Tips for preventing workhardening when boring

One of the challenges when boring workpiece materials susceptible to workhardening is recognizing whether the prebored hole is already workhardened.

“The biggest issue is that most of the time the people who are boring that workhardened material don’t realize it,” said Harvey Patterson, product development manager for Scientific Cutting Tools Inc., Simi Valley, Calif.

As a result, they typically program the machine to bore with parameters appropriate for a metal in a nonhardened condition, such as soft steel, and experience degraded tool life, he added. However, the hole may have already workhardened because it was produced with a drill that was dull or inadequate for the job, and the end user troubleshoots the wrong operation.

Image courtesy of Scientific Cutting Tools.

“A lot of time, the manufacturer of the boring bar will get a call and hear that tool life isn’t as good as it used to be—’What’s going on? Are you making substandard tools now?’—when that’s not even the issue,” Patterson said.

Sarang Garud, product manager for turning, boring and indexable drills at Walter USA LLC, Waukesha, Wis., agreed. “A dull or misapplied drill will leave the bore in a very nasty workhardened state,” he stated. “No matter how many of the best practices you use during boring, you will pay the price for a bad roughing cut. Instead of blaming the boring tool manufacturer, a good applications engineer should be able to spot workhardening based on insert condition after the cut.”

Materials prone to workhardening include nickel-base superalloys, such as Inconel 718, and austenitic and duplex stainless steel, Garud noted.

He explained that the tendency for a material to workharden increases in the presence of heat, which machining puts into the workpiece. Materials even tend to workharden when their crystal structure is realigned during a cold-working process, such as stressing, pressing or burnishing. However, “the crystal structure of the material gets rearranged much more easily in a heated state.”

Chip Check

To know if drilling caused the material to workharden might require visually examining the chips generated during that operation, according to Patterson. Because signs of workhardening can’t be determined during boring, as chips are flying and coolant is splashing, a machinist will need to dig through the chips and examine the larger ones that drilling creates compared to the ones boring produces.

When boring a workpiece that tends to workharden, Walter recommends applying a tool with a sharp edge radius and sharp corner radius to reduce the radial cutting force and take a DOC that is well-past the corner radius. Image courtesy of Walter USA.

“The chips might appear a lot darker,” he said about the chips from workhardened material, “so I’m creating more heat, and that heat creation is workhardening the material. Looking at the chips of the prior operation is probably the quickest check.”

Another visual indication of workhardening is inconsistent shine and slipperiness on the bore’s surface, according to Garud. When boring causes the problem, workhardening is usually detected as a symptom, such as insert DOC notching and chipping, after the first pass. In addition, when boring a workhardened, thin-walled part feature, the wall tends to deflect because of excessive forces.

“Some advanced nondestructive techniques, such as ultrasonics, may help, but not all shops are equipped and trained for it,” Garud stated. “Lab testing or destructive testing, such as splitting open the bore and testing the workpiece before and after machining on a hardness testing machine, is also possible.”

Ben Morrett, product manager for Allied Machine & Engineering Corp., Dover, Ohio, added that chip formation and control provide further clues.

“When breaking a chip in a material known to be poor chipping,” Morrett said, “your material is likely workhardened.”

Geometries at Work

To prevent workhardening while effectively boring a hole, Morrett said Allied offers an ISO-standard insert with a positive cutting edge geometry that gets underneath the chip to shear it and reduce the amount of heat generated during boring.

“This geometry is a good starting point for materials that are likely to workharden and is available in three different ISO sizes” he added.

A slight hone on the cutting edge is also beneficial when boring materials susceptible to workhardening. Patterson pointed out that a notch can form at a tool’s DOC line when boring a workhardened hole, and the hone helps stop the notching.

Garud recommends varying the DOC of each pass if the process and the available amount of stock to be removed allow for it. When that approach isn’t possible, apply a tool with a sharp edge radius and sharp corner radius to reduce the radial cutting force and take a DOC that is well-past the corner radius, he added. “Plan the depth of cut in advance.”

Morrett added that the Allied insert also has a PVD multilayer titanium-aluminum-nitride coating for finish-machining difficult-to-cut high-temperature alloys and stainless steel.

“This coating is excellent at resisting heat and minimizing material adhesion, like built-up edge,” Morrett said. “Our WHC111 coating and substrate combination does really well for materials up to 58 Rockwell.”

Patterson concurred that a TiAlN-coated tool is effective for boring workhardened material. However, many parts manufacturers apply uncoated tools because they cost less and are suitable for softer metals. “But if you’re workhardening it, you may need the coating.”