Qualification of inspection for SCC3 August 2003
Inspection using non-destructive testing (NDT) methods should be able to detect, characterise and size any defects before they grow to sizes of structural concern. However, the combination of complex morphology and material properties presents significant challenges for NDT. By Will Daniels, Peter Roscoe, David Tice, Chris Waites and Christopher Udell
Stress corrosion cracking (SCC) has been observed in both BWR and PWR systems. In BWRs, cracking has occurred in both stainless steels and nickel-based alloys, whereas the latter appear to have been most susceptible in PWRs. Recent examples are:
• In BWR plant, extensive cracking has been observed in both the stainless steel core shroud and in Alloy 600 core shroud support structures (for example the H9 weld at Tsuruga, Japan).
• In PWR plant, extensive cracking has affected Alloy 600 components in the steam generators. Control rod drive mechanism (CRDM) penetrations have been affected in a number of plants worldwide, with both parent tubing (such as at Bugey 2 in France) and J-welds (at Oconee and Davis Besse, USA) having suffered cracking. Cracking has also recently been observed in primary circuit nozzle transition welds (V C Summer, USA and Ringhals, Sweden).
For non-destructive testing (NDT) to play an effective part in management of susceptible plant, it must be capable of detecting and sizing cracks of a size below that which could grow to compromise component integrity within the inspection interval. Once initiated, SCC tends to have a relatively high propagation rate. This means that NDT needs to reliably detect and size relatively small defects.
Detection and sizing of SCC defects present significant challenges to NDT due to the complex nature of the materials and defects. The challenge is particularly acute in nickel-based alloy welds.
The conditions for initiation and propagation of SCC are a susceptible material that is in contact with coolant and is subject to tensile stress. The difference in coolant chemistry between BWRs and PWRs results in different microstructure requirements for SCC to occur. Initiation of cracking often appears as generalised intergranular attack (IGA) with multiple surface
initiation sites. Cracking occurs when this generalised attack coalesces under stress into larger scale damage (IGSCC). It has been observed in some cases that the surface IGA is less easily detected than the coalesced cracking below the exposed surface. Indeed, there have been instances where SCC has been incorrectly reported by NDT as non-surface breaking.
The crack is constrained to near planar by the requirement for tensile stress the crack plane being perpendicular to the direction of the tensile stress. This said, cracking propagates along susceptible grain boundaries. The susceptibility of the grain boundary is determined by the local chemistry at the grain boundary; depending on the material and
environment, chromium depletion at the boundary may enhance crack propagation (sensitised stainless steel or
nickel alloys in BWRs) or carbide decoration may impede it (nickel alloys in PWR primary coolant). The grain boundary composition usually varies throughout the material and so the crack exhibits surface characteristics consistent with the crystal dimensions. It is also the case that the crack may branch or meander as the crack encounters new grain boundary intersections.
This branching process occurs in both the length and depth directions. In some cases, the crack may bypass some regions leaving uncleaved zones on the crack plane. When materials such as stainless steel or nickel-based alloys are used as weld filler, there is a tendency for the weld to manifest a strong texture with long columnar grains. These were formed as grains sought to align themselves with the direction of maximum heat flow as the weld metal solidified. This structure contributes strongly to the final surface morphology of SCC.
It is also characteristic for the defects to exhibit an irregular depth profile.
Influence on NDT performance
The main methods for detection of defects in these types of components are:
• Visual testing (VT).
• Eddy current testing (ECT).
• Ultrasonics (UT).
VT and ECT are principally surface inspection methods with little or no capability for detecting defects which have any ligament. SCC defects are always surface breaking so these methods do offer some capability, but it should be noted that the observed behaviour of the damage at the
initiation surface sometimes being slight means that both techniques must be sensitive and the possibility of missed defects and/or false calls is high.
Additionally, the fairly large scale ordering of crystals in weld material can cause ECT performance to be degraded by excessive noise.
UT is used for both detection and sizing of defects. However there are some issues for application of UT in these types of materials and for these types of defects. In forged plate form, the materials' crystalline structure tends to be suppressed. In weld metal, however, strong texture is generally observed. The main practical implication of this for inspection is that ultrasound beams can be distorted. Figure 2 shows a prediction of ultrasonic beams in a typical BWR instrumentation RPV penetration. Note the bending of beams in the Inconel 182 weld material.
