Table of Contents Forums Dictionary Recommended Reading Marketplace Giftshop What I Ate Michael's Blog
Latest Post on Michael's Blog: Fall Television Season 2014 Grid
Equipment & Gear

Knife and Sharpening Steel Hardness

by cynicalb
Normal view
Next »
« Prev
A Quick Analysis of Knife and Sharpening Steel Hardness

Surface hardness of through hardened steel products is a quick test used for quality assurance to determine if the proper heat treatment properties have been attained. Surface hardness can also be used as a component of a reverse engineering process to determine relative quality compared to a known product and to determine steel grade and heat treatment. In this case we have tested eight knives by five manufacturers in carbon steel and stainless steel and four sharpening steels to compare the relative hardness and provide insight into the performance of the knives.

Rockwell C hardness (HRC) is the typical hardness scale used for through hardened steel products. As HRC testing does leave a noticeable indent in the surface of the tested part, the Rockwell 15N (HR15N) scale was chosen to perform the test with the results converted to HRC. The 15N scale is the lightest of the superficial hardness tests and the indents are barely noticeable.
Hardness testing of different metals can be compared as long as those metals are relatively close in physical and mechanical properties. Thus, comparing the hardness of through hardened carbon steel and hardened stainless steel is usually acceptable. To do a complete metallurgical analysis of a component, chemical and micrographic analysis should also be employed. However, both of those tests are (typically) destructive in nature and I did not want to destroy my knives.

Table 1 - Selected Knife and Sharpening Steel Hardness
Hardness, HR15N
KnifeSteel Type123Avg.Equivalent HRC
Wusthof Classic ParingSS88.288.888.388.456.5
Wusthof Dreizack ParingSS87.587.187.387.354.1
Sabatier 4-Star 10-in. Chef'sCS88.889.289.989.358.3
Sabatier 4-Star 14-in. Chef'sCS80.379.279.479.637.8
Sabatier Two Lions CleaverSS83.484.183.883.846.6
Sabatier 4-Star 6-in. Nogent SlicerSS84.184.183.984.047.1
Hoffritz (Henckel's) 8-in. Chef'sSS84.785.087.785.850.9
Anton-Wingen Othello SlicerSS81.980.681.981.541.7
Sharpening Steels12345Avg.Equivalent HRC
Wusthof89.790.688.291.590.890.260.1
Hoffman90.391.589.891.290.761.2
F. Dick91.291.691.090.291.061.9
Sabatier91.290.491.391.691.091.162.1

All of the knives tested are forged steel with the exception of the Wusthof Dreizack and the Sabatier Two Lions which are stamped steel. All are stainless steel with the exception of the two Sabatier 4-Star Chef's knives which are carbon steel. The sharpening steel chemistries are unknown, but it is guessed that the Hoffman and F. Dick are almost assuredly carbon steel and the Wusthof and Sabatier might be stainless steel.

By convention, hardness tests are usually done in groups of three or more with the results averaged. More indents were done on the sharpening steels because there appeared to be slightly greater variation most likely due to the longitudinal striations (grooves) found in sharpening steels.

A note should be made regarding the various tested knives and sharpening steels. For the knives, the Sabatier Nogent is brand new; the Wusthofs are relatively new (probably less than 5 years old); the Sabatier 4-Star 10-in. is at least 30 years old (perhaps even older), the Sabatier 4-Star 14-in. and Hoffritz are probably 20 years old; and the Sabatier Cleaver and Anton-Wingen are probably 30 years old. For the sharpening steels, the Sabatier is brand new; the Wusthof is probably less than 5 years old; and both the F. Dick and Hoffman steels are at least 40 years old (probably older).

