EDGE STABILITY IN BUTCHER’S AND KITCHEN KNIVES

[KnifeGrinders]

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

Prof. Foell may have drawn his oversimplified catchy "What is commonly known as 'hardness' is nothing more than the yield stress, just measured in different units."  from the same Verhoeven's plot.
The almost linear correlation in that plot looks suspicious to us who've been following Jan's study on non-linearity of the HRC scale.
Verhoeven comments in his work that "The correlation becomes less reliable at HRC values above 55." (page 41) - in other words that plot is for soft and medium steels, not used in knives.
Again, the outcome depends on the way the steel is Q & T -ed - for low-alloy steels you may get a springy or stiff piece.

Hardness test indirectly tells of the strength, but hardness and strength are not the same properties of the steel, though relation between these two is closer than that of each of them with toughness. Approximation is too far to say "just measured in different units"

In view of all that, moreover impatiently we wait for Mike's experiments on the extent of edge rolling by steel hardness.