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.
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
|Steel (Quenched and tempered)||270-350||29-38|
|Stainless Steel (Austenitic)||70-180||<10|
|Heat resistance superalloys (Inconel)||130-450||<46|
Effect of Material Hardness on Machining
|60-120||0||Too Soft, almost impossible to break the chips, build-up-edge.|
|120-200||<15||Ideal, good chance to break chips, not too much wear.|
|200-400||16-44||Wear rate gradually increases as the hardness rises|
|400-550||45-55||Too hard to machine with carbide inserts, switch to ceramic or CBN inserts.|
|550-750||55-68||Too hard to machine with carbide or ceramic inserts, switch to CBN inserts.|
To cut metal, the cutting edge of the tool must be significantly harder than the raw material. Cutting materials are measured in HV units
|Material||Hardness (HV)||Main Applications|
|Carbide||1200-2000||Machining of all metals up to 47 HRC.|
|Ceramics||1500-3000||* Machining hardened steel at 45-55 HRC.|
* Machining cast iron and superalloys with higher cutting speeds.
|CBN||2000-4000||* Machining Steel above 55 HRC.|
* machining of cast iron with higher cutting speeds.
|PCD||4000-6000||Machining 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 Hardness||Hardness [Hv]||Application|
|Hard||1700-1900||Continuous turning in stable conditions|
|Balanced||1500-1700||Turning at less favorable conditions and Grooving|
|Tough||1300-1500||Milling, 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.