Researchers from Technical University of Denmark (DTU) (Lyngby, Denmark) and Colorado School of Mines (Golden, Colorado, USA) are using neutron analysis at the Oak Ridge National Laboratory (ORNL) (Oak Ridge, Tennessee, USA) to validate a new laser welding method on steel plates.
Neutrons have highly penetrating properties—more so than x-rays—and can probe almost any material in a nondestructive fashion, they say. Neutron analysis enables the researchers to study the welds on steel plate samples of varying thicknesses at the atomic scale. Their findings could lead to faster and more efficient production methods, as well as stronger welds.
“We’re studying residual stresses in supersized monopiles—enormous steel cylinders that form the underwater foundations for wind turbines,” explains DTU professor Michael Joachim Andreassen. “We want to look at the relationship between residual stress and varying thicknesses in the steel plates used in construction by comparing two different welding methods.”
The steel plates can be up to 130-mm thick and are typically welded together via submerged arc welding, in which electric arcs melt the joining materials. The weld’s molten seam is submerged in a granular flux of compounds used to support the weld and protect it from contaminants. “In the end, you have a huge groove with lots of introduced residual stresses,” Andreassen says.
Colorado professor Zhenzhen Yu is helping compare that technique to a new laser method. “The hybrid laser-arc welding technique introduces a more focused heat source that allows us to mitigate residual stress,” Yu says. “If you have high degrees of tensile residual stress, the faster corrosion occurs, and the greater the likelihood of fractures or cracks propagating through welded regions.”
Neutron measurements can compare changes in residual stress using both methods as researchers increase the steel plate sample sizes from 10 to 20, 40, and 60-mm thick.