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What are the best inserts for turning hardened steel?

Although hardened steel (H in the ISO material chart) is one of the smallest material groups, it can often be one of the most difficult types to machine. A main contributor to this is that hardened and tempered steels are usually associated with hardness levels of 45HRc – 70HRc.

To combat this, developments in turning solutions have led to special hard part turning solutions. These solutions have been proven to reduce insert cost by up to 70% when using the correct style insert and substrate.

Different levels of hardness should be tackled using the appropriate tooling:

  • For materials from HRc55 up to HRc70, the best solution would be CBN (Cubic Boran Nitride) turning inserts, which are designed specifically for cutting hardened steels.
  • Ceramic and Cermet inserts would cover the lower hardness hardened steels from HRc45 up to HRc55.

Using CBN on hardened steel can eliminate the need for grinding operations whilst providing an impressive mirror surface finish. These styles of insert come in a variety of common sizes and styles which fit in your standard tool holders (for instance CN, CC, DN, DC, TN, VC & VN styles).

Components where hardened steels are commonly found are gear box housing, gears, die stamping and drive pinions.

One thing to note is, when turning hardened steels in the region of 65HRc and 70HRc, your depth of cut will be reduced significantly compared to standard steel turning operations. You would expect depths of cuts around 0.3mm for the hardest materials, which will be reduced further to finish components accurately and increase surface roughness. Below are some cutting data examples of CBN turning grades.

It is also advised that, when hard part turning, you do not use coolant and ensure that the material is cut dry. This will drastically improve tool life due to reducing thermal shock whilst improving surface finish.

For any advice on turning hardened steels, or for more information on the range Cutwel supplies, please call the Cutwel Technical Team on 01924 869610, email sales@cutwel.net or visit www.cutwel.co.uk.