Sumitomo vs Kyocera: Best Inserts for Hardened Steel Turning HRC 50+

Hard turning has revolutionized the finishing of hardened steel components, replacing grinding operations in many applications and enabling complete part machining on a single CNC lathe. When workpiece hardness exceeds HRC 50, conventional carbide inserts fail rapidly through abrasive and diffusive wear, requiring either CBN (cubic boron nitride) tipped inserts or advanced ceramic grades. Sumitomo Electric and Kyocera are two Japanese manufacturers at the forefront of hard turning technology, each offering specialized grades for machining hardened steels from HRC 50 to HRC 68. This article provides a detailed technical comparison of their hard turning solutions.

The Hard Turning Challenge

Machining hardened steels above HRC 50 places extreme demands on the cutting tool. The cutting forces are 2-3 times higher than in conventional turning, temperatures at the cutting edge exceed 800 degrees Celsius, and the abrasive martensitic microstructure causes rapid flank wear. The key requirements for successful hard turning include high hot hardness to resist thermal softening, chemical stability to prevent diffusion wear, and sufficient fracture toughness to withstand the high mechanical loads. Both CBN and ceramic inserts address these requirements, but with different performance characteristics.

Sumitomo’s Hard Turning Solutions

Sumitomo Electric Hardmetal is a global leader in CBN insert technology, with a product range that spans from standard tipped inserts to solid CBN tools. Their BNX25 grade is a high-content CBN grade (approximately 90% CBN by volume) with a ceramic binder, designed for continuous turning of hardened steels at HRC 55-65. The BNX25 provides exceptional abrasive wear resistance and can maintain dimensional accuracy of +/- 5 um over production runs of several hundred parts.

For interrupted hard turning, Sumitomo’s BNC200 grade features a lower CBN content (approximately 65%) with a tougher ceramic-metallic binder that absorbs impact energy from interruptions such as keyways, splines, and oil grooves on hardened shafts. The BNC300 grade further increases toughness for heavy interrupted cuts, making it suitable for machining hardened gears with tooth gaps and bearing races with mounting slots.

Sumitomo also offers the KBN series of whisker-reinforced ceramic inserts for hard turning, specifically the KBN250 grade which uses silicon nitride whiskers in an alumina matrix to provide high hot hardness and thermal shock resistance. This grade is effective for continuous turning of hardened steels at HRC 50-60 at cutting speeds of 150-350 m/min, offering a lower-cost alternative to CBN for applications where the surface finish requirements are less stringent.

Kyocera’s Hard Turning Solutions

Kyocera’s hard turning lineup centers on their advanced ceramic and CBN grades. The KBN520 grade is Kyocera’s premium CBN offering, featuring a high-CBN-content structure (approximately 85%) with a titanium carbonitride binder that provides both abrasion resistance and chemical stability. The KBN520 is optimized for continuous hard turning of bearing steels (100Cr6, HRC 58-64) and case-hardened steels (20MnCr5, HRC 58-62) at cutting speeds of 120-250 m/min.

For interrupted cuts, Kyocera offers the KBN535 grade with reduced CBN content and a more compliant binder system that provides better thermal shock resistance. The KBN535 handles moderate interruptions such as oil holes and light splines without chipping, extending the application range of CBN inserts beyond purely continuous cuts.

Kyocera’s ceramic option for hard turning is the KS6000 series, an alumina-based ceramic reinforced with titanium carbide particles. The KS6025 grade covers hardened steel turning at HRC 50-62, providing a cost-effective alternative to CBN for non-critical operations. Kyocera’s advanced silicon nitride ceramic grade, the KS8000 series, offers superior thermal shock resistance for applications where coolant application is intermittent or where the tool enters and exits the cut frequently.

CBN Grade Comparison

Ceramic Grade Comparison

Application Scenario: Bearing Race Hard Turning

Hard turning of bearing races from 100Cr6 (SAE 52100) at HRC 60-64 is one of the most common hard turning applications. In this scenario, Sumitomo’s BNX25 grade with a CNGA insert and honed cutting edge achieves surface finishes of Ra 0.2-0.4 um at Vc = 150 m/min and f = 0.10 mm/rev, meeting the typical Ra 0.4 um requirement for bearing raceways. The tool life reaches 500-600 inner races per edge before dimensional drift exceeds the +/- 5 um tolerance band.

Kyocera’s KBN520 under the same conditions achieves Ra 0.3-0.5 um surface finish with 400-500 parts per edge. The slightly broader surface finish range is attributed to Kyocera’s edge preparation, which uses a larger hone radius (approximately 20 um vs Sumitomo’s 15 um) that provides better edge security but slightly increases cutting forces and affects the minimum achievable surface roughness.

Application Scenario: Hardened Shaft with Keyway

When turning hardened shafts (42CrMo4, HRC 55-58) with longitudinal keyways, the interrupted cut at each keyway entry and exit demands high toughness from the CBN insert. Sumitomo’s BNC200 handles this application at Vc = 100-150 m/min and f = 0.12 mm/rev, with the lower CBN content and tougher binder absorbing the impact energy. Typical tool life is 80-120 shafts per edge. Kyocera’s KBN535 performs comparably at similar speeds, achieving 70-110 shafts per edge, with the primary failure mode being micro-chipping at the keyway entry point.

Budget Alternatives for Hard Turning

For operations where CBN insert costs are prohibitive, ceramic inserts from brands like Korloy and ZCC-CT offer a viable entry point for hard turning at HRC 50-58. Korloy’s ceramic grade provides basic hard turning capability for non-critical finishing passes on hardened components where Ra 1.6-2.0 um surface finish is acceptable. While these ceramics do not match the tool life or surface finish of Sumitomo or Kyocera CBN grades, they enable shops to begin hard turning operations without the initial investment in CBN tooling.

When to Choose Which

Choose Sumitomo when you require the highest dimensional accuracy and surface finish in continuous hard turning, when machining bearing steels above HRC 60, or when you need a complete CBN product range that includes dedicated interrupted-cut grades. Sumitomo’s BNX25 is the benchmark for precision hard turning in the bearing and automotive industries.

Choose Kyocera when you need higher cutting speeds in hard turning, when thermal shock resistance is important for applications with intermittent coolant or frequent air cuts, or when you want the flexibility of both CBN and advanced silicon nitride ceramic grades from a single supplier. Kyocera’s KS8000 Si3N4 ceramic is unique in the market for its thermal shock performance.

Conclusion

Sumitomo and Kyocera both offer world-class solutions for hardened steel turning above HRC 50, with Sumitomo leading in precision CBN performance and Kyocera providing broader ceramic options and competitive CBN grades. The choice between CBN and ceramic depends on your surface finish requirements, production volume, and budget. Both Sumitomo and Kyocera hard turning inserts, along with cost-effective ceramic alternatives from Korloy, are available at hooguu.com with expert technical support to help you select the optimal grade for your hard turning challenges.

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