Cermet Cutting Tools: Complete Guide to Grades, Geometries and Applications

What a cermet insert is - and how it differs from carbide

Cermet is a powder-metallurgy material built from a hard ceramic phase bonded by a metal binder. In the Koves range the hard phase is titanium carbonitride, written Ti(C,N), carried in a Co/Ni binder with tungsten carbide and TaNbC additions.

Tungsten carbide is technically a cermet as well (WC plus Co), but in tooling the name is reserved for the TiC and Ti(C,N) families. TiC-based cermet was first produced in Germany in 1929 for steel finishing. From the mid-1970s Japanese makers raised its toughness and Ti(C,N) cermet moved into milling, and the older TiC family has since been largely replaced by Ti(C,N).

Compared with a P10 carbide, cermet is far lighter at 7.1 instead of 12.4 g/cm3, of comparable hardness at HV 1600 against HV 1550, but with a lower transverse rupture strength of 1800 against 2100 N/mm2. The biggest physical difference is thermal conductivity: 12 W/m.K against 28 W/m.K. That single property explains most of the process rules around cermet, because a material that conducts heat away slowly is sensitive to thermal shock.

• Better machined surface finish, often good enough to replace grinding with turning or milling
• Strong tool life in finishing and in stable semi-finishing conditions
• High oxidation resistance, which suits dry cutting
• Good chemical stability that suppresses built-up edge
• Good hot hardness for dry, high-speed cutting
• Low density and competitive cost
• Lower consumption of tungsten and cobalt, the two strategic raw materials

KT20 and KCT25: choosing the grade

Koves supplies two cermet grades. KT20 is the uncoated grade. KCT25 uses the same Ti(C,N) platform with a nano-scale AlTiN coating. Both have a 1.5 um grain size and a hardness of HV30 1620.

KT20 is specified as Co/Ni 12.5%, WC 20.0%, TaNbC 12.0%, balance TiCN. KCT25 is specified as Co/Ni 14%, WC 23.0%, TaNbC 6.0%, balance TiCN plus the nano-AlTiN layer. Both grades are described as combining high wear resistance with high toughness and as tolerant of dry cutting.

The choice is straightforward in most jobs. KT20 aims at the sharpest possible edge, which is what drives surface finish and dimensional consistency in a finishing pass. KCT25 adds coating-driven wear resistance in steel, stainless steel and cast iron turning and milling, so it is the better starting point when tool life and cost per part matter more than the last increment of edge sharpness.

How to read an ISO insert code - CCGT060202

The catalogue uses the ISO 1832 identification system, and the order of the symbols is fixed: insert shape, clearance angle, tolerance, chipbreaker or mounting type, insert size, insert thickness and corner radius.

Walking through CCGT060202 shows how the system works.

• C - insert shape: an 80 degree rhombic (diamond) insert
• C - clearance angle: a 7 degree relief
• G - tolerance class: a precision ground G class
• T - the mounting and chipbreaker field: hole and clamping style
• 06 - insert size, the 06 inscribed-circle size
• 02 - insert thickness
• 02 - corner radius: 0.2 mm

Chipbreaker and geometry codes

Every turning insert carries a chipbreaker suffix. The M grade, which is the uncoated KT20 family, and the G grade, which is the coated KCT25 family, share many prefix geometries but are tuned for different depths of cut.

A chipbreaker is not a cosmetic detail. It controls where the chip breaks, how much cutting force reaches the edge and whether the chip clears the work zone. Most complaints about surface finish and chip tangling trace back to a chipbreaker that does not match the depth of cut and feed in use.

Setting cutting conditions: Vc, feed and depth of cut

The catalogue publishes recommended starting values for every family. Treat them as a starting point and let the chip form and the surface finish guide the final numbers.

For M-grade turning with negative inserts, the MT chipbreaker runs at a depth of cut of 1-5 mm and 0.15-0.55 mm/rev. The finer JF and FG chipbreakers in the positive range take 0.3-2.0 mm and 0.05-0.25 mm/rev. In grooving, steel runs at 80-220 m/min and stainless steel at 70-160 m/min for both grades, with the feed set by the insert width. In threading, carbon, low-alloy and tool steels run at 100-150 m/min and stainless steel at 60-80 m/min.

The pattern across the range is consistent: carbon steel is fastest, stainless steel is slowest, and the feed window is narrow because cermet is a finishing material.

Cutting conditions for the G grade, grooving, threading and milling

The coated G grade takes a lighter depth of cut than the M grade in its finishing chipbreakers. NP runs at 0.05-0.6 mm and 0.02-0.20 mm/rev; TF runs at 0.1-3.0 mm and 0.05-0.25 mm/rev.

One summary table is enough to set a first trial across the main operations.

Materials you can machine - and the no-go zones

The catalogue is explicit about what cermet is designed to cut. Staying inside that list is the single most important condition for a good result.

• Carbon and alloy steel up to HRC45; semi-finishing is possible when the operation is stable
• Aluminium alloys in turning and milling, finishing and semi-finishing, but not alloys with more than 10% silicon
• Stainless steel - ferritic, martensitic and austenitic - in finishing with a depth of cut below 0.1 mm
• Pure iron for electromagnetic components
• Grey cast iron and malleable cast iron in finishing
• Not recommended: duplex stainless steel
• Not recommended: heavy interrupted cuts and roughing, where cermet's lower toughness limits it

Two process rules that matter more than any parameter

First, manage thermal shock. Cermet should be run either dry or under full flood cooling, never with intermittent, misted or insufficient coolant, because its low thermal conductivity makes rapid temperature change the main cause of edge damage. Cermet is well suited to dry cutting, so this is usually a simplification rather than a constraint.

Second, remember what the material is. Cermet is a finishing and light semi-finishing tool. Pushing it into roughing, into deep interrupted cuts, or past the HRC45 and 0.1 mm stainless limits will produce chipping rather than a better surface.

Troubleshooting: failure modes and corrections

The catalogue includes a turning and milling trouble-shooting guide. Most failures resolve to four root causes: the grade is wrong, the cutting conditions are wrong, the tool geometry is wrong, or the machine setup is not rigid enough.

The table below condenses the turning section of that guide.

When to step up to CBN or ceramic instead of cermet

Cermet has a clear boundary, and it is better to recognise it early than to fight it with parameters. Where the workpiece is harder than about HRC45, where the cut is genuinely interrupted, or where impact loads reach the edge, a different tool material is the correct answer.

• CBN for hardened steel above roughly HRC45, for high-volume hard turning that replaces grinding, and where the edge must survive interrupted cuts in hard material
• Ceramic for high-speed roughing and finishing of cast iron and for nickel-based superalloys, where chemical wear at high temperature dominates
• Coated carbide for roughing, heavy interrupted cuts and general turning, where toughness and cost per edge matter more than surface finish
• Cermet for finishing steel, stainless steel and cast iron, where surface finish, edge sharpness and dimensional consistency drive the process