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| COLD WORKING AND HARDNESS | |
| Our second basic characteristic making copper and many of its alloys valuable is the ability to withstand severe cold working, i.e., high ductility and malleability. In other words, they can be drawn or rolled to a remarkable degree at room temperature. As we work copper in the cold condition, hardness is imparted to it. In fact, the hardness imparted to the metal increases as the amount of cold working increases. Incidentally, this is the only way that copper or an alpha brass may be hardened. It cannot be heat-treated to obtain hardness as can the steels. This is true not only of copper but any pure (unadulterated or unalloyed) metal, i.e., aluminum, iron, magnesium. There is no “lost art” of tempering copper despite the wide publicity given its alleged existence. Any “hard” copper made by the ancients was made hard by cold work, by the presence of impurities making an alloy, or both. As we are considering the ability of copper to be cold worked, we should have some basic understanding as to: |
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| A piece of soft copper may be considered as being made up of innumerable crystals of copper. A crystal of copper, if it could be produced under ideal conditions, would take the form of a perfect cube. This cube might be compared with one of a pair of dice. At each corner of this die is a molecule of copper. On each face of the die—located in the position of the one spot—or “snake eye” —is another molecule of copper. This arrangement of molecules is called a face-centered lattice. Acting between these molecules are stresses—each molecule having an attraction for those about it. When the opportunity arises for the molecules to move about freely, they will arrange themselves in the position in
which the minimum of these stresses occurs—much as the molecules in a rubber ball return to their natural position when you stop squeezing it. This freedom of movement is brought about by heating processes, and during this condition the molecules take the position of least stress, i.e., the cubical, face-centered lattice described above. Now starting with this condition—namely, a material with maximum freedom from stress (or in other words, soft metal), let us see what happens when any cold work—or cold deformation—occurs. With sufficient stress (application enough to overcome the stresses holding the molecules in position) the crystal is fractured and part moves or slips along on the other part. To illustrate, if we consider our crystal as a pack of cards we have taken half the deck and slid it along the other half for a slight distance. In this area where slippage has occurred, the orderly molecular arrangement has been destroyed and the intermolecular stresses are increased. Because of this increased stress there is now a greater resistance to further deformation and the metal in this area is harder. As further work is done on the crystal, more and more fractures and slippages occur until we have completely broken up the former orderly or crystalline arrangement of the molecules. The metal may now be said to be “full hard” or completely cold worked. Referring to our deck of cards once again, all the cards have been moved in relation to those about it as a result of numerous cold workings. The fractures just described are called slip-planes”, an amorphous material that has no crystal structure. This then is the theory of cold working, and refers not only to copper and its alloys but other metals as well. To digress for a moment into factors other than cold working, which affect the hardness of metals, let us consider the case of the brasses wherein the addition of zinc to copper increases hardness. Let us return to the illustration of the copper crystal wherein the molecules align themselves in a cubic manner—a molecule at each corner of the cube and one centrally located on each face. With the addition of zinc, one or more of the molecules of copper is displaced by molecules of zinc. The molecules are still in approximately the same position—but the forces of attraction of a zinc molecule are not the same as those of a copper molecule. The crystal is not as well balanced with greater stresses set up between molecules. As usual, where stresses are increased so is the resistance to deformation increased proportionally. To put it another way, we now know that the metal is harder. If we should continue to add zinc, we would eventually cause such increases in stresses that the molecules would no longer take the position described before. We would still have our cube with a molecule at each corner, but instead of one on each face, we now have one deep inside the body of the cube. This is a ‘body-centered lattice”. The crystal might be said to be compact with very great internal stresses—in other words—hardness. When this condition is reached, we have beta brass. Other metals will substitute for copper in our original cube as does zinc—but the stresses that are set up vary with different metals. For example, aluminum sets up strong stresses in low percentages making the strong aluminum bronze. Silicon is similar—making the silicon bronzes or Herculoys. The cold working principle used with these alloys is practically the same as that of copper itself insofar as the crystalline derangement is concerned. Of course, with greater inherent hardness, stronger external forces are required for fabrication. Up to now, we have described the basic changes taking place within the metal when it is cold worked. Let’s consider the practical effect of cold working. In our discussion, we carried the cold working to the point of complete crystal deformation. In practice this is seldom done. In addition to certain effects this might have upon subsequent processing, it is not an economical procedure as the metal becomes very hard and processing is difficult. Copper may be cold reduced more than 95% without annealing. It is clear by now surely, from all the preceding comments, that with increased cold work, we obtain increased hardness. This means that with limited work. we can produce a metal slightly harder than soft and by additional work, we can increase this hardness all the way up to the point of the maximum possible. With this control, we can supply a complete range of tempers within the limits for a given metal, to provide for successful use in a wide variety of applications. Let us consider some of these applications. As an example, a relatively soft material might be desired with which the manufacturer wishes