Table of Contents

Materials and Structure




Recrystallization and Grain Growth

When materials are altered in shape by plastic deformation their grain shape tends to change. This is illustrated below for a wire being "drawn" into a dimension suitable for a bicycle spoke. While passing through the drawing die the material decreases in cross section and is elongated (material volume is conserved). This process also work hardens the material and makes it less ductile. Heat treatment is used to counter these effects.

By heating the steel wire to 650 C, the process of recrystallization occurs under the influence of the strain fiends produced during deformation. Small approximately equiaxed grains are produced from the elongated grains of the deformed material by this stage of heat treatment. The grain size of the material is then controlled by heating it at 1000 C where diffusion processes are more rapid. These processes cause the growth of large grains and the dissolution of small grains. Time and temperature are used to control the final grain size and achieve the desired mechanical properties: yield stress, hardness, ductility and toughness. The graph below shows the variation in hardness of the spoke.

From: McMahon and Graham, "Introduction to Engineering Materials," Merion (1992)