What Does Steeling Do? Part 1

Even the name is confusing.  Knife steel.  Butcher’s steel.  Honing rod.  Sharpening steel.  Do they remove metal or just “re-align” the edge? What does “re-align” mean?   Are “honing” and “sharpening” different things?  Scanning through opinions found on the internet will leave you more confused than enlightened.  This series examines what actually happens at microscopic scale.

It is a common misconception that steeling does not remove metal, but simply “re-aligns the edge.”  I have shown that re-alignment is one of the four results of stropping in What Does Stropping Do?  However, in my experience this type of re-alignment rarely occurs.  In the vast majority of cases, the steel near the apex is too damaged to be straightened, and instead simply breaks away rather than realign.  Secondly, with typical use in slicing applications, blades usually become “dull” through abrasion (including micro-chipping) or blunting and thickening of the apex, rather than by rolling (or “major deflection”) of the edge.  Rolling-like deflection can occur in a limited number of cases; for example by cutting into the non-abrasive lip of a glass.  More commonly, cutting non-abrasive materials produces a blunted, mushroom-shaped apex, as shown in the image below.

Delica_cardboard_02

Cross-section SEM image of a typical dull knife blade. The apex is blunted and “mushroomed” through both abrasion and movement of softened steel.

One of the first articles I wrote on this blog was to define the terms “sharp” and “keen” as they relate to sharpening/honing/stropping blades.  In this series of articles, I will demonstrate the effect of steeling with the various types of honing rods, and show that steeling improves keenness by removing metal.   In simple terms, steeling primarily produces a micro-bevel.  To be consistent with these definitions, steeling does not sharpen the blade – if we accept that sharpening requires thinning the blade by grinding the bevel.  For example, we may choose to sharpen a knife at 30 degrees (15 degrees per side) creating a millimeter wide bevel and then maintain the cutting ability of the knife by steeling at 20 degrees per side (40 degrees inclusive) to form a micro-bevel.

For this study I primarily use Olfa carbon steel cutting blades and dull them with a few cutting passes into the edge of sharpening stone.  This produces a blunt apex with minimal damage to the underlying steel.

When “steeling” the edge, I use the traditional edge-leading, heel to tip slicing motion with light force applied at an angle slightly higher than that of the existing bevel.  A range of honing rods are compared; a traditional ribbed steel (Wustof), a smooth ceramic rod, a smooth/polished “butcher’s” steel (Victorinox), a tungsten carbide sharpener (Chestnut tools Universal Sharpener), the Spyderco Sharpmaker (204MF) and a diamond-embedded rod.   In part 1 of this study, I will demonstrate that steeling improves keenness through metal removal rather than “re-alignment of the edge.”

Ribbed Steel

The set of images below demonstrate that the traditional ribbed knife steel does not “re-align” a “rolled” or deflected apex.  The rod simply produces a micro-bevel.  In the first image, the profile of the “factory” edge of the blade is shown and in the second image, the same blade after “rolling the edge.”

Olfa_new_05

Cross-section view of the factory edge on the Olfa blade.  The blade has been sharpened/ground at angle just below 30 degrees and buffed or stropped to produce a micro-convex with excellent keenness.

rolled_glass_03

Cross-section view of a “rolled” edge, produced by attempting to cut into the lip of a glass beaker at an off-normal angle to push the blade to one side.

The blade was then steeled in the traditional fashion, edge leading at an angle of approximately 30 degrees.  The blade was imaged, confirming that keenness had been restored to the apex and revealing that this was achieved by removal of steel; creating a new micro-bevel.  There is some evidence of softened metal being redistributed; however, a micro-bevel is unquestionably formed through metal removal.

rolled_glass_steel_EL_06

Edge-view of the rolled edge after steeling on a conventional ribbed steel rod (edge leading) demonstrates keenness has been restored.

rolled_glass_steel_EL_02

Cross-section view of the steeled blade with the white overlaid lines showing the factory geometry.  Comparison with the cross-section image of the factory edge proves that keenness has been restored by metal removal with the formation of a micro-bevel, not by re-alignment of the rolled edge.  Some subtle redistribution/burnishing has also occurred, particularly visible on the lower left of the bevel where metal is observed outside the white triangle.

Ceramic Rod

Unlike the traditional metal butcher’s steel, it is undeniable that a ceramic honing rod removes metal, since the black steel swarf is visible on the white surface after use.  In the following example, the blade was dulled by cutting into an abrasive stone, leaving a cleanly blunted apex.  Honing with the ceramic rod is also observed to produce a micro-bevel with a keen apex.

