Yes, Mr. Mark you are asking a good question. 
As mentioned by prof. Herring in the article quoted above, grain size has a measurable effect on most mechanical properties of steels. Hardness, yield strength, tensile strength, fatigue strength and impact strength all increase with decreasing grain size.
Quantitatively this improvement of mechanical properties is described as:
Example: In table 8.2 on page 69 prof. Verhoeven describes grains refinement from very fine grains with average diameter of 8 microns to ultrafine grains with average diameter of some 2 microns.
In this case we can expect following percentage performance improvement (ppi):
![[Image: ppi.jpg?dl=1]](https://www.dropbox.com/s/ohibk17q7xtmebr/ppi.jpg?dl=1)
You surely understand that there is considerable uncertainty regarding the detailed mechanism of the grain size effect, and also appropriate definition of the average grain size.
Jan
P.S.: Mr. Mark’s question is a perfect opportunity to use the Rockwell Hardness Difference Calculator (http://bessex.com/forum/showthread.php?tid=287).
Imagine, that via 3 cycle heat treatment we reduced the grain size and the blade hardness increased from 55 HRC to 60 HRC. The Rockwell calculator tells us that the change in hardness was 23.6% (ppi = 100 + 23.6 = 123.6%). The question is, what happened with average size of grains?
Using the ppi formula above we can calculate that: initial grain size / final grain size ≈ 1.5
As mentioned by prof. Herring in the article quoted above, grain size has a measurable effect on most mechanical properties of steels. Hardness, yield strength, tensile strength, fatigue strength and impact strength all increase with decreasing grain size.
Quantitatively this improvement of mechanical properties is described as:
Example: In table 8.2 on page 69 prof. Verhoeven describes grains refinement from very fine grains with average diameter of 8 microns to ultrafine grains with average diameter of some 2 microns.
In this case we can expect following percentage performance improvement (ppi):
You surely understand that there is considerable uncertainty regarding the detailed mechanism of the grain size effect, and also appropriate definition of the average grain size.
Jan
P.S.: Mr. Mark’s question is a perfect opportunity to use the Rockwell Hardness Difference Calculator (http://bessex.com/forum/showthread.php?tid=287).
Imagine, that via 3 cycle heat treatment we reduced the grain size and the blade hardness increased from 55 HRC to 60 HRC. The Rockwell calculator tells us that the change in hardness was 23.6% (ppi = 100 + 23.6 = 123.6%). The question is, what happened with average size of grains?
Using the ppi formula above we can calculate that: initial grain size / final grain size ≈ 1.5