Work Hardening Annealing

9.5 Work Hardening Annealing ها if you cold work a component to around 80-90%, it is no longer possible to form this material as furiner deformation would cause it to fracture ها you can heat treat (specifically anneal) a cold worked sample & this can remove some of the cold working defects & have a dramaic impact on material properties Limitations of Cold Working high dislocation density makes the component very strong & brittle ها Clongated grains demonstrate. anisotropic properties (properties of the component are not the same in all directions if the component is heat treated, the grains. con become equared & the component is no longer anisdiagh Annealing Steps Recovery Recrystallisation Grain Growth Recovery Driving force for Recovery original structure dislocation density ~10"m" weale/ ductile cold-worked structure dislocation density~10m² Strang/less ductile avound dislocations there are Localised lattice strains. this strain energy (as) provides the driving force for the recovery process 2 dislocation Burger's density vector Shear modulus ه recovery is completed at an elevated temperature Le strain energy associated with the dislocations is relieved by: dislocation motion reduction in number of dislocanons (dislocation density) change in dislocation configurations Original Cold-worked: After recovery: Low dislocation density High dislocation New dislocation density configuration Lo even after recovery is complete, the grains are still in a relatively high strain energy state Recrystallisation Lo materials are held at elevated temperatures 41 recrystallisation temp: temp at which recrystallisation is completed after one hour typically 0.3-0.5Tm (alloy) 0.3m (pure metal) 0.7Tm (commercial alloys) recrystallisation formation of a new set of strain-free & equiaxed grains with low disiocation densities to recrystallisation driving force difference in internal energy between strained & unstrained material During After recrystallization recrystallization Formation of small Egolaxed grains ها as the key for recrystallisation is the deformation from the cold working process, there needs to be a minimum amount of cold work before a recrystallised (known as the critical cwως typically around 10% component can be recrystallisation temp is a function of the ow as % Cw fes, temp bas