Carbon and Stainless Steel Blade Question

[TedGamble]

OK, this seems to me a contradiction of terms.

Everyone seems to unanimously state that the carbon blades take an edge and sharpen much easier than the stainless blades. But, it appears that most of you saying that the carbon blades also hold the edge better than the stainless blades.

I don't understand how that can happen. If a blade sharpens easier, isn't it softer? And if it is softer, shouldn't is lose its edge quicker?

 

[Don Luis]

I'm no metallurgist but I know that there are many factors involved, not just hardness, you could make a lot of blades from different steels, same size, shape, blade and edge geometry, etc. harden them to the same RC, sharpen them at the same angle, and they would perform different.

Some steels may resist abrasion but loose shape under pressure, these would be hard to sharpen but not hold the edge much, some may have a fine grain structure and take a good edge for push cuts but others with coarse grain may take a better toothy edge for slicing.

If you have the time you may try searching around the forums, including the archives and you may learn more or get more confused.

Just my 2 cents.

Luis

 

[lt632ret]

They seem to hold an edge better because they sharpen better and take an edge quicker. If someone is sitting at home slowly honing a hunting knife they can put a pretty good edge on a stainless steel knife with a 58 hardness on the rockwell scale. However in the field the chances are they will not be as fastidious. What the knife is used for is quite important, as well, if the knife is being used to skin electrical wire chances are they will both dull at about the same speed perhapes the carbon a bit faster being softer ( ever notice how most tools of this nature are usually stainless ). In the field on flesh the illusion of the carbon lasting longer is usually due to it being sharper to begin with and getting an edge back much quicker. Next time in hunting camp take a new stainless steel skinner with a good factory edge and a carbon knife. I think you will find both will keep there edge about the same amount of time but after that put the stainless away until you get back home. It will simply take to long in the field to bring it back to the same degree of effectiveness. The carbon can be brought back with a few swipes on a steel and last almost as long, over and over.

 

[Sword and Shield]

Actually, it's not that much of a stretch.

Let's go with two steels- 1095 and, say, 420HC.

1095 is straight carbon steel. ~0.95% carbon, 0.4% Manganese, balance iron. 420HC is basic stainless, 0.54% Carbon, 14% Chromium, pinch of manganese, molybdenum, phospohorus and vanadium.

Important part is the carbon and chrome. Carbon is a hardener of steel. Higher carbon= higher hardness. Higher hardness= better edgeholding, to a point. Chrome adds stain resistance, but chromium carbides are a bit softer than true iron carbides. It adds wear resistance, but the edge is more likely to roll over.

So, there. Carbon steel has the carbon in its favor for hardness, while stainless has chrome pulling it down. There are semi-exotic stainlesses out there (154CM, BG42, VG10) that approach carbon-like edgeholding, and some crossover steels (D2, 440V) that have carbon-like edgeholding with some stain resistance.

However, those, you pay heavy for. In a cheap knife, carbon will hold an edge longer.

 

[MarkPinTx]

Actually, it's not that much of a stretch.

Let's go with two steels- 1095 and, say, 420HC.

1095 is straight carbon steel. ~0.95% carbon, 0.4% Manganese, balance iron. 420HC is basic stainless, 0.54% Carbon, 14% Chromium, pinch of manganese, molybdenum, phospohorus and vanadium.

Important part is the carbon and chrome. Carbon is a hardener of steel. Higher carbon= higher hardness. Higher hardness= better edgeholding, to a point. Chrome adds stain resistance, but chromium carbides are a bit softer than true iron carbides. It adds wear resistance, but the edge is more likely to roll over.

So, there. Carbon steel has the carbon in its favor for hardness, while stainless has chrome pulling it down. There are semi-exotic stainlesses out there (154CM, BG42, VG10) that approach carbon-like edgeholding, and some crossover steels (D2, 440V) that have carbon-like edgeholding with some stain resistance.

However, those, you pay heavy for. In a cheap knife, carbon will hold an edge longer.

