Heat Treatment Processes
Heat treating is a general term for a collection of processes meant to affect the microstructural or chemical configuration of a material. These processes are most often used with metals but can be used with other materials such as glass and plastics. Below is a list of common processes used.
Annealing is used to eliminate residual stresses left from a previous process. In metals, this process "restores" a material through the elimination of dislocations from plastic deformation by inducing recrystallization. This process can be divided into three processes: recovery, recrystallization, and grain growth.
Recovery occurs when the physical properties of a cold-worked metal are restored without changing the microstructure. Properties sensitive to point defects are commonly most affected by this process.
Recrystallization occurs when the microstructure of a cold-worked metal is replaced by a new, strain-free microstructure. Typically, recrystallization can be detected by standard metallographic methods. Dislocation density decreases significantly during recrystallization effectively eliminating any prior strain hardening.
Grain Growth is a strongly temperature-dependent process. Grain growth is driven by a decrease in free energy as a result of decreasing grain-boundary area. This process is inhibited by second-phase particles, which restrict grain-boundary motion. Abnormal grain growth can occur in some cases, where a fine-grained, recrystallized metal begins to grow grains by consumption of surrounding grains.
Precipitation hardening, also known as age hardening, is a method used to strengthen a metallic alloy by causing the formation of secondary particles. These particles act as barriers to dislocation motion by causing dislocation pile-ups to either stop dislocation motion or slow motion by forcing dislocations expend their energy by moving around the inclusion. For precipitation hardening to occur the second phase must be soluble at elevated temperatures and must exhibit decreasing solubility as temperature decreases. This requirement places a restriction on the number of useful precipitation hardening systems.
Quenching is rapid cooling process that strengthens a metal by causing the formation of a quasi-stable phase with an overabundance of solute atoms that act as barriers to dislocations. These phases (martensites) tend to be strong yet brittle.
Tempering is a process applied to metal alloys that lowers hardness (mild annealing) while recovering lost ductility and is usually performed after quenching. The amount of hardness reduction depends on the temperature of the tempering process.