Edge retention slicing carpet with 3Cr13, O1, K390, S30V and 121REX


Introduction

Carpet, especially when used is a very demanding material to cut because the supporting layer can be very hard and the material itself, especially when dirty contains enough abrasive to produce a very high wearing effect on knife edges. The nice thing about cutting it then is that it allows an examination of edge retention to a very low state of sharpness in a reduced time frame. Care has to be taken however as the type of blunting seen in cutting this type of material is very different than carving wood for example so it would not be sensible to conclude directly that steels which excel at this would excel at holding a sharp edge in cutting wood.

There is also a very large challenge because the carpet, especially when used is very variable and thus the results could easily be scatted due to the influence of the carpet and not well represent the steels used. The two main ways to combat this are :

Method

The knives were used to slice the carpet using a two inch section of blade and enough force to make a two inch cut into the carpet. The sharpness was measured by cutting light cord under a specific tension, two types were used depending on the extent of blunting :

The edge retention statistic used was the TCE or total cutting efficiency which is calculated very simply as the sum of :

The more intervals used, the more precise the TCE.

Two comparisons where made, the first was with the following knives :

The knives had a 6-8 dps edge bevel and a microbevel at 15 dps with a 600 DMT plate. The stopping point was set at 1.5% of the initial sharpness.

The second comparison was an extended run on the S30V and 121REX blade with the following modifications :

This was intended to see if there was any relative difference in performance if the edge geometry was changed to make it more stable and if the higher wear resistance of the 121REX would show an advantage when the knives were used to a much duller state. The image on the right shows the initial edge finish and microbevel.

Results

The results from the first comparison are as shown on the right which includes the ranking from both the edge retention and an edge retention efficiency ranking which is calculated as :

Where PPS is the number of passes per side to reapex the edge after a round of cutting. This ranking was suggested by Kyley Harris who argued that it might be of practical interest to look at edge retention from the point of view of how much work (time) was required to maintain the edges.

The performance of the steels follows previous patterns where the edge retention increases with increasing carbide content of the steel up to a point and then it starts to decrease. In this comparison it peaks around S30V.

The results from the second comparison are also shown on the right. The results are consistent with the previous comparison :

Discussion

Previous work on hemp and cardboard had shown that edge retention with the edge set at 15 dps / 600 DMT tended to peak at a high but not extreme carbide volume and then it fell off after that. The explanation for this may be due to the argument made by Roman Landes that carbide volume decreases apex stability and leads to edges chipping around and through the carbides 1. The image at the right shows this effect at work. The point is basically that even though such very high carbide steels have a high abrasive wear resistance, if the loads are high they can fracture which can lead to more rapid material loss at the apex.

The second comparison was an attempt to show / investigate if a higher apex angle and finer finish would allow the 121REX blade to have increased edge retention however it didn't materialize. However the curious thing is that the apex did resist damage stronger visually as it would cut the carpet for longer without showing evidence of light reflecting from the edge, but the sharpness was not maintained at a higher level. It might be that this point, the thickness required for the apex to produce visible reflection (which is about 20 microns) is enough for the apex to stabilize the carbides.

Recap

The following steels were compared for edge retention on carpet :

In regards to edge retention, the maximum performance was found to be S30V. However in regards to edge retention efficiency, the maximum performance was found to be with the 3Cr13 stainless steel.

More details and discussion can be found on the forum threads on :

and the YT video.

References

1 : Dr. R. Landes, Messerklingen und Stahl, 2. Auflage, Wieland Verlag, Bruckmühl, Germany. Copyright 2006


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Written: 17/01/2015 Updated: Copyright (c) 2015 : Cliff Stamp