Olfa_dulled_02

Cross-section view SEM image of an Olfa blade dulled by cutting into an abrasive stone.

Olfa_dulled_ceramic_hone_x5_02

Cross-section view SEM image of the dulled Olfa blade following honing on a the ceramic rod. A keen micro-bevel has been formed.

In the SWARF! article, it was shown that traditional hones remove metal by cutting furrows into the metal of the blade producing curled metal chips – microscopic versions of the chips found in any metal fabrication shop.  This type of metal removal is generally termed abrasive wear and occurs when a hard, sharp abrasive cuts into a softer metal surface.

SG2k_swarf_04

Micrograph of metal swarf produced by a conventional (Shapton Glass 2k) hone. The sharp abrasive ejects curls of metal waste.

Traditional honing rods do not have sharp protrusions, and the metal swarf found on the surface is observed as flattened or smeared patches of metal.  This type of metal removal (or transfer) is generally termed adhesive wear.  Adhesive wear occurs at points of very high pressure that occur when the contact area is very small.

Ceramic “steels” have become common and are often preferred to traditional ribbed knife steel.  Although these ceramics are composed of micro-sized sintered grains, the surfaces are relatively smooth and do not display a “grit-like” texture.  These rods appear to remove metal predominantly via adhesive wear rather than abrasive wear (grinding).

ceramic_hone_scalpel_swarf_02

Conventional Electron image of the ceramic honing rod shows the surface roughness of the ceramic and steel debris transferred from the knife blade.

ceramic_hone_scalpel_swarf_01

Back-Scattered electron image of the ceramic honing rod.  In this image, the ceramic is dark and the metal removed from the blade is bright.  Comparison with the previous image allows identification of the metal particles on the hone surface.

ceramic_hone_scalpel_swarf_03

Higher magnification image of the metal swarf on the surface of the ceramic rod.  The morphology of the metal “chips” is typical of adhesive wear rather than abrasive wear.

It is commonly suggested that honing rods are “only effective for simple and soft steel” blades. In the following example, steeling a high hardness vanadium steel blade (Buck, S30V) is shown to produce a microbevel in same manner as observed in the carbon steel Olfa blade.  In this case, the knife was freehand sharpened at about 30 degrees (inclusive) and used as an every day carry for routine tasks until it was in need of sharpening.  The first two images, below, show that the blade deteriorated by both blunting and chipping and also some slight deflection occurred in one area.  It is noteworthy that the deflection occurs at a much larger length scale and independent of the blunting.

S30V_dull_07

Cross-section view of the dull S30V steel knife displays mushrooming of the apex.

S30V_dull_02

Edge view image of the dull S30V knife apex displays areas that are chipped (top), slightly deflected (middle) and blunted (bottom).

The knife was “steeled” with edge leading strokes in a slicing motion at about 25 degrees  on the ceramic rod.  The image below shows that, once again, steel has been remove to form a microbevel with a relatively keen apex.  Note that this image is taken with the same magnification (scale) as the mushroomed apex image.

S30V_ceramic_steeled_03

Cross-section view of the S30V steel knife apex after steeling with the ceramic rod. A micro-bevel with an included angle of about 45 degrees was created with a relatively keen apex.

Polished (Smooth) Steel Rod

Abrasive wear typically requires ‘sharp’ grit-like features to scratch or cut into the steel, while adhesive wear does not.  Instead, adhesive wear occurs when the local pressure at a relatively smooth “bump” is very high.  Pressure is defined by the applied force divided by the contact area.  As a result, when the contact area is microscopically small, the local pressure will be extremely large.  When steeling a dull knife, at an angle higher than that of the existing bevel, the contact area is microscopically small since contact occurs only along the width of the newly formed micro-bevel.  While this bevel is less than a few microns wide, the local pressure (even with light applied force) will be sufficiently high to enable adhesive wear to occur.

In the example below, the Olfa blade was dulled by cutting into an abrasive stone, and then steeled with a smooth, polished steel rod (Victorinox Honing Steel – round smooth polish).  Once again, a obvious micro-bevel has been formed through metal removal.

olfa_dull_04

Cross-section view SEM image of an Olfa blade dulled by cutting into an abrasive stone.

olfa_dull_smooth_steel_08

Cross-section view SEM image of the dulled Olfa blade following honing on a the polished steel rod. A keen micro-bevel has been formed.

olfa_dull_smooth_steel_07

Cross-section view SEM image of the dulled Olfa blade following honing on a the polished steel rod. A keen micro-bevel has been formed.

