Hardness Conversion Charts: HRC, HV, HBW and UTS Relationships Explained

Hardness conversion between Rockwell C (HRC), Vickers (HV), Brinell (HBW) and approximate tensile strength (UTS) is one of the most routine — and most misunderstood — tasks in materials engineering practice. This tutorial explains the physical basis of each hardness scale, the limits of inter-scale conversion, how to read ASTM E140 conversion tables correctly, and how to estimate tensile strength from hardness data with appropriate caveats.

The Physical Basis of Each Hardness Scale

All hardness tests share a common principle: force an indenter of defined geometry into the material surface, then measure the resulting deformation. However, each scale measures a different geometric outcome of that deformation, which is precisely why inter-scale conversion cannot be exact.

Rockwell Hardness (HRC, HRB, HRA)

The Rockwell test measures the incremental depth of penetration of an indenter under a defined major load after a minor pre-load has been applied. For the C scale (HRC), a 120° diamond Brale cone indenter is loaded to 150 kgf (1471 N) after a 10 kgf minor load. The hardness number is derived from the additional depth h caused by the major load:

Each Rockwell unit therefore represents 0.002 mm (2 μm) of depth. The C scale is valid between approximately 20 and 70 HRC. Below 20 HRC the depth increment becomes large relative to the minor load depth, reducing repeatability; above 70 HRC, most materials fracture rather than indent uniformly. For intermediate hardness levels (HRC < 20), the Rockwell B scale (1.588 mm steel ball, 100 kgf) is more appropriate, as detailed in the Hardness Testing Methods article.

Vickers Hardness (HV)

The Vickers test employs a square-based diamond pyramid with a face angle of 136°. Load is applied for 10–15 seconds, and after removal both diagonals of the residual indent are measured optically. Hardness is calculated as:

The Vickers test is geometrically self-similar: the hardness number is theoretically independent of load (provided the material is homogeneous). In practice, at very low loads (<10 gf), the indentation size effect (ISE) causes apparent hardness to increase with decreasing load. The test is uniquely suited to thin sections, case-hardened layers, weld HAZ profiles, and individual microstructural constituents when used as microhardness (HV 0.01 to HV 1). The geometric self-similarity of the Vickers pyramid makes it the preferred scale for scientific work and cross-material comparison.

Brinell Hardness (HBW)

The Brinell test uses a tungsten carbide ball (diameter D = 10 mm in the standard test) pressed into the surface under a specified load (F = 3000 kgf for steel). Hardness is the ratio of force to the curved surface area of the indent:

The Brinell test averages over a relatively large area (indent diameter 2–6 mm), making it suitable for heterogeneous materials such as castings, forgings, and weld deposits. Its upper limit with carbide ball is approximately 650 HBW; above this the ball deforms. The test is not suitable for case-hardened surfaces, thin sections, or small components. The abbreviation HBW (tungsten carbide ball) has superseded HBS (steel ball) in modern standards following ISO 6506 and ASTM E10 revisions.

Why Hardness Conversions Are Always Approximate

The fundamental reason that no mathematically exact conversion exists between hardness scales is that each scale is sensitive to different material properties in different proportions. The Rockwell depth measurement is strongly influenced by elastic recovery (modulus-dependent), while the Vickers area measurement is dominated by plastic flow (yield-stress and work-hardening dependent). The Brinell measurement involves larger deformation volumes and is thus more sensitive to second-phase particles and microstructural heterogeneity.

Consider two steels, both measuring 40 HRC: a tempered martensite (low work-hardening rate, high elastic modulus contribution) versus an austenitic stainless steel (high work-hardening rate). The same HRC reading will correspond to different Vickers values because the austenitic steel work-hardens during the Vickers indent, elevating the HV reading relative to the carbon steel. ASTM E140 accounts for this by publishing entirely separate tables for different material classes.

ASTM E140: The Definitive Conversion Reference

ASTM E140, Standard Hardness Conversion Tables for Metals, is published by ASTM International and revised periodically. The current version (E140-12b, with 2019 reapproval) provides separate tables for:

• Table 1: Steel — non-austenitic (carbon, alloy, tool steels)
• Table 2: Cartridge brass (70% Cu, 30% Zn)
• Table 3: Copper and copper alloys
• Table 4: Austenitic stainless steels
• Table 5: Nickel alloys (Monel, Inconel, etc.)
• Table 6: Titanium alloys (limited range)

Each table is derived from regression analysis of interlaboratory data. The standard explicitly states that values are approximate and that direct hardness measurement in the specified scale is always preferable to conversion. Uncertainty bands (approximately ±2 HRC across the mid-range) are not typically published in the tables but are documented in the research annex of the standard.

