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Strain Hardening |
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Pure iron, wrought iron, and extremely low carbon steels cannot be appreciably hardened by heat treatment, since they contain no hardening element. Cast iron can be hardened, but its heat treatment is limited. When cast iron is cooled rapidly, it forms white iron, which is hard and brittle. When cooled slowly, it forms gray iron, which is soft but brittle under impact.
In plain carbon steel, the maximum hardness depends almost entirely on the carbon content of the steel. As the carbon content increases, the ability of the steel to be hardened increases. However, this increase in hardenability with an increase in carbon content continues only to a certain point. In practice, that point is 0,85 percent carbon content. When the carbon content is increased beyond 0,85 percent, there is no increase in wear resistance.
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For most steels, the hardening treatment consists of heating the steel to a temperature just above the upper critical point, soaking or holding for the required length of time, and then cooling it rapidly by plunging the hot steel in to oil, water, or brine. Although most steels must be cooled rapidly for hardening, a few may be cooled in still air hardening increases the hardness and strength of the steel but makes it less ductile. When hardening carbon steel, it must be cooled to below 1000*F, in less than 1 second. Should the time required for the temperature to drop to 1000*F, exceed 1 second, the austenite begins to transform into fine pearlite. This pearlite varies in hardness, but is much harder than the pearlite formed by annealing and much softer than the martensite desired. After the 1000*F, temperature is reached, the rapid cooling must continue if the final structure is to be all martensite. When alloys are added to steel, the time limit for the temperature drop to 1000*F, increases above the 1-second limit for carbon steels. Therefore, a slower quenching medium will produce hardness in alloy steel. Because of the high internal stresses in the "as quenched " condition, steel must be tempered just before it becomes cold. The part should be removed from the quenching bath at a temperature of approximately 200*F, since the temperature range from 200*F, down the room temperature is the cracking range.
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