🔩 Rockwell hardness of metals — measurement of the permanent increase in indentation depth (scales A, B, C and superficial)

Materials and NDT PN-EN ISO 6508-1

Overview

The standard specifies the method for the Rockwell hardness test in the regular scales A, B, C, D, E, F, G, H, K and in the superficial scales 15N, 30N, 45N, 15T, 30T and 45T, for metallic materials; it applies to stationary and portable hardness testers. The fifth edition, ISO 6508-1:2023, replaces the 2016 edition; the changes included, among others, removal of the note on steel and tungsten balls, addition of a clause with terms, and information on one-piece indenters with a spherical tip.

The individual scales differ in the type of indenter, the forces and the range of application: HRA (diamond cone, 588.4 N, 20–95 HRA), HRBW (ball 1.5875 mm, 980.7 N, 10–100 HRBW), HRC (diamond cone, 1.471 kN, 20–70 HRC, with possible extension down to 10 HRC when the cone and tip surfaces are polished), HRD, HREW, HRFW, HRGW, HRHW, HRKW. The superficial scales work with a preliminary force of 29.42 N and total forces of 147.1 N, 294.2 N or 441.3 N and have a scaling constant of 0.001 mm instead of 0.002 mm.

In the laboratory exercises of the Lublin University of Technology the measurement is carried out on a Rockwell hardness tester (type KP-15001) on steel test pieces after hardening and tempering, after thermochemical treatment (carburising, carbonitriding, diffusion chromising), on thin steel strips and hard casting alloys; the choice of scale (A, B, C or F) depends on the thickness of the test piece and the expected hardness.

Method principle

An indenter of specified size, shape and material is forced into the surface of the test piece at two force levels. First the preliminary force F0 is applied and the initial indentation depth is measured, then the additional force is applied and removed, returning to the preliminary force, after which the final indentation depth is measured. Rockwell hardness is determined from the difference h between the final and the initial indentation depth and from two constants: the full range N and the scaling constant S, using the formula H = N − h/S. For regular scales with a diamond indenter hardness = 100 − h/0.002, for scales with a ball = 130 − h/0.002, and for superficial scales = 100 − h/0.001.

Applications

• Rapid hardness checks of steel, cast steel and non-ferrous metals in production and acceptance testing
• Assessment of the effects of hardening, tempering and thermochemical treatment
• Testing of thin strips and sheets (superficial scales 15T, 30T, 45T)
• Measurements with portable hardness testers on large components
• Conversion of hardness to other scales and estimation of tensile strength (comparison tables)

Key parameters

Standard

Standard number

PN-EN ISO 6508-1:2024-06

Title (PL)

Metale — Pomiar twardości sposobem Rockwella — Część 1: Metoda badania

Title (EN)

Metallic materials — Rockwell hardness test — Part 1: Test method

Step-by-step procedure

1. Surface preparation

   Test a flat, smooth surface, free of scale, contaminants and lubricants. The exception is reactive metals, e.g. titanium, which may adhere to the indenter — a suitable lubricant (e.g. kerosene) is permitted, and its use is stated in the report. The preparation should minimise any change in hardness through heating or cold working.

2. Selection of the scale

   Select the scale (e.g. A, B, C or F) depending on the thickness of the test piece or layer and the expected hardness; check the minimum thickness in Annex B, determine the type of indenter, the total force and the minimum distances between indentations and from the edge of the test piece.

3. Daily check

   Before the first test of each day carry out the check according to Annex E for each scale used; check the condition of diamond indenters according to Annex F. After each change of indenter, ball or support carry out at least two measurements and discard the results, then repeat the daily check.

4. Positioning the test piece

   Place the test piece on a rigid support so that the tested surface is perpendicular to the axis of the indenter and to the line of action of the force; support cylindrical products on V-blocks. Pay attention to the seating and coaxiality of the indenter, test piece and holder — deviation from perpendicularity falsifies the result.

5. Preliminary force

   Bring the indenter into contact with the surface without impact, vibration, oscillation or overload and apply the preliminary force F0; its duration 3 s (from 1 s to 4 s). Measure the initial indentation depth.

   ⏱ Time: 3 s

6. Total force and reading

   Apply the additional force until the total force is reached, then remove it, returning to the preliminary force, and measure the final indentation depth; hardness is calculated using the formula H = N − h/S.

7. Number of indentations

   Carry out at least three trial measurements, then 3 indentations on each test piece; summarise the results in a table (practice of the Lublin University of Technology exercises).

8. Corrections and comparisons

   When testing convex cylindrical and spherical surfaces take into account the corrections from Annexes C and D of the standard; where necessary compare the results with Vickers and Brinell hardness using comparison tables.

Required equipment and apparatus

Health and safety (OHS)

• High test forces (up to 1.471 kN) — keep fingers away from the indenter, clamp test pieces securely
• Cylindrical products on V-blocks may rotate — secure them against rolling
• Grinding of test pieces: dust and sparks — safety glasses, extraction
• Chips when testing brittle materials (hardmetals, hardened steels) — guard on the workstation

Frequently asked questions