Rockwell Hardness Testing: Method, Procedure, and ASTM E18
Representative Infinita Engineering Visual explaining the four-step workflow for Rockwell Hardness Testing Guide.What Is the Rockwell Hardness Test?
The Rockwell hardness test, per ASTM E18, is a differential-depth indentation hardness test — unlike Brinell or Vickers, which measure the size of an indentation optically, Rockwell measures the residual depth the indenter penetrates under load, converting that depth directly into a hardness number.
Test Procedure
- Surface preparation: ensure the test surface is clean, flat, and reasonably smooth; specimen thickness should be at least 10 times the expected indentation depth
- Scale selection: choose the appropriate Rockwell scale (e.g., HRC for hardened steel, HRB for softer metals) based on expected material hardness and thickness
- Preliminary (minor) load application: the indenter is brought into contact with the surface and a minor load (typically 10 kgf, or 3 kgf for superficial scales) is applied to establish a stable datum position and minimize the effect of surface roughness
- Major load application: the full major load (60, 100, or 150 kgf for standard scales; 15, 30, or 45 kgf for superficial scales) is applied, driving the indenter deeper into the material
- Major load removal: the major load is removed while the minor load is maintained, allowing partial elastic recovery
- Reading the result: the permanent increase in indentation depth (compared to the minor-load datum) is converted to a Rockwell hardness number using the relationship HR = E minus h/s, where E is a constant (100 for diamond indenters, 130 for ball indenters) and s = 0.002 mm per Rockwell unit
Verification and Calibration
Rockwell testing machines require periodic verification using calibrated test blocks traceable to NIST standards, covering the specific scale(s) being used. Multiple test blocks at different hardness levels (high, medium, low) within each scale are typically used to confirm machine accuracy across its working range.
Common Errors to Avoid
Selecting the wrong scale for the material (e.g., applying HRC to soft annealed steel) produces excessive indenter penetration and invalid, unrepeatable results. Testing too close to a specimen edge or to a previous indentation can also distort readings — minimum indentation spacing requirements are specified in ASTM E18 to prevent this.
Industry Specifications Referencing Rockwell Testing Procedure
- ASTM E18: governing standard for the test procedure and machine verification requirements
- ISO 6508-1: international equivalent, including correction factors for curved specimen surfaces (convex cylinders and spheres)
Also Read – Metal Hardness Testing: Rockwell vs Brinell vs Vickers — Comparison Guide
Conclusion
Rockwell’s speed and simplicity — direct reading, minimal specimen prep — make it the default hardness test for production and incoming-material QC, but that speed only produces valid data when the scale, load, and specimen thickness are matched correctly to the material being tested.
How is a Rockwell hardness test performed? A minor force is first applied to establish a reference position. A larger major force is then applied and removed, after which the permanent indentation depth is converted into a Rockwell hardness value.
What are the common Rockwell hardness scales? Common scales include HRC for hardened steels and HRB for softer steels, copper alloys, and similar metals. Other scales use different combinations of indenters and test forces for specific materials.
Which indenters are used in Rockwell testing? Rockwell testing uses either a diamond cone indenter or a hardened ball indenter. The selected indenter depends on the chosen scale, material hardness, and specimen characteristics.
What factors can affect Rockwell hardness results? Results may be influenced by surface roughness, specimen thickness, curvature, vibration, indentation spacing, machine alignment, scale selection, and instrument verification. Consistent test conditions improve repeatability.
How is a Rockwell hardness tester verified? The tester is commonly checked using certified hardness reference blocks. Verification confirms that the machine, force system, indenter, and depth-measuring system produce results within acceptable limits.
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