Carbon Vs Stainless

J

JDBLADE

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Dec 8, 2003
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Why is it that Carbon Steels appear to be better than Stainless Steels. If we were to take Carbon Steels such as 1095 and 0-1 and compare them with 440c (which has a similar carbon content) – all heat-treated to say 58hrc why does it appear that the Carbon Steels are sharper and easier to sharpen. My hypothesis is as follows:

Carbon Steels appear sharper because due to the nature of the steel a more toothier edge is created in the process of sharpening and using. Whereas due to the high level of chromium in stainless steels a more polished clean edge is created in the sharpening and using processes. In fact by through the process of actually cutting the edge of stainless steel is polished significantly more than carbon steel. Another factor that helps create a toothier edge is the microscopic erosion that occurs with Carbon Steel esp. if kept in an un-sealed condition.

As to the assertion that Carbon Steels are easier to sharpen this is because with minimal chromium there is less resistance created between the edge and the sharpening medium and the fact that less polishing takes place and more steel is removed over the same number of strokes.

It may not be fair to Carbon Steels to compare them with some of the Boutique stainless steels eg VG10 and S30V as there are other factors involved although it may be fair to make similar comparisons between D2 and S30V.

So with the exception of the “stainless” quality of SS it is fair to say that at the levels I stated in the aforementioned that Carbon Steel is a better Cutlery Steel than Stainless.
 
There is no difference in "sharpness" at similar grit levels and quality of honing. However since most stainless steels are much more difficult to machine, the problem is that often a fully honed carbon steel blade is compared to a stainless blade that has futher to go.

If the steels are very different the times required can be many to one times different. So if they are compared at similar times the stainless blades will appear to be no where near as sharp.

Note that this only applies to low allow carbon steels, high alloy non-stainless steels like D2, M2, etc., have the same machinability problems.

Another factor, and one which is usually more significant is the edge geometry and suitability of steel.

If the edge is highly optomized (minimized profile) and the properties of the steel well suit the tasks of the knife, it will be easy to sharpen. This is one of the reasons why VG-10 got a reputation for high sharpenability.

It is used by Spyderco on blades like the Temperance which have a very high cutting ability and minimal edge profile. The steel is well suited to the blades intended use and the NIB edge profile allows easy touchups on a Sharpmaker without any profiling.

Compare this to ATS-34 at 62+ HRC on the thick edged Benchmades. They didn't cut well NIB, chipped out easy, edges were too thick to sharpen on v-rods and many jigs. The steel thus got a reputation as being difficult to sharpen.

[note if you want to be really picky there is a difference in sharpness, but you have to polish below the grit size of the steel, which is sub-micron, and the edge angles have to be really low, <10 degrees per side, even then, even coarse grain steels readily take hair popping edges so it isn't a significant point]

-Cliff
 
It seems to me from my experience, all things being equal, that Carbon Steels are sharper and easier to sharpen. This is esp. evident in butchers knives eg the old Green River style Skinner. Such knives normally have a hardness of around 55-56hrc, due to consumer demand for ease of sharpening, and in my experience Carbon Steel is easier to sharpen then Stainless Steel and once sharpened Carbon Steel is easier to steel then Stainless Steel. I believe the main factor is the high level of Chromium which in fact inhibits steel removal. If stainless steels are much more difficult to machine then it must stand true that stainless steels are much more difficult to sharpen as both processes involve the removal of steel.

When the Chromium molecules rub together they create the “polish” of stainless steel. It seems to me to be folly to “steel” a stainless steel knife on a stainless steel steel/sharpening rod as the process may realign the edge but also increase the “polish” of the edge. Whether or not you want a polished or toothy edge is a matter of contention – some will argue that a polished edge will cut meat easier and with less effort whilst others will maintain that a toothy edge will cut quicker esp. through tendons and sinuous fibres. From my experience the latter is the best edge whilst a polished edge is better on vegetables etc.
 
