Hardness Scales Compared: HRC, HBW and HV and the Limits of Conversion

Three indenters, three philosophies

• Rockwell C (HRC) — a diamond cone under a 150 kgf major load; the reading comes from indentation depth, so it is fast and direct-reading. It is the default scale for hardened steels, useful over roughly 20–70 HRC.
• Brinell (HBW) — a 10 mm tungsten-carbide ball, typically at 3000 kgf for steel. The wide impression averages over grains and phases, which makes it forgiving on castings, forgings and inhomogeneous material. Its practical ceiling is around 650 HBW.
• Vickers (HV) — a diamond pyramid (136° face angle) at loads from a few grams to 100 kgf. One continuous scale runs from soft to very hard material, and at low loads it is the only one of the three that can profile thin case layers and weld heat-affected zones point by point.

Approximate equivalence (non-austenitic steels)

Orientation values consistent with ASTM E140, rounded — for comparison only, never for acceptance:

HRC	HBW	HV	Approx. tensile strength
20	226	238	~760 MPa
30	283	294	~950 MPa
40	371	392	~1,240 MPa
50	481	513	~1,650 MPa

A common shop approximation links the mid-range values: tensile strength in MPa ≈ 3.3–3.5 × HBW for plain and low-alloy steels. It is a rule of thumb, not a specification relation.

Why conversions wander

• The scales interrogate different volumes. Brinell averages microstructure over a wide dent; Rockwell depth responds to the hardest continuous path under a point; a Vickers indent at low load can sit inside a single phase or carbide.
• Conversion tables are empirical fits per material class. ASTM E140 tables were built on specific steel populations; applying them to austenitic stainless steels, coarse-carbide tool steels or case-hardened surfaces can miss by several HRC points.
• Above roughly 55 HRC, Brinell readings compress (the ball itself deforms), so the top of the table is Vickers and Rockwell territory only.
• A purchase specification should therefore name the test method and scale, not a converted value: "≥ 38 HRC per ASTM E18" is enforceable, "≈ 360 HBW equivalent" is an argument waiting to happen.

Choosing a scale

• Receiving inspection of bar, plate and forgings: HBW — robust and averaging.
• Hardened dies, tools and components: HRC — or HV where the part is small or thin.
• Case depth profiles, nitrided layers, weld and HAZ traverses: HV at micro/macro low loads.
• Very hard or thin material: avoid Brinell; the impression is either unreliable or destructive relative to the section.