Work hardening is a process where metals harden when mechanically flexed, useful for increasing strength in metals that cannot be heat treated. Dislocations form in the crystalline structure, increasing yield strength but decreasing ductility. Cold forming can intentionally initiate work hardening, but excessive bending can cause unwanted work hardening, leading to damage or breakage.
Work hardening is a process in which metals harden when they are mechanically flexed. Work hardening is a process that takes place in the crystalline lattice of a material. This form of hardening is useful for increasing the strength of metals that cannot be hardened by heat treatments. Many metals and alloys that can be heat hardened can, however, also be work hardened. Work hardening, also called work hardening, can be applied intentionally as a hardening process during the forming of the part or occur unintentionally during machining or abnormal operation.
Prior to work hardening, materials typically exhibit a uniformly distributed, defect-free crystalline structure. When the material is subjected to mechanical stress, microscopic defects known as dislocations form in the crystalline structure. If the stress continues, these dislocations propagate and interact with each other, forming new internal structures that resist further bending. These formations – or nail points – increase the materials yield strength, or ability to withstand stress, resulting in a decrease in ductility or softness. One of the most common ways to intentionally initiate the work hardening process is by cold forming parts.
As mentioned above, work hardening can be a desirable or an undesirable process. When work hardening is an expected end result, cold working or forming parts is one of the most effective ways to achieve it. This is especially useful when machining metals that cannot be heat hardened. These include low carbon steel, aluminum and pure copper. When these metals are pressed, drawn, bent or hammered during forming, the stresses involved induce the formation of crystalline dislocations which harden the material.
Unwanted work hardening occurs when ductile or soft materials are machined incorrectly or bent excessively during their duty cycles. If, during machining, the workpiece is exposed to excessively deep cuts, the resulting stress can cause the formation of crystalline dislocations resulting in hardening. This unintentional work hardening can therefore prevent further machining or even damage the tool tips. When machining ductile parts, tool tips must be advanced carefully to prevent unwanted work hardening from occurring.
Metal parts that are bent beyond their design parameters during regular work can also experience some degree of work hardening. Small deviations within these parameters are easily absorbed by a material which returns to its original shape without any changes to its internal structure. When it is flexed beyond these limits, however, the process of dislocation formation begins and the material hardens. This causes a consequent resistance to any bending, which can lead to eventual cracking or breakage of the piece.
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