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What is Hardness?

Hardness is a measurement of the resistance to localized plastic deformation caused by force or abrasion. Materials with high hardness would generally be stronger and more wear-resistant, but on the other hand, more brittle and sensitive to fracture. Hardness is a broad topic, that well covered. In this post, we will cover it from the machining perspective.

Hardness in Machining

Hardness is one of the most critical parameters when it comes to any machining discussion. It is the primary parameter to know for both the raw materials and the cutting materials.

Raw Materials

Although counter-intuitive, it is not ideal to machine very soft material. It is impossible to break chips below a certain hardness, and the raw material tends to stick on the cutting edge causing BUE. From a certain hardness point, increasing the hardness further increases the wear and will either require you to reduce the cutting speed or settle for lower tool-life. Above a certain threshold, it will become impossible to machine the material with a conventional carbide insert, and advanced materials such as Ceramics and CBN will be mandatory.

Material Groups Hardness Range

Material GroupHBHRC
Steel (Annealed)70-270<28
Steel (Quenched and tempered)270-35029-38
Steel (Hardened)350-75039-68
Stainless Steel (Austenitic)70-180<10
Cast Iron180-30010-32
Heat resistance superalloys (Inconel)130-450<46

Effect of Material Hardness on Machining

60-1200Too Soft, almost impossible to break the chips, build-up-edge.
120-200<15Ideal, good chance to break chips, not too much wear.
200-40016-44Wear rate gradually increases as the hardness rises
400-55045-55Too hard to machine with carbide inserts, switch to ceramic or CBN inserts.
550-75055-68Too hard to machine with carbide or ceramic inserts, switch to CBN inserts.

Cutting Materials

To cut metal, the cutting edge of the tool must be significantly harder than the raw material. Cutting materials are measured in HV units

MaterialHardness (HV)Main Applications
Carbide1200-2000Machining of all metals up to 47 HRC.
Ceramics1500-3000* Machining hardened steel at 45-55 HRC.
* Machining cast iron and superalloys with higher cutting speeds.
CBN2000-4000* Machining Steel above 55 HRC.
* machining of cast iron with higher cutting speeds.
PCD4000-6000Machining aluminum with better surface finish and longer tool-life.

Harder is not always better!

The high hardness comes hand in hand with lower toughness and brittleness. Choosing a too hard grade can cause catastrophic failure from inserts breakage or lower tool-life due to the cutting edge’s chipping.

Carbide inserts recommended hardness per application

Grade HardnessHardness [Hv] Application
Hard1700-1900Continuous turning in stable conditions
Balanced1500-1700Turning at less favorable conditions and Grooving
Tough1300-1500Milling, parting off or Turning with interrupted cut

Hardness measurement and units

Hardness is measured in several methods and units. The standard units used in the machining space are:

  • Brinell (HB) Used to measure the hardness of raw materials in all hardness levels.
  • Rockwell C (HRC) Used to measure the hardness of raw materials at medium to high hardness levels.
  • Rockwell B (HRB)- Used to measure the hardness of raw materials at medium to low hardness levels.
  • Vickers (HV) – Used to measure the hardness of cutting materials (Carbide, Ceramic, CBN).
  • Gpa Used to measure the hardness of PCD.
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