Steel Hardness Conversion Chart — HRC, HV, HB, HRB

HRC — Rockwell C. The Rockwell C scale uses a 120° diamond cone indenter with a 10 kgf preliminary load and a 150 kgf major load. It is designed for hardened steel above approximately 20 HRC (roughly 225 HB / 226 HV). Below 20 HRC the indentation is too shallow for reliable reading and the scale loses resolution — the diamond cone does not penetrate deeply enough to average over the microstructure, and readings become sensitive to individual carbide particles and surface roughness. HRC is the standard scale for quenched and tempered parts, tool steel, and case-hardened surfaces. It is not appropriate for as-delivered fine blanking strip, which is almost always below 20 HRC.

HV — Vickers. The Vickers test uses a square-based diamond pyramid indenter (136° between opposite faces) under a selectable load ranging from 10 gf to 100 kgf. For steel strip, a 30 kgf load (HV30) is common for bulk hardness, while lower loads (HV1, HV0.3, HV0.1) are used for microhardness profiling across decarburized layers, case-hardened zones, or thin strip below 1.5 mm where a Brinell indentation would be too large relative to the thickness. Vickers covers the entire hardness range from soft annealed steel to the hardest tool steel, and the same indenter geometry is used at all loads, making HV the most versatile scale. The indentation diagonal is measured under a microscope and converted to HV via a formula.

HB — Brinell. The Brinell test uses a 10 mm diameter tungsten carbide ball under a 3000 kgf load for steel (lower loads of 500 or 1500 kgf are used for softer materials). The resulting indentation is typically 2–6 mm in diameter, which averages the hardness over a large area and makes HB insensitive to local microstructural variation — a valuable property for fine blanking strip where carbide distribution varies at the micrometer scale. Brinell is the standard incoming-inspection scale for as-delivered fine blanking steel in the 120–200 HB range. The practical upper limit is approximately 650 HB; above that the ball indenter itself deforms. HB is not suitable for strip thinner than approximately 1.5 mm because the indentation depth (roughly 0.2–0.5 mm) becomes a significant fraction of the strip thickness and the anvil backing influences the reading.

HRB — Rockwell B. The Rockwell B scale uses a 1/16 inch (1.588 mm) diameter hardened steel ball indenter with a 10 kgf preliminary load and a 100 kgf major load. It is designed for soft steel, annealed copper, and aluminum alloys up to approximately 100 HRB (roughly 200 HB / 20 HRC). Above 100 HRB the ball indenter penetrates too deeply and deforms, producing inaccurate readings. HRB is convenient for incoming inspection of low-carbon strip (SAE1010, SPCC, DC01) because it is fast, requires no indentation measurement, and leaves a small mark. For medium- and high-carbon spheroidized strip in the 160–200 HB range, HRB readings of 85–96 are valid, but HB is preferred because the larger Brinell indentation averages over more microstructure.

Which scale for steel strip incoming inspection? The choice depends on strip thickness and hardness range:

• Strip ≥ 1.5 mm, hardness 80–650 HB: Use Brinell (HB, 3000 kgf). This covers the vast majority of as-delivered fine blanking strip.
• Strip < 1.5 mm, or surface-layer / decarburization profiling: Use Vickers (HV, load selected so indentation depth ≤ 1/10 of thickness or layer depth).
• Soft low-carbon strip, quick production check: Use Rockwell B (HRB). Fast and non-destructive enough for coil-side verification.
• Heat-treated parts or hardened tool steel: Use Rockwell C (HRC) above 20 HRC, or Vickers (HV) for all ranges.

When reading a Mill Test Certificate (MTC), always note which scale and load was used — "HV" without a load is ambiguous, and "HB" without specifying the ball material (tungsten carbide vs. steel) and load can differ by several points. The MTC should report hardness at head, middle, and tail of the coil for uniformity verification. For guidance on interpreting the full MTC, refer to the companion guide on how to read a mill test certificate.

How to use this table. When a supplier's MTC reports hardness in one scale and your purchase specification is written in another, find the row closest to the reported value and read across. For example, if the MTC reports 180 HB for a coil of SAE1078 steel strip, the approximate equivalent is 91.5 HRB and below 20 HRC — confirming that the material is in the as-delivered spheroidized condition, not hardened. If a heat-treatment supplier reports 48 HRC on a quenched clutch plate made from 65Mn spring steel strip, the approximate equivalent is 481 HV and 465 HB — useful for comparing against a drawing that specifies HV.