While use of experience and NDT modelling is becoming increasingly important for determining NDT capability, performance in experimental trials has been the main tool for assessing NDT capability. Experimental demonstration is particularly important for SCC in nickel-based alloy components given the complex nature of the inspection problem and the requirement for detection and sizing of small defects.
The conventional method of performing experimental trials for qualification is to manufacture testpieces containing artificial 'defects' which comply with the inspection scope in terms of their morphology, extent, location and orientation. While sophisticated approaches have been established, the normal approach is for the testpiece to closely resemble the plant item to be inspected.
Commercially available artificial defects used in these
testpieces have been manufactured by a variety of processes. There are a number of issues with the use of such testpieces. Defects are either manufactured in situ or implanted. Implantation is usually by welding. In ferritic materials this process has been used to great effect. This is because this type of weld material generally does not form large crystals during welding and so the implant weld can be practically indistinguishable from the component material using NDT. However, in materials such as the nickel-based alloy welds discussed here, which exhibit strong texture, any implant weld will influence the inspection performance. The influence of the implant weld is a move away from the plant condition. In this way, the linkage between inspection qualification results and performance in plant becomes difficult or impossible to establish.
Another issue is that the artificial defects may have very low levels of similarity to plant defects either in their morphology, orientation or location. Again, results from an inspection qualification based upon these types of defects are of dubious worth when trying to assess performance for real defects.
The main options to reduce the impact of these problems are:
• In some cases it may be possible to make use of actual plant data in trials.
• To improve understanding of the linkage between commercially available defect testpieces and real plant defects.
• To improve the representative nature of defects used in trials.
While all three of the options have merit, one method (MISTIQ) to address the last option is discussed here.
An alternative to the majority of processes used for defect manufacture for qualification purposes is to generate defects in very much the manner they arise in plant, but in an accelerated timescale. The resulting defect has the crack morphology of an SCC defect because it is an SCC defect (see Figure 3). There are no artefacts associated with implantation because the defects are grown directly in the testpiece which will be used for qualification.
Serco Assurance has been developing a process known as MISTIQ, which has been used to produce defects for inspection qualification and performance demonstration programmes worldwide.
For a particular application, use of the MISTIQ process involves:
• The manufacture of a testpiece, which is representative from an inspection perspective of the plant item using typical materials. A condition for the success of the process is that the materials used must be susceptible to SCC. Where detailed knowledge of the material is available, this can be predicted. Susceptibility can also readily be tested for small material samples.
• Subjecting the component to tensile stress.
• Exposure of the component to a corrosive agent.
• Careful monitoring of the crack initiation and progression using NDT.
Both the cost and time involved in production of specimens using the process can vary depending upon the complexity of the testpiece that is manufactured. Costs tend to be comparable to more conventional specimen manufacture. Defect growth rates under the MISTIQ process are typically several millimetres per day once cracking has initiated.
Using this method a range of testpieces have been manufactured for use in qualification and performance demonstration programmes for ultrasonic and eddy current inspection of:
• PWR CRDM tube cracking initiating in the J-weld or tube.
• PWR CRDM J-weld cracking shallow defects initiating on the wetted surface and larger defects propagating towards the J-weld triple point.
• Generic transition welds both parallel and transverse to the welding direction (see Figure 4).
It has been demonstrated that the process is applicable to production of large-scale testpieces for use in assessment of cracking in BWR core shroud structures. The process has also been used to produce a series of Inconel 182 weld and stainless steel testpieces suitable for use in generic investigation of SCC inspection performance assessment and qualification programmes.
Use of these types of testpiece does require some evolution of the normal qualification process. A feature of the MISTIQ process is that the absolute size of the defect is known only to the precision of the inspections used as part of the testpiece fabrication process. This means that in cases where size measurement is of paramount importance, it may be necessary to destructively examine testpieces at the conclusion of trials. Since the process is relatively inexpensive, inspection qualification with some level of destructive examination is a practical option particularly if plant owners with similar plant can share the benefits and costs of the exercise.
Another option in inspection qualification is to use a mixture of highly realistic defects and defects with lower levels of realism but well defined sizes. This means that the capability of the inspection to work with genuine SCC defects can be assessed as can critical sizing.
Since defects are grown in situ, it is possible to inspect throughout the manufacturing process. This means inspection can be applied to the defects through life from initiation to failure (if required) and can give useful information about the absolute capability of inspection methods.