recovery

A cold-worked metal is full of tangled dislocations and locked-in stress, like a rope knotted and twisted under tension. Heat it gently and the first thing that happens, before anything visible changes, is recovery: the dislocations shuffle around, some cancel out, and the trapped stresses ease, so the rope untwists a little without untying its knots. The grains keep their deformed shape; only the internal clutter is tidied.

During recovery the metal's atoms gain just enough thermal wiggle to let dislocations glide and climb into lower-energy arrangements. Opposite-sign dislocations meet and annihilate, and the rest line up into neat walls (a process called polygonization), lowering the stored energy. This restores properties that depend on the lattice being orderly, so electrical conductivity climbs back toward the annealed value and residual stresses drop sharply, but strength and hardness fall only slightly, because the overall dislocation density is still high.

Recovery is the mild, low-temperature step before recrystallization. It is used as a stress-relief anneal: a cold-worked or welded part is warmed just enough to remove damaging residual stresses (which cause warping and stress-corrosion cracking) while keeping most of the strength that cold work gave it. Get the temperature a bit higher and recovery gives way to recrystallization, where brand-new soft grains erase the deformation entirely.