MINING AND METALLURGY
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Published b y the American Institute of Mining and
Metallurgical Engineers
JANUARY, 1990
Section 9
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fielationship o f Physical a n d Chemical Properties of Copper
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(New York Meeting, February. 1920)
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CERTAIN
physical .and chemical properties of copper are so intimately
related that a change in variation of the physical properties indicates a
"certain chemical change. The standard specifications of copper of the
American Society for Testing Materials are based upon physical characteristics with one exception, namely, the copper contents. The physical
standard includes electrical properties, defects in the "set or casting,"
variation in dimensions, weight, etc. Paragraph 4 of the specifications
sthtes, "Wire bars, cakes, slabs and billets shall be substantially free from
&ink holes, cold sets,. pits, sloppy edges, concave tops, and similar
,defects in set or casting." Shrink holes and concave tops indicate both
physical and metallurgical defects; cold sets, sloppy edges, etc., indicate
physical defects only; small pit holes may be indicative of either or both
ph'ysical and metallurgical defects, depending on conditions under which
they originated.
To the refiner, a copper casting, no matter how made, is known as a
"shape, irrespective of whether it is wire bar, ingot, cake, slab, billet,
etc. As practically all shapes are cast in an open mold, that is a mold
without cope, the surface of the shape coolsin contact with the atmosphere
and has a coating of oxide. Certain phenomena of the surface indicate
the adaptability of the metal for forging or rolling. The writer believes
t h t it i s much simpler to produce shapes that are metallurgically correct
tha those that are mechanically satisfactory. When copper is to be
rolled into tnin sheets or drawn into very fine wire, the slightest physical
defect becomes evident. It is, therefore, of the utmost importance to
consider every characteristic of the shape and ascertain the physical and
ohemical relationship.
Pitch and Set.-The appearance of the surface of the copper when
cooled is considered as to its "pitch," or the general contour of the surface
of the shape (which may vary from a concave to a convex) ;and the "set,"
or the detail appearance of the surface of the shape (the wave-like strucJ
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Superintendent Raritan Copper Works.
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Copyiright, 1920, by the American Instilute of Mining anpl Metallurgical Engineers, Inc.
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MINING A N D METALLURGY
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ture). When the shape presents a concave surface, it is known as "low *
pitch;" a convex surface is known as "high pitch" or "tough pitch;"
and a level surface is called "flat pitch." The set of the copper is, in
a very large measure, directly related to the pitch; and so intimately are
these characteristics connected that they are often used as synonymoug
terms by the refiner when he is speaking of well-refined copper. If,
however, he is speaking of unhealthy or improperly refined copper, he
will define both the pitch and the set. The character of the set will
show whether the copper is low pitch but healthy, or low pitch b d '
unhealthy and must be refined again. High-pitch copper is usually
described in terms of the set. If the set is very close and even, the
copper contents are about 99.95 per cent.; this close set cannot bec
produced on a shape with a low pitch.
As the shape cools, there is a constant contraction with some degassing of the metal. As the top surface of the shape freezes, there is k
tendency for the thin layer of copper to assume a finely corrugated surface a t right angles to the longitudinal axis of the shapes; while the gassing
of the metal tends toward the formation of a coarse wave-like structure.:
This structure varies, with the metallurgical properties of metal, between,
the extremes of a finely corrugated irregular surface, set or close grain>
to a comparatively smooth surface in which the ridges are a considerable.
distance apart and the grain is coarse.
Influence of Oxygen on Copper.-Metallurgically, the copper presents
the best general characteristics when shapes of a certain size present a
slightly convex contour, high pitch, with a good set and close grain.
While a shape of a certain weight presents a satisfactory appearance, it
does not follow that larger or smaller shapes will be the same. As a general rule, the thicker the shape, the less oxygen is permissible in it; the
thinner the shape the more oxygen may be present. Oxygen is, therefore, often spoken of as a medicine for copper, i t being used to regulatk
the pitch. While the pitch may be flat or slightly convex, i t may b6.
decidedly inferior if it presents a coarse grain.
Copper containing an excessive amount of oxygen that is introduced
during the refining period is known as "set" or "dry copper." As tbk?
amount of oxygen decreases and approaches the analysis of satisfactorp
metal, it is known as underpoled; and when it is thoroughly refined, if:
is known as tough pitch. The fracture of tough copper must exhibit a!
decided metallic luster; a brick-red shade is indicative of undesirable
oxygen contents.
With low oxygen contents, copper may be heated over a long peridd
with the formation of a very slight coating of oxide, while with h i g h e ~
oxygen contents, the copper oxidizes quickly.
Influence of Sulfur on Physical Properties.-The
general physical
properties of copper change in a marked manner with the increase gf
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MINING A N D METALLURGY
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sulfur. An increase in the sulfur contents affects the number of bends in
a very much greater ratio than does oxygen.
Unhealthy Copper.-Unhealthy copper contains slight quantities of
gases, such as hydrogen, carbon monoxide, sulfur dioxide, and carbon dioxide. If an attempt is made t o remove the excessive oxygen, the copper
will spew; that is, a miniature volcanic action will occur in t.he shape and the
copper will be known as overpoled. Copper in this state will be low in
electrical conductivity and have a small reduction in area, due to several
reason6 particularly the excess of oxygen. These properties can be
improved only by subjecting the entire charge t o a refining process,
which consists of oxidizing the copper, thus removing the reducing gases,
and then poling the copper up to a tough pitch. Unhealthy copper may
present either a high or a low pitch, but if an attempt is made oo raise the
low pitch, the copper will spew.
