Methods for volumetric hardness measurement of grinding balls

There are several methods for volumetric hardness measurement of grinding balls. The method for hardenability determining, namely, the method of end hardening with a hardness measurement interval from 1.5mm to 3mm (GOST 5657-69) takes as the basis of all methods.

Hardenability – the steel ability of tempering at a certain depth from the surface to the center.

We will consider the 4 most common methods for the volumetric hardness measurement of grinding balls in this article:

1. The method of volumetric hardness determination proposed in DSTU 3499-97 / DSTU 8538: 2015.

This method consists of hardness measurement at all radius parts of grinding balls, averaging the values and determining the hardness by the formula:

VH= 0,289 Тsurf+ 0,436 Т0,25 + 0,203Т0,5 + 0,063Т0,75 + 0,009 Тc
Where:
VH – volumetric hardness of grinding balls;
Тsurf – surface hardness of grinding balls;
Т0,25 – hardness at 0,25 of the radius depth;
Т0,5 – hardness at 0,5 of the radius depth;
Т0,75 – hardness at 0,75 of the radius depth;
Тc – hardness at center of grinding ball;

The hardness measurement scheme

2. The second method of volumetric hardness measurement used in Bulgaria.

Measurement and calculation takes place by zones (4 zones correspond to radius parts as at DSTU 3499-97 / DSTU 8538: 2015, but measurements provide by center of zones).

The volumetric hardness determination calculates by the formula:

VH = (48.8 * T1 Zone + 29.6 * T2 Zone + 15.2 * T3 Zone + 5.6 * T4zone + 0.8 * Center) / 100

The hardness measurement scheme

This method has a small deficiency, because it isn’t takes into account the surface hardness of grinding balls.

3. The third method used in Turkey.

This method consists of the hardness measurement from the surface to the center with R/12 step (for example: the radius R is 30 mm for a 60mm grinding ball, then R divide into 12 dimensions and get a 2.5 mm pitch).

The volumetric hardness determination calculates by the formula:

VH = 22,97%*Т_1zone+19,16% Т_{2 zone} +15,68% Т_{3 zone} +12,56% Т_{4 zone} +9,78% Т_{5 zone} +7,35% Т_{6 zone} +5,27% Т_{7 zone} +3,53% Т_{8 zone} +2,14% Т_{9 zone} +1,1% Т_{10 zone} +0,41% Т_{11 zone} +0,06% Т_{12 zone}
Where: T1 – 12 zones – the hardness value at each zone of grinding ball from the surface to the center, measured at one radius.

The hardness measurement scheme

This method has small deficiencies, because measurements perform at the same radius and don’t exclude the spotted hardness presence.

The fourth method of volumetric hardness measurement used in Czech Republic.

This method is similar to the previous one: measurement and calculation occurs by zones (10 zones with a layout from the surface A to the center J, the radius equal 1).

The volumetric hardness determination calculates by the formula:

VH = 0,271*А+0,217*В+0,169*С+0,127*D +0,091*Е+0,061*F +0,037*G +0,019*Н+0,007*I +0,001*J

The hardness measurement scheme

Summarizing, we can say all methods analogous and lead to the same goal – determination the hardened layer depth from the surface to the center of grinding ball.

Steel grinding balls with cross-cutting hardenability (i.e. high volumetric hardness) have the same structure and, as a result, the same properties (yield strength, relative narrowing and toughness) through the entire section, provides high operational properties: the ability to wear resist during work in the mill and the grinding balls reduce consumption per one ton of raw materials.