As infrastructure spending is set to increase, so will the opportunities for metal manufacturers. The winners will offer (ideally at least) products designed to last. Unfortunately, the very act of manufacture puts the material to the test, so manufacturers will have to use a variety of techniques to mitigate the effects of this stress. Stores can apply heat (as in post-weld heat treatments); relief of vibratory stress; shot blasting; or even the high-tech cousin of shot blasting, laser shot blasting. Another option is burnishing, an old-fashioned process that, like shot blasting, has a high-tech cousin. This is called low plasticity burnishing, or LPB.
“Conventional burnishing has been around for decades. Burnishing of railway axles is a common application. Basically, you roll a tool – in old-fashioned burnishing, it’s usually a wheel-type tool – over a piece of metal to smooth the surface. And you can impose residual compressive stresses while you’re doing it.
It was Mike Prevey, vice president of engineering 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. “From the start, we learned that there are a lot of benefits to being able to apply compressive stresses while keeping cold work low. “
From there was born LPB. The process may at first glance resemble traditional burnishing, in which a tool applies pressure and smooths the surface of the material. “You plastically deform [the workpiece] in tension to rebound with residual compression, ”explained Prevey. “As long as you do this as little as possible, you can keep most of the cold work in the material.”
He added that control is what differentiates LPB from traditional burnishing and other stress relief processes like shot blasting. “With shot peening you have random splashes of shot all over the place, and you can hit a dot half a dozen times before you hit the spot next to it. So you end up with a high level of cold working with your compressive stress.
LPB can meet a variety of surface conditioning requirements, including some welding applications, especially those susceptible to stress corrosion cracking. Welds are inherently tensile. Because metal is an excellent thermal conductor, heat does not stay in one place for long. The energy of the arc induces heat to create molten molten metal. As the arc moves, the heat dissipates extremely quickly as the molten metal solidifies. This creates high tensile stress which leaves the weld under tension and, in some applications, vulnerable to stress corrosion cracking.
In these cases, the tension must be treated. Prevey described applications involving certain welds in 304L and 316L stainless steel, as well as nickel-based alloys, including Alloy 22. Certain welds in these materials can result in residual tensile stresses of up to 100 KSI at the surface. welding and nearby.
This is where LPB can play a role. The burnishing tool used depends on the application, but common tools involve a ball supported in a spherical hydrostatic bearing. It is manipulated in various ways, for example on a tool holder in a CNC milling machine, by a robot or in a custom mechanized system.
The pressure of the ball rolling on the surface of the workpiece causes a very slight deformation. A displacement of 0.0001 to 0.0006 inch is typical. Immediately after deformation, however, the material returns to a compressive state. An additional advantage: LPB leaves a near-mirror finish. According to Lambda, the process leaves a surface with roughness values between 5 and 8 microinches.
Control, which determines how hard the burnishing tool must push to achieve the desired results, is the heart of the system. As Prevey explained, “Each tool is calibrated to determine the pressure needed to achieve a certain force. The control system, which we build in-house, has software that is also calibrated to send the correct hydraulic force. And it is closed loop servo control. We collect data every 5 milliseconds or so, so we can change the force with the position as needed.
“Ultimately, what determines the force that goes into the room [is] empirical testing, ”Prevey continued. “Our company started out as a surface testing treatment laboratory, and we’re still very attached to it. We send out 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 stresses.
Today Lambda builds and sells custom LPB systems and offers its own LPB (and other surface treatments) services. Most of its customers come from high-end manufacturing sectors such as medical, power generation, nuclear and aerospace, markets that apply strict welding standards. Once a part is submitted to the LPB, pre-testing (usually as per code requirements) has ensured that the work is free from internal defects.
“Internal defects will become tensile stress raisers regardless of the means by which the compressive stress is applied to the weld,” explained Prevey.
When compressive stress is applied, by burnishing, shot blasting, laser blasting or the like, the stress is absorbed as the surface returns to residual compression. Again, this prevents subsequent cracking because material under compression is much more stable than material under tension.
But if the weld has porosity, inclusions, or other discontinuities large enough to qualify as a defect, the story changes. These faults cause high levels of voltage inside the weld. Under pressure, these defects become important stress elevators where internal cracking can begin.
Prevey pointed out that weld defects are usually not a problem, mainly because non-destructive exams like x-rays or ultrasound tests are usually built into the process. After all, LPB generally applies to applications where service conditions are critical (think landing gear) or where welds are designed to last a long time, or a combination of both.
For example, Idaho National Labs used the LPB to relieve stress on the closure welds of nuclear waste containers. “We did the finishing process on these 4 inch thick welds on top of these waste containers,” said Prevey. “Of course they had to last, and they wanted to be sure that they weren’t susceptible to any kind of corrosion. “
With infrastructure spending expected to increase dramatically, more and more manufacturing operations will design welded constructions to last. Managing material stress, whether through LPB or whatever, will continue to be a key ingredient.