GlossaryKind of term: Manufacturing termHardness (HRC, HB)
What is hardness?
Hardness is a material's resistance to permanent indentation by a harder body, measured by a standardised method such as Rockwell, Brinell or Vickers.
Each method presses its own indenter in under a defined force, so a hardness number belongs to a method. Rockwell C (HRC) presses in a diamond cone and is used for hardened steel. Brinell (HBW) presses in a carbide ball and suits softer and quenched and tempered steel, cast iron and non-ferrous metals. Vickers (HV) uses a diamond pyramid and also measures small zones and thin layers such as a nitrided case. Older drawings say HB or HBS; today the designation is HBW.
What does hardness depend on?
Hardness follows from composition, structure and treatment: carbon content, heat treatment, cold work and, after surface hardening, the distance from the surface. A hardened part therefore often shows a hardness gradient from case to core. The measured value also depends on the method, the test force, the surface condition, the thickness of the specimen and where it is measured. A reading on a curved surface or too close to an edge can be off. So a hardness requirement without a method and a location is ambiguous.
What does the standard say about hardness?
The methods are set out in ISO 6508-1 (Rockwell), ISO 6506-1 (Brinell) and ISO 6507-1 (Vickers). Conversion between scales is possible with the tables in ISO 18265, but only approximately and only for the material groups those tables cover. ISO 15787 describes how to indicate heat treatment and hardness on a drawing, and ISO 18203 how to determine case depth. A hardness with a case depth also needs the limit hardness at which that depth is determined.
How does hardness work in practice?
A hardness requirement often adds a sequence of steps to the route: rough machining soft with stock allowance, hardening, usually subcontracted, then grinding or hard machining on the dimensions that count, with extra setups and measurements. How much allowance is needed depends on distortion during hardening, the operation that follows and the size; with a hardened case, enough case depth must remain after grinding. Hardened steel falls in ISO material group H. Hard turning usually uses CBN or ceramic, hard milling often coated carbide, and grinding remains the usual route for tight sizes and low roughness; all three are slower than in soft steel. If the requirement is noticed late in the estimate, route, lead time and price can change considerably.
What is hardness often confused with?
Hardness is not strength: in steel the two are related and ISO 18265 gives an approximate conversion, but tensile strength in MPa comes from a different test. Nor is hardness machinability: soft austenitic stainless such as 1.4404 can be harder to machine than harder, quenched and tempered 42CrMo4, because work hardening and chip formation count for more.
Frequently asked questions
What is the difference between HRC and HB?
HRC is Rockwell C, using a diamond cone, mainly for hardened steel. HB, now HBW, is Brinell, using a carbide ball, for softer steel, quenched and tempered steel, cast iron and non-ferrous metals. Conversion is only approximate, using the tables in ISO 18265.
Can hardened steel still be machined?
Yes, by hard turning, usually with CBN or ceramic, by hard milling, often with coated carbide, or by grinding. It is slower and the tooling costs more. For dimensions that must hold after hardening, leave stock allowance at the roughing stage.
What does 58 to 62 HRC on a drawing mean?
That the measured Rockwell C hardness must lie between 58 and 62. If a case depth is also given, the hardness applies at the surface and the depth runs to a defined limit hardness. Where to measure belongs on the drawing or in the agreement.