ANS:B - refinement

The improvement in mechanical properties of metals during hot working is primarily attributed to grain refinement. Hot working involves shaping or forming metals at elevated temperatures, typically above their recrystallization temperature but below the melting point. During hot working processes such as forging, rolling, or extrusion, the metal undergoes plastic deformation, causing changes in its microstructure. Grain refinement occurs as a result of the plastic deformation during hot working. The original coarse grains in the metal are broken down into smaller, more numerous grains. This finer grain structure improves the mechanical properties of the metal by enhancing its strength, toughness, and ductility. Recrystallization, on the other hand, occurs after hot working when the metal is allowed to cool. It involves the formation of new, strain-free grains within the material, which helps relieve internal stresses and further improves mechanical properties. Recovery is an early stage in the process where some of the dislocations introduced during plastic deformation are removed, leading to a reduction in internal stresses. However, recovery alone does not significantly improve mechanical properties. Grain growth, conversely, typically occurs at higher temperatures and involves the enlargement of grains over time. This can lead to a coarser grain structure and may degrade mechanical properties. In summary, during hot working, the improvement in mechanical properties of metals is primarily due to grain refinement resulting from plastic deformation.