The U.S. Departments of Energy and Defense teamed up to create a series of weld filler materials that could dramatically improve high-strength steel repair in vehicles, bridges and pipelines. This novel weld wire could help revitalize America’s aging infrastructures, which in 2021 received a C- grade from the American Society of Civil Engineers.
The invention from DOE’s Oak Ridge National Laboratory and the U.S. Army enables onsite welding without costly, laborious heat treatments typically used to reduce residual stresses and material distortion. It solves a major problem of welded steels that occurs when hydrogen atoms enter the metal during welding and reduce the metal’s ductility, toughness and strength. Subsequently, high tensile residual stress leads to perilous cracking.
“The filler material that ORNL and the U.S. Army invented is a unique and game-changing solution for residual stress control, distortion reduction and avoidance of hydrogen-induced cracking for a wide range of structural steels,” said Zhili Feng, who leads ORNL’s Materials Joining Group. He heads research and development programs to advance materials joining and manufacturing for automotive, nuclear energy, fossil energy, hydrogen and defense technologies.
“About 80% of welded structures in the United States are made of steels, so applications for our innovative fill metal are extensive,” said Stan David, an ORNL corporate fellow emeritus who led the lab’s welding program for 25 years before retiring. “It is cheaper to repair a structure than to replace it. Our filler provides high-quality weld joints for increased service life of welded structures in demanding environments. The invention could potentially save U.S. industry hundreds of millions to billions of dollars each year.”
If stronger steel is used to make a welded structure, then less of it is needed, reducing weight, saving energy and cutting carbon dioxide emissions during materials manufacturing and vehicle operation. That results in more fuel-efficient cars, lighter-weight combat and support vehicles, and more durable fuel pipelines. However, strong steels are especially prone to hydrogen-induced cracking.
To overcome this challenge, scientists at ORNL and DOD’s former U.S. Army Tank Automotive Research, Development and Engineering Center — now called the Ground Vehicle System Center — partnered to invent an alloy with a unique chemical composition that can join strong steels while reducing residual stresses.
The alloy’s ability to resist hydrogen-induced cracking comes from a novel phase transformation in the weld. As a weld cools, the filler material combats tensile stress, or “bad stress,” which pulls at steel’s crystalline microstructure to lengthen and break it. The phase transformation introduces compressive stress, or “good stress,” to compensate for bad stresses as the weld cools.
No part of a welded structure is exposed to more stress than the weld region, where metal is heated and expands, then cools and contracts. Long after thermal expansion and contraction are over, stress remains in the material to distort it, creating the structure’s weakest link.
A weld filler needs to be at least as strong as the steel panels it joins. To develop the chemical composition of their pioneering, stress-compensating filler, ORNL researchers ran a theory-based model on high-performance computers. With more efficient algorithms, the computing code ran a thousand times faster than a comparable commercial code, identifying problems in one day versus nearly three years.