Inside Extreme Scale Tech|Wednesday, June 3, 2015
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Modeling a Stronger Nuclear Weapon 

When it comes to systems that you really want to model out carefully, nuclear weapons likely come out at the top of that list. With that much destructive potential, every component must be exactly right.

So when the National Nuclear Security Administration’s Kansas City Plant (KCP) found that their weapons’ metal tubing wouldn’t meet specifications, they turned to Sandia National Laboratories, hoping the lab’s computer modeling team could find a workaround in case the KCP couldn’t find any suitable replacement parts in time.

To be clear, the Kansas City Plant, managed and operated by Honeywell Federal Manufacturing & Technologies, produces the nonnuclear material used in our nuclear weapons arsenal. Nonetheless, ensuring that each nonnuclear component works as it should is essential.

Specifically, the problem the KCP faced was that their stainless steel tubing was too hard, which (perhaps unintuitively) means that it can become brittle. So Lisa Deibler and Arthur Brown set out to design an annealing process that would soften the metal while keeping its shape intact.

According to Brown, this project fit right in with Sandia’s existing Nuclear Weapons program, thanks to a project called Predicting Performance Margins. There, Sandia researchers have been at work for some time studying how a material’s microstructure affects its properties, just as the KCP needs to refine the properties of stainless steel tubing.

What arose from the Predicting Performance Margins project was a thermal-mechanical modeling tool that could predict how forging and welding change a material’s microstructure.

Using their existing research as a springboard, Deibler and Brown set out to devise a heat treatment solution to the KCP’s stainless steel problem. To do this, they plugged in data to Brown’s stainless steel recrystallization model, which looks at the process of replacing grains in deformed microstructures with strain-free grains.

The process in question takes place during “annealing” — heating a metal to allow built up energy to escape, which results in a softer, more ductile material.

In Sandia’s case, solving this problem was relatively quick and easy because they already had the models in place. For KCP, this provided them with a solution they could count on immediately without needing to order and wait for replacement tubing.

The researchers centered their model around both forging and welding because the two processes have opposing effects on the properties of the stainless steel used in nuclear weapons (or in anything, for that matter). Forging sets the metal up with a strong microstructure, whereas welding adds heat that can undo that strength.

“If you were able to model that process, that would provide a lot more confidence in the overall modeling that their parts aren’t going to fail,” said Deibler. And when it comes to nukes, failure certainly is not an option.

But still, it wasn’t exactly ‘plug and chug.’ After some initial modeling, the team found that their model wasn’t accurately predicting microstructure properties by looking at recrystallization alone. The solution, as Deibler’s experiments showed, was to include an additional softening mechanism called recovery, which had previously gone unaccounted for.

“It was important to model both softening mechanisms because we were seeing microstructures that contained no new recrystallized grains, but which had changed properties from the initial deformed material,” Deibler said. “By failing to include the effects of recovery, our model couldn’t predict why the properties weren’t the same as the initial deformed material. Adding in recovery allowed us to account for the changed properties in microstructures with no recrystallization.”

So why was recovery so important? It turns out that recovery happens first when a material is heated and then softens. To account for this extra phenomenon, Deibler measured hardness and recrystallization to determine how much of the softening could be attributed to the recovery process.

Making this all possible were a series of experiments that served to develop a baseline model, which boiled down to testing all sorts of metal tubing with a vast array of heat treatments. The tubing was then sectioned off, polished, etched and analyzed to assess how the microstructure of each sample had recrystallized. From their, the best fit heat treatment for the KCP’s tubing could be decided upon.

As far as the specifics of the treatment, the furnace was an additional worry. Even if the correct heat treatment was chosen, the speed at which the furnace is heated could potentially lead to errors in temperature that would not sufficiently strengthen the brittle stainless steel.

The final test was to fill the furnace with tubing, heat it, and measure how much variation in microstructure occurred due to temperature fluctuations.

Although this helped the KCP to meet their deadline with a well-engineered project, this isn’t the end for such research at Sandia. Upcoming efforts are slated to investigate the effects of laser welding, resistance welding and gas tungsten arc welding, each of which could lead to key differences in microstructural properties.

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