The most interesting aspect of the testing is the great variation between the knives and the lack of hardness variation between the sharpening steels. A complete metallurgical analysis would be able to determine why this is the case, but as previously noted, this was not done. Also of interest is that the hardest knife was carbon steel and is the oldest knife tested (the 10-in. Sabatier), while the least hard is the moderately old carbon steel 14-in. Chef's knife. Anecdotally, both knives are relatively easy to sharpen (this is one of carbon steel's attributes), and the 10-in. does appear to be the best holder of an edge of all of the knives that I have (probably equal to the Hoffritz). The most difficult knives to sharpen are the Nogent, the cleaver, and the Anton-Wingen. This is not surprising as they are all stainless steel and the intrinsic toughness of stainless does result in a knife that is more difficult to sharpen. Also, they do not appear to hold an edge as long as the 10-in. Chef's, the Hoffritz, or the Wusthofs.

Higher hardness does not necessarily mean a knife is better. What it means is that through proper sharpening you should be able to attain a finer edge but that it will be harder to sharpen than a less hard knife. Depending on the chemistry of the knife, stainless steel should hold an edge longer as it is tougher than carbon steel, i.e. less susceptible to wearing away. However, in my experience with toughness testing I am not convinced that the typical toughness tests (e.g. Charpy impact testing) are equivalent to the type of wear that knife experiences. A good experiment for that would be to examine the edge of a knife after proper sharpening and honing with a scanning electron microscope (SEM) to establish a baseline condition, then subject the knife to some sort of slicing or chopping regimen that is repeatable and examine with an SEM at regular intervals. Thus, you could build up a history of edge deformation vs. cycles of chopping, thus determining which knife (steel) holds its edge the best.

The hardness of the sharpening steels relative to the knives indicates that there probably is some sharpening going on of the less hard knives regardless of which steel that I use. It also indicates that, at least from a hardness perspective, it doesn't really matter which steel I use. What is probably more important is the surface roughness of the steels (which was not measured) and that, coupled with the geometry of the striations, probably affect the final quality of the knife's edge. Microphotography of the knife edge before and after steeling would be a good experiment to determine which steel is the most effective at deburring and realigning the edge.


Next »
« Prev
Written by cynicalb
Published on September 07, 2006 at 10:15 PM
28 comments on Knife and Sharpening Steel Hardness:(Post a comment)

On September 08, 2006 at 11:14 AM, Craig (guest) said...
Subject: Sharpening
You might enjoy "The Complete Guide to Sharpening" by Leonard Lee. It's aimed more at woodshop tools than kitchen knives, but it does cover kitchen knives briefly, and the general parts of the book are quite applicable.


On September 08, 2006 at 01:29 PM, FranksPlace2 said...
Subject: Ceramic rods
I use the steel to dress the knife edges. Periodically I use an old Chicago Cutlery sharpener with ceramic rods. There are two positions to get the right edge shape. It is fast and effective for me.

Frank


On December 04, 2006 at 03:13 AM, humbucker (guest) said...
Subject: ROCKWELL hardness tests!?!
the Rockwell scale is for wussies! :P why not use Vickers or Knoop microhardness -- or better yet, nanoindentation, with a Berkovich diamond tip?

(just kidding!)


On November 08, 2007 at 05:45 PM, PaulR (guest) said...
Subject: Steel and steels
Toughness in knife steel corresponds to resistance to chipping, not to edge holding. This is why your carbon steel knives hold edges longer. The harder Sabatier should an edge much longer than the stainless knives, even if sharpened to a more accute bevel angle. It would also probably be more fragile ... better suited to fine slicing than to hacking up chickens and pineapples.\

Ease of sharpening is hard to correlate to either hardness or toughness (there are some hard and tough steels that sharpen pretty easily, and some that fight you every step of the way). I'm not sure what physical qualities equate to sharpenability, but, subjectively they're easy to feel. Some stainless steels feel "gummy." Some might be too springy ... the edges seem to flex when you try to grind them on a stone.

Something that contributes to steel's abilty to take a very fine edge is carbide size, which is determined by the alloy, and possibly heat treatment. Smaller carbides allow for a finer, more stable edge. Carbon steels tend to have much smaller carbides than stainless steels, but some of the newer "super steels" that you see in expensive Japanese and custom knives come close.


On December 20, 2007 at 04:03 PM, Mr. K (guest) said...
Subject: Work Hardening
Will work hardening have a noticeable impact on knives that have seen more than their fair share of kitchen action?


