What is the effect of grain size on mechanical properties of metals? All solid metals are crystalline and the crystals or grains are made up of several atoms. The grain size has an important effect on the mechanical properties of a metal. The size of the grains depends upon a number of factors, but the principal one is the heat treatment to which the metal has been subjected.
When a low carbon steel is heated, there is no change in grain size upto the *lower critical point and it is same for all steels (723° C). At this temperature, birth of new grains takes place. At the upper critical point, the average grain size is a minimum. Further heating of the steel causes an increase in the size of the grains, which in turn governs the final size of the grains when cooled. Some steels like medium carbon steel and many alloy steels when heated to a higher temperature, known as coarsening temperature, the grain size increases very rapidly. The coarsening temperature is not a fixed temperature and may be changed by prior hot or cold working and heat treatment.
*The temperature point at which the change starts on heating is called lower critical point and the temperature point where this change ends in heating is called upper critical point. It varies according to the carbon content in steel.
The quenching of steel from the upper critical point results in a fine grained structure, whereas slow cooling or quenching from a higher temperature yields a coarse grained structure. The coarse grained steels are less tough and have greater tendency for distortion than those having a fine grain. A fine grained steel, in addition to being tougher, are more ductile and have less tendency to distort or crack during heat treatment.