in metallurgy, increase in hardness of a metal induced, deliberately or accidentally, by hammering, rolling, drawing, or other physical processes. Although the first few deformations imposed on metal by such treatment weaken it, its strength is increased by continued deformations. The reason for this seeming paradox lies in the crystalline structure of metal. As stresses are exerted, the crystals slip against each other; but, because of the complexity of the crystal structure, the more such slips are multiplied, the more they tend to place obstacles in the way of further slippage, because the various dislocation lines crisscross each other. See also tempering.