It has a remarkable effect on the mechanical properties of the metal. The temperature, the alloying elements and the thermal impregnation time affect the grain size.
As a general rule, a small grain size is better as it has higher tensile strength, higher hardness and is less deformed during quenching, being less vulnerable to cracking.
The grain size is determined microscopically by counting the grains in each square inch under an increase of 100X.
The specified grain size is usually the austenitic grain size.