How to Specify Hardness on a Drawing: A Checklist for Engineers and Purchasing

Through-hardening example: SCM435 dowel pin — harden and temper to 40–45 HRC, measured on the cylindrical surface per ISO 6508 (HRC) or converted from ISO 6507 (HV1); runout/TIR ≤ 0.05 mm; no complete surface decarburization; supply a hardness certificate with lot traceability.

Case-hardening example: SCM415 pin — carburize/carbonitride to a surface hardness of 58–63 HRC, effective case depth (CHD) 0.30–0.50 mm to the 550 HV1 limit per ISO 18203, core 30–40 HRC.

These numbers are not universal defaults; choose them from the material, function and dimensional tolerance. The goal is one requirement everyone reads the same way before production starts.

"Harden it" has no number to accept against. Always give a range — e.g. 40–45 HRC or 550–620 HV. A single value like "45 HRC" forces impossible precision, because hardness scatters naturally both between locations and between pieces.

Pick the scale to suit the part: HRC (Rockwell C) for sections thick enough to support the indenter, HV (Vickers) for thin surfaces, hardened cases or very small parts where an HRC indenter would punch through. State the load too (e.g. HV1, HV0.3) because the value depends on it.

Keep the band realistic for the process. A window of about ±2–3 HRC is repeatable; anything tighter should be discussed in advance because it affects cost and rejection rate.

Hardness changes with position — surface, sub-surface and core differ, especially on case-hardened parts. Mark the measurement point on the drawing (e.g. "on the bearing face of the head" or "at mid-section of the core") so there is nothing to argue about later.

State the method/standard — ISO 6508 for HRC, ISO 6507 for HV — and be careful with cross-scale conversion. Conversion tables (e.g. ISO 18265) are approximations, not measurements in that scale. If the spec is HRC, measure HRC directly where thickness allows.

On small or thin parts an HRC test may punch through or read low from the anvil effect. In that case specify HV on a prepared cross-section.

For carburizing/carbonitriding/induction work, specify the effective case depth, not just "hard surface". The current measurement method is ISO 18203:2016, which cancelled and replaced the earlier ISO 2639:2002.

Definitions differ by process: CHD (case hardening depth) for carburized parts is measured to a 550 HV1 limit unless otherwise agreed; NHD (nitriding hardness depth) is measured to core hardness + 50 HV; SHD (surface hardening depth) for induction/flame is measured to 80% of the minimum surface hardness.

Give the depth range, the reference hardness (HV limit) and the load — e.g. "CHD 0.3–0.5 mm to 550 HV1". Otherwise depth figures cannot be compared between suppliers.

Quenching always creates stress and distortion, worst on thin, long or unevenly sectioned parts. State a checkable limit — runout/TIR, straightness per length, flatness, or thread-gauge acceptance — and allow grind stock when tolerances are tight.

Decarburization (loss of surface carbon) leaves the surface softer than designed and cuts fatigue life. For fasteners, ISO 898-1 sets decarburization limits, such as the height of the unaffected base-metal zone (E) and the maximum depth of complete decarburization (G). Reference those limits instead of writing "no surface damage".

Both distortion and decarburization are controlled by furnace atmosphere and correct loading. Clear, measurable limits make the lot auditable and prevent disputes.

For process review and quotation, send: drawing/photo, material grade (e.g. S45C, SCM435, SUJ2), function, hardness range and scale, measurement location, case depth and its definition, distortion limits, decarburization requirement, weight/count per lot and required documents.

If there is no callout yet, state the real function (wear, impact or clamp load) and the team can convert it into initial acceptance criteria for a trial lot. For bolts quoting a property class, tie the requirement to ISO 898-1 so hardness, strength and decarburization stay consistent.

A spec-builder tool on the website walks through these items so nothing is missed before the job is sent.