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Iron, Steel & Pianos
Continued from Previous Page
All steels and cast irons also contain other elements and
materials. Some of these, such as sulphur and phosphorus,
are residual impurities which generally have been reduced
in manufacture to the economic minimum. Others, such as
silicon and manganese, may be purposely left in in controlled
amounts, or be purposely added, to give the material
special qualities. Examples are gray cast iron, containing
considerable silicon, and the many alloy steels containing
chromium, nickel, molybdenum and so on.
The steel category includes a large spectrum of materials,
classified by carbon content, as well as by the percentage
ranges of other alloying elements. Music wire is generally
made from carbon steel, as distinguished from alloy steel,
which means that no deliberate alloy additions are used.
(Except manganese. Almost all steels, either carbon or alloy,
contain appreciable amounts of manganese.)
Apart from the chemical distinction between steel and
cast iron, one of the most important differences is that the
various steels are generally ductile and malleable in varying
degrees, while cast iron generally is not. (The amenability of
a material to plastic deformation under stress without
fracture is called ductility or malleability, according as the
stress is tensile or compressive.)
Wire could not be made from cast iron because the
manufacturing process and most wire usages demand a
ductile material. On the other hand, piano plates could be
made of steel by forging, casting or welding, but among
other disadvantages they would be costly far beyond any
strength superiority they would have over plates of gray cast
iron.
We will return to some of the other properties and
reactions of these materials after a short excursion into iron
and steel making.
The manufacture of steel divides rather naturally into
two stages:
1) making the material and 2) making the product from
the material (product meaning bars, structural shapes, sheets,
wire, etc.). With gray cast iron on the other hand, the
material is generally turned out in product form, as we shall
see.
Pig Iron — The Blast Furnace
The first step for either steel or cast iron is to recover iron in
usable form from iron ore, which is iron oxide (rust) in
varying mixtures with earth, sand and rock. This recovery is
mostly a process of getting rid of the oxygen by heating the
ore in the presence of carbon and limestone. This takes place
in a blast furnace, which is a shaft, typically 25 feet or more
in diameter by 75 feet or more in height, charged with
18 Piano Technicians Journal / November 2000
layers of coke, ore and limestone, which form a descending
column. Air is forced in at the bottom, and the coke burns
partially to carbon monoxide, which in turn reduces the
iron oxide ore. The limestone forms a molten, fluid slag,
which, floating on the molten iron, accumulates and carries
off much of the waste matter. The blast furnace operates
continuously, with materials charged at the top and the slag
and molten pig iron drawn off at the bottom. This is a hot
process, with a temperature gradient in the furnace from a
few hundred degrees at the top to about 2,750 degrees F. at
the bottom.
Pig iron, the blast furnace product, contains fairly large
percentages (totaling seven percent or more) of impurities
such as silicon, manganese, sulphur, phosphorus and an
excess of carbon. “Impurities” is a relative term, as some of
these inclusions are impurities only as they are in excess for
the purpose at hand.
Gray Cast Iron — The Cupola
If the end product is to be gray cast iron, the pig iron from
the blast furnace is refined in an oxidizing furnace known as
a cupola. In foundry practice the cupola charge usually
includes cast iron and steel scrap and ferro-silicon, as well as
the pig iron. Coke for fuel and limestone for flux are also
included. Because little or no chemical correction is possible
in the cupola after melting, the charge must be carefully
planned as to proportions of entering materials, based on
the constitution of these materials and the desired constitution
of the product. A typical melt for piano plates might
contain 3.5 percent carbon and 2.4 percent silicon, about
which more will be said later.
The molten “cast iron” is drawn off and poured into
molds, usually of sand, in which it takes the shapes of the
patterns used in preparing the molds — piano plates, for
example.
Steel — Making the Material
If the end material is to be steel, the pig iron from the blast
furnace is refined in one of various types of oxidizing
furnace permitting closer control than the cupola of the
iron foundry. The reader will have heard of the Bessemer
converter and the open-hearth furnace, both long used in
steel making. The electric furnace, once limited to special
steel manufacture, is now used extensively in the production
of more common grades. Steel music wire may be made of
either open hearth or electric furnace steel, never Bessemer.
(The very fast Bessemer process is not deliberate enough to
allow the analysis and chemical corrections necessary to the
careful manufacture of high carbon steel.)