Question

Copper Lab

1. Briefly describe the effect of annealing temperature on hardness. Why does annealing have this effect? (Describe both the driving force and the physical mechanism for the change in hardness with annealing).

2. Does annealing have a greater effect on the relative change in hardness in the specimen with a 40% target CW or 6% target CW? Why? (Describe both the driving force and the physical mechanism for the change in hardness with annealing).

Answer 1

Annealing

Annealing is a heat treatment in which the metal is heated to a temperature above its recrystallization temperature, kept at that temperature some time for homogenization of temperature followed by very slow cooling to develop equilibrium structure in the metal or alloy. The cooling is done in the furnace itself.

Aims of Annealing are:-

Increase ductility, Reduce hardness and brittleness, Alter microstructure to soften the metal prior to shaping by improving formability, Recrystallize cold worked (strain hardened) metals, Remove internal stresses, Increase toughness, Increase machinability, Decrease electrical resistance, Improve magnetic properties

Based on the temperature of operation during annealing, annealing is further categorised into the various process:-

1. Full annealing
2. Stress relief annealing
3. Process annealing
4. Spheroidizing annealing

Now As coldwork is performed on specimen its grain orientation changes and leads to increase in hardness and brittleness. So to reduce hardness and induce ductility Annealing is performed.

It is followed in three phases:_

1. Recovery - The relief of some of the internal strain energy of a previously cold-worked material. recovery occurs in which there is a change in the stored energy without any obvious change in the optical microstructure. Excess vacancies and interstitials anneal out giving a drop in the electrical resistivity but little change in hardness. Dislocations become mobile at a higher temperature, eliminate and rearrange to give new orientation.
2. Recrystallization - the formation of a new set of strain-free grains within a previously cold-worked material. The dislocation density decreases only a little during recovery and the deformed grain structure is largely unaffected by recovery. It takes the growth of new grains to initiate a much larger change, i.e. recrystallization. The nucleation of new grains happens in regions of high dislocation density. Nucleation begins in a jumble of dislocations. The recrystallized grain will essentially be free from dislocations. A greater nucleation rate leads to finer ultimate grain size. There is a critical level of deformation below which there will be no recrystallization at all. A critical strain anneal can lead to a single crystal on recrystallization.
3. Grain Growth - the increase in the average grain size of a polycrystalline material. A large concentration of grain boundaries (fine grain structure) is reduced by grain growth that occurs by high temp. annealing. The driving force for grain growth is the reduction in the grain boundary surface energy.

NOW,

Yes due to different % of CW there is change in hardness of specimen after annealing. Because %of CW relates to different recrystallization temperature. Different temperature is needed for annealing and grain growth pattern and grain sizes will affect hardness of the specimen.

Below image shows the variation of hardness upon annealing of various %CW specimens of Al-based alloys (CR- cold rolling).

Clearly understandable form above image is that all specimen is cold-worked from same ~1400 degree Celcius, still shows different hardness as they have been initially subjected to different %of CW.