05-11-2018, 12:41 PM
Mr. Mark, your interest pleases me! 
The graph you are mentioning was copied from the famous book by prof. Verhoeven and so I hope it is a representative example of the correlation between UTS and Rockwell hardness.
I cannot tell you exactly why prof. Foell considers "yield strength" to be just another term for hardness. May be the reason is the absence of good definition what hardness is from the first physical principles. Hardness is defined by an indentation procedure only. This is in contrast with clear physical definitions of strength or toughness as example.
The original contribution of prof. Foell you can find here: https://www.tf.uni-kiel.de/matwis/amat/iss/kap_3/backbone/r3_1_4.html#Ultimate%20tensile%20strength
Jan
The graph you are mentioning was copied from the famous book by prof. Verhoeven and so I hope it is a representative example of the correlation between UTS and Rockwell hardness.
I cannot tell you exactly why prof. Foell considers "yield strength" to be just another term for hardness. May be the reason is the absence of good definition what hardness is from the first physical principles. Hardness is defined by an indentation procedure only. This is in contrast with clear physical definitions of strength or toughness as example.
The original contribution of prof. Foell you can find here: https://www.tf.uni-kiel.de/matwis/amat/iss/kap_3/backbone/r3_1_4.html#Ultimate%20tensile%20strength
Jan