Alloy Hardness Converter

Overview: Why Convert Alloy Hardness?

Hardness testing is one of the fastest ways to assess how an alloy will behave in service. A harder metal generally resists indentation, wear, and abrasion, while a softer one deforms more easily during forming, machining, or impact. Because different labs, standards, and industries use different hardness tests, you often need to translate a value from one scale to another.

This alloy hardness converter helps you approximate equivalent values between common scales so that machinists, engineers, inspectors, and students can compare reports or select materials even when test methods differ. The tool focuses on steels and common structural alloys, where simple empirical relationships are reasonably reliable in the mid-hardness range.

Common Hardness Scales and Notation

Hardness numbers are dimensionless quantities derived from indentation tests. The main scales supported here are:

• Brinell Hardness Number (BHN) — Uses a hard steel or carbide ball pressed into the surface under a known load. The diameter of the indentation is measured and converted into BHN.
• Vickers Hardness (HV) — Uses a diamond pyramid indenter. The diagonals of the small square indentation are measured. Vickers is common for lab work and microhardness testing.
• Rockwell B (HRB) — Uses a hardened steel ball with a relatively light major load. It is suited to softer metals, such as mild steels, some stainless steels, and non-ferrous alloys.
• Rockwell C (HRC) — Uses a diamond cone (Brale) with a higher load and is standard for harder steels, tool steels, and case-hardened surfaces.

These scales are not interchangeable by definition. Each test uses a different indenter geometry and load, probes a different effective depth below the surface, and responds differently to microstructure. That is why you need empirical relationships or tables to convert one hardness number to another.

Approximate Conversion Formulas Used

The converter is based on simple empirical relationships that work reasonably well for carbon steels and many general-purpose structural steels in the mid-hardness range. They are not official standards but are commonly used for quick engineering estimates.

The core relationships expressed in terms of Brinell hardness number (BHN) are:

• Vickers from Brinell: HV ≈ 0.95 × BHN
• Rockwell B from Brinell: HRB ≈ (BHN − 20) / 4.7
• Rockwell C from Brinell: HRC ≈ (BHN − 70) / 9.5

In MathML form, a typical relationship can be written as:

To convert between non-Brinell scales, the calculator internally converts your input back to an approximate Brinell value and then applies the appropriate formula to reach the target scale.

Typical Ranges for Each Scale

The following ranges are general guidelines for steels and similar alloys. Actual ranges depend on alloy composition and heat treatment, but these bands can help you sanity-check converted values:

• BHN: Roughly 120–220 for low-carbon structural steels, up to around 600 for very hard tool steels or hardened surfaces.
• HV: For many engineering steels, values between about 130 HV and 700 HV are common. Microhardness tests on thin coatings may exceed this range.
• HRB: Typically 40–100 HRB for mild steels and some non-ferrous alloys. Above this range, HRC is usually preferred.
• HRC: Around 20–40 HRC for many quenched-and-tempered structural steels, 50–65 HRC for high-hardness tool steels or case-hardened layers.

If your converted result falls far outside these typical bands for the material you expect, re-check the input scale and consider whether the formulas are applicable to your alloy.

Worked Example: From Brinell to Other Scales

Suppose you have a steel specified as approximately 250 BHN from a supplier datasheet, but you want to know the approximate Vickers and Rockwell hardness values.

1. Convert BHN to HV
   Using HV ≈ 0.95 × BHN:
   HV ≈ 0.95 × 250 ≈ 238.
2. Convert BHN to HRB
   Using HRB ≈ (BHN − 20) / 4.7:
   HRB ≈ (250 − 20) / 4.7 ≈ 230 / 4.7 ≈ 48.9.
3. Convert BHN to HRC
   Using HRC ≈ (BHN − 70) / 9.5:
   HRC ≈ (250 − 70) / 9.5 ≈ 180 / 9.5 ≈ 18.9.

These values match typical conversion tables for medium-hard steels and illustrate how a single Brinell measurement can be expressed on multiple scales.

Worked Example: From Rockwell C Back to Brinell

Now consider a hard tool steel measured as 60 HRC. To convert this to BHN and then to HV, we can algebraically rearrange the HRC formula.

