What is the Brinell Hardness Number (HBW or BHN)?
The Brinell Hardness Number (HBW or BHN) was the first widely accepted and standardized indentation hardness test for metals and was proposed by J.A.Brinell in 1900. The test consists of indenting the surface of the metal with a 10mm diameter tungsten carbide ball using a load of up to 3000kg for a fixed period of time (~15seconds). After the ball is retracted the diameter of the impression is measured twice at 90 degrees apart with a low powered microscope having a calibrated graticule. The readings are averaged and the hardness number calculated by dividing the applied load by the surface area of the impression. This calculation is however somewhat tedious and it is more usual and far simpler to refer to a set of standard tables, from which the Brinell hardness number can be read directly. The smaller the impression the harder is the steel. In terms of the alloys we use every day, here are some common hardness ranges:
217 - 235HBW = Impression Diameter of 4.10 – 3.95mm
285 - 341HBW = Impression Diameter of 3.60 – 3.30mm
While this test method is widely used for bulk metal hardness measurements, it does have its limitations – the large indentation diameter being the single biggest issue. However, this too can have its benefits, in that because of its size, it is less affected by surface roughness and tends to average out any local heterogeneity within the steel.
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