By C Allen, A Barnes, M Dodge, E Davison, A Addison, C Carpentier and N Capar
In the power generation industry, there are a number of applications where transitions between dissimilar metals are required. Significant plant outage and repair costs are often encountered due to failures or degradation of dissimilar metal joints, typically associated with inadequate joining strategies, processes or manufacturing methodologies for such transitions.
Particularly, in the nuclear sector, it has been a hot topic for many years and yet no alternative solution has been derived. Ferritic steel reactor pressure vessels (RPVs), pressurisers and steam generators are joined to stainless steel piping systems through complex dissimilar metal welds (DMWs). Typically, these joints comprise several Ni Alloy buttering layers on the ferritic side, followed by a sequence of Ni alloy and/or stainless steel arc welds, until finally reaching the stainless steel pipe. Ferritic grades such as 2.25Cr 1Mo (T22) offer a cost effective combination of strength, creep performance and oxidation resistance at lower temperatures (<600°C), compared to austenitic grades. However at higher temperatures, where enhanced oxidation resistance and higher creep strength is required, more expensive austenitic grades such as 316 or 347 austenitic stainless steel, alloy 800H and/or alloy 600 are required, and thus dissimilar metal transitions become necessary.
Additive manufacturing (AM) techniques have developed considerably in the last decade, and through the use of novel techniques known as direct energy deposition (DED) such as twin power source tandem arc also or laser beam delivery systems combined with coaxial wire and/or powder feeding nozzles , offer the potential to produce functionally graded materials (FGMs). However, in addition to the design of successful process, it is essential that metallurgical issues for the various FGM transitions should be given consideration to ensure the quality of the fabricated FGMs, their properties and fitness for service.
- The literature review is showing, metallurgical issues encountered in FGMs produced by DED powder or wire feed, most notably pointing out brittle intermetallic and/or crack-sensitive compositions. Use of one or more intermediate layers or a ‘bond coat’ during grading might be one solution, but it is then arguable to what extend this reintroduces an undesirable discontinuity in to the overall part. Removal of such discontinuities is precisely one of the drivers for moving away from conventional industrial welding solutions (i.e. DMWs) for dissimilar joints, and towards FGMs.
- In conventional dissimilar metal welds (DMWs) microstructural inhomogeneity, and the loss of creep strength in the carbon-depleted zone that develops, exacerbated by differences in thermal expansion coefficient between the dissimilar materials, can lead to premature service failures. Studies carried out using FGM to avoid these abrupt changes, considered a variety of material combinations. Literature review shows that it was possible to create a carbon steel to stainless steel transition piece, with a gradation in both microstructure and toughness; in the DMW the compositional change occurred over a distance of ~80µm, compared to ~50mm for their graded T22 to 347 joint (i.e. three orders of magnitude greater).
- The literature review has revealed there was a notable absence of detailed microstructural characterisation within the transition pieces. Although some solidification cracking was reported, the potential for formation of deleterious phases when subsequent layers are deposited, or for in-service degradation, was not addressed. The use of predictive tools, either computational models or compositional diagrams, to predict from the layer chemistries and thermal cycle the point(s) in the AM sequence when such issues would arise, could allow modifications to be engineered, to mitigate such issues.
- The review proposes that modelling be used to determine whether a build is likely to be free of defects and acceptable, and to assess the potential fitness for service, prior to practical trials. This approach will save time in the laboratory and reduce experimentation, instead making use of intelligent tools such as microstructural modelling and finite element analysis (FEA). This may also have the added benefit of being able to reduce the number of layers required to transition from one material to another, and therefore the overall amount of material required to produce a deposit, thereby improving cost-efficiency.
- The literature review has revealed the important to consider in process testing and non-destructive testing immediately after fabrication in order to verify the quality of the built before any further high value activities are done to the built. Several methods have been identified such as Electromagnetic Acoustic Transducer (EMAT), Eddy Current Array (ECAT) and Laser Ultrasound (LUT), with arguably the first two of these the easiest to apply once specifically developed for the carbon steel to stainless steel FGMs on interested to the power generation and chemical plant industries. The work in the project will provide understanding of the conditions for which the ECAT and EMAT inspection techniques identified can be applied for the testing of the transition piece during manufacturing.