The equivalent international standard is ISO 18265, Metallic Materials — Conversion of Hardness Values, which uses the same empirical basis but reports slightly different values in some ranges due to different regression datasets. For European engineering documentation, ISO 18265 is commonly cited alongside EN ISO 6506 (Brinell), EN ISO 6507 (Vickers), and EN ISO 6508 (Rockwell).

Hardness Conversion Table — Carbon and Alloy Steels (ASTM E140-12b)

The following table presents key conversion values extracted from ASTM E140-12b, Table 1, covering the range from very hard tool steel and case-hardened surfaces through to normalised structural steel. Values for intermediate HRC levels follow from the published table.

* Brinell values above ~450 HBW require a carbide ball (HBW) and may involve ball deformation errors; values marked with * should be confirmed by Vickers. UTS values based on ASTM A370 correlation (UTS ≈ 3.45 × HBW MPa). — indicates range exceeds Brinell validity. Based on ASTM E140-12b Table 1 and ASTM A370. © metallurgyzone.com

The Hardness–Tensile Strength Relationship

Physical Basis of the Correlation

Both hardness and tensile strength are measures of a material’s resistance to plastic deformation. In uniaxial tension, this resistance is quantified as the load per unit area at maximum load (UTS). In hardness testing, resistance is measured as load per unit projected or curved area of the indent. For strain-hardening metals, both measurements sample the same underlying dislocation mechanisms.

Tabor (1951) showed theoretically and experimentally that for materials following a power-law strain-hardening relationship (Hollomon: σ = Kεⁿ), the Vickers hardness is proportional to the representative flow stress at a characteristic strain of approximately 8% (corresponding to the geometry of the Vickers pyramid):

For many steels, the flow stress at 8% strain is reasonably close to the UTS, which occurs at a true strain equal to the strain-hardening exponent n. This is why the empirical HBW-to-UTS correlation works surprisingly well for a wide range of carbon and alloy steels.

The ASTM A370 Correlation

ASTM A370, Standard Test Methods and Definitions for Mechanical Testing of Steel Products, formalises the correlation:

Since HBW and HV are approximately equal up to about 350 HBW, a similar relationship holds: UTS (MPa) ≈ 3.3 × HV. This 5% difference (3.45 vs 3.3) arises from the slight divergence in HBW and HV scales noted in the ASTM E140 conversion table.

Upper and Lower Bounds of the Correlation

Below approximately 100 HBW (very soft annealed steels), the relationship underestimates UTS because necking occurs at very low strain. Above approximately 400 HBW, the correlation overestimates UTS because fracture increasingly competes with continued plastic flow, particularly in high-carbon martensitic steels where brittleness limits the area under the stress-strain curve. For these high-hardness applications, direct tensile testing on witness coupons remains mandatory.

Special Cases and Industry Limits

NACE MR0175 / ISO 15156 Sour Service Limit

In environments containing hydrogen sulphide (H₂S) — sour service — high-hardness microstructures are susceptible to sulphide stress cracking (SSC), a form of hydrogen-induced stress corrosion cracking. NACE MR0175 / ISO 15156-2 restricts hardness of carbon and low-alloy steel base metal, weld metal, and HAZ to a maximum of 22 HRC (248 HV, 237 HBW).

This limit exists because untempered or under-tempered martensite, present when welding is not followed by PWHT, is highly susceptible to hydrogen embrittlement. The 22 HRC threshold was established empirically through industry experience rather than from a single clean fracture mechanics boundary; materials just above 22 HRC have been observed to crack in sour service while those just below have generally performed acceptably. The topic is discussed further in the Hydrogen Induced Cracking article and in the context of HAZ microstructure development.

