Challenge: What to do if cemented carbides do not deliver in HRSA machining?
Solution: Move to ceramic insert grades, which when correctly applied offer a powerful alternative.
Sialon and Whisker Ceramics are viable alternatives for both turning and milling heat-resistant super alloys (HRSA). One subgroup is the Inconel alloys, typically used for high-temperature applications in aircraft engines and energy production. Another increasing application is welded-on hard materials in the oil and gas industry.
Ceramics are strong in hot-hardness and low reactivity with workpiece materials; their downside is lower toughness compared with cemented carbides. This requires a rigid process with minimal vibrations. Tool overhangs should be short, entries and exits smooth and, in milling, full slotting should be avoided.
Milling vs. Turning
Ceramic turning and milling operations show some major differences. In turning, the cutting speeds top at around 985 ft/min, while milling cutters may reach as high as 3,300 ft/min. Ceramic turning requires abundant, uninterrupted coolant flow, whereas milling tolerates no coolant at all to avoid thermal stress.
Round inserts are optimal for both turning and milling, and low depths of cut are recommended.
The principal guideline for ceramic milling is to use relatively low feed rates (o.oo2-0.003 in/tooth) compared with carbides to achieve very high cutting speeds. The heat around the cutting zone enhances the cutting action and efficient removal of the resultant small, red-hot chips.
Originally published in Metalworking World 2.2011, a business magazine published by Sandvik Coromant.
Text: Turka Kulmala