Grinding balls hardness is one of the most decisive features when discussing grinding media business. Different balls sizes and hardness groups accomplish different missions for particles size reduction to allow mineral recovery.
Smaller balls enjoy an easier life facing much less challenges. They have a rather uniform hardness from surface to the core and roll over using their kinetic energy resulted from ball mill torque as attrition forces to powderize the already broken ores.
As the grinding balls get larger in size, their responsibility will also be different. They will fall down on freshly extracted big mineral ores to break them into smaller pieces using deceleration impact forces. For this purpose, while large grinding balls should be tough on the surface, their core hardness shall be relatively lower in a way that it forms a hardness profile that reaches its minimum at the very central point that I call heart of the ball here. This slightly lower hardness helps a forged grinding ball damp the impact forces not to break. Imagine a 120 mm forged grinding ball going through a SAG mill without any hardness gradient from surface to core. It will certainly break in no time, because as impact forces go through the ball, static molecules cannot damp the vibrations and at some point, this high hardness causes the ball to break. Here comes the great role of heat treatment. Balls need to get so hot for forging and after that to get the required mechanical properties, they shall be quenched in steps and over time. This process aims at a volumetric hardness which in turn improves milling efficiency.
This is story of life! The bigger you get in terms of knowledge, experience, socioeconomical rank, position, etc., the more responsibility you have to accept, simply because you now have a greater role in comparison with the ones who are considered beneath you and for that reason you should be more accountable! You cannot shoulder great responsibilities of life without getting tougher on the outside to endure punches that life throws at you. Nevertheless, as you get hard on the surface, you should be relatively softer inside, otherwise you will be damaged and broken badly! This process happens by enduring pain over time, just like heat treatment steps for grinding balls!