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Introduction
Quenching is a widely-used method for heat treatment of metals and alloys, which involves heating the material to a suitable temperature followed by rapid cooling. This technique is often used in order to obtain a desired mechanical or microstructural change within the material. When quenching is applied, a transformation will occur where excess heat is released as the material suddenly cools, leaving it with greater hardness and strength. While this technique can obviously be incredibly useful, it also leads to a number of potential problems. One of the most challenging issues that practitioners of heat treatment may face is the introduction of unwanted internal stresses.
Internal Stresses in Quenched Metals
Internal stresses are generated in metal components every time they undergo quenching processes. These stresses, also known as retained stresses, can be result from various sources including, but not limited to: molecular diffusion, shear work and thermal contraction. In addition, it can often be difficult to predict the magnitude of the internal stresses that will be developed as a result of quenching.
When stresses are retained, a number of different deformation patterns can arise. These patterns are usually different from the ones that are presented by the full quenching process due to the difference in cooling rates. Additionally, internal stresses can also lead to increased or decreased mechanical properties, as well as distort a material’s chemical composition. Moreover, internal stresses can also cause a material to become brittle and may even lead to cracks or fractures.
Methods to Reduce Internal Stress in Quenched Metals
Many measures exist to reduce the level of internal stress that is created when a metal undergoes quenching. These methods are generally categorized into two different approaches: those that occur before the quenching process and those that happen after the quenching has been completed.
Pre-Quenching Measures
The implementation of pre-quenching measures is often the best way to reduce unwanted internal stresses. A variety of techniques can be used to accomplish this, including tempering, annealing, aging, and metal alloying.
Tempering: During tempering, the metal is heated for an extended period of time at a relatively low temperature. This method is typically used to reduce the amount of retained stresses in metals by softening those that are already present.
Annealing: This process involves heating a metal to a certain temperature in order to form a desired microstructure. After annealing, the material should have a homogenous microstructure, which is more malleable and less prone to internal stresses.
Aging: This heat treatment also works to improve a material’s microstructure by allowing it to slowly age over time. By aging the metal, it can become more resistant to internal stresses.
Metal Alloying: By introducing different types of metal alloys into a material, it can become harder and less prone to internal stresses.
Post-Quenching Measures
Often, post-quenching measures are the only option when it is too late to reduce stresses before quenching. The most common approach is known as post-quenching tempering, which is used to soften the metal and ease retained stresses.
Conclusion
The introduction of internal stresses can be a major concern for practitioners of quenching, as it can drastically alter the performance of the material and potentially lead to fractures. Given the severity of this problem, various methods have been established to minimize the amount of retained stress that is generated by quenching. By taking pre-quenching measures such as tempering, annealing, aging, or alloying, the amount of generated internal stress can be reduced substantially. When this is not possible, post-quenching tempering can be used to soften the metal and ease any retained stresses.