On April 17, 2008 at 06:20 AM, cynicalb said...
Subject: Re: Work Hardening
Mr. K wrote:
Will work hardening have a noticeable impact on knives that have seen more than their fair share of kitchen action?


My guess would be only for knives that are not properly maintained. Work hardening causes metal to become brittle. So, I would expect that if you have significant amounts of work hardening on a blade, you might start to create a microscopic jagged edge, rather than a nicely honed edge. However, that would be easily remedied by a good sharpening and honing.

Interesting metallurgical thought experiment.


On September 20, 2008 at 03:51 AM, RMC (guest) said...
Subject: Work Hardening
Will work hardening have a noticeable impact on knives that have seen more than their fair share of kitchen action?

No. You would have to bend or otherwise deform the metal - repeatedly - to such an extent that the knife would be useless before it work hardens. Just abusing it in the normal way (dishwasher, banging into other metal objects, etc.) won't accomplish any metalurgical changes.

Here's how to observe true work hardening: take a section of soft copper tubing - the kind used for plumbing - and coil it up tightly, perhaps around a stick or something. Now try to uncoil it. Seems much stiffer now? Has a tendency to kink? This is work hardening. The way to reverse this is to anneal the copper (heating it to dull red and letting it cool off in still air). One anealed it's soft again.

The only possible metalurgical issue I can imagine besides rust or staining that might affect kitchen knives is chloride stress corrosion cracking in stainless knives. I suppose if you immersed your 300-series stainless knife in a bucket of chlorine bleach and put some tension on the knife, after a while the kinfe would crack. So... don't do this and you'll be fine.

I've inherited a 60 year old carving knife that was rusty and quite abused. It's now the sharpest knife I own - shaving sharp - and hasn't work-hardened.


On February 17, 2009 at 03:18 AM, PaulR (guest) said...
Subject: Work Hardening
I'm not sure if work hardening is a component of this or not, but fatigue definitely comes into play. Luckily it only effects the very edge of the knife. The primary source of stress is the cycle of bending from use and bending back from steeling. Eventually the edge just fatigues and breaks off. That's when you have to sharpen.

This is all for knife metals that have a ductile failure mode. Many of the higher end Japanese and custom knives have a brittle failure mode. The edges don't tend to roll at all. These knives (which are typically above HRC 60) hold edges much longer than more malleable ones, but also tend to be more fragile. They're best used by by people with more refined cutting and sharpening skills.


On February 23, 2009 at 08:36 AM, eric g (guest) said...
Subject: carbon steel
There are different types of carbon steel.

Best steel comes from japan. they use veryhigh carbon steel.

stainless uses chromium which doesn't add strength but prevents rust.


On March 04, 2009 at 04:46 AM, Steve (curious) (guest) said...
Subject: Steel on Steel
Has anyone seen any evidence of small steel shavings as a result of using a sharpening steel on a knife? I haven't, but I haven't yet studied it either. It just seems possible because of the mechanical action of the harder serations on the knife edge. For this reason, I carefully wipe down the knife and the steel after sharpening.


On March 04, 2009 at 04:42 PM, Dilbert said...
Steve -

yup.

I have one of those dastardly "grooved steels" the knife geeks tell me should never be used. twenty years on same knives with no issues I detect, so I kinda have to discount the theory.

that said, if the knife is in middling to poor condition, a couple passes on the steel followed by a wipe with a tissue will probably show very fine particles.

thusforth need I no iron supplements <g>


On November 09, 2009 at 09:33 PM, johnliu (guest) said...
Subject: Steeling A Knife
I've looked at my knife edges after using a grooved steel, with a microscope.

An edge freshly sharpened on a fine stone shows a uniform pattern of diagonal grooves on the bevel but also a slight jagged-ness to the very edge with occasional bits of metal hanging off. Gentle steeling smooths the jaggies (very technical term here) and removes the dangling bits, while also knocking down the bevel grooves a bit. Basically the edge becomes "smoother".

