From composition to performance, two recent studies show how additively manufactured steels measure up to their conventionally produced counterparts.
Stainless steel has long been a workhorse material for the nuclear industry. It fortifies walls and forms crucial components throughout nuclear reactors, where it withstands decades of extreme heat, pressure and irradiation.
Compared to conventional methods for steelmaking, techniques in additive manufacturing — or 3D-printing — offer a way to produce complex stainless steel parts more efficiently and with greater design flexibility. But additive manufacturing processes can leave behind defects in the microscopic structures of steel parts, impacting their performance.
Before they can be trusted in reactor environments, the nuclear industry needs a deeper understanding of 3D-printed steels and how to control them.
In two recent studies, researchers at the U.S. Department of Energy’s (DOE) Argonne National Laboratory used X-ray diffraction and electron microscopy to investigate steels made with an additive manufacturing process called laser powder bed fusion (LPBF).
With LPBF, they printed samples of two stainless steel alloys that are of interest to the nuclear industry. One study focused on 316H, an established type of stainless steel for structural components in nuclear reactors. Another study focused on Alloy 709 (A709), a newer alloy designed for advanced reactor applications.
Both studies uncovered important differences between printed steels and their wrought — or conventionally produced — counterparts. They also revealed how printed steels responded 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. ?“They also provide foundational knowledge of printed steels that will help guide the design of next-generation nuclear reactor components.”
Healing and Fortifying Printed Steels with Heat
During LPBF, a laser melts precise designs into a metal powder one layer at a time to construct a solid, 3D metal object. The rapid heating and cooling caused by the laser creates unique features in the microstructures of steel.
For example, printed steels show higher numbers of dislocations — defects where an otherwise consistent pattern in a material’s structure suddenly shifts. 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. During heat treatment, a material begins to heal through a process called recovery, where high temperatures allow atoms to shift and repair dislocations. This can lead to recrystallization, where new, strain-free grains replace the original structure altogether. But keeping some dislocations can be beneficial; they promote the precipitation of particles that, in the right quantities, can further improve a material’s performance.
The Argonne researchers are studying the delicate balance between these processes in 3D-printed steels, paving the way toward their adoption in nuclear applications.
3D-Printing 316H, A Nuclear Industry Veteran
In one of the studies, the researchers focused on 316H, a well-known structural material in its wrought form. The researchers compared the microstructures of wrought and LPBF-printed samples of 316H using capabilities at Argonne’s Center for Nanoscale Materials (CNM), a DOE Office of Science user facility, including scanning electron microscopy (SEM) and scanning transmission electron microscopy.
They also performed in situ X-ray diffraction experiments at a second DOE Office of Science user facility at Argonne, the Advanced Photon Source (APS). At beamline 1-ID, the team probed the samples with high-energy X-rays as they underwent variations of a heat treatment called 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.”
The experiments showed that recovery and recrystallization were inhibited by nano oxides — nanoscale defects common in 3D-printed materials.
“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 researchers took the detailed structural data they obtained at the CNM and APS and related it to mechanical properties, including strength under tension and resistance to creep. A major consideration for the nuclear industry, creep is the slow deformation of a material under a constant mechanical load.
3D-Printing A709, A Promising Up-and-Comer
The other study focused on A709, a newer, more advanced stainless steel designed for high-temperature environments like those inside sodium fast reactors — next-generation reactor systems that operate at higher efficiencies. In this study, Argonne researchers investigated samples of A709 printed with LPBF, marking the first experimental look at an additively manufactured form of the alloy.
The researchers used capabilities at CNM — including SEM and transmission electron microscopy — to peer deep inside samples of printed and wrought A709 during multiple heat treatments.
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. 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.”
Both studies were funded by DOE’s Office of Nuclear Energy’s Advanced Materials and Manufacturing Technologies program. Work performed at the CNM and APS, DOE Office of Science user facilities, was supported by DOE’s Office of Basic Energy Sciences.