CNC Knowledge

Material Hardness: What the Tool Engineer Must Understand First

Hardness decides which insert survives the first pass, how much stock you can take, and why a part passes inspection but fails in service. This page covers the common scales, how to convert between them, and where hardness stops being a useful number. Read it if you specify materials, plan processes, or sign off on incoming stock.

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Rockwell / Brinell / VickersHRC vs HV conversionTool life and feedsCase depth vs core

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What hardness actually tells you

One number, three different meanings: resistance to indentation, resistance to scratching, and resistance to rebound. Know which one you measured.

Definition

Hardness is not one property

Hardness measures how a material responds to a localized load. Push a hard indenter into the surface and you get a number. That number describes resistance to permanent deformation, not toughness, not fatigue life, and not corrosion resistance. A 60 HRC die insert and a 60 HRC cutting tool can behave very differently under impact.

For a tool engineer, the useful question is not what the hardness value is, but what load case it represents. A bearing race cares about rolling contact fatigue. A punch cares about compressive yield. A spring cares about elastic recovery. Each one needs a different part of the hardness story.

Hardness is also cheap to check. A bench tester in the inspection room gives a reading in seconds, where a tensile test needs a machined coupon and a day. That is why hardness is used as a proxy for strength, heat treatment condition, and wear resistance on the shop floor.

1
Indentation hardnessRockwell, Brinell, Vickers. The number most drawings quote.
2
Scratch hardnessMohs scale. Rarely used for metals, common for coatings and minerals.
3
Rebound hardnessLeeb and Shore. Portable, useful on large parts, less accurate.

Measurement

Reading the common hardness scales

Rockwell C uses a diamond cone under 150 kgf. It suits hardened steel from about 20 HRC up to 70 HRC. Below 20 HRC the indentation gets large and the scale loses resolution, so switch to Rockwell B with a 1/16 in ball. Aluminum, brass, and mild steel in the annealed state belong on the B scale or on Brinell.

Brinell presses a 10 mm carbide ball under 3,000 kgf and measures the impression diameter with a microscope. The large impression averages out local variation, which makes it a good fit for castings and forgings with coarse grain. The tradeoff is a visible dent, so Brinell is usually a sample test, not a finished-part test.

Vickers uses a diamond pyramid and works across the full range from soft aluminum to carbide. Because the indenter is a pyramid, the geometry stays similar at every load. That makes Vickers the practical bridge when you need to convert HRC to HV or compare a thin case to a soft core. Micro-Vickers with loads under 1 kgf lets you map a carburized case in steps of 0.1 mm.

Reference

Scale selection by material and feature

Pick the scale before you pick the specification limit. The wrong scale gives a number that cannot be compared to the drawing.

Material / feature	Scale	Typical load	Watch out for
Hardened tool steel, 55–65 HRC	Rockwell C	150 kgf diamond	Cracks near edges on thin sections
Annealed aluminum, brass	Rockwell B	100 kgf ball	Cold worked surface reads high
Castings, forgings	Brinell	3,000 kgf ball	Visible dent, sample only
Carburized case	Micro-Vickers	0.3–1 kgf	Case depth must exceed 3× indent
Large shafts, in place	Leeb rebound	Portable	Needs polished, flat surface
Ceramic and carbide	Vickers or Knoop	1–30 kgf	Edge chipping skews reading

Process

How hardness changes the cut

Cutting speed drops as hardness rises. A 6061-T6 bracket at 95 HRB machines at 400–600 m/min with uncoated carbide. A 4140 pre-hard at 30 HRC wants coated carbide and roughly half that speed. At 50 HRC the same geometry needs CBN or ceramic, and the toolpath should favor constant engagement over sharp corners.

Feeds follow a similar rule, but not linearly. Hard materials shear in a narrow zone, so the chip is thinner and hotter. Reduce feed per tooth too far and you rub instead of cut, which work-hardens the surface and shortens tool life. On 17-4PH in the H900 condition, a light finishing pass with a worn insert can raise surface hardness by 10 to 15 percent in the top 0.05 mm.

Depth of cut is where hardness and machine rigidity meet. A 60 HRC die block on a small 3-axis machine will chatter before the insert fails, and the chatter marks will show up in the finish. That is a setup limit, not a material limit. On a 5-axis center with a rigid holder and short gauge length, the same block can be finished with a 0.2 mm stepover and Ra 0.8–1.6 μm.

Judgment

When hardness is the wrong spec

Hardness alone will not tell you if a part will survive fatigue. Two 4140 shafts at 28 HRC can have very different fatigue lives depending on inclusion content and surface finish. A hardness check passes both. Only a fatigue test or a controlled cleanliness spec separates them.

Case-hardened parts are the classic trap. A drawing that says 58–62 HRC on a carburized gear says nothing about case depth or core hardness. A part with a 0.3 mm case and a 25 HRC core will chip under shock load, while the same surface spec with a 0.8 mm case and a 35 HRC core holds up. Specify both, and specify the transition zone.

Very soft materials swing the other way. Annealed copper and pure aluminum read so low on Rockwell C that the test is meaningless. Use Brinell or Vickers, and check that the indentation does not punch through the wall of a thin part. On a 1 mm wall, even a micro-Vickers indent can deform the back face.

1
Add core hardnessFor any case-hardened or induction-hardened part.
2
Add case depthEffective depth at a stated limit, usually 50 HRC or 550 HV.
3
Add surface finishRa drives fatigue life as much as hardness does.
4
Add test locationEdge, midsection, or core. State the distance from the surface.

FAQs

Questions engineers ask after the first cut

Can I convert HRC to HV directly?

Approximately, yes, using standard conversion tables. The conversion is empirical, not physical, so it holds best in the mid range from 20 to 60 HRC.

Below 20 HRC and above 65 HRC the tables drift by several points. If the specification is tight, test the actual material on both scales rather than converting.

Why does the same alloy read differently on two machines?

Most often it is surface condition. Decarburized or cold-worked skin reads soft or hard relative to the core. Remove 0.2–0.5 mm before testing.

Anvil seating and part support also matter. A thin part that flexes under load gives a low reading on every scale.

Does hardness affect machined tolerance?

It affects the process, not the achievable limit. We hold ±0.005 mm on hardened 17-4PH and on soft 6061 alike, but the hardened part needs more passes, sharper tools, and often a stress-relief step between roughing and finishing.

Hardened thin walls move after cutting. If the drawing has a tight flatness call, plan a semi-finish, a stress relief, then a light finish.

What hardness should I specify for a wear part?

Start from the wear mechanism. Abrasion wants high surface hardness, usually 55 HRC or above. Adhesive wear and galling want dissimilar materials or a coating, and hardness alone will not fix them.

Impact wear needs a tough core with a hard surface. A through-hardened 60 HRC part will crack where a case-hardened part survives.

How many hardness readings should a lot have?

For a normal production lot, one reading per heat or per batch is common, taken from the same location on each part. Add a second location for any heat-treated part.

For safety-critical parts, map the case at three depths and record the values. We keep those reports on file and can send them with the shipment.

Can you machine a part after it is hardened?

Yes, within limits. Up to about 45 HRC we mill and turn with coated carbide. From 45 to 62 HRC we use CBN or ceramic inserts, or grind.

Small internal features and sharp corners are the hard cases. Drill them before hardening and use the hardened state only for finishing the functional surfaces.