Conversion accuracy. No conversion table is exact. The relationship between hardness scales depends on the material's elastic modulus, yield strength, and work-hardening behavior, which vary by grade and heat-treatment condition. ASTM E140 and ISO 18265 provide conversion tables for specific material groups (carbon steel, alloy steel, stainless steel) because the equivalents differ slightly between groups. The values in the table above are the widely used carbon / low-alloy steel equivalents. For stainless steel or high-alloy tool steel, consult the material-specific conversion table in ASTM E140. When hardness is a critical acceptance parameter, specify the test method and scale directly on the purchase order rather than relying on cross-scale conversion.

Why delivery condition matters more than grade. Two coils of the same grade can have very different hardness depending on the delivery condition. SAE1078 in the normalized condition can measure 220–250 HB — too hard for fine blanking — while the same grade in the fully spheroidized annealed condition measures 160–200 HB and blanks well. This is why purchase specifications for fine blanking steel must state the delivery condition explicitly ("spheroidized annealed, 160–200 HB") rather than just the grade name. C67S / C75S spring steel strip is commonly supplied in both the spheroidized condition (for fine blanking) and the hardened-and-tempered condition (for direct spring use) — the two are not interchangeable.

Carbon content and hardness relationship. In the annealed or spheroidized condition, hardness increases roughly linearly with carbon content up to approximately 0.85% C: each 0.10% C addition raises annealed hardness by roughly 15–25 HB. This is why low-carbon steel (≤0.12% C) tops out around 140 HB, while high-carbon steel (0.70–0.85% C) reaches 180–200 HB even when fully spheroidized. Above the eutectoid point (~0.77% C), additional carbon forms excess cementite networks that increase hardness and brittleness without improving hardenability — which is why most fine blanking grades cap at 0.85% C. For a detailed treatment of the spheroidizing process that produces these hardness ranges, refer to the spheroidizing annealing complete guide.

The hardness band concept. For fine blanking, the as-delivered hardness must fall inside a specified band with both an upper and a lower bound. The band depends on carbon content:

• Low carbon (≤0.12% C): 120–160 HB. The lower bound prevents excessive roll-over and dimensional drift; the upper bound is naturally limited by the low carbon content.
• Medium carbon (0.30–0.50% C), spheroidized: 160–200 HB. The lower bound of 160 HB ensures the material has enough strength to resist excessive flow; the upper bound of 200 HB keeps press load and die wear manageable.
• High carbon (0.70–0.85% C), fully spheroidized: 160–200 HB, with 160–180 HB preferred for parts thicker than 4 mm. The longer deformation path at heavy gauge generates more work hardening mid-stroke, so starting at the lower end of the band prevents the effective hardness from rising above the blankable range during the stroke.

Why too hard causes die wear and tearing. When strip hardness exceeds approximately 210 HB, two failure modes accelerate. First, the higher yield strength requires greater blanking force, which increases contact pressure between the punch edge and the material and accelerates abrasive wear — every 20 HB increase above the band can roughly halve punch and die life. Second, harder material is less ductile and more likely to fracture rather than flow during the shear deformation, producing a torn zone on the shear face. In high-carbon steel with incomplete spheroidization, lamellar pearlite colonies act as crack initiation sites even when the bulk hardness is within the band — which is why hardness alone is an insufficient acceptance criterion and microstructure must also be verified. The companion fine blanking material requirements guide covers the full three-parameter window (carbon, hardness, microstructure) in detail.

Why too soft causes roll-over and dimensional drift. Steel below approximately 150 HB in the medium- and high-carbon range is over-annealed — the ferrite grains have coarsened and the material yields too easily under the punch. The result is excessive roll-over (the rounded zone at the top of the shear face, where the material is pushed down and outward by the punch entry), a smeared rather than cleanly cut shear surface, and part dimensions that drift from blank to blank because the material flows unpredictably. In low-carbon steel below 120 HB, the same effect appears as excessive burr and poor edge definition. A lower bound on hardness is therefore as important as an upper bound — always specify a band such as 160–200 HB or 170–190 HB, never just "≤200 HB."

