General effects of annealing

6.8.1 General effects of annealing

When a metal is cold worked, by any of the many industrial shaping operations, changes occur in both its physical and mechanical properties. While the increased hardness and strength which result from the working treatment may be of importance in certain applications, it is frequently necessary to return the metal to its original condition to allow further forming operations (e.g. deep drawing) to be carried out for applications where optimum physical properties, such as electrical conductivity, are essential. The treatment given to the metal to bring about a decrease of the hardness and an increase in the ductility is known as annealing. This usually means keeping the deformed metal for a certain time at a temperature higher than about one-third the absolute melting point.

Cold working produces an increase in dislocation density; for most metals ρ increases from the value of 10 10 −10 12 lines m −2 typical of the annealed state, to 10 12 –10 13 after a few percent deformation, and up to 10 15 –10 16 lines m −2 in the heavily deformed state. Such an array of dislocations gives rise to

a substantial strain energy stored in the lattice, so that the cold-worked condition is thermodynamically unstable relative to the undeformed one. Consequently, the deformed metal will try to return to a state of lower free energy, i.e. a more perfect state. In general, this return to a more equilibrium structure cannot occur spontaneously but only at elevated temperatures, where thermally activated processes such as diffusion, cross-slip and climb take place. Like all non-equilibrium processes the rate of approach to equilibrium will be governed by an Arrhenius equation of the form:

Rate = A exp[ − Q/kT ], where the activation energy Q depends on impurity content, strain, etc.

The formation of atmospheres by strain ageing is one method whereby the metal reduces its excess lattice energy, but this process is unique in that it usually leads to a further increase in the

Mechanical properties I 349

Vickers hardness

) (mW) ∆ P 100

50 a′

Power difference (

Temperature (K)

Figure 6.48

and hardness (VPN) for specimens of nickel deformed in torsion and heated at 6 K min −1 (Clareborough, Hargreaves and West, 1955).

structure-sensitive properties rather than a reduction to the value characteristic of the annealed condition. It is necessary, therefore, to increase the temperature of the deformed metal above the strain-ageing temperature before it recovers its original softness and other properties.

The removal of the cold-worked condition occurs by a combination of three processes, namely: (1) recovery, (2) recrystallization and (3) grain growth. These stages have been successfully studied using light microscopy, transmission electron microscopy or X-ray diffraction; mechanical property measurements (e.g. hardness); and physical property measurements (e.g. density, electrical resistivity and stored energy). Figure 6.48 shows the change in some of these properties on annealing. During the recovery stage the decrease in stored energy and electrical resistivity is accompanied by only a slight lowering of hardness, and the greatest simultaneous change in properties occurs during the primary recrystallization stage. However, while these measurements are no doubt striking and extremely useful, it is necessary to understand them to correlate such studies with the structural changes by which they are accompanied.