Articles

12.12.2014

Cold Working Wrought forming refers to shaping metals by mechanical deformation. This includes bending, forging, stamping, and many other deformation operations.

We have seen that the typical stress-strain curve taken at room temperature slopes upwards. If we are part way up the curve, we would describe the current sample as stronger than when the test began, since it now possesses a higher yield strength. It is also more brittle, in that it now requires less strain to fracture than the original sample. This phenomenon is called strain hardening or work hardening, and when it occurs, the process is called cold working. Approximately, this is the case when the forming temperature is: 0.2 < T/Tm < 0.4, where T is the test temperature, and Tm is the melting temperature of the metal. Temperature ratios are only meaningful with absolute temperature scales – kelvin in SI units. Room temperature forming falls in the above range for almost all metals. Exceptions are low melting lead and tin alloys. The explanation for this phenomenon is one of the detective successes in metallurgy research. First, it was observed that plastic deformation is internal shear deformation. Then, the force required to push one layer of close packed atoms over another was calculated. Observed yield stresses are ten to a hundred times weaker than the calculated values. Although the calculations were approximate, this much discrepancy suggests that something else is going on. An analogy is to consider the floor to be one plane of atoms and a rug to be another. One way to move the rug is to grab hold of an edge and pull. This takes a lot of work. A more clever way is to put a bump in the rug along one edge, and then push the bump across the room. The net result is the same. The rug has been displaced, but the second way takes much less work. Long before they were seen directly with electron microscopes, it was speculated that there were, in metal crystals, line defects of atomic arrangement labeled dislocation lines. It would take less force to move a dislocation line than to move an entire plane of atoms. The result of a dislocation line sweeping across a crystal plane would be a net displacement of planes, analogous to pushing the bump in the rug. The process of strain hardening is explained by observing that plastic deformation itself generates dislocations in large numbers. As the density of dislocations builds up, the dislocation lines become entangled and movement becomes more difficult. That is, a higher applied stress is required. When we break a metal wire by repeated bending, we are work hardening it to the point of it becoming brittle. Plastic deformation at the bend is required.

Src: http://www.saylor.org/