Good analysis Sword. From a lot of what you read around the fora, hardness would seem to be the sine qua non of edge-holding. It's not. Hardness is typically accompanied by brittleness (lack of ductility/malleability), which leads to fractured edges and dullness just as quickly (possibly faster) as a soft steel in which the edge rolls. Edge-holding is an almost magical combination of hardness and malleability. Carbon steel achieves it probably better than most stainless, but the heat treat of the steel is also a factor.

Sword is correct that it is iron-carbon molecules, called carbides, that make a steel hard and abrasion resistant. The size of the carbide molecules can make it brittle (if large), because carbide molecules are themselves brittle (think tiny rocks). The addition of chromium to steel causes the formation of chromium carbide, which is hard and brittle (and large, therefore displacing malleable iron in the composition, think concrete with too many large rocks in it). The various iron carbides (the size and distribution of which are controlled by the heat treat: smaller is usually better) in a matrix of steel (iron) is what achieves the magical balance. Chromium, among other alloying elements in stainless, only disturbs the balance and makes it more difficult to achieve a tough yet hard blade that sharpens reasonably easily and holds an edge.

 

[lt632ret]

Both of you seem quite correct and a lot more specific than myself. I was speaking in generalitys your analyisis is a lot better, just saying stainless steel is like saying alcohol there are alot of variations for a lot of purposes. Thankyou for the input I enjoyed it. This is part of the subject that I for one would like to know more about. In fact you did clear up one point for me regarding D2 steel. LT

 

[Carl64]

This is one of those things I always have trouble saying without too many words.

First, given Carbon and stainless blades of the same hardness, don't think of the Carbon steel blade as easier to sharpen, think of the stainless blade as harder to sharpen. Left at that there wouldn't seem to be a difference in those two thoughts, but it will make more sense later.

If wear resistance were the only factor that ever changed in a knife blade, it would be correct to say that a longer-lasting edge would always be harder to sharpen, or that an easier to sharpen edge would always get dull quicker.

But there is more to sharpening than wearing down the steel. You want the steel to wear down AND hold the shape you are making it into. For this to work, you need the steel to be hard.

Therefore, ease of sharpening has to do mainly with the relationship between hardness and wear resistance. Try taking a piece of soft leather and sharpening it. You basically can't. The same goes for a piece of chewing gum. Both actually have a fair amount of wear resistance, but virtually no hardness. They will deform before you grind away much.

But what if both steels have the same hardness and wear resistance? Well, those two ratings don't tell the whole story in enough detail.

Stainless steel tends to behave like gum or leather, even when the hardness rating is the same. I notice that many such blades tend to have a flaking behavior if not sharpened carefully (ie, with very low pressure), such as my Microtech mini SOCOM. It's 154cm at 60rc. My Eye Brand with a Carbon steel blade, that is almost certainly a lower RC rating, can be sharpened with high pressure very quickly and end up with a very sharp edge. But if I sharpen the harder SOCOM with too much pressure it developes little flakey bits along the edge, and when they break off they leave a dull area. These flakes also tend to be springy, much more so than you would expect from the supposed hardness. I have seen the same happen with other stainless blades.

It's weird, but stainless that is officially hard acts as if it is soft to a small degree. Maybe you could describe it as a hard substance that bleeds a softer substance. If none of this makes sense, it's because stainless steels usually don't make any sense in the way they act and that's why sharpening them is more difficult. Does that make sense?

Many years ago, after my first attempt at sharpening a stainless knife (spyderco with G2), my first comment was that the steel felt "gummy." That was only the second knife I had every sharpened, the first being an Old Timer (Carbon steel), but a few other knife users agreed that gummy was the word. Does anyone else think this reflects their own observations?

 

[mete]

It's difficult to generalize about stainless steels since there are so many of them.While there are some that are junk like 420 there are some that are very fine and not gummy at all.The finest in my opinion is S30V.

 

[Carl64]

s30v is one of the few I have not tried, but ats34/ats55/154cm, vg10, 440 series, and many other common stainless steels do. There are also many tool steels that don't fit into either category well and don't have the same problem.

I haven't tried s30v except for a Spyderco Native with lower hardness than I would like, so I really "need" to get a good knife with s30v. The one I had definitely did not act ideally, although it was better than most.