to do considerable cold work himself—such as producing deep drawn or spun articles. A hard material would be indicated where maximum strength is required and little or no cold work is to be done. In between these two extremes lies a wide variation of possible tempers—any of which may be desired. Consider the case of switch clips. Here a strip of metal is bent to form a “U”. It must be soft enough to withstand the bend—and yet springy enough to insure good contact with the switchblade when it is pushed into the “U” of the clip. All this means that the customer and the supplier must have a common ground of understanding. The former must have some means of indicating to the supplier the degree of hardness he requires. The latter must have means of insuring that his product meets the degree of hardness specified. We must have, therefore, a set of standards understood by everyone concerned. There are several organizations that issue specifications which serve as standards for various products, among them, The American Society for Testing and Materials (ASTM), the Society of Automotive Engineers (SAE), the American Society of Mechanical Engineers (ASME) and the U.S. Government Military (MIL), and Federal. ASTM is most used in the brass mill industry with about 200 specifications covering many alloys and types of products. Some typical ones are: |
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| Various methods of testing are also established such as: | |
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| The standards for Copper Metals all have the prefix B, as are those for other nonferrous materials as aluminum, magnesium, zinc, etc. Steels would be A, for Cement, C, and many organic compounds as rubber, textiles, etc., would be D. The copper metal standards, as well as others, are written and approved by both producers and users, thus representing a consensus of all interested parties. There may also be specifications set up by individual users, which may deviate from similar ASTM specifications to accommodate peculiar requirements of the buyer. It has been the custom in the industry to describe cold worked tempers for sheet (strip) and wire as one eighth (1/8) hard, one quarter (¼) hard, half (½) hard, etc. This represents the amount of cold reduction in thickness for strip (by area for sections as rod, wire and tube) in reducing metal to specified gauge. In strip rolling it is described in terms of Brown and Sharp gauge numbers. For example, .064” thick strip is 14 B & S gauge. If we cold reduce it 6 B & S numbers, the final gauge will be .032. It will be called Extra Hard and represents about 50% cold work. (The B & S gauge system has about 11% difference in thickness between each gauge number. Consequently, counting gauge numbers from the initial thickness to the final one is an easy way of describing the amount of cold work.) The majority of the specifications in use require these various conditions of cold work to be measured by various means, which will now be described. A test for surface hardness is the easiest one to make and may be used as a complete control device or as an advisory one. The most common in our industry is the Rockwell hardness test which measures the degree of penetration of a ball or diamond cone into the metal in a certain way; the deeper the penetration the softer the metal. The values are expressed as B (100 kilogram load), F (60 kilogram load), 30T, 15T and so on. The higher the number from 1 to 100, the harder the material, but one must know the load or scale used to understand the value. There are others as Brinell, Knoop, Vickers, etc., for other materials or for special measuring needs. Within certain limits, they may be converted one to the other. The most complete description of a metals’ properties are obtained by determining the Ultimate Tensile Strength, the Yield Strength (various), the elongation and, for solid sections like rod and wire, the reduction of area. All of these will result in using a tensile testing machine which applies a steady controlled force to a suitably prepared specimen. It first stretches - the amount of which may be recorded automatically on a graph called a stress-strain curve - and then breaks the piece. Ductile materials may stretch, or “neck down” to a remarkable extent; brittle materials will not. Ultimate tensile strength is the pounds required to break the metal divided by the area of the sample before fracture and expressed as pounds per square inch (PSI), though it is usually expressed as KSI;. . . 1 KSI equals 1000 lbs/sq. in. For annealed copper the value will be between 30,000 and 33,000 PSI, and for Full Hard (4 No’s) about 45,000 PSI (or 45 KSI); spring temper 5% Phosphor Bronze (510) would be about 100,000 PSI (or 100 KSI); Beryllium Copper (170) about 200,000 PSI (200 KSI). The Yield Strength is not determined for specification purposes as often as tensile strength, but is needed in design of many things since it is a measure of the ability to withstand a load or stress without being permanently deformed - in other words it absorbs the load without permanent damage. The methods of determining the several values as 0.1 and 0.2% offset yield strengths are outlined in ASTM specification E 8. The same sample used for getting the tensile strength is marked in the center portion so that the amount of total stretch to breakage can be measured. The difference between the original distance (usually 2”) and that after breaking, divided by the original (x 100) is called % of Elongation - the original marked distance must always be specified since it is not necessarily always 2”; and the meaning is different for different gauge lengths. Very ductile metals as copper, cupro-nickel, etc., have values over 50%. The number becomes less as material is cold worked; after 95% cold work, copper will be only about 1%. The reduction in area represents the amount the specimen “necks down” - or stretches out like taffy candy being pulled. Measurement is made by matching the broken pieces and getting the diameter at the smallest place. The difference between the area at this point and the area of the smallest part of the original sample divided by the original (x 100) is the % of Reduction in Area. For very ductile materials the value is over 60% and for brittle materials, less than 10%. Both the % of Elongation and the % of Reduction Area are useful in determining the ability of a metal to be cold worked to make strip by cold rolling, bolts by cold heading, rods by cold drawing and so on. |
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