Hardness Conversion Chart — HRC, HB, HV and Tensile Strength

Rockwell C presses a diamond cone into the surface and reads the depth of penetration at a 150 kgf load; Brinell presses a 10 mm ball at 3000 kgf and reads the diameter of the impression. The Brinell indentation is much larger, so it averages out surface roughness and is the usual shop method on castings, forgings and annealed stock, while Rockwell C is faster on finished, hardened parts. Because the two tests measure different volumes of material at different loads, the conversion between them is empirical — that is exactly what this table is.

The diamond cone in Rockwell C barely penetrates soft steel, so a small reading error becomes a large hardness swing — the standard lists values in that range only as estimates. Below roughly 20 HRC the correct move is to report HB, HV or HRB instead. The second table on this page covers that band on the scales suited to it, so a soft annealed batch is quoted on a scale the mill can actually hit.

Only roughly, and only for carbon and low-alloy steels in a quenched and tempered condition. The rule of thumb is tensile strength in MPa = 3.45 x HB, valid within about 15% in the 150-450 HB band. The tensile column on this page follows the published conversion tables rather than that single rule, but treat every figure in it as an indication: for a certified value the material has to be tensile tested to the relevant standard, not converted from a hardness reading.

They are separate standards built from different data sets and slightly different rounding, so equivalent rows can sit a unit or two apart — for example around HV 390 the ASTM-based table reads about 369 HBW while the ISO table reads about 371 HB. That is not a typo in either document, it is the scatter of an empirical conversion. Pick one system, state which one you used on the drawing or certificate, and do not mix rows from the two on the same part.

HRC for through-hardened or case-hardened steel that lands between about 20 and 68 HRC — that is what a heat-treatment supplier will measure by default, and 42-48 HRC or 58-62 HRC are the ranges most often seen on drawings. Below 20 HRC use HB or HRB; for very thin hardened layers, cemented carbide and case depths under about 0.5 mm, use HRA or HV instead, because the Rockwell C load reaches deeper than the layer itself.

No, and this is the most common way a converted number goes wrong. The table above is for non-austenitic carbon, alloy and tool steels. Austenitic stainless (304, 316), nickel and high-nickel alloys, cast iron, copper alloys and aluminum each sit on their own conversion curve, and applying the steel table to them can misread by well over ten percent. Send the material grade with the hardness figure and we will confirm which table applies before anything is ordered against it.

Find the HV value in the left column of the main table and read across to the HRC, HBW and HRA columns on that row. Vickers is the most linear of the common scales, which is why both ASTM E140 and ISO 18265 index their steel tables on hardness values that include HV. If the value falls between two rows, quote the nearer row rather than interpolating: hardness relationships are not linear between rows, and an interpolated mid-point can be further from the truth than either neighbour.

Only if the ball was tungsten carbide. Above roughly 450 HB a hardened steel ball deforms under the 3000 kgf load, so the standards require a tungsten carbide ball — written HBW — and readings taken with a steel ball in that range understate the hardness and are not comparable. The main table is in HBW for that reason; older certificates that simply print HB may be either, so check the indenter before converting a value from the high range.