Cabon steel vs Stainless steel

[SHARPCO]

Though sharpened in the same way, carbon steel has a different feel when cut compared to stainless steel. So the Japanese sushi knives are often made of carbon steel.

Where does this difference come from?

Can this difference occur even when the BESS scores are the same?

[Mark Reich]

Carbon steel has been in use for thousands of years. Only a century ago, someone discovered that adding a lot of chromium to carbon steel kept blades from rusting. 

Very simply, steel is "stainless" because it has at least 14% chromium. Stainless only rusts under the worst conditions.

The only benefit of so much chromium alloy is that it prevents rust. Otherwise more than about 1.5% chromium is an impurity that severely degrades the performance of regular carbon steel. Mainly it makes steel softer, more malleable, with larger grain. These things lead to difficult burr removal and less edge stability, which kill cutting performance.

The oldest, most common Japanese steels are White #1 and #2, which is basically purified iron with a single alloying element, an abundance of carbon. With these two, clean ingredients, iron gets very hard, with finer than fine grain. Actually, cementite isn't an alloy, it's a phase of steel with no grain. 

Cementite isn't perfect. It's fragile. It breaks and chips easily, and of course it rusts easily. But it is easy to sharpen to an extremely keen edge, mainly because of it's so hard and brittle that it's a cinch to completely deburr.    

This is just a rudimentary explanation. but it's probably mostly accurate. 

[SHARPCO]

Carbon steel has been in use for thousands of years. Only a century ago, someone discovered that adding a lot of chromium to carbon steel kept blades from rusting. 

Very simply, steel is "stainless" because it has at least 14% chromium. Stainless only rusts under the worst conditions.

The only benefit of so much chromium alloy is that it prevents rust. Otherwise more than about 1.5% chromium is an impurity that severely degrades the performance of regular carbon steel. Mainly it makes steel softer, more malleable, with larger grain. These things lead to difficult burr removal and less edge stability, which kill cutting performance.

The oldest, most common Japanese steels are White #1 and #2, which is basically purified iron with a single alloying element, an abundance of carbon. With these two, clean ingredients, iron gets very hard, with finer than fine grain. Actually, cementite isn't an alloy, it's a phase of steel with no grain. 

Cementite isn't perfect. It's fragile. It breaks and chips easily, and of course it rusts easily. But it is easy to sharpen to an extremely keen edge, mainly because of it's so hard and brittle that it's a cinch to completely deburr.    

This is just a rudimentary explanation. but it's probably mostly accurate. 

Thank you for your explanation. 

But stainless steel has made great strides in recent decades. As a result, some stainless steels outperform carbon steel in edge retention. And powder steel made from CPM is also very fine.

So if it were such high-end stainless steel, wouldn't it be hard to say that carbon steel is better?(When BESS scores are the same)

[Mark Reich]

Well, you're asking big questions, which is fine, it just takes a lot of answering. I'm starting out as "generally speaking".

If you compare apples to apples, CPM M4 is a third generation Carbon Powdered steel. If you compare it to 3rd generation stainless, you will see that carbon steel is still superior for sharpening and cutting. M4 still has the drawback of rusting easily though.

If you are comparing certain high carbon, high vanadium, stainless steels, such as S90V and the like, they are almost prohibitively hard to sharpen, but they hold an edge close to M4.

There are still many comparisons to be made. If you are trying to compare S35VN (a highly regarded 3rd generation) to high carbon tool steel, I am well qualified to give you real life experience.

The biggest difference is the difficulty in sharpening. Edge holding is similar.

[SHARPCO]

Well, you're asking big questions, which is fine, it just takes a lot of answering. I'm starting out as "generally speaking".

If you compare apples to apples, CPM M4 is a third generation Carbon Powdered steel. If you compare it to 3rd generation stainless, you will see that carbon steel is still superior for sharpening and cutting. M4 still has the drawback of rusting easily though.

If you are comparing certain high carbon, high vanadium, stainless steels, such as S90V and the like, they are almost prohibitively hard to sharpen, but they hold an edge close to M4.

There are still many comparisons to be made. If you are trying to compare S35VN (a highly regarded 3rd generation) to high carbon tool steel, I am well qualified to give you real life experience.

The biggest difference is the difficulty in sharpening. Edge holding is similar.

You are right. Comparisons should be made between the same classes. I would like to use a CPM-M4 knife if I have a chance. 

[Mark Reich]

I have one for you to try.

[me2]

Inexpensive stainless steels often have inexpensive heat treatments.  This leaves a high percentage of retained austenite, which is soft and gummy.  However, it was also easy to get stainless steels not suitable for cutting tools.  This combination gave stainless a less than stellar reputation.

