Hardness Conversion Charts: Why They Are Not Universal

How are hardness conversion charts built?#

Hardness conversion charts are built by measuring a large set of test coupons from a single alloy family on two or more hardness scales, then fitting an empirical regression curve to the paired data. Because hardness is not a fundamental property but a response to a specific indentation procedure, the resulting curve is a statistical correlation valid only for that alloy family and metallurgical condition.

Hardness is a response to a specific indentation procedure: indenter geometry, applied load, and dwell time. Rockwell B measures depth of penetration with a 1/16-inch steel ball under 100 kgf. Rockwell C uses a diamond cone under 150 kgf. Brinell uses a 10 mm tungsten carbide ball under 3000 kgf and measures the diameter of the residual impression. Vickers uses a diamond pyramid and reports a value derived from the diagonal length of the impression. Each test probes a different volume of material, imposes a different strain field, and reflects a different combination of elastic and plastic deformation.

Conversion tables are built by taking a large population of test coupons from a single alloy family, measuring hardness on two or more scales on the same coupons, and fitting a curve through the data points. The curve is an empirical regression, not a physics-based equation. The relationship between a 10 mm Brinell impression in annealed 1020 steel and a Rockwell C reading in the same steel is a statistical correlation valid only for the metallurgical condition and composition range represented in the data set.

ASTM E140, “Standard Hardness Conversion Tables for Metals,” is the authoritative document in the United States. It contains separate tables for:

• Carbon and alloy steels in the annealed, normalized, and quenched-and-tempered conditions
• Austenitic stainless steels (annealed and cold-worked grades)
• Nickel and high-nickel alloys
• Cartridge brass (70% Cu, 30% Zn)
• Copper alloys
• Aluminum alloys (wrought and cast)

Each table covers a defined hardness range. Outside that range, the regression is an extrapolation with unknown uncertainty. The standard states plainly that conversions should not be used for acceptance or rejection of a product unless the conversion is specified in the governing product standard or purchaser-supplier agreement.

Why are hardness conversion charts specific to each alloy family?#

Hardness conversion charts are alloy-specific because the work-hardening behavior and elastic modulus of a metal govern its response to indentation, and these properties vary widely across alloy families. Austenitic stainless steels work-harden rapidly, aluminum alloys work-harden slowly, and copper alloys fall in between, causing the same indenter load to produce different hardness readings on different materials.

The core issue is work-hardening behavior. When an indenter penetrates a metal, the material beneath it deforms plastically and hardens. The rate and extent of that work hardening differs dramatically across alloy systems.

Austenitic stainless steels, such as 304 and 316, exhibit rapid work hardening. A Rockwell B reading of 80 on 304 stainless corresponds to a Brinell hardness roughly 10% higher than the same Rockwell B reading on carbon steel, because the stainless steel hardens more under the indenter, reducing the penetration depth for the same applied load. The conversion curve shifts upward.

Aluminum alloys work-harden at a lower rate. An aluminum alloy that reads 60 on the Rockwell E scale (1/8-inch ball, 100 kgf) will produce a Vickers hardness number that does not map to the steel conversion table. Attempting to force the aluminum reading through a steel-based chart yields a Vickers value that can be off by 20 HV or more.

Copper and brass sit in yet another regime. Cartridge brass (C26000) has a conversion table in ASTM E140 that differs from the copper-nickel table and from the phosphor bronze table. A single “non-ferrous” chart does not exist because the work-hardening coefficients, elastic moduli, and stacking-fault energies that govern indentation response vary too widely.

The elastic modulus also matters. Indentation involves elastic recovery when the load is removed. Rockwell measures under load; Brinell and Vickers measure after unloading. The difference between loaded and unloaded impression dimensions depends on the elastic modulus. Steel has a modulus around 200 GPa. Aluminum is roughly 70 GPa. Copper is near 110 GPa. This alone shifts the relationship between depth-reading scales (Rockwell) and impression-diameter scales (Brinell) across alloy groups.

What is the error when using a steel hardness conversion chart for aluminum or copper?#

Using a carbon steel hardness conversion chart for aluminum or copper can produce errors exceeding 30%. For example, a Rockwell B 75 reading on 6061-T6 aluminum converts to approximately 95 Brinell using the correct ASTM E140 aluminum table, but a steel-based chart would give 130 Brinell, a 37% overestimate. Similar discrepancies occur for brass and copper.

To make the problem concrete, consider a Rockwell B reading of 75. Using the ASTM E140 table for carbon steel, that converts to approximately 130 Brinell (3000 kgf, 10 mm ball). If the material is actually 6061-T6 aluminum, the same Rockwell B 75 reading corresponds to a Brinell hardness closer to 95. The steel-based conversion overestimates the Brinell value by roughly 35 points, an error of about 37%.

The error flows the other direction as well. A Brinell measurement of 100 on annealed copper corresponds to a Rockwell F reading of approximately 65. If a steel conversion chart is used, the predicted Rockwell F reading would be near 55. The discrepancy is not a small calibration offset; it is a systematic error rooted in the wrong material model.

The table illustrates a critical point: even within the steel family, austenitic stainless steels diverge from carbon and low-alloy steels. The error for 304 stainless is smaller than for aluminum or copper, but at higher hardness levels, where cold-worked stainless can reach 35 HRC and above, the discrepancy widens. ASTM E140 provides a dedicated table for austenitic stainless steels for this reason.

When must you perform a direct hardness test instead of using a conversion chart?#

A direct hardness test on the required scale is mandatory for product acceptance or rejection, for materials not covered by a recognized conversion table (such as titanium or zinc alloys), for thin coatings or case-hardened layers, for welds and heat-affected zones, and for work-hardened or anisotropic conditions. Conversions are for estimation only and should not drive material disposition decisions.

