Hardness Conversion (HRC / HV / HB)
Cross-convert Rockwell C, Vickers, and Brinell for non-austenitic steel per ASTM E140 Table 1, plus a tensile-strength estimate.
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The engineering
Hardness scales don't convert by physics — they convert by correlation. Rockwell C reads differential indentation depth under a diamond cone, Vickers reads the diagonal of a pyramid impression, and Brinell reads the diameter of a ball crater. ASTM E140 published the empirical correlation tables that let you compare a heat-treat cert quoting HRC against a drawing calling out HB. This card linearly interpolates the E140 Table 1 dataset for non-austenitic steels over the 20–65 HRC range.
The big gotcha: these conversions are only valid for the material class the table was built from. Aluminum, copper alloys, austenitic stainless, and case-hardened parts each have their own E140 tables — steel conversions applied to 6061-T6 will be wrong. The tensile-strength row uses the classic UTS ≈ 3.45·HB rule of thumb (≈500·HB in psi); treat it as an estimate good to roughly ±10%, never a substitute for a pull test on anything flight or fracture critical.
Sanity check: 40 HRC ≈ 392 HV ≈ 371 HB, and quenched-and-tempered 4340 at that hardness pulls around 1,280 MPa (~186 ksi). If your numbers land far from that ratio, check which Brinell load (3000 kgf standard here) the cert used.
Where this math comes from
Johan August Brinell, chief engineer at Sweden's Fagersta ironworks, needed a fast shop-floor check of steel quality and showed his ball-indentation test at the 1900 Paris Exposition — press a hardened ball into the part, measure the crater. It worked, but the optical measurement was slow and the ball useless on hardened steel. Stanley P. Rockwell, a metallurgist at a Connecticut ball-bearing plant, and his unrelated colleague Hugh M. Rockwell needed to verify bearing-race heat treatment quickly; their 1914 patent read indentation depth directly off a dial, no microscope required. Robert Smith and George Sandland at Vickers Ltd. added the diamond pyramid in 1921, giving one continuous scale from soft brass to tool steel.
Three tests, three incompatible numbers — and by mid-century every drawing, cert, and purchase spec mixed them freely. ASTM stitched the scales together statistically, issuing E140's conversion tables in 1958 from thousands of paired measurements on common steels. Those tables, essentially unchanged, are what this card interpolates.
- 1900Johan August BrinellDemonstrates the ball-indentation hardness test at the Paris Exposition.
- 1914Stanley P. & Hugh M. RockwellPatent the differential-depth tester for rapid bearing-race checks.
- 1921R. L. Smith & G. E. Sandland (Vickers Ltd.)Introduce the diamond-pyramid (Vickers) test spanning soft to hardened metals.
- 1958ASTMFirst issues E140 standard hardness conversion tables from paired-measurement data.
- 2012ASTME140-12 (reapproved 2019) — current conversion tables for steels and nonferrous alloys.
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