either side of the weld however, the parent material might have a predominantly banded structure as a consequence of having been mechanically worked. As a result of this, two effects are immediately apparent when applying sound waves to such a weld using traditional UT procedures: Attenuation The ultrasound does not penetrate far.
Backscatter The A-scan display is seemingly full of indications that are related to the probes' position but appear right across the time-base ( Fig.2). Because the grain size of the material is of the same order as the wavelength of the ultrasound, the beam reflects from the grains themselves.
Another less obvious effect is:
Beam skew The velocity of the ultrasound is a function of the orientation of the beam with respect to the grains. Also, in the same way as the ultrasonic beam is refracted at the metal to probe-shoe interface, the beam tends to refract at the boundaries between grains. The net effect is that the beam tends to skew towards preferred directions ( Fig.3). Whilst the beam is being skewed, its velocity is changing. Where beam skewing occurs, the flaws can plot out in the wrong position.
Project objectives
The primary objective of the GSP is to ascertain empirically the limitations of the UT technique (and, to a lesser extent, MPI) in at least two material/thickness/weld procedure combinations. A 25% chromium superduplex material with thick (about 70mm) section will be used in the study along with a thin (about 20mm) section 22% chromium duplex. The thicker section material is representative of that which might be found in pressure vessel manufacture, while the thinner section material is more representative of pipeline applications.
Clearly, the thinner section material will have a well banded, wrought structure, outside the weld, because of the extent of rolling during its manufacture. Conversely, the thicker section material will more closely resemble cast components. Regarding UT, the optimum UT probe characteristics will be ascertained and the benefits of automation in terms of imaging and data processing will be quantified. The frequency, size, wave-mode and configuration of probes can have a marked effect on the technique's efficacy.
Imaging allows the operator to discriminate immediately between reflectors in the material that have significant length or width and reflectors that are grain-like. Signal processing can prove valuable in filtering out flaw-like echoes from grain-like echoes using some characteristic unique to the flaw echo.
During the investigation into signal processing, TWI will be collaborating with Uppsala University, Sweden, which is developing its split spectrum processing technique. This is a technique whereby a number of received ultrasonic waveforms are digitally sampled and analysed in the frequency domain. By comparing the magnitude of the signals at certain frequencies it is possible to discriminate between flaw echoes and grain response.
A conversion of the data back to the time domain allows the range of these resolved echoes to be ascertained. With sufficient experimental data and practical experience to validate the process, it is envisaged that split spectrum filtering could ultimately be performed in real-time with minimum operator interaction. A secondary objective is to perform some MPI on welded testpieces produced to investigate the efficacy of the technique compared with ferritic steels.
With the resultant data, the sponsors of the project will be able to determine whether UT and/or MPI could be viable for their application and will have the essential basic information on which to base an NDT specification. The project is therefore of greatest interest to organisations involved in: development of duplex stainless steel as a material; specification of NDT of duplex components; licensing/insurance of operators using duplex; and development of ultrasonic probes and instrumentation.
