1. Functionally graded components for nuclear applications
- Author
-
Stavroulakis, Emmanouil, Preuss, Michael, and Pickering, Edward
- Subjects
austenitic-ferritic ,Functional grading ,Hot Isostatic Pressing ,SA508 ,316L - Abstract
Functionally graded components are considered for the next generation nuclear reactors, where two dissimilar metals are required to operate in the framework of a single component. High quality dissimilar metal welds are difficult to produce and suffer from significant residual stresses. Functional grading may address the problem by creating a gradual transition in alloy chemistry and therefore in mechanical properties between the two parent materials. This work concerns in particular the joining of the SA508 grade 3 low-alloy ferritic steel of the reactor pressure vessel and the 316L austenitic stainless steel of the piping. In the present work functional grading is simulated by mixing various fractions of the respective steels in powder form to create theoretical layers of a transition zone. The mixed fractions are then consolidated by Hot Isostatic Pressing (HIP). Detailed phase fraction analysis reveals that the resulting ferrite over austenite phase ratio after HIP processing is not linear but increases dramatically in the midrange of the powder mix. Chemical and microstructure mapping reveals that this is a result of the dilution of austenite stabilising elements from the stainless into the low-alloyed steel powder, which is enhanced at the high temperature conditions of the HIP cycle. The extend of element diffusion and its effect on phase transformation is studied by performing diffusion couple experiments and diffusion simulations. Furthermore, refinement of the ferritic microstructure takes place due to the alteration of the Continuous Cooling Transformation (CCT) diagram by diffusing alloying elements. The latter results in an increase of the hardness and decrease of the ductility of the component. The findings are discussed in view of the suitability of functionally grading components for achieving a linear transition of mechanical properties that could potentially substitute part of the dissimilar metal welding in nuclear reactors.
- Published
- 2021