Edge retention by hardness, carbon content and wear-resistant alloys

[KnifeGrinders]

In the below chart we've put together SET-tested steels which composition we know.
SET Results are sorted from the best to the worst.

Even without fancy graphs, just looking at the numbers, it is clear that edge retention correlates primarily with the content of wear-resistant alloys, then with the carbon content, and finally with the HRC.



However, when we look at the resistance to initial rolling in the first 5 impact cycles, we see that, though wear-resistant steels do withstand rolling by about 30% better, there is no correlation between the wear resistance and resilience to initial rolling.



The high-vanadium edge sharpness quickly moves beyond the shaving range to just sharp. In the first impacts a 10% vanadium edge apex rolls to the same extent as a 3%, and both the 3% and 10% vanadium edges lose their initial keenness almost at the same rate as a mainstream knife. Considering that the impact assembly weighs just 150 grams, isn't that astonishing?
Higher wear-resistant blades win as stayers, but are equal sprinters.
As one of our readers has commented: "Which may explain why s110v loses it's keenness rather quickly but is able to keep a working edge for a long time"

To make things worse, you cannot steel "supersteels" back to shaving sharp as you can with softer steels – they are too hard for this, and the bent apex stays there as a tiny scraper.
For this reason, professional meat cutters prefer mainstream steel for their knives, as a meat plant veteran commented: "When working in the boning rooms as a boner, after sharpening my knife it needed to shave after steeling for necessary sharpness to work with. Steeling is necessary to get through the day."

It is getting really intriguing what Mike's SET testing of the A2 steel hardened to a range of HRCs will show.
The main lesson we've learnt so far is not to assume anything, yet I wonder if the A2 data will show similar pattern of equal initial keenness loss, but better long-run retention with the increase in HRC or not.
A2 is a high carbon and primarily Molybdenum steel, and if the pattern is different, it will tell us that what we've seen is vanadium-specific and shouldn't be generalized to other wear-resistant alloys.

[Larrin]

It is getting really intriguing what Mike's SET testing of the A2 steel hardened to a range of HRCs will show.
The main lesson we've learnt so far is not to assume anything, yet I wonder if the A2 data will show similar pattern of equal initial keenness loss, but better long-run retention with the increase in HRC or not.
A2 is a high carbon and primarily Molybdenum steel, and if the pattern is different, it will tell us that what we've seen is vanadium-specific and shouldn't be generalized to other wear-resistant alloys.

A2 has a much lower carbide volume than Vanadis 10, M390, or Elmax so that would be expected to be a more significant difference than simply the vanadium content. M390 and Elmax don’t form much in the way of vanadium carbides but rather vanadium enriched chromium carbides. D2 has a similar carbide volume to Vanadis 10 but with chromium carbide instead. Though it is conventional rather than powder metallurgy so CPM-D2 would be a better comparison for vanadium vs chromium. Vanadis 4 would be a good comparison for lower carbide volume but maintaining the vanadium carbides of Vanadis 10.

[Mark Reich]

Really a lot of outstanding work, KG!  

Do you use CBN wheels for everything, then finish on 0.5µ diamond on paper wheels? What grit CBN wheels do you have? Is it about the same difficulty for you to sharpen any type of steel to the same amazing sharpness?

It takes a Lot more effort for me to sharpen high vanadium steels. M360 isn't bad, and it gets very sharp. I even think it holds a a keen edge longer than S90V, but S90V is difficult to sharpen to the point that I don't use it. 

The 9%-10% range of vanadium is too difficult to keep sharp, but they hold mediocre for a long time. Thanks for including it. That went a long way in confirming my feelings. 

I doubt there's much difference between S90V and Vanadis 10 in sharpening or use, but Vanadis 10 is quite a bit harder than any of the other steels tested (and S90V). It seems like a lot of it's capabilities could come from that as much as the high alloy. 

There are a lot of different opinions on what steel works best, but multi-multi-millions of dollars have been spent tailoring knives and steel to meat cutters. The type of edges they like are pretty different than average, as is the way they maintain their edges. And they're limited to stainless (SR 101 is looking pretty good without chromium or anything).

[KnifeGrinders]

Molybdenum

It is getting really intriguing what Mike's SET testing of the A2 steel hardened to a range of HRCs will show.
The main lesson we've learnt so far is not to assume anything, yet I wonder if the A2 data will show similar pattern of equal initial keenness loss, but better long-run retention with the increase in HRC or not.
A2 is a high carbon and primarily Molybdenum steel, and if the pattern is different, it will tell us that what we've seen is vanadium-specific and shouldn't be generalized to other wear-resistant alloys.

A2 has a much lower carbide volume than Vanadis 10, M390, or Elmax so that would be expected to be a more significant difference than simply the vanadium content. M390 and Elmax don’t form much in the way of vanadium carbides but rather vanadium enriched chromium carbides. D2 has a similar carbide volume to Vanadis 10 but with chromium carbide instead. Though it is conventional rather than powder metallurgy so CPM-D2 would be a better comparison for vanadium vs chromium. Vanadis 4 would be a good comparison for lower carbide volume but maintaining the vanadium carbides of Vanadis 10.

Appreciate that info, Larrin.
I understand that the volume of carbides is as important as what carbides are of,  but since I could use only the knives I own and my collection of blades does not have every steel, of what I have I picked those that have close content of wear-resistant elements other than Vanadium, but couldn't equal them in chromium content. Nevertheless, chromium-rich M390 and Elmax do not fall out of the general pattern we've revealed.

When you sort them by chromium content you see no pattern at all, when you sort by carbon content you see a pattern similar to vanadium content but less regular, and when you sort by HRC you see only a hint of a tendency.
I had done all that thinking before concluded that "edge retention correlates primarily with the content of wear-resistant alloys, then with the carbon content, and finally with the HRC", but it does not correlate with the volume of chromium carbides.

Larrin, could you advise please what Molybdenum/Cobalt content in a vanadium-free or low-vanadium steel would give a carbides volume comparable to the high-vanadium steels we've tested? Would really appreciate if you could name the steels.
I need this final test as we are most curious if these Molibdenum/Cobalt steels display the same quick initial rolling as high-vanadium, or this is a vanadium-specific phenomenon.

Mark, yes all knives  I sharpen on CBN wheels, my usual sequence for knives is CBN #400 >> #1000, then paper wheel with 5 micron diamonds (or felt wheel with 6 micron) - all the above at the exact edge angle resulting in a razor edge with a foil burr, which I deburr on a paper wheel with 0.5 micron diamonds at +0.4 degree higher angle. As I see it the diamonds on this final wheel cut off the foil burr at the very apex.
We revealed this +0.4 degree in our study detailed in the USA Sharpeners' Report:
http://knifegrinders.com.au/Manuals/USA_...rticle.pdf
We call it the Best Honing Angle (BHA), but actually it is gentle deburring.
Tormek confirmed later that the best honing angle on their leather wheel for chisels is by 0.46 degree higher than the edge angle; Tormek hones with 3-1 micron alumina honing paste.
If you think over our diamonds and Tormek honing paste, so different and common only by the best angle, it becomes clear that deburring at this angle goes along the very apex, giving a cleaned keen edge.