Question

I have Rockwell and Brinell hardness test data. How determine what kind of material I have from the hardness test?

Answer 1

Hardness testing of materials helps us to evaluate material’s properties, such as strength, ductility and wear resistance, and so helps you determine whether a material or material treatment is suitable for the purpose you require. The principal purpose of the hardness test is to determine the suitability of a material for a given application or the particular treatment to which the material has been subjected.

The Hardness testing is normally done by changing the applied force on the indentor, and by changing the sizes of the indentor (changing dia of the ball indentor or by changing the diagonal lengths of the diamond indentor). These are the two parameters which can be varied and the data what we get from the Brinell hardness is the Brinell hardness number (HB / BHN) and Rockwell Hardness number (HRC / HRB / HRA) with changing sizes of indentors, applied loads and the depth of indentation. The material properties can be found by using the above data:

• When the load applied on the indentor is less and the depth of indentation is more then the material is soft in both the cases.
• These hardness numbers are used for finding Ultimate Tensile strength (UTS) of the material as
  • UTS = k*BHN, k= constant = 3.38 to 3.55 for steels
  • UTS = k*BHN, k= constant = 3.48 to 3.21 for brass
  • UTS = k*BHN, k= constant = 2.86 to 3.63 for nodular Iron
• The tensile strength (TS) of the material can be found by
  • TS = 3.55*BHN if BHN175 and TS = 3.38*BHN if BHN175
• The Yield strength (YS) of the material can be found by using
  • YS = (BHN*9.816) / 3

There will be a common relation between Brinell hardness number and Rockwell Hardness number. But mostly all the material properties will be based on the Brinell Hardness Number. By knowing the Ultimate Tensile strength, Tensile Strength and the Yield Strength of the materials we can use them at the required purposes. Thus by the Brinell hardness and Rockwell hardness numbers we can know properties like its hardness, Ultimate tensile stress, yield strength, tensile strength.

NOTE: The above relations for finding the material properties is an experimental derivation but not mathematical. So the material property values will be accurate to the known property values.