Compare precipitation hardening (Section 11.9) and the hardening of steel by quenching and tempering (Sections 10.5, 10.6, and 10.8) with regard to the following: (a) The total heat treatment procedure (b) The microstructures that develop (c) How the mechanical properties change dur- ing the several heat treatment stages

Precipitation hardening, also known as age hardening, is a heat treatment technique used to increase the yield strength of malleable materials, including most structural alloys of aluminium, magnesium, nickel, titanium, and some steels and stainless steels. The process enhances the metal's mechanical properties by the formation of very fine precipitates that impede dislocation movement, leading to increased hardness and strength. Precipitation hardening consists of three integral stages: solution heat treatment, rapid cooling or quenching, and aging.

During solution heat treatment, the alloy is heated until its constituent elements dissolve to form a solid solution. Upon quenching, the solution is rapidly cooled to preserve the high-temperature phase structure. Finally, during aging, typically at a lower temperature, fine precipitates form within the structure, strengthening the material. Aging can be natural at room temperature or artificial at elevated temperatures. The key here is the control of precipitation size and distribution to enhance mechanical properties without compromising material ductility.

Quenching and tempering are two stages of heat treatment commonly applied to steel to enhance its mechanical properties. Austenitization, the first stage, involves heating the steel above its critical temperature to form austenite, with homogeneous distribution of carbon and other elements. Quenching then rapidly cools the steel, usually in water or oil, leading to the formation of martensite, a super-saturated solution of carbon in iron. This phase is hard and brittle.

The subsequent tempering stage is crucial as it relieves the stresses induced by quenching and reduces brittleness, leading to a tougher microstructure. By carefully controlling the tempering temperature, manufacturers can fine-tune a steel's final mechanical properties to achieve an ideal balance of hardness, toughness, and ductility. The resulting tempered martensite is less hard but more durable than its untempered counterpart.

The development of microstructures during heat treatment is fundamental to the changes in a material's mechanical properties. Microstructure refers to the arrangement of phases and structural features on a microscopic scale. In precipitation hardening, the microstructure evolution involves the uniform distribution of fine precipitate particles that block the movement of dislocations, thereby hardening the alloy.

Conversely, in the quenching and tempering of steel, martensite forms first—a hard, highly stressed, and tetragonal structure. Temperament then modifies the martensitic structure, reducing internal stresses and resulting in the development of tempered martensite, along with secondary phases like cementite, which enhance toughness. These microstructural changes are directly correlated to the material's performance and can be observed and quantified using techniques like microscopy, which provides valuable insights into the effectiveness of the heat treatment process.

Mechanical property changes during the heat treatment of alloys are dramatic and are essential for tailoring materials for specific applications. Precipitation hardening is effective in significantly enhancing the hardness and tensile strength of alloys. Specifically, the aging process increases these properties due to the impediment of dislocation motion by newly formed precipitates.

In the case of quenching and tempering steel, the mechanics of transformation are different. Quenching leads to a hard but brittle material due to the formation of martensite, which lacks ductility. The tempering stage partially relaxes the martensitic structure and typically involves the precipitation of carbides—fine-tuning the steel's hardness, yield strength, and ductility. Understanding these mechanical property changes is crucial for engineers and material scientists to manipulate the heat treating processes optimally to yield desired material characteristics.