A Rockwell hardness number only means something with its scale letter, because each scale uses a different indenter and a different test force. HRC, the scale used for hardened steel, presses a diamond cone into the surface with a total force of 150 kgf. HRB, used for softer steel and copper alloys, uses a 1/16 inch ball at 100 kgf. This chart lists the indenter and forces for all 15 regular scales and all 15 superficial scales.
Regular Rockwell scales
Every regular scale applies a preliminary force of 10 kgf to seat the indenter, then the total force. The hardness number comes from how much deeper the indenter sits after the extra force is removed.
| Scale |
Symbol |
Indenter |
Preliminary force (kgf) |
Total force (kgf) |
Total force (N) |
Typical materials |
| A | HRA | Diamond cone | 10 | 60 | 588.4 | Cemented carbides, thin steel and shallow case-hardened steel |
| B | HRBW | 1/16" ball (1.5875 mm) | 10 | 100 | 980.7 | Copper alloys, soft steel, aluminium alloys, malleable iron etc. |
| C | HRC | Diamond cone | 10 | 150 | 1471 | Steel, hard cast irons, pearlitic malleable iron, titanium, deep cased hardened steel |
| D | HRD | Diamond cone | 10 | 100 | 980.7 | Thin steel and medium case-hardened steel and pearlitic malleable iron |
| E | HREW | 1/8" ball (3.175 mm) | 10 | 100 | 980.7 | Cast iron, aluminium and magnesium alloys, bearing metal |
| F | HRFW | 1/16" ball (1.5875 mm) | 10 | 60 | 588.4 | Annealed copper alloys, thin soft sheet metals |
| G | HRGW | 1/16" ball (1.5875 mm) | 10 | 150 | 1471 | Phosphor bronze, beryllium copper, malleable irons |
| H | HRHW | 1/8" ball (3.175 mm) | 10 | 60 | 588.4 | Aluminium, zinc, lead |
| K | HRKW | 1/8" ball (3.175 mm) | 10 | 150 | 1471 | Bearing metals and other very soft or thin materials including plastics |
| L | HRL | 1/4" ball (6.35 mm) | 10 | 60 | - | Bearing metals and other very soft or thin materials including plastics |
| M | HRM | 1/4" ball (6.35 mm) | 10 | 100 | - | Bearing metals and other very soft or thin materials including plastics |
| P | HRP | 1/4" ball (6.35 mm) | 10 | 150 | - | Bearing metals and other very soft or thin materials including plastics |
| R | HRR | 1/2" ball (12.7 mm) | 10 | 60 | - | Bearing metals and other very soft or thin materials including plastics |
| S | HRS | 1/2" ball (12.7 mm) | 10 | 100 | - | Bearing metals and other very soft or thin materials including plastics |
| V | HRV | 1/2" ball (12.7 mm) | 10 | 150 | - | Bearing metals and other very soft or thin materials including plastics |
Superficial Rockwell scales
Superficial scales use a 3 kgf preliminary force and total forces of 15, 30 or 45 kgf. The lighter loads make a shallower indentation, which suits thin sheet, case-hardened surfaces and small parts that a regular scale would punch through. N scales use the diamond cone and T, W, X and Y scales use balls.
| Scale |
Indenter |
Preliminary force (kgf) |
Total force (kgf) |
Total force (N) |
| 15N | Diamond cone | 3 | 15 | 147.1 |
| 30N | Diamond cone | 3 | 30 | 294.2 |
| 45N | Diamond cone | 3 | 45 | 441.3 |
| 15T | 1/16" ball (1.5875 mm) | 3 | 15 | 147.1 |
| 30T | 1/16" ball (1.5875 mm) | 3 | 30 | 294.2 |
| 45T | 1/16" ball (1.5875 mm) | 3 | 45 | 441.3 |
| 15W | Ball | 3 | 15 | - |
| 30W | Ball | 3 | 30 | - |
| 45W | Ball | 3 | 45 | - |
| 15X | Ball | 3 | 15 | - |
| 30X | Ball | 3 | 30 | - |
| 45X | Ball | 3 | 45 | - |
| 15Y | Ball | 3 | 15 | - |
| 30Y | Ball | 3 | 30 | - |
| 45Y | Ball | 3 | 45 | - |
Which range each scale is valid over
A scale is only reliable over part of its length. Below the range the indenter goes too deep for the scale to resolve, and above it the indentation is too shallow. The ranges below are the applicable ranges given in ISO 6508-1.
| Scale |
Symbol |
Applicable range |
| A | HRA | 20 to 95 |
| B | HRBW | 10 to 100 |
| C | HRC | 20 to 70 |
| D | HRD | 40 to 77 |
| E | HREW | 70 to 100 |
| F | HRFW | 60 to 100 |
| G | HRGW | 30 to 94 |
| H | HRHW | 80 to 100 |
| K | HRKW | 40 to 100 |
This is why two scales are needed for steel. HRB is valid up to 100 and HRC from 20, so soft and annealed steels are tested on B and hardened steels on C. A reading outside the range should be retaken on the neighbouring scale, not quoted as it stands.
How to choose a Rockwell scale
Start from the material and its thickness. Hardened steel and anything harder than HRB 100 goes on C. Soft steel, brass, bronze and aluminium alloys go on B. Thin hard material and cemented carbide go on A, which uses the diamond with a light 60 kgf load. Very soft metals, bearing alloys and plastics use the larger ball scales from E onwards. When the part is too thin or the hardened case too shallow for the regular scale, move to the matching superficial scale.
To compare a Rockwell figure with Vickers or Brinell, use the hardness conversion chart. Conversions between scales are approximate and depend on the material.
What the letters after HR mean
ISO 6508-1 writes the ball scales with a W, for example HRBW, because the ball is now tungsten carbide. Older reports and most workshop charts still write HRB. The scale and the forces are the same.
References
- ISO 6508-1 - Metallic materials, Rockwell hardness test, test method
- ASTM E18 - Standard test methods for Rockwell hardness of metallic materials
Every MLC reference tool in one place: engineering calculators and charts.