Hardness Conversion Table: The Complete Guide to Rockwell, Brinell & Vickers
Hardness conversion charts are indispensable tools for engineers, metallurgists, and quality control professionals who work across multiple material testing standards. Translating between Rockwell, Brinell, and Vickers hardness scales is a routine challenge in manufacturing environments. Getting it wrong can mean misread material specifications, failed components, or rejected parts.
The need for accurate conversions is especially pressing when sourcing materials from international suppliers or working from legacy specifications written to a different standard. Each hardness scale brings its own strengths: Rockwell suits finished components, Brinell is more reliable for cast iron and non-homogeneous materials, and Vickers offers precision on thin or small specimens. Knowing how to convert between them and when each method is most appropriate is a core competency in materials engineering.
This guide provides a comprehensive reference for hardness conversions, covering the principles behind each scale, practical conversion tables, tensile strength correlations, and the limitations engineers must keep in mind.
Understanding Steel Hardness and Why Conversion Matters
Steel hardness testing underpins material verification across virtually every engineering and manufacturing discipline. At its core, hardness quantifies a material's resistance to plastic deformation, a property that directly determines how a component will perform under load and wear.
What Is Indentation Hardness?
Indentation hardness measures how much a material resists permanent deformation when a concentrated load is applied to its surface. A precisely dimensioned indenter is pressed into the specimen under a known force, and hardness is calculated from either the depth of penetration or the size of the resulting impression.
Dimensionally, indentation hardness is expressed as force per unit area (typically kg/mm²), which is why hardness values tend to correlate roughly with tensile strength in metals. This relationship is imperfect and varies depending on the indentation geometry and material type. Hardness is best understood as a comparative measure of material behaviour under standardised conditions, not as a standalone fundamental property.
Why Multiple Hardness Scales Exist
Different scales emerged to solve different testing problems. Each uses a distinct combination of indenters, applied forces, and measurement techniques:
Rockwell was developed as a faster, less destructive alternative to earlier methods. It measures the additional depth caused by applying a major load beyond the minor preload, giving a direct readout with no need for optical measurement or surface preparation. This makes it the go-to method in production environments.
Brinell, first standardised in 1900, presses a 10mm carbide ball into the material under loads of up to 3000 kgf. Its large indentation area makes it particularly well suited to non-homogeneous materials such as castings and forgings, where localised measurements could be misleading.
Vickers uses a square pyramid-shaped diamond indenter and lighter loads, offering greater precision and the ability to test very small or thin specimens. It requires optical measurement and specimen preparation, so it carries a higher cost, but it delivers excellent accuracy across a wide range of materials.
When Are Conversions Needed?
Hardness conversions are required in several common scenarios:
- When engineering drawings specify a scale that your available equipment cannot test directly
- When a component's geometry or thickness prevents use of the required method. Thin or fragile samples often need Vickers or Knoop rather than Rockwell
- When cross-referencing materials tested under different standards or comparing against historical data
- When verifying heat treatment results using the equipment on hand rather than the specified tester
It is critical to understand that conversions between scales are approximations, not exact equivalents. Different hardness tests measure different combinations of material properties, which means perfect mathematical conversion is not possible. ASTM E140, the primary standard governing hardness conversion, explicitly states that conversions should only be used when direct testing under the specified conditions is impossible, and should always be applied with care.
The Rockwell Hardness Scale
Rockwell is the most widely used hardness testing method globally, valued for its speed, repeatability, and minimal specimen preparation requirements. The test directly measures indentation depth after applying a specified load, giving an immediate hardness readout.
HRC vs HRB: Key Differences
The two most common Rockwell scales differ in both their indenters and applied loads:
HRC (Rockwell C) uses a diamond cone indenter with a 120° apex angle and a 150 kgf major load. It is the correct choice for harder materials including hardened and through-hardened steels, titanium alloys, and pearlitic malleable iron. Typical knife blades and cutting tools fall in the 55–66 HRC range.
HRB (Rockwell B) uses a 1/16-inch (1.588mm) steel ball indenter with a 100 kgf major load. It is suited to softer materials such as aluminium alloys, copper alloys, and mild steels. Brass, for example, typically measures between HRB 55 and HRB 93 depending on alloy and temper.
Both scales apply a 10 kgf preload before the major load to seat the indenter and eliminate surface irregularities.
