Thermal Fatigue of Additive Manufacture Laser Powder Bed Fusion And Wrought 316LN Stainless Steel Prototypic Small-Bore Globe Valves – A Comparative Study

Additive Manufacture (AM) of plant components using Laser Powder Bed Fusion (LPBF) technology, offers opportunities to reduce costs and improve production lead-times, in addition to enhancing performance if Design for AM (DfAM) can be used. However, utilisation of AM technology in safety critical applications is limited and is currently a key area of development for high integrity pressure vessel codes such as Section III of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code. Robust understanding of fatigue behaviour is imperative for the safety and longevity of nuclear components in a Pressurised Water Reactor (PWR) environment. This paper outlines the findings from the testing and subsequent destructive examination of two 316LN stainless steel test articles prototypic of small-bore globe valve bodies, subjected to thermal transient cycling. One valve body was manufactured from AM LPBF followed by Hot Isostatic Pressing (HIP) consolidation and the other from traditional wrought material. In this testing, loading was applied purely from thermal cycling at constant pressure. Application of a cycle number far in excess of the code allowable cycles resulted in the initiation and growth of fatigue cracks, but short of functional failure or leaking. This enables comparisons to be made between the AM LPBF and wrought test articles and assessment of margins between code allowable cyclic service and total useful life. The destructive examination, incorporating striation counting of the fracture surfaces, suggests no significant difference in fatigue crack initiation or growth behaviour between AM LPBF and wrought material. This work supports efforts to develop the code and utilise these modern manufacturing methods in safety critical applications. The observation of stable, tolerable flaws following cycling well beyond code allowable, indicates the conservatism of current code for fatigue and potentially supports future development towards a total life approach.