Article Digest: 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
Wusthof Classic Paring
Wusthof Dreizack Paring
Sabatier 4-Star 10-in. Chef's
Sabatier 4-Star 14-in. Chef's
Sabatier Two Lions Cleaver
Sabatier 4-Star 6-in. Nogent Slicer
Hoffritz (Henckel's) 8-in. Chef's
Anton-Wingen Othello Slicer
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.
Posted: Fri Sep 08, 2006 11:14 am Post 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.
Posted: Thu Nov 08, 2007 5:45 pm Post 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.
Posted: Thu Apr 17, 2008 6:20 am Post 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.
Posted: Sat Sep 20, 2008 3:51 am Post 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.
Posted: Tue Feb 17, 2009 3:18 am Post 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.
Posted: Wed Mar 04, 2009 4:46 am Post 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.
Posted: Mon Nov 09, 2009 9:33 pm Post 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.
Posted: Sun Dec 27, 2009 12:50 am Post 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.