Brinell scale

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The Brinell scale characterises the indentation hardness of materials through the scale of penetration of an indenter, loaded on a material test-piece. It is one of several definitions of hardness in materials science.

Brinelling refers to surface fatigue caused by repeated impact or overloading. It is a common cause of roller bearing failures, and loss of preload in bolted joints when a hardened washer is not used. Engineers will use the Brinell hardness of materials in their calculations to avoid this mode of failure. Fretting corrosion can cause a similar-looking kind of damage and is called false brinelling since the mechanism is different.

Proposed by Swedish engineer Johan August Brinell in 1900, it was the first widely used and standardised hardness test in engineering and metallurgy. The large size of indentation and possible damage to test-piece limits its usefulness.

The typical test uses a 10 mm diameter steel ball as an indenter with a 3,000 kgf (29 kN) force. For softer materials, a smaller force is used; for harder materials, a tungsten carbide ball is substituted for the steel ball. The indentation is measured and hardness calculated as:

$\mbox{BHN}=\frac{2P}{\pi D ({D-\sqrt{(D^2-d^2)})}}$

where:

P = applied force (kgf)

D = diameter of indenter (mm)

d = diameter of indentation (mm)

[edit] Common values

When quoting a Brinell Hardness Number (BHN or, more commonly, HB), the conditions of the test used to obtain the number must be specified. The standard format for specifying tests can be seen in the example "HBW 10/3000". "HBW" means that a tungsten (chemical symbol W, short for German Wolfram) carbide ball indenter was used, as opposed to "HBS", which means a hardened steel ball. The "10" is the ball diameter in millimetres. The "3000" is the force in kilograms force.

Brinell Hardness Numbers (standard test conditions unless otherwise stated)
Material	Hardness
Softwood (e.g., pine)	1.6 HBS 10/100
Hardwood	2.6 to 7.0 HBS 1.6 10/100
Aluminium	15 HB
Copper	35 HB
Mild steel	120 HB
18-8 (304) stainless steel	250 HB
Glass	550 HB
Hardened tool steel	650 - 700 HB

[edit] Standards

International (ISO) and European (CEN) Standard

EN ISO 6506-1 : Metallic materials - Brinell hardness test - Part 1 : test method.
EN ISO 6506-2 : Metallic materials - Brinell hardness test - Part 2 : verification and calibration of testing machine.
EN ISO 6506-3 : Metallic materials - Brinell hardness test - Part 3 : calibration of reference blocks