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Strain hardening is a process in which metals harden when flexed mechanically. Strain hardening is a process that takes place in the crystal lattice of a material. This form of hardening is useful to increase the strength of metals that cannot be hardened by heat treatments. Many metals and alloys that can be heat hardened may, however, also be strain hardened. Strain hardening, also called work hardening, can be purposefully applied as a hardening process during part forming or occur unintentionally during machining or abnormal operation.
Prior to strain hardening, materials typically exhibit an evenly distributed, defect-free crystalline structure. When the material is subjected to mechanical stress, microscopic defects known as dislocations form in the crystal structure. If the stress continues, these dislocations propagate and interact with each other, forming new internal structures that resist further deflection. These formations — or pinning points — increase the materials yield strength, or ability to resist being stressed, with a subsequent decrease in ductility or softness. One of the most common ways of intentionally initiating the strain hardening process is to cold form parts.
As mentioned previously, strain hardening can be a desirable or undesirable process. When work hardening is an intended end result, cold working or forming of parts is one of the most effective ways of getting it done. This is particularly useful when working metals that can not be heat hardened. These include low-carbon steel, aluminum and pure copper. When these metals are compressed, drawn, bent or hammered during forming, the stresses involved induce the formation of the crystalline dislocations that harden the material.
Undesirable strain hardening occurs when ductile or soft materials are machined incorrectly or flexed excessively during their work cycles. If, during machining, the part is exposed to excessively deep cuts, the resultant stress can cause the formation of crystalline dislocations with resultant hardening. This inadvertent hardening can then prevent further machining or even damage the tool bits. When ductile parts are machined, tool bits should be advanced cautiously to prevent unwanted work hardening from occurring.
Metal parts that are flexed beyond their design parameters during regular work may also experience a degree of strain hardening. Small deflections within those parameters are easily absorbed by a material that returns to its original shape without any changes to its internal structure. When flexed beyond those limits, however, the process of dislocation formation begins, and the material hardens. This causes a resultant resistance to any flexing, which can lead to eventual cracking or fracturing of the part.