Abstract
The Additive Manufacture (AM) of nuclear plant components using Laser Powder Bed Fusion (LPBF) technology, offers opportunities to reduce costs and improve production lead-times. Cost reductions can be achieved by reducing the amount of raw material required, reducing the amount of machining conducted, and eliminating the welding together of sub-assemblies.
However, AM is a relatively new manufacturing technology, and although now seeing greater usage in industry generally, there are still very few examples of where the technology has been applied to components used in safety critical applications. Furthermore, it is not currently covered by the American Society of Mechanical Engineers (ASME), Section III, nuclear design code. For nuclear plant applications, it is imperative that accurate and robust material property data is established to input into design assessment and analysis.
This paper presents Rolls-Royce’s approach to the derivation of a fatigue design curve specific to AM LPBF 316LN stainless steel with Hot Isostatic Press (HIP) process step, for use in ASME III, Subsection NB-3200 or NB-3600 fatigue crack initiation assessments on nuclear plant applications within its portfolio, including small bore globe valves and pipework tees and reducers.
As part of a safety justification strategy for the implementation of AM LPBF small bore globe valves for nuclear plant Rolls-Royce carried out an initial suite of materials testing, including in-air uni-axial fatigue endurance tests on AM LPBF 316LN stainless steel samples. The limited data generated was seen to fall within the expected scatter when compared to the data upon which the NUREG/CR-6909 air best-fit curve is based. However, on completion of further testing in various test orientations on multiple powder batches and builds a more complex fatigue behaviour was observed.
This paper provides an overview of the investigation into the orientation effects on the material fatigue behaviour, especially at high strain amplitudes, and how a combined best fit curve has been constructed for this material using either a fit to the limiting orientation data or the NUREG/CR-6909 mean curve at each point across the S-N curve, whichever is lower. A fatigue design curve is then produced from the best fit curve by first applying a mean stress correction and then applying transference factors on stress and on cycles to account for material variability, component size and surface finish, and load history.
The fatigue design curve generated specific to AM LPBF 316LN stainless steel is judged to be suitably conservative for the design assessment and analysis of the material on nuclear plant applications, including the AM small bore globe valves which have also undergone supporting ASME, Section III, Appendix II, thermal cyclic testing.
