Argonne National Laboratory investigates effects of heat treatments on 316H and A709 alloys.
Setting aside the aerospace and medical device industries, energy and power generation represent perhaps the most fecund set of industrial applications for additive manufacturing (AM). The combination of extreme environmental conditions and the need for complex geometries (such as in impellers and other power-generating components) means that the design and material flexibility of AM is a perfect fit for this sector.
Add in the recent interest in the nuclear power industry (driven by the incredibly power-hungry data centers supporting artificial intelligence), and it should be no surprise that interest in AM applications for nuclear power appears to be growing at a commensurate rate. The latest example of this enthusiasm comes from Argonne National Laboratory, where researchers have been investigating the viability of using 3D printed components in the next generation of nuclear reactors.
More specifically, they’ve been using X-ray diffraction and electron microscopy to investigate steels made using laser powder bed fusion (L-PBF). In one case, they examined L-PBF 316H as a candidate for structural components in nuclear reactors and, in a separate study, looked at Alloy 709 (A709), which is specifically designed for reactor applications.
According to the researchers, both studies revealed important differences between the 3D printed steels and their conventionally forged counterparts, as well as how the former respond to heat treatments typically used for wrought materials.
“Our results will inform the development of tailored heat treatments for additively manufactured steels,” said Argonne materials scientist Srinivas Aditya Mantri, a co-author on both studies, in a press release. “They also provide foundational knowledge of printed steels that will help guide the design of next-generation nuclear reactor components.”
In comparing the microstructure of 3D printed 316H, Mantri and his colleagues used X-ray diffraction from Argonne’s Advanced Photon Source (APS) to reveal that the recovery and recrystallization of the steel were inhibited by nano oxides.
“Nano oxides act as a sort of barrier to the movement of dislocations and the growth of new grains, causing some dramatic differences between the response of L-PBF-printed and wrought steels to heat treatment,” said co-author and Argonne materials scientist Xuan Zhang, in the same release. “For example, the printed samples started to recrystallize at temperatures several hundred degrees higher than their wrought counterparts.”
The more recently developed alloy, A709, is designed for high-temperature environments such as those inside sodium fast reactors, a next-generation nuclear technology that operates at higher efficiencies compared to conventional designs. According to Argonne, this is the first experimental investigation of the material properties of additively manufactured A709.
They also studied the strengths of the heat-treated samples under tension. At both room temperature and 1022 F (550 C) – a temperature relevant to sodium fast reactor applications – the printed A709 displayed higher tensile strengths compared to the wrought A709. The Argonne researchers believe this was likely because the printed samples began with more dislocations, which also promoted the formation of more precipitates during heat treatment.
“Our research is providing practical recommendations for how to treat these alloys,” said Zhang, “but I believe our biggest contribution is a greater fundamental understanding of printed steels.”
The studies on 316H and A709 are published in the journals Materials & Design and Materials Science and Engineering, respectively.