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| Electrical Conductivity of Copper |
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When we speak of electrical conductivity as being high or low we are referring to electrolytic tough pitch copper (110) as the standard at 100% IACS (International Annealed Copper Standard). Present day purity is high enough that conductivity may be over 100% and for certified OFHC (101) it is close to 102%. The only other metal over this is pure silver at about 106%. A low value among Revere alloys would be 4% as for 70/30 Cupro-Nickel (715). Metal alloys used for resistance heating (electric stoves, irons, etc.) have conductivities far less than 1%, and measurements are made in terms of resistance rather than conductivity. (A few examples are listed in table that follows). Heat conductivity numbers are mathematically related to electrical conductivity so that for much practical testing and checking, the electrical conductivity is determined because it is so much easier to perform. |
| Electrical Conductivity | ||
| Material | CDA Alloy No. | % IACS. |
| Electrolytic Tough Pitch | (110) | 100.8 |
| Deoxidized Copper - DLP | (122) | 85 |
| Gilding Metal - 95% | (210) | 56 |
| Commercial Bronze - 90% | (220) | 44 |
| Red Brass - 85% | (230) | 37 |
| Cartridge Brass - 70% | (260) | 28 |
| Admiralty | (443) | 25 |
| Aluminum Bronze (D) | (614) | 14 |
| Cupro-Nickel - 10% | (706) | 9 |
| Aluminum | 50-64 | |
| Low Carbon Steel | 14-17 | |
| Stainless Steel 18-8 | 4 | |
| Pure Nickel | 25 | |
| Pure Zinc | 28 | |
| Pure Tin | 15 | |
| The beta brasses (64% copper and less) are especially noted for their ease of working in the hot condition. However, they are not susceptible to extreme cold work and in the lowest range commonly used (about 55% copper) the amount of cold work that can be done without annealing is relatively small. These brasses exhibit comparatively high tensile strength and qualities of hardness. They are not used where maximum corrosion resistance or electrical conductivity is desired although they are better than many other materials in this respect. Their color is a surprising feature. As the copper content decreases under 62% (the beta content increases) they become more and more red so that at 58% copper the color is a close match for Commercial Bronze, 90%. This is useful in that complicated architectural shapes may be made by hot extrusion, and used with rolled panel sheets of Commercial Bronze, 90%. Common beta brasses and their commercial names are: | |
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Muntz
Metal 60% (280) |
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next common series of brasses are those intended for free-machining
purposes. These leaded brasses are numbered from (300) to (398)
although the several “Manganese Bronzes” are in the (600) series. The free-machining property is brought about (in the case of the brasses) by the addition of lead. This is not soluble in, nor does it enter into, chemical combination with the brass. Instead, it collects in small particles throughout the brass, Upon machining, these particles cause the metal being cut off to form into small chips rather than in long spirals. This prevents fouling of tools and the lead helps lubricate the tool face. All the leaded brasses may be hot worked but those with about 1.5% lead or higher must have support as in a die (extrusion or forging). At high temperatures, the lead within the metal is molten and in the ordinary processes of hot rolling or piercing, breakage would occur. Leaded brasses show relatively poor cold working properties. They will all withstand a limited amount of cold work, but as the lead content increases the workability decreases. Some alpha brasses with limited lead content will withstand cold working and still show fair machining properties. These might be called “compromise” alloys. In general, however, leaded alloys are not intended for severe cold working and should be avoided if this characteristic is paramount. Of special interest in the free-machining metals is Tellurium Copper (145). Unlike lead, tellurium forms a chemical compound with copper—a copper telluride—which is in turn insoluble in copper and lies in small particles dispersed through the copper as does the lead in brasses. Tellurium Copper may be hot worked, and will accept cold working but does not compare with other coppers in this respect. Tellurium, in the quantities used, does not appreciably affect corrosion resistance, electrical or heat conductivity or color. Sulfur copper (147) is similar to Tellurium Copper in properties and has an oxygen-free base. A third series of alloys are those containing tin, and are numbered in the (500) series, including those with lead for free machining. Tin is added for one of two effects—added strength or corrosion resistance. In this series are the true bronzes. These are called Phosphor Bronze and contain up to 10% tin. The term phosphor is derived from the fact that the alloy is deoxidized with phosphorus. However, the metal is essentially tin and copper. These alloys show excellent cold-working properties and will exhibit greatly added tensile strength over that ot copper. Their resistance to fatigue (alternating stresses) is very high while their resistance to corrosion is excellent. Their electrical and heat conductivity is low. Tin is added to other alloys in this series in small percentages (about 1%) to increase corrosion resistance. When added to 70-30 brass, we have Admiralty (443)—long used for resistance to both fresh and salt water corrosion, as in heat exchanger tubes. When added to Muntz Metal, we have Naval Brass (464). These alloys do not differ greatly from the original brasses to which the tin has been added except to increase resistance to corrosion. The alloy may be slightly harder. There are a number of special alloys which have particular properties such as high strength or corrosion resistance, or both, which are numbered in the 600 series. Of these there are: |
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| The last series of Revere alloys is the (700) group—the
Cupro-Nickels and the Nickel Silvers. Two alloys are important - Cupro-Nickel 10% (706) and CuproNickel 30% (715). They are standard alloys for marine use in resisting salt water corrosion and/or erosion.
Cupro-Nickel 10% (706) is also non-fouling. They hot and cold-work well and are produced in the form of sheet, plate, tube and rod. The Nickel Silvers are copper-nickel-zinc alloys and are generally used as a base for silver plate. Their strength and corrosion resistance are good and, except for the leaded Nickel Silvers, their workability, hot or cold, is good. The color is silver-white. In all cases, the presence of nickel reduces, electrical and heat conductivity. Nickel Silvers are excellent spring materials and are so used in quantity. The coppers themselves may be considered to be a separate and distinct series of alloys. The most common form is electrolytic tough pitch copper (110) used for electrical work in bus bar, commutator, wire and strip. It is also used to a great extent in sheet form for roofing, general sheet metal work and processing equipment where both corrosion resistance and heat conductivity are required. Strip copper in large quantities is used for automobile radiators and heaters. Its unique color is important for decoration. There are also special Coppers: |
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| *OFHC is the registered trademark of the high purity oxygen-free copper produced by American Metal Climax, Inc. |
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