Cermets

Modern Machine Shop, Jan, 1991 by Henry Stier

The tendency to cold or warm form the shape of mass produced parts to a "Near-Net-Shape" can be seen throughout industry. The small amount of material left for machining requires specific properties of the cutting tool material. The contact between the tools, cutting edge, and part to be machined is limited to the comer of the insert. During the finishing cut, with small feeds per revolution, the total cutting force is concentrated on the insert's small nose radius. Thus, it must have high thermal and mechanical-shock resistance.

Under these circumstances, typically encountered in finish turning, the insert wear must be small in order to retain dimensional stability and good surface finish combined with long tool life.

Cutting materials specially created for these applications are the cermets. The first cermets on the market were made of titanium carbide with a nickel binder. Today, the base material is mostly titanium carbonitride. Depending on grade and manufacturer, 60 percent of titanium carbo- nitride is mixed with different amounts of molydenum, tantalum, tungsten or vanadium carbides. Lately, the binder is a combination of nickel and cobalt. A higher percent of binder increases the toughness of the insert and reduces the possibility of cutting edge fracture. The higher heat conductivity reduces the effects of thermal shock.

Cermets are specially suited for finish turning of steel due to their resistance to abrasive wear and notching. Since the material is more slippery than tungsten carbide, built-up edges are almost non-existent when the surface speed is maintained in the 500 to 1,000 surface feet per minute range.

All manufacturers of cermets are working hard on future developments. One of them is a partial substitution of titanium carbo-nitride with tantalum nitride. This is designed to increase the oxidation. resistance and lower the diffusion wear during steel machining. Tougher grades are under development and some have reached the market. These grades can be used for interrupted cuts during turning and also for finish milling.

In the United States and Europe, cermets are a small percentage of cutting inserts--approximately 2 to 3 percent. In Japan, this material constitutes more than 25 percent of all inserts. It is to be expected the same trend will follow in the United States.