Question

The water quenched 1045 sample had a higher hardness and tensile strength than the air cooled sample? Explain in term of microstructural theory as to why this is the case.

Answer 1

In simple language we can say, when the metal is cooled after heating upto certain temperature there will be multidirectional crystal growth called dendrite formation. If the cooling rate is slow nucleation takes place at smaller places which produces coarse grain structure. If the cooling rate are very high, nucleation will takes place at large number of places ,this produces fine grain structure.

In case of quenching the workpiece is cooled in mediums like water, oil or aqueous sollution which gives faster rate of cooling. Therefore quenching gives finer garain structure as compared to air cooling which results in higher hardness and tensile strength.

When the grain boundaries are less than that of atoms are called coarse atoms and when they are more they are called fine atoms.As we know the region of orientation mismatch are called grain boundaries. When the dislocation starts moving at the grain boundary it changes its direction which requires more amount of stresses. As soon as the dislocation approaches the grain boundary it has to change its orientation because atoms are arranged in a different direction ,this requires extra stress. So grain boundaries creates an obstacle in the movement of dislocation. So, fine the grain better will be the strength and hardness of the material.