Good stainless has been available for a while, 90 years or so.  It has higher wear resistance than the low alloy and plain carbon steels like 1095, 52100, O1, and similar steels, though the difference is not large.  Several alloys were developed for cutting applications.  They behave very similarly to carbon steels.  They have carbide sizes similar to low alloy and plain carbon steels, but I have been unable to find grain size data from a reliable source.

The extra chromium in stainless makes stainless corrosion resistant.  The minimum necessary is between 10 and 12 percent, depending on the source of information.  When a stainless has enough carbon, it combines with the extra chromium to make chromium carbides, which are harder and more wear resistant than iron carbide.

[Bud]

This makes sense to me Mr. me2. I've picked up a number of cheap stainless knives in past years at sales and so forth. Cheap not just in price but cheaply made as well. That being the case some have turned out to be not to bad and others are only good to get the mud off my boots. The difference to me is always how hard it is to get the burr off the edge after grinding. There have been a number of them that I have just given up on. Your comment about sticky steel really hits home because that's just how the burr acts. They may be the same or similar steel but hardened differently. Good point you make and thank you for it.

[me2]

It can really be confusing without some practice. 440 stainless is hardenable, 431 is not. If overheated a bit, the amount of retained austenite increases substantially in hardenable grades. It wouldn't surprise me if some knives were made of non-hardenable grades. Poorly treated stainless will hold on to a burr like grim death. On the other hand, I have 2 inexpensive stainless knives that get extremely sharp with just the most basic sharpening stones and procedures. Haven't measured them, but by feel it's pretty surprising.

Several of the Sandvic stainless steels, and varieties of modified 420HC can behave very much like carbon steel if properly hardened. Also, I have heard of the chromium oxide layer on stainless being noticeable when sharpening. I haven't experienced it myself, but typically don't rely on tactile feedback like many do.

[Mark Reich]

Inexpensive stainless steels often have inexpensive heat treatments.  This leaves a high percentage of retained austenite, which is soft and gummy.  However, it was also easy to get stainless steels not suitable for cutting tools.  This combination gave stainless a less than stellar reputation.

Good stainless has been available for a while, 90 years or so.  It has higher wear resistance than the low alloy and plain carbon steels like 1095, 52100, O1, and similar steels, though the difference is not large.  Several alloys were developed for cutting applications.  They behave very similarly to carbon steels.  They have carbide sizes similar to low alloy and plain carbon steels, but I have been unable to find grain size data from a reliable source.

The extra chromium in stainless makes stainless corrosion resistant.  The minimum necessary is between 10 and 12 percent, depending on the source of information.  When a stainless has enough carbon, it combines with the extra chromium to make chromium carbides, which are harder and more wear resistant than iron carbide.

Mr. Me2, what old stainless are you speaking of? This is news to me, and shouldn't be. I certainly don't know of any stainless, old or new, that is as easy to sharpen as plain carbon. 

I know by old definition a steel can be considered stainless with as little as 10-11% chromium, but I think that must mean free chromium. I don't know of any steel with that amount of chromium, and I don't know of any steel that is considered stainless (at this time) with less than 14-15% chromium, but there are a lot of variables. 

I don't think wear resistance is a very good indicator of edge holding. S30V has high wear resistance, but I don't know anyone who thinks it performs well. Most consider it very hard to sharpen. Most say it needs high refinement, and holds a mediocre edge for quite a while. I've had an S30V Sebenza for a decade, and it's been sharpened a lot (almost ready for a new blade), because I don't care for mediocre sharpness. 

I think D2 is a good steel for comparisons. If you read what is commonly written about D2's rust resistance, it's never considered "stainless". With 11-13% chromium, the best I've ever heard is "semi-stainless". I have D2 knives, and they are not very stainless, but it does have very high carbon, so I know a lot of the chromium is tied up in it's very large carbides (which are a recognized drawback). 

I have a lot of real world experience with hunting knives. I've been sharpening all sorts of hunting knives for over 40 years, with an emphasis on pre 86 Gerbers and Bark River knives, which I consider benchmarks for production knives. Gerber ran 440C at HRC 58-60, but it seems a point higher to me. Their old M2 is fabulous, and hard. I think HRC 63-64 is fair. It's better than Benchmade's old M2, which was another benchmark of it's own. These knives have appreciated dramatically, and are super collectible, which should mean something.

Obviously, I favor 52100, and I've carried a Bark River in 52100. It outperforms Mike's favored A2 , but maximized with an exact heat treat, it is simply unbelievable. The plentiful real world feedback is stunning. I carry two knives at all times, one of mine, and a high end folder with the best steels. I make comparisons on a daily basis, and I do a Lot of sharpening and BESS testing.