Tackling Stainless Steel

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by Mary Scianna

New cutting technologies help to overcome machining challenges

Manufacturers have been machining stainless steel since the early 1900s and for just as long, it has presented machining challenges.

“The biggest challenge with machining stainless steel is that traditionally manufacturers have used slower spindle speeds,” notes Steve Geisel, senior product manager Iscar Tools, Oakville, ON. “Parameters were not as aggressive as those for machining carbon and alloy steels so productivity was not very high. Now companies are looking for faster and less expensive ways to increase productivity on stainless and they’re asking cutting tool companies if we can increase speeds, improve chip control, achieve better cut quality and reduce the overall time to make a part.”

There are three main challenges when machining stainless steel: chip control, work hardening and insert life. Be aware though, different stainless steel alloys react differently depending on the nickel and chromium content, warn suppliers.

The main types of stainless steel are austenitic, ferritic/martensitic/PH stainless and duplex (austenitic/ferritic).

“Austenitic alloys have higher amounts of nickel in them, which makes them tougher and have a higher tendency to create build-up on the cutting edge,” says Kurt Ludeking, product manager for turning, Walter USA, Waukesha, WI. Ferritic/martensitic/PH alloys are lower in nickel content, but high in chromium. The higher chromium content makes these alloys stronger and more abrasive, which leaders to faster and higher insert wear.

Higher alloy duplex steels are difficult to machine, adds Kevin Burton, product specialist for Sandvik Coromant, Mississauga, ON, “especially when it comes to heat generation, cutting forces and chip control.” He says common wear mechanisms are flank and crater wear, plastic deformation, chip hammering and notch wear.

Tool selection for stainless is also dependent on the application, says Alex Livingston, product manager, Tungaloy Americas, Brantford, ON.

“Some processes can move from interrupted turning to continuous turning and with each one you may need a different type of chipbreaker and a different type of insert grade. The key with machining stainless is to ensure you have a rigid set-up because higher rigidity will help the performance of the tool.”

A common problem is using tooling not specifically designed for stainless steel. “In the field, people misapply grades and geometry combinations,” says Chad Miller, product manager, turning, Seco Tools, Troy, MI. “We have grades and chipbreakers designed specifically for stainless steel turning that address the common problems associated with turning this alloy, such as work hardening and tool wear.”

Chip control
Turning, by its nature, generates long stringy chips, and chips, as everyone knows, will wreak havoc on the machining process if not removed. And since stainless steel has a tendency to work harden, you need more aggressive chipbreaker geometries and high pressure coolants to effectively remove chips.

For instance, in high alloyed duplex steels, “chip control and coolant are important to avoid plastic deformation,” says Burton. He suggests using cutting tools with internal high pressure coolant supply for several reasons: it provides more efficient cooling of the insert close to the hot contact zone; it forces the chip away from the insert face quickly, reducing insert wear; and it breaks the chips into smaller pieces to evacuate it from the cutting area.

Different types of stainless steel require different cutting tools.                         Image: Walter USAChipbreaker design is an important consideration. “When possible, you want to use a positive chipbreaker to reduce the heat. Positive chipbreakers will reduce work hardening and built-up edge, which are key failure modes in machining stainless,” advises Livingston.

More importantly, according to Geisel, is to ensure you have a chipbreaker designed for stainless steel. Iscar recently redesigned its entire cutting tool line for stainless steel and created new tooling for finishing, roughing and medium machining of stainless steel.

“A lot of chipbreakers are designed to work on a broad range of materials, but what we did with our new chipbreakers is that we specifically targeted stainless steel. When you try to design a generic chip former, it may not be the best for stainless but when you have something that people know is for stainless, they also know they will get the best performance from it and it makes the choice of cutting tools for stainless much easier to make.”

Work hardening
Austenitic stainless steel in particular, tends to work harden easily, making it difficult for cutting tools in roughing, finishing and medium machining operations. The harder surface leads to insert wear. To overcome the problem, cutting tool suppliers have designed inserts with sharp edges and high wear resistance surfaces.

“The sharp edges help to prevent built-up edge and work hardening, while coatings enhance wear resistance,” says Ludeking.

The biggest problem occurs in applications where multiple passes are required.

“If you do have to take more than one pass, you can change the depth of cut. For instance, if you took 200 thou [thousandths of an inch] off your part you could do one pass at 100 thou and another at 100 thou and that’s very common, but with these types of materials, it’s better to split that up differently. I would like to see 125 thou depth of cut and then 75 thou depth of cut to work around the work hardening issue,” suggests Miller.

Insert life
Work hardening impacts tool life. To overcome wear issues, suppliers have redesigned geometries with sharper edges and positive rake features and introduced new coatings to help users run at higher speeds and feeds.

“Creating cutting tools for stainless is a bit of a balancing act,” explains Ludeking. “Thicker coatings [CVD] will enhance wear resistance and enable higher speeds for better productivity. However thicker coatings make it more difficult to keep the edges sharp.”

PVD coatings, typically used for austenitic stainless steel, are thinner, so they keep the edges sharp and provide a smoother surface, but your speeds and feeds are slower and since it’s a thinner coating, edges can get more easily damaged and lead to faster tool wear.

Some suppliers have introduced new variations of CVD and PVD coatings to overcome the challenges, while others have developed post-process treatments to enhance wear resistance properties.

“We found with our new coating technology, users can now do medium machining of stainless steel pretty much as they would machine carbon and alloy steels,” says Geisel. “We can run much faster sfm; we used to run around 400 to 450 sfm and now we can run up to 900 sfm.”

Tungaloy’s Livingston says the company “recently came out with new stainless steel grades with our PremiumTec post process for CVD and PVD coated tools which creates a smoother surface and helps reduce micro chipping of the coating.”

Some suppliers suggest using wiper inserts for higher feed rates that won’t compromise surface finish quality.

“Typically to achieve a good surface finish you have to feed at low rates,” says Miller, “but with the wiper insert, you can go three times faster on feed rate and still produce a better surface finish than you would achieve with a standard insert without a wiper. When you feed faster, you also get better chip control.”

While suppliers have made significant strides, some say there are still some issues that need to be addressed. One is the growing need for higher speed operations.

“Productivity hinges on what you can do with speed and there is always room for improvement,” says Ludeking.

Another area of change will likely come from chipbreaker technologies, adds Ludeking. He expects to see “continued development of geometries to provide chip control in wider feed ranges; it would simplify choices for end users if the could use the same insert at high and low feed rates.” SMT

Iscar Tools

Sandvik Coromant

Seco Tools

Tungaloy Americas

Walter USA

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