Hardness uniformity requirement. A hardness band is meaningless if the hardness varies across the coil. For fine blanking, the difference between the hardest and softest readings in the same coil should not exceed 20 HB. Hardness should be measured at the head, middle, and tail of each coil, and at both the center and edge of the strip width. Edge-to-center variation greater than 15 HB indicates uneven annealing — often caused by coil packing density or furnace temperature stratification — and produces uneven V-ring penetration, uneven counterpressure, and part-to-part dimensional variation as the strip feeds through the die. This is particularly critical in automotive seat recliner fine blanking, where gear tooth pitch must be consistent for safe engagement.

Relationship between hardness, carbon content, and spheroidization. Hardness in the as-delivered condition is the product of two factors: carbon content (which sets the volume of hard cementite phase) and microstructure (which determines how that cementite is distributed). Fully spheroidized cementite — globular carbide particles in a soft ferrite matrix — produces the lowest possible hardness for a given carbon content because the globular particles do not impede dislocation motion as strongly as lamellar pearlite. For SAE1078 (0.78% C), the fully spheroidized condition yields 160–180 HB, while the normalized condition (lamellar pearlite) yields 220–250 HB. Spheroidizing is therefore not just a microstructural preference — it is the process that brings high-carbon steel into the fine blanking hardness window. SAE1035 / SAE1050 steel strip and SK5 carbon tool steel strip both rely on spheroidizing to reach their blankable hardness ranges.

Test locations. Hardness should be measured at multiple positions to verify both longitudinal and transverse uniformity:

• Longitudinal: Head (outer wrap), middle, and tail (inner wrap) of the coil. Hardness variation along the coil length indicates uneven annealing or cooling.
• Transverse: Center and both edges of the strip width. Edge-to-center variation indicates uneven furnace temperature or coil packing density during annealing.
• Minimum six readings per coil (3 longitudinal × 2 transverse) for incoming inspection. For safety-critical parts or new suppliers, increase to nine readings (3 × 3, adding quarter-width points).

Sample preparation. The test surface must be flat, clean, and free of oxide, oil, rust, and shear deformation. For Brinell testing on strip, the sample should be cut from the coil, flattened if necessary (but cold working from flattening can raise hardness by 5–15 HB — use a large enough sample and test away from the flattened edges), and the surface lightly ground or polished to remove the decarburized layer if surface hardness is not the target. For Vickers microhardness profiling, the sample must be mounted in epoxy, ground, and polished to a 1 micrometer finish, then etched if the indentation must be located relative to microstructure. The sample must be thick enough that the indentation depth does not exceed 1/10 of the sample thickness — otherwise the anvil backing influences the reading.

Scale selection by strip thickness. The indenter geometry and load must be matched to the strip thickness to avoid anvil effects:

• Strip ≥ 1.5 mm, bulk hardness: Brinell HB (3000 kgf, 10 mm tungsten carbide ball). Indentation depth is typically 0.15–0.35 mm, well below 1/10 of thickness for strip ≥ 1.5 mm.
• Strip 0.5 – 1.5 mm, bulk hardness: Vickers HV with load selected so indentation diagonal ≤ 1/3 of strip thickness. Typically HV1 to HV5. Alternatively, use HRB if the material is soft and the indentation depth is acceptable.
• Strip < 0.5 mm or surface-layer profiling: Vickers microhardness HV0.1 to HV1, with load reduced so indentation depth ≤ 1/10 of layer thickness. Used for decarburization depth measurement, case-hardened layer profiling, and coated strip.
• Soft thin strip, quick check: Rockwell B (HRB) with the superficial Rockwell variant (HR15T, HR30T) if conventional HRB indentation is too deep for the strip thickness.

How to read MTC hardness values. The MTC should report hardness with the scale, load, test standard, and locations. Look for: (1) the scale designation — "HBW 10/3000" means Brinell with tungsten carbide ball, 10 mm diameter, 3000 kgf load; "HV30" means Vickers at 30 kgf; "HRB" means Rockwell B; (2) the number of readings and their locations — a single hardness value without location data is not sufficient for uniformity verification; (3) the test standard — ASTM E10 for Brinell, ASTM E92 for Vickers, ASTM E18 for Rockwell, or the ISO equivalents; (4) whether the hardness was measured on the strip surface or on a cross-section — surface readings on decarburized strip will read lower than bulk hardness. If the MTC reports only "HB 180" without load, ball type, or location, request a corrected certificate.

