Influence of burnishing on edge retention


Introduction

Using burnishing as a method to maintain the sharpness of a knife is a commonly promoted technique to both extend the time between sharpening on a stone and to prolong the life of a knife by minimizing metal loss.

When used properly, a steel will realign the edge of the blade such that the sharpened bit is all facing the right direction. You should steel your knife every time you use it to ensure that you're getting the best edge possible.

Of course no article on burnishing would be complete without mentioning one of the first people to advocate it in the West and be outspoken about experimental results :

The steeled edge has been smoothed out (top) into an even sharper cutting edge, as though someone had wiped the frosting on a cake with the side of a knife. Notice that there are no primary furrows (caused by the hone) left in the steeled area. Bottom photo shows a more heavily steeled edge. The metal has actually flowed back as though it were molten. This is an edge no hone will ever equal. But be careful not to over steel. In some microphotographs we've taken, you can see a thin hair of metal peeling away from such an over steeled edge. That ruins the edge and the blade has to be honed again.

However burnishing as it doesn't abrade but only deforms leaves worn and fatigued metal on the apex of the knife and doesn't replace any torn out carbides and so there are obvious concerns about durability and edge retention. As well burnishing, as it produces a very high polish can issues when it is used to maintain sharpness on low grit finishes as it replaces the coarse finish with a very high polish.

Results

Case #1 : using burnishing to maintain sharpness on two common knife steels using a pair of production knives on a very abrasive material (dirty polypropylene rope) :

the following was observed1 :

In more detail in regards to how the apexes behaved during use there was significant scatter. The knives would show a combination of wear, chipping, and deformation in rolling and dents.

Note the images to the right which show

That is the result of one trial and the results were not consistent with repeated trials. In general given that the 15n20 is significantly softer in the knife in question (52 to 54 HRC vs 61/63 HRC) and has much lower carbide steel it would be expected that it would tend to deform and the 10V would be expected to chip however given the large random influence it would take quite a few trials to reproduce those expected results.

Case #2 : using burnishing to maintain sharpness on another two common knife steels using a production knives and small shop knife on moderate material (cardboard)

the results were a little different2 .

In this case the as-sharpened edges had a much finer grit (6 micron/DMT) and the loss in edge retention in burnishing is much less. Using the TCE statistic, the results with the as-sharpened edge :

and after burnishing :

There was again no significant advantage to either steel, however the loss in edge retention was much less than when the 15 micron DMT plate was used to set the apex. This supports the fact that one of the main reasons why burnishing produces less edge retention in slicing is that it simply is applying a much finer grit and finer grits have lower edge retention on a slice 3 .

Again, the apex showed a mixture of deformation, wear and fracture. As there were only two trials it wasn't possible to see if there was a difference between AUS-6 and 10V in that regard.

Case #3 : using burnishing to maintain sharpness on two common knife steels using two production knives on non-abrasive material (hemp rope)

The results are consistent with the above and expand on the utility of burnishing dependent on the hardness of the media being cut.

With three repeated runs with the basic kitchen knife :

Then note :

Interestingly enough the VG-10 blade did one run and had very similar response to the simpler stainless blade however an attempt to repeat the trial saw the edge just collapse behind the apex. This is not that unexpected given :

and this produces fatigue type failures. Now it is interesting that VG-10 showed this type of problem here but it wasn't seen in 10V which has a much higher carbide volume. This could be due to the slightly lower edge angle here or it could be due to the fact that modern PM steels had a much cleaner structure because of use of VIM/Var and other processes to produce very clean steels.

An interesting possibility would be to compare 154CM in the ingot and powder version to see if there is any difference due to the way the steel is manufactured.

Summary discussion

A brief recap :

References

1 : Influence of burnishing edge on retention slicing polypropleyne: 10V and 15N20

2 : Edge retention : slicing cardboard (AUS-6, 10V-64 HRC)

3 : Influence of apex grit (finish) on edge retention slicing half inch hemp


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Written: 30/07/2015 Updated: Copyright (c) 2015 : Cliff Stamp