Conversions are a convenience for estimation, not a substitute for measurement. There are several situations where a direct test on the required scale is mandatory.

Acceptance or rejection of product. If the purchase order or governing specification states a hardness requirement in Brinell, a Rockwell test with conversion to Brinell is not acceptable for final acceptance unless the specification explicitly permits it. ASTM E140 states this in the scope. The risk is that a batch of material near the acceptance limit may pass on a converted value but fail on a direct test, or vice versa, leading to disputes.

Material not covered by a recognized table. There is no ASTM E140 table for titanium alloys, zinc die-casting alloys, or precious metals. Any conversion attempted for these materials is an extrapolation with no validated statistical basis. Direct testing is the only defensible option.

Thin coatings, case-hardened layers, or decarburized surfaces. Conversions assume a homogeneous material with a thickness sufficient to prevent the indentation stress field from interacting with the substrate. For nitrided cases, induction-hardened layers, or chrome plating, a microhardness test (Vickers or Knoop) is required. For more on this, see Microhardness Testing for Case Depth and Coatings.

Welds and heat-affected zones. Weld metal, base metal, and HAZ can have different microstructures and hardness responses within a few millimeters. A Rockwell test averages over too large a volume. Direct Vickers testing with a low load is standard practice. For weld qualification requirements, refer to Weld Procedure and Welder Qualification: Test Checklist.

Work-hardened or non-homogeneous conditions. Cold-drawn bar stock, shot-peened surfaces, and materials with significant crystallographic texture can show anisotropic hardness. Converting from one scale to another compounds the uncertainty from the texture effect with the uncertainty from the empirical correlation.

A practical rule of thumb: if the converted value will be used to make a decision about material disposition, perform the direct test. If the conversion is for internal tracking, trending, or rough sorting, it may be acceptable with a documented understanding of the uncertainty.

How do you use ASTM E140 hardness conversion tables correctly?#

To use ASTM E140 conversion tables correctly, first identify the exact alloy family and condition, verify the measured hardness falls within the table’s stated range, check that the conversion direction is covered, document the specific table used on the test report, and never chain multiple conversions. These steps prevent extrapolation errors and ensure traceability.

Applying conversion charts without introducing unnecessary risk requires discipline. Follow these steps:

1. Identify the alloy family and condition. “Steel” is not specific enough. Determine whether it is carbon steel, low-alloy steel, tool steel, or austenitic stainless steel. For non-ferrous metals, determine whether it is wrought aluminum, cast aluminum, cartridge brass, or another copper alloy.
2. Verify that the measured hardness falls within the stated range of the appropriate ASTM E140 table. Each table has minimum and maximum values. Extrapolating beyond the table range produces values with unknown uncertainty.
3. Check whether the conversion direction is covered. Some tables provide conversions in both directions (e.g., Rockwell C to Brinell and Brinell to Rockwell C), but the regression may be fitted for one direction only. Use the table as published; do not invert it mathematically.
4. Document the table used. On a test report, a converted value should cite the standard and table number, for example: “Converted per ASTM E140-24, Table 1 (Carbon and Alloy Steels).”
5. Never chain conversions. Converting Rockwell C to Brinell and then Brinell to Vickers compounds the error from two separate regressions. If a Vickers equivalent is needed, measure Vickers directly or use the single-step conversion table if one exists.

For a broader comparison of hardness test methods and their appropriate applications, see Rockwell vs Brinell vs Vickers: Hardness Test Comparison.

Why do hardness conversions create risk in material specifications?#

Hardness conversions create risk in material specifications because acceptance criteria for safety-critical properties, such as sulfide stress cracking resistance per NACE MR0175/ISO 15156, are based on direct measurements. Converting a Brinell test to Rockwell C introduces the conversion chart’s uncertainty into a pass/fail decision, potentially allowing non-conforming material to be accepted or conforming material to be rejected.

Material specifications and fabrication codes tie hardness to mechanical properties, wear resistance, and sometimes to maximum hardness limits for sulfide stress cracking resistance (e.g., NACE MR0175/ISO 15156). A specification that limits hardness to 22 HRC maximum for carbon steel in sour service is based on a direct Rockwell C measurement. Converting a Brinell test to 22 HRC using a conversion chart introduces the conversion uncertainty into a safety-critical acceptance criterion.

The same logic applies to welding procedure qualification. ASME Section IX and AWS D1.1 often specify maximum hardness limits in the HAZ. A Vickers microhardness traverse is the standard method because it provides spatial resolution. Converting a Rockwell B reading on the weld cap to an estimated Vickers value and comparing it to a HAZ limit is technically invalid and will not withstand audit scrutiny.

Procurement teams face this problem when suppliers from different regions report hardness on different scales. A European supplier may report Vickers HV10, while the drawing calls for Rockwell B. Rather than accepting a converted value, update the purchase specification to accept the scale the supplier can provide, with an agreed-upon acceptance range validated by testing actual samples. Alternatively, require that the supplier perform the direct test on the specified scale.

Key takeaways#

• Hardness conversions are empirical correlations valid only for the specific alloy family and condition for which they were developed.
• ASTM E140 provides separate tables for carbon steels, austenitic stainless steels, nickel alloys, cartridge brass, and aluminum alloys. There is no universal chart.
• Converting across alloy groups can produce errors exceeding 30%, particularly between steel and aluminum or copper alloys.
• Direct testing is mandatory for product acceptance, materials not covered by a recognized table, thin cases or coatings, and safety-critical applications.
• Document the specific ASTM E140 table used when reporting any converted value, and never chain multiple conversions together.