However, if you take a dull knife and steel it vigorously and with much pressure, something different happens. The microscope shows that the steel has smeared and torn the edge into many dangling jaggies, almost like a microscopic saw blade. This knife will seem sharp and at first it will cut effectively due to those microscopic saw teeth, but will quickly lose its apparent sharpness, as the jaggies are worn off (presumably lodging in the food).

I think this is why old-time butchers were frequently whaling away with their steels, and their soft carbon steel knives get worn so badly that the blades can even become concave. It is not the ideal way to maintain your knife.

After learning this, I started using my steel differently. I gently and slowly draw the knife over the steel at nearly a 0 degree angle - the flat of the blade laying on the steel. Feel for slight resistance, that is an uneven or wavy or jaggy part of the blade. Work it with gentle strokes until the resistance goes away. Then steepen the angle just a bit, maybe to 5 degrees, and repeat. Do this until the angle is at whatever angle the edge bevel is. And don't go any further. If you really want to do a nice job, use the smooth (ungrooved) bit of the steel (or buy a smooth steel).

I think you can do even better by stropping the edge, but I haven't tried.


On November 09, 2009 at 11:25 PM, Dilbert said...
>>do better

interesting ideas.

if one looks into how manufacturers "sharpen" the knife, it's typically done on a sanding/grinding belt concept - 90 degres to the cutting edge.

in that configuration, the micro groves are [nearly] perpendicular to the knife edge - creates a "mini" serrated effect.

sharpening on a stone will very likely produce the "diagonal" effect you mention. it' a short width stone and a long length blade - not likely to produce perpendicular grit scratches.

whether diagonal or perpendicular, those "mini" grooves produce a serrated edge.

which begs the question: does a serrated edge produce a finer, sharper, easier, smoother, [ . . . . . ] cutting knife?

okay, why are bread knives serrated?
and what's with them there serrated tomato knives?

I personally have no questions (magnifying loop, white tissue....) that a grooved steel removes small particles.

theory holds that a "steel" only 'straightens' up any rolled over cutting edge. this I find to be 'true' - but it also knocks off any of the 'steel fuzzies' hanging around on the edge.

indeed, a steel can't 're-profile' the cutting edge geometry in a few light/heavy passes - but it sure does do wonders in terms of putting a good edge geometry back to 'spec'

my personal experience says the micro-serrations fresh from sharpening on a stone makes for an easier cutting edge in - for example - meats.


On December 27, 2009 at 12:50 AM, Guest808 (guest) said...
Subject: Holding an Edge
Not only is hardness important to how long an edge is maintained, but also corrosion resistance. Although plain carbon steels have the potential to be harder than the stainless steels, if they are not kept dry and conditioned, the edge will corrode and become dull. This is obviously, much less of a problem for the stainless steels while, although not as hard, will not corrode as much.



"I suppose if you immersed your 300-series stainless knife in a bucket of chlorine bleach and put some tension on the knife, after a while the kinfe would crack."

I don't believe that you can find 300-series stainless steels in knives--austenitic stainless steel is just too soft. Instead, you will find 400-series stainless in the martensitic structure.


On March 19, 2010 at 09:09 PM, gowings (guest) said...
Subject: knife sharpeners
Hello. I'm kind of late to this discussion but wanted a recommendation on knife sharpeners (steels and strops will come later). I use a Lansky and have borrowed a Syderco sharpener. I prefer the Lansky because of the stone-edge angle being maintained in my novice hands (blade clamped in bracket with stone on guided rod, captured in a bracket guide hole). The Spyderco (free hand keeping blade vertical, drawing down angled rods) left too much room for error, for my tastes. Problem is each of these takes quite a bit of time to go through our sets a few times per year. I understand sharpening frequency may be decreased with steeling/stropping but until then can anyone suggest a powered sharpener which will yield good results? -Thanks.


On March 19, 2010 at 10:12 PM, Dilbert said...
most knife nuts don't recommend "power sharpeners" -

blink and you've got a ruined dinged up gouged out knife edge.

in use the really pointy part of the cutting edge will bend over, wear down, etc.

steeling 're-aligns' the cutting edge - that is if it is rolled over, it straightens it out - if using a grooved/pebble type steel, the steel can remove a minor amount of metal to 'restore' a sharp bit to the edge.

stropping 'polishes' the cutting edge to an even finer 'thickness'

unless you're doing 1 mm thick sushi slices or insist that a grape dropped from 1/2 inch must slice through under its own weight, stropping is likely a bit overboard for the average user.

