I have made this thread really to respond to many questions I received concerning hardness testing. Questions like:
Which hardness test method should I use?
Can I convert HV/0.3 to HRC or HR15N to HRC?
I just want to make a few important comments:
Selection of best test method, scale and force will be dependent on:
* Material hardness level - Methods like Rockwell require the use of specific scale/loads to suit certain hardness ranges.
* Material size, thickness and geometry
* Thickness of surface treatments/coatings
* Macro/averaged hardness or hardness of individuals constituents (micro-hardness)
* Finally, It is important to use the same test method, scale and force stated in specifications/standards when testing for compliance.
Hardness is not an intrinsic material property dictated by precise definitions in terms of fundamental units of mass, length and time. A hardness property value is the result of a defined measurement procedure. Hardness can not be treated like measuring length. A measuring method like ruler, vernier, micrometer etc.. will result in the same fundamental unit value metres, inches or whatever (only accuracy being effected), while hardness measurement with different instruments will not - Vickers hardness is not the same as Rockwell hardness nor is HRC the same as HRB.
There is now a trend towards reporting Vickers and Brinell hardness in SI units (MPa or GPa, typically units for pressure and strength) particularly in academic papers. I am not keen on this and unfortunately, this can cause confusion. Vickers hardness (e.g. HV/30) value should normally be expressed as a number only (without the units kgf/mm2). Rigorous application of SI is a problem. Most Vickers hardness testing machines use forces of 1, 2, 5, 10, 30, 50 and 100 kgf and tables for calculating HV. SI would involve reporting force in newtons (compare 700 HV/30 to HV/294 N = 6.87 GPa) which is practically meaningless and messy to engineers and technicians.
Converting values between different hardness test methods, scales and force should really only be done for rough comparison. Conversions are not universal and can only be approximate. If stating a converted hardness value for ease of visualisation ie most people may have a better sense of 60 HRC than say 700 HV/30 or 90HR 15N, then this fine as long as it is stated that it is converted/equivalent and reference is made to the original test and conversion charts used.
More info
https://www.gordonengland.co.uk/hardness/