On the correlation of hardness with tensile and yield strength

The relations between hardness and tensile properties of common metals have been reviewed and compared with available experimental data. Using the Cahoon formula the correlation of hardness with tensile strength is generally good (difference usually less than ± 10 %) provided that the strain hardening exponent n is known or can be estimated. The corresponding correlation with yield strength is somewhat less impressive but still reasonable (difference usually less than ± 15 %) if the value of n remains approximately constant and characterises the original material. Less satisfactory correlations may be obtained for multiphase materials or materials showing microstructural changes during plastic straining. Reliable determination of n from hardness-type indentations has not proved to be easy, and seems especially difficult to obtain from the same standard type hardness indentation which is used for hardness measurement. In spite of these limitations the available expressions correlating hardness with yield and tensile strength can be considered very useful.