# Rockwell hardness testers

Depth-based hardness measurement, scale selection, specimen support and verification of Rockwell testing systems.

Reviewed: 2026-11-01

Tags: inspection, instruments, hardness

## What the result means

Rockwell hardness uses permanent indentation depth after the additional force is removed and the preliminary force remains. Retain the scale with the result. [Mitutoyo, Product Fundamentals, Rockwell hardness section](https://www.mitutoyo.com/webfoo/wp-content/uploads/Product-Fundamentals.pdf)

For the regular C scale, the relationship is **HRC = 100 − h/0.002 mm**, where h is the permanent depth increase measured under the preliminary force after removal of the additional force. An **illustrative 60 HRC result** therefore corresponds to (100 − 60) × 0.002 mm = **0.080 mm**. This is not the maximum penetration while the total force is applied. The regular-scale depth increment is 2 µm per hardness unit; superficial scales use a different increment. [Struers, Rockwell calculation](https://www.struers.com/en/knowledge/hardness-testing/rockwell)

The reported number requires its scale: **60 HRC** and **60 HRB** are different results. The scale identifies an indenter and force combination. A hardness test locally characterizes resistance to indentation; it does not independently identify an alloy or establish every mechanical property of a part.

For metallic materials, ISO **6508-1:2023**, fifth edition published **December 2023**, covers specified regular and superficial Rockwell scales on stationary and portable machines. Material- or product-specific standards may also apply. Specify the required method and edition from the drawing or contract rather than assuming every instrument mode meets the same requirement. [ISO 6508-1:2023, scope](https://www.iso.org/standard/84343.html)

## Regular and superficial scales

Regular Rockwell uses a **10 kgf preliminary force**; common total test forces are **60, 100 and 150 kgf**. Superficial Rockwell uses a **3 kgf preliminary force** with common totals **15, 30 and 45 kgf**. The total already includes the preliminary force: 150 kgf on the C scale is not an additional 150 kgf on top of 10 kgf. [Mitutoyo, Rockwell and superficial Rockwell fundamentals](https://www.mitutoyo.com/webfoo/wp-content/uploads/Product-Fundamentals.pdf)

| Example scale | Indenter | Total force |
| --- | --- | ---: |
| HRA | Diamond cone | 60 kgf |
| HRC | Diamond cone | 150 kgf |
| HR15N | Diamond cone | 15 kgf |
| HR30N | Diamond cone | 30 kgf |
| HR45N | Diamond cone | 45 kgf |

Superficial scales reduce indentation depth and can suit thinner specimens, but suitability still depends on thickness, hardness and geometry. Select a scale within the method's valid range and retain the appropriate ball-material designation where applicable. Struers describes diamond and tungsten-carbide-ball indenters and the different regular/superficial force families. [Struers, Rockwell hardness testing](https://www.struers.com/en/knowledge/hardness-testing/rockwell)

## Thickness, surface and support

The indented volume must be representative of the material being assessed. Surface scale, coatings, decarburization or a hardened layer may change what is measured. Prepare the specified surface without materially changing its hardness through heating or mechanical working. A case-depth question may require a different preparation and test method from bulk Rockwell acceptance.

EMCO-TEST's guidance, referring to ISO 6508, gives minimum thickness relationships of **ten times residual indentation depth for cone indenters** and **fifteen times for ball indenters**, and provides scale-dependent charts. Apply the governing standard and actual specimen conditions; the relationship is not a universal permission to test every coating or case of that thickness. [EMCO-TEST, minimum specimen thickness](https://www.emcotest.com/en/hardness-knowledge/applications-tips)

Support the specimen rigidly on a clean, appropriate anvil. Rocking, dirt underneath or unsuitable support can contribute movement to a depth measurement. Curved parts require secure alignment and the correction, if any, permitted for their scale and curvature. EMCO-TEST describes cylindrical-specimen corrections as dependent on both radius and scale. Do not apply a generic correction to an unsupported or misaligned part. [EMCO-TEST, cylindrical-specimen correction](https://www.emcotest.com/en/hardness-knowledge/applications-tips)

