HRC/HB Hardness specification
Hardness specification · Tratamiento térmico · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
On the drawing, “58-62 HRC · or 270-300 HB” states a hardness specification requirement.
What controls
Mandatory hardness range the part must show after heat treatment, expressed on the scale given on the drawing (HRC, HB, HV or other) and, where applicable, in a specific area. It is the quintessential result requirement of heat treatment: while the furnace cycle is the heat treater's business, the hardness range is what gets accepted or rejected at receiving inspection. A single value without a range cannot be specified: there is always scatter, and the drawing must give a minimum and a maximum or a tolerance.
How to verify
Hardness tester on the drawing scale, on a clean surface free of scale and decarburization, flat and properly supported, with several indentations spread over the functional area and spaced apart from each other. When the part cannot be marked, witness coupons treated in the same batch are used, backed by the heat treatment certificate. Portable hardness testers are practical on large parts, but they need enough mass and support to give reliable readings.
Workshop guideline values
- General
- Wide ranges, in the order of 6–10 HRC points or the HB equivalent, on general-purpose parts.
- Accuracy
- Ranges of 4–5 HRC points on wear parts and tooling.
- High precision
- Ranges of 2–3 HRC points, requiring batch control and a certified witness coupon.
Guidance only: the drawing's value always wins.
Common mistakes when interpreting it
- Converting between hardness scales using a table when the drawing does not allow it: the conversion is approximate and depends on the material.
- Measuring on scale, decarburization or white layer and accepting the reading as valid.
- Using too high a load on a thin case and actually measuring core hardness.
How to get it in the workshop / if it comes out of tolerance
Measure where it matters: the hardness of a heat-treated part is not the same at the surface as at the core, nor in a thick section as in a thin one. On case-hardened parts, the scale and load must suit the case; with too high a load you end up measuring the core without realizing it. And on nitrided surfaces or very thin cases, the valid measurement is microhardness, not the usual shop-floor scale.
Scattered readings on the same part: poorly prepared surface, inadequate support or indentations too close together; before rejecting, repeat the measurement properly. Hardness too high: tempering missing, corrected by tempering. Hardness too low: cannot be fixed by tempering and normally requires repeating the full cycle. If hardness meets the requirement at the surface but fails at depth, the requirement to look at is case depth, not this one.
Related symbols
Other symbols in Heat treatment
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