ANS:B - strain-ageing.

Blue brittleness in mild steel between 230 and 370°C is caused by strain-ageing. Blue brittleness refers to a phenomenon observed in certain types of steel, particularly mild steel, where the material becomes brittle and susceptible to fracture at elevated temperatures, typically between 230 and 370°C (446 and 698°F). This phenomenon occurs due to the interaction between strain (deformation) and aging (time-dependent changes in the material's microstructure). During strain-ageing, dislocations in the crystal lattice of the steel become immobilized or pinned by the presence of small solute atoms (such as carbon and nitrogen) or precipitates within the material. This immobilization of dislocations reduces the material's ability to deform plastically, leading to increased brittleness and susceptibility to fracture under applied stress. Therefore, strain-ageing is the primary mechanism responsible for blue brittleness in mild steel within the specified temperature range. The other options listed—immobility of dislocations, increase in Young's modulus, and strain hardening—do not directly explain the phenomenon of blue brittleness in mild steel at elevated temperatures.