Lansky and EdgePro are two very well respected names in the 'knife sharpener gizmo' business.

>>go through our sets a few times per year.

at a loss... "sets"? are you attempting to resolve a 'home use" issue or a 'commerical kitchen/restaurant' issue?

I have some ten knives in fairly heavy/active home use - 10 inch to 3 inch - I freehand sharpen using a medium and a fine stone - I break out the stone and check/sharpen them twice a year. I use a grooved steel per manufacturer item # for in between and I don't abuse the knives.

may take 45-60 minutes to 'go thru the set'

once you've got the Lansky system "setup" and having two smidgens of practice, really should not take all too long to put a good edge on a knife.

unless of course they're abused, used to hack open tin cans, bash bones, chop up glass cutting boards....and the like.


On March 30, 2010 at 07:05 PM, gowings (guest) said...
Subject: sharpeners
Thank you, Dilbert. I'll also check the Edge Pro. Thanks also for helping me understand the difference between stropping and steeling.


On April 12, 2010 at 11:52 AM, BLouis79 said...
My preferred major sharpening method is by hand using a flat diamond stone. Oilstones work well but their wear causes problems with flatness over time that diamond stones do not suffer (more important with flat blades like chisels and planes).

Steels are recommended for daily light use. The diamond stone gets used once a year or so whenever bluntness becomes annoying.

In a pinch (eg on holidays), the unglazed fine ceramic edge of a bowl or mug base is a great sharpener if no others are available. Grandfather and mother used to use that method.

Angle of the knife edge makes a big difference to performance - a blunter angle resists damage, a more acute angle improves sharpness at the expense of edge fragility. Beyond that a slightly curved bevel profile more like an axe is probably optimum as opposed to a single angle to the blade face.

I'm wary of ceramic or sapphire coated rods which will remove metal and alter the knife blade shape at the point of first contact.


On April 12, 2010 at 02:32 PM, Jim Cooley said...
I'd just add that it took me years to realize that couple swipes on a steel does wonders for an already sharp knife. So if you're not in the steel habit, I'd give it a test.


On April 12, 2010 at 04:04 PM, Dilbert said...
Jim -

do you use the steeling toward the body or away from the body technique?

I never felt comfortable steeling toward me, so for years I used the steeling away technique. then I decided I was going to master the technique of flashing knives and singing steel and worked on steeling toward the body.

It could certainly be my imagination, but I think it gets better results - the edge seems sharper steeled toward the body than away. the "geometry" of the situation says 'it's the same' but there must be other factors - consistency of angle / pressure / whatever that is different. might take some hi-speed video ala MythBusters to 'document' it tho . . .


On November 11, 2010 at 06:11 AM, Joe Marfice (guest) said...
Subject: Honing, not grinding the edges
The discussion here seems partly focused on grinding a new edge on one's knives, which sadly is the case on most knife forums. In reality, this is rarely what your knife needs (unless you've damaged it by trying to cut something too hard).

Honing - drawing the blade edge along a honing steel - will only remove a neglible amount of steel, if any, and is what a dull knife needs 95% of the time. I have 4-5 knives I use on a daily basis, doing everything from cutting twigs to butchering small game, and I probably regrind edges on a stone no more than once a year per knife (except for my pocket knife, that I abuse...).

Honing is the act of straightening the nearly microscopic bend-overs at the very edge of the knife. It does not require removal of material. In fact, a perfectly smooth honing steel would probably work as well as a grooved steel, but I've never seen one. The bottom of a coffee cup (unglazed porcelain) is a perfectly functional hone (though it does grind away tiny bits of steel, but this is trivial: its real efficacy is through honing). I have a cheapo $10 steel, and it sharpens - hones - my $90 Henckel Twin Chef with ease. It only has to be harder than the knife; "superlative craftsmanship and balance" are a pretentious waste of $$$ on hones.

