Mechanical Engineering - Engineering Materials - Discussion

When metal is heating to corresponding crystallization temperature of metal the internal stresses of metal are relieved then start re-crystallization that means forming of another crystal structure then metal will cooling during this process grain grow-thing is takes place.

After annealing the metal is heated above the crystallization temper and then stress relief.

The micro structure contains deformed grains from cold-work and there are a number of dislocations when we first heat the metal, for example, the additional thermal energy permits the dislocations to locomote.

This treatment removes residual stresses due to cold-working without changing the dislocation density of but causing a rearrangement in the dislocations therefore stress is relieved. Then new small grains begin to nucleate at the cell boundaries, eliminating most of the dislocations thereby creating the new grains we are recrystallizing the metal.

If the thermal energy is high enough the grains begin to grow, with favored grains consuming the smaller ones, therefore grain growth takes place.

After cold working of metals are subjected to annealing to relieve internal stresses are induced during cold working and that annealing carried out by subsequent re-crystallization and grain growth.

The Recovery occurs at low temperatures and reduces or eliminates work-hardening effects. Interestingly, the grain structure is not affected, leaving existing grain boundaries (size) intact. As a result of recovery at low temperatures, small changes in hardness occur due to a decrease in the density of microstructural characteristics created by the initial cold work.

Recrystallization occurs when sufficient thermal energy is available to drive the creation and strain-free growth of new grains in the existing matrix. It is a diffusion process, driven by increased thermal energy. The growth of these new grains eliminates the distortions in the existing matrix, effectively erasing the increase in mechanical properties that had resulted from the cold work. This returns ductility to the material as well as reducing tensile and yield strength. For our shops, this means lower hardness and gummier, less brittle chips. In very high carbon grades, the diffusion into a spheroidal form of carbides results in an improvement in machinability and surface finish by increasing the mean free path of ferrite between the carbide spheroids.

Grain growth is the result of continued high temperatures past recrystallization as the grain boundaries are eliminated, increasing the present grain size regardless of its prior austenitic grain size. Large oversize grains observed in nominally fine-grained steel are evidence of time and temperature at or above the recrystallization temperature.

Please give an example for the answer.

When heated steel 1st at of all relieves the internal stress caused by cold working and after it will be re crystallization of steel and than upper reaction. Temperature will be grain growth.