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| Figure 4: After annealing, the high-yield, high-tensile, low-elongation metal becomes elastic and has low yield, low tensile strength, and low elongation. |
Some material can be formed with a zero inside radius (folded back on itself) without outside cracks. Others may require an inside radius equal to one, two, or more times the metal thickness. Steel with 80,000- to 100,000-PSI yield strength and about 2 percent elongation may require a minimum inside radius equal to or larger than four times the material thickness, or r = 4t.
The microstructure of the material also can affect cracking. Large, rough inclusions and certain alloying elements can contribute to cracking, but with proper alloying and treatment, the inclusions can be controlled and the minimum bending radius of some high-strength steels (up to 200,000-PSI yield strength) can be formed with an inside radius equal to four times the metal thickness.
Cold Working and Hot Working
The yield and tensile strength of a material usually can be increased by cold working, alloying, heat treating, and combinations thereof. For example, bending a piece of wire several times back and forth at the same place will harden and eventually break the wire. The reason for the breakage is the repeated bending, which is cold working. With each bend, the wire gets harder and its elongation is reduced until it cannot take any more cold work and it breaks.
The same phenomenon occurs when metals are rolled. To achieve the specified material thickness, the metal strip is passed through a pair of rolls with decreasing gaps between them. After a certain amount of cold reduction, the metal becomes so hard that rolling cannot be continued without cracking the strip.
Some of the cold-worked metals—often called skin-rolled, 1/4, 1/2, 3/4, or full hard, depending on the degree of cold work—are used by the industry. Most of the metals used, however, are heated to a higher temperature, or annealed, before they are shipped to users. During annealing above a specific temperature and time, the long, elongated, and hardened crystals recrystallize and acquire new and more elastic properties. As a result, the high-yield, high-tensile, low-elongation hardened metal after annealing is again elastic and has low yield, low tensile strength, and long elongation (see Figure 4).
