abeles@mhuxi.UUCP (10/12/83)
On the subject of the steels used for knives and other cooking utensils: I have seen some pretty uninformed people masquerading as experts on the net, and those who have been explaining the difference between stainless steel and carbon steel are among the worst of them. This article is pretty long. For those who just want to know the main points, they are that: (1) Stainless steel is not harder than carbon steel. Carbon steel is known to be considerable harder than stainless steel. (2) Paradoxically, stainless steel is harder to sharpen than carbon steel. (3) Knives made of carbon steel are sharper and can be re-sharpened but should be dried carefully following usage to prevent them from rusting. Invisible "micro-rusting" may be perceived by the owner as a reduction in sharpness. (4) Carefully maintained, carbon steel knives should continue to be sharp longer than any stainless steel blade of identical shape and profile. How- ever, stainless steel blades are manufactured differently so as not to be as sharp in the first place but to maintain their ability to cut in spite of the use of the softer material. Stainless steel is any steel that resists rusting. Certain stainless steels also are resistant to other corrosion. Most well known are the so-called 18/8 stainless steels, which contain 8% and 18% of chromium and nickel (I admit I don't for sure remember whether it's 8%Cr18%Ni or 18%Cr8%Ni). Those are the so-called "austenitic" stainless steels and are also referred to by numbers such as "304 stainless steel". Any three digit number beginning with 3 (3xx) denotes an austenitic alloy. The name "austenitic" is derived from the fact that these alloys assume the crystal structure known as austenite. Austenite is only one of a number of crystal structures assumed by pure iron-carbon alloys (depending upon percentage of carbon and temperature). The way to identify austenitic stainless steel is with a magnet, since it does not exhibit ferromagnetism. It is true, however, that certain of the austenitic stainless steels of lower quality *will* exhibit ferromagnetism because of the improper alloying of the constituent elements into a single phase. There are other alloys to choose besides the 18/8 alloys if one is interested in maintaining non-magnetic behavior. These are generally made with more Ni and Cr, and mechanical strength is, I believe, compromised as a result. Certain non-austenitic stainless steels also exist. They are magnetic and are supposed to be magnetic. Their identification numbers are four digits beginning with a 4 (4xxx). Now to the point: What is carbon steel? Basically carbon steel is any steel that will rust. That is, it is any alloy of iron (a.k.a. Fe) which will rust and contains carbon. However, since for practical purposes *all* steel contains residual carbon which is costly to remove, we may define carbon steel as any steel which will rust. Note that the term steel is not (as many have been erroneously taught) applied only to alloys of Fe with C, since the austenitic stainless steels described above don't contain carbon, except as an impurity. As a matter of fact, did you know that the term "cast iron" is applied to an alloy of iron containing about 2% carbon? This is a good deal more carbon than is contained in the high-quality carbon steel used for knife blades. Iron is very sensitive to the inclusion of small quantities of carbon in it. About 0.5% is the amount in what is normally called steel, I believe, and 2% is so much that the material has lost the qualities of hardness and strength that are exploited in so many applications. Iron is not as sensitive to the inclusion of other impurities as it is to the addition of carbon, probably because the carbon atoms end up placed between the orderly arrangement of iron atoms in the crystal lattices which make up the tiny crystallites which are found in the bulk iron. Other metals, such as manganese, *substitute* for the iron, rather than jamming into the lattice unwelcome (as it were) like the caraway seed on the denture adhesive commercials. As readers of this newsgroup know, stainless steel knives are easier to care for since they don't rust, but many authorities on cooking prefer non-stainless steel knives (which they correctly call carbon steel) for reasons which are apparently not known to at least one contributer to the newsgroup. That explanation is in the mechanical properties (as opposed to the chemical properties) of the two types of steel. Stainless steel is not, repeat NOT, harder than carbon steel. Quite the opposite. And that is the reason that carbon steel is preferred for knives IF you can keep them from rusting. But some people are confused about why it is harder to sharpen stainless steel knives if they are not harder than than carbon steel. Allow me to explain that final point: Hardness and toughness are two separate qualities of metals. Hardness refers to the ability of the metal to resist, as one accepted test requires a sample of the metal to do, the imprint of a diamond spherical point into the surface of the metal. Hardness tests are quite standardized and you may rest assured that carbon steel is harder than stainless steel. (Incidentally you may be interested to know that tungsten is the hardest of all metals--alloys or pure elements.) But toughness is another matter and not quite so easily quantified. The term as I mean it refers to the resistance of the metal to being machined or filed or sharpened or drilled or tapped or cut in any way. In this sense, ordinary copper is one of the toughest of all common metals, although it is also one of the softer metals. Why? Because it has a quality which is like "gumminess". It is too malleable, and rather than cut, it likes to bend. Imagine to make a sharp knife from chewing gum. I know that is a ridiculous idea, but it sticks to the cutting tool or file that would be used to shape it. However it is incredibly soft. Stainless steel is much tougher in the present sense than carbon steel. If you know anyone who has worked with machine tools, ask him/her, and you will find I am telling you the truth. It is harder to form parts from stainless steel than it is from carbon steel. However, the cutting tools themselves are made from (somewhat specialized for that application) carbon steel. The carbon steel drills find it harder to drill a hole in stainless steel than other carbon steel, but still carbon steel is the harder material--that's why the drill is made of it! And that's why you might prefer the carbon steel knives to the stainless steel ones, as long as you are able to take the time to keep them dry and prevent rust. The carbon steel blades will (paradoxically) remain sharper and harder (all else being equal) than the stainless steel blades, but will also be more easily sharpened when they do degrade. Of course, knowing this the manufacturers of knives use a different cut for the blade of a stainless steel knife blade so that it is never as sharp as the carbon steel version, but does not degrade from being buffeted about (it is more vulnerable since it is both softer and cannot be re-sharpened as easily, or as well). I hope this information is of use. --Joe Abeles BTL MH