Hardness Requirements in ASME and API Codes

ASME Section IX and ASME B31.3 reference post-weld heat treatment (PWHT) requirements that are indirectly linked to hardness. Most PWHT requirements for low-alloy steels are triggered at thicknesses above a threshold to ensure the HAZ tempers adequately below the NACE limit. API 5L, 5CT, and 6A all impose hardness limits on line pipe, casing, and wellhead equipment in sour service, cross-referencing NACE MR0175. These requirements affect the materials engineer’s selection of alloy composition, preheat, PWHT temperature, and holding time.

Hardness and Weldability

For ferrous alloys, the hardness of the as-welded HAZ is closely linked to carbon equivalent (CE), which governs the martensite start temperature and thus the proportion of untempered martensite in the HAZ after cooling. High as-welded HAZ hardness (>350 HV) signals high CE and elevated hydrogen cracking risk. The relationship between hardness and weldability is discussed in depth in the hardness testing methods article and cross-referenced to hydrogen-induced cracking prevention strategies.

Microhardness for Case Depth Determination

Case depth (effective case depth, ECD, or total case depth, TCD) is defined by the depth at which hardness drops to a specified value below the case surface — commonly 550 HV (50 HRC equivalent) for carburised bearing surfaces, per ISO 2639. Microhardness traverses on polished metallographic cross-sections using HV 0.3 or HV 1 provide the spatial resolution needed to resolve steep hardness gradients across case depths of 0.1–2 mm. Conversion of microhardness data to HRC using ASTM E140 is permitted when the indent is wholly within the case layer.

Conversion Table: Austenitic Stainless Steel (ASTM E140-12b, Table 4)

Because austenitic stainless steels work-harden at a higher rate than carbon steels during indentation, their Vickers values are systematically higher than the carbon steel table would predict for the same HRC reading. The following abbreviated table illustrates this divergence:

For austenitic stainless steels, using the carbon steel ASTM E140 table systematically underestimates the equivalent HRC by 2–4 HRC above 350 HV. Use ASTM E140 Table 4 for all austenitic grades. © metallurgyzone.com

Practical Guide to Scale Selection

When to Use HRC

Use Rockwell C when: the material is harder than 20 HRC; the component is large enough to accommodate the large indent without edge effects; rapid production-floor testing is required (no optical measurement needed); and the material is a monolithic solid with no coatings or thin sections. HRC is the preferred scale in production inspection of heat-treated components — gears, dies, springs, shafts — because of its speed and repeatability.

When to Use Vickers

Use Vickers when: the material is thin (<2 mm) or has a surface treatment; microstructural-level hardness mapping is needed (individual phase, HAZ gradient, case depth profile); the material is a non-ferrous alloy; or you require the widest possible hardness range with a single scale (HV 5 to HV 3000). Vickers is the reference scale in most scientific literature because of its geometric self-similarity. Vickers is also required by ASTM E140 and ISO 18265 to serve as the common pivot point for cross-scale conversion. The connection between Vickers HAZ hardness profiles and microstructural evolution is explored further in the HAZ microstructure article.

When to Use Brinell

Use Brinell when: the material is heterogeneous (cast iron, forgings with banding, weld deposits); the component is large and rough enough that a large indent area is needed to average microstructural variability; or the hardness is below 650 HBW. Brinell is standard for rough-machined or as-cast components, and for incoming material inspection where surface preparation is limited. It should not be used for case-hardened surfaces, thin wall components, or hardness above 650 HBW.

Using Portable / Leeb Rebound Testers

Leeb (dynamic rebound) hardness testers are invaluable for in-situ inspection of large structures — pressure vessels, pipe spools, structural fabrications — where the component cannot be moved to a laboratory testing machine. The Leeb HL value (impact / rebound velocity ratio × 1000) is converted internally within the instrument to HRC, HV, or HBW using regression curves specific to material class. The user must select the correct material class (steel, cast iron, stainless steel, etc.) and probe orientation to obtain valid conversions. Accuracy is typically ±2–3 HRC compared with a fixed laboratory Rockwell machine.

Converting Between HRC, HV and HBW: Step-by-Step Worked Examples

Example 1 — HRC to HV and HBW

A quenched-and-tempered 4340 alloy steel shaft measures 42 HRC on the Rockwell C scale. Convert to Vickers and Brinell, and estimate tensile strength.

