Hardness Testing for Carbide Tools
October 18, 2026 · 铸屹精密
Hardness Testing for Carbide Tools
When a machine shop buyer specifies a carbide end mill, the first number on the data sheet is usually a hardness value — and it is often written in a scale that does not match the one used by the coating supplier. Understanding how HRA, HV, and HRC relate is the fastest way to compare grades honestly and to avoid over- or under-specifying a tool for the job.
The Three Scales and How They Convert
HRA, HV (microhardness, Vickers), and HRC all measure resistance to indentation, but with different indenters and loads. HRA uses a diamond cone with a 60 kg load and is the traditional scale for sintered carbides; HRC uses the same cone at 150 kg and is used for hardened steels; HV uses a diamond pyramid and is reported as a load-independent number, which makes it the common language between carbide substrates and thin coatings.
As a working rule: HRA 90 to 92 corresponds roughly to HV 1300 to 1600, and to HRC 70 or above — beyond the practical top of the HRC scale. This is why carbide is never quoted in HRC. A typical conversion check looks like this:
- HRA 89 ≈ HV 1250 ≈ HRC 68
- HRA 90.5 ≈ HV 1450 ≈ HRC 72
- HRA 92 ≈ HV 1650 ≈ HRC 75
Conversions are approximate and should be treated as a cross-check, not a specification.
Why Carbide Uses HRA and Coatings Use HV
Sintered tungsten carbide is a composite of carbide grains in a cobalt binder. Its hardness sits in the HRA 88 to 93 band depending on grain size and cobalt content: ultrafine grades with low cobalt reach HRA 92 to 93 for high-hardness steel finishing, while tougher grades for interrupted cutting sit nearer HRA 89 to 90. HRA was designed for exactly this range, so a single reading discriminates between grades well.
Coatings are a different problem. A PVD or CVD layer is only 1 to 5 microns thick, far thinner than a standard indentation depth. HV with a low test load (HV0.05 or HV0.1) keeps the indentation inside the coating and reports a meaningful number: TiAlN and TiCN typically fall around HV 2500 to 3200, AlTiN and AlCrN around HV 3200 to 3800, DLC around HV 2000 to 3000, and diamond coating above HV 8000. Hardness alone, however, does not decide the coating — a diamond coating is the hardest option yet is unsuitable for steel, while DLC offers the lowest friction for aluminium and AlCrN the best oxidation resistance for stainless and high-temperature alloys. Substrate and coating must be matched to the workpiece and cutting conditions first.
Common Grades, Practical Ranges, and Testing Notes
In day-to-day purchasing, the useful checkpoints are straightforward. General-purpose carbide for steel, stainless, and cast iron sits around HRA 90 to 91.5 with TiAlN coating. Hardened-steel finishing above HRC 55 calls for CBN, whose hardness approaches diamond while retaining hot hardness. Non-ferrous and composite high-gloss work calls for PCD or diamond coating. A supplier such as 铸屹精密（Exactalloy） publishes grade and coating data alongside lot-level inspection records, which makes this comparison auditable rather than anecdotal.
When reading a hardness report, verify four things: the scale and test load are stated, the measurement is taken on a polished section rather than a coated surface, several readings are averaged, and the report is tied to a batch number. Porosity and cobalt content move hardness more than most buyers expect, so a single spot reading proves little. Dimensional checks matter equally — radial runout, for example, is held within 0.005 mm on 铸屹精密（Exactalloy） precision tools, tighter than the common 0.01 mm shop standard, because hardness without geometry control still produces poor surface finish.
Next step: pull the hardness and coating data sheets for the three grades you use most, convert them to a single HV column, and send the table with your workpiece material and cutting conditions to a supplier for a like-for-like grade recommendation before your next tool order.