Our Sites

Burnishing technology combats stress corrosion cracking of welds

Low-plasticity burnishing process can help welds last and last

Weld stress corrosion cracking

The untreated portion of this weld has evidence of stress corrosion cracking (SCC).

With infrastructure spending set to rise, so will opportunities for metal fabricators. Those that win the work will (ideally, at least) propose fabrications designed to last. Unfortunately, the very act of fabrication stresses the material, so fabricators will need to employ various techniques to mitigate the effects of that stress. Shops might apply heat (like in postweld heat treatments); vibratory stress relief; shot peening; or even shot peening’s high-tech cousin, laser shock peening. Another option is burnishing, an old-school process that, like shot peening, has a high-tech cousin. It’s called low-plasticity burnishing, or LPB.

“Conventional burnishing has been around for decades. Burnishing of railroad axles is a common application. You’re basically rolling a tool—in old-school burnishing, it’s usually a wheel-type tool—across a piece of metal to smooth the surface. And you can impart some residual compressive stresses while you’re doing it.”

That was Mike Prevey, vice president of enigneering and manufacturing at Lambda Technologies Group, Cincinnati. Lambda’s roots go back to the 1970s, when it was primarily a laboratory that studied surface treatments and residual stresses. “Early on, we learned that there is a lot of benefit to being able to put in the compressive stresses while keeping cold-working low.”

From this came LPB. The process might at first glance look like traditional burnishing, during which a tool applies pressure and smooths the material surface. “You plastically deform [the workpiece] in tension in order for it to rebound with residual compression,” Prevey explained. “As long as you do that as little as possible, you can keep the bulk of the cold-working low in the material.”

He added that control is what differentiates LPB from traditional burnishing and other stress-relief processes like shot peening. “With shot peening, you have a random spattering of shot going everywhere, and you might hit one point a half-dozen times before you hit the spot next to it. So you end up with a high level of cold-work along with your compressive stress.”

LPB can fulfill a variety of surface conditioning requirements, including certain welding applications—especially those susceptible to stress corrosion cracking. Welds are by their nature full of tension. Because metal is such a great thermal conductor, heat doesn’t stay in one place for long. Energy from the arc induces heat to create molten weld metal. When the arc moves on, the heat dissipates extremely quickly as the weld metal solidifies. This creates high tensile stress that leaves the weld in tension and, in some applications, vulnerable to stress corrosion cracking.

In these cases, the tension must be dealt with. Prevey described applications involving certain weldments in stainless 304L and 316L, along with nickel-based alloys including Alloy 22. Some welds in these materials can result in tensile residual stresses that can exceed 100 KSI at and near the weld surface.

This is where LPB can play a role. The burnishing tool used depends on the application, but common ones involve a ball supported in a spherical, hydrostatic bearing. It’s manipulated in various ways, such as on a toolholder in a CNC mill, by a robot, or within a custom mechanized system.

Pressure from the ball rolling over the workpiece surface causes an ever-so-slight amount of deformation. A displacement of 0.0001 to 0.0006 in. is typical. Immediately after deformation, though, the material springs back into a compressive state. An added benefit: LPB leaves a near-mirror finish. According to Lambda, the process leaves a surface with roughness values between 5 and 8 microinches.

The control, which determines how hard the burnishing tool should push to achieve the desired results, is the heart of the system. As Prevey explained, “Every tool is calibrated to determine how much pressure is needed to achieve a certain force. The control system, which we build in-house, has software that is also calibrated to send out the right hydraulic force. And it’s a closed-loop servo-based control. We’re getting data back about every 5 milliseconds, so we can change the force with the position as needed.

burnishing

In low-plasticity burnishing (LPB), a ball supported in a hydrostatic bearing applies a precise amount of pressure to a weld joint, replacing tension with compression and reducing the chance of stress corrosion cracking.

“Ultimately, what determines the force that goes into the part [is] empirical testing,” Prevey continued. “Our company began as a surface testing treatment laboratory, and we’re still very closely attached to it. We send down various test coupons of different materials and different welds to see what forces and types of tools work to achieve a specific range of residual stress.”

Today Lambda builds and sells custom LPB systems and offers its own LPB (and other surface treatment) services. Most of its customers come from high-end fabrication sectors like medical, power generation, nuclear, and aerospace—markets that have stringent welding standards. Once a workpiece undergoes LPB, prior testing (usually per code requirements) has ensured the work is free from internal defects.

“Internal defects will become tensile stress risers regardless of the means in which compressive stress is put into the weld,” Prevey explained.

When compressive stress is applied—using burnishing, shot peening, laser shock peening, or anything else—the stress is absorbed as the surface springs back in residual compression. Again, this prevents later cracking because material under compression is much more stable than material under tension.

But if the weld has porosity, inclusions, or other discontinuities significant enough to qualify as a defect, the story changes. These defects cause high levels of tension inside the weld. Under pressure, those defects become significant stress risers where internal cracking can initiate.

Prevey emphasized that weld defects usually aren’t an issue, mainly because nondestructive examinations like radiography or ultrasonic testing are usually built into the process. After all, LPB usually applies to applications in which in-service conditions are critical (think landing gears) or where welds are designed to last a long time, or a combination of both.

For instance, Idaho National Labs used LPB to relieve stress on nuclear waste container closure welds. “We performed the finishing process on these 4-in.-thick welds on the top of these waste containers,” Prevey said. “They of course needed to last, and they wanted to be sure they wouldn’t be susceptible to any sort of corrosion.”

With infrastructure spending set to ramp up in a big way, more fabrication operations will be designing weldments to last. Managing material stress—be it through LPB or anything else—will continue to be a key ingredient.

About the Author
The Fabricator

Tim Heston

Senior Editor

2135 Point Blvd

Elgin, IL 60123

815-381-1314

Tim Heston, The Fabricator's senior editor, has covered the metal fabrication industry since 1998, starting his career at the American Welding Society's Welding Journal. Since then he has covered the full range of metal fabrication processes, from stamping, bending, and cutting to grinding and polishing. He joined The Fabricator's staff in October 2007.