Thermal Processing of Bobby Pins
Objective: To show the difference that processing, especially thermal processing, can have on the properties of a material.
Background Information: A material can possess a variety of different mechanical properties such as strength, stiffness, plasticity, elasticity, and deflection. Deflection is the amount of displacement experienced by a material when placed under a load. This property is of great importance in building construction, and building codes often specify a maximum allowable deflection, generally as a fraction of the length of the beam. A material’s mechanical properties are dependent on the material’s microstructure, including the phases present, the number and arrangement of dislocations, and the grain size and shape.
Thermal processing is used to change the crystal structure, defect structure (dislocations), and/or grain structure of a material. Annealing is a process used to weaken metals, such as steel, to make them easier to form into desired shapes. To anneal metal, it must be heated above a critical temperature, maintained at that temperature, and then allowed to cool. For steel, that critical temperature is the transformation temperature to austenite or austenite/cementite. If a metal is annealed for too long it is considered to be “over-aged,” and in this state, it has very few dislocations and is very ductile. Heating the metal to a red-hot temperature causes the atoms to move faster and more freely. By slowly cooling from this high temperature, the atoms are able to adopt more ordered arrangements and create a more perfect crystal. The more perfect the crystal of the metal, the easier atoms can “slide” past one another, and thus, the more easily the metal can bend. The material also tends to have large grains after annealing and slow cooling (Figure 1), and this leads to a more malleable material. In contrast, to make a metal hard and brittle through thermal processing, it must be rapidly quenched from high temperature to room temperature. This quick cooling of the metal from red-hot temperatures freezes the atoms into a disordered phase with many defects. Due to the large number of defects, the atoms cannot move easily, and the metal is considered hard to bend and brittle. Quenching a metal such as steel will also cause it to change phases (or atomic arrangements) and sometimes form a phase called martensite (Figure 2), which is very hard and brittle. See the Introductory presentation for examples of real-world applications where thermal processing is used to modify materials.