Cermet vs Carbide vs CBN: Choosing a Cutting Tool Material

The three materials in one paragraph each

Cermet uses a titanium carbonitride hard phase in a Co/Ni binder with tungsten carbide and TaNbC additions. It delivers a very sharp edge and a fine surface finish, and it is intended for finishing and light semi-finishing.

Coated carbide uses tungsten carbide grains in a cobalt binder with a wear-resistant coating. It is the toughest of the three and the default for roughing, general turning and interrupted cuts.

CBN (cubic boron nitride, usually supplied as a PCBN compact) is second only to diamond in hardness and is designed for hard turning, typically above HRC45, where it can replace grinding.

Hardness, toughness and thermal behaviour

Against a P10 carbide, cermet is lighter (7.1 against 12.4 g/cm3) and of similar hardness (HV 1600 against HV 1550), but its transverse rupture strength is lower (1800 against 2100 N/mm2) and its fracture toughness is lower. It also conducts heat less than half as well, at 12 against 28 W/m.K.

That low thermal conductivity is the practical dividing line. Cermet holds its hardness at temperature and produces excellent surfaces, but it is sensitive to thermal shock and to impact, which is why it belongs in stable finishing operations.

CBN takes the opposite trade. It brings much higher hardness at temperature and better thermal behaviour in hard material, at a much higher tool cost.

Where cermet wins

• Finishing of carbon and alloy steel up to HRC45, where the finish can be good enough to eliminate a grinding step
• Finishing of ferritic, martensitic and austenitic stainless steel at a depth of cut below 0.1 mm
• Finishing of grey cast iron and malleable cast iron
• Aluminium alloy finishing and semi-finishing, provided silicon content stays below 10%
• Dry cutting, where cermet's oxidation resistance and hot hardness are an advantage
• Jobs where built-up edge is the persistent problem, because cermet has good chemical stability

Where carbide wins

• Roughing and heavy interrupted cuts, where toughness dominates
• Materials harder than the cermet range, or where scale, inclusions or hard spots are present
• General-purpose turning where one grade must cover many operations
• Applications where cost per edge, not cost per part, is the deciding factor

Where CBN wins

• Hardened steel above roughly HRC45, where cermet is out of its range
• High-volume hard turning that replaces grinding on shafts, gears and bearing components
• Interrupted cutting in hard material, where a cermet edge would chip
• Cases where the required surface finish and dimensional consistency must be held in hardened material

Cost is cost per part, not cost per insert

Cermet has a lower density than carbide and uses less tungsten and cobalt, so the material cost is competitive. The measure that matters, however, is cost per part, which is the insert cost divided by the number of parts produced per edge.

In the published field tests, KCT25 and KT20 inserts were run against reference Japanese cermet grades at identical parameters. On 45# steel the KT20 HQ insert produced 1200 pieces per edge against 1000 for the reference, a 1.2x gain; on 20Cr the MT insert produced 400 against 300, a 1.33x gain; and on 20# steel in interrupted cutting it produced 140 against 90, a 1.56x gain. Several other tests were level at 300 against 300 or 500 against 500, which is a reminder that a cermet gain is real only when it is measured on the actual part.

A short decision rule

• Harder than HRC45, genuinely interrupted, or impact-heavy: choose CBN or coated carbide
• Finishing steel, stainless steel or cast iron, need finish and consistency: choose cermet
• Roughing and general turning, one grade for many jobs: choose coated carbide
• Above all, test the candidate material on your own part before committing a production line to it