Instead of wondering which high-tech, high-cost sharpening system to use, buy a cheap hone (or grab the steel you probably already own), and slowly, gently, stroke the edge into line. Three strokes on the left, three on the right, repeat, repeat, quit. Gentle - you don't want to overbend the metal to the other side! The amount of metal you're trying to bend is tiny - added pressure is probably too much. Slow - the showy swish!swish! motions you see most people use are sloppy, and you can't bend something straight with sloppy moves.


On July 07, 2011 at 12:34 AM, Joe Test (guest) said...
Subject: Damascus Steel
I found a source of low cost Damascus steel kitchen knives. I think I can get a Chef's knife for under $100. The manufacturer asked what hardness I wanted and I thought Rockwell 85 sounded good per your analysis. I won't be able hold or examine the knife before it arrives, is there anything I should look out for?

Thanks!


On July 07, 2011 at 03:35 PM, Jim Cooley said...
Dilbert wrote:
Jim -

do you use the steeling toward the body or away from the body technique?

I never felt comfortable steeling toward me, so for years I used the steeling away technique. then I decided I was going to master the technique of flashing knives and singing steel and worked on steeling toward the body.

It could certainly be my imagination, but I think it gets better results - the edge seems sharper steeled toward the body than away. the "geometry" of the situation says 'it's the same' but there must be other factors - consistency of angle / pressure / whatever that is different. might take some hi-speed video ala MythBusters to 'document' it tho . . .


Could it be that the edges of the steel itself are worn when you steel "away" but not when you steel "towards" you?

I have two steels, and usually use the old worn one because the newer one seems just a bit too coarse.


On July 07, 2011 at 04:42 PM, Dilbert said...
Jim Cooley wrote:

Could it be that the edges of the steel itself are worn when you steel "away" but not when you steel "towards" you?

I have two steels, and usually use the old worn one because the newer one seems just a bit too coarse.


dunno - my steel is round, so an edge on the steel itself isn't 'possible'

in both cases, the stroke is 'into' the knife edge - the only difference I can detect is the angle of the knife-to-steel is more perpendicular stroking away from the body - stroking toward the body the angle seems more acute...


On July 07, 2011 at 04:46 PM, Dilbert said...
Subject: Re: Damascus Steel
Joe Test wrote:
I found a source of low cost Damascus steel kitchen knives. I think I can get a Chef's knife for under $100. The manufacturer asked what hardness I wanted and I thought Rockwell 85 sounded good per your analysis. I won't be able hold or examine the knife before it arrives, is there anything I should look out for?

Thanks!


85R would be about 'normal' - a lot of the Japanese style knives are a couple points harder - this in theory allows a finer edge when stoned/polished/stropped to the nth degree - but the downside to the hard thin edge is: "oops, it chipped!" . . . and that does happen.

as for the rest - basically you're left with the knife maker's reputation -


On July 08, 2011 at 03:31 PM, Jim Cooley said...
Dilbert wrote:


dunno - my steel is round, so an edge on the steel itself isn't 'possible'

in both cases, the stroke is 'into' the knife edge - the only difference I can detect is the angle of the knife-to-steel is more perpendicular stroking away from the body - stroking toward the body the angle seems more acute...


Doesn't your steel have tiny vertical ridges running down its length? Mine do.

Say I'm holding the steel in my left hand. When steeling away from myself, the knife would contact the left edge of the ridges; and vice versa if I were steeling toward myself.


On July 08, 2011 at 03:57 PM, Dilbert said...
>>groves -

yes - see what you mean - need to take the mag glass to the steel and see if I can detect any geometry differences....


On August 06, 2012 at 07:56 AM, essentialfoodstorage02 (guest) said...
Subject: sharpening knives
Well, it is better to keep your kitchen knife sharp as possible to avoid ruining your recipes and your cooking skills, too... thanks for sharing this stuffs... it's a good help for me especially now that I am planning to cook some fresh sushi....or maybe tomorrow...

About CfE Contact User Agreement FAQ's In the Press Write for CfE