Hardness Conversion
Converts a hardness reading on one scale to the other two: Rockwell C (HRC), Brinell (HBW) and Vickers (HV). It uses the ASTM E140 conversion table for non-austenitic steels: carbon, alloy and tool steels [1]. Engineers, heat treaters and inspectors use it to compare a mill certificate or a test report with a drawing that calls for a different scale.
How the conversion works
The page holds the E140 rows for HRC 20 to 65 in steps of 5 [1]. Brinell runs from 226 to 739 HBW over that span and Vickers from 238 to 832 HV. The HBW values above about 650 are for a tungsten-carbide ball. Between two rows the page interpolates in a straight line:
y = ya + (x − xa)·(yb − ya)/(xb − xa)
x is your reading on the input scale, xa and xb the table values on that scale just below and above it, and ya and yb the values in the same rows on the scale you want. Outside the table the page extends the end rows’ line, adds a note that the values are extrapolated, and refuses a reading so far out that the result would be zero or negative.
Limits and common mistakes
- The conversions are empirical. Each scale measures a different indentation, so a converted value is an estimate, not a test result. If a specification calls for a scale, test on that scale.
- The table is for non-austenitic steels only. Don’t use it for austenitic stainless steel, cast iron, aluminum, copper alloys or case-hardened surfaces; E140 has separate tables for some of them [1].
- Hardness is reported as a whole number on the HBW and HV scales, and HRC to a whole number or one decimal at most. The page shows more digits only because it interpolates. Round the result before you write it down.
- Below HRC 20 the C scale isn’t used. Start from Brinell or Vickers for softer steels, and treat anything below 226 HBW as extrapolated.
Worked example
A mill certificate gives 300 HBW for a bar of pre-hardened 4140. The drawing calls for HRC 28–32. Does the bar pass?
- Choose Input Scale Brinell (HBW).
- Enter Brinell Hardness (HBW) 300 and press Solve. The Solution shows:
Rockwell C (HRC): 31.7073
Vickers (HV): 316.024
That rounds to HRC 32 and 316 HV, inside the drawing’s range but near its top. Check by hand: 300 HBW lies (300 − 286)/(327 − 286) = 0.341 of the way from the HRC 30 row to the HRC 35 row, so HRC = 30 + 0.341 × 5 = 31.7. In this part of the table HV is about 1.05 × HBW, and 1.05 × 300 = 315. A part this close to the limit is worth a Rockwell test of its own.
