Metal hardness converter

Vickers presses in a diamond pyramid with a 136-degree face angle and measures the diagonals of the impression. Because the impression is geometrically the same shape whatever the load, Vickers gives one continuous number series from an annealed block all the way up to a nitrided case. That is what makes it the scale for anything thin and anything small.

Brinell presses in a ten-millimetre tungsten-carbide ball at three thousand kilograms-force. The impression is large, so it averages across segregation and coarse structure — ideal for stock, useless on a hardened case, and above 650 the ball itself starts to flatten.

Rockwell measures no area at all, only the depth that remains after the load is removed. That is fast, needs no optics and can be read off the machine on the shop floor — the reason hardened tool steel is called out in HRC almost every time. The price is that each Rockwell scale only holds inside its own window: below 20 HRC the C scale loses its resolution, and above 100 HRB the B scale is finished.

A conversion table is not a calculation, it is a record of measurements. ASTM E140 was built by testing real specimens with several methods in turn and writing down what the relationship turned out to be. So the table holds exactly for the material group it was measured on — and for no other.

What pulls the numbers apart is work hardening: how much a material stiffens under the indenter depends on its structure and its history. Austenitic stainless does this markedly, tempered tool steel barely at all. That is precisely why the standard carries separate tables for austenitic steel, aluminium and brass — and why this calculator says so rather than quietly handing back steel figures.

So never convert twice over. Carrying an HRC reading through Vickers into Brinell stacks the same error on top of itself. And where an acceptance decision hangs on the number, measure what the drawing asks for.

Both standards do the same job and are not identical. ASTM E140 is the older and, in the tooling world, the more widely used collection; it keeps separate tables per material group and gives tensile strength in ksi. ISO 18265 covers the same ground metrically and tabulates tensile strength directly in megapascals.

Where both describe the same hardness, the hardness figures sit close together; the derived tensile strength differs by a few per cent, because the underlying tensile tests were different ones. This calculator works from the ASTM E140 tables and says so everywhere the two standards diverge. Where an acceptance specification names ISO 18265 explicitly, its table is the one that governs.

On a nitrided or coated part, the surface and the core are two different materials. A nitrided case reaches roughly a thousand to twelve hundred Vickers; the tempered core underneath stays around five hundred — one component, two numbers with nothing to do with each other.

For PVD coatings the gap widens further: TiN, TiAlN and DLC all sit far above anything ASTM E140 tabulates at all, and they are only a few thousandths of a millimetre thick. Measure them at Rockwell C loads and you measure the substrate straight through the coating. That is why a coating hardness is nearly always quoted as a Vickers or Knoop figure at a small load — and why the hardness ladder ends in a region no conversion reaches.

So the drawing has to carry both: which value applies, and where it is measured. The coatings on the materials page state their hardness figures individually for exactly this reason.

The usual form is value, tolerance, scale — 58 ± 2 HRC, say. The tolerance band belongs there: a hardness figure without limits is a wish, not a requirement, and two points of HRC is the spread a sound heat treatment realistically holds.

For case-hardened and nitrided surfaces, two more entries make the first one checkable: the test force and the location. A nitriding hardness is given as a surface hardness with the load named explicitly, often alongside the nitriding hardness depth; a case-hardened part also needs its core hardness, or it stays open what the component actually carries in service.

And the scale itself is part of the requirement. Where the drawing says HRC, a figure measured in HV and converted is not evidence — it is an indication that everything is probably fine.

For hardened tool steel in solid section, HRC is the obvious call: fast to measure, present in every heat-treatment shop, and its 20-to-68 range covers everything between tempered and through-hardened. For steel as delivered — annealed so it can be machined at all — the C scale no longer reaches down; there the callout is HBW or HRB.

As soon as the wall is thin, the area small, or the layer of interest only a few hundredths of a millimetre deep, the route runs through Vickers or Knoop, because the test force can be chosen freely there. And for cemented carbide, which sits above the Rockwell C scale entirely, HRA is the usual figure.

One habit saves more argument than any conversion: use the same scale for the requirement, the in-process check and incoming inspection. Three scales on one part produce three numbers that are all correct and still do not agree.