….to continued in part 2….

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15 responses to “What Does Steeling Do? Part 1

  1. Another interesting entry. One suggestion – use a bit more quantitative description of the pressures you’re using. I never would have expected that a steel actually removed
    much material.

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    • The pressure is enormous, but only because the contact area is small. The applied force, however, is just enough to make contact. I work with these blades loose, holding the end between my thumb and forefinger, so there is really no way to apply much lateral force.

      Liked by 1 person

  2. Very interesting. I was definitely one of the ones who thought the steels just realigned. As I look at these pics, I don’t know if I can really tell a difference between the ribbed steel, ceramic rod and the polished steel. Is there one that makes for a better edge as far as use goes?

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    • That’s an important question, and I plan to discuss that in detail in part 3. The key point is that the nano-scale keenness and structure of the apex primarily determines the cutting efficacy.

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  3. Awesome stuff as usual. I’ve been waiting for this for months. I knew the imaging would show more than just aligning an edge. I see you focussed on edge leading strokes, any input on the extract same thing but with edge trailing? I’ve always assumed steeling and stropping should be done in the same method, but everyone always says edge leading only for ribbed steels, but in practice I have found 5 or 6 very firm & slow edge trailing strokes, per side, works best in practice for kitchen knives for draw cutting.

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    • I haven’t looked at edge trailing in detail, other than to confirm that it doesn’t straighten a rolled edge, or observably change the micro-bevel. The cleanliness of the apex does seem to be affected, and I’m not surprised that you observe a more aggressive draw cutting performance with edge-trailing strokes.

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  4. congratulation scienceofsharp. Again weldocumnentated news!

    Den tor. 23. aug. 2018 kl. 00.03 skrev scienceofsharp :

    > scienceofsharp posted: “Even the name is confusing. Knife steel. > Butcher’s steel. Honing rod. Sharpening steel. Do they remove metal or > just “re-align” the edge? What does “re-align” mean? Are “honing” and > “sharpening” different things? Scanning through opinions found on” >

    Like

  5. GREAT REPORT AS ALWAYS, I WILL LITE’N UP ON MY STEEL. OFF SUBJECT, I AM GOING TO USE YOUR METHOD FOR S RAZOR SHARPENING , TWO QUESTIONS: 1. ON LINEN PASTE STROP, DO YOU STROP WET OR DRY? HOW OFTEN DO YOU ADD PASTE( FRITZ). 2. ON LEATHER WITH DIAMOND SPRAY, DO YOU STROP WET OR DRY( DRY I THINK, SPRAY, LET DRY, RE SPRAY). BOTH ARE HANGING STROPS. ANY SUGGESTIONS ON HOW TO SET UP BLUE JEAN STROP IN ORDER FOR IT TO HANG? I REMAIN AN AVID SUPPORTER OF YOUR OUTSTANDING WORK. THANK YOU; ROB H

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    • I always let the paste dry on the denim and then work it in with the shank of a screwdriver, but I prefer pastes that have a waxy base like Mother’s Mag polish, not so much the dry ones like Wenol. I probably get one or two thousand stropping laps out of a strip of denim – i’ll replace it when its black. The diamond spray on leather should also dry – these last a very long time, probably tens of thousands of laps.

      The denim can be duct-taped to the bench, or make a loop by gluing with white glue and clamping it until it’s dry. I hang mine from the bottom of the pegboard.

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  6. As usual some PhD level stuff in here. I am curious about the hardness of blade materials versus yield strength as a predictor of edge holding ability. While only Hardness is marketed, yield strength seems to me much more important in holding a fine edge as the failure modes are both tension and compression related. The low yield strength of ceramics might be behind the chipping experienced while sharpening and use. Also high carbide blades seem inferior to simple HT alloy steel in their edge holding and their thickness is huge to compensate for their low yield strength. I do not doubt the huge advantage of high hardness ceramics in machining, but there is no sharp edge used there and most tools are designed for compression stress. Do you perhaps have any thoughts on this Hardness vs. Yield?

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    • That’s a complex question. In general, hardness and yield strength are MACRO-scopic properties while “edge holding” is determined by MICRO-scopic properties. Although the macroscopic properties are determined by the microstructure, I’m not convinced that there is a simple correlation. On top of this, it is not obvious that the sharpened edge or near-apex metal has the same microscopic properties as the bulk of the steel. And further, there are several different mechanisms by which the edge is “lost.”

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