Carbon steel blades do not take a toothier edge than stainless, they take a finer/smoother edge than stainless (most stainless). It is relatively easy to get a fine shaving edge with simple carbon steel, like 1095, or a vanadium carbon steel like O1 or carbon-V. Stropping a carbon steel greatly enhances this smooth shaving edge. It is true that this can be more conspicuous on a softer carbon steel, but I've also noticed it on harder carbon steel.

The problem with a lot of stainless is that the grain structure is rather coarse and full of lumpy chromium. They don't take a toothier edge due to bad distribution and shape of the chrome. The chrome is hard to shape and often breaks out of a thin edge. There are exceptions to this, but that's the general trend. If you get a highly refined stainless like 12C27 or a vanadium stainless like AUS-8, VG-10, or BG42 the grain structure gets finer and starts to approach basic carbon steel. You still have a problem that the chrome makes it harder to abrade these steels to sharpen them. They also don't benefit as easily from stropping as the basic carbon steel.

You can put a toothy edge on either carbon or stainless steel. I don't know many alloys that favor a toothy edge particularly. D2 is supposed to have intrinsic toothiness, but I have done pretty well getting a razor edge on it.
 
A couple of notes :

1) finish sizes are not on the scale of atoms of Cr, or even the carbides, this is a property which is almost macroscopic in size, it has to do with the alignment of the steel as a whole, you can see it readily under magnification which is *FAR* above the level needed to see actual elements of Cr

2) machinability is only one element in ease of sharpening, and if it is the critical one then you have picked the wrong steel for the job, and/or the wrong geometry. When edges blunt, the amount of metal which is actually damaged is about 1/10 of a mm deep, this doesn't take any significant time to remove if it is CPM-15V or 1095. Of course if you don't just remove it, but try to do something else which is far less efficient then machinability is a problem, but this isn't the blades fault, the method of sharpening is just highly inefficient (puukko's etc).

Jeff noted that some stainless steels don't take a high polish well, I have noted this with ATS-34, you can see it under mag at low edge angles (5 degrees per side), there are chips in the edge which will not go away even when you switch to high grit abrasives.

However, for the vast majority of people this isn't a significant factor because edge angles in general are not commonly that low, and even that blade that I noted could still push shave readily.

As for aggression, that is more hype than fact, carbide sizes are of microns, a rough belt finish can be 10-100 times greater than the size of the carbides, the scratch pattern of the abrasive is all that matters.

-Cliff
 
Cliff -- been meaning to ask: can you recommend any sources or references that describe and illustrate steel grain and structure, and perhaps go into some detail that would be useful for a layman seeking to better visualize grain size, carbides of different types as they are embedded in the steel matrix, etc.? Your reviews and posts have been extremely helpful in giving me a better sense of all this, and especially an appreciation of scale, but illustrations would be very helpful at this point.

Dave
 
If you want the full detail, try something like :

"Heat Treating and Processing Principles", Krauss

"Steels : Microstructures and Properties", Honeycombe

However these are very *dense* works, an excellent introduction is :

"Heat treatment, Selection, and Application of Tool Steels", Bryson

Bryson's book has shots of annealed, steel as well of D2 showing the austenite grain boundries and carbides, and the carbide distribution in HSS.

This is more of a cook book, as in take this steel, do this temp/time, the other two works are very comprehensive metallurgy books which go into considerable detail on the physics behind the behavior.

Even if you are not interesting in knowing the heat treatment specifics, Bryson's book contains a wealth of information on the different abilities of various steels, what they are intended to do, where they should be used, he deals with lots of side issues like stress relief, cryo, grinding and over heating, etc. .

-Cliff
 
thanks for the book links, Cliff - been meaning to ask. Email sent.
 
Thanks, Cliff. Bryson's book actually sounds like it would be of added interest.

After that, maybe I'll decide I'm dense enough to tackle those other two dense tomes.

Dave
 
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