Keep indentations sufficiently separated from each other and edges. NIST explains that indentation creates a surrounding deformation zone, so a nearby test can be influenced by prior work-hardening. Use the spacing rules of the applicable method and specimen, rather than placing tests wherever unused surface is visible. [NIST SP 960-5, 2001, section 3.4.7](https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication960-5.pdf)

## A depth calculation, with limits

Under the cited cone-thickness relationship, ten times 0.080 mm is 0.80 mm. This arithmetic illustrates why scale, hardness and thickness interact. It is not sufficient on its own to approve a particular thin part: surface condition, support, substrate influence and the governing method still need evaluation.

## Named machine example

ZwickRoell's **DuraJet 10 G5** product sheet, identified **PI 190 2.0321**, lists electronic force application over **9.8–2,450 N**, a **260 mm maximum test height**, **175 mm throat depth** and **100 kg maximum workpiece weight**. It supports clamped or unclamped testing and data export. These are characteristics of that configuration, not generic Rockwell tester limits. Check clearances with the selected indenter, nose cone, anvil and actual part. [ZwickRoell product information, pages 1–4](https://www.zwickroell.com/fileadmin/content/Files/SharePoint/user_upload/PI_EN/15_190_DJ10_Rockwell_hardness_PI_EN.pdf)

The same sheet labels its HVD and HBD depth-measurement modes **non-standard**. Do not report those modes as standard optical Vickers or Brinell tests merely because the machine offers related labels. The PDF prints the document code above rather than a written calendar revision date. [DuraJet 10 G5 product information, area of usage](https://www.zwickroell.com/fileadmin/content/Files/SharePoint/user_upload/PI_EN/15_190_DJ10_Rockwell_hardness_PI_EN.pdf)

## Verify the complete measurement system

NIST distinguishes **direct verification** of parameters such as force, depth measurement and hysteresis from **indirect verification** using standardized reference blocks and indenters. Indirect checks assess repeatability and bias. NIST's guide was published in **2001**, so use its good-practice discussion alongside the applicable current test standard. [NIST SP 960-5, sections 5.1–5.2](https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication960-5.pdf)

Routine reference-block checks monitor changes between formal verifications. Use a suitable certified block for the scale and hardness region, respect usable surface and spacing, and retain results for trending. Follow the procedure after setup changes. Investigate failed checks before releasing production results. [NIST SP 960-5, section 6](https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication960-5.pdf)

Record the indenter and block identity as well as the machine. Buehler's Wilson block catalog distinguishes blocks certified with tungsten-carbide balls from steel-ball blocks. The certificate and scale designation matter; an approximate catalog hardness is not the assigned certified value of an individual block. [Buehler, Wilson Rockwell test blocks](https://www.buehler.com/products/hardness-testing/hardness-testing-blocks/wilson-rockwell-test-blocks/)

## Interpretation and reporting

Resolution, repeatability, bias and uncertainty answer different questions. Report the actual scale and result, with uncertainty and acceptance rules appropriate to the task. Retain specimen location and preparation, thickness/curvature considerations, support, cycle settings, corrections and reference checks. Do not silently discard inconvenient readings or replace individual results with an unqualified mean.

Conversions between hardness scales are approximate correlations, not identities. TWI explicitly cautions that conversions are not absolute. Keep the originally measured result and method visible, identify any conversion source and its material scope, and obtain agreement before using a converted result for acceptance. A converted hardness also does not substitute for a required tensile test. [TWI, converting between hardness scales](https://www.twi-global.com/technical-knowledge/faqs/faq-how-can-i-convert-between-hardness-scales)

## Related reference

[Inspection and quality documents](../../sourcing/documents/index.md)