Rockwell Hardness Converter

A Rockwell hardness converter translates a reading on one indentation scale into its approximate equivalents on the others: Rockwell C, B, and A, Brinell, Vickers, and Knoop. Each scale presses a different indenter (a diamond cone, a hardened ball, or a diamond pyramid) into the metal under a different load and measures either depth or indentation size, so no exact mathematical conversion exists; the equivalents are empirical relationships measured on steel. The hardness value accepts decimals, fractions such as 3/4, and mixed numbers such as 5 1/2.

Each scale serves its own band. Rockwell C (150 kgf, diamond cone) covers hardened steel from 20 to 68 HRC; Rockwell B (100 kgf, 1/16 inch ball) covers softer steel and nonferrous metal up to 100 HRB; Rockwell A (60 kgf, diamond cone) spans both and is the usual scale for cemented carbide. Brinell here is the 3000 kgf, 10 mm ball value, and Vickers and Knoop are the diamond-pyramid scales also used at light loads for thin layers and coatings. The converter covers 20 to 68 HRC, 55 to 100 HRB, 37.5 to 85.6 HRA, 99 to 739 HB, and about 104 to 940 HV, and it reports no conversion outside those bands rather than extrapolating.

Conversions run through Vickers. The tool holds a sparse set of equivalent rows from published conversion charts for non-austenitic steels, the relationships tabulated in ASTM E140 and SAE J417, such as 40 HRC = 392 HV = 371 HB = 70.4 HRA, and interpolates linearly between the two rows that bracket the input. Below 82 HRB the charts give no Vickers figure, so Vickers is taken as Brinell / 0.95, the ratio the charts show through the soft range. Knoop is taken as Vickers / 0.95, since Knoop runs about 5 percent higher for steels at test loads of 500 gf and above. Settings shows each interpolation step and controls the decimal places.

The equivalents describe carbon and alloy steels. Austenitic stainless steel, aluminum, brass, copper, and titanium have their own conversion behavior, and their presets carry a note that the figures are indicative only. Material presets are typical values; real hardness depends on alloy, temper, and heat treatment. Diamond, at roughly 10,000 HV, lies far beyond every Rockwell and Brinell scale, and hard anodized coatings, nominally 0.002 inch thick at 350 to 600 HV, are tested by microhardness on a cross-section because a Rockwell indenter passes straight through to the aluminum. A converted value suits comparison and screening; a specification that names a scale is met only by a test on that scale.

A reading of 47 HRC lies between the 45 HRC row (446 HV) and the 50 HRC row (513 HV), two fifths of the way up. Vickers is 446 + 0.4 x 67 = 472.8 HV, Brinell is 421 + 0.4 x 54 = 442.6 HB, Rockwell A is 73.1 + 0.4 x 2.8 = 74.2 HRA, and Knoop is 472.8 / 0.95 = 497.7 HK. There is no Rockwell B value, because the B scale ends near 23 HRC.

Hardness conversion bridges the gap between testing equipment, material certifications, and engineering specifications that reference different scales. Below are practical scenarios where cross-scale translation proves essential.

• Quality Assurance: A purchase order that specifies Brinell hardness can be screened with a Rockwell C reading from the shop floor tester. The Brinell equivalent shows whether incoming material sits near the order tolerance before a Brinell test settles acceptance.
• Heat Treatment Verification: Confirm that a batch of quenched and tempered parts meets the target hardness window. Converting the Rockwell C reading to Vickers allows comparison against process specification documents written in HV.
• Tool and Die Manufacturing: Select the correct tool steel grade by comparing published Rockwell C ratings across suppliers. A tempered tool steel at 45 HRC sits near 446 HV and 421 HB, while a fully hardened 60 HRC part reaches about 697 HV.
• Coatings and Hard Layers: Hard anodizing, nitrided cases, and plated layers are too thin for a Rockwell indenter, so they are specified in Vickers or Knoop microhardness. The anodized aluminum preset shows where a 450 HV coating would sit on the Rockwell C scale, about 45 HRC, while explaining why that figure cannot be read on the part.
• Aerospace Engineering: Titanium test coupons are often specified in Rockwell C. The Ti-6Al-4V preset shows a typical 36 HRC with its approximate Brinell and Vickers equivalents, flagged as indicative because steel relationships only approximate titanium, so acceptance testing stays on the specified scale.
• Education: Teach students the relationship between indentation-based hardness scales. The formula view shows each interpolation between chart rows, making visible how the same physical property maps to different numeric ranges on each scale and why the B and C scales overlap only near 20 HRC.
• Machining and CNC Programming: Estimate machinability by converting a workpiece Brinell hardness to Rockwell B or C. Harder materials may require reduced feed rates and different insert grades, so confirming the cross-scale value supports toolpath and cutting parameter decisions.
• Welding Inspection: Heat-affected zone readings from a portable Rockwell tester can be compared against code limits stated in Vickers. The common sour-service limit of 22 HRC for carbon steel converts to about 249 HV, close to the 248 HV10 figure usually quoted beside it.