Lehigh software yields clues to welding of stainless steel

Software developed at Lehigh is helping researchers learn how to control the behavior of stainless steel when it is welded, Herman Nied, professor and chair of mechanical engineering and mechanics, said recently in an invited talk and power-point presentation in Canada.

Nied told an audience of engineers and scientists at McMaster University in Hamilton, Ontario, that he and his students have developed specialized codes to predict the fracture behavior of welded stainless steel, which warps during welding more than regular steel does.

His talk was part of the Distinguished Lecturer Series sponsored by the McMaster Manufacturing Research Institute and Materials and Manufacturing Ontario.

Nied's work is part of a larger multidisciplinary effort at Lehigh's ATLSS (Advanced Technology for Large Structural Systems) Research Center, which has received major funding from the U.S. Office of Naval Research. Researchers are studying the design, fabrication and operation of ships whose double hulls are made of stainless steel and fiber-reinforced polymers. Stainless steel is valued for its corrosion resistance and for its non-magnetic properties, which enable a ship to avoid detection by mines.

The welded joints of a stainless steel structure are the parts most likely to crack under stress. A precise understanding of the welding behavior of stainless steel is also crucial, says Nied, because ships today are built in modular fashion. When separately fabricated components are assembled into a final product, they must fit exactly.

Hundreds of variables must be taken into account to ensure optimal welding, says Nied. These include welding temperatures, cooling rates, welding stresses, the interaction of solid and fluid mechanics, and the types of preventive measures that fabricators can use.

The ATLSS researchers test their theories with physical experiments, but, says Nied, numerical calculations performed with the Lehigh-developed software offer control of a greater number of variables.

"To control the numerous variables during physical experiments is extremely difficult," says Nied. "We use solid-mechanics calculations to determine the stresses that arise during welding and to analyze the warping that those stresses cause."

The calculations, he says, represent "some of the most computationally intense work I have ever done. Some problems take days to run. There are huge storage requirements. What we do is to create a model, run it, analyze the results, and then change the parameters in a very systematic way."

Nied is particularly interested in a welding techniques known as "skip welding" and "backstep welding." His simulations show that the traditional continuous welding process imposes different types of stresses and causes greater distortion in stainless steel than does the back-step welding process.

Performing numerical calculations also help Nied and his students determine which techniques for preventing warping - fixturing (clamping), pre-heating, pre-bending, pre-warping and bar-support - are the most effective.

"Modeling and simulation can tell you when and what kind of preventive steps, such as fixturing or pre-bending, are most effective to avoid distortion," he says.

A better understanding of the behavior of welded joints, especially their strength and fatigue resistance, is important not just for ship-hull fabrication but also for the construction of nuclear power plants and power-plant boilers, says Nied.

In addition, the same Lehigh-developed software that models cracking in ship-hull joints is used by semiconductor packaging companies, says Nied. That is because the underlying physical mechanisms do not change significantly from the macrostructural to the microstructural level, even though the material properties differ.

Recent advances in computer technology have enabled the Lehigh software to achieve a high level of sophistication, says Nied.

"Even a few years ago, computers were not capable of doing this kind of computation. The speed, memory size, and hard drive storage capacities have enabled this level of sophisticated simulation."

Nied and his students also use Sysweld, a commercial software package that is the number-one welding simulation program available today. This enables students to continue to apply advanced welding simulation concepts after they've graduated and are working in industry.

Other Lehigh faculty members active in the stainless-steel double-hulled ship research include Joachim Grenestedt, associate professor of mechanical engineering and mechanics, who studies hybrid and composite hulls; Hugo Caram, professor of chemical engineering, who is attempting to ameliorate the fumes that result from stainless-steel welding; and John DuPont, associate professor of materials science and engineering, who investigates the metallurgy of welding and the filler metals used in stainless steel to prevent weld degradation.