Quenching and tempering is a heat treatment process used to improve the mechanical properties of metals and alloys. The process involves heating the material to a high temperature, then cooling it rapidly (quenching), followed by reheating it to a lower temperature (tempering). This combination of heating, cooling, and reheating produces significant changes in the microstructure and properties of the material. This post will take a look at exactly what is happening inside the material and why we might want to do it!
A Summary Of The Process
Quenching is the process of rapidly cooling a hot metal to lock the atoms in place, creating a very hard but brittle microstructure. This is typically done by plunging the hot metal into a cooling bath, such as oil or water. The rapid cooling rate prevents the atoms from rearranging into a more stable, lower-energy state, which results in a high density of defects in the crystal lattice. These defects increase the hardness of the material, but also make it more susceptible to cracking.
Tempering is the process of reheating the quenched metal to a lower temperature, which increases the toughness of the material. During the tempering process, the defects in the crystal lattice rearrange into a more stable, lower-energy state, reducing the density of defects as well as the hardness.
How Does It Work?
During the quenching and tempering process, a number of phase changes occur, resulting in changes in the microstructure of the material. These phase changes involve the transformation of one type of crystal structure to another, and they are driven by changes in temperature and cooling rate.
Austenite to Martensite Transformation
During the quenching process, the austenite crystal structure transforms into a martensite crystal structure. Austenite is a face-centered cubic (FCC) crystal structure that is stable at high temperatures in ferritic steels. During quenching, the rapid cooling rate prevents the austenite from transforming into a more stable, low-temperature crystal structure, such as ferrite (BCC) or pearlite. Instead, the austenite transforms into martensite, a body-centered tetragonal (BCT) crystal structure. This transformation results in a hard and brittle microstructure with a high density of defects, such as dislocations, which are responsible for the increased hardness and strength of the material. The image below shows a typical martensitic microstructure, which is characterized by narrow lath and block patterns bounded by the prior austenite grain boundaries.
The plot below shows the Time-Temperature-Transformation (TTT) diagram of a typical steel. This allows us to understand the effect of cooling rate on the structures that will form during the cooling process from fully austenite. As can be seen in the schematic, fast cooling rates result in the formation of either martensite or bainite (similar to martensite), while significantly slower cooling rate allows the growth of a ferrite-pearlite microstructure.
Ferrite-Pearlite Regrowth
During the tempering process, the martensite crystal structure begins to transform back into a more stable, low-temperature crystal structure, such as ferrite or pearlite. This transformation is driven by the rearrangement of defects in the crystal lattice into a more stable, lower-energy state. The specific crystal structure that forms depends on the temperature, the cooling rate during the tempering process, and the chemical composition of the material. The image below shows ferrite/pearlite regrowth (white) in a largely martensitic (grey) microstructure. The formation of these new crystal structures also results in a reduction in the hardness of the material, which improves the toughness and fatigue performance of the material.
Why Do We Do It?
Improved Strength: The high-energy defects created during the quenching process increase the hardness and strength of the material. This increase is a result of the increased density of defects in the crystal lattice, which prevent dislocations from easily moving through the material, which is the mechanism through which plasticity occurs. This results in a material that is stronger and more resistant to deformation.
Increased Hardness: High-energy defects also result in a material that is much harder than the original material. Hardness is a measure of a material's resistance to indentation and abrasion, and is closely related to strength. Hardness and wear resistance is essential in applications such as gears and drills, in which parts are constantly rubbing together.
Improved Fatigue Performance: The rearrangement of defects during the quenching and tempering process can also improve the fatigue performance of the material. Fatigue is a failure mode that results from the repeated application of stress, and the improved fatigue performance is a result of increased toughness of the material. This can be very valuable in applications such as shafts and axles, where load is cycling often and failure would be catastrophic.
Final Thoughts
Hopefully this post provides some insight into the process and why we might want to do it in our engineering components. In conclusion, the quenching and tempering process involves a series of phase changes, including the transformation from austenite to martensite and the regrowth of ferrite-pearlite. These phase changes result in changes in the microstructure of the material and the atomic movement of atoms within the crystal lattice. The specific mechanisms of these phase changes are driven by changes in temperature and cooling rate, and they result in significant improvements in the strength, hardness, and fatigue performance of the material.
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