(Oak Ridge National Laboratory: Oak Ridge, TN) -- America’s national security relies in part on the U.S. Navy’s nuclear-powered submarines, which can operate more than 800 ft below the ocean’s surface. At such dark, unforgiving depths, the crew’s survival depends on the ability of the sub’s steel hull—and its welded joints—to withstand immense water pressures that can exceed 50,000 lb/sq ft.

As much as 200 yards in length, the submarines’ pressure hulls consist of many steel plates measuring up to 100 ft long, all joined by precision manual or robotic welding methods. The long-term integrity of the welded joints, or seams, where the plates meet are the subject of neutron experiments at the U.S. Department of Energy’s Oak Ridge National Laboratory.

Despite meticulous quality control during welding on submarines, the problem of ductility dip cracking (DDC) has not been completely eliminated. It can occur when welded metals cool and solidify, causing small cracks to form and propagate around a weld, which weakens the joint over time—even with the latest steel alloys.

Neutrons tackle welding-related cracking in US nuclear subs

One advanced alloy, comprising 70% copper and 30% nickel, is widely used in naval applications. Yet DDC in this material remains a critical concern of submarine builders worldwide. To address the issue, the U.S. Navy, Electric Boat (the Navy’s primary submarine builder), and the University of Connecticut are collaborating to determine the internal stresses and mechanisms responsible for DDC behavior in 70/30 copper-nickel.

A key component of their research uses neutron scattering at the High Intensity Diffractometer for Residual Stress Analysis (HIDRA) at ORNL’s High Flux Isotope Reactor.

“Our study is the first to consider nonmicrostructural aspects of DDC, including residual stress or internal distortions caused by heat from the welding process,” says Lesley Frame, assistant professor of materials science and engineering at the University of Connecticut. “The HIDRA neutron instrument is optimized for strain measurement and is ideal for analyzing residual stress in engineered materials such as copper-nickel alloys. Its neutrons can penetrate deeply into bulk materials, making them suitable for inspecting thick metals and welds.”

Neutrons can reveal weld quality through techniques such as neutron diffraction and neutron radiography, which provide insights into residual stresses and internal defects. Neutron diffraction methods can measure changes caused by stress in crystallized materials’ molecular lattice spacings, and neutron radiography provides images based on density variations in materials that attenuate, or weaken, neutron energies as they pass through a sample.

Another researcher who was working at HIDRA, Matt Caruso, is a member of the Lesley Frame Research Group and a doctoral student in materials science and engineering at UConn. His research focuses on quantifying the effects of residual stress that can lead to cracking in welded metals.

“Welding is such a dynamic process that it’s usually easier just to map the stresses after the welding has been completed,” Caruso says. “We can then correlate the conditions that occurred at each point along the weld with the computer model being developed by our colleagues. The goal is to have a model that accurately predicts when welding conditions can cause cracking and understand how to avoid or minimize those conditions.”

The scientists are looking at the gaps between the atoms in the alloy and using the HIDRA diffraction data to learn how the crystallographic planes within a crystalline structure have been distorted by residual stresses.

Frame says, “We’re trying to nondestructively map out the stresses in the welded material. At HIDRA, we don’t need to cut up the sample, which is one of the big benefits of working with neutrons. Another benefit is that HFIR’s neutrons come directly from the reactor, so they all have the same energy, and they are produced in such large amounts that we get faster counts, so we’re not sitting there counting for weeks. It just takes a few hours of beam time.”

Frame also mentioned that the neutron data complements the X-ray testing they are conducting at DOE’s Brookhaven National Laboratory. “We’re using multiple complementary techniques to measure the same materials to better understand all of the physics and mechanisms at play during the welding.”

Caruso says, “Our research will increase our understanding of ductility dip cracking and offer approaches to help mitigate its occurrence. Preventing DDC will result in safer vessels for the Navy.”

HFIR is a Department of Energy Office of Science user facility. For more information, visit energy.gov/science.

The pressure hulls of U.S. Navy nuclear submarines consist of many steel plates joined by precision manual or robotic welding methods. The long-term integrity of the welded joints is the subject of neutron experiments at ORNL. Artistic rendering. Credit: Phoenix Pleasant/ORNL, U.S. Dept. of Energy