EDGE RETENTION/ROLLING PART II

[KnifeGrinders]

Effect of Hardness on Edge Retention

Many thanks to Mike B. for sending us samples of A2 tool steel.
A2 is a high carbon, high molybdenum tool steel.
The A2 blade #7 has been hardened to HRC 54, while the A2 blade #11 to HRC 62 - they represent extremes of the common knife hardness range.


We sharpened these two blades exactly the same way at 15 degrees per side (dps) on Tormek using CBN wheels, the edge set on CBN #1000 edge-leading. Deburring was done on a paper wheel with 5 micron diamonds with the help of our support for controlled-angle honing and computer software.

The initial sharpness was very close, 109 and 114 BESS.
The 2 blades have been subjected to controlled edge rolling on the BESS SET tester.

Rolling cycle explained
The impact roller is a linear bearing slant at 10° to the horizontal base or in other words at 80° to the plane of the blade clamped vertically.
Standard impact assembly weight is 150 grams.
The impact roller is lowered at "A", then moved (rolled) over to "B" and then back to "A".
A-B-A is one cycle.


The results have shown a tremendous difference in edge holding.

It took 45 rolling cycles to blunt the HRC-54 blade to 500+ BESS.

The HRC-62 blade holds a working sharp edge at least 10 times longer.
I say "at least" because even after 500 rolling cycles the HRC-62 kept scoring a working edge of under 500 BESS. By then I had been rolling the HRC62 for 1 hour, and just gave up on blunting it. For comparison, mainstream knives blunt to 500+ BESS within the first 100 rolling cycles.

Link to raw data >>

Chart


However, despite that extraordinary edge holding, the HRC-62 blade won't hold the super-sharp edge any better.
The initial blunting rate is relatively rapid regardless of hardness.
Initially both the HRC54 and HRC62 blades were scary sharp near 100 BESS, but it took only 1 rolling cycle to blunt them to just sharp, and 5 rolling cycles to working edge.
The divergence of sharpness graphs in the chart starts at about 350-400 BESS - the HRC-62 sharpness stays at this level through continued rolling impacts, while the HRC-54 sharpness deteriorates almost linearly till blunted to practically unusable.
400 BESS is what real world working edges score, and the A2 @ HRC62 holds this edge extremely well - we've seen that good edge retention only in Vanadis-10 and ceramic knives.


Key Indicators