Hardness is an empirical response to a defined test, not a single intrinsic number that can be compared without naming the method, scale, load, indenter, and specimen condition. For metals, common methods include Rockwell, Brinell, Vickers, and Knoop. Shore durometer scales are principally used for elastomers and some plastics, not as substitutes for metallic hardness scales.
A drawing or purchase specification should state the required hardness value and test method/scale—for example, 58–62 HRC under the applicable Rockwell standard—not merely “hardness 60.” Select the method from the material, expected range, section thickness, surface condition, case depth, permitted indentation, and acceptance purpose.
Hardness can support material and heat-treatment verification, but a single indent is local evidence. It may not represent the entire part, and hardness does not by itself prove tensile strength, wear life, case depth, or microstructure.
What Are the Main Methods of Hardness Testing?
| Method | Measurement principle | Typical use | Important limitation |
|---|---|---|---|
| Rockwell | Depth difference under minor and major test forces | Fast production checks on suitable metallic sections; many regular and superficial scales | Scale, thickness, curvature, spacing, support, and surface condition must comply |
| Brinell | Diameter of a relatively large ball indentation | Bulk/average response of castings, forgings, and less uniform metals | Large indent; load/ball combination and thickness must suit the material |
| Vickers | Optical measurement of two diagonals from a diamond-pyramid indent | Wide hardness range; macro or micro loads; case profiles and small areas | Preparation and optical measurement become critical as indents get small |
| Knoop | Optical measurement of an elongated diamond indent | Thin layers, brittle materials, and hardness gradients at low force | Force and preparation affect results; not a universal bulk-hardness replacement |
| Shore durometer | Spring-loaded indentation depth | Elastomers and selected plastics using the specified Shore type | Different Shore types are not directly interchangeable and are not metal scales |
ASTM maintains separate standards for Rockwell metallic hardness (E18), Brinell metallic hardness (E10), and Knoop/Vickers microindentation (E384). Use the edition required by the drawing, product specification, or contract.
What Is the Difference Between Rockwell and Brinell Hardness Methods?
Rockwell derives a value from penetration-depth change and gives a direct reading after the prescribed force sequence. Brinell optically measures the diameter of a larger residual impression and calculates an HBW value for the stated ball and force condition. Their numbers are not the same quantity.
Rockwell scale selection matters
ISO 6508-1:2023 covers regular scales A, B, C, D, E, F, G, H and K plus superficial N and T scales. HRC uses a diamond indenter and is commonly applied to suitable hardened metallic specimens; HRB uses a ball indenter for an appropriate lower hardness range. Do not select a scale by habit: verify the expected range, specimen thickness, curvature correction, support, spacing, and surface condition under the governing method.
Brinell averages a larger area
Brinell can be useful when a larger impression better represents a heterogeneous or coarse material. Modern reporting uses HBW for a tungsten-carbide ball. The ball diameter and test force are part of the test condition; a generic statement such as “Brinell uses 3,000 kgf” does not cover softer materials, smaller balls, or every standard-permitted combination.
What Is the Difference Between HRC and Brinell Hardness?
HRC is one particular Rockwell scale. HBW is a Brinell result. A requirement such as 58–62 HRC should normally be tested and reported on HRC unless the controlling specification authorizes another method or conversion. Conversion tables are material-dependent approximations, not mathematical identities.
Do not use conversion to hide an unsuitable test
- Name the material and heat-treatment condition covered by the conversion table.
- Confirm the original test is within the valid range for its scale.
- Report which value was measured and which was converted.
- Use the specified method for acceptance whenever practical.
- Do not convert a surface or case-hardness result as though it represented bulk material.
When Should You Use Vickers or Knoop?
Vickers and Knoop are useful when the area is too small or thin for a macroindentation test, or when a hardness traverse must resolve a case, coating, weld, or local constituent. ASTM E384 notes that very small indents are increasingly sensitive to surface preparation, optical measurement, force application, and indenter geometry; a single microindentation may not represent bulk hardness.
For a case-depth profile, define specimen sectioning and preparation, test force, spacing from edges and adjacent indents, traverse locations, number of indents, and the criterion used to determine effective or total case depth. “Microhardness” alone is not a complete inspection instruction.
What Is the Difference Between Rockwell Hardness and Shore Durometer?
Rockwell is a family of metallic indentation tests with defined indenters and force sequences. Shore durometer methods use spring-loaded indenters for rubber, elastomers, and some plastics. “Rockwell D” is not the normal name for Shore D; they are different methods and scales.
ASTM D2240 covers multiple durometer types, including A and D, and warns that there is no simple relationship between results from different durometer types or other hardness instruments. For a polymer part, specify the Shore type, conditioning, thickness/stacking allowed by the method, temperature, dwell or reading time, and sampling locations.
How Should Hardness Be Specified on a CNC Part?
- State material and condition: grade, product form, heat-treatment route, and whether hardness applies before or after finishing.
- Name method and scale: governing ASTM/ISO or product standard, scale, load where relevant, and required range.
- Locate the test: surface, cross-section, distance from edge, case-depth traverse, or sacrificial coupon.
- Protect function: identify cosmetic, sealing, fatigue-critical, or finished surfaces where an indent is not acceptable.
- Define sampling and records: parts per lot, number of indents, averaging/reporting rule, calibration or verification status, and certificate content.
- Resolve conversion: say whether conversion is prohibited or which controlled table and material family is allowed.
Hardness requirements belong with the complete material and inspection plan. The material specification page and quality assurance page show the related RFQ information.
Conclusion
Rockwell, Brinell, Vickers, Knoop, and Shore durometer results answer different questions. Select and report the actual method, scale, load and specimen condition; avoid casual conversions; and define sampling and test location. That turns a hardness number into usable acceptance evidence.