Cermet Inserts for Hardened Steel Finishing

Hardened and pre-hardened steel is the job cermet was invented for. A sharp Ti(C,N) edge holds its hardness at temperature, resists oxidation without coolant and produces a finish fine enough to remove a grinding step, provided the hardness stays inside the cermet range.

Where cermet fits in hardened steel

The catalogue sets the cermet range at carbon and alloy steel up to HRC45, with semi-finishing possible when the operation is stable. Within that range cermet is a finishing and light semi-finishing tool: it removes the last small allowance and sets the final size and finish.

Above HRC45 the material is outside the stated cermet range. That is the point at which CBN becomes the candidate, because it keeps much more of its hardness in hard material and tolerates the loads that would chip a cermet edge.

• Pre-hardened mould and die steels, and tool steels, in finishing passes
• Hardened shafts, bushings, pins and rings below the HRC45 limit
• Parts where the finishing pass is stable and the allowance is small
• Operations where a ground finish is needed but a grinding machine is not available or not economic

Recommended geometries

For external finishing, the positive G-grade inserts give the sharpest edges and the lightest cuts. CCGT, DCGT, VCGT and VBGT cover the common turning profiles, with TCGT for smaller work. For heavier semi-finishing passes at larger depth of cut, the negative M-grade MNMG and WNMG/CNMG inserts carry more edge strength.

The clean way to start is to pick the insert shape that matches the part geometry - a 55 or 35 degree point for profiling, an 80 degree rhombic or a 60 degree triangle for generating and facing - then set the chipbreaker to the depth of cut.

• CCGT, CCGT06 and CCGT09 sizes for general finishing and generating
• DCGT and VBGT for profiling where the approach angle must clear a shoulder
• VCGT for small-radius internal and external profiles
• TNMG and WNMG negative inserts with MT or FS chipbreakers for semi-finishing passes

Cutting conditions

Use the catalogue values as the window. For M-grade negative turning the MT chipbreaker runs at 1-5 mm depth and 0.15-0.55 mm/rev, and speeds rise as the steel gets softer: 220-460 m/min for low-carbon, 150-270 m/min for medium-carbon and 130-200 m/min for high-carbon steel.

For G-grade finishing the NP chipbreaker takes 0.05-0.6 mm depth and 0.02-0.20 mm/rev, at 240-480 m/min in low-carbon, 160-260 m/min in medium-carbon and 140-240 m/min in high-carbon steel. The TF chipbreaker runs 0.1-3.0 mm deep at 0.05-0.25 mm/rev.

Process rules for hardened steel

• Run dry or with full flood cooling, never with partial or misted coolant
• Hold the depth of cut inside the chipbreaker window: too light a pass causes rubbing and chatter
• Match the corner radius to the pass; larger radii spread the load but need more depth to cut cleanly
• Check tool overhang before blaming the insert - chatter in a hardened part is usually rigidity, not the grade
• Confirm the hardness of the batch; cermet assumes a stable, consistent material

Verifying the result

The published tests on hardened and medium-carbon steels show that results vary by part. On 20Cr the KT20 MT insert produced 400 pieces per edge against 300 for a reference Japanese cermet grade, a 1.33x gain, while other tests on 40Cr and 45# steel were level. That is why a cermet grade for hardened steel finishing should be proven on the actual part.

Measure finish and size across a full edge rather than a few parts, and compare cost per part against the existing route, whether that is grinding or another insert.

Related

• CCGT Coated Cermet Turning Inserts — G Grade — 27 models
• DCGT Coated Cermet Turning Inserts — G Grade — 17 models
• VCGT Coated Cermet Turning Inserts — G Grade — 7 models
• VBGT Coated Cermet Turning Inserts — G Grade — 20 models
• TCGT Coated Cermet Turning Inserts — G Grade — 11 models
• TNMG Negative Cermet Turning Inserts — M Grade — 14 models
• WNMG Negative Cermet Turning Inserts — M Grade — 10 models
• CNMG Negative Cermet Turning Inserts — M Grade — 6 models
• VNMG Negative Cermet Turning Inserts — M Grade — 6 models

Planning note

Set the finishing allowance from the catalogue depth-of-cut window for the chosen chipbreaker - 0.05-0.6 mm for the G-grade NP insert, 0.3-2.0 mm for the positive M-grade finishing breakers. Run dry or full flood, and set Vc inside the band for the steel type; low-carbon steel tolerates the highest speeds, high-carbon the lowest.