Gray cast iron, another mainstay of vehicular manufacturing, frequently is being replaced by stronger, tougher nodular cast irons in components such as housings, crankshafts and camshafts, However, the strength and toughness that make nodular irons desirable workpiece materials also make them difficult to machine. Tools to machine these irons must resist abrasive wear and endure interruptions in the cut, as well as be capable of productive cutting speeds and feed rates.

Nodular irons typically would be machined with carbide inserts featuring chemicalvapordeposition (CVD) coatings. CVD coatings have been commercially available for about 30 years, and the fact that more than half of the inserts sold are CVDcoated testifies to the effectiveness of these coatings. However, the high temperatures (about 1,000°C) involved in the CVD process create an embrittlement called “eta phase” at the coating/substrate interface. Depending on its extent, the embrittlement can affect performance in operations involving interruptions of cut and inconsistency of workpiece microstructure such as found in some nodular irons. Recently developed mediumtemperatureCVD (MTCVD) coatings have shown a reduced tendency to formation of eta phase. MTCVDcoated tools offer increased resistance to thermal shock and edge chipping compared to conventional CVDcoated tools. The result is greater tool life as well as increased toughness compared to hightemperature CVD coatings.