Step 1: Locate 42 HRC in ASTM E140-12b, Table 1 (steel)
        HRC 42 → HV = 409, HBW = 390

Step 2: Estimate UTS via ASTM A370 correlation
        UTS (MPa) = 3.45 × HBW = 3.45 × 390 = 1,346 MPa
        UTS (ksi) = 0.5 × HBW  = 0.5 × 390 = 195 ksi

Step 3: Note uncertainty
        ASTM E140 table resolution = 1 HRC
        ±1 HRC at 42 HRC corresponds to HV range 400–420 (ASTM Table 1)
        UTS uncertainty ≈ ±35 MPa (~2.5%)
  

Example 2 — HV to HRC (Austenitic Stainless Steel)

A 316L stainless steel weld overlay measures 380 HV during microhardness testing. The inspector wants to compare against a 38 HRC maximum specification.

Step 1: Select correct ASTM E140 table
        Material = austenitic stainless steel → Table 4

Step 2: Locate 380 HV in Table 4
        380 HV → HRC ≈ 39.5 (ASTM E140 Table 4)

Step 3: If carbon steel Table 1 had been used incorrectly:
        380 HV in Table 1 → HRC ≈ 37.6 (2 HRC error)

Step 4: Conclusion
        Correct value (38.5 HRC, Table 4) exceeds the 38 HRC specification.
        Incorrect table (37.6 HRC, Table 1) would pass the specification.
        Using the wrong table produces a non-conservative assessment error.
  

Industrial Applications

Quality Control in Heat Treatment

Hardness testing is the most common post-heat-treatment quality check because it is fast, cheap, and non-destructive to the component in most cases. In a typical Q+T production line for alloy steel bars, Brinell hardness testing of each bar end provides a rapid indication that the target microstructure (tempered martensite) has been achieved within specification. Outliers trigger destructive coupon sampling, metallographic examination, and comparison with quenching and tempering process parameters. The connection between tempering temperature and hardness drop is quantified by hardness traverse data referenced to the martensite formation model.

Incoming Material Inspection

When receiving steel plate, bar, or forgings to a hardness specification (e.g., 200–275 HBW for S355 Q+T, or max 22 HRC for sour service pipe), portable Leeb testers or bench Brinell machines are used to verify compliance without consuming material. Conversion to HRC is then used to compare against design specifications expressed in that scale.

Failure Analysis

Post-failure hardness mapping on fracture cross-sections and adjacent material is a standard element of metallurgical failure analysis. Hardness data is used to infer the heat treatment history of a failed component, identify softening in the HAZ, and detect improper PWHT. Hardness conversion between scales enables comparison across reports generated in different laboratories or periods. The methodology is described in the failure analysis tutorial.

Weld Procedure Qualification

ASME Section IX and AWS D1.1 weld procedure qualification records (PQRs) routinely include hardness surveys of the weld cross-section. Traverses are taken in the weld metal, fusion line, coarse-grain HAZ, fine-grain HAZ, and base metal. Results are reported in HV (typically HV 10) for precision, then converted to HRC if the engineering specification uses that scale. Maximum HAZ hardness limits in sour service (22 HRC per NACE MR0175) are verified from these traverses. This topic is also addressed in the WPS writing tutorial.

Limitations and Common Errors

• Wrong material table: Using the carbon steel ASTM E140 table for austenitic stainless steel or nickel alloys introduces systematic non-conservative errors of 2–4 HRC.
• Extrapolation beyond scale limits: Rockwell C below 20 HRC or above 70 HRC gives meaningless values. Brinell above 650 HBW is invalid. Vickers below HV 5 (indentation too small to measure reliably with most machines) is unreliable.
• Surface condition: Rough surfaces, scale, decarburisation, and residual compressive stress all bias hardness readings. Surfaces must be prepared per ASTM E10, E18, or E384 before testing.
• Edge effects and thin sections: Testing within 2.5 × indent diameter of an edge or on a section thinner than 10 × indent depth will give artificially low readings.
• Confusing HBW with HBS: HBS (steel ball Brinell) is no longer recommended for materials above 450 HBW because the ball deforms. Modern standards use HBW exclusively; historical data reported as HB without a suffix should be clarified.
• Using hardness as a proxy for toughness: Hardness and Charpy impact energy (toughness) are essentially uncorrelated beyond the broad observation that harder = generally more brittle. Do not infer toughness from hardness; perform Charpy testing per Charpy impact testing procedures.