Machining of hard steels

Materials with high hardness fall under the ISO H group of materials. Their hardness makes them difficult to machine: the materials generate heat during cutting and are very abrasive for the cutting edge. Because of that, hardened steel is not a common work piece material for machining. Finishing is the most common machining operation for these materials. Typically, however, hard part turning falls within the range of 45-68 HRC.


The specific cutting force needed for hardened steel is  2,550–4,870 N/mm², which is quite high and also comes with a high power requirement. The operation usually produces fair chip control.

The cutting tool material needs to have good resistance to plastic deformation (hot hardness) and good chemical stability (at high temperatures), mechanical strength and resistance to abrasive wear. Cubic boron nitride (CBN) has these characteristics and makes turning possible, instead of grinding.

Cemented carbide dominates in milling and drilling applications and is used up to approx. 60 HRC. Mixed or whisker-reinforced ceramic is also used in turning when the workpiece has moderate surface finish demands and the hardness is too high for carbide.

Because of the tough conditions involved in machining of ISO H materials, the right coating can prolong the lifetime and improve the performance of the cutting tools involved. Ionbond engineers would be happy to discuss your specific application and guide you towards the most appropriate coating.

Application examples