The disadvantages of hardening include increased brittleness, dimensional changes, compromised quality of the hardened case, and potential conflicts between desired surface properties and bulk properties.
Increased Brittleness: Hardening processes, such as heat treatment, can significantly increase the strength of materials. However, this increase in strength often comes at the cost of reduced toughness and increased brittleness. For instance, case hardening or through hardening enhances strength but can make the material more brittle. To mitigate this, parts are often tempered or drawn back to decrease brittleness. The degree of tempering required depends on the desired ultimate strength of the material. If the material is too brittle after hardening, it may need to be annealed or re-tempered to improve its ductility and usability.
Dimensional Changes: Hardening processes can lead to significant dimensional changes in materials. This requires larger material stock allowances to accommodate these changes during post-processing operations. The dimensional changes can affect the final fit and function of components, especially in precision applications.
Compromised Quality of the Hardened Case: The quality of the hardened case can be compromised during post-heat-treatment operations such as grinding. These operations can remove the area of highest hardness, which was the primary objective of the hardening process. Additionally, issues like intergranular oxidation/intergranular attack (IGO/IGA) and surface oxidation (dealloying) can occur to a certain depth, affecting the integrity and performance of the hardened surface.
Conflicts Between Surface and Bulk Properties: Hardening often focuses on improving surface properties such as hardness and wear resistance. However, these improvements can conflict with the bulk properties of the material, which might be required to maintain high mechanical stability, ease of manufacturing, or low material cost. In many high-tech applications, components are made from composite materials where the surface properties significantly differ from the bulk properties. This can lead to challenges in achieving a balance between surface hardness (for wear resistance) and bulk toughness (for resistance against crack propagation), as a single material may not fulfill both demands effectively.
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