HEAT TREAT NEWS

Argonne scientists have been investigating 3D-printed steels for use in next-generation nuclear reactors. In two studies, they used X-ray diffraction and electron microscopy to reveal how heat treatments can help 3D-printed steels endure nuclear service.

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Crucial components within nuclear reactors are often made using stainless steel; it fortifies falls and withstands decades of extreme heat, pressure, and irradiation. Additive manufacturing — or 3D-printing — offers a way to produce complex stainless steel parts more efficiently, however it can leave behind defects in the microscopic structures of steel parts, impacting their performance. Two recent studies have shown how additively manufactured steels compete with their conventional counterparts.

Researchers at the U.S. Department of Energy’s (DOE) Argonne National Laboratory used X-ray diffraction and electron microscopy to discover nanostructures of steel made with an additive manufacturing method called powder bed fusion (LPBF).

They printed two samples of stainless steel alloys with LPBF. In one study, they focused on 316H, an established type of stainless steel for structural components in nuclear reactors, and in the other they focused on Alloy 709 (A709), a newer alloy designed for advanced reactor applications. Both studies revealed the differences between 3D-printed steel and conventionally wrought steel, and also highlighted how printed steels responded to various heat treatments.

Before these steels can be relied upon in reactor environments, the nuclear industry has discovered their growing need for a deeper understanding of how to control 3D-printed steels.

“Our results will inform the development of tailored heat treatments for additively manufactured steels,” said Srinivas Aditya Mantri, an Argonne materials scientist who co-authored both studies. ​“They also provide foundational knowledge of printed steels that will help guide the design of next-generation nuclear reactor components.”

Using Heat Treatment for Repair

In LPBF, a laser melts precise designs into a metal powder one layer at a time until a solid, 3D metal object is formed. The repeated heating and cooling caused by the laser changes the microstructures of the steel.

Printed steels, for example, show higher numbers of dislocations — a defect of non-uniform shifts in a steel’s structure. Dislocations strengthen steel, but they also increase its internal stress, leaving it more vulnerable to fracture.

Heat treatment is a way to relieve this stress. Heat treatment changes the microstructures of a metal and can repair dislocations as high temperatures allow atoms to shift. Recrystallization is a product of heat treatment where new, strain-free grains replace the original structure. However, keeping some dislocations can have benefits of promoting precipitation of particles that can improve a material’s performance.

3D-printing 316H, a Standard for the nuclear industry

In one of the studies, researchers focused on 316H, comparing the microstructures of wrought and LPBF-printed samples by scanning electron microscopy (SEM) and scanning transmission electron microscopy.

At a second office, they used in situ X-ray diffraction experiments. At beamline 1-ID, the team evaluated the samples using high-energy X-rays while they underwent varying heat treatments of solution annealing.

“The high flux of photons provided by the APS allowed us to track the evolution of the microstructures in real time during the dislocation recovery process,” said Xuan Zhang, another materials scientist at Argonne and co-author on both studies. ​“That’s something you can only achieve with a synchrotron X-ray facility like the APS.”

What was revealed is that recrystallization was inhibited by nano oxides, which are nanoscale defects common in 3D-printed steel.

“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 LPBF-printed and wrought steels to heat treatment,” Zhang said. ​“For example, the printed samples started to recrystallize at temperatures several hundred degrees higher than their wrought counterparts.”

The effects of these discoveries on the mechanical properties of the metal are essential, particularly strength under tension and resistance to creep. Creep is the slow deformation of a material under consistent mechanical load, which is relevant for nuclear applications.

3D-printing A709, An Up-And-Coming Heavy-Hitter

The other study focused on A709, a newer advanced stainless steel designed for high-temperature environments such as inside sodium fast reactors. Researchers studied samples of A709 printed with LPBF, making it the first experimental look at an additively manufactured form of the alloy.

While investigating the strengths of the heat treated samples under tension, both at room temperature and 1022°F (550°C) — a temperature relevant to sodium fast reactors — the printed A709 showed higher tensile strengths than the wrought A709. This is most likely on account of the printed samples beginning with more dislocations, which also led to 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.”

Press release is available in its original form here.

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