Steels such as D2 and ATS-34 are commonly described as hard to sharpen as it takes a long time to remove metal from the edge. At an extreme, steels like S90V and ZDP-189 are so difficult to grind that some will label such steels as impossible to sharpen without specialized equipment1. To the right is an image of ATS-34 at high magnification which shows the main reason for such complaints2. The blocky white regions are chroimum carbides which are much harder than the steel itself. The large amount of carbide in such steels lowers the grindability which means it takes a long time to remove material with an abrasive. This is the cause of complaints about sharpening such steels as noted frequently3. However, just how much steel needs to be removed when a knife is sharpened assuming that the edge was just dulled and not grossly damaged?
To the right is an image of a knife showing the effects of seven weeks of use in the kitchen4. Even after almost two months it requires high magnification to see significant effects on the edge. Brent Beach has completed much work on plane irons measuring edge wear and noted that even after planing hundrends of feet of Douglas Fir the worn edge can be less than 0.0005" wide or about one half of a thousand of an inch5. Personal comparisons of knives has shown that even after cutting large amounts of abrasive materials until the sharpness is as low as 10% of optimal results in edge wear which is again typically just 1-2 thousands of an inch wide6. Even when larger knives are used extensively for heavy chopping in thick woods, the extent of blunting will again typically be of the order of a thousand of an inch.
When sharpening a knife Leonard Lee proposed using what he termed as micro-bevels to speed up the sharpening process 7 :
Leonard Lee, The Complete Guide to Sharpening
The sole purpose of a micro-bevel is to get the finest edge in the shortest period of time
The idea proposed by Lee is quite simple, if the required edge angle for durability is for example 25 degrees, then shape the main bevel at 24 degrees with a coarse abrasive and only hone the very edge itlsef at 25 degrees with the finishing grits. Since only a small strip of metal is polished during the final sharpening it radically reduces honing time. Brent Beach recommends a slightly more complicated procedure in which several angles are used. Each time the grit is changed so is the angle as shown in the image on the right8. When resharpening, Beach recommends starting with the abrasive which formed the coarse microbevel to minimize sharpening time. Beach's method of just honing the micro-bevels is similar to using ceramic or diamond rods for "touch-ups".
Of course, repeated honing of just the microbevel will in time reduce cutting efficiency and each subsequent sharpening will take longer as the micro-bevel will grow in size with each "touch-up". However it takes a significant amount of sharpening to have a noticable effect as the micro-bevel grows slowly with each sharpening. When such a detrimental effect has been noted then it simply a matter of recutting the primary edge bevel again with a suitable coarse stone. As an example, on a Heafner fixed blade in D2 with a three inch long blade, with an edge bevel of almost 1/4" wide, it took less than a minute with with an x-coarse DMT to reset the primary. The edge was then refined on a 600 DMT benchston and the final micro-bevel applied using the Sharpmaker medium rods. The entire process took less than five minutes6.
Complaints about steels being hard to sharpen because they are either very hard or high in carbides are unfounded with proper abrasive selection and micro-beveling. One of the real causes of hard to sharpen steels is when the heat treatment is less than optimal such as on the low carbide steels often used in inexpensife knives. For example 440A stainless steels can reach an optimal hardness of 59 HRC after tempering but that requires an oil quench and deep cooling to obtain maximum martensite formation9. However in production knives this steel is frequently ran at 55 HRC or less and with the hardness so low burr formation will be an issue with sharpening and will make obtaining a high quality edge time consuming. It is in fact because of the burr issue due to improper hardening that that the edge retention of such steels is only a fraction of optimal10. Note that makers such as Roman Landes, who made the Damascus knife on the right actually favor such low carbide steels as superior for high performance knives.
Another cause of hard to sharpen knives is when steel is not well matched to the type of knife, one of the most frequent examples is high carbide steels used for knives which see frequent impact such as wood chopping. Such steels are optomized for wear resistance at the expense of toughness and edge strength. There is little use for high wear resistance with such knives but great deamdn for toughness and edge strength as the edges will blunt by deformation or fracture during the frequent encounters with debris. The high carbide volume will then ensure maximum damage in such encounters and maximum grinding time to remove the damage. S30V for example is one of the worst steels for large knives for exactly this reason. Some makers will reduce the hardness of the steel to try and increase ease of sharpening but this has the exact opposite effect. The reduction in hardness has little effect on grindability as that depends more on carbide volume and the softer and thus weaker edge deforms more in use and is prone to heavy burr formation when sharpening both making it harder to sharpen. In addition the lower hardness will significantly reduce edge retention11.
Improper geometry will also cause issues with ease of sharpening, one of the most extreme missmatched steels is found in Kershaw's used of 13C26. This steel has maximum edge stability and is thus designed for edges less than 0.010" thick sharpened at 15-20 degrees with optimal hardness is 63/65 HRC 12. Yet in folding knives such as the Cyclone, the included edge angle is as much as 40 degrees and the edge as much as 0.030" thick13. In comparison, a primary edge of 30 degrees included and 0.015" thick is enough for the heavy wood work on a large bowie such as shown in the small movie clip to the right14. The hardness on the Kershaw 13C26 is well under optimal which causes problems with excess burr formation15 and is at such an extreme underhardening that the hardness can reached by 12C27M. At that hardness 12C27M would be tougher and more corrosion resistant than underhardened 13C26 as well as be easier to sharpen and even cheaper.
Complaints about steels being hard to sharpen because they are either very hard or high in carbides are unfounded if proper sharpening techniques are used, specifically micro-beveling. The real cause of hard to sharpen steels are when steels and geometry are miss-matched in the knife causing an edge to suffer premature major damage or the heat treatment is far less than optimal which induces heavy burr formation.
1 : Jay Fisher, www.jayfisher.com, 2007
2 : Dr. R. Landes, Messerklingen und Stahl, 2. Auflage, Wieland Verlag, Bruckmühl, Germany. Copyright 2006
3 Left Hand Path Sharpening ZDP-189 Caly 3, www.spyderco.com, 2007.
4 : www.zknives.com
5 : Brent Beach, Testing Plane Irons, www3.telus.net, 2002-2007
6 : Cliff Stamp, Lil Bear : Heafner Knives, 2007
7 : Leonard Lee, The Complete Guide to Sharpening, Tauton Press, Inc.1995
8 : Brent Beach, Bevels New and Used, www3.telus.net, 2002-2007
9 : Cliff Stamp, Blade Materials, www.cutleryscience.com, 2007
10 : Cliff Stamp, M16 (Zytel) from CRK&T, www.cutleryscience.com, 2006
11 : Cliff Stamp, Green Beret, www.cutleryscience.com, 2006
12 : Dr. J. Verhoeven, Steel Metallurgy for Non-Metallurgists, ASM, 2007
13 : Mike Cheshareck, Private Communication, 2007
14 : Cliff Stamp, Ratweiler, www.cutleryscience.com, 2007
15 : Kevin Xie, Private Communication, 2007
|Written: Sept. 2007||Updated: Aug. 2007||Copyright (c) 2007 : Cliff Stamp|