THE INTERNAL STRAINS IN GRINDING BALLS

In the selection of grinding balls, the consumer is mostly based on hardness (hardness group) and chemical composition of the balls. The selection does not take into account such factors as the geometrical parameters (design) of the ball mill, the possibility of changing the technological parameters of the production process (quantity of material supply to the mill, dense modes, etc.). These factors have a significant influence on the grinding process, the grinding balls and the formation of internal stresses in the balls.

Energosteel specialists constantly carry out research in the direction of production improvement. In this publication we will discuss the topic of intensity of internal stresses depending on the grade of steel from which the steel grinding balls were made.

The steel (analogue Gr.2) was chosen as the material to be analysed and is ideal for the production of 25-80mm high hardness steel balls, i.e. grinding balls that operate in cascade or mixed mode. However, the use of this grade of steel is not always appropriate for the production of 90-125mm diameter steel balls.

So what is the secret? As shown in Figure 1, steel Gr.2 has a critical point Mh – the beginning of the 200 ̊С martensitic transformation, with 100 ̊С the transformation only occurs at 50% and only at 20 ̊С does it complete.

Isothermal thermokinetic diagram of the decomposition of supercooled austenite

When used in the mill, the balls experience impact loads. The point impact of the ball surface on another ball and armour creates stress concentrators on the surface layer of the impact sites. This is due to structural transformations of steel that occur at a micro level. Since the temperature reaches 200 C and above at the point of impact, the atoms move in a crystal lattice, accompanied by a certain energy. The cyclicality of these shocks results in the accumulation of this energy, leading to microfractures, and at one point, a chip of the surface layer is produced.

It follows from the above that 90-125mm balls made of the steel under investigation are suitable for cyclic impact only if there is enough shredding material to minimize the ball impact.

This is achieved by reducing the mill load with grinding balls, increasing the mill’s performance or changing the design of the armour.

It is also not recommended to use this type of grinding ball in ball mills with approximately the same diameter as its length.

In the case of balls with a diameter of 25-80mm made of the above-mentioned grade of steel, which operate in a mill or in a mixed mode, or in a cascade (abrasion), no specific chips are observed because they do not receive or accumulate excess energy.

To sum up, we can safely say that the selection of grinding balls is a rather complex and time-consuming process that requires certain skills and knowledge. Our specialists have all the necessary knowledge and will always help our partners to make the right choice.