Text: From: zebpiano@worldnet.att.net.geentroep (Richard Vance) To: rolls@foxtail.com Date: Sat, 24 Feb 2001 18:23:34 -0500 Subject: Making Steel for Piano Strings [ I asked Richard to write about the steel from which [ piano strings are made. -- Robbie I can't address the matter of comparing old steel to the modern product, since I know nothing of the history of when the things I mention were first introduced. But I do know that steel metallurgy is a surprisingly old science, well developed as far back as the 19th century, so one would have to go back pretty far to find piano wire that is 'worse' in some way when compared to today's product. I am not a metallurgist, so anything I say is only what I have picked up hanging around steel mills. The thing that makes steel metallurgy so fiendishly complex, compared to most other metals, is that molten iron (and only a very few other metals) is a true electrolytic solvent like water. It supports all the complex ionic reactions and solution phenomena just like water does. Comparing the chemistry of iron to, say, copper, is like comparing water to kerosene. Steel is even worse, because steel is not just a mixture, but is also a complex linkage of iron, iron carbide in several species, oxygen, carbon, and up to 10 or more other elements in significant amounts. All this stuff influences the 'shape' of the steel. In the steel business, shapes mean steel beams or other rolled profiles, but 'shape' means the way the various molecules in steel are piled up and linked in the crystalline form of the metal, which profoundly influences the strength, elasticity, and hardness of the metal. The thing that struck me, while reading the catalogs of piano string makers, is that piano wire is made from cast steel. The significance of this will be explained further on. This is woefully simplistic, but there are three fundamental things that can go wrong with steel: First, it could have the incorrect composition, or include the wrong 'other' elements. Suffice it to say, that careful selection of the raw materials and additives, and constant testing with the mass spectrometer, has gotten this problem pretty much licked. Second, gas easily dissolves in iron, and such dissolved gas must be carefully controlled or eliminated. O2 and CO percent must be not zero, but just right. H2 and N2 are no-no's, but are easily absorbed from atmospheric N2 or water vapor that breaks down into H2 and O2 on the hot surface. It may surprise you to learn that molten steel is not allowed to touch air once it is made. Any ladle, tundish*, or mold is provided with a layer of slag on top, but some unwanted gas gets in along with the raw materials or additives. (Another 'term of the art': in iron making, slag is the layer of oxide and other crap that floats to the top, as everybody thinks of it. But in steel making, slag is an artificial material, essentially powdered glass, which is sprinkled on the liquid surface, where it melts and forms an air impregnable layer.) All good steels are finally degassed at the ladle met station, which is the last stage where a ladle of molten steel is prepared for casting. Aluminum is added which 'gets' (links to) the excess O2, which floats off as light aluminum oxide. H2, N2, and excess CO are then sucked off under very high vacuum. The third 'problem', of particular interest when discussing piano wire, is inclusions; that is, either voids or tiny bits of insoluble matter that end up in the finished metal. One can see how these could spoil a piano wire. Of course all steel must be cast, to make it into a solid product. Most steel is now continuously cast by pouring it into a narrow mold as a liquid at the top, and pulling it out the bottom of the mold as a partially hardened strip of essentially infinite length, analogous to the traditional way of making sheets of organ metal. However carefully they try to eliminate them, continuously cast steel still contains a few inclusions. But when steel is advertised as cast steel, as for piano wire, this usually means that the steel is solidified the old fashioned way, by pouring it into individual ingot molds, and then rolling, drawing or machining the resultant billets individually. The Timken Steel company in Canton, Ohio, has the only modern mill that I know of which still casts much of its production as ingots. Many people don't know that Timken makes steel not only for its own bearings, but they also sell a large quantity of super-high quality steel to tool makers, knife smiths, and any anybody else who needs steel that is virtually free of inclusions. Timken's procedure is as follows: The steel is made, adjusted in composition, and degassed just like in any other mill. But then the ladle of molten steel is poured into a row of ingot molds instead of into the tundish (distributor box) of a caster. The molds are bottom poured; that is, they are filled from the bottom up, through a refractory tube. The full molds are allowed to cool very slowly. This is important. Any voids settle out or rise to the top. Inclusions either migrate to the walls of the mold and stick there, sink to the bottom, or float to the top. Then the cooled billets are cropped -- the bottom foot and the top two or three feet are sawn off and discarded. Finally the billets are scarfed; an inch or so is planed off the four sides. What is left is the ultra-pure center part, free from any inclusion. This process is wasteful and labor intensive, like eating soft-shell crabs from Chesapeake Bay. No wonder Timken steel is very expensive, and used only by those who need its special properties. But I wouldn't be surprised to learn that Maps Piano String Co. buys its billets for piano wire from Timken or someone like them. Richard Vance

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