Welding is an essential part of manufacturing, and the key to making crack-free welds relies on the ability to understand how the weld is put together atom by atom.
Before the COVID-19 pandemic, graduate students at the Center for Welding, Joining and Coatings Research of Colorado School of Mines, Tim Pickle and Ben Schneiderman, used neutrons at the Department of Energy’s (DOE’s) Oak Ridge National Laboratory (ORNL) to improve that understanding. They’re part of a project supported by DOE’s SunShot division and the National Renewable Energy Laboratory (NREL). The goal is to investigate the performance of welds used to build large thermal energy storage tanks at concentrating solar plants—facilities with vast networks of mirrors used to collect solar energy, some stretching several million square feet in size.
“What we’re trying to do is compare the differences in stress profiles between two manufacturing approaches, with and without post-weld heat treatment, used to create the storage tanks,” said Pickle. “We’re also trying to validate a finite element model that can be used by NREL and potential manufacturers to help them determine the best welding and post-weld heat treatment procedures to mitigate and find solutions to cracking problems.”
Specifically, the team is studying stress relaxation cracking (SRC)—the susceptibility of welds to cracking over time due to factors such as internal stress and high temperatures. Thermal fatigue created by alternating stress between room and extremely high temperatures may also contribute to SRC. Each time the metal experiences a change in temperature during the welding process, new stress is added. Those lasting changes, or deformations, called residual stresses, can have a big impact on the performance of the weld during service.
The storage tanks are large structures about 100 feet wide by 30 feet tall. They’re used to store molten salt material that is heated and liquified to store energy captured by solar panels. When energy is needed, the molten salt is pumped into a steam system that boils water, which then spins a turbine that generates electricity.
Essentially, a tank is made by rolling large plates of stainless steel into a cylinder. The ends are then fused together using seam welds, which require multiple layers of weld metal to fill the space in between the weld joints.
“When the welded areas of the wall joints go from room temperature to above 550 or 600 degrees Celsius, they develop stresses around the weld,” said Pickle. “We want to