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.
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| Hardness, HR15N | ||||||||
| Knife | Steel Type | 1 | 2 | 3 | Avg. | Equivalent HRC | ||
| Wusthof Classic Paring | SS | 88.2 | 88.8 | 88.3 | 88.4 | 56.5 | ||
| Wusthof Dreizack Paring | SS | 87.5 | 87.1 | 87.3 | 87.3 | 54.1 | ||
| Sabatier 4-Star 10-in. Chef's | CS | 88.8 | 89.2 | 89.9 | 89.3 | 58.3 | ||
| Sabatier 4-Star 14-in. Chef's | CS | 80.3 | 79.2 | 79.4 | 79.6 | 37.8 | ||
| Sabatier Two Lions Cleaver | SS | 83.4 | 84.1 | 83.8 | 83.8 | 46.6 | ||
| Sabatier 4-Star 6-in. Nogent Slicer | SS | 84.1 | 84.1 | 83.9 | 84.0 | 47.1 | ||
| Hoffritz (Henckel's) 8-in. Chef's | SS | 84.7 | 85.0 | 87.7 | 85.8 | 50.9 | ||
| Anton-Wingen Othello Slicer | SS | 81.9 | 80.6 | 81.9 | 81.5 | 41.7 | ||
| Sharpening Steels | 1 | 2 | 3 | 4 | 5 | Avg. | Equivalent HRC | |
| Wusthof | 89.7 | 90.6 | 88.2 | 91.5 | 90.8 | 90.2 | 60.1 | |
| Hoffman | 90.3 | 91.5 | 89.8 | 91.2 | 90.7 | 61.2 | ||
| F. Dick | 91.2 | 91.6 | 91.0 | 90.2 | 91.0 | 61.9 | ||
| Sabatier | 91.2 | 90.4 | 91.3 | 91.6 | 91.0 | 91.1 | 62.1 | |
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.
Frank
(just kidding!)
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.
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.