Known Limitations
Readings below 20 HRC or above 100 HRB are considered unreliable. These scales overlap precisely to avoid the need for measurements in those problematic ranges. Additional considerations include:
- Small errors in depth measurement can produce disproportionate swings in hardness values
- Test surfaces must be clean, flat, and free of contamination
- Specimen thickness must be sufficient. If the indentation affects the backing anvil, readings will be falsely elevated
Common Applications
Rockwell testing is the standard choice for production-line quality control, post-heat-treatment verification, and Jominy end-quench testing. As a general rule, the heaviest applicable load should be used, since larger indentations are less sensitive to surface condition variability.
The Brinell Hardness Scale
The Brinell test remains one of the most reliable methods for characterising structural steels and other materials with coarse or heterogeneous microstructures.
Test Method
In its standard form, a 10mm diameter tungsten carbide ball is pressed into the specimen under 3000 kgf for 10–15 seconds. For softer non-ferrous metals, reduced loads of 1500 kgf or 500 kgf may be used. After unloading, the diameter of the indentation is measured optically, and the hardness value is calculated by dividing the applied force by the curved surface area of the impression. Modern testing machines perform this calculation automatically.
Because the indentation is relatively large, the specimen must have adequate thickness of at least eight times the depth of the impression. This limits Brinell testing on thin stock but makes it more representative for bulk and near-net-shape components.
HBW Notation
Modern standards use the designation HBW to indicate a tungsten carbide ball indenter. A complete result is expressed in the format: 250 HBW 10/3000, meaning a hardness of 250 using a 10mm ball under 3000 kgf. Including the test parameters is essential, as Brinell values obtained under different conditions are not directly comparable.
Why Brinell Suits Structural Steel
Three factors make Brinell the preferred method for structural applications:
- The large indentation averages across microstructural variation, making it more representative for castings, forgings, and weld zones
- Brinell hardness correlates well with ultimate tensile strength in steel, enabling indirect strength estimation when destructive testing is impractical
- The method covers the 80–600 HBW range effectively, encompassing most structural and engineering steels
Master Hardness Conversion Chart
The following tables provide updated conversion data based on current testing standards. All values are most accurate for non-austenitic steels; deviations should be expected for other alloy types.
Brinell to Rockwell C Conversion
| Brinell (HBW) | Rockwell C (HRC) |
|---|---|
| 300 | 30.5 |
| 350 | 34.0 |
| 400 | 39.1 |
| 450 | 44.5 |
| 500 | 49.1 |
Conversion accuracy is highest in the 200–350 HBW range, which covers the majority of structural steel grades.
Rockwell B to Vickers Conversion
| Rockwell B (HRB) | Vickers (HV) |
|---|---|
| 80 | 153 |
| 85 | 169 |
| 90 | 185 |
| 95 | 210 |
| 100 | 240 |
Tensile Strength Correlation
For most carbon and low-alloy steels, the following approximations apply:
- Tensile strength (MPa) ≈ 3.45 × Brinell hardness
- Tensile strength (psi) ≈ 500 × Brinell hardness
A steel at 300 HBW therefore has an estimated tensile strength of approximately 1035 MPa or 150,000 psi. These correlations are useful for indicative purposes but should not substitute for direct tensile testing in critical applications.
How to Read and Use These Tables
- All conversions are approximations, so treat them as estimates rather than exact equivalents
- Material-specific factors such as alloy chemistry, heat treatment, and grain size can cause significant deviations
- ASTM E140 is the primary reference standard; even its published values are acknowledged as approximate
- Always prefer direct testing under the specified scale wherever conditions allow
- When reporting converted values, document the original reading and the conversion standard used (e.g., 35 HRC, converted from 330 HBW per ASTM E140)
Accuracy, Limitations, and Best Practices
Why Conversions Are Always Approximate
Each hardness test evaluates a different combination of material properties under distinct loading conditions. There is no single universal equation that perfectly maps one scale onto another. Conversion tables are derived from empirical testing, often conducted on equipment that may no longer be in service, and represent statistical averages rather than exact relationships.
Material-Specific Variation
Differences in chemical composition, heat treatment history, grain structure, and processing route can all shift conversion results significantly. Two steels with nominally identical grades can yield meaningfully different converted values if their thermal histories diverge. Conversion accuracy also varies across the hardness range, adding further complexity.
Adherence to ASTM E140
ASTM E140 is the recognised international reference for hardness conversion tables, covering specific material categories and providing formulae for approximate cross-scale conversions. The standard is explicit that conversion should only be carried out when direct testing is not feasible, and it provides no confidence limits for conversion errors. This serves as a reminder that even standardised conversions carry inherent uncertainty.