Koves offers two cermet grades, and the difference between them is not a matter of quality but of where the edge spends its life. Both share the same Ti(C,N) platform, the same 1.5 um grain size and the same HV30 1620 hardness. The decision comes down to coating, cut continuity and workpiece material.
One substrate family, two grades
KT20 is the uncoated grade, specified as Co/Ni 12.5%, WC 20.0% and TaNbC 12.0%, balance TiCN. KCT25 is the coated grade, specified with a slightly different composition of Co/Ni 14%, WC 23.0% and TaNbC 6.0%, balance TiCN, and a nano-scale AlTiN coating.
The stated ISO application ranges differ accordingly. KT20 is listed as P05-P25, M10, K05-K10 and N05-N10. KCT25 is listed as P10-P25, M10-M20, K10 and N10. Both are described as high-toughness grades that combine wear resistance with stability in complex working conditions.
| KT20 | KCT25 | |
|---|---|---|
| Coating | Uncoated | Nano AlTiN |
| Composition | Co/Ni 12.5%, WC 20.0%, TaNbC 12.0%, balance TiCN | Co/Ni 14%, WC 23.0%, TaNbC 6.0%, balance TiCN |
| Grain size | 1.5 um | 1.5 um |
| Hardness | HV30 1620 | HV30 1620 |
| ISO application range | P05-P25 / M10 / K05-K10 / N05-N10 | P10-P25 / M10-M20 / K10 / N10 |
| Typical role | Precision finishing and sharp-edge turning and milling | Finishing and light interrupted cutting with longer tool life |
What the nano-AlTiN coating changes
The coating does not make the substrate harder - the substrate is the same HV30 1620 in both cases. It protects the surface of the edge from the two mechanisms that end an insert's life first: abrasive wear on the flank and chemical wear, or cratering, on the rake face.
The catalogue states the coating extends tool life in precision finishing and in light interrupted cutting of steel and stainless steel, and that KCT25 in combination with the nano-scale coating is used for turning and milling of steel, stainless steel and cast iron.
- Choose KCT25 when the operation is repetitive and edge life drives cost per part
- Choose KCT25 when there is any light interruption, where substrate wear accelerates
- Choose KT20 when the sharpest possible edge is the priority, as in very light finishing
- Choose KT20 for aluminium and other non-ferrous finishing, where a sharp uncoated edge cuts cleanly
- Consider both when in doubt - the same geometry is offered in both grades in most series
Continuous versus interrupted cutting
A continuous finishing pass loads the edge smoothly. Either grade will run, and the choice is usually made on life, which favours the coated grade.
Light interruption - a keyway, a cross-hole or a scale-broken surface - produces impact and thermal cycling at the edge. Here the coating's wear resistance is worth having, but the underlying toughness of the substrate is what prevents chipping, and both grades are described as high-toughness. For heavy interruption, neither grade is the right answer; a tougher carbide or CBN should be used.
Steel versus stainless steel
Steel is the primary application. Carbon and alloy steels up to HRC45 can be finish and semi-finish turned, and in stable conditions the catalogue states cermet can withstand semi-finishing. The cutting speeds are the highest in the programme: 220-460 m/min for low-carbon steel with the M-grade MT chipbreaker.
Stainless steel behaves differently. It is gummy, work-hardens, and conducts heat poorly, so the catalogue sets a lower speed and a tighter limit: ferritic, martensitic and austenitic stainless steel in finishing with a depth of cut below 0.1 mm. In grooving, stainless runs at 70-160 m/min against 80-220 m/min for steel; in threading, 60-80 m/min; in small-bore work, 30-80 m/min. Duplex stainless steel is not recommended at all.
What life gains look like in test data
The published field tests compare Koves cermet inserts against reference Japanese cermet grades at identical parameters, on the customer's own part. The results are a mixture of gains and level results, which is the honest picture for any tool material.
- 45# steel, continuous, TNMG160408 KT20 HQ: 1200 pieces per edge against 1000 for the reference, a 1.2x gain
- Carbon steel, continuous, TNMG160404 KT20 R-VF: 1000 against 900, a 1.11x gain
- 20Cr, continuous, TNMG160404 KT20 MT: 400 against 300, a 1.33x gain
- 20# steel, interrupted, WNMG080404 KT20 MT: 140 against 90, a 1.56x gain
- 20# steel, continuous, WNMG080404 KT20 MT: 600 against 550, a 1.09x gain
- Several tests were level: 300 against 300 on 40Cr with VBMT11T304 KT20 HQ and with TNMG160404 MT in interrupted cut, and 500 against 500 and 300 against 300 on 20# steel
A simple selection rule
Start with KCT25 for production finishing of steel, stainless steel and cast iron, because the coating extends life at no cost in edge geometry. Move to KT20 where the sharpest edge is the objective, where the material is non-ferrous, or where a test shows it produces the better finish or the longer life on your specific part.
In both cases, hold the operation inside the cermet band: finishing or light semi-finishing, stable conditions, and cooling run dry or as full flood.
FAQ
Is KCT25 harder than KT20?
No. Both grades are quoted at HV30 1620 with a 1.5 um grain size. The difference is the nano-AlTiN coating on KCT25, which raises wear resistance at the surface rather than changing the hardness of the substrate.
Which grade should I use for stainless steel?
KCT25 is the usual starting point because of its coating and its M10-M20 stainless range, with the depth of cut held below 0.1 mm and speeds set well below steel - for example 70-160 m/min in grooving. KT20 is also listed for stainless finishing and can be trialled where edge sharpness matters most.
Can the same insert geometry be ordered in both grades?
In most series, yes. The catalogue lists KT20 as the preferred (standard stock) grade and KCT25 as an available option for the same model, so geometry can be held constant while the grade is changed for a test.
Related
- Cermet Cutting Tools: Complete Guide to Grades, Geometries and Applications
- Cermet vs Carbide vs CBN: Choosing a Cutting Tool Material
- Hard Turning vs Grinding: When Cermet Turning Replaces Grinding
- Cermet Insert Troubleshooting: Causes and Fixes for Common Failures
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