Hardness Units - MPa

[mrfailure]

I recently dealt with a modern version of an SAE cast iron spec (SAE J431) which used the preferable units for hardness as MPa. I had never seen this before. I understand the logic of how one could use these units, but I have never seen hardness testers report in MPa either. SAE in the spec would prefer you to use metric MPa/MPa ratios for tensile strength/hardness (luckily, they also specify psi/HB, though they consider these secondary units and really prefer the MPa hardness). Am I out of date here? Does anyone else use MPa instead of HB for cast iron hardness?

 

[CoryPad]

No, you are not out of date, SAE is just ahead of the curve regarding SI units. Conversion from gram-force or kilogram-force to newtons is straightforward, and multiple standards already show either N/mm² or GPa (e.g. ASTM E384, ISO 6506-1, ISO 6507-1). I expect that within our lifetime we will see a migration towards hardness expressed in pascals.