Hardness Testing Methods and Conversion

[Gordon]

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/

[attin]

Thanks for all information and attention...

[Stuart Milton]

Some great points raised here. Currently dealing with a situation where a customer is looking to coat crankshaft journals. The wrought journals are hardened and have a hardness of around 60HRc. The customer is specifying that the coating should be 60HRc also but are looking at the age old arcpsrayed 75E bond, 60E top coat which has been used for many years (and flamesprayed before arc was around). We're in the process of explaining that although a coating may not have a hardness of 60HRc, the micro-hardness of the particles are likely to exceed 1000HV and hence the important thing, e.g. how the coating wears, will be more than sufficient for their needs.

Just an example of where an understanding (or more a misunderstanding) of hardness data and it's relevance to application can cause a lot of confusion. We have used one of the HV to HRc conversion tables to give the potential customer an indication of the 'equivalent' HRc but this is done with some caution.

Thanks for highlighting again the minefield of hardness testing and it's relevance to real life.

[Gordon]

Just for fun and to highlight further comments from above posts consider:

Typical plasma sprayed Tungsten carbide/cobalt coating compared to hardened steel.

WC/Co 750-950 HV/0.3. Claimed macro-hardness 50-55 HRC ( I have issues with this, as I believe HRC would not have been original test and that it is converted from HR 15N. No reference to original test method and conversion charts used. example of bad practice)

Steel with roughly same 750-950 HV/0.3 and HRC around 62-65.

Both roughly in the same ball park as measured by Vickers, the nod going to the steel for higher Rockwell.

Now, if we evaluate these using Mohs scale, would you expect them to be equal? Would you expect steel to scratch WC/Co, the other way round or no difference? Bear in mind an individual tiny WC particle with in the WC/Co could possibly have micro-hardness values of 2 -3 times (on Vickers scale) that of the bulk HV/0.3 or HRC indicate.

Really, just trying to get the point over that Vickers hardness is not the same as Rockwell hardness is not the same as Mohs hardness etc. They are just test methods with their own specific interpretation of "hardness" value. Units of hardness like HV/0.3 and HRC are not like units of mass where value can be accurately converted between different units (Kg and lb). Conversion of hardness units really only works when we practically compare results of different methods under the exact same conditions and material. This leads to the point that two materials, say with the same HRC hardness value will not necessarily have equal hardness values when tested with another method/scale and also probably more important, these two materials may have quite different wear properties.