Hardness scale conversion

Hardness scales measure different things. Brinell and Vickers divide the test force by the surface area of a permanent impression and return a pressure. Rockwell measures the depth difference between a minor and a major load and returns a dimensionless number on an inverted scale. There is no physical law linking them, so every conversion between scales is an empirical correlation established by testing the same material on both scales — which is precisely what ASTM E140 tabulates.

The consequence is that a conversion is only as good as the material it was established on. ASTM E140 gives separate tables for non-austenitic steels, austenitic stainless steels, nickel and cobalt alloys, cartridge brass, copper, aluminium and wrought aluminium products. This tool uses the non-austenitic steel tables — carbon, low-alloy and martensitic steels in the wrought or heat-treated condition. Applying them to austenitic stainless, duplex, cast iron, nickel alloys or a heavily cold-worked surface will give a wrong answer, sometimes by tens of points.

Each scale also has a working range outside which it returns nothing meaningful, and the tool leaves those cells empty rather than extrapolating. Rockwell C is not reported below 20 HRC because the diamond cone is barely penetrating and the reading becomes unrepeatable. Rockwell B is not reported above about 100 HRB because the 1/16 in ball starts to flatten. Brinell with a 10 mm ball loses validity above roughly 650 HBW as the ball itself deforms.

The most important practical rule: if a specification states an acceptance limit on a particular scale, test on that scale. A weld procedure qualification that requires 248 HV10 maximum for sour service is not satisfied by a converted value from a portable Leeb or Rockwell instrument. Conversions are for interpreting historical records, comparing data from different sources, and sanity-checking — not for acceptance. The indicative tensile strength, UTS[MPa] ≈ 3.45 × HBW, carries the same caveat and is a screening figure for wrought carbon and low-alloy steel only.