Two identical steel samples are cold-rolled: Sample A to 70% reduction, Sample B to 20% reduction. Both are slowly heated in a furnace. Which sample begins recrystallization at a lower temperature?
ASample B — less damage means dislocations can reorganize more easily at lower temperatures
BSample A — more stored energy from greater cold work provides a larger driving force, lowering the recrystallization temperature
CBoth samples recrystallize at the same temperature, since recrystallization temperature is a fixed material property
DSample B — fewer dislocations provide cleaner nucleation sites for new grains
Recrystallization is driven by stored elastic strain energy from dislocations. The 70%-reduced sample has a far higher dislocation density and therefore more stored energy. This larger thermodynamic driving force means recrystallization can proceed at a lower temperature — less thermal activation is needed. Recrystallization temperature is not a fixed property; it decreases with increasing prior cold work, typically spanning a range of 100–200°C for the same alloy.
Question 2 Multiple Choice
A machined precision component must have residual stresses removed after machining, but its dimensional accuracy and grain structure must be preserved. Which annealing treatment is most appropriate?
AFull anneal — heat to the austenite region and furnace-cool to produce the softest possible condition
BStress-relief anneal — heat below the recrystallization temperature to allow dislocation rearrangement without forming new grains
CNormalizing — air-cool from the austenite region to produce a fine, uniform pearlite microstructure
DProcess anneal — heat just above recrystallization temperature to restore ductility for further forming
A stress-relief anneal targets the recovery stage: temperatures are high enough for dislocation rearrangement (reducing residual stresses) but below recrystallization, so no new grains form and dimensional changes are minimal. A full anneal or normalizing would alter the microstructure and potentially cause distortion from the phase transformation. The process anneal restores ductility for forming operations — not the right objective here.
Question 3 True / False
Recrystallization is a solid-state transformation in which new strain-free grains nucleate and grow to replace the cold-worked microstructure — the metal does not melt during this process.
TTrue
FFalse
Answer: True
Recrystallization occurs entirely in the solid state, typically at 0.3–0.5 of the absolute melting temperature — hundreds of degrees below the melting point. New grains with very low dislocation density nucleate at dislocation tangles (sites of highest stored energy) and grow by consuming the deformed matrix. The crystal structure type does not change; only the dislocation density and grain morphology change. This distinction from melting matters because the dimensional and surface characteristics of the part are preserved.
Question 4 True / False
A full anneal of plain-carbon steel produces a finer, stronger microstructure than normalizing because the furnace provides more controlled cooling.
TTrue
FFalse
Answer: False
This is backwards. A full anneal uses slow furnace cooling, which allows more time for pearlite lamellae to coarsen — the result is coarse pearlite, the softest and most machinable condition. Normalizing uses faster air cooling, which produces finer pearlite with closely spaced lamellae — yielding higher strength and hardness than a full anneal. Full anneal maximizes softness; normalizing maximizes uniformity and moderate strength. For applications requiring consistent mechanical properties (rather than minimum hardness for machining), normalizing is preferred.
Question 5 Short Answer
Explain why the degree of prior cold work affects the recrystallization temperature, and identify the thermodynamic driving force for recrystallization.
Think about your answer, then reveal below.
Model answer: Cold working (plastic deformation below the recrystallization temperature) dramatically increases the density of dislocations in the metal, storing elastic strain energy in the lattice. This stored energy is the thermodynamic driving force for recrystallization — the system reduces its free energy by replacing the high-dislocation-density deformed structure with strain-free grains. Because the driving force scales with stored energy, which scales with dislocation density, more severe cold work creates a larger driving force that can overcome the activation barrier at a lower temperature. A lightly cold-worked sample requires higher temperatures (more thermal activation) to nucleate and grow new grains; a heavily cold-worked sample recrystallizes more readily and at lower temperatures.
This principle has practical consequences: a part that has been bent or formed slightly may not recrystallize under the same anneal that fully recrystallizes a heavily drawn wire. Process engineers must account for the actual reduction percentage when specifying anneal temperatures, not just the alloy composition.