1. Rearrange the HRC equation
   Starting from HRC ≈ (BHN − 70) / 9.5, solve for BHN:
   BHN ≈ 9.5 × HRC + 70.
2. Compute BHN from 60 HRC
   BHN ≈ 9.5 × 60 + 70 ≈ 570 + 70 ≈ 640.
3. Compute HV
   Using HV ≈ 0.95 × BHN:
   HV ≈ 0.95 × 640 ≈ 608.

This gives approximate values of 640 BHN and 608 HV for a 60 HRC tool steel, which are in the expected range for very hard, wear-resistant alloys.

How to Interpret Converted Hardness Values

Hardness is often used as a proxy for other properties:

• Higher hardness typically means better wear and abrasion resistance but reduced ductility and increased brittleness. Very high HRC or HV values usually require slower machining speeds, rigid setups, and specialized cutting tools.
• Lower hardness corresponds to easier machinability and formability but reduced resistance to indentation, scratching, and surface damage.

When you use the converter, ask how the new value compares to typical ranges for your application. For example, going from 25 HRC to 40 HRC is a substantial increase in strength and wear resistance that may also require changes in tooling and heat treatment practice.

Comparison Table: Example Approximate Conversions

The table below shows representative conversions for several Brinell values using the empirical formulas above. These are not official standards but can help you check that your calculator inputs are in a plausible range.

If your converted values differ slightly from other references, that is expected: many published conversion charts are based on proprietary data and may use slightly different curve fits.

Who This Calculator Is For

This tool is designed as a quick-reference aid in situations such as:

• Machine shops comparing hardness specifications across drawings that reference different standards.
• Quality and inspection labs that need a rough equivalence between the hardness scale used on a test certificate and the scale requested by a customer.
• Educational settings where students are learning how Brinell, Vickers, and Rockwell tests relate and how to move between them.
• Preliminary design or material selection when you have hardness data in one scale but need a rough check against guidelines in another.

It is not intended to replace certified test reports, detailed metallurgical analysis, or official conversion tables in standards.

Limitations and Assumptions

The simplicity of the formulas used in this calculator makes them fast and convenient, but it also introduces important limitations. Keep the following points in mind whenever you rely on converted hardness values:

• Material type: The relationships are tuned to carbon and low-alloy steels and many common structural steels. Accuracy drops for high-alloy tool steels, stainless steels with complex microstructures, cast irons, aluminum alloys, copper alloys, and other non-ferrous metals.
• Hardness range: The equations work best in the mid-hardness range typical of quenched-and-tempered steels. At very low hardness (soft annealed materials) or very high hardness (carbide-rich tool steels or thin hard coatings), the error can become significant.
• Surface versus bulk: Rockwell C tests probe a relatively shallow depth. Case-hardened or surface-treated components may show high HRC near the surface while the core remains much softer. Simple conversions assume uniform hardness and cannot account for gradients through the thickness.
• Test method variability: Differences in test machine calibration, indenter condition, applied load, surface preparation, and operator technique all affect measured hardness. Converting an already uncertain measurement to another scale compounds that uncertainty.
• Non-standard conditions: Values obtained using modified test methods, microhardness tests on very thin layers, or tests at non-ambient temperatures may not be compatible with these relationships.

For safety-critical components (such as pressure-containing parts, aerospace hardware, lifting equipment, or automotive safety parts), always rely on certified hardness test results and, where possible, on conversion tables published in recognized standards (for example, ASTM or ISO) rather than on simple empirical formulas.

Practical Tips and Best Practices

To get the most value from the converter, consider the broader context of your hardness data:

• Check the test scale on the report: Before converting, confirm whether the original value is in BHN, HV, HRB, or HRC. Misreading the scale is a common source of error.
• Compare with specification limits: After conversion, compare the result to the acceptance criteria in your drawing or material standard. Allow for some tolerance due to the approximate nature of conversions.
• Use conservative assumptions: Where uncertainty is high, assume the material is slightly softer (for strength-critical designs) or harder (for machining/process planning) than the converted value to stay on the safe side.
• Combine with other data: Where available, consider tensile strength, yield strength, and impact toughness alongside hardness, especially when assessing suitability for dynamic or fatigue-loaded applications.

Disclaimer

The hardness conversions provided by this calculator are approximate and are based on simplified empirical relationships. They are intended for educational use, preliminary design checks, and non-critical engineering decisions only. For certification, regulatory compliance, or safety-critical design, always refer to official conversion tables, applicable standards, and direct hardness testing on the actual component or material lot.