Sandia National Laboratories researchers Lisa Deibler and Arthur Brown had a ready-made problem for their computer modeling work when they partnered with the National Nuclear Security Administration’s Kansas City Plant (KCP) to improve stainless steel tubing that was too hard to meet nuclear weapon requirements. When steel is too hard it becomes brittle, so the plant ended up getting new tubing. However, Deibler said KCP needed a backup in case it couldn’t find replacements in time to meet deadlines.
Sandia’s modeling, coupled with experiments, allowed the rapid design of an annealing process to soften the tubing while keeping the metal’s desired structure. The model predicted how the microstructure would be affected by variations in the process, which improved researchers’ confidence that the heat treatment would produce parts that met specifications.
Deibler provided experimental data that Brown fed into his model of stainless steel recrystallization. Recrystallization, in which grains in deformed microstructures are replaced by strain-free grains, occurs during annealing — the process of heating metal to dissipate energy built up while the metal is compressed, twisted or otherwise worked. Heat makes the metal softer and more ductile. Deibler and Brown were able to solve the plant’s real-life problem since recrystallization is part of the annealing process. And they were able to do it quickly because the model already existed.
Deibler’s experiments indicated it was important to model two softening mechanisms, recovery and recrystallization. Recovery happens first within a microstructure when material is heated and softens. By measuring the hardness and the amount of recrystallization after each heat treatment, the team identified how much softening was due to recovery.
The team first developed a baseline for the model. Deibler performed heat experiments on the steel tubing since she didn’t know the conditions under which it was manufactured. She put tubing samples in Sandia’s thermal-mechanical experimental system at various temperatures for different lengths of time. Then she had the tubing sectioned, polished and etched, and analyzed the images to see how much the microstructures had recrystallized. Brown fit her data with the model to simulate different heat treatments.