Hardness testing is a non-destructive (or minimally destructive) material characterization process that quantifies a material’s resistance to permanent deformation, scratching, or penetration under a standardized applied force. Unlike tensile strength (which measures a material’s ability to resist breaking under tension) or yield strength (the point at which a material stops deforming elastically), hardness is a relative, empirical property—its value depends entirely on the specific test method used.
Material verification, by contrast, is the broader process of confirming that incoming raw material, heat-treated components, or finished parts meet specified material properties (including hardness, composition, and microstructure) before they enter production or are shipped to customers. For CNC manufacturers, hardness is the most widely used proxy for verifying material performance because it correlates strongly with critical properties like wear resistance, fatigue strength, and machinability.
Hardness and material verification directly impact three core aspects of CNC manufacturing: part function, assembly compatibility, and production cost.
Hardness is a primary driver of a component’s ability to perform its intended role. For example:
Hardness differences between mating parts can cause uneven wear or failure. For example, a hardened steel shaft (60 HRC) paired with a softer bearing race (50 HRC) will cause the bearing to wear out 3–5 times faster than a properly matched pair. This is especially critical for components machined on high-precision machines like the GA-UHD500, which holds positioning accuracy of 0.006 mm—misaligned or mismatched hardness can negate the machine’s precision.
Incorrect hardness can lead to costly rework or scrap:
The three most common hardness test methods used in CNC manufacturing are Rockwell, Brinell, and Vickers. Each is designed for specific material types, hardness ranges, and part geometries, and all follow standardized procedures defined by organizations like ASTM International or ISO.
The Rockwell test measures the depth of penetration of an indenter under two applied forces: a minor preload (to seat the indenter and eliminate surface irregularities) and a major test load. The resulting hardness value is derived from the difference in penetration depth between the two loads.
There are two primary Rockwell scales used in CNC manufacturing:
Shop-floor method: The part is placed on the test machine’s anvil, the indenter is lowered to apply the minor load, then the major load is applied and held for 10–15 seconds. The hardness value is displayed directly on the machine’s digital readout. Accuracy limits: ±1 HRC for properly calibrated machines, ±2 HRB.
The Brinell test measures the diameter of the indentation left by a hard steel or tungsten carbide ball indenter under a fixed major load (typically 3000 kg for steels, 500 kg for non-ferrous metals). The hardness value is calculated as the applied load divided by the surface area of the indentation.
Shop-floor method: The indenter is pressed into the part for 10–30 seconds, then the indentation diameter is measured with a microscope or digital caliper. The Brinell hardness number (BHN) is calculated using a formula or lookup table. Accuracy limits: ±5 BHN for steels, ±3 BHN for non-ferrous metals.
The Vickers test uses a square-based diamond pyramid indenter, with the major load ranging from 1 kg (for thin coatings or small parts) to 120 kg (for bulk materials). The hardness value is calculated as the applied load divided by the surface area of the indentation, measured from the diagonal lengths of the square impression.
Shop-floor method: The indenter is pressed into the part for 10–15 seconds, then the two diagonals of the indentation are measured under a microscope. The Vickers hardness number (HV) is calculated automatically by modern digital test machines. Accuracy limits: ±2 HV for loads ≥10 kg, ±5 HV for loads <10 kg.
Since hardness values are method-specific, conversion between scales is often required. For example, a part specified as 220 BHN can be converted to HRC to verify compliance with a heat treatment requirement.
Conversion is based on empirical data and is only valid for specific material groups (e.g., carbon steels, alloy steels, aluminum). Standard conversion tables (ASTM E140 or ISO 18265) are used, but they have inherent error:
A material certification (or mill test report, MTR) is a document issued by the raw material supplier that verifies the material’s composition, mechanical properties (including hardness), and compliance with standards like ASTM A36 or SAE 4140. For CNC manufacturers, MTRs are required for critical components to trace material properties back to the source.
Hardness test results can be skewed by a range of factors, leading to incorrect material verification:
Hardness testing and material verification have critical limitations that are often overlooked:
| Method | Accuracy (Typical) | Effort (Shop-floor) | Typical Use Case |
|---|---|---|---|
| Rockwell C | ±1 HRC | Low | Hardened steels, tool steels, heat-treated components |
| Rockwell B | ±2 HRB | Low | Soft steels, aluminum, brass |
| Brinell | ±5 BHN | Medium | Large parts, castings, forgings, soft and medium-hard steels |
| Vickers | ±2–5 HV | Medium-High | Thin parts, coatings, small components, micro-hardness testing |
Q: Can I use a Rockwell test on a part machined on a GA-UHD500 that is only 1 mm thick? A: No. Rockwell tests apply high loads that will cause the indenter to penetrate through thin parts, leading to inaccurate results. For 1 mm thick parts, use a low-load Vickers test (1 kg or less) instead.
Q: My part is specified as 220 BHN. Can I convert this to HRC to test it? A: Yes, but with caution. For carbon steels, 220 BHN converts to approximately 19 HRC per ASTM E140, with an error margin of ±3 HRC. If your part is made of a non-standard material, test directly in BHN for accuracy.
Q: Why do I need to verify hardness if I have an MTR for the raw material? A: MTRs are batch-specific and do not account for changes in material properties during machining or heat treatment. For example, heat treatment can alter hardness by ±10 HRC, so in-house testing is required to verify the final part’s properties.
Q: Can I test hardness on a curved surface? A: Testing on curved surfaces requires special correction factors to account for the curvature. For example, a Rockwell test on a 10 mm diameter steel rod requires a correction factor of +2 HRC to get the accurate hardness value.
Q: What is the maximum part size I can test with a Rockwell machine? A: The maximum part size depends on the machine’s throat depth. For example, a standard Rockwell machine can test parts up to 300 mm in diameter and 200 mm in height. Larger parts, such as those machined on a GA-GF4028 gantry machining center, require portable hardness test methods like ultrasonic contact impedance (UCI) testing.
In summary, hardness testing and material verification are critical processes for ensuring part quality and performance in CNC manufacturing. By understanding the strengths and limitations of each test method, following proper procedures, and accounting for sources of error, manufacturers can effectively verify material properties and avoid costly production issues.
Send us your part, batch size and target port. We reply with a configuration, a binding lead window and a total-cost comparison.
Request a quote →