We have considered the steel grinding balls surface hardness measurement and the steel grinding balls hardness measurement at 1/2 radius depth in previous articles. We will consider the steel grinding balls volumetric hardness measurement in this article. The grinding balls volumetric hardness provides to confirm their compliance to the fifth hardness group.

The volumetric hardness determines in two mutually perpendicular directions on the templates cut from the central part of the grinding ball. Their surface prepared in accordance with the requirements GOST 9012 and GOST 9013.

It is necessary remove metal to the R distance from one side of grinding ball and to 1/2R distance from the other side in order to the sample stably stood on hardness tester table. The measurement points must be drawn with a pencil. We draw two mutually perpendicular straight lines for this, then each radius we halved and the rest radius parts we halved also.

As a result, we obtain 6 measurement points on each straight line. They correspond to 0,25R; 0,5R; 0,75R and one common point R (the grinding ball center). We get 4 values on each part of the radius, except the center.

The minimum and maximum values of the obtained hardness are not taken into account. Hardness defines as the arithmetic mean of the two remaining measurements.

**Volumetric hardness (VH) calculated by formula: **

**VH= 0,289 ****Т**_{surf}**+ 0,436 ****Т**_{0,25}** + 0,203****Т**_{0,5}** + 0,063****Т**_{0,75}** + 0,009 ****Т**_{c}

where **Т**_{surf}**, ****Т**_{0,25}** , ****Т**_{0,5}** ,** **Т**_{0,75}** , ****Т**** _{c}** the hardness values at a distance from the grinding ball surface in the radius parts

The value of T** _{surf } **takes from the surface hardness measurement for a given grinding ball