Reasons jor Rejecting Copper.-Reasons for rejecting refined copper on
physical examination are: Low pitch; overpoling; cold sets; spewing;
shot; fish (slivers of foreign copper, as the finsfrom the pouring ladle, etc.) ;
holes of several characters, as water holes, shake holes, shrink holes, gas
holes, spew holes, general porosity; foreign substances, such as charcoal,
bone ash, dirt, etc.; splashes; fins; cracks; large set marks; water bursts;
ridges on surface; shapes cast in rough molds; wrong dimensions; and
collective defects. By the last term are meant two or more slightly
developed defects, one of which would not be a sufficient cause for the
' rejection of the shape.
A definite individual hole in the set, or a "spot" as it is termed, is
indicative of excess of oxygen; while the series of small pit-like holes
along the center of fhe shape is an indication of overpoled copper.
If the chemical analysis is standard and the pitch and set are good,
there should be an examination for porosity, which is discovered just
under the skin of the shape by cutting with a sharp chisel-the metal
should show a dense surface and substantially be free from porosityand the character and reason for the holes in the shape. If the holes
were shaken in, the shape will not be satisfactory for drawing into fine
wire or for rolling into very thin sheets. This-defect is caused by the
vibration of the mold allowing liquid copper to surge back and forth
when it is cooling, resulting in slip bands when the shape contracts in
cooling. This internal fracture between the grains of copper may be
present even though there may not be any outward indications.
Copper, generally, is cast in molds that are carried by a machine
under a ladle and i t was deemed impossible a t one time t o move a mold
of molten copper with sufficient smoothness to eliminate this washing of
the metal in the mold and produce a perfectly sound casting. ,Prof.
A. L. Walker has invented an apparatus with which shapes can be successfully poured. So important is this question of moving the molds of
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molten metal that casting machines have been introduced in the refineries that weigh as much as 300,000 lb. (Clark machine) in which the
vibration. is reduced to a minimum by a highly organized mechanical
design.
Analysis of Electrolytic Copper.--Good electrolytic copper should
analyze approximately as follows: copper, 99.950 per cent.; silver,
0.0010 pcr cent.; oxygen, 0.0390 per cent.; sulfur, 0.0030 per cent.;
arsenic, 0.0015 per cent.; antimony, 0.0020 per cent.; nickel, 0.0015 per
cent.; iron, 0.0025 per cent.; lead, nil; bismuth, nil; selenium, trace;
tellurium, trace. The specifications of the American Society for Testing
Materials permit a metallic contents of copper plus silver of 99.88 per
cent. In every refinery, no matter how carefully cohducted, an occasional furnace charge will not do bet.ter than to meet this analysis,
the decrease being due generally to the combination of sulfur, oxygen,
and iron.
Electrical Conductivity.-The
electrical conductivity of copper is
considered the most convenient method of determining its general properties. The conductivity varies in inverse ratio to its oxygen contents,
and is directly proportional to the number of bends, a function of its
reduction in area. The bends are determined on 12-gage wire by the
Capp machine, a simple device that bends a wire at an angle of 90" until
it is broken. The general relationship of various physical properties of
the oxygen contents are shown in Table 1. This table is based upon a
series of tests in which the sulfur contents is about 0.002 per cent. The
copper in the series J analyzes 99.97 per cent.
TABLEl.-General Law Relating to the Influence of Oxygen on Properties
of Annealed Copper. Determined on No. 12 Sojt Wire
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Contents,
per Cent.
Pounds
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Square nch
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E l o ~ a t i o ~ C o Number
n d ~ of
Per Cent.
Per Cent.
Bende
A ................
B ................
C.:... . . . . . . . . . . .
D ................
E ................
F . . ............ .;
G ................
H ................
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Laws Governing Tough-pitch Period.-A few general laws may be
stated for the tough-pitch period, with reference to the relative properties
of copper, which are applicable over a short range only.
MINING AND METALLURGY
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1. As the tensile strength increases, the conductivity decreases.
2. As the oxygen contents increases, the tensile strength increases
3. As the tensile strength increases the elongation decreases.
4. As the oxygen contents increase, the resistance of bending
decreases.
Annealing Copper.-Proper care in the annealing of copper is often
neglected, and it is seldom that the maximum elongation is realized,
which is very necessary where the metal must stand a large amount of
work in being fabricated. Extremely tough-pitch copper exhibits great
ductility, and if it is given improper treatment in the mill it may tend to
shred or tear the surface of the copper. These metallic shreds or slivers
will not be entirely torn from the main body, but will adhere 40 it a t one
end.
When such a piece of copper is pickled, the shreds, not being oxide,
do not dissolve in the solution, thus resulting in an imperfect surface.
If the copper is not so tough, these slivers may be entirely detached or
will not be formed at all, due to the thick veneer of scale. While copper
may occasionally be preferred with such properties, it is a t the expense of
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the physical characteristics of copper.
DISCUSSION OF THIS PAPER IS INVITED. It should preferabl b e presented in person at
the New York Meeting Feb. 16 to 19 1920. when an abstract of the paper w& be read. If this is impossible, discussion in writ& m& be aedt t o the Editor American Institute of Mining and Metallurgical
Engineers, 29 Weat 39th Street, New York N. Y., for'preaentation by the Seoretary or other representative of ~taauthor. U n l w special arranglment is made the d~scuasionof this paper will close Apr. 1,'
1920. Any diecussion offered thereafter should preferably be in the form of a new paper.
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