European Technology Development

Steel will be an essential part of any commercial fusion reactor design. Applications in this area involve extreme conditions, imposing particular performance requirements, such as high operational temperature and creep resistance, and also a limitation on the elements that can be used due to the activation that occurs on interaction with irradiation. This work begins with a steel developed for conventional power plant applications, the IBN1 grade developed by IMPACT (a UK consortium of industrial and academic research organisations). This grade has shown excellent properties at high temperature due to high temperature-stable precipitate phases, but contains several elements that would become radiologically active to a degree that is incompatible with the required disposal routes after exposure to the fusion reactor environment. In this study, modifications of the composition are made to remove these elements, and thermodynamic modelling and experimental assessment of the phases that form are undertaken. In this, we have paid particular attention to the prediction of transformation temperatures (to understand if normalisation and tempering can be applied successfully) and the precipitates, to see if suitable phases that are likely to impart creep strength and other desirable properties would be formed. The modifications made include the removal of Nb, Mo, Ni, Co, Cu and Al from the starting alloy, and the substitution of Ta (intended to form carbides, replacing the effect of Nb). Modifications of the amount of retained elemental components, such as C, Mn and Cr, have been made with Thermo-Calc modelling, to ensure preservation of comparable phase transformation temperatures and microstructures. The predicted changes to the alloy are compared to the observations from experimental investigation, finding that tantalum can substitute for niobium in these systems and form similar carbides with similar distribution in the material, and that reduction of Cr to 8 wt% and increase of C to 0.12 wt% raises the Ae 4 temperature to allow a high-temperature heat treatment without δ-ferrite formation. While assessment of the mechanical properties of this alloy would be required, the perspectives for these alloys to perform at high temperature that can be inferred from the microstructure are discussed.

ABSTRACT To help determine remaining lifetime of pressure vessels suffering creep, the authors have previously developed a method and presented promising results using a combination of AC and DC electrical potential drop (EPD) on-line monitoring, detecting both final cracking as well as incipient creep damage. The latter was tentatively ascribed to the development of cavitation damage, but recent modelling and separate off-line measurements have shown that cavitation is unlikely to provide enough of a change in electrical properties to explain all of the variations previously observed. Here we gather the results obtained to date, and review their likely relationships in an attempt to obtain a greater insight into the mechanisms at play. Whilst changes in both on-line and off-line EPD are largely in accord, the belief now is that the changes seen cannot be fully explained by cavitation development and that EPD is responding to other creep induced phenomena as well.

The practical use of defect assessment procedures for industrial component integrity assessment is described through two practical examples in this paper. In the first example, the procedure is used to perform low temperature fitness-for-service (FFS) analysis of a longitudinal seam welded vessel manufactured from a duplex stainless steel. The impact energy obtained from the Charpy impact test performed on the weld of the vessel was found lower than the value of the minimum impact energy criteria given by the British Standard BS 5500. Charpy impact energy and fracture toughness correlation and failure assessment diagram (FAD) methodology were used in this FFS analysis to determine the fracture resistance of the vessel. For the assumed defect size used in the assessment, the weld was found to meet the fracture resistance criteria and therefore would still be fit-for-purpose. The FAD analysis was, however, repeated using the J-values obtained from the CTOD test to gain better confidence as the Charpy impact test does not provide direct measurement of the fracture toughness. The FAD was later used to determine the critical surface crack length which was then represented as a function of crack depth. The Charpy impact energy correlation method was found to be more conservative than the method of evaluation using the CTOD J-values. The lack of side wall fusion (LOF) which is a typical defect in this type of vessel would usually be influenced by the size/diameter of the electrode wire used in the weld and the number of runs. In this case, the fracture resistance of the 15mm thick vessel with the longitudinal seam weld should be adequate if less than 5mm diameter of electrode wire is used.The second example illustrates defect assessment of a high temperature plant component. A defect was found in the high pressure final superheater header. A defect assessment incorporating FAD on the header showed that the defect was non-critical. This led to the need to perform creep and fatigue crack growth calculations and remaining life assessment in order to determine the mitigation plan for the engineers. The deterministic approach, which mainly considers the worst case scenario, suggested that the remaining life of the header was approximately 4.5 years. Probabilistic analysis showed that the component could still be fit for service up to 6 years. This will allow the engineers to mitigate a more efficient plan with a decision to either repair or replace the header and when. The use of probabilistic lifing methodologies and algorithms could consequently bring considerable financial benefits to the plant owners/operators e.g. by avoiding premature component repair or replacement. Nevertheless, it would be in the management’s interest to avoid a forced outage. The recommendation would be that the component could still be fit to use until their next minor outage (in 3 years). From then, the remedy option would be to grind down to a depth of suspected crack, excavate a small area/surrounding which might be affected, followed by correct and regular monitoring, or alternatively replace the component.

Steels such as P91 and P92 (9Cr martensitic steels) with higher creep strength were initially developed for use in the newer ultra supercritical power plants at temperatures above about 1050°F (570°C). However, soon these steels became popular materials as replacement components for conventional power plant and for HRSGs (Heat Recovery Steam Generators) operating at relatively lower temperatures. The main attraction was in terms of their lower weight per unit volume thus reducing construction, transportation and welding costs. Furthermore, due to privatization and restructuring of the electricity industry many of the existing power plants are now required to operate in cycling mode and this requires the use of materials with high resistance to thermal fatigue. Here high strength steels offer an additional benefit in that the lower section thickness reduces the level of through wall temperature gradients in thick section components. Because of this envisaged additional benefit a number of operators/owners of the existing plant have been replacing some of the ageing components in their plant with those made from these higher strength steels. For the HRSGs, there is a requirement to produce more compact units and here the higher strength steels are used to make smaller size components. This paper discusses these issues and the perceived benefits with the actual plant and more recent R&D experience. It further refers to the potential future techniques for damage detection and life estimation.