Common testing errors. The most frequent mistakes in strip hardness testing are: (1) testing on a curved coil surface without flattening — the indentation is elliptical and the reading is inaccurate; (2) using Brinell on strip thinner than 1.5 mm — the indentation bottoms out on the anvil and reads too high; (3) testing through oil or oxide — the surface layer cushions the indenter and reads too low; (4) placing indentations too close together or too close to the sample edge — the minimum spacing is 3× the indentation diameter from center to center and 2.5× from center to edge; (5) converting between scales without noting the material group — ASTM E140 conversions differ for carbon steel, alloy steel, and stainless steel; (6) using a worn or chipped indenter — diamond cones and Vickers pyramids chip with use and must be inspected periodically. For a complete walkthrough of MTC interpretation including hardness, chemistry, and dimensional results, see the how to read a mill test certificate guide.

As-delivered hardness vs. final hardness. These are two independent specifications and should not be confused. The as-delivered hardness (typically 160–200 HB for spheroidized medium/high carbon) is chosen for blankability — it ensures the material flows cleanly in the fine blanking press and does not wear the die excessively. The final hardness (typically 40–60 HRC after quenching and tempering) is chosen for part function — wear resistance, spring load, or structural strength. A fully spheroidized SAE1078 coil at 175 HB will blank cleanly and then, after quenching in oil or polymer and tempering at 200–300°C, reach 58–62 HRC. The as-delivered hardness has almost no effect on the final achievable hardness, which is determined primarily by carbon content, section size, quench severity, and tempering temperature. The only exception is that severe decarburization in the as-delivered strip reduces the surface carbon available for hardening, resulting in a lower surface hardness after quenching.

Decarburization effect on surface hardness. Decarburization is a surface layer where carbon has diffused out during hot rolling or annealing, leaving a softer, lower-carbon zone. After quenching, this decarburized layer fails to reach the required hardness because it lacks sufficient carbon to form martensite. A 0.05 mm decarburized layer on a 2.0 mm SAE1078 clutch plate can result in a surface hardness of 45–50 HRC instead of the required 58–62 HRC, causing premature wear and loss of spring load. For fine blanking steel, total decarburization depth should be limited to 1.5% of strip thickness per side (maximum 0.10 mm for strip ≤ 2 mm, 0.15 mm for 2–8 mm). Decarburization is measured by Vickers microhardness profiling across the cross-section — a series of HV0.1 or HV0.3 indentations from the surface inward reveals the hardness gradient and the depth of the decarburized zone. This is the most reliable method because it directly measures the functional property (hardness) rather than inferring it from microstructure.

Case hardening vs. through hardening. Two heat-treatment routes produce different hardness profiles and require different as-delivered strip specifications:

• Through hardening (quenching and tempering of medium/high-carbon steel): The entire part cross-section reaches a uniform hardness determined by carbon content and section size. As-delivered strip must be spheroidized to 160–200 HB for blankability. Typical grades: SAE1045, SAE1078, C67S, C75S, 65Mn, 42CrMo. Final hardness 40–62 HRC depending on temper.
• Case hardening (carburizing or carbonitriding of low-carbon steel): The part is blanked from soft low-carbon strip (120–160 HB), then carburized to raise the surface carbon to 0.70–0.90%, quenched, and tempered. The result is a hard, wear-resistant case (58–62 HRC, 0.3–1.5 mm deep) over a tough, low-carbon core (20–30 HRC). Typical grades: 20MnCr5, 16MnCr5, SAE1010 carburizing grade. Case hardening is used for gears, shafts, and synchronizer parts where both surface wear resistance and core toughness are required.

The choice between through hardening and case hardening affects both the as-delivered strip specification and the fine blanking process. Case-hardening grades are lower carbon and therefore blank more easily (lower press load, longer die life), but require an additional carburizing step after blanking. Through-hardening grades blank at higher hardness but skip the carburizing step. For parts with complex geometry and thin sections, case hardening is often preferred because the lower as-delivered hardness reduces the risk of cracking during blanking. For simple flat parts such as clutch plates and saw blade bodies, through hardening from high-carbon strip is more economical. The fine blanking steel product range covers both routes, and our engineers can recommend the optimal grade and heat-treatment path based on your part drawing and functional requirements — contact our engineering services for a material specification.