MANUFACTURING HEAT TREAT

Scientists at several research institutions recently reported a breakthrough in 3D printing a marine grade stainless steel — a low-carbon type called 316L — that promises high-strength and high-ductility properties. Researchers at Lawrence Livermore National Laboratory (LLNL), along with collaborators at Ames National Laboratory, Georgia Tech University, and Oregon State University, published their findings online October 30, 2017, in the journal Nature Materials.

“Marine grade” stainless steel is valued for its performance under corrosive environments and for its high ductility — the ability to bend without breaking under stress — making it a preferred choice for oil pipelines, welding, kitchen utensils, chemical equipment, medical implants, engine parts and nuclear waste storage. However, conventional techniques for strengthening this class of stainless steels typically comes at the expense of ductility.

“In order to make all the components you’re trying to print useful, you need to have this material property at least the same as those made by traditional metallurgy,” said LLNL materials scientist and lead author Morris Wang. “We were able to 3D print real components in the lab with 316L stainless steel, and the material’s performance was actually better than those made with the traditional approach. That’s really a big jump. It makes additive manufacturing very attractive and fills a major gap.”

To successfully meet, and exceed, the necessary performance requirements for 316L stainless steel, researchers first had to overcome the porosity which causes parts to degrade and fracture easily during the laser melting (or fusion) of metal powders. Researchers addressed this through a density optimization process involving experiments and computer modeling, and by manipulating the materials’ underlying microstructure.

“This microstructure we developed breaks the traditional strength-ductility tradeoff barrier,” Wang said. “For steel, you want to make it stronger, but you lose ductility essentially; you can’t have both. But with 3D printing, we’re able to move this boundary beyond the current tradeoff.”

Using two different laser powder bed fusion machines, researchers printed thin plates of stainless steel 316L for mechanical testing. The laser melting technique inherently resulted in hierarchical cell-like structures that could be tuned to alter the mechanical properties, researchers said.

Wang called stainless steel a “surrogate material” system that could be used for other types of metals. The eventual goal, he said, is to use high-performance computing to validate and predict future performance of stainless steel, using models to control the underlying microstructure and discover how to make high-performance steels, including the corrosion-resistance. Researchers will then look at employing a similar strategy with other lighter weight alloys that are more brittle and prone to cracking.

Read more from the Lawrence Livermore National Laboratory here: “Lab Researchers Achieve Breakthrough in